Aryl ketones, cyclodehydration

Intramolecular alkylation of aryl ketones with the concomitant elimination of water, also called cyclodehydration,313 has been studied by Shudo and co-workers.314 Cyclodehydration of 1,3-diphenylpropanones to give 1 -phenyl- l//-indenes [Eq. (5.119)] shows strong dependence on acidity. 

A -Acylamino aromatic ketones 790 can be prepared by arylation of saturated oxazolones in the presence of Lewis acids. Cyclodehydration of 790 leads to 2,5-diaryloxazoles 791. For example, saturated 5(4//)-oxazolones 789 from 7/-benzoyl-alanine or A-benzoylvaline undergo Friedel-Crafts arylation to afford substituted A-benzoylphenacylamines 790. In the presence of POCI3, 790 cyclizes to produce 5-aryl-2-phenyloxazoles 791 (Scheme 7.241). ... 

The carbonyl unit involved in the cyclization process is not restricted to aldehydes and ketones. The carbonyl of acid derivatives, such as amides can also be utilized. One of the more important cyclodehydration reactions is applied to the formation of heterocychc systems via cyclization of p-aryl amides, in what is called the Bischler-Napieralski reaction In this reaction amides of the type 47 are... 

The reaction of 3-(l,l,3,3-tetramethyl-l,3,2-disilaazolidin-2-yl)propylmagnesium bromide (1) with aromatic nitriles gives 2-aryl-l-pyrrolines, due to intramolecular cyclodehydration of the released amino ketones. 

Dialkyl- and 2-alkyl-2-aryl-3,4-dimethyl-5-phenyloxazolidines were formed from ketones and pseudoephedrine or ephedrine by cyclodehydration in the... 

In principle, the heterocyclization between these p-substituted ketones and monosub-stitnted hydrazines proceeds by a Michael addition-elimination on the p-carbon atom by the more nncleophilic nitrogen of the hydrazine, followed by cyclodehydration to give mixtnres of 3-triflnoromelhyl and 5-trifluoromethylpyrazoles. In general, condensation reactions with melhylhydrazine lead to the 3-trifluoromethylpyrazole derivatives as only or major regioisomers, whereas with aryl or heterocyclic aromatic hydrazines, the formation of 5-trifluoromethylpyrazole prevails. 

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