Reaction of compound 26 with an electrophilic cyanating agent (e.g. cyanogen bromide, N-cyanobenzotriazole or cyanogen bromide/4-dimethylaminopyridine complex) in a suitable solvent (THF acetonitrile or DMF, optionally in the presence of a base such as pyridine or sodium hydride or potassium tert-butoxide) may then afford the desired compound 27.