Stereocontrol in organic synthesis using silicon-containing compounds. A formal synthesis of prostaglandins controlling the stereochemistry at C-15 using a silyl-to-hydroxy conversion following a stereochemically convergent synthesis of an allylsilane

Abstract

Hydrosilylation of isoprene with chloro(diphenyl)silane gave (Z)-chloro(2-methylbut-2-enyl)diphenylsilane 7. The cuprate reagent derived from this chloride underwent conjugate addition to methyl cinnamate 11, 1,2-silylcupration with hex-1-yne 16 and allene 18, and allylic displacement reactions with 1-vinylcyclohexyl acetate 20 and (Z)-1-cyclopentyloct-2-en-1-yl acetate 22. The silyl group in each of the products was converted into a hydroxy, with the removal of the 2-methylbut-2-enyl group taking place under much milder acidic conditions than those needed to remove the phenyl group from the dimethyl(phenyl)silyl group, and making this group suitable for the conversion of an allylsilane into an allyl alcohol. A stereospecifically anti conjugate displacement of the allylic benzoate group in (Z)-(1S,5R,6R,7R,1′S)-7-benzoyloxy-6-(1′-benzoyloxyoct-2′-enyl)-2-oxabicyclo[3.3.0]octan-3-one 52, and a stereospecifically syn conjugate displacement of the carbamate group in (Z)-(1S,5R,6R,7R,1′R)-7-benzoyloxy-6-(1′-N-phenylcarbamoyloxyoct-2′-enyl)-2-oxabicyclo[3.3.0]octan-3-one 51, gave stereoconvergently the same allylsilane (1′E,2″Z)-(1S,5R,6R,7R,3′S)-7-benzoyloxy-6-[3′-(2″-methylbut-2″-enyl)diphenylsilyloct-1′-enyl]-2-oxabicyclo[3.3.0]octan-3-one 53. Silyl-to-hydroxy conversion gave the allyl alcohol (E)-(1S,5R,6R,7R,3′S)-7-benzoyloxy-6-(3′-hydroxyoct-1′-enyl)-2-oxabicyclo[3.3.0]octan-3-one 54, having the relative and absolute stereochemistry at C-15 of the prostaglandins.