Abs&uct: 2-akoxy-2-flume-glycosyl fluc&h la and lb react with tenzyl alcdml dexivatives in the presence of the fluorine activating system Cp2HfC~AgClO4 giving isochrmnane or O-glycosides derivatives depending on tbe glycosyl fhnk&/alcobol ratio. A mechanism of the consecutive reactions leading to tk isochromane dexivatives is projwed.
Glycosyl fluorides are useful glycosyl donors because of their stability with regard to other glycosyl halides, ease of handling and the availability of specific activation meth0ds.t We recently reported2 that methyl pyranosid-Zuloses react with diethylaminosulfur trifhroride PAST) to give good yields of 2-alkoxy-2-fluoro-glycosyl fluorides. The 1,24ifluoro carbohydrate la2 is a stable compound and it behaves like a typical electrophilic glycosyl donor when it reacts with bis-(trimethylsilyl)uracile to give the 2-alkoxy-2fluoronucleoside 2 (Scheme 1).3 Gn the other hand, la can also behave lie a 1.2~dielecuophilic synthon. When activatedtf-j by Cp$lfCl2/AgClO4 it reacts with dinucleophilic compounds such as benzyl alcohol to give the isochromane derivatives 3 and 4.4 This reaction appeared to be dependent on the metallocene derivative used, on the reagents' molar ratio and on the solvent. The best results in the preparation of isochromane derivatives were obtained when a hafnocene dichloride/silver salt ratio of 1:2 and a sugar/benxyl alcohol ratio of 1:2 were used in dichloromethane as the solvent. When xirconocene dichloride was used as activator, principally non cyclixed compounds were obtained. Various features of this reaction are oustanding, the stereospecificity, the exclusive formation of the C-C bond at C-l position and not at C-2 and the obtention of compound 4 where the OMe group present in the starting material has been substituted by a OBn group. Uracil~h Cp$lfCl~AgC104 2 la 3 4 scheme 1
In this article we describe the behaviour of thii reaction with different alcohols and provide evidence of the structum of new compounds and for the mechanism of this process.