Glycals epoxidation

The glycal epoxide method turned out to be useful for the construction of complex 2-branched [3-aryl glycosides, which are salient features of the potent antibiotic vancomycin [75a]. Glycal epoxide glycosylation with sodium salts of indoles pro-... 

Scheme 8.41 Utilization of glycals and a-glycal epoxides in carbohydrate chemistry.

Evans developed a new method for the synthesis of [(-C-allylglycosides, based on BusSnOTf-mediated ring-opening of glycal epoxides with allylstannanes as nucleophiles [81a], This methodology has been efficiently used in the (3-stereoselective introduction of the side chain (C44-C51) of spongistatin 2 (Scheme 8.43) [81b,c]. 

The outstanding chemoselectivity of Cp2TiCT was amply demonstrated by Merlic [50,51] and by Dotz [52] who employed a, /3-unsaluraled tungsten and chromium carbenes as radical traps for C - C bond formation. In the latter contribution the very acid sensitive glycal epoxides were used with good success. An example is shown in Scheme 8. 

R. J. Patch, H. Chen, and C. R. Pandit, Multivalent templated saccharides Convenient syntheses of spacer-linked 1, l -bis -and 1, 1, 1 "-tris-/i-glycosides by the glycal epoxide glycosidation method, /. Org. Chem., 62 (1997) 1543-1546. 

C-Allyl glycosides can be prepared by the reaction of glycal epoxides with allyltributyltin in the presence of tributyltin triflate as a Lewis acid,281 and aldonitrones can be allylated with trimethylsilyl triflate as a catalyst (Equations (101) and (102)).282... 

The opening of epoxides with zinc reagents is a difficult reaction. However, activated epoxides such as the glycal epoxides 225 react with diorganozincs in the presence... 

G. Bellucci, G. Catelani, C. Chiappe, and F. D Andrea, A simple and highly diastereoselective preparation of glycal epoxides using the MCPBA-KF complex, Tetrahedron Lett., 35 (1994) 8433-8436. 

A Ti(Oz -Pr)4-mediated kinetic spirocyclization (with C-l retention) for the stereocontrolled synthesis of spiroketals from glycal epoxides such as 485 has been reported (Scheme 87) <2006JA1792>. A complementary methanol-induced kinetic cyclization (with C-l inversion) allows for a synthetically systematic approach for accessing stereo-diversified spiroketals, for example, 486 and 487 from glycals (e.g., 484) <2005JA13796>. 

T. Hiroshi, N. Matoba, and T. Takahashi, Effective one-pot synthesis of H type 1 and 2 trisaccharide derivatives using glycal epoxide, Chem. Lett., 34 (2005) 400-401. 

Scheme 5.2.31 Studies of allylation reactions via glycal epoxides...

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