The reactions of 3,4-epoxytetrahydropyran and of its cis- and trans-2-methyl derivatives with hydrogen halides and with lithium aluminum hydride have been investigated in order to assess the influence of an O atom in the β position on the regioselectivity of the epoxide ring opening. All these reactions exhibit a high preference for nucleophilic attack at position 4, which decreases moderately only when the inductive effect of the O atom and the stereoelectronic requirements of the attack act in opposite directions. Similar trends are observed in the reactions of the 5,6-dihydro-2H-pyrans with NBA, which occur with preferential nucleophilic attack by water at position 4 of the intermediate epibromonium ions. A remarkably high preference (96%) for electrophilic attack syn to the 2-Me group is observed in the latter type of reaction, in accordance with a previous proposal of a mechanism in which the nucleophilic step is rate determining.

Regio- and stereoselectivity of the 3-membered ring opening of some 3,4-epoxytetrahydropyrans and of the corresponding epibromonium ions.

CATELANI, GIORGIO;
1974

Abstract

The reactions of 3,4-epoxytetrahydropyran and of its cis- and trans-2-methyl derivatives with hydrogen halides and with lithium aluminum hydride have been investigated in order to assess the influence of an O atom in the β position on the regioselectivity of the epoxide ring opening. All these reactions exhibit a high preference for nucleophilic attack at position 4, which decreases moderately only when the inductive effect of the O atom and the stereoelectronic requirements of the attack act in opposite directions. Similar trends are observed in the reactions of the 5,6-dihydro-2H-pyrans with NBA, which occur with preferential nucleophilic attack by water at position 4 of the intermediate epibromonium ions. A remarkably high preference (96%) for electrophilic attack syn to the 2-Me group is observed in the latter type of reaction, in accordance with a previous proposal of a mechanism in which the nucleophilic step is rate determining.
Catelani, Giorgio; Catelani, G; Ferretti, M; Monti, L.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11568/555
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