Α-oximono-esters as precursors to heterocycles – generation of oxazinone n-oxides and cycloaddition to alkene dipolarophiles
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Heaney, Frances; Fenlon, Julie; O′Mahony, Colm; McArdle, Patrick; Cunningham, Desmond (2003). Α-oximono-esters as precursors to heterocycles – generation of oxazinone n-oxides and cycloaddition to alkene dipolarophiles. Org. Biomol. Chem. 1 (23), 4302-4316
Preparation of a series of terminally and internally substituted delta-alkenyl and delta-alkynyl esters 6,7 and 9, potential precursors to oxazin-2-one nitrones, has been attempted. Condensation between pyruvic or benzoylformic acid and the appropriate alcohol proceeded smoothly in some cases whilst allylic transposition was a major feature in other cases most especially during reactions with alpha-vinylbenzyl alcohol. Oximation of piyruvic acid derivatives furnished E-oxime isomers whilst benzoylformic acid derivatives afforded mixed geometrical isomers. The E-oxime of 4a(1) carrying an internal Me group undergoes facile thermal cyclisation affording nitrones 1c and 1d in good yield. Oximes E-5a,b with a terminal methyl substituent on the alkene moiety furnish nitrone only under the influence of an external electrophile [PhSeBr/AgBF4]. A terminal Ph substituent on 5c,d prohibits formation of the cyclic dipole irrespective of reaction conditions, and whilst 5d reacts to afford a bicyclic isoxazolofuranone 13 by an IOOC reaction (intramolecular oxime olefin cyclisation) 5c remains thermally inert. Finally delta-alkynyl oximes 9c,d also failed to cyclise. The regio- and stereochemical characteristics of the cycloadditions between the new dipoles and electron poor olefinic dipolarophiles have been investigated. The conditions needed for reaction were rather forcing since the dipoles are somewhat stabilised by the adjacent alkoxycarbonyl group. All reactions proceeded regiospecifically to give adducts with 5-substituted isoxazolidine rings whilst diastereoselectivity varied with the choice of dipolarophile and the steric demands of the nitrone substituents. The phenylselenyl dipole 10a could not be trapped by any dipolarophiles bar dimethyl acetylenedicarboxylate.