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Some recent developments in stereoelectronic theory. Reevaluations of ALPH and the reverse anomeric effect

Chandrasekhar, Sosale (2005) Some recent developments in stereoelectronic theory. Reevaluations of ALPH and the reverse anomeric effect. In: ARKIVOC: Online Journal of Organic Chemistry (13). pp. 37-66.

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Abstract

A critique of objections to the 'antiperiplanar lone pair hypothesis' (ALPH) and the 'reverse anomeric effect' (RAE) is presented. Whilst early fears that ALPH is incompatible with the Curtin-Hammett principle (CHP) were apparently unfounded, objections to ALPH based on the reactions of cyclic hemiorthoesters and amidinium salts are debatable. (The unreactivity of Kirby's bicyclic bridgehead acetal supports ALPH convincingly.) Objections to RAE were based on its apparent incompatibility with ALPH, studies on several models indicating (dubiously) the existence of a weak anomeric effect rather than RAE. However, this may well suggest that a normal anomeric effect is being offset by RAE. This apparently indicates different bases for the ground state and kinetic anomeric effects: the classical 'electrostatic gauche repulsive interaction' (EGRI) and the 'orbital interaction' (OI) respectively. The evidence against free oxocarbenium ions, but favouring the ion pair and the 'exploded transition state' (ETS) models apparently raises the question 'Whither ALPH?'. A rigorous application of the CHP to the stereochemistry of displacements at glycosyl anomeric centres, indicates that the alpha anomers react retentively via ion pairs, whereas the beta anomers react with inversion via the ETS. (A substantial reassessment of the 'in situ anomerisation technique' is also indicated.)

Item Type: Journal Article
Additional Information: Copyright of this article belongs to Arkat USA Inc.
Keywords: ALPH;anomeric;Curtin-Hammett Principle;oxocarbenium;reverse-anomeric;stereoelectronic.
Department/Centre: Division of Chemical Sciences > Organic Chemistry
Date Deposited: 08 Jan 2010 06:17
Last Modified: 19 Sep 2010 04:56
URI: http://eprints.iisc.ernet.in/id/eprint/17366

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