Electrocyclizations nitrile imines

Thus in the N-silyl substituted series, 17 and 18, which rearrange thermally to the corresponding diazo compounds, the stability increases through the series R=Me, Ph, i-Pr. As discussed below, these compound undergo the usual cycloaddition and electrocyclization reactions of nitrile imines and are not simply overstabilized curiosities. The usefulness in synthesis of those with P—C bonds is probably limited since these bonds are not easily broken, but products derived from those with C—Si and C—B bonds (e.g., 21 and 22) should be capable of further... 

Reaction of dichorobenzaldoxazine with sodium azide followed by 1-propa-nethiol-triethylamine gave the tetrazine imide 367 as the major product. Its formation is thought to involve the reaction sequence shown which includes an unprecedented electrocyclization of the nitrile imine 366 as the final step (210). 

The oxidation of hydrazones 143 provides, in principle, a very convenient route to nitrile imines from easily accessible starting materials. However, the earliest reagent used, lead tetraacetate, was of limited effectiveness as yields were low and the reaction often gave high yields of diacylhydrazides as byproducts. Work has been done on the application of several other oxidants to this process to produce a more effective general route. The one that has proved most popular is chloramine T (A-chloro-iV-sodio-p-toluenesulfonamide, CAT) which is used under mild conditions and has been shown to work well for both cycloaddition (79) (e.g., in the preparation of 146 from 145) and electrocyclization (80) reactions. 

The 1,7-electrocyclization of nitrile imines 47 has been proposed as a key step in the conversion of the stable phosphorus ylides 45 to pyrazolo[4,3-d] [2,3]benzodiazepines 48, upon refluxing in xylene (Scheme 16). Ringopening of the triazoles 45 and recyclization is postulated to give the pyra-zoles 46. Migration of the triphenylphosphine group, followed by the elimination of triphenylphosphine oxide, would then give the nitrile imine 47 (95TL5637). 

The 1,7-electrocyclization of nitrile imines with an a,13 C=N bond 51 has also been observed by Boyd and co-workers [87JCS(CC)99], Thermolysis of the tetrazoles 49 led to an initial migration of the imidoyl group. Loss of nitrogen from tetrazole 50, followed by 1,7-electrocyclization and a [1,5]-hydrogen shift, gave the 1,3,4-benzotriazepines 52 (Scheme 17). 

Garanti and Zecchi have extended the range of their synthesis of 1,2-benzodiazepines via 1,7-electrocyclization of the nitrile imines (54). Earlier work used 1-chloro-hydrazones to produce diazepines bearing electron-withdrawing substituents, but now the use of 1-nitro-hydrazones (53) allows the preparation of unsubstituted and 3-alkyl-substituted diazepines (55) in moderate yield.The... 

The action of chloramine T on the acylhydra-zones 79 (R, R = aryl or 2-furyl) generates nitrile imines 80, which undergo a 1,5-dipolar electrocyclization to the 1,3,4-oxadiazoles 81 (94SC1879). 

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