The First Enantioselective Approach to 13a-Methyl-14-hydroxyphenanthroindolizidine Alkaloids - Synthetic Studies towards Hypoestestatin 2

Modular α-tertiary amino ester synthesis through cobalt-catalysed asymmetric aza-Barbier reaction

Unnatural chiral α-tertiary amino acids containing two different carbon-based substituents at the α-carbon centre are widespread in biologically active molecules. This sterically rigid scaffold is becoming a growing research interest in drug discovery. However, a robust protocol for chiral α-tertiary amino acid synthesis remains scarce due to the challenge of stereoselectively constructing sterically encumbered tetrasubstituted stereogenic carbon centres. Herein we report a cobalt-catalysed enantioselective aza-Barbier reaction of ketimines with various unactivated alkyl halides, including alkyl iodides, alkyl bromides and alkyl chlorides, enabling the formation of chiral α-tertiary amino esters with a high level of enantioselectivity and excellent functional group tolerance. Primary, secondary and tertiary organoelectrophiles are all tolerated in this asymmetric reductive addition protocol, which provides a complementary method for the well-exploited enantioselective nucleophilic addition with moisture- and air-sensitive organometallic reagents. Moreover, the three-component transformation of α-ketoester, amine and alkyl halide represents a formal asymmetric deoxygenative alkylamination of the carbonyl group.

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.

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.

Organolithium-mediated cyclization reactions: a practical way to access hetero- and carbocycles

Cyclization reactions are considered as one of the most important reactions in organic synthesis due to the fact that natural molecules contain cyclic components either as part of the molecule or molecular skeleton.

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.

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.

Asymmetric synthesis of (S)-tylophorine and (S)-cryptopleurine via one-pot Curtius rearrangement and Friedel–Crafts reaction tandem sequence

A practical total synthesis of phenanthroindo/quinolizidine alkaloids was developed, featuring an enantioselective alkylation and a one-pot Curtius rearrangement/intramolecular cyclization cascade.