Enantioselective approach to 13a-methylphenanthroindolizidine alkaloids

Chiral Aldehyde Catalysis-Enabled Asymmetric α-Functionalization of Activated Primary Amines

ConspectusThe development of catalytic activation modes provides a reliable and effective platform for designing new enantioselective reactions and preparing chiral molecules with diverse structures. Chiral aldehyde catalysis is an attractive concept in asymmetric catalysis, which utilizes a chiral aldehyde catalyst to promote the asymmetric hydroamination of allylic amines, the asymmetric α-functionalization of primary amines, or the asymmetric transamination of α-keto esters. Typically, the chiral aldehyde-catalyzed asymmetric α-functionalization of primary amines provides an efficient and straightforward method for the synthesis of α-functionalized chiral amines, which does not require any additional protection or deprotection manipulations of the amine group. However, achieving catalytic stereoselective transformations with high efficiency and enantioselectivity by this strategy has remained an intractable challenge.This Account summarizes our endeavors in the development and application of chiral aldehyde catalysis. Using a chiral aldehyde as a catalyst, we reported the catalytic asymmetric α-C alkylation of 2-aminomalonate with 3-indolylmethanol in 2014, which represents the first chiral aldehyde-catalyzed asymmetric α-functionalization of an activated primary amine. Subsequently, several axially chiral aldehyde catalysts were continuously prepared by using chiral BINOL as the starting material, and their applications in asymmetric synthesis were explored. On the one hand, they were used as organocatalysts to realize the various transformations of α-amino acid esters, such as asymmetric 1,4-addition toward conjugated enones/α,β-unsaturated diesters and cyclic 1-azadienes as well as asymmetric α-arylation/allylation and benzylation with corresponding halohydrocarbons. Notably, taking advantage of the difference in the distribution of catalytic sites between two chiral aldehyde catalysts, we disclosed chiral aldehyde-catalyzed diastereodivergent 1,6-conjugated addition and Mannich reactions. On the other hand, the potential for the cooperative catalysis of a chiral aldehyde with a transition metal has also been demonstrated. Enabled by the combination of a chiral aldehyde, a palladium complex, and a Lewis acid, the enantioselective α-allylation of amino acid esters with allyl alcohol esters was established. Moreover, the ternary catalytic system has been successfully used for the α-functionalization of amino acid esters with 1,3-dienes, allenes, allenylic alcohol esters, 1,3-disubstituted allyl alcohol esters, and arylmethanol esters as well as the asymmetric cascade Heck-alkylation reaction. The combination of a chiral aldehyde and nickel complex allows for the asymmetric α-propargylation of amino acid esters with propargylic alcohol esters and provides excellent enantioselectivities. These transformations provide a large library of optically active amines and amino acids. With those chiral amino acid esters as key building blocks, the synthesis or formal synthesis of multiple natural products and biologically significant unnatural molecules was accomplished. This includes the stereodivergent synthesis of natural pyrrolizidine alkaloid NP25302 and the formal synthesis of natural product (S)-hypoestestatin 1 and manzacidin C, clinical candidate compound (+)-AG-041R, and somatostatin mimetics. It is fully anticipated that chiral aldehyde catalysis will soon witness rapid expansion both in the development of novel asymmetric transformations and in innovative applications for constructing optically active nitrogen-containing molecules with significant values.

Catalytic asymmetric Tsuji-Trost α-benzylation reaction of N-unprotected amino acids and benzyl alcohol derivatives

Catalytic asymmetric Tsuji–Trost benzylation is a promising strategy for the preparation of chiral benzylic compounds. However, only a few such transformations with both good yields and enantioselectivities have been achieved since this reaction was first reported in 1992, and its use in current organic synthesis is restricted. In this work, we use N-unprotected amino acid esters as nucleophiles in reactions with benzyl alcohol derivatives. A ternary catalyst comprising a chiral aldehyde, a palladium species, and a Lewis acid is used to promote the reaction. Both mono- and polycyclic benzyl alcohols are excellent benzylation reagents. Various unnatural optically active α-benzyl amino acids are produced in good-to-excellent yields and with good-to-excellent enantioselectivities. This catalytic asymmetric method is used for the formal synthesis of two somatostatin mimetics and the proposed structure of natural product hypoestestatin 1. A mechanism that plausibly explains the stereoselective control is proposed.

The catalytic asymmetric benzylations of prochiral nucleophiles are very limited. Here, the authors disclose an asymmetric α−benzylation of N-unprotected amino acids with benzyl alcohol derivatives by a chiral aldehyde-involved catalytic system.

Oxazaborolidine-catalyzed reductive parallel kinetic resolution of ketones from β-nitro-azabicycles for the synthesis of chiral hypoestestatins 1, 2

A novel approach for the synthesis of 13a-methyl tylophora alkaloids has been reported. The key features included two different synthetic pathways targeted at transforming the β-nitro-azabicycle to the phenanthrene core. The successful steps involved the oxidation of the nitro-piperidine moiety to the corresponding α,β-unsaturated ketone, and an oxidative biaryl coupling reaction for phenanthrene ring formation. Finally, the desired product was obtained via a formal reductive removal of the hydroxyl group. This methodology has been applied for the synthesis of 13a-methyl tylophora alkaloids in up to 65% yield over six steps from β-nitro-azabicycles. Both natural and unnatural enantioenriched hypoestestatins 1 and 2, and related compounds were synthesized using parallel kinetic resolution of the CBS-oxazaborolidine-catalyzed reduction of racemic ketones to provide two separable diastereomeric alcohols in combined yields up to 91% and with high enantioselectvity (up to 89% ee). In addition, the catalytic asymmetric reduction to seco-hypoestestatins 1 and 2 has been reported for the first time. Thus, the ability to develop the racemic mixtures to both enatioenriched forms offers benefit for various biological assays in the future.

The Pictet-Spengler Reaction Updates Its Habits

The Pictet-Spengler reaction (P-S) is one of the most direct, efficient, and variable synthetic method for the construction of privileged pharmacophores such as tetrahydro-isoquinolines (THIQs), tetrahydro-β-carbolines (THBCs), and polyheterocyclic frameworks. In the lustro (five-year period) following its centenary birthday, the P-S reaction did not exit the stage but it came up again on limelight with new features. This review focuses on the interesting results achieved in this period (2011-2015), analyzing the versatility of this reaction. Classic P-S was reported in the total synthesis of complex alkaloids, in combination with chiral catalysts as well as for the generation of libraries of compounds in medicinal chemistry. The P-S has been used also in tandem reactions, with the sequences including ring closing metathesis, isomerization, Michael addition, and Gold- or Brønsted acid-catalyzed N-acyliminium cyclization. Moreover, the combination of P-S reaction with Ugi multicomponent reaction has been exploited for the construction of highly complex polycyclic architectures in few steps and high yields. The P-S reaction has also been successfully employed in solid-phase synthesis, affording products with different structures, including peptidomimetics, synthetic heterocycles, and natural compounds. Finally, the enzymatic version of P-S has been reported for biosynthesis, biotransformations, and bioconjugations.

Total synthesis of the reported structure of 13a-hydroxytylophorine

The first total synthesis of the reported structure of 13a-hydroxytylophorine was accomplished. The key step was an unprecedented NaBH₄-promoted one-pot reductive cyclization cascade that efficiently yielded a hydroxyl azonane intermediate. The indolizidine framework was obtained by means of oxidation and a subsequent unexpected protecting-group migration. This total synthesis revealed that the reported structure of the naturally isolated compound is incorrect.

Sequential Oxidative and Reductive Radical Cyclization Approach toward Asperparaline C and Synthesis of Its 8-Oxo Analogue

The most advanced approach, so far, to the asperparalines is developed. Consecutive oxidative and reductive radical cyclizations serve as the key steps to stereoselectively access the complex fully elaborated skeleton containing the cyclopentane and spiro-succinimide units.

Spatial Configuration and Three-Dimensional Conformation Directed Design, Synthesis, Antiviral Activity, and Structure-Activity Relationships of Phenanthroindolizidine Analogues

Our recent investigation on the antiviral activities against tobacco mosaic virus (TMV) of phenanthroindolizidine alkaloid analogues preliminarily revealed that the basic skeleton and substitution pattern at the C13a position of the molecule, which are closely related to the spatial arrangement of the molecule, have great effects on the biological activity. To further study the in-depth influence of spatial configuration and three-dimensional (3D) conformation of the molecules on their anti-TMV activities and related structure-activity relationship (SAR), a series of D-ring opened derivatives 3, 4, 5a-5j, 6, and 7, chiral 13a- and/or 14-substituted phenanthroindolizidine analogues 10-12 and 18-20, and their enantiomers ent-10-ent-12 and ent-18-ent-20 were synthesized and evaluated for their anti-TMV activities. Bioassay results showed that most of the chiral phenanthroindolizidines displayed good to excellent in vivo anti-TMV activity. Among these compounds, ent-11 showed more potent activity than Ningnanmycin (one of the most successful commercial antiviral agents), thus emerging as a potential inhibitor of the plant virus. Further SARs were also discussed for the first time under the chiral scenario, demonstrating that both spatial configuration and 3D conformation of the molecules are crucial for keeping high anti-TMV activity.

Cyclic enaminones. Part II: applications as versatile intermediates in alkaloid synthesis

Among many other strategies, the enaminone approach is an important strategy to construct and diversify the azacyclic core in various alkaloids syntheses. In this brief review we discuss the application of cyclic enaminones as building blocks, as well as potential intermediates in the total synthesis of selected alkaloids.

Concise asymmetric syntheses of novel phenanthroquinolizidines

The first preparation of enantioenriched phenanthroquinolizidines with a quaternary center at C14a was accomplished in seven steps from readily available starting materials. Key steps were an efficient dynamic kinetic allylation of a diastereomeric mixture of chiral tert-butylsulfinyl ketimines and the construction of a piperidine E ring by rhodium catalyzed hydroformylation. The Stevens rearrangement of the corresponding N-benzyl derivatives took place smoothly, allowing the installation of a benzyl moiety at C9 in a trans relationship with the methyl group. The cytoxicity of the prepared phenanthroquinolizidines was evaluated against different human cancer cell lines.

An analysis of the complementary stereoselective alkylations of imidazolidinone derivatives toward α-quaternary proline-based amino amides

Alkylations of proline-based imidazolidinones are described based on the principle of self-regeneration of stereocenters (SRS), affording high levels of either the cis or trans configured products. Stereoselectivity is dictated solely on the nature of the "temporary" group, where isobutyraldehyde-derived imidazolidinones provide the cis configured products and 1-naphthaldehyde-derived imidazolidinones afford the complementary trans configured products. These stereodivergent products can be readily cleaved to afford both α-alkylated proline enantiomers from readily available L-proline. A series of imidazolidinones were alkylated to investigate the origin of the anti-selectivity. Potential contributions toward the observed anti-selectivity are discussed on the basis of these experiments, suggesting a refined hypothesis for selectivity may be in order.

Design, synthesis, antiviral activity, and SARs of 13a-substituted phenanthroindolizidine alkaloid derivatives

On the basis of our previous structure-activity relationship (SAR) and antiviral mechanism studies, a series of 13a-substituted phenanthroindolizidine alkaloid analogues (3a-16a, 3b, 4b, 6b, 7b, 10b, and 14b) were designed targeting tobacco mosaic virus (TMV) RNA, synthesized, and evaluated for their antiviral activity against TMV for the first time. The bioassay results showed that most of the synthesized compounds (such as 4a, 6a, 7a, 11a, 14a, 6b, and 14b) exhibited good to excellent antiviral activity against TMV both in vitro and in vivo. Especially, for inactivation effect and curative effect, compounds 4a, 6a, 7a, 11a, 14a, and 14b showed higher activity at both concentrations (500 μg mL(-1) and 100 μg mL(-1)) than commercial Ningnanmycin. Preliminary SARs showed that the substituted groups with hydrogen donor at 13a position were found to be favorable for keeping high antiviral activity. The present work demonstrates that 13a-substituted phenanthroindolizidines can be used as possible lead compounds for developing anti-TMV agents.

Design, synthesis, antiviral activity, and structure-activity relationships (SARs) of two types of structurally novel phenanthroindo/quinolizidine analogues

To investigate the influence of the variation of the original skeletons of natural phenanthroindo/quinolizidine alkaloids on antiviral activities, two types of structurally totally novel analogues 7a, 7b, 16a, and 16b were designed, synthesized, and evaluated against tobacco mosaic virus (TMV) for the first time. Bioassay results indicated that all four of the newly designed analogues showed good to excellent antiviral activities, among which analogue 16a dispalyed comparable activity with that of ningnanmycin, perhaps one of the most successful commercial antiviral agents, thus emerging as a potential inhibitor of plant virus and serving as a new lead for further optimization. Further structure-activity relationships are also discussed, demonstrating for the first time that the same changes of the original skeletons of phenanthroindolizidine and phenanthroquinolizidine exihibted totally different antiviral activities results, providing some original and useful information about the preferential conformation for maintaining high activities.

An enantioselective strategy for the total synthesis of (S)-tylophorine via catalytic asymmetric allylation and a one-pot DMAP-promoted isocyanate formation/Lewis acid catalyzed cyclization sequence

A novel total synthesis of (S)-tylophorine is reported, featuring asymmetric allylation and cascade isocyanate formation and cyclization.

Total synthesis of phenanthroindolizidine alkaloids via asymmetric deprotonation of N-Boc-pyrrolidine

A concise synthesis of two typical phenanthroindolizidine alkaloids via an asymmetric deprotonation/diastereoselective carbonyl addition strategy is described.