Epimine regioselectivity

Saeki s group examined the possibility of synthesizing exocyclic monosaccharide epimines by reduction of azido sulfonates. Regioselective displacement of... 

For reactions of furanoid epimines, the configuration of the epimine does not determine the regioselectivity. Other factors, such as steric interactions between nucleophile and epimine, or soft or hard character of the nucleophile, may control the regioselectivity. 

Exocyclic aziridines react without the influence of a tetrahydropyran or tet-rahydrofuran ring on the regioselectivity, and thus these epimines resemble nonsugar aziridines in their cleavage reactions. 

Fukase and coworkers described the deoxygenation of butirosins A and B via hydrogenation of their epimine-based derivatives over Raney nickel catalyst. Free (335) and A-anisoylated (336) epimines were cleaved by hydrogen to form mixtures of 3-deoxy- (337-338) and 2-deoxybutirosins (339-340) in 71 20 and 56 26% yields, respectively. The authors explained the regioselectivity in terms of less steric hindrance at C-2 than C-3 at the catalyst surface. 

Unsubstituted benzyl 4-(9-benzyl-2,3-dideoxy-2,3-epimino-(3-D-/yv o- (341) and -ot-D-n )o-pyranoside (342) have been used by Paulsen and Patt in 1981 in cleavage reactions with azide. By the action of a mixture of sodium azide and ammonium chloride in a butanol-water system, the aziridine-ring cleavage proceeded exclusively at C-3, thus exhibiting anti-Furst-Plattner regioselectivity for both epimines. No explanation for this preference was given in the paper. 

The observed regioselective attack was at C-3, while the cleavage of the corresponding aziridino lactones proceeded exclusively at C-2, despite the configuration of the epimine (D-lyxo and D-ribo). 

In contrast to these reactions, attempts" at aziridination of 2-methoxy-5,6-dihydro-(2//)-pyran led only to products of trans cleavage of the epimine. This regioselectivity can be explained by predominance of Fiirst-Plattner-type cleavage over SNl-type because of destabilization of the carbonium ion at C-3 by the acetal moiety. 

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