Studies on the Enantioselective Synthesis of E-Ethylidene-bearing Spiro[indolizidine-1,3′-oxindole] Alkaloids

molecules Article Studies on the Enantioselective Synthesis of E-Ethylidene-bearing Spiro[indolizidine-1,30-oxindole] Alkaloids Nihan Yayik 1, Maria Pérez 2, Elies Molins 3, Joan Bosch 1and Mercedes Amat 1,*   Citation: Yayik, N.; Pérez, M.; Molins, E.; Bosch, J.; Amat, M. Studies on the Enantioselective Synthesis of E-Ethylidene-bearing Spiro[indolizidine-1,30-oxindole] Alkaloids. Molecules 2021,26, 428. https://doi.org/10.3390/ molecules26020428 Academic Editor: David Diez Received: 29 December 2020 Accepted: 12 January 2021 Published: 15 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Laboratory of Organic Chemistry, Faculty of Pharmacy and Food Sciences, and Institute of Biomedicine (IBUB), University of Barcelona, 08028 Barcelona, Spain; [email protected] (N.Y.); [email protected] (J.B.) 2Department of Nutrition, Food Sciences and Gastronomy, Faculty of Pharmacy and Food Sciences, and Institute of Nutrition and Food Safety (INSA-UB), University of Barcelona, 08921 Santa Coloma de Gramanet, Spain; [email protected] 3Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Cerdanyola, Spain; [email protected] *Correspondence: [email protected]; Tel.: +34-93-402-4540 Abstract: A synthetic route for the enantioselective construction of the tetracyclic spiro[indolizidine1,3 0 -oxindole] framework present in a large number of oxindole alkaloids, with a cis H-3/H-15 stereochemistry, a functionalized two-carbon substituent at C-15, and an E-ethylidene substituent at C-20, is reported. The key steps of the synthesis are the generation of the tetracyclic spirooxindole ring system by stereoselective spirocyclization from a tryptophanol-derived oxazolopiperidone lactam, the removal of the hydroxymethyl group, and the stereoselective introduction of the E-ethylidene substituent by acetylation at the α -position of the lactam carbonyl, followed by hydride reduction and elimination. Following this route, the 21-oxo derivative of the enantiomer of the alkaloid 7(S)-geissoschizol oxindole has been prepared. Keywords: oxindoles; alkaloids; spiro compounds; ethylidene 1. Introduction The spiro[pyrrolidine-3,3 0 -oxindole] ring system is a structural moiety found in a large number of natural products, pharmaceuticals, and biologically active compounds. This moiety is present in oxindole alkaloids, which constitute a large group of monoterpenoid alkaloids characterized by a spiro-fusion to a pyrrolidine ring at the 3-position of the indole core [ 1 – 3 ]. Oxindole alkaloids possess a variety of pharmacological properties [ 4 – 12 ] and have served as an inspiration for the development of new therapeutic agents [ 13 ]. One of the most important substructural classes of these alkaloids incorporates a tetracyclic spiro[indolizidine-1,3 0 -oxindole] framework, in which the pyrrolidine nucleus is embedded in an indolizidine ring. Structurally, these tetracyclic oxindole alkaloids differ in the functionalized substituent at C-15 and the two-carbon substituent at C-20 (usually ethyl, vinyl or E-ethylidene), as well as in the configuration at the C-3, C-7, C-15, and (in some cases) C-20 stereocenters (biogenetic numbering) [ 14 ], thus giving rise to a diversified array of relative stereochemical relationships. Some representative examples are depicted in Figure 1. Molecules 2021,26, 428. https://doi.org/10.3390/molecules26020428 https://www.mdpi.com/journal/molecules Molecules 2021,26, 428 2 of 14 Molecules 2021, 26, x FOR PEER REVIEW 2 of 14 Figure 1. Representative tetracyclic spiro[indolizidine-1,3′-oxindole] alkaloids. The members of this group of alkaloids often occur in nature as pairs of C-7 epimers [15,16], interconvertible under acidic or basic conditions by retro-Mannich/Mannich reactions via a ring-opened intermediate [17–20]. Scheme 1 illustrates this equilibrium for geissoschizol oxindoles [16]. Scheme 1. Equilibrating a C-7 epimeric pair of geissoschizol oxindoles. The appealing structure of these alkaloids and their significant biological activities have attracted considerable synthetic attention, resulting in a large number of total syntheses, both in the racemic series and in enantiopure form [21–24]. However, the access to E-ethylidene-bearing spiro[indolizidine-1,3′-oxindoles] with a cis H-3/H-15 stereochemistry has been little explored and to our knowledge no total syntheses of tetracyclic oxindole alkaloids with this substitution and stereochemical pattern have been reported so far. In fact, although the oxidative rearrangement of tetrahydro-β-carbolines is one of the most frequently used methods to assemble the spiro[pyrrolidine-3,3′-oxindole] system [18,19,21], its application to either cis or trans E-ethylidene-bearing Corynanthe-type indolo[2,3-a]quinolizidine amines (but not amides) leads only to C-7 epimeric mixtures of tetracyclic spirooxindoles with a trans H-3/H-15 stereochemistry due to an equilibration of the C-3 and C-7 stereocenters via a reversible Mannich reaction (Scheme 2) [25,26]. The severe A1,3-interaction between the C-15 side chain and the methyl group on the ethylidene moiety in the cis isomers, which is absent in the trans isomers, could account for the exclusive formation of the latter [26]. Scheme 2. Oxidative rearrangements of indolo[2,3-a]quinolizidines reported by S. F. Martin [26]. N N H O H MeO2C O N N H H MeO2C O NN MeO2C H HX X=O X=H,H mixture of C-7 epimers t-BuOCl, Et3N then AgClO4, HClO4 aqueous MeOH t-BuOCl then aq. HOAc cis H-3/H-15 trans H-3/H-15 33 15 15 7 Figure 1. Representative tetracyclic spiro[indolizidine-1,30-oxindole] alkaloids. The members of this group of alkaloids often occur in nature as pairs of C-7 epimers [ 15 , 16 ], interconvertible under acidic or basic conditions by retro-Mannich/Mannich reactions via a ring-opened intermediate [ 17 – 20 ]. Scheme 1illustrates this equilibrium for geissoschizol oxindoles [16]. Molecules 2021, 26, x FOR PEER REVIEW 2 of 14 Figure 1. Representative tetracyclic spiro[indolizidine-1,3′-oxindole] alkaloids. The members of this group of alkaloids often occur in nature as pairs of C-7 epimers [15,16], interconvertible under acidic or basic conditions by retro-Mannich/Mannich reactions via a ring-opened intermediate [17–20]. Scheme 1 illustrates this equilibrium for geissoschizol oxindoles [16]. Scheme 1. Equilibrating a C-7 epimeric pair of geissoschizol oxindoles. The appealing structure of these alkaloids and their significant biological activities have attracted considerable synthetic attention, resulting in a large number of total syntheses, both in the racemic series and in enantiopure form [21–24]. However, the access to E-ethylidene-bearing spiro[indolizidine-1,3′-oxindoles] with a cis H-3/H-15 stereochemistry has been little explored and to our knowledge no total syntheses of tetracyclic oxindole alkaloids with this substitution and stereochemical pattern have been reported so far. In fact, although the oxidative rearrangement of tetrahydro-β-carbolines is one of the most frequently used methods to assemble the spiro[pyrrolidine-3,3′-oxindole] system [18,19,21], its application to either cis or trans E-ethylidene-bearing Corynanthe-type indolo[2,3-a]quinolizidine amines (but not amides) leads only to C-7 epimeric mixtures of tetracyclic spirooxindoles with a trans H-3/H-15 stereochemistry due to an equilibration of the C-3 and C-7 stereocenters via a reversible Mannich reaction (Scheme 2) [25,26]. The severe A1,3-interaction between the C-15 side chain and the methyl group on the ethylidene moiety in the cis isomers, which is absent in the trans isomers, could account for the exclusive formation of the latter [26]. Scheme 2. Oxidative rearrangements of indolo[2,3-a]quinolizidines reported by S. F. Martin [26]. N N H O H MeO2C O N N H H MeO2C O NN MeO2C H HX X=O X=H,H mixture of C-7 epimers t-BuOCl, Et3N then AgClO4, HClO4 aqueous MeOH t-BuOCl then aq. HOAc cis H-3/H-15 trans H-3/H-15 33 15 15 7 Scheme 1. Equilibrating a C-7 epimeric pair of geissoschizol oxindoles. The appealing structure of these alkaloids and their significant biological activities have attracted considerable synthetic attention, resulting in a large number of total syntheses, both in the racemic series and in enantiopure form [ 21 – 24 ]. However, the access to E-ethylidene-bearing spiro[indolizidine-1,3 0 -oxindoles] with a cis H-3/H-15 stereochemistry has been little explored and to our knowledge no total syntheses of tetracyclic oxindole alkaloids with this substitution and stereochemical pattern have been reported so far. In fact, although the oxidative rearrangement of tetrahydroβ -carbolines is one of the most frequently used methods to assemble the spiro[pyrrolidine-3,3 0 -oxindole] system [ 18 , 19 , 21 ], its application to either cis or trans E-ethylidene-bearing Corynanthe-type indolo[2,3-a]quinolizidine amines (but not amides) leads only to C-7 epimeric mixtures of tetracyclic spirooxindoles with a trans H-3/H-15 stereochemistry due to an equilibration of the C-3 and C-7 stereocenters via a reversible Mannich reaction (Scheme 2) [ 25 , 26 ]. The severe A 1,3 -interaction between the C-15 side chain and the methyl group on the ethylidene moiety in the cis isomers, which is absent in the trans isomers, could account for the exclusive formation of the latter [26]. Molecules 2021, 26, x FOR PEER REVIEW 2 of 14 Figure 1. Representative tetracyclic spiro[indolizidine-1,3′-oxindole] alkaloids. The members of this group of alkaloids often occur in nature as pairs of C-7 epimers [15,16], interconvertible under acidic or basic conditions by retro-Mannich/Mannich reactions via a ring-opened intermediate [17–20]. Scheme 1 illustrates this equilibrium for geissoschizol oxindoles [16]. Scheme 1. Equilibrating a C-7 epimeric pair of geissoschizol oxindoles. The appealing structure of these alkaloids and their significant biological activities have attracted considerable synthetic attention, resulting in a large number of total syntheses, both in the racemic series and in enantiopure form [21–24]. However, the access to E-ethylidene-bearing spiro[indolizidine-1,3′-oxindoles] with a cis H-3/H-15 stereochemistry has been little explored and to our knowledge no total syntheses of tetracyclic oxindole alkaloids with this substitution and stereochemical pattern have been reported so far. In fact, although the oxidative rearrangement of tetrahydro-β-carbolines is one of the most frequently used methods to assemble the spiro[pyrrolidine-3,3′-oxindole] system [18,19,21], its application to either cis or trans E-ethylidene-bearing Corynanthe-type indolo[2,3-a]quinolizidine amines (but not amides) leads only to C-7 epimeric mixtures of tetracyclic spirooxindoles with a trans H-3/H-15 stereochemistry due to an equilibration of the C-3 and C-7 stereocenters via a reversible Mannich reaction (Scheme 2) [25,26]. The severe A1,3-interaction between the C-15 side chain and the methyl group on the ethylidene moiety in the cis isomers, which is absent in the trans isomers, could account for the exclusive formation of the latter [26]. Scheme 2. Oxidative rearrangements of indolo[2,3-a]quinolizidines reported by S. F. Martin [26]. N N H O H MeO2C O N N H H MeO2C O NN MeO2C H HX X=O X=H,H mixture of C-7 epimers t-BuOCl, Et3N then AgClO4, HClO4 aqueous MeOH t-BuOCl then aq. HOAc cis H-3/H-15 trans H-3/H-15 33 15 15 7 Scheme 2. Oxidative rearrangements of indolo[2,3-a]quinolizidines reported by S. F. Martin [26]. Molecules 2021,26, 428 3 of 14 2. Results and Discussion We present herein a procedure for the enantioselective construction of cis H-3/H-15 tetracyclic spiro[indolizidine-1,3 0 -oxindoles] bearing an E-ethylidene substituent at the C-20 position. The cis H-3/H-15 relationship was secured using the spirocyclization procedure we have recently reported for the direct generation of spirooxindoles from tryptophanolderived oxazolopiperidone lactams [ 27 ], whereas the E-ethylidene substituent was stereoselectively installed, taking advantage of the piperidone carbonyl present in the resulting tetracyclic spirooxindole. Scheme 3outlines the initial steps of the synthesis. Molecules 2021, 26, x FOR PEER REVIEW 3 of 14 2. Results and Discussion We present herein a procedure for the enantioselective construction of cis H-3/H-15 tetracyclic spiro[indolizidine-1,3′-oxindoles] bearing an E-ethylidene substituent at the C20 position. The cis H-3/H-15 relationship was secured using the spirocyclization procedure we have recently reported for the direct generation of spirooxindoles from tryptophanol-derived oxazolopiperidone lactams [27], whereas the E-ethylidene substituent was stereoselectively installed, taking advantage of the piperidone carbonyl present in the resulting tetracyclic spirooxindole. Scheme 3 outlines the initial steps of the synthesis. Scheme 3. Initial steps of the synthesis. The required bicyclic lactam 2 was prepared, as previously reported [28], by cyclocondensation of prochiral aldehyde-diester 1 with (S)-tryptophanol in a process that involves the stereoselective desymmetrization of two enantiotopic acetate chains. A subsequent bromination with pyridinium perbromide, with careful control of the reaction time (10 s) and operating under strictly anhydrous conditions to minimize the formation of the corresponding oxindole, afforded 2-bromoindole 3 in an improved yield (92%). The latter underwent smooth spirocyclization on treatment with TFA to provide (71%) tetracyclic spirooxindole 4 as a single stereoisomer, whose absolute configuration was unambiguously confirmed by X-ray crystallographic analysis (Figure 2; see Supplementary Materials). Figure 2. ORTEP plot of the X-ray structure of spiro[indolizidine-1,3′-oxindole] 4. Removal of the hydroxymethyl substituent, which plays a decisive role as an element of stereocontrol in the spirocyclization reaction, was initially accomplished in four steps: oxidation to carboxylic acid 6 via the corresponding aldehyde 5, subsequent generation of selenoester 7, and finally radical reductive decarbonylation (Scheme 4) [29,30]. Following this procedure, tetracyclic oxindole 8 was obtained in 45% overall yield from aldehyde 5. Scheme 3. Initial steps of the synthesis. The required bicyclic lactam 2 was prepared, as previously reported [ 28 ], by cyclocondensation of prochiral aldehyde-diester 1 with (S)-tryptophanol in a process that involves the stereoselective desymmetrization of two enantiotopic acetate chains. A subsequent bromination with pyridinium perbromide, with careful control of the reaction time (10 s) and operating under strictly anhydrous conditions to minimize the formation of the corresponding oxindole, afforded 2-bromoindole 3 in an improved yield (92%). The latter underwent smooth spirocyclization on treatment with TFA to provide (71%) tetracyclic spirooxindole 4 as a single stereoisomer, whose absolute configuration was unambiguously confirmed by X-ray crystallographic analysis (Figure 2; see Supplementary Materials). Molecules 2021, 26, x FOR PEER REVIEW 3 of 14 2. Results and Discussion We present herein a procedure for the enantioselective construction of cis H-3/H-15 tetracyclic spiro[indolizidine-1,3′-oxindoles] bearing an E-ethylidene substituent at the C20 position. The cis H-3/H-15 relationship was secured using the spirocyclization procedure we have recently reported for the direct generation of spirooxindoles from tryptophanol-derived oxazolopiperidone lactams [27], whereas the E-ethylidene substituent was stereoselectively installed, taking advantage of the piperidone carbonyl present in the resulting tetracyclic spirooxindole. Scheme 3 outlines the initial steps of the synthesis. Scheme 3. Initial steps of the synthesis. The required bicyclic lactam 2 was prepared, as previously reported [28], by cyclocondensation of prochiral aldehyde-diester 1 with (S)-tryptophanol in a process that involves the stereoselective desymmetrization of two enantiotopic acetate chains. A subsequent bromination with pyridinium perbromide, with careful control of the reaction time (10 s) and operating under strictly anhydrous conditions to minimize the formation of the corresponding oxindole, afforded 2-bromoindole 3 in an improved yield (92%). The latter underwent smooth spirocyclization on treatment with TFA to provide (71%) tetracyclic spirooxindole 4 as a single stereoisomer, whose absolute configuration was unambiguously confirmed by X-ray crystallographic analysis (Figure 2; see Supplementary Materials). Figure 2. ORTEP plot of the X-ray structure of spiro[indolizidine-1,3′-oxindole] 4. Removal of the hydroxymethyl substituent, which plays a decisive role as an element of stereocontrol in the spirocyclization reaction, was initially accomplished in four steps: oxidation to carboxylic acid 6 via the corresponding aldehyde 5, subsequent generation of selenoester 7, and finally radical reductive decarbonylation (Scheme 4) [29,30]. Following this procedure, tetracyclic oxindole 8 was obtained in 45% overall yield from aldehyde 5. Figure 2. ORTEP plot of the X-ray structure of spiro[indolizidine-1,30-oxindole] 4. Removal of the hydroxymethyl substituent, which plays a decisive role as an element of stereocontrol in the spirocyclization reaction, was initially accomplished in four steps: oxidation to carboxylic acid 6 via the corresponding aldehyde 5 , subsequent generation of selenoester 7, and finally radical reductive decarbonylation (Scheme 4) [29,30]. Following this procedure, tetracyclic oxindole 8was obtained in 45% overall yield from aldehyde 5. Molecules 2021,26, 428 4 of 14 Molecules 2021, 26, x FOR PEER REVIEW 4 of 14 Scheme 4. Removal of the hydroxymethyl substituent. Alternatively, 8 was prepared in a more efficient manner by direct rhodium(I)-induced decarbonylation of aldehyde 5. Thus, treatment of 5 with the rhodium(I) complex generated in situ from the chloro(1,5-cyclooctadiene)rhodium(I) dimer {[RhCl(cod)]2} and the bidentate phosphine ligand 1,3-bis(diphenylphosphino)propane (dppp) [31] satisfactorily provided oxindole 8 in 79% yield. The use of the rhodium(I) complex derived from carbonyl(chloro)bis(triphenylphosphine)rhodium(I) [RhCl(CO)(PPh3)2] and dppp [32] gave less satisfactory results (46% yield). The preparation of 8 in four steps and 40% overall yield from tryptophanol-derived lactam 2 represents a notable improvement with respect to the previously reported eightstep route from the same starting lactam 2, involving the generation of a spiroindoline and the final oxidation of an indoline [27,28], which gave 8 in 13% overall yield (Scheme 5). Scheme 5. Previous synthesis of oxindole 8 via oxidation of the indoline moiety. Having secured an efficient and reliable sequence to access the tetracyclic scaffold 8, we focused on the introduction of the C-20 two-carbon substituent. Chemoselective LiBH4 reduction of the ester group and subsequent successive protection of the resulting primary alcohol 9 with tert-butydimethylsilyl chloride (TBDMSCl) and the indole nitrogen with either methoxymethyl chloride (MOMCl) or di-tert-butyl dicarbonate (Boc2O) afforded the diprotected intermediates 12 and 13 (Scheme 6). Under the LiBH4 reduction conditions, only minor amounts (5%) of the corresponding indoline 10 were formed. Scheme 4. Removal of the hydroxymethyl substituent. Alternatively, 8 was prepared in a more efficient manner by direct rhodium(I)-induced decarbonylation of aldehyde 5 . Thus, treatment of 5 with the rhodium(I) complex generated in situ from the chloro(1,5-cyclooctadiene)rhodium(I) dimer {[RhCl(cod)] 2 } and the bidentate phosphine ligand 1,3-bis(diphenylphosphino)propane (dppp) [ 31 ] satisfactorily provided oxindole 8 in 79% yield. The use of the rhodium(I) complex derived from carbonyl(chloro)bis(triphenylphosphine)rhodium(I) [RhCl(CO)(PPh 3 ) 2 ] and dppp [ 32 ] gave less satisfactory results (46% yield). The preparation of 8 in four steps and 40% overall yield from tryptophanol-derived lactam 2 represents a notable improvement with respect to the previously reported eightstep route from the same starting lactam 2 , involving the generation of a spiroindoline and the final oxidation of an indoline [27,28], which gave 8in 13% overall yield (Scheme 5). Molecules 2021, 26, x FOR PEER REVIEW 4 of 14 Scheme 4. Removal of the hydroxymethyl substituent. Alternatively, 8 was prepared in a more efficient manner by direct rhodium(I)-induced decarbonylation of aldehyde 5. Thus, treatment of 5 with the rhodium(I) complex generated in situ from the chloro(1,5-cyclooctadiene)rhodium(I) dimer {[RhCl(cod)]2} and the bidentate phosphine ligand 1,3-bis(diphenylphosphino)propane (dppp) [31] satisfactorily provided oxindole 8 in 79% yield. The use of the rhodium(I) complex derived from carbonyl(chloro)bis(triphenylphosphine)rhodium(I) [RhCl(CO)(PPh3)2] and dppp [32] gave less satisfactory results (46% yield). The preparation of 8 in four steps and 40% overall yield from tryptophanol-derived lactam 2 represents a notable improvement with respect to the previously reported eightstep route from the same starting lactam 2, involving the generation of a spiroindoline and the final oxidation of an indoline [27,28], which gave 8 in 13% overall yield (Scheme 5). Scheme 5. Previous synthesis of oxindole 8 via oxidation of the indoline moiety. Having secured an efficient and reliable sequence to access the tetracyclic scaffold 8, we focused on the introduction of the C-20 two-carbon substituent. Chemoselective LiBH4 reduction of the ester group and subsequent successive protection of the resulting primary alcohol 9 with tert-butydimethylsilyl chloride (TBDMSCl) and the indole nitrogen with either methoxymethyl chloride (MOMCl) or di-tert-butyl dicarbonate (Boc2O) afforded the diprotected intermediates 12 and 13 (Scheme 6). Under the LiBH4 reduction conditions, only minor amounts (5%) of the corresponding indoline 10 were formed. Scheme 5. Previous synthesis of oxindole 8via oxidation of the indoline moiety. Having secured an efficient and reliable sequence to access the tetracyclic scaffold 8 , we focused on the introduction of the C-20 two-carbon substituent. Chemoselective LiBH 4 reduction of the ester group and subsequent successive protection of the resulting primary alcohol 9 with tert-butydimethylsilyl chloride (TBDMSCl) and the indole nitrogen with either methoxymethyl chloride (MOMCl) or di-tert-butyl dicarbonate (Boc 2 O) afforded the diprotected intermediates 12 and 13 (Scheme 6). Under the LiBH 4 reduction conditions, only minor amounts (5%) of the corresponding indoline 10 were formed. Molecules 2021,26, 428 5 of 14 Molecules 2021, 26, x FOR PEER REVIEW 5 of 14 Scheme 6. Stereoselective introduction of the E-ethylidene chain. Initial attempts to directly introduce a 1-hydroxyethyl chain by an aldol-type reaction between 12 or 13 and acetaldehyde (LDA, −78 °C) were unsuccessful, and the corresponding alcohols were only formed in trace amounts. Satisfactorily, the E-ethylidene substituent was installed by acylation at the α-position of the lactam carbonyl of 12, followed by hydride reduction and stereoselective elimination. Thus, acetylation of 12 with methyl acetate in the presence of LDA afforded ketone 14 as a single stereoisomer in 52% yield, with 29% of the starting material being recovered. Although a subsequent reduction with NaBH4 gave a nearly equimolecular mixture of epimeric alcohols 15a and 15b, they could be separated by column chromatography and independently converted to the E-ethylidene derivative 16 in excellent overall yield. Alcohol 15a was dehydrated by treatment with DCC and CuCl as a catalyst in refluxing toluene [33–35] to stereoselectively provide the E-ethylidene derivative 16 in 91% yield. This is a useful method for the syn elimination of β-hydroxycarbonyl compounds that proceeds via an isourea intermediate through a six-membered cyclic transition state [36]. In turn, alcohol 15b was subjected to an anti-elimination sequence [34,35] by treatment of the corresponding mesylate with DBU to give the expected E-ethylidene derivative 16 (60% yield) along with minor amounts of its Z isomer (20% yield). The stereochemistry of both isomers was assigned from their 1H-NMR spectra, in which the olefinic proton of the E isomer appears at a lower field (δ 6.67) than that of the Z isomer (δ 5.94) due to the anisotropic effect of the lactam carbonyl group. A final treatment of 16 under smooth conditions brought about the simultaneous deprotection of the oxindole and alcohol moieties to provide the cis H-3/H-15, E-ethylidene-bearing spiro[indolizidine-1,3′- oxindole] 17. In summary, we have developed a procedure for the stereoselective construction of the spiro[indolizidine-1,3′-oxindole] framework characteristic of tetracyclic oxindole alkaloids, bearing a cis H-3/H-15 stereochemistry and incorporating a functionalized two-carbon substituent at C-15 and an E-ethylidene substituent at C-20. These studies could open synthetic routes to tetracyclic oxindole alkaloids featuring this substitution and stereochemical pattern, for instance, 7(S)-kopsirensine A or 7(S)-isositsirikine oxindole. In fact, compound 17 can be envisaged as the enantiomer of 21-oxo-7(S)-geissoschizol oxindole. Scheme 6. Stereoselective introduction of the E-ethylidene chain. Initial attempts to directly introduce a 1-hydroxyethyl chain by an aldol-type reaction between 12 or 13 and acetaldehyde (LDA, − 78 ◦ C) were unsuccessful, and the corresponding alcohols were only formed in trace amounts. Satisfactorily, the E-ethylidene substituent was installed by acylation at the α -position of the lactam carbonyl of 12 , followed by hydride reduction and stereoselective elimination. Thus, acetylation of 12 with methyl acetate in the presence of LDA afforded ketone 14 as a single stereoisomer in 52% yield, with 29% of the starting material being recovered. Although a subsequent reduction with NaBH 4 gave a nearly equimolecular mixture of epimeric alcohols 15a and 15b , they could be separated by column chromatography and independently converted to the E-ethylidene derivative 16 in excellent overall yield. Alcohol 15a was dehydrated by treatment with DCC and CuCl as a catalyst in refluxing toluene [ 33 – 35 ] to stereoselectively provide the E-ethylidene derivative 16 in 91% yield. This is a useful method for the syn elimination of β -hydroxycarbonyl compounds that proceeds via an isourea intermediate through a six-membered cyclic transition state [ 36 ]. In turn, alcohol 15b was subjected to an anti-elimination sequence [ 34 , 35 ] by treatment of the corresponding mesylate with DBU to give the expected E-ethylidene derivative 16 (60% yield) along with minor amounts of its Zisomer (20% yield). The stereochemistry of both isomers was assigned from their 1 H-NMR spectra, in which the olefinic proton of the Eisomer appears at a lower field ( δ 6.67) than that of the Zisomer ( δ 5.94) due to the anisotropic effect of the lactam carbonyl group. A final treatment of 16 under smooth conditions brought about the simultaneous deprotection of the oxindole and alcohol moieties to provide the cis H-3/H-15, E-ethylidene-bearing spiro[indolizidine-1,3 0 - oxindole] 17. In summary, we have developed a procedure for the stereoselective construction of the spiro[indolizidine-1,3 0 -oxindole] framework characteristic of tetracyclic oxindole alkaloids, bearing a cis H-3/H-15 stereochemistry and incorporating a functionalized two-carbon substituent at C-15 and an E-ethylidene substituent at C-20. These studies could open synthetic routes to tetracyclic oxindole alkaloids featuring this substitution and stereochemical pattern, for instance, 7(S)-kopsirensine A or 7(S)-isositsirikine oxindole. In fact, compound 17 can be envisaged as the enantiomer of 21-oxo-7(S)-geissoschizol Molecules 2021,26, 428 6 of 14 oxindole. The use of (R)-tryptophanol instead of the Senantiomer in the synthetic sequence outlined in Scheme 6would provide access to the natural enantiomeric series. 3. Experimental Section 3.1. General Information All air sensitive manipulations were carried out under a dry argon or nitrogen atmosphere. THF and CH 2 Cl 2 were dried using a column solvent purification system. Analytical thin-layer chromatography was performed on SiO 2 (Merck silica gel 60 F254), and the spots were located with 1% aqueous KMnO 4 . Chromatography refers to flash chromatography and was carried out on SiO 2 (SDS silica gel 60 ACC, 35–75 mm, 230–240 mesh ASTM). NMR spectra were recorded at 400 ( 1 H) and 100.6 ( 13 C), and chemical shifts are reported in δ values downfield from TMS or relative to residual chloroform (7.26 ppm, 77.0 ppm) as an internal standard. Data are reported in the following manner: Chemical shift, multiplicity, coupling constant (J) in hertz (Hz), integrated intensity, and assignment (when possible). Assignments and stereochemical determinations are given only when they are derived from definitive two-dimensional NMR experiments (HSQC-COSY). IR spectra were performed in a spectrophotometer Nicolet Avantar 320 FT-IR, and only noteworthy IR absorptions (cm −1 ) are listed. High resolution mass spectra (HMRS) were performed by Centres Científics i Tecnològics de la Universitat de Barcelona. 3.2. Improved Preparation of Tetracyclic Oxindole 4 (3S,7R,8aS)-3-[(2-Bromo-3-indolyl)methyl]-7-(methoxycarbonylmethyl)-5-oxo-2,3,6,7,8,8ahexahydro-5H-oxazolo[3,2-a]pyridine ( 3 ). A solution of pyridinium tribromide (PyHBr 3 , 4.20 g, 13.1 mmol) in anhydrous THF (57 mL) was added at 0 ◦ C by a large transfer, under an argon atmosphere, to a stirred solution of lactam 2 [ 28 ] (3.21 g, 9.36 mmol) in anhydrous CH 2 Cl 2 (57 mL). Saturated aqueous solutions of Na 2 S 2 O 3 (19 mL) and NaHCO 3 (10 mL) were added 10 s later. Distilled H 2 O (10 mL) was added, and the mixture was extracted with CH 2 Cl 2 . The combined organic extracts were dried over anhydrous mgSO 4 , filtered, and concentrated under reduced pressure. Flash chromatography (hexane to 7:3 hexane-EtOAc) of the resulting residue gave bromo derivative 3 (3.63 g, 92%) as a white foam: [ α ] 22D = − 74.1 (c1.4, CHCl 3 ); IR (KBr): 3171 (NH), 1630, 1734 (CO) cm −1 ; 1 H-NMR (400 MHz, CDCl 3 , COSY, g-HSQC): δ = 1.31–1.40 (qm, J= 11.6 Hz, 1H, H-8), 2.17 (dd, J= 17.6, 10.4 Hz, 1H, H-6), 2.32 (dm, J= 11.6 Hz, 1H, H-8), 2.37 (m, 1H, H-7), 2.40 (s, 2H, CH 2 CO 2 ), 2.65 (dd, J= 17.6, 4.8 Hz, 1H, H-6), 2.69 (dd, J= 14.0, 10.4 Hz, 1H, CH 2 -Ind), 3.65–3.69 (m, 2H, CH 2 -Ind, H-2), 3.69 (s, 3H, CH 3 O), 4.08 (d, J= 8.8 Hz, 1H, H-2), 4.31 (ddd, J= 9.6, 6.8, 3.2 Hz, 1H, H-3), 4.70 (dd, J= 10.0, 3.2 Hz, 1H, H-8a), 7.07 (td, J= 8.0, 0.8 Hz, 1H, H AR ), 7.13 (td, J= 8.0, 0.8 Hz, 1H, H AR ), 7.28 (dd, J= 8.0, 0.8 Hz, 1H, H AR ), 7.76 (dd, J= 8.0, 0.8 Hz, 1H, H AR ), 9.28 (br. s, 1H, NH); 13 C-NMR (100.6 MHz, CDCl 3 ): δ = 26.4 (CH 2 -Ind), 27.2 (C-7), 34.4 (C-8), 37.6 (C-6), 40.1 (CH 2 CO), 51.8 (CH 3 O), 55.5 (C-3), 70.2 (C-2), 88.3 (C-8a), 108.8 (C-Br), 110.4 (CH AR ), 112.0 (C AR ), 118.7 (CH AR ), 120.3 (CH AR ), 122.5 (CH AR ), 127.9 (C AR ), 136.1 (C AR ), 166.8 (CO), 171.8 (CO); HRMS (ESI) calcd for [C 19 H 22 BrN 2 O 4 + Na+]: 433.0577, found: 433.0577. (1’R,3 0 S,7 0 R,8a’R)-3 0 -(Hydroxymethyl)-7 0 -(methoxycarbonylmethyl)-2,5 0 -dioxospiro[indoline3,1 0 -indolizidine] ( 4 ). TFA (3.15 mL, 40.9 mmol) was added at room temperature under an argon atmosphere to a solution of bromo derivative 3 (556 mg, 1.32 mmol) in anhydrous CH 2 Cl 2 (25 mL). The resulting mixture was stirred at room temperature for 16 h and then concentrated under reduced pressure. Flash chromatography (silica previously washed with Et 3 N; 1:1 to 1:9 hexane-EtOAc) of the resulting residue gave spirooxindole 4 (337 mg, 71%) as a white foam: [ α ] 22D = + 46.1 (c1.0, CHCl 3 ); IR (KBr): 3401 (NH), 1617, 1724 (CO) cm −1 ; 1 H-NMR (400 MHz, CDCl 3 , COSY, g-HSQC): δ = 0.69 (q, J= 12.4 Hz, 1H, H-14), 1.70 (dm, J= 12.4 Hz, 1H, H-14), 1.96 (dd, J= 18.0, 12.0 Hz, 1H, H-20), 2.14–2.26 (m, 4H, H-6, CH 2 CO), 2.38 (m, 1H, H-15), 2.67 (ddd, J= 18.0, 3.6, 1.2 Hz, 1H, H-20), 3.63 (s, 3H, CH 3 O), 3.82 (dd, J= 12.0, 2.4 Hz, 1H, CH 2 OH), 3.89 (dd, J= 12.0, 7.2 Hz, 1H, CH 2 OH), 4.10 (dd, J= 12.4, 4.4 Hz, 1H, H-3), 4.64 (m, 1H, H-5), 6.92 (d, J= 7.6 Hz, 1H, H AR ), 7.00 (d, J= 7.6 Hz, Molecules 2021,26, 428 7 of 14 1H, H AR ), 7.07 (td, J= 7.6, 0.8 Hz, 1H, H AR ), 7.29 (td, J= 7.6, 1.2 Hz, 1H, H AR ), 7.88 (br. s, 1H, NH); 13 C-NMR (100.6 MHz, CDCl 3 ): δ = 29.0 (C-15), 29.6 (C-14), 36.7 (C-6), 37.5 (C-20), 39.6 (CH 2 CO), 51.7 (CH 3 O), 56.2 (C-7), 60.0 (C-5), 64.5 (C-3), 66.5 (CH 2 OH), 110.7 (CH AR ), 123.2 (CH AR ), 123.5 (CH AR ), 128.9 (CH AR ), 129.4 (C AR ), 140.3 (C AR ), 170.9 (CO), 171.6 (CO), 177.2 (CO); HRMS (ESI) calcd for [C19H22N2O5+ H+]: 359.1601, found: 359.1601. 3.3. Removal of the Hydroxymethyl Substituent (1’R,3 0 S,7 0 R,8a’R)-3 0 -Formyl-7 0 -(methoxycarbonylmethyl)-2,5 0 -dioxospiro[indoline-3,1 0 - indolizidine] ( 5 ). Dess-Martin periodinane (DMP, 649 mg, 1.53 mmol) and NaHCO 3 (257 mg, 3.06 mmol) were added at room temperature under an argon atmosphere to a solution of spirooxindole 4 (367 mg, 1.02 mmol) in anhydrous CH 2 Cl 2 (38 mL). The resulting mixture was stirred at room temperature for 4 h. Saturated aqueous solutions of Na 2 S 2 O 3 (30 mL) and NaHCO 3 (30 mL) were added and the mixture was stirred for 30 min. The aqueous layer was extracted with CH 2 Cl 2 . The combined organic extracts were dried, filtered, and concentrated under reduced pressure. Flash chromatography (1:1 hexane-EtOAc to EtOAc) of the resulting residue gave aldehyde 5 (280 mg, 77%) as a white foam: 1 H-NMR (400 MHz, CDCl 3 , COSY, g-HSQC): δ = 0.75 (q, J= 12.4 Hz, 1H, H-14), 1.73 (dm, J= 12.4 Hz, 1H, H-14), 1.99 (dd, J= 17.8, 12.0 Hz, 1H, H-20), 2.14–2.33 (m, 3H, CH 2 CO, H-6), 2.44–2.53 (m, 2H, H-6, H-15), 2.70 (ddd, J= 17.8, 5.4, 1.9 Hz, 1H, H-20), 3.63 (s, 3H, CH 3 O), 4.15 (dd, J= 11.4, 4.2 Hz, 1H, H-3), 4.88 (t, J= 9.6 Hz, 1H, H-5), 6.90 (d, J= 7.4 Hz, 1H, H AR ), 6.97 (d, J= 7.8 Hz, 1H, H AR ), 7.07 (td, J= 7.5, 1.0 Hz, 1H, H AR ), 7.30 (td, J= 7.6, 0.4 Hz, 1H, H AR ), 7.69 (br. s, 1H, NH), 9.73 (d, J= 1.7 Hz, 1H, CHO); 13 C-NMR (100.6 MHz, CDCl 3 ): δ = 29.5 (C-15), 29.7 (C-14), 34.6 (C-6), 37.2 (C-20), 39.7 (CH 2 CO), 51.7 (CH 3 O), 56.7 (C-7), 62.9 (C-5), 65.0 (C-3), 110.8 (CH AR ), 123.3 (CH AR ), 123.5 (CH AR ), 128.8 (CH AR ), 129.1 (C AR ), 140.2 (C AR ), 169.1 (CO), 171.6 (CO), 176.4 (CO), 197.4 (CHO); HRMS (ESI) calcd for [C 19 H 20 N 2 O 5 + H+]: 357.1445, found: 357.1450. (1 0 R,3 0 S,7 0 R,8a’R)-7 0 -(Methoxycarbonylmethyl)-2,5 0 -dioxo-3 0 -(phenylselenocarbonyl)spi-ro[indoline3,1 0 -indolizidine] ( 7 ). First step: 2-Methyl-2-butene (2 M in hexane, 6.4 mL) and t-BuOH (25.2 mL) were added at room temperature to a solution of aldehyde 5 (321 mg, 0.9 mmol) in CH 3 CN (8.1 mL). A solution of NaClO 2 (472 mg, 5.22 mmol) and NaH 2 PO 4 (733 mg, 5.31 mmol) in distilled H 2 O (8.7 mL) was added at 0 ◦ C to the above mixture, which was stirred at room temperature for 1 h. Then, 0.1 M Na 2 S 2 O 3 and 2 N HCl solutions were added until pH = 1, and the resulting mixture was extracted with EtOAc, dried over anhydrous mgSO 4 , filtered, and concentrated under reduced pressure to give carboxylic acid 6 , which was used in the next step without purification. Second step: Diphenyl diselenide [(PhSe) 2 , 478 mg, 1.53 mmol] and tri-n-butylphosphine (n-Bu 3 P, 0.63 mL, 2.52 mmol) were added at room temperature under an argon atmosphere to a solution of crude acid 6 in anhydrous CH 2 Cl 2 (5.4 mL). The resulting mixture was stirred at reflux for 16 h. Distilled H 2 O was added. The aqueous layer was extracted with CH 2 Cl 2 , and the combined organic extracts were dried over anhydrous mgSO 4 , filtered, and concentrated under reduced pressure. Flash chromatography (hexane to 7:3 hexane-EtOAc) of the resulting residue gave seleno ester 7 (216 mg, 50% overall yield for the two steps) as a yellow foam: [ α ] 22D = − 4.4 (c1.04, CHCl 3 ); IR (film): 3242 (NH), 1726, 1698, 1660, 1635 (CO) cm −1 ; 1 H-NMR (400 MHz, CDCl 3 , COSY, g-HSQC): δ = 0.73 (q, J= 12.8 Hz, 1H, H-14), 1.75–1.82 (dm, J= 12.8 Hz, 1H, H-14), 1.99 (dd, J= 18.0, 12.0 Hz, 1H, H-20), 2.17 (dd, J= 15.6, 7.6 Hz, 1H, CH 2 CO), 2.30 (dd, J= 15.6, 6.4 Hz, 1H, CH 2 CO), 2.46 (dd, J= 12.8, 8.4 Hz, 1H, H-6), 2.46–2.58 (m, 1H, H-15), 2.62 (dd, J= 12.8, 8.4 Hz, 1H, H-6), 2.73–2.79 (dm, J= 18.0 Hz, 1H, H-20), 3.64 (s, 3H, CH 3 O), 4.37 (dd, J= 11.2, 4.4 Hz, H-3), 5.13 (t, J= 9.2 Hz, 1H, H-5), 6.84 (d, J= 7.6 Hz, 1H, H AR ), 6.99 (d, J= 7.6 Hz, 1H, H H AR ), 7.05 (td, J= 7.6, 1.2 Hz, 1H, H AR ), 7.29 (td, J= 8.0, 1.2 Hz, 1H, H AR ), 7.36–7.39 (m, 3H, H AR ), 7.52–7.55 (m, 2H, H AR ), 8.74 (s, 1H, NH); 13 C-NMR (100.6 MHz, CDCl 3 ): δ = 29.3 (C-15), 29.6 (C-14), 37.5 (C-20), 38.0 (C-6), 39.6 (CH 2 CO), 51.8 (CH 3 O), 57.1 (C-7), 65.6 (C-3), 66.4 (C-5), 110.8 (CH AR ), 123.3 (CH AR ), 123.4 (CH AR ), 124.8 (C AR ), 128.8–129.4 (4CH AR , C AR ), 136.1 (2CH AR ), 140.2 (C AR ), 169.4 (CO), 171.6 (CO), 176.1 (CO), 200.5 (CO); HRMS (ESI) calcd for [C25H24N2O5Se + H+]: 513.0923, found: 513.0927. Molecules 2021,26, 428 8 of 14 (1 0 R,7 0 R,8a’R)-7 0 -(Methoxycarbonylmethyl)-2,5 0 -dioxospiro[indoline-3,1 0 -indolizidine] ( 8 ). Method A: from seleno derivative 7 : Azobisisobutyronitrile (AIBN, 9 mg, 0.05 mmol) was added under an argon atmosphere to a solution of seleno derivative 7 (216 mg, 0.43 mmol) in anhydrous benzene (20 mL). The mixture was heated to reflux, and a solution of tributyltin hydride (TBTH, 180 µ L, 0.65 mmol) in anhydrous benzene (4 mL) was added very slowly (over 30 min). The resulting mixture was stirred at reflux for 1 h, and the solvent was evaporated. Flash chromatography (6:4 hexane-EtOAc to 100% EtOAc) of the resulting residue gave spirooxindole 8 (127 mg, 90%). Method B: from aldehyde 5 : Argon was bubbled through anhydrous diglyme (3.2 mL) for 30 min. Chloro(1,5cyclooctadiene)rhodium(I) dimer (3 mg, 0.006 mmol) and 1,3-bis(diphenylphosphino) propane (dppp, 10 mg, 0.023 mmol) were weighed in corning tubes and introduced into the reaction flask under an argon flow using inert glovebox equipment. Anhydrous diglyme (2.2 mL) was transferred into the reaction flask and the bubbling of argon was continued for 15 min. Aldehyde 5 (80 mg, 0.23 mmol) was dissolved in anhydrous diglyme and transferred into the flask. The mixture was stirred at reflux for 24 h. Distilled H 2 O (2.2 mL) and CH 2 Cl 2 (2.2 mL) were added, the layers were separated, and the aqueous phase was extracted with CH 2 Cl 2 . The combined organic extracts were washed with brine, dried over anhydrous mgSO 4 , filtered, and concentrated under reduced pressure. Flash chromatography (1:1 to 1:9 hexane-EtOAc) of the resulting residue gave compound 8 (57 mg, 75%) as a white foam and minor amounts of its 5,6-dehydro derivative, which was hydrogenated (10% Pd/C, absolute EtOH) to give additional compound 8 (3 mg, 4%) after flash chromatography: [ α ] 22D = + 63.0 (c0.55, CHCl 3 ); IR (film): 3194 (NH), 1727, 1619 (CO) cm −1 ; 1 H-NMR (400 MHz, CDCl 3 , COSY, g-HSQC): δ = 0.72 (q, J= 12.4 Hz, 1H, H-14), 1.67 (dm, J= 12.4 Hz, 1H, H-14), 1.95 (dd, J= 17.6, 12.0 Hz, 1H, H-20), 2.09 (dd, J= 12.4, 8.4 Hz, 1H, H-6), 2.14 (dd, J= 15.6, 7.6 Hz, 1H, CH 2 CO), 2.23 (dd, J= 15.6, 6.4 Hz, 1H, CH 2 CO), 2.35 (m, 1H, H-15), 2.52 (q, J= 12.4 Hz, 1H, H-6), 2.61 (dd, J= 17.6, 4.8 Hz, 1H, H-20), 3.61 (s, 3H, CH 3 O), 3.83 (t, J= 11.2 Hz, 1H, H-5), 3.99 (tm, J= 11.2 Hz, 1H, H-5), 4.04 (dd, J= 11.2, 4.4 Hz, 1H, H-3), 6.91 (d, J= 7.2 Hz, 1H, H AR ), 6.99 (d, J= 7.6 Hz, 1H, H AR ), 7.05 (t, J= 7.6 Hz, 1H, H AR ), 7.28 (t, J= 7.6 Hz, 1H, H AR ), 8.92 (br. s, 1H, NH); 13 C-NMR (100.6 MHz, CDCl 3 ): δ = 29.6 (C-15), 29.7 (C-14), 33.3 (C-6), 37.3 (C-20), 39.9 (CH 2 CO), 43.9 (C-5), 51.7 (CH 3 O), 56.9 (C-7), 64.3 (C-3), 110.5 (CH AR ), 123.1 (CH AR ), 123.7 (CH AR ), 128.6 (CH AR ), 129.7 (C AR ), 140.2 (C AR ), 168.4 (CO), 171.7 (CO), 177.5 (CO); HRMS (ESI) calcd for [C18H20N2O4+ H+]: 329.1496, found: 329.1497. 3.4. Introduction of the E-Ethylidene Chain (1’R,70S,8a’R)-70-(2-Hydroxyethyl)-2,50-dioxospiro[indoline-3,10-indolizidine] (9): Lithium borohydride (LiBH 4 , 20 mg, 0.9 mmol) was added at 0 ◦ C under an argon atmosphere to a solution of compound 8 (49 mg, 0.15 mmol) in anhydrous THF (5 mL). The resulting mixture was stirred at room temperature for 72 h. The reaction was quenched at 0 ◦ C by distilled H 2 O (5 mL), and the mixture was concentrated under reduced pressure using a rotary evaporator with a dry ice condenser. Flash chromatography (95:5 EtOAc-MeOH) of the resulting residue gave oxindole 9 (34 mg, 76%) as a white foam and minor amounts of indoline 10 (3 mg, 5%). Oxindole 9 : [ α ] 22D = + 58,0 (c1.12, MeOH); IR (film): 3100–3600 (NH, OH), 1731, 1714 (CO) cm −1 ; 1 H-NMR (400 MHz, CDCl 3 , COSY, g-HSQC): δ = 0.64 (q, J= 12.0 Hz, 1H, H-14), 1.43 (m, 2H, CH 2 CH 2 O), 1.64 (dm, J= 12.0 Hz, 1H, H-14), 1.90 (dd, J= 17.6, 12.0 Hz, 1H, H-20), 2.05 (m, 2H, H-6, H-15), 2.49 (dt, J= 10.4 Hz, 1H, H-6), 2.58 (dd, J= 17.6, 5.2 Hz, 1H, H-20), 3.60 (t, J= 6.4 Hz, 2H, CH 2 O), 3.80 (dd, J= 11.6, 9.2 Hz, 1H, H-5), 3.98 (m, 2H, H-5, H-3), 6.91 (d, J= 7.6 Hz, 1H, H AR ), 6.97 (d, J= 8.0 Hz, 1H, H AR ), 7.04 (t, J= 7.6 Hz, 1H, H AR ), 7.26 (td, J= 7.6, 1.2 Hz, 1H, H AR ), 8.83 (br. s, 1H, NH); 13 C-NMR (100.6 MHz, CDCl 3 ): δ = 29.5 (C-15), 29.9 (C-14), 33.2 (C-6), 37.9 (C20), 38.4 (CH 2 CH 2 O), 43.8 (C-5), 57.0 (C-7), 59.7 (CH 2 O), 64.6 (C-3), 110.3 (CH AR ), 123.1 (CH AR ), 123.8 (CH AR ), 128.6 (CH AR ), 129.9 (C AR ), 140.1 (C AR ), 169.2 (CO), 177.5 (CO); HRMS (ESI) calcd for [C 17 H 20 N 2 O 3 + Na + ]: 323.1366, found: 323.1371. (1 0 S,7 0 S,8a’R)-7 0 - (2-Hydroxyethyl)-5 0 -oxospiro[indoline-3,1 0 -indolizidine] ( 10 ): IR (film): 3346 (NH, OH), 1606 Molecules 2021,26, 428 9 of 14 (CO) cm −1 ; 1 H-NMR (400 MHz, CDCl 3 , COSY, g-HSQC): δ = 0.69 (q, J= 12.4 Hz, 1H, H-14), 1.48 (m, 2H, CH 2 CH 2 O), 1.83–1.92 (m, 2H, H-14, H-20), 1.97–2.08 (m, 2H, H-15, H-6), 2.25 (ddd, J= 12.8, 8.4, 2.0 Hz, 1H, H-6), 2.54 (ddd, J= 17.6, 5.2, 1.6 Hz, 1H, H-20), 3.50–3.66 (m, 2H, H-5, H-3), 3.51 (d, J= 9.2 Hz, 1H, H-2), 3.57 (d, J= 9.2 Hz, 1H, H-2), 3.65 (t, J= 6.4 Hz, 2H, CH 2 CH 2 O), 3.83–3.91 (m, 1H, H-5), 6.66 (d, J= 7.6 Hz, 1H, H AR ), 6.71 (t, J= 7.6 Hz, 1H, H AR ), 6.77 (dd, J= 7.6, 1.6 Hz, 1H, H AR ), 7.08 (td, J= 7.6, 1.2 Hz, 1H, H AR ); 13 C-NMR (100.6 MHz, CDCl 3 ): δ = 29.7 (C-15), 30.3 (C-14), 35.9 (C-6), 38.2 (C-20), 38.7 (CH 2 CH 2 O), 43.5 (C-5), 54.9 (C-2), 55.4 (C-7), 60.0 (CH 2 CH 2 O), 66.1 (C-3), 110.1 (CH AR ), 119.5 (CH AR ), 124.0 (CH AR ), 128.5 (CH AR ), 131.0 (C AR ), 151.2 (C AR ), 169.2 (CO); HRMS (ESI) calcd for [C17H22N2O2+ H+]: 287.1754, found: 287.1758. (1 0 R,7 0 S,8a’R)-7 0 -{2-[(tert-Butyldimethylsilyl)oxy]ethyl}-2,5 0 -dioxospiro[indoline-3,1 0 -indolizidine] ( 11 ). tert-Butyldimethylsilyl chloride (TBDMSCl, 27 mg, 0.18 mmol) and imidazole (25 mg, 0.36 mmol) were added at 0 ◦ C under an argon atmosphere to a solution of alcohol 9 (27 mg, 0.09 mmol) in anhydrous DMF (1 mL). The resulting mixture was stirred at room temperature for 16 h. Brine was added, and the mixture was extracted with EtOAc. The combined organic extracts were dried over anhydrous mgSO 4 , filtered, and concentrated under reduced pressure. Flash chromatography (7:3 hexane-EtOAc to 1:1 hexane-EtOAc) of the resulting residue gave silyl derivative 11 (30 mg, 80%) as a white foam: [ α ] 22D = + 37.09 (c0.23, CHCl 3 ); IR (film): 3181 (NH), 1725, 1620 (CO) cm −1 ; 1 H-NMR (400 MHz, CDCl 3 , COSY, g-HSQC): δ = − 0.01 (s, 3H, CH 3 Si), 0.00 (s, 3H, CH 3 Si), 0.65 (qd, J= 12.4, 3.6 Hz, 1H, H-14), 0.83 [s, 9H, C(CH 3 ) 3 ], 1.39 (m, 2H, CH 2 CH 2 O), 1.65 (dm, J= 12.4 Hz, 1H, H-14), 1.89 (dd, J= 16.4, 11.6 Hz, 1H, H-20), 2.03–2.09 (m, 2H, H-15, H-6), 2.50–2.58 (m, 2H, H-6, H-20), 3.55 (m, 2H, CH 2 O), 3.81 (t, J= 12.4 Hz, 1H, H-5), 3.95–4.03 (m, 2H, H-5, H-3), 6.91–6.95 (m, 2H, H AR ), 7.03–7.07 (m, 1H, H AR ), 7.27 (m, 1H, H AR ), 7.68 (br. s, 1H, NH); 13 C-NMR (100.6 MHz, CDCl 3 ): δ = − 5.5 (2CH 3 Si), 18.2 [C(CH 3 ) 3 ], 25.8 [C(CH 3 ) 3 ], 29.7 (C-15), 30.1 (C-14), 33.3 (C-6), 37.9 (C-20), 38.5 (CH 2 CH 2 O), 43.8 (C-5), 57.0 (C-7), 60.1 (CH 2 O), 64.8 (C-3), 110.2 (CH AR ), 123.1 (CH AR ), 123.9 (CH AR ), 128.5 (CH AR ), 129.9 (C AR ), 140.0 (C AR ), 169.2 (CO), 177.5 (CO); HRMS (ESI) calcd for [C 23 H 34 N 2 O 3 Si + H + ]: 415.2411, found: 415.2419. (1 0 R,7 0 S,8a’R)-7 0 -{2-[(tert-Butyldimethylsilyl)oxy]ethyl}-1-(methoxymethyl)-2,5 0 -dioxospiro[indoline3,1 0 -indolizidine] ( 12 ). A solution of compound 11 (386 mg, 0.93 mmol) in anhydrous THF (2.5 mL) was transferred at 0 ◦ C under an argon atmosphere to a suspension of NaH (95%, 36 mg, 1.4 mmol) in anhydrous DMF (2.5 mL). The resulting mixture was stirred at 0 ◦ C for 30 min. Methoxymethyl chloride (MOMCl, 0.12 mL, 1.4 mmol) was added, and the resulting mixture was stirred at room temperature for 1.5 h. The mixture was cooled to 0 ◦ C and saturated aqueous NaHCO 3 (11.2 mL) was added. The mixture was extracted with EtOAc and the combined organic extracts were dried over anhydrous mgSO 4 , filtered, and concentrated under reduced pressure. Flash chromatography (hexane to 7:3 hexane-EtOAc) of the resulting residue gave the N-MOM derivative 12 (283 mg, 67%) as a white foam: [ α ] 22D = + 50.6 (c2.26, CHCl 3 ); IR (film): 1725, 1651 (CO) cm −1 ; 1 H-NMR (400 MHz, CDCl 3 , COSY, g-HSQC): δ = − 0.02 (s, 3H, CH 3 Si), − 0.03 (s, 3H, CH 3 Si), 0.64 (q, J= 12.4 Hz, 1H, H-14), 0.82 [s, 9H, C(CH 3 ) 3 ], 1.37 (dd, J= 12.8, 6.4 Hz, 2H, CH 2 CH 2 O), 1.55 (dm, J= 12.4 Hz, 1H, H-14), 1.89 (dd, J= 17.6, 12.0 Hz, 1H, H-20), 2.00–2.07 (m, 2H, H-15, H-6), 2.50–2.57 (m, 2H, H-20, H-6), 3.32 (s, 3H, CH 3 O), 3.54 (t, J= 6.0 Hz, 2H, CH 2 CH 2 O), 3.81 (t, J= 11.6 Hz, 1H, H-5), 3.96–4.03 (m, 2H, H-5, H-3), 5.15 (s, 2H, NCH 2 O), 6.94 (d, J= 7.6 Hz, 1H, H AR ), 7.08–7.11 (m, 2H, H AR ), 7.32 (t, J= 7.6 Hz, 1H, H AR ); 13 C-NMR (100.6 MHz, CDCl 3 ): δ = − 5.5 (2CH 3 Si), 18.2 [C(CH 3 ) 3 ], 25.8 [C(CH 3 ) 3 ], 29.6 (C-15), 30.2 (C-14), 33.7 (C-6), 37.8 (C-20), 38.6 (CH 2 CH 2 O), 43.8 (C-5), 56.3, 56.9 (C-7, CH 3 O), 60.0 (CH 2 CH 2 O), 64.8 (C-3), 71.5 (NCH 2 O), 110.0 (CH AR ), 123.6 (CH AR ), 123.7 (CH AR ), 128.6 (CH AR ), 129.1 (C AR ), 141.2 (C AR ), 169.1 (CO), 176.3 (CO); HRMS (ESI) calcd for [C 25 H 38 N 2 O 4 Si + H + ]: 459.2674, found: 459.2675. (1 0 R,7 0 S,8a’R)-7 0 -{2-[(tert-Butyldimethylsilyl)oxy]ethyl}-1-(tert-butoxycarbonyl)-2,5 0 -dioxospiro[indoline3,1 0 -indolizidine] ( 13 ). NaH (95%, 25 mg, 0.64 mmol) and (Boc) 2 O (70 mg, 0.32 mmol) were added at 0 ◦ C under an argon atmosphere to a solution of compound 11 (33 mg, 0.08 mmol)