This invention provides a method for preparing compounds having the structure [Formula (I)] wherein L is [Formula (A)] or [Formula (B)] in which R.sub.1 is hydrogen or optionally substituted alkyl, n is zero or 1 and m is 1; wherein R is H, halogen, optionally substituted C.sub.1-6alkyl or C.sub.1-6alkoxy; wherein Z is a bond, O or S; wherein p is an integer from 1 to 5; wherein Q is a bond provided that Z is not a bond when p is 1; or Q is O, S or --C(O)NR.sub.6-- in which R.sub.6 is hydrogen, optionally substituted alkyl or cycloalkyl; or Q is --NR.sub.6--, --NR.sub.5C(O)NH-- or --NR.sub.5C(O)O-- in which R.sub.5 is hydrogen, alkyl or aralkyl provided that p is not 1; wherein W is cycloalkyl, aryl or heterocyclyl; or W and R.sub.6 form a 9- to 10-membered bicyclic ring, which may be optionally substituted or may contain oxygen, nitrogen or sulfur.