Extended enolates Thermodynamic product

Alkylation is essentially irreversible, so thermodynamic products are difficult to make, but the aldol reaction is reversible, and the proportion of reaction at the y position of ester 11 and acid 17 derivatives in aldol reactions increases both with temperature and time. Hence the a-aldol 27 is the product with the extended enolate of ester 11 if the reaction is worked up at low temperatures. At higher temperatures the y-aldols 25 and -26 are the only products,10 the lactone 25 coming from cyclisation of Z-26. 

The neutral 1,4- and 1,2-quinone methides react as Michael acceptors. However, the reactivity of these quinone methides is substantially different from that of simple Michael acceptors. The 1,6-addition of protonated nucleophiles NuH to simple Michael acceptors results in a small decrease in the stabilization of product by the two conjugated 7T-orbitals, compared to the more extended three conjugated 7T-orbitals of reactant. However, the favorable ketonization of the initial enol product (Scheme 1) confers a substantial thermodynamic driving force to nucleophile addition. By comparison, the 1,6-addition of NuH to a 1,4-quinone methide results in a large increase in the -stabilization energy due to the formation of a fully aromatic ring (Scheme 2A). This aromatic stabilization is present to a smaller extent at the reactant quinone methide, where it is represented as the contributing zwitterionic valence bond structure for the 4-0 -substituted benzyl carbocation (Scheme 1). The ketonization of the product phenol (Scheme 2B) is unfavorable by ca. 19 kcal/mol.1,2... 

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