[2 + 2] cycloaddition of optically active ketenes. Synthesis of (−)‐blastmycinone. Preliminary Communication

Abstract [2 + 2] Cycloaddition of (menthyloxy)methylketene (2) to cis‐ethyl propenyl ether showed a ca. 3:1 diastereo‐selectivity. A two‐step transformation of the thermodynamically more stable adduct 4 furnished the (2R*, 3R*)‐compound 5 with ca. 50% e.e. (Scheme 1). The analogous reaction sequence with cis‐ benzyl 1‐hexenyl ether (9) and 2 showed a ca. 5:1 diastereoselectivity in 10 and 11 (Scheme 2). Absolute configuration of C(3) and C(4) in 11 has been proved by converting it to (−)‐blastmycinone. Accordingly, the preferred transition state has been deduced as 15.