mean the groups may have either isomer form or are mixtures thereof.Examples of R are alkyl such as methyl, ethyl, propyl, isopropyl, n-butyl, sec.-butyl, tert.-butyl, pentyl and hexyl and aralkyl such as benzyl and phenethyl and a silylated group such as trimethylsilyl, tert.-butyldimethylsilyl, triphenylsilylor diphenyltert.-butylsilyl.A preferred group of compounds of formula I are those where