Further, when the B linking group is a carboxylate group such that the compound has a urethane functionality, a reaction sequence as shown in scheme 3, below will be useful: ##STR12## wherein all of the variables are as set forth above and R18 is a group susceptible to transesterification, e.g. an alkoxy group or an aryloxy group, preferably lower alkoxy (e.g. a methoxy group).