" of formula (IIA) is obtained, but that N-alkylation at the 7-position is minimised or avoided.A compound of formula (II) may be prepared by reaction of a compound of formula (III): wherein R1 is as hereinbefore defined for a compound of formula (I), P is a protecting group, typically a tetrahydro-2H-pyran-2-yl group, and R3 is as hereinbefore defined for a compound of formula (IIA), with a suitab" . . . . .