A virtually complete asymmetric control in the synthesis of 2, 3-disubstituted butan-4-olide (10) was demonstrated by employing the butenolide (12) as the chiral acceptor for the conjugate 1, 4-addition. Highly efficient asymmetric total synthesis of natural (-)-and unnatural (+)-steganacin was accomplished. The absolute stereostructure of natural antitumor steganain was determined to be 1.
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