Stereoselective Alkylation of the Vinylketene Silyl N,O-Acetal and Its Application to the Synthesis of Mycocerosic Acid

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

Total Syntheses and Absolute Configuration Assignment of (+)-Sootepdienone, (-)-Jambolanin C, (-)-Jambolanin I, and (-)-Gibberodione

Total syntheses of the sesquiterpenes (+)-sootepdienone, (-)-jambolanin C, (-)-jambolanin I, and (-)-gibberodione have been accomplished in 10 steps each from R-(+)-pulegone, allowing assignment of the absolute configuration of the natural products. A key step in the synthetic pathways involves the one-carbon ring expansion of a cyclic allylic phosphonate to a substituted cycloheptenone by a tandem oxidative cleavage/intramolecular Horner-Wadsworth-Emmons reaction.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Synthesis of C3-C21 Segment of Aflastatin A Using Remote Asymmetric Induction Reactions

The C3-C21 segment of aflastatin A has been synthesized by converging three segments including the C3-C8 segment, the C9-C15 segment, and the C16-C21 segment. Each segment has been synthesized from a vinylketene silyl N,O-acetal possessing a chiral auxiliary by a wide-range stereocontrol strategy. The C3-C8 segment was constructed in seven steps including the stereoselective vinylogous Mukaiyama alkylation, while the C9-C15 segment and the C16-C21 segment were synthesized using the vinylogous Mukaiyama aldol reaction in seven and eight steps, respectively.

Remote Asymmetric Induction Reactions using a E, E-Vinylketene Silyl N, O-Acetal and the Wide Range Stereocontrol Strategy for the Synthesis of Polypropionates

The construction of libraries of acyclic polyketides remains a challenging topic, mostly due to the difficulties associated with finding the right balance between diversity and brevity for the synthetic routes leading to polyketides. Recently, relatively short methods have been developed and applied to the synthesis of natural products. However, these short routes often suffer from limited diversity with respect to the arrangement of functional groups and stereochemistry, as these usually require reactions that direct multiple functional groups simultaneously in one step. Therefore, methods that combine a small number of reaction steps with structural diversity remain an attractive research target for the construction of acyclic polyketide libraries. In 2004, we reported a remote asymmetric induction reaction using chiral vinylketene silyl N, O-acetal 1, which is commensurate to an anti-selective vinylogous Mukaiyama aldol reaction. Ever since, this reaction has been applied to the synthesis of numerous natural products, as this synthetic route is short and efficient on account of the simultaneous introduction of both asymmetric centers and the multiply functionalized carbon chain. Recently, we have developed a variety of this remote asymmetric induction reaction based on the E, E-vinylketene N, O-acetal 1, which includes syn-selective vinylogous Mukaiyama aldol reactions, as well as alkylation, acylation, and bromination reaction. These reactions provide polypropionates in a highly stereoselective manner. The proposed transition states of these reactions are discussed in this Account. Additionally, we have developed a new short synthesis of polypropionates by combining reactions for the remote asymmetric induction and the functionalization of double bonds (wide-range stereocontrol, WRS). The remote asymmetric induction reaction simultaneously constructs the stereogenic centers at the central part of the products and introduces the α,β-unsaturated imide, while the new strategy is based on the initial construction of the central part of the molecule and a subsequent functionalization of the surroundings (WRS). This strategy successfully furnished stereoisomers in a few steps, and the stereodivergent synthesis of 2,4,6-trimethyloctanoic acid derivatives was accomplished. This strategy should also be feasible to construct an acyclic polyketide library. Moreover, we applied this method to the concise synthesis of natural products. In this Account, the development of remote asymmetric induction reactions and the new WRS strategy are described. Applications of the WRS strategy as well as reactions for the stereodivergent synthesis of polypropionates and natural products are also described. The aforementioned acyclic polyketide library should be constructed in the future with the help of the WRS strategy and become a powerful tool in drug discovery.

Diastereoselective and Catalytic α-Alkylation of Chiral N-Acyl Thiazolidinethiones with Stable Carbocationic Salts

Direct nickel-catalyzed alkylation of chiral N-acyl-4-isopropyl-1,3-thiazolidine-2-thiones using a commercially available nickel(II) complex, (Me₃P)₂NiCl₂, has been developed for tropylium and trityl tetrafluoroborate salts. The reaction provides a single diastereomer of the corresponding adducts in good to high yields, which, in turn, can be easily converted into a wide array of enantiomerically pure compounds that are difficult to obtain by other asymmetric procedures.