CHELATION-CONTROLLED REGIOSELECTIVE ENDO CLEAVAGE AND STEREOSELECTIVEC-1 ALKYLATION OF PENTOFURANOSIDES

Combinations of Lewis acids and nucleophilic reagents trigger endo-ope ning of the furanoside ring of methyl furanosides 1, 10 and 13, result ing in the attachment of the nucleophilic group at C-1 of the carbohyd rate. The stereoselectivity in the C-C bond-forming step is low for th e 2-deoxyfuranosides but very high (dr 1:99) for the furanosides carry ing a methoxy group in the 2-position when a combination of TiCl4 and Me2Zn is used. Different selectivities are obtained with Me2Zn as comp ared with Me3Al. Reagents based on several organometallic reagents of Al, Si, Ti and Zn in combination with TiCl4 can be used.