The alcohol group of 3 is activated by conversion into a leaving group such as mesylate, tosylate or triflate by treating the alcohol with a sulfonyl chloride or (preferably) sulfonic anhydride, such as trifluoromethanesulfonic anhydride, in the presence of a hindered amine base such as triethylamine, diethyl isopropylamine or 2,6 lutidine, to afford a compound such as compound 4.