Groups R1, R2, R3 and particularly R4, may be modified after the preceding condensation reactions if required; for example a benzyloxy group can be converted to a hydroxy group by hydrogenation, an ester can be hydrolysed to an acid, a benzyl ester can be hydrogenated to yield an acid, a salt of an acid can be esterified by reaction with a reactive chloride, bromide or tosylate, an acid can be est