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Synthesis of 6-(S)-carbomethoxysancycline (26) by Michael–Claisen cyclization using the methyl phenyl diester 25 and enone 1 as substrates, with deprotonation in situ, followed by deprotection; in one experiment conducted with the corresponding benzyl phenyl diester 27 as the D-ring precursor, a diastereomeric mixture of cyclization products was obtained.
