Endo benzyloxy group

Synthesis from xylose The xylose derivative 47, obtained from 5,5-bis-benzyloxy-7-oxa-bicyclo[2.2.1]hept-2-ene (45), has been used in the synthesis of (-P)-castanospermine (1) (Scheme 6). Bromination of 45 occurred exclusively on the less hindered convex face of 45, followed by stereoselective migration of the endo OBn group of the acetal to give 46, which subsequently converted to 47. Mesylation of 47 followed by cyclization with ammoifia gave 48, whose protection, hydrolysis, acetylation and cyclization by an intramolecular Wittig-Homer condensation gave 49. Conversion of 49 into epoxide 50... 

A methylol acetate was heated to afford a single bicyclic Diels-Alder adduct in 93% yield. The stereoselectivity in this process was rationalized by assuming a transition state in which the carbonyl group of the acyl imine is endo to the diene and the bridging chain has a chair-like conformation with the large benzyloxy methyl group quasi-equatorial. Such a conformation, which minimizes nonbonded and maximized secondary orbital interactions, would afford the observed product. 

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