Cyclization agents/cyclizations thionyl chloride

Suitable agents are chlorosulfonic acid, monohydrate, or concentrated sulfuric acid. Other methods include cyclization of phenylthioglycolic acid chlorides, which is afforded by Friedel-Crafts reaction with aluminum chloride, possibly in the presence of sulfuric acid/sodium chloride. The acid chloride is obtained from phenylthioglycolic acid with thionyl chloride or phosphorus trichloride. 

This class of compounds has been prepared only as the S-oxides. In most cases thionyl chloride serves as the cyclization agent. Thus the first A3-thiatriazoline (129) was prepared by Huisgen et al. from diphenylbenzamidrazone and SOCl2, and from 1,3-dipolar cycloaddition between diphenylnitrileimine and thionylaniline (equation 78) (62LA(658)169>. 

Benzyl-4-methyl-6/7-l,2,6-thiadiazin-3(2i7)-one 1,1-dioxide (203) is obtained in poor yield (22%) by cyclization of the sulfamide (202) with thionyl chloride in dichloromethane containing a catalytic amount of dimethylformamide. Yields are much improved (61 %), however, with trimethyl-silyl iodide as the cyclization agent (Equation (25)) <84CCC840>. 

Subsequent developments, including cyclizations by means of benzene-sulfonyl chloride or silver benzenesulfonate in pyridine, and by phosphoryl or thionyl chloride have already been reviewed by Cornforth.2 4 Of the above cyclodehydrating agents, phosphorus pentoxide is generally preferred to effect cyclizations smoothly 40 for example, phosphorus pentachloride tends to give oxazoles chlorinated in the side chain,60 as shown in Eq. (4). 

Hydroxyethylamino)quinoxalines 112a-e are cyclized into l,2-dihydroimidazo[l,2-a]quinoxalines 113a-e under the action of dehydrating agents, such as thionyl chloride, HCI or phosphorus(V) oxochloride (Scheme 4.52) (Hirotaka et al. 1970 Ohmori et al. 1997 Borchardt et al. 2010, 2011b). 

In addition to the perfluorosulfonic acid derivatives, carboxylic acid forms have been developed. These may be obtained by modification of Nafion films or by direct synthesis. In the case of modification, the ionomer is converted from a thionyl chloride to a carboxylic acid. Typical reducing agents are hydriodic acid, hydrobromic acid and hydrophosphonic acid. The carboxylic acid form can be made directly by copolymerization of tetrafluoroethylene and a carboxylated perfluorovinyl ether. The synthesis of the ether is a difficult task, however, as cyclization can occur. One synthetic route to the ether is shown in Scheme 5. After copolymerization with tetrafluoroethylene, the ester group can be quantitatively hydrolyzed to yield the sodium salt. 

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