B-Allenyl- and B-(γ-Trimethylsilylpropargyl)- 10-phenyl-9-borabicyclo[3.3.2]decanes: Asymmetric Synthesis of Propargyl and α-Allenyl 3° -Carbinols from Ketones

摘要

Simple and efficient Grignard procedures are reported for the syntheses of S-allenyl-10-(phenyl)-9-borabicyclo[3.3.2]decane (1) and its B-(γ-trimethylsilylpropargyl) counterpart (2) in both enantiomeric forms. Both add selectively to ketones, providing propargyl- and α-silylallenyl 3°-carbinols, respectively (i.e., 6 (61-93% ee) and 9 (64-98% ee)). The air-stable boron byproduct is efficiently recovered and recycled back to either 1 or 2. The ozonolysis and bromination of 9 provide nonracemic α-hydroxy acids and γ-bromopropynyl carbinols, respectively.