Flash vacuum thermolysis azomethine ylide

Enaminone 128 (Scheme 33) is obtained, together with an isomeric indo-lizine derivative, by flash vacuum thermolysis of aminomethylene Meldrum s acid derivative through intermediate ketene and delocalized azomethine ylide (85TL833). The thermally induced cyclization of semi-cyclic dienamines to afford, for instance, tricyclic 129 is also believed to start with an azomethine ylide (97JOC7744) the p-chlorophenyl substituent is essential for the reaction. Unstabilized ylide 130, on the other hand, is generated from pipecolinic acid and /1-phenylcinnamaldehyde by the decarboxylation method target base 131 is formed by 1,7-electrocycliza-tion and [l,5]-hydrogen shift (99J(P1)2605). 

Azomethine ylide generation from oxazolidines has also been achieved by flash vacuum thermolysis (20,21). During synthetic efforts toward alkaloid central skeletal cores, Joucla and co-workers (22) revealed that flash vacuum thermolysis of oxazolidine (84) led to an intramolecular [3 + 2] cycloaddition furnishing pyrrolidine 85 in 82% as a single regio- and stereoisomer. Subsequent Dieckmann... 

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). 

Joucla has reported another example of azomethine ylide generation from oxazolidines by a technique of flash vacuum thermolysis (FVT) (87TL2973). Allyl esters of N-substituted a-amino acids react with 2 molar equivalents of paraformaldehyde, under reflux in toluene, to give rise to oxazolidines 166... 

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