2 + 21 Cycloaddition of (menthy1oxy)methylketene (2) to cis-ethyl propenyl ether showed a ca. 3:l diastereoselectivity. A two-step transformation of the thermodynamically more stable adduct 4 furnished the (2R*,3R*)compound 5 with ca. 50% e.e. (Scheme I). 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 (-)-hlastmycinone. Accordingly, the preferred transition state has been deduced as 15.
Yeast reduction of rac -ethyl 2-methyl-6-oxocyclohexanecarboxylate (rac -1) yielded selectively (+)-ethyl 2-hydroxy-6-methylcyclohexane carboxylate (+)-2 (Scheme 1 ) which has been alkylated with 5-iodo-2-methylbut-2-ene by the dianion method to furnish the 4-methylbut-3-enyl derivat 3 (Scheme 3). NaBH4 reduction of (+)-l led to three hydroxy-carboxylates (-)-2, (+)-5, and (-)-6 (Scheme 4). Allylation of the dianion of (+)-5 afforded (+)-7.Introduction. -
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