Formal synthesis of (+)-varitriol. Application of Pd(II)/Cu(II)-catalysed bicyclisation of unsaturated polyols

A formal and improved synthesis of natural (+)-varitriol from d-glucose and dimethyl l-tartrate, respectively, are reported. The key steps are the Pd(II)/Cu(II)-catalysed bicyclisation of O-benzyl protected triols l-xylo-15 and l-xylo-15/l-lyxo-15, respectively, followed by ring opening of intermediate dianhydro-l-gulitol 16. The syntheses of key intermediate of the furanoside portion 17 proceed in 13 steps with 5% (from bisacetone-d-glucose), and in 12 steps with 7.6% over-all yield from dimethyl l-tartrate, respectively.