C., whereupon the alkylating agent of choice, R.sup.1 X is added (R.sup.1 is as described above and X is chloro or bromo; alternatively the alkylating agent may be R.sup.1 -tosylate, R.sup.1-mesylate or an aldehyde or ketone such as acetaldehyde and the like) to provide mono-alkylated species 18.