Hydrobromides

Anilines react with ct-haloacetophenones to give 2-arylindoles. In a typical procedure an W-phenacylaniline is heated with a tw o-fold excess of the aniline hydrobromide to 200-250°C[1]. The mechanism of the reaction was the subject of considerable investigation in the 1940s[2]. A crucial aspect of the reaction seems to be the formation of an imine of the acetophenone which can isomerize to an aldimine intermediate. This intermediate apparently undergoes cyclization more rapidly (path bl -> b2) than its precursor (Scheme 7.3). Only with very reactive rings, e.g, 3,5-dimethoxyaniline, has the alternative cydiz-ation (path al a2) to a 3-arylindole been observed and then only under modified reaction conditions[3],... 

The crystal and molecular structures of 2-amino-4-phenylthiazole hydrobromide have been determined by radiocrystallography the angle between the thiazole and phenyl rings was found to be 19 . The major features are reported in Fig. VI-4 (142). 

Fig. Vl-4. Geometry of thiazole ring in 2-arniro-4-phenylthiazole hydrobromide. Italicized values are interatomic distances (A) other numbers are angles in degrees. From Ref. 342.

Bromination of 2,4-dimethylselenazole with cold bromine gives an unstable monobrominated derivative initially (m.p. 168°C). which is easily converted to a product [m.p. 205°C (decomp.)] considered by Haginiwa to be 5-bromO-2,4-dimethylselenazole hydrobromide (19). 

Unlike nitration. 2-amino-4-methylselenazole can be directly bromi-nated, using bromine in carbon tetrachloride solution, to give 2-amino-5-bromo-4-methylselenazole hydrobromide [m.p. 180°C (decomp.)] (19). The free base cannot be isolated. Use of excess of bromine can lead to destruction of the molecule. 

Starting from 2-bromoethylamine hydrobromide and selenourea, Chu and Mautner (60) prepared the salt of 2-(aminoethyl)-selenopseudourea, which cyclizes upon heating in aqueous solution to give 2-aminoselenazoline (Scheme 58). 

Amino-A.-selenazoline and its 5-methyl derivative have previously been obtained by Baringer (61) starting from 2-bromoethylamine hydrobromide and l-amino-2-bromopropane hydrobromide, respectively, by reaction with potassium selenocyanate. 

DiaminO 4,4-dimethyl-l,3,5-thiadiazine hydrobromide was isolated as by-product (418). Benzene sulfonates of cyanohydrin prepared from sodium cyanide and an halobenzoaldehyde, when treated with thiourea or its derivatives, afford 2,4-diamino-5-(p-halogenophenyl)-thiazole benzene sulfonates (447). Similarly, cyanoamido thiocarbamates obtained from cyanamide and isothiocyanates yield substituted 2,4-diaminothiazoles (598). 

Scopolamine (42), an optically active, viscous Hquid, also isolated from Solanaceae, eg. Datura metell. decomposes on standing and is thus usually both used and stored as its hydrobromide salt. The salt is employed as a sedative or, less commonly, as a prophylactic for motion sickness. It also has some history of use ia conjunction with narcotics as it appears to enhance their analgesic effects. BiogeneticaHy, scopolamine is clearly an oxidation product of atropiae, or, more precisely, because it is optically active, of (—)-hyoscyamiae. 

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