Straightforward synthesis of α-substituted prolines by cross-metathesis

The synthesis of several a-substituted N-Boc-protected prolines has been achieved by cross metathesis (CM) of N-Boc-allylproline 5 with terminal long chain alkenes and alkenes bearing hydroxy, silyloxy, ester, and O-acetylglucosamido groups. The CM occurred with good selectivity and short reaction time under microwave heating conditions, affording yields in the range of 40-92 %. Addition of Ti(OiPr)(4) as a Lewis acid allowed a slight increase of the yield in the case of alkenes with Lewis basic substituents. The CM was also successfully applied to allylproline protected with trichloroacetaldehyde 4, but the intermediate products were less practical for further deprotection and elaboration. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).