Non‐chelate‐controlled addition of 1‐Bromo‐1‐lithio‐1‐alkenes to O‐protected lactaldehydes and 3‐alkoxybutyraldehydes

Non‐chelate control is found to be the favored stereochemical outcome in the addition of the 1‐bromo‐1‐lithio‐1‐alkenes 2b and ( S )‐ 5b to O ‐protected lactaldehydes ( S )‐ 1 . Thus diastereoselectivities of 84 to 96% d.e. are reached in the formation of alcohols 3 and 6 , which are converted into the O ‐protected 2,3‐dihydroxyaldehyde 31 or O ‐protected 1,2,3‐triols 24 and 27. In an analogous way, a non‐chelate‐controlled addition of ( R )‐ 5b to ( R )‐3‐(benzyloxy)butyraldehyde ( 14a ) leads to the predominant formation of the diastereomer 15a (92% d.e. after enrichment by chromatography) which yields the alcohol 42a upon debromination, O ‐protection, and ozonolysis. Thus, the carbenoids 2b and 5b enable the introduction of the synthons − CH 2 OH and − CHO into enantiomerically pure aldehydes ( S )‐ 1 and ( R )‐ 14 in a highly stereoselective manner. Relatively low diastereoselectivities are reached in the „mismatched”︁ combination of ( R )‐ 5b with ( S )‐ 1 as well as ( S )‐ 5b with ( R )‐ 14 . The dilithium compound 20b , generated at −78°C, reacts in a retro‐1,4‐Brook rearrangement at −20°C to give vinylsilane 44b after protonation.