Stereoselective Synthesis of 4-Hydroxy-1-Silyl-1-Allenylboranes and Access to 2-Silylethynyl-1,3-Diols

The highly stereoselective gem-silylboration of chiral 2-substituted 1-ethynylepoxides is reported herein. The reaction involves first the deprotonation of the epoxides in the acetylenic position followed by transmetallation with Et3SiBpin. The transient silylborane ate-complexes generated undergo the stereoselective 1,2-migration of the Et3Si-group to the sp-hybridized terminus carbon of the ethynyl-moiety in a stereospecific anti-addition. The moisture-sensitive and thermally unstable 4-hydroxy-1-triethylsilyl-1-allenyl(pinacolato)boranes thus obtained react with aldehydes through an SE2'-mode of addition, affording highly functionalized 2-triethysilylethynyl-1,3-diols with a good diastereoselectivity which is rationalized by a Yamamoto-Houk type transition state model. The stereoselective and stereospecific gem-silylborylation of chiral trans 2-substituted 1-ethynylepoxides affords (aS*,S*)-4-hydroxy-1-silyl-1-allenylboranes. The reaction of theses gem-bimetallic species with aldehydes gives access to highly functionalized (anti,syn)-2-triethysilylethynyl-1,3-diols with a good diastereoselectivity. image