STEREOSELECTIVE SYNTHESIS OF ALCOHOLS .50. STEREOSELECTIVE SYNTHESIS OF A C-15 C-27 SEGMENT OF THE VENTURICIDINES/

Chain extension of an aldehyde by two ''propionate'' units has been at tained by stereoselective allylboration with the chiral 1-methylbuteny l boronate 3 to give, e.g., the homoallylic alcohol 6, followed by a r egioselective hydroboration/carbonylation procedure to give, e.g., the epimeric aldehydes 8. The latter were converted into the lactols 10, which equilibrated to the desired epimer. The lactols could again be s ubjected to an allylboration reaction, initiating a second round of th e chain extension protocol. This technique has been used to synthesize in 15 steps in the venturicidene C-15/C-27 segment 23, containing 8 s tereogenic centers with the proper absolute configuration.