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.
|Autori:||Berti G; Catelani G; Ferretti M; Monti L|
|Titolo:||Regio- and stereoselectivity of the 3-membered ring opening of some 3,4-epoxytetrahydropyrans and of the corresponding epibromonium ions.|
|Anno del prodotto:||1974|
|Digital Object Identifier (DOI):||10.1016/S0040-4020(01)97376-4|
|Appare nelle tipologie:||1.1 Articolo in rivista|