. . "In formulae (I), (II) and (III), R is a hydrogen atom or an acetyl radical; R1 is a benzyl radical or a radical of the general formula R2???O???CO???, wherein R2 is an optionally substituted alkyl radical, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, bicycloalkyl, phenyl or heterocyclyl; each of R3 and R4, which are the same or different, is hydrogen, alkyl, alkoxy, optionally substituted aryl or o" . . .