Isomer formation, azomethine ylide generation

The thermolytic preparation by De Shong et al. (74) of azomethine ylides from aziridines and their intermolecular reactions are the first examples of singly stabilized ylides of this type. However, the protocol has been further extended to include intramolecular processes. Aziridines tethered to both activated and unactivated alkenes were subjected to flash vacuum thermolysis generating cycloadducts in moderate-to-excellent yields. While previously singly activated alkenes had furnished low material yields via an intermolecular process, the intramolecular analogue represents a major improvement. Typically, treatment of 222 under standard conditions led to the formation of 223 in 80% yield as a single cis isomer. Similarly, the cis precursor furnished adduct 224 in 52% yield, although as a 1 1 diastereomeric mixture (Scheme 3.77). 

Decarboxylative condensations of N-substituted or N-unsubstituted a-amino acids with carbonyl compounds offer the most convenient generation of a wide variety of azomethine ylides (Section II,E). The intermediacy of 5-oxazolidinones is ascertained by the fact that in several cases 5-oxazolidinones have been actually isolated and their thermolysis leads to the generation of azomethine ylides (76CSR377 77MI1 87BCJ4079). As the decarboxylation of the 5-oxazolidinone intermediates would take place stereospecifically in a concerted manner, 2,4-trans- and 2,4-cis-isomers of the 5-oxazolidinones give rise to anti- and syn-azomethine ylides, respectively. Condensations of cyclic a-amino acids with aldehydes produce 2,4-trans bicyclic oxazolidinones as thermodynamically more stable products. Accordingly, the exclusive formation of anti-ylides is expected in the decarboxylation route (Section II,E). 

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