bols B, R3 and R81 used in the Schemes have the significance given above, while R19 is alkyl or cycloalkyl, B' is an amino protecting group, P is an O-protecting group and Q is a leaving group such as alkylsulphonyl or arylsulphonyl, for example methylsulphonyl, trifluoromethylsulphonyl, p-toluenesulphonyl and the like.