Syntheses of the Lower Portions of the Pamamycins from .gamma.-(Silyloxy)allenes Using Stereoselective Cyclization, Reduction, and Aldehyde Addition Methodologies
作者:Robert D. Walkup、Sang Woong Kim
DOI:10.1021/jo00091a037
日期:1994.6
The pamamycins are a group of homologous macrodiolides produced by Streptomyces alboniger and Streptomyces aurantiacus JA 4570 which are remarkable for their autoregulatory, antifungal, antibacterial, and anion-transfering activities. The syntheses of the C-1'-C-11' (''lower'') portions of pamamycin-607, pamamycin-649A, pamamycin-635A, pamamycin-649B, and pamamycin-635B as the methyl esters 8 and 10-12 are reported. The C-9'-C-11', portions of these esters were introduced by chelation-controlled allylations of C-8' aldehyde intermediates derived from the C-8' alcohols 9 (whose synthesis has been previously reported) and 13-15 using allyltrimethylsilane in the presence of titanium(IV) chloride. The alcohols 13-15 were synthesized via cis selective cyclizations of the trimethylsilyl ether derivatives of the nonracemic gamma-hydroxy allenes 24, 30, and 31 using a one-pot oxymercuration/transpalladation/methoxycarbonylation reaction followed by a chelation-controlled conjugate reduction of the resulting acrylate intermediates. The gamma-hydroxy allene 24 was synthesized via an enzymatic resolution of the gamma-acetoxy allene 20, and the gamma-hydroxy allenes 30 and 31 were synthesized by asymmetric aldol reactions. During the syntheses of the products 14 and 15, chemoselective hydrolyses and reduction reactions were employed in order to differentiate a C-8' carboxyl group from the C-1' carboxyl group. Starting from simple allenyl alcohol starting materials, overall yields from 2% (in 14 steps, for 10) to 14% (in 9 steps, for 12) were observed for the production of the C-1'-C-11' portions of the pamamycins.