Asymmetric conjugate reductions with samarium diiodide: asymmetric synthesis of (2S,3R)- and (2S,3S)-[2-2H,3-2H]-leucine-(S)-phenylalanine dipeptides and (2S,3R)-[2-(2)H,3-2H]-phenylalanine methyl ester.

The highly diastereoselective samarium diiodide and D(2)O-promoted conjugate reduction of homochiral (E)- and (Z)-benzylidene and isobutylidene diketopiperazines (E)-5,7 and (Z)-6,8 has been demonstrated. This methodology allows the asymmetric synthesis of methyl (2S,3R)-dideuteriophenylalanine 27 in > or = 95% de and >98% ee, and (2S,3R)- or (2S,3S)-dideuterioleucine-(S)-phenylalanine dipeptides 37 and 38 in moderate de, 66% and 74% respectively. A mechanism is proposed to account for this process.