Concise epoxide-based synthesis of the C14-C25 bafilomycin A(1) polypropionate chain

Epoxide-Based Synthetic Approaches toward Polypropionates and Related Bioactive Natural Products

Polypropionate units are a common structural feature of many of the natural products in polyketides, some of which have shown a broad range of antimicrobial and therapeutic potential. Polypropionates are composed of a carbon skeleton with alternating methyl and hydroxy groups with a specific configuration. Different approaches have been developed for the synthesis of polypropionates and herein we include, for the first time, all of the epoxide-based methodologies that have been reported over the years by several research groups such as Kishi, Katsuki, Marashall, Miyashita, Prieto, Sarabia, Jung, McDonald, etc. Several syntheses of polypropionate fragments and natural products that employed epoxides as key intermediates have been described and summarized in this review. These synthetic approaches involve enatio- and diastereoselective synthesis of epoxides (epoxy-alcohols, epoxy-amides, and epoxy-esters) and their regioselective cleavage with carbon and/or hydride nucleophiles. In addition, we included a description of the isolation and biological activities of the polypropionates and related natural products that have been synthetized using epoxide-based approaches. In conclusion, the epoxide-based methodologies are a non-aldol alternative approach for the construction of polypropionate.

Reiterative epoxide-based strategies for the synthesis of stereo-n-ads and application to polypropionate synthesis. A Personal Account

The enantioselective synthesis of polypropionates continues to be an attractive realm for the synthetic chemists mostly due to the challenges presented by the number of consecutive stereogenic centers contained within the aliphatic chain. Over the years, our laboratory has developed an epoxide-based three-step reiterative methodology for the construction of these targets, with the ultimate goal that the approach could be extended to the synthesis of polypropionate-containing natural products. The key steps include the diastereoselective epoxidation of allylic and homoallylic alcohols, and the regioselective cleavage of 2-methyl-3,4-epoxy alcohols. The choice of the organometallic reagent, and the cis/trans geometry of the chiral epoxide can be used to control both the relative and absolute configuration of the resulting propionate unit, allowing our approach to be applied in the synthesis of advanced fragments. Additionally, the combination of our first- and second-generation methodologies permits the incorporation of different variations at the methyl moiety.

Diastereoselective synthesis of vicinal tertiary and N-substituted quaternary stereogenic centers by catalytic hydroalkylation of dienes

An efficient and diastereoselective (CDC)-Rh-catalyzed hydroalkylation of dienes with 1,3-oxazol-5(4H)-ones is reported. Aryl and alkyl substituted dienes are converted to α,α-substituted oxazolones (24 examples) by the formation of N-substituted quaternary carbon stereogenic centers in good yields (up to 96%) and with high diastereoselectivity (>20 : 1 dr). The reaction is tolerant of a range of dienes and oxazolones bearing various functional groups. Utility of the oxazolone products is illustrated through hydrolysis to form α,β-substituted α-amino acid analogues and stereoselective epoxidation of the resultant alkene to create four contiguous stereocenters.

Crystal structure of (-)-(2R,3S,4R,5R)-5-(1,3-di-thian-2-yl)-3-methyl-1-(triiso-propyl-sil-yloxy)hexane-2,4-diol

The title compound, C20H42O3S2Si, crystallized with two independent mol-ecules (A and B) in the asymmetric unit. They consist of syn,anti,anti-stereo-tetrads with a 1,3-di-thiane motif and a primary alcohol protected as the triisopropyl silyl ether. The 1,3-di-thiane ring adopts a chair conformation, while the rest of each mol-ecule displays a common zigzag conformation. There is an intra-molecular O-H⋯O hydrogen bond in each mol-ecule. In the crystal, the A and B mol-ecules are linked via O-H⋯O hydrogen bonds, forming -A-B-A--B-- chains along [010]. The absolute structure was determined by resonant scattering (anomalous scattering) [Flack parameter = 0.035 (8)].