from hydroxyl which when present in an enol system may be represented in the tautomeric keto form c 1 6 alkyl c 1 6 alkoxy oxo which may be represented in the tautomeric enol form carboxyl c 1 6 alkoxycarbonyl c 1 6 alkylcarbonyl formyl aryl aryloxy aryloxycarbonyl arylcarbonyl heteroaryl amino mono and di c 1 6 alkyl amino carbamoyl mono and di c 1 6 alkyl aminocarbonyl amino c 1 6 alkyl aminocarbonyl mono and di c 1 6 alkyl amino c 1 6 alkyl aminocarbonyl c 1 6 alkylcarbonylamino cyano guanidino carbamido c 1 6 alkanoyloxy sulphono c 1 6 alkylsulphonyloxy nitro sulphanyl dihalogen c 1 4 alkyl trihalogen c 1 4 alkyl halogen where aryl and heteroaryl representing substituents may be substituted 1 3 times with c 1 4 alkyl c 1 4 alkoxy nitro cyano amino or halogen preferred examples are hydroxyl c 1 6 alkyl c 1 6 alkoxy carboxyl c 1 6 alkoxycarbonyl c 1 6 alkylcarbonyl aryl amino mono and di c 1 6 alkyl amino and halogen wherein aryl may be substituted 1 3 times with c 1 4 alkyl c 1 4 alkoxy nitro cyano amino or halogen in the present context the term u201c tri alkyl aryl silyl u201d means a silyl group substituted with 0 3.