Cyclization trimethylsilylating agent

Several successful cyclizations of quite complex structures were achieved using polyphosphoric acid trimethylsilyl ester, a viscous material that contains reactive anhydrides of phosphoric acid.58 Presumably the reactive acylating agent is a mixed phosphoric anhydride of the carboxylic acid. 

Aminobenzenesulfonamides are ring closed directly to AH- 1,2,4-benzothiadiazine 1,1-dioxides (500) with a variety of carboxylic acids, employing polyphosphoric acid trimethylsilyl ester as the cyclization agent (83S851, 85JAP(K)6025984, 90JMC172l). 

There is one more way for conversion of ort/to-nitroarylacetonitriles into indoles. Alkylation of such nitriles with allyl or benzyl halides followed by treatment of the compounds obtained with basic agents results in a multistep transformation, which is likely to proceed via intermediate nitrosoarenes, to produce 1-hydroxyindoles. For instance, alkylation of ort/io-nitroarylacetonitriles with 3-phenylallyl bromide gives the compounds that in the presence of chlorotri-methylsilane and triethylamine undergo cyclization into 3-cyano-l-hydroxy-2-vinylindoles (Scheme 70) [188]. Presumably, this reaction proceeds via 0-silylation of the nitronate anion and 1,5-elimination of trimethylsilanol from the intermediate trimethylsilyl nitronate, followed by cyclization and a hydrogen shift. 

The synthesis of l-oxa-3,8-diazospiro[4,5]decan-2-ones 749 from 4-pyridones by addition of trimethylsilyl cyanide, reduction with LAH, and cyclization of the thus obtained amino alcohol with triphosgene has been reported [538]. l-Oxa-3,8-diazo-spiro[4,5]decan-2-ones are structurally related to the antihypertensive agent Indor-amine, a known postsynaptic ai adrenoceptor blocker. 

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