Retro-aldol type cleavage

Attempted hydrolysis of the acetate function of the 10a-8jS,9j5-epoxide (303) with dilute methanolic potassium hydroxide afforded, via retro-aldol type cleavage, the 8,9-seco-steroid (304). 

Evidence for this postulate was provided by epimerization of deuterio-or tritio-labeled derivatives of (148) 117). As the result, a retro-aldol type mechanism in which stereoelectronic requirements are satisfied in the transition state was suggested. The process is depicted in Figure 5. This concept was also applied to the retro-Dieckmann type cleavage of ketone (151) to carboxylic acid (152) (Scheme 31), the facile epimerization of the 3p-axial alcohol (153) to the 3 a-equatorial alcohol (154) under the influence of dilute alkali (followed by esterification) and the cleavage of the 3-ketone (155) to methyl ester (156) on treatment with sodium hydroxide in methanol at 0°C for one hour (Scheme 31). 

The first step in the nonreversible degradation reactions is the formation of a reactive a-dicarbonyl species through the p-elimination of a hydroxide ion. The subsequent reaction pathways to all degradation products can be described by just five reaction types, namely, p-elimination, benzilic acid rearrangement, a-dicarbonyl cleavage, aldol condensation, and retro-aldol condensation (see Fig. 7).31 Retro-aldol condensation and a-dicarbonyl cleavage involve C-C bond... 

From a preparative point of view the heteroatom in P-position has an influence because important latent cyclobutane cleavage pathways exist in the product. The prototypical reaction of this type is the [2 + 2]-photocycloaddition/retro-aldol reaction sequence (de Mayo reaction) [55-57], the course of which is illustrated for substrate A3 (Q = O, PG = protecting group) in the reaction with ethylene as a generic olefin. 

TA is another fold-type I enzyme, which catalyzes the cleavage of the Ca—C/3 bond of hydroxyamino acids. TA and SHMT have very similar catalytic properties, being able to catalyze the same set of reactions, that is, the retro-aldol cleavage of several different erythro and threo L-/3-hydroxyamino acids and the transamination... 

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