be prepared starting with chiral starting components or separated from mixtures of isomers by methods well known in the art a further embodiment of compounds of formula 3 is represented in formula 3 b below r 1 and r 2 are h or ch 3 x u2212 is any of the pharmaceutically acceptable counter ions and these compounds of formula 3 b exist as a single stereoisomer of l r u2014 configuration a preferred embodiment of formula 4 is represented by formula 4 a below and its pharmaceutically acceptable salts r 1 and r 2 are h or ch 3 r 3 r 4 r 5 r 6 are independently selected to be hydrogen methyl ethyl hydroxyl alkoxyl and r 2 is an easily cleavable group under acidic or neutral conditions such as a t butyl group or an alkylaryl group wherein the aryl group may be unsubstituted or substituted with straight chain or branched c 1 c 6 alkyl c 3 c 6 cycloalkyl c 1 c 6 alkoxyls 1 3 dioxolanyl and c 1 c 6 dialkylamino a further embodiment of formula 4 is represented by formula 4 b below and its pharmaceutically acceptable salts r 1 and r 2 are h or ch 3 these compounds of formula 4 b exist as a single stereoisomer of l r u2014 configuration a preferred embodiment of formula 5 is represented by formula 5 a below and its pharmaceutically acceptable salts u0155 1 u0155 2 u0155 3 and u0155 4 are independently.