Fused ring C-glycoside

Although the previous reaction involved the use of an unactivated olefin, this was apparently not the source of the lack of stereochemical induction. This is illustrated by work reported by Herscovici et al. [70,71], shown in Scheme 7.13, where several unactivated olefins were successfully used to produce a-C-glycosides in good to excellent yields. Additional work in this area resulted in a facile route to fused-ring C-glycosides [72] similar to the core structures of natural products including the halichrondrins [73], the herbicidins [74-76], and octosyl acid [77]. 

Levy, D E, Dasgupta, F, Tang, P C, Synthesis of novel fused ring C-glycosides, Tetrahedron Asym., 5, 2265-2268, 1994. 

In a final example demonstrating the utility of unfunctionalized olefins, Levy, et al.,43 reported a novel adaptation in which the formed C-glycoside cyclizes to an adjacent oxygen with loss of a benzyl group. As shown in Scheme 2.3.17, l-0-acetyl-2,3,4-tri-0-benzyl-L-fucose was treated with isobutylene and an excess of TMSOTf giving the illustrated fused ring C-glycoside. This reaction proceeded in >70% yield in both methylene chloride and acetonitrile. Additionally, borontrifluoride etherate could also be used. The proposed mechanism, shown in Scheme 2.3.17, involves the initial elimination of the... 

Scheme 2.3.17, Scheme 2.3.18, and Table 2.3.1 (Daniel E. Levy, Falguni Dasgupta, Peng Cho Tang "Synthesis of Novel Fused Ring C-Glycosides Tetrahedron Asymmetry 1994,5,1937). 

Fused ring C-glycosides. Ionization < ate alkenes leads to C-C bond formation. able group, formation of a tetrahydrofuran ri the axial direction, the C-2 oxygen functioi new ring. 

Anomeric effects were first demonstrated in furanosides by means of X-ray diffraction [61,68, 294]. The importance of the anomeric effect for furanosides in solution was shown by studies of fused ring systems [295,296], from studies of nucleosides [297,298], and by comparing C-, N-, and 0-furanosyl glycosides in the solid state and solution [299]. Interestingly, the magnitude of the anomeric effect for nucleosides is pH dependent with the largest effect being observed under the most acidic conditions [300]. 

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