940. Heptasulphur N-acylimides

Abstract

A mild method for preparing heptasulphur N-acylimides is the reaction of heptasulphur imide with carboxylic acids in the presence of dicyclohexylcarbodi-imide or ethoxyacetylene.HEPTASULPHUR IMIDE (I) has been reported to react with acetyl chloride in pyridine at O", to give the N-acetylimide (11; R = Me) in 70% yield, and with benzoyl chloride in the presence of pyridine at 60" to give the N-benzoylimide in low yield. We found that free carboxylic acids and dicyclohexylcarbodi-imide or ethoxyacetylene can be used to prepare compounds of type (11).When molar amounts of the imide, a carboxylic acid, and dicyclohexylcarbodi-imide are allowed to react between 0" and 20" in tetrahydrofuran, dicyclohexylurea is precipitated quantitatively. The N-acylimides can be isolated in 50--85y0 yield (see Table ).The reaction of ethoxyvinyl esters, prepared in sit% from ethoxyacetylene and carboxylic acids, and the imide at room temperature was sluggish, but proceeded readily in boiling chloroform. The N-acylimides prepared in this way were identical with those obtained by the carbodi-imide method.The structures (11) are based on elemental analysis, infrared spectra, hydrolysis, and reduction. The infrared spectra4 show bands at 795-838 cm.-l, attributed to the S,N ring. All spectra show a carbonyl band which is shifted by 40-50 cm.-l towards a higher frequency compared with the unsubstituted amide RCO-NH,; this indicates that the known ionic contributions to the structure of R*CO*NH, are absent in R*CO*NS,. The carbonyl frequencies of R*CO*NS, thus correspond very closely to the frequencies of the analogous methyl ketones RCOCH,. This " abnormal CO-amide bond " could be demonstrated chemically by the extreme ease of hydrolysis. Heptasulphur N-acyl imides are quantitatively hydrolysed when shaken with basic aluminium oxide in methylene