1,3-cycloelimination, anionic

Preparative routes to aziridines and 1-azirines are derived from cycloelimination processes in which one, and sometimes two, bonds are formed directly to the nitrogen atom (Scheme 1). For aziridines these include the two intramolecular cyclization pathways involving either nucleophilic displacement by the amine nitrogen (or nitrenium anion) on the /3-carbon (route a) or nucleophilic displacement by a /3-carbanionic centre on the amine nitrogen... 

Methylenation. This phosphonate ester is generally regarded as unsuitable for methylenation of ketones, even though the anion forms the intermediate adduct in satisfactory yield. However, under suitable experimental conditions, this reagent was used for methylenation of 1 to give 2 in 90% yield, higher than that obtained with methylphosphonic acid bis(dimethylamide) (78%).1 The optimum yield is obtained if exactly 2 equiv. of base and 1 equiv. of water are used to quench the adduct before cycloelimination. 

Alkyttdenation. Methylenation of ketones can be effected in 50 99% yield by reaction with the lithium anion (2) of 1 with 0=CR R2 to give a stable adduct (3) that undergoes cycloelimination to alkenes (4) on methylation. When applied to aldehydes, the yield is low except for benzaldehydes or a,/ -unsaturated enals. 

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