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

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.

Photochemically induced binding of Rh(phen)2Cl2+ to DNA.

The photoinduced covalent binding of the title compound to native and heat denatured DNA is described. The level of binding has been measured by UV (for DNA) and atomic absorption (for Rh) analysis. Quantum efficiencies of 6.4 x 10(-4) mol Rh per mol photons and 1.6 x 10(-3) mol Rh per mol photons have been determined for binding to native and denatured calf thymus DNA, respectively. Levels of bound rhodium as high as 1 molecule per five bases have been achieved. There is no binding of the complex in the absence of light, and there is evidence that at least a portion of the binding may be due to the photolytic conversion of the complex into one or more stable intermediates. Studies with polyribonucleotides indicate a strong preference for binding to the purine bases.