Multiple and long-range participation of benzyl groups in intramolecular C-arylation reactions of benzylated glycosides

Abstract

The intrinsic reactivity of furanosides bearing activated O-benzyl substituents (3-methoxybenzyl), in the presence of bidentate Lewis acids such as tin(IV) chloride, was explored. These glycosides were found to exhibit extremely interesting chemical properties. Thus, with three reactive substituents (at O-2,3,5), the corresponding glycosides ( 1 and 7) underwent a novel internal bis- C-arylation process, which involved successive alkylations of the benzyl groups at O-2 and O-3 (“multiple participation”), leading to the formal replacement of the two CO bonds at the anomeric center of the glycoside by two CC bonds. The bis- C-arylated constitution of the resulting polycyclic compounds 4 and 8, and the cis configuration of their fused ring system (a tetrahydro-[2]benzopyrano[3,4- d][2]benzoxepin derivative), were determined on the basis of their n.m.r.-spectral parameters. With two 3-methoxybenzyl substituents (at O-3 and O-5, compound 6), intramolecular alkylation of the benzyl group at O-3 or O-5 occurred when glycoside 6 was reacted with titanium(IV) chloride or tin(IV) chloride, respectively, thereby leading to novel bicyclic internal aryl C-glycosides ( 9 and 12) as major products (“long-range participation”). The constitution of compounds 9 and 12 was unambiguously established by the reactions of analogs of 6 bearing only one 3-methoxybenzyl substituent at a specific position (at O-3: 15; at O-5: 20). The unexpected divergent behavior of 6 in the presence of titanium(IV) and tin(IV) chloride remains to be explained. The availability of compound 9 made it possible to independently prepare the bis- C-arylated derivative 8 (by way of the reverse sequence of internal C-arylation reactions) and thereby to definitively demonstrate its constitution. These unprecedented reactions extend the scope of the intramolecular C-glycosidation of substituted sugars and provide novel methodologies in synthetic carbohydrate chemistry.