Patent ID: 7312343

Claim:
A process for the preparation of a compound of formula I comprising reacting a compound of formula II with a compound of formula III in the presence of a Lewis acid and an organic solvent; wherein R 1 is C 1-4 alkyl, C 3-6 cycloalkyl-C 1-4 alkyl or aryl-C 1-4 alkyl; R 2 is C 1-4 alkyl, benzyl, substituted benzyl or allyl; R 3 is C 1-4 alkyl or allyl; R 4 is C 1-4 alkyl, allyl, benzyl or substituted benzyl; R 4′ is benzyl or substituted benzyl; or alternatively, R 1 and R 4 together with the nitrogen to which they are bound form a pyrrolidine group; and R 5 is a trialkylsilyl group.