Regioselective benzoylation of 6-O-protected and 4,6-O-diprotected hexopyranosides as promoted by chiral and achiral ditertiary 1,2-diamines

Monobenzovlation of triols (6-O-silylated glycopyranosides) or diols (4,6-O-benzylidenated glycopyrano- sides) with benzoyl chloride and triethylamine at - 60degrees to 23degrees is promoted by catalytic amounts of ditertiary 1,2-diamines. The regioselectivity depends mostly on the structure of the alcohols: it is modulated by the configuration and constitution of the diamines, as shown by comparing the effect of Oriyama's catalyst ((S)-1 and (R)-1), N,N,N',N'-tetramethylethylenediamine (TMEDA), N,N,N',N'-tetraethylethylenediamine (TEEDA), Et3N and EtNMe2. The effect of the catalysts on the reactivity is impaired by their steric hindrance. In agreement with the modest enantioselectivity of the mono- and dibenzoylation of rac-cyclohexane-1,2-diol in the presence of Oriyama's catalyst, the influence of these diamines on the regioselectivity is rather limited. While associated with procedural simplicity. these catalysts lead, in a few cases, to higher yields of a single benzoate than established methods, viz, in the preparation of the 3-O-benzoyl beta-D-glucopyranoside 4, the 2-O-benzoyl alpha-D-galactopyranoside 22, the 3-O-benzoyl alpha-D-galactopyranoside 23, and the benzylidenated 2-O-benzoyl alpha-D-galactopyranoside 44. The regioselective benzoylation of the benzylidenated beta-D-mannopyranoside 47, leading to 48, appears to be new