A compound of formula I: ** see formula ** in which R1 is -C (= O) ORa, cyano, (C1-C6) alkyl, (C1-C6) alkanoyl, (C2-C6) alkenyl or alkynyl ( C2-C6), where any (C1-C6) alkyl, (C1-C6) alkanoyl, (C2-C6) alkenyl or (C2-C6) alkynyl may be optionally substituted with ...