A process for the preparation of a compound of formula (1):
##STR1##
is provided, which comprises the reaction a compound of formula (2):
##STR2##
with a compound of formula Al(OR)3 under substantially anhydrous
conditions. X, and X1 are each independently H or a protecting group,
B is a base; R is an alkyl, alkoxyalkyl, alkenyl or alkynyl group, each of which
may be optionally substituted, and L is a leaving group.
