Analogous retro-aldol reaction

Fig. 10 Why decarboxylation is inherently slower than an analogous retro-aldol reaction (itself a slow reaction). The angular distortion for sp2 to sp geometry is much larger than for sp3 to sp2 geometry, and the energy cost is proportional to the square of the angular distortion. Reproduced with permission from ref.9...

The P-acetoxy esters 2 were obtained in yields over the range 69-75%, based on the aldehyde and with a high enantiomer excess (ee). In addition, in this case the absence of a base as a co-catalyst is essential in order to avoid retro-aldol reaction of the product. The scope of this methodology is not restricted to aromatic hydroxy acids, as the non-aromatic cyclohexyl analog was also successfidly deracemized (Scheme 13.3). 

The hydroxide-catalyzed addition of a-acyldiazomethane derivatives, while quite facile with aldehydes to produce stable aldol adducts, is poor with aliphatic ketones. However, the lithium or magnesium anion (26) adds smoothly to produce the corresponding aldol adducts (27) in good yield (Scheme 8). On treatment with acid, thermolysis or metal catalysts, these adducts lose nitrogen and rearrange to the 3-keto esters (28) and (29). The analogous reaction with a-diazo ketones has also been carried out however, with bulkier substituents, the retro-aldol reaction competes with rearrangement. 

It has previously been shown that in the triplet state a nitro group on a benzene ring is highly electron withdrawing at the 3- and 4- positions and induces decarboxylations, retro-Aldol type processes and a novel intramolecular redox-type reaction by way of nitrobenzyl carbanion-type intermediates in all cases. In this context, the irradiation of the three 4-nitrobiphenyl derivatives (217), (218) and (219) has been examined in order to assess the potential of photoexcited nitro groups to induce such processes across the biphenyl system.Indeed analogous reactions are observed in these biphenyls but with enhanced quantum efficiencies to those for the reactions in benzenoid compounds. Thus (217) yields the aldehydes (220) and (221) at pHs of 2 and 7, respectively, and (218) gives (222) and (223) under similar conditions, whereas (223) is also formed from (219) at pH 13 but no reaction is observed in neutral solution. The authors note that these results provide more evidence that the... 

The appropriately substituted vinylogous amides can imdergo an intramolecular photocycloaddition-retro-Mannich-Mannich sequence. This sequence is analogous to the photocycloaddition-retro-aldol-aldol sequence shown in the formation of 19 from 15 (vide supra). Thus, irradiation of 110 leads to the formation of ketoimine 112, the product of photoaddition followed by rerra-Mannich fragmentation. Reaction of 112 with 1 equiv of trimethyloxonium tetrafluoroborate, followed by treatment of the resulting iminium ketone with aqueous hydrochloric acid, provides the photocycloaddition-re/ro-Mannich-Mannich product 113 in 50% yield from the acyclic photosubstrate 110. ... 

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