Asymmetric Total Synthesis of Fasicularin by Chiral N-Alkoxyamide Strategy

Dyotropic Rearrangement of β-Lactams: Reaction Development, Mechanistic Study, and Application to the Total Syntheses of Tricyclic Marine Alkaloids

An unprecedented dyotropic rearrangement of β-lactams has been developed, which provides an enabling tool for the synthesis of structurally diverse γ-butyrolactams. Unlike the well-established dyotropic rearrangements of β-lactones, the present reaction probably proceeds through a dual-activation mode, and thus displays unusual reactivity and chemoselectivity. The combined computational and experimental results suggest that the dyotropic rearrangement of β-lactams may proceed through different mechanisms depending on the nature of migrating groups (H, alkyl, or aryl). Hinging on a chemoselective H-migration dyotropic rearrangement of β-lactams, we have completed the divergent synthesis of tricyclic marine alkaloids (-)-lepadiformine A, (+)-cylindricine C, and (-)-fasicularin within 11-12 longest linear steps.

Total Synthesis of (-)-Fasicularin

Asymmetric total synthesis of (-)-fasicularin was achieved in nine steps from a commercially available inexpensive material, by leveraging (1) an aryl radical-mediated, copper-catalyzed Sonogashira-type cross-coupling, (2) a Au-catalyzed tandem intramolecular alkyne hydroamination/iminium formation/intramolecular allylation, and (3) a tandem hydrogenation/hydrogenolysis/intramolecular reductive amination as key transformations.

Asymmetric Synthesis of Arboduridine

The first asymmetric total synthesis of the monoterpenoid indole alkaloid arboduridine has been accomplished. The tricyclic A/B/D ring system was constructed by an enantioselective Michael reaction followed by intramolecular nucleophilic addition. Intramolecular α-amination of a ketone forged the piperidine ring, while a Horner-Wadsworth-Emmons (HWE) reaction was used to form the pyrrolidine ring. A reduction cyclization cascade led to formation of the tetrahydrofuran ring.

Computational Study Focusing on a Comprehensive Conformational Analysis of Transition States for Aza-Spiro Ring Formations with N-Alkoxyamides

An accurate understanding of conformations in transition states is a critical piece in the theoretical analysis of complex molecular reactions. In this study, we investigated conformationally diverse transition states during intramolecular aza-spiro ring formation with an allylsilane moiety and N-alkoxy iminium ion, a key reaction in the synthesis of fasicularin by Sato and Chida et al., and identified the origins of stereoselectivity of the cyclization. A large number of conformational isomers with forming C-C bonds were comprehensively analyzed using Cremer-Pople puckering parameters. It was found that the conformations of the transition states had different puckering preferences depending on the reactant's double-bond geometry and the product's stereochemical configuration. Furthermore, an asymmetric aza-spiro cyclization with a tolyl group as a chiral auxiliary was investigated, showing that conformational anchoring by both a CH-O hydrogen bond and the CH-π interaction was critical for the asymmetric induction.

Total Synthesis of (±)-Fasicularin through Double Consecutive Epimerizations

The total synthesis of tricyclic marine alkaloid fasicularin (1b) has been accomplished from a novel sterically well-defined α-aminonitrile 7, featuring a novel double consecutive epimerization process and Ir-catalyzed reductive functionalization of a tertiary γ-lactam. The required configuration is obtained through the thermodynamically stereoselectively driven isomerization of a readily available 8a-cyanodecahydroquinoline framework. The strategy allows us to achieve the tricyclic core structures efficiently from affordable starting materials through simple operations.

Enantioselective total syntheses of marine natural products (+)-cylindricines C, D, E and their diastereomers

On the basis of the amide strategy, we have achieved the concise enantioselective total synthesis of (+)-cylindricines D, C, and E, and their 2-epimers in seven, eight, and nine steps, respectively.

Development of a Chiral N-Alkoxyamide Strategy and Application to the Asymmetric Total Synthesis of Fasicularin

The asymmetric total synthesis of fasicularin by a chiral N-alkoxyamide strategy is reported. Incorporation of the chiral alkoxy group onto an amide nitrogen changes the original reactivity of the amide, enabling two key transformations: aza-spirocyclization and the reductive Strecker reaction. DFT calculations indicate that pyramidalization of the N-alkoxyamide nitrogen is crucial to produce a cyclic hemiaminal in equilibrium, which undergoes aza-spirocyclization. The chiral alkoxy group is also used as a stereocontrol element to establish two consecutive stereocenters. The iridium-catalyzed reductive Strecker reaction of the N-alkoxylactam provides the aminonitrile with high diastereoselectivity.

General α-Amino 1,3,4-Oxadiazole Synthesis via Late-Stage Reductive Functionalization of Tertiary Amides and Lactams*

An iridium-catalyzed reductive three-component coupling reaction for the synthesis of medicinally relevant α-amino 1,3,4-oxadiazoles from abundant tertiary amides or lactams, carboxylic acids, and (N-isocyanimino) triphenylphosphorane, is described. Proceeding under mild conditions using (<1 mol %) Vaska's complex (IrCl(CO)(PPh3 )2 ) and tetramethyldisiloxane to access the key reactive iminium ion intermediates, a broad range of α-amino 1,3,4-oxadiazole architectures were accessed from carboxylic acid feedstock coupling partners. Extension to α-amino heterodiazole synthesis was readily achieved by exchanging the carboxylic acid coupling partner for C-, S-, or N-centered Brønsted acids, and provided rapid and modular access to these desirable, yet difficult-to-access, heterocycles. The high chemoselectivity of the catalytic reductive activation step allowed late-stage functionalization of 10 drug molecules, including the synthesis of heterodiazole-fused drug-drug conjugates.

Reductive Functionalization of Amides in Synthesis and for Modification of Bioactive Compounds

In this review, applications of the amide reductive functionalization methodology for the synthesis and modification of bioactive compounds are covered. A brief summary of the different protocols is presented in the introduction, followed by the synthetic application of these in late-stage functionalization, leading to known pharmaceuticals or to their derivatives, including bioisosteres, with potential higher activity as the main axis of the article. The synthetic approach to natural products based on amide reduction is also discussed; however, this is given in a condensed form focusing on recent or as yet unexplored applications due to a number of recently published excellent reviews covering this topic. The aim of this review is to illustrate the potential of reductive functionalization of amides as an elegant and useful tool in the synthesis of bioactive compounds and inspire further work in this field.

Total Synthesis of (-)-Lepadiformine A via Radical Translocation-Cyclization Reaction

Total synthesis of (-)-lepadiformine A featuring construction of the 1-azaspiro[4.5]decane skeleton by a highly diastereoselective radical translocation-cyclization reaction of a γ-lactam derivative bearing a chiral butenolide moiety is described. The enantioselective construction of butenolide is conducted via Krische's catalytic asymmetric allylation protocol. After the radical translocation-cyclization reaction, a hydroxymethyl group at the C-13 position was stereoselectively introduced by a one-pot partial reduction-allylation protocol of the unprotected lactam derivative. Finally, the total synthesis is completed by formation of a C ring.

Chemoselective Synthesis of α-Amino-α-cyanophosphonates by Reductive Gem-Cyanation-Phosphonylation of Secondary Amides

A novel approach to α-amino-α-cyanophosphonates has been developed. The method features a Tf₂O-mediated reductive geminal cyanation/phosphonylation of secondary amides. Mild reaction conditions, high bond-forming efficiency, inexpensive readily available starting materials, and good to excellent yields with wide functional group compatibility constitute the main advantages of this method. The protocol can be run on a gram scale.