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 Et 3 SiBpin. The transient silylborane ate‐complexes generated undergo the stereoselective 1,2‐migration of the Et 3 Si‐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 S E 2’‐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.