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Asymmetric Tandem Mannich-Michael Reactions of Amino Acid Ester Imines with Danishefsky's Diene
Waldmann, H.,Braun, M.
, p. 4444 - 4451 (2007/10/02)
Imines 1 derived from aromatic, aliphatic, and functionalized aldehydes and various amino acid esters react with Danishefsky's diene under Lewis acid catalysis via a tandem Mannich-Michael mechanism to give cyclic 6-substituted 2,3-didehydro-4-piperidinones in good to high yields and with diastereomeric ratios reaching from 92:8 up to 97:3.The chiral auxiliary is removed by conversion of the α C atom of the amino acid into an acetalic center, employing a Curtius reaction as the key step.For the elucidation of the absolute configuration, the alkaloids (S)-coniine and (R)-δ-coniceine are synthesized from the enaminones 5i and 5r.
