A method according to claim 14 wherein (a) includes: (a1) providing a compound of the formula (B3): ##STR00439## wherein L is a leaving group and P.sup.c is a carboxyl protecting group, (a2) displacing the leaving group with an amine offormula R to produce a compound of the formula (B4): ##STR00440## wherein R.sup.1 is as defined in claim 1 or a protected form thereof, and P.sup.c is a carboxyl pr