Compounds of formula (III) can be prepared from a compound of (IIIa) according to the general method described in EP 400,971 and Ojima et al, Tetrahedron, 48:6985-7012, 1992. ##STR21## Thus a compound of formula (IIIa) is first treated with a base such as n-butyllithium or triethylamine, and then followed by a compound of the formula R4 (O)p CO-L where L is a leaving group to provide a compound of