Synthesis of β-methyl-β-alanine-l-proline-XAA tripeptides by Yb(OTf)3 catalysed Michael addition of amines to N-crotonyl-l-proline-XAA: a versatile route to cyclic β-methyl-β-alanine-derived tripeptides via ring closing metathesis

N-Crotonyl-l-proline derived peptides can be transformed to the corresponding β-methyl-β-alanine-l-proline peptides by Yb(OTf)3 catalysed Michael addition of aliphatic amines to the crotonyl residue. The Michael adducts derived from addition of allylamine are versatile precursors to the corresponding cyclic peptides obtainable via ring closing metathesis.