O- to C-Glycoside rearrangement

Obtained (via O fi C glycoside rearrangement) by adding boron trifluoride etherate to a mixture of 2-(benzyloxy)-6-hydroxy-4-methoxy-acetophenone, 2,3,4,6-tetra-O-benzyl-a-D-glucopyranosyl fluoride (R) and powdered molecular sieves 4 A in methylene chloride and stirring at -78° for 2 h under an argon atmosphere after the disappearance of (R) on TLC, the reaction temperature was raised from -78° to r.t. and the mixture stirred for 45 min (78%) [3198]. 

The utility of glycosyl phosphates in the synthesis of C-glycosides was demonstrated in the synthesis of the natural product 8,10-di-O-methyIbergenin 39. Utilizing the O-to-C rearrangement, the key C-aryl linkage was installed and further elaboration afforded the product in good yield. 

Matsumoto, T, Hosoya, T, Suzuki, K, Improvement in O — C-glycoside rearrangement approach to C-aryl glycosides. Use of 1-0-acetyl sugar as stable but efficient glycosyl donor. Tetrahedron Lett., 32, 4629-4632, 1990. 

Scheme 6.12 Rearrangement to C-aryl glycosides from aryl O-glycosides following a-selective synthesis using glycosyl phosphates.

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