Internal Asymmetric Induction Research Articles

Stereoselective Mannich Reactions in the Synthesis of Enantiopure Piperidine Alkaloids and Derivatives

AbstractPiperidine alkaloids are members of the alkaloid family that is characterized by the presence of a six‐membered nitrogen‐containing heterocycle. Piperidine alkaloids are found mainly in plants and often exhibit interesting biological and pharmacological activities. Despite the accumulation of these natural products in plants, relatively low quantities of alkaloids are produced in absolute terms and thus synthesis of alkaloids and derivatives thereof remains relevant to identify targets for drug discovery. Throughout the years, researchers have come up with a myriad of methods to synthesize piperidine derivatives. This review describes methods that employ stereoselective Mannich reactions to create the core of piperidine alkaloids. Asymmetric induction in the Mannich reaction has been achieved by a range of methods that have been divided into three conceptual approaches: (1) chiral pool‐based (internal asymmetric induction), (2) chiral auxiliary‐based (relayed asymmetric induction) and (3) asymmetric catalysis‐based (external asymmetric induction). Of each approach, we describe the reaction mechanism and rationalize the stereochemical outcome of the Mannich products.

The Stereoselective Total Synthesis of Axially Chiral Naphthylisoquinoline Alkaloids.

The naphthylisoquinoline (NIQ) alkaloids are a thrilling class of natural biaryls─structurally, biosynthetically, and pharmacologically. A common feature of these metabolites is the biaryl bond between their naphthalene and isoquinoline moieties, which in most cases is rotationally hindered, leading to the phenomenon of axial chirality. Depending on their individual structures, including the respective axial configurations, NIQs show promising bioactivities. Their total synthesis is a challenging but rewarding goal, with the stereocontrolled construction of the biaryl linkage as the key step.The position of the biaryl axis and its configuration determine the overall molecular shape and thus the choice of the best possible method for efficient asymmetric aryl-aryl bond formation. The axis in NIQs can cover a broad range of steric hindrance, from freely rotating to configurationally stable. For dioncophylline B (1) and dioncophylline F (2a/b), with only two ortho-substituents next to the axis, the synthesis is easy to accomplish by direct coupling of the intact naphthalene moiety with the isoquinoline unit, and no atropo-selectivity is required.Naphthylisoquinolines with a configurationally stable biaryl axis are the focus of the present Account. They are more difficult to synthesize because, in addition to the problem of decreased chemical yields with increasing steric hindrance at the axis, the synthesis needs to proceed stereoselectively. Within this class of NIQs, 5,8'-coupled representatives, such as korupensamine A (3a), have received considerable synthetic attention because the rotational barrier is high enough for the existence of atropisomerism without being too excessive, and they show potent bioactivities. Their synthesis, as systematically presented herein, thus occupies a central role in this report. For their aryl-aryl bond formation, both intra- and intermolecular approaches can be successfully applied. Axial stereoinformation is introduced by internal asymmetric induction from stereogenic elements already present in the isoquinoline or its precursors, from chiral auxiliary elements artificially introduced, or by external asymmetric induction using chiral catalysts.To overcome even higher steric hindrance, as in ancistrocladine (4a), innovative approaches were developed. A most successful strategy is the "lactone concept" developed by the Bringmann group, which allows the directed synthesis of any desired atropisomer in high chemical and optical yields, thus permitting the atropo-divergent preparation of the two isomers from a single joint precursor. In this approach, the two formal tasks of stereoselective biaryl synthesis, which are usually done simultaneously─the C-C linkage and the asymmetric induction─are achieved consecutively. The coupling step is performed intramolecularly after prefixation of the coupling partners by an ester bridge. The resulting biaryl lactone already possesses the biaryl axis but is still configurationally unstable; it can then, with internal or external asymmetric induction, be cleaved atropo-divergently with high stereoselectivities. Besides its unique concept, the procedure excels by its broad applicability; among all presented methods, it has been used for the synthesis of the largest number of NIQs, more than 20 representatives, including those with the highest steric hindrance.This Account gives comprehensive insight into the plethora of conceptual approaches for the efficient formation of the hindered biaryl bond of NIQs.

Unprecedented Stereocontrol in the Synthesis of 1,2,3‐Trisubstituted Tetrahydro‐β‐carbolines through an Asymmetric Pictet–Spengler Reaction towards Sarpagine‐Type Indole Alkaloids

The asymmetric Pictet–Spengler (P‐S) reaction of chiral Nb‐ethynyl‐substituted tryptophan methyl ester derivatives (from both d‐ and l‐tryptophan) with a simple aliphatic aldehyde, exhibited unprecedented selectivity towards either of the diastereomeric products. A simple variation of conditions could alter the outcome of the cyclization from either 100 % trans‐selective to 100 % cis‐selective originating entirely from internal asymmetric induction under mild conditions. This resulted in a highly efficient access to both 1,3‐cis‐(1,2,3‐trisubstituted tetrahydro‐β‐carbolines, THβCs) and 1,3‐trans‐(1,2,3‐trisubstituted THβCs). To the best of our knowledge, this type of stereocontrol has never been observed from tryptophan methyl ester derivatives (either D or L) in accessing either 1,3‐disubstituted or 1,2,3‐trisubstituted THβCs. By exploiting this very useful ambidextrous diastereoselectivity, we have set the crucial C‐3 and C‐5 stereocenters of C‐19 methyl‐substituted sarpagine/macroline/ajmaline alkaloids beginning with the DNA‐encoded and cheaper l‐(–)‐tryptophan, as well as optionally from commercially available d‐(+)‐tryptophan.

Total Synthesis of Sarpagine-Related Bioactive Indole Alkaloids.

Extension of the asymmetric Pictet-Spengler reaction to bulkier Nb -alkylated tryptophan derivatives resulted in an improved stereospecific access to the key bicyclo[3.3.1]nonane core of bioactive C-19 methyl substituted sarpagine/macroline/ajmaline indole alkaloids with excellent diastereoselectivity by internal asymmetric induction. Complete stereocontrol of the C-19 methyl function in either the α- or β-configuration was achieved, which enables the total synthesis of any member from this group of thirty alkaloids. We report herein, the total synthesis of macrocarpines (A-C) 1-3, talcarpine 4, N(4)-methyl-N(4),21-secotalpinine 5, dihydroperaksine 8 and deoxyperaksine 9.

Concise Total Synthesis of (-)-Affinisine Oxindole, (+)-Isoalstonisine, (+)-Alstofoline, (-)-Macrogentine, (+)-Na -Demethylalstonisine, (-)-Alstonoxine A, and (+)-Alstonisine.

A highly enantio- and diastereoselective strategy to access any member of the sarpagine/macroline family of oxindole alkaloids via internal asymmetric induction was developed from readily available d-(+)-tryptophan. At the center of this approach was the diastereospecific generation of the spiro[pyrrolidine-3,3'-oxindole] moiety at an early stage via a tert-butyl hypochlorite-promoted oxidative rearrangement of a chiral tetrahydro-β-carboline derivative. This key branching point determined the spatial configuration at the C-7 spiro center to be entirely 7R or 7S. Other key stereospecific processes were the asymmetric Pictet-Spengler reaction and Dieckmann cyclization, which were scalable to the 600 and 150 gram levels, respectively. Execution of this approach resulted in first enantiospecific total synthesis of (+)-isoalstonisine and (-)-macrogentine from the chitosenine series (7R), as well as (+)-alstonisine, (+)-alstofoline, (-)-alstonoxine A and (+)-Na -demethylalstonisine from the alstonisine series (7S).

Internal asymmetric induction by the C-6 substituent on the oxidation reaction of interglycosidic sulfur atom of thiodisaccharides

Complete diastereoselectivity was archived in the oxidation of 3-deoxy-S-(1-4)-disaccharides due to anchimeric assistance of the terminal CH2OH group.

1,2‐Asymmetric Induction in Diastereo­selective Zwitterionic Aza‐Claisen Rearrangements: Key Steps in Optically Active Alkaloid Synthesis

AbstractThe zwitterionic aza‐Claisen rearrangement of optically active N‐allylpyrrolidines and α‐phenoxyacetyl fluorides proceeds with complete simple diastereoselectivity (internal asymmetric induction) and complete 1,2‐asymmetric induction to generate a new C–C bond adjacent to a chiral C‐N‐Boc functionality. The resulting γ,δ‐unsaturated amides were cyclised to give the corresponding pyrrolizidinones, which enabled the determination of the relative configuration of the stereotriads. Vinyl group degradation and a final lactam reduction gave an optically active analogue of (+)‐petasinine (a pyrrolizidine alkaloid). Furthermore, the stereotriad‐containing amides should be useful key intermediates for the total synthesis of optically active elaeocarpin (an indolizidine alkaloid).

Convergent approach to complex spirocyclic pyrans: practical synthesis of the oxa-pinnaic acid core

The enantioselective synthesis of the oxa-pinnaic acid framework has been achieved through internal asymmetric induction. The synthetic strategy pursued illustrates the adaptability of the Achmatowicz oxidative rearrangement for the synthesis of complex spirocyclic pyrans starting from tertiary alcohols.

Nature-inspired stereospecific total synthesis of P-(+)-dispegatrine and four other monomeric sarpagine indole alkaloids.

All five: The first total synthesis of the C2-symmetric indole alkaloid 1 involved a late-stage thallium(III) acetate-mediated intermolecular oxidative coupling to construct the C9C9′ bond with complete regio- and stereocontrol. The formation of a single atropodiastereomer in this critical step arises from internal asymmetric induction. The first total synthesis of four other monomeric sarpagine indole alkaloids is also described.

Total Syntheses of (+)‐Polygalolide A and (+)‐Polygalolide B: Elucidation of the Absolute Stereochemistry and Biogenetic Implications

The total syntheses of (+)-polygalolide A and (+)-polygalolide B have been completed by using a carbonyl ylide cycloaddition strategy. Three of the four stereocenters, including two consecutive tetrasubstituted carbon atoms at C2 and C8, were incorporated through internal asymmetric induction from the stereocenter at C7 by a [Rh(2) (OAc)(4)]-catalyzed carbonyl ylide formation/intramolecular 1,3-dipolar cycloaddition sequence. The arylmethylidene moiety of these natural products was successfully installed by a Mukaiyama aldol-type reaction of a silyl enol ether with a dimethyl acetal, followed by elimination under basic conditions. We have also developed an alternative approach to the carbonyl ylide precursor based on a hetero-Michael reaction. This approach requires 18 steps, and the natural products were obtained in 9.8 and 9.3 % overall yields. Comparison of specific rotations of the synthetic materials and natural products suggests that polygalolides are biosynthesized in nearly racemic forms through a [5+2] cycloaddition between a fructose-derived oxypyrylium zwitterion with an isoprene derivative.

ChemInform Abstract: Synthetic Studies in the Lysocellin Family of Polyether Antibiotics. The Total Synthesis of Ferensimycin B.

A convergent asymmetric synthesis of the polyether antibiotic ferensimycin B has been completed. Chiral enolate bond constructions were employed to establish seven of the 16 stereocenters of the subunits 35 and 52, which comprise the C& and CI&3 portions of ferensimycin B. The stereogenic centers at C3, C4, Cg, Clo, CI6, C,,, and CIS were incorporated through internal asymmetric induction, while those at Cm and C1l were established by using asymmetric epoxidation methodology. In this transformation, a vanadium-catalyzed internal epoxidation of a bis-homoallylic alcohol was employed to relay chirality from the C13 to the CI6 oxygen-bearing stereocenter. A final aldol addition reaction on intermediates devoid of protecting groups united the fragments 52 and 35 to provide synthetic ferensimycin B, whose absolute configuration was found to be

ChemInform Abstract: Asymmetric Desymmetrization by Enantioselective Catalysis of Carbonyl- Ene Reaction: Remote Internal Asymmetric Induction.

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ChemInform Abstract: Asymmetric Induction in Aza-Claisen Rearrangement of Carboxamide Enolates. Effect of Chiral Auxiliary on Nitrogen.

Abstract Aza-Claisen rearrangement of enolates of N-alkyl-N-(2E)-butenylpropanamides with chiral alkyl groups proceeded with high relative asymmetric induction as well as excellent internal asymmetric induction to give optically active N-alkyl-Psyn-2,3-dimethylpent-4-enamides.

Mechanistic studies on the cis to trans epimerization of trisubstituted 1,2,3,4-tetrahydro-beta-carbolines.

It is well-known that N(b)-benzyltryptophan alkyl esters undergo the Pictet-Spengler reaction with aldehydes to furnish both cis- and trans-1,2,3,4-tetrahydro-beta-carbolines, with the trans isomer predominating. Epimerization at C-1 took place under acidic conditions to produce, exclusively, the thermodynamically more stable trans diastereomer via internal asymmetric induction. Recent kinetic experiments provided insight into the cis to trans epimerization mechanism involved in the Pictet-Spengler reaction of 1,2,3-trisubstituted tetrahydro-beta-carbolines. Since the epimerization reaction had been shown to be sensitive to electronic effects at C-1, the rate data for a series of 1-phenyl-substituted 1,2,3,4-tetrahydro-beta-carbolines was investigated via a Hammett study. Analysis of the data supported the presence of a positively charged intermediate with a rho value of -1.4, although the existence of an iminium ion intermediate or a carbocationic intermediate could not be determined from this data alone. Analysis of the rate of epimerization demonstrated first-order kinetics with respect to TFA following the initial protonation of the substrate. This observation was consistent with the formation of a doubly protonated intermediate as the rate-determining step in the carbocation-mediated cis to trans epimerization process. In addition, the observed first-order rate dependence was inconsistent with the retro-Pictet-Spengler mechanism since protonation at the indole-2 position was not rate determining as demonstrated by kinetic isotope effects. Based on this kinetic data, the retro-Pictet-Spengler pathway was ruled out for the cis to trans epimerization of 1,2,3-trisubstituted 1,2,3,4-tetrahydro-beta-carbolines, while the olefinic mechanism had been ruled out by experiments carried out in TFA-d.

Total synthesis of the antimalarial naphthylisoquinoline alkaloid 5- epi-4′- O-demethylancistrobertsonine C by asymmetric Suzuki cross-coupling

The first total synthesis of the antimalarial naphthylisoquinoline alkaloid 5- epi-4′- O-demethylancistrobertsonine C ( 1a) and its—as yet unnatural—atropo-diastereomer, 1b, is described. The key step of the synthesis is the construction of the rotationally hindered and thus stereogenic biaryl axis, which was built up by a Suzuki reaction. The use of chiral ligands in the palladium-catalyzed cross-coupling permitted to increase the low internal asymmetric induction up to a diastereomeric ratio of 74:26. The assignment of the axial configurations of the atropo-diastereomers was achieved by 2D NMR experiments and corroborated by quantum chemical CD calculations.

Stereoselective Synthesis of the C9—C19 Lactone‐Dipropionate Fragment of Calyculin C.

A highly diastereoselective synthesis of the title fragment of calyculin C has been developed based on an internal asymmetric induction between a chiral aldehyde and Z-crotyl trifluorosilane.

Stereoselective synthesis of the C 9–C 19 lactone-dipropionate fragment of calyculin C

A highly diastereoselective synthesis of the title fragment of calyculin C has been developed based on an internal asymmetric induction between a chiral aldehyde and Z-crotyl trifluorosilane.

Polyhydroxylated indolizidine alkaloids—an efficient synthesis of 1-deoxy-8,8a-di- epi-castanospermine

A new and efficient enantioselective total synthesis of the title deoxycastanospermine derivative has been developed, based on amino acid and β-ketophosphonate chemistry, as well as employment of internal asymmetric induction for the creation of the new chiral centers proved successful. With proper choice of reaction conditions, the approach can also be applied in selective preparation of several isomers of deoxycastanospermine. The length (9 steps) and overall yield of the title compound trihydroxyindolidine 1 , 7.3%, compares well with the literature syntheses of similar compounds.

The lactone concept—a novel approach to the metal-assisted atroposelective construction of axially chiral biaryl systems

The atroposelective synthesis of axially chiral biaryls via configurationally unstable, lactone-bridged biaryls is reviewed. These key molecules are easily accessible by regioselective intramolecular cross-coupling of ester-linked, even sterically hindered aromatic portions and can be cleaved highly atropo-enantio- or -diastereoselectively by three principal options, either (a) by using a wide range of chiral metalated nucleophiles (usually with external asymmetric induction), (b) after Lewis acid activation of the lactone CO function using uncharged chiral or achiral nucleophiles, or (c) with internal asymmetric induction, using the stereoelement of planar chirality originating from η 6-coordination (typically involving Cr or Ru complexes). The resulting ring-opened configurationally stable biaryls are obtained in mostly excellent chemical and optical yields. By the choice of the respective enantiomer of the nucleophile, the method allows the atropo-divergent synthesis of both atropisomers from the same immediate biaryl precursor and, if required, a recycling of the undesired minor atropisomer is possible, too. Such advantages are otherwise well-known for the stereoselective preparation of centrochiral compounds.

Application of Complex Aldol Reactions to the Total Synthesis of Phorboxazole B

The synthesis of phorboxazole B has been accomplished in 27 linear steps and an overall yield of 12.6%. The absolute stereochemistry of the C4−C12, C33−C38, and C13−C19 fragments was established utilizing catalytic asymmetric aldol methodology, while the absolute stereochemistry of the C20−C32 fragment was derived from an auxiliary-based asymmetric aldol reaction. All remaining chirality was incorporated through internal asymmetric induction, with the exception of the C43 stereocenter which was derived from (R)-trityl glycidol. Key fragment couplings include a stereoselective double stereodifferentiating aldol reaction, a metalated oxazole alkylation, an oxazole-stabilized Wittig olefination, and a chelation-controlled addition of the fully elaborated alkenyl metal side chain.