Type A Syntheses

Type A (S—C—N + C—C) Syntheses.—4-Hydroxy-A -thiazolines are isolable as intermediates in certain Hantzsch reactions (see p. 357). Condensation of oxalyl chloride with hydrazones R R C=NNPhC(S)NHa [R z= = Me or R R = ( 112)4] yields the A -thiazoline-4,5-diones (56), whilst the gem-dicyano-epoxides (6 R = Ar, R = H, R = R = CN) react with thiourea 

Type A (S—C—N + C—C) Syntheses.—4-Hydroxy-A -thiazolines have been produced by Hantzsch s method. The synthesis of A -thiazolines by the interaction of substituted thioureas and diethyl acetylenedicarboxylate (see these Reports, Vol. 2, p. 617) has been applied to the production of 5-alkoxycarbonylmethylene-2-(2-pyridylamino)-A -thiazolin-4-ones (85).  

The synthesis of A -thiazolin-4-ones from glycidic esters and thiourea, first reported by Culvenor et al., has been reinvestigated by modem spectroscopic methods, and the mechanism originally proposed has been confirmed experimentally. Phenyl dithiocarbamate reacts analogously. 

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No practical type B syntheses of quinoxalines are commonly in use, largely because of the fact that type A syntheses are more facile however, some phenazine syntheses of this type are known, particularly those described in the older chemical literature. Hillemann (38CB42) has effected dimerization of 0-bromoaniline by heating its solution in nitrobenzene with K2CO3 and copper powder. The reaction is believed to proceed through the intermediacy of 5,10-dihydrophenazine, but the latter has not been isolated (Scheme 68). 

Sexual conjugation in yeast is also induced by pheromones (mating factors).325-327 Yeast cells of mating type a synthesize the 12-residue mating factor a which contains a C-terminal cysteine methyl ester S-alkylated with a frans,frans-farnesyl group (Table 30-5). Cells of type a synthesize a 13-residue factor ol 327a Cells are attracted to the pheromone produced by cells of the opposite type. The tremerogens, sex hormones of certain basidiomycetes, have related structures (Table 30-5)328... 

The formation of the diketopiperazine ring from proline-containing peptides can occur in two ways (see type A and type B). Entropy considerations predict that type A syntheses should result in relatively easier cyclization than type B syntheses. In both approaches, the presence of a proline residue in the peptide allows ready cyclization to the diketopiperazine under conditions which would leave other nonproline-containing dipeptides unaffected. ... 

An interesting example of intramolecular catalysis of type A syntheses is provided by the ready condensation of certain histidylproline peptides. Treatment of the ditrityl derivative of the tripeptide 121 with 50% acetic acid at room temperature results in rapid detritylation followed by cyclization to give the diketopiperazine 122 with elimination of... 

Synthesis. - Type A Syntheses (S-C-N + C-C). Thiourea derivatives or dithiocarbamates react with0-bromo or chlorocarbonyl compounds... 

Synthesis. - Type A Syntheses (S-C H -N + C). The C-nucleoside (182) is formed by condensation of 2-amino-4-chlorobenzenethiol with an acetylated aldonic nitrile or glycosyl cyanide followed by deacetylation23. 2-Aminobenzenethiol reacts with RC0CsCPh(Rs2-thienyl) to give (183). The latter cyclises to 2-phenylbenzo-thiazole on attempted recrystallisation (MeOH). With terminal acetylenic ketones a mixture of benzothlazolines results232. 

Synthesis.—Type A Syntheses (S—C—N + C—C). The reaction between thioureas RNHCSNH2 and /3-aroylacrylic acids gives the A -thiazolin-5-ones (47), which may be converted into bicyclic compounds on treatment with hydrazine or hydroxylamine (see later).However, the reaction with acrylic acid amides simply leads to transamidation, giving ArCOCH=CHCONHCSNH2. ... 

Synthesis.—Type A Syntheses S—C—N + C—C). The reaction between thioamides, e.g., MeC(0)CH2C(S)NHPh, and bis(perfluoroketimines) (F3CN=CF)2 in the presence of a fluoride acceptor gives 4,5-bis(trifluoro-... 

Other Type A Syntheses.—Successive treatment of lactone (4) with hydrochloric acid and thiourea yields 2-amino-5-hydroxyethyl-4-methylthiazole, an intermediate in the synthesis of vitamin j8j8-Dichloro-a-amino-acrylonitrile,... 

Type A Syntheses [N—C—S + C—N]. From 1,2,3A-thiatriazoles. Two independent reports of the production of a variety of 1,2,4-thiadiazolidines from a reactive species (37) arising from the thermolysis of 4-alkyl-5-arylsulphonyl-imino-4,5-dihydro-l,2,3,4-thiatriazoles (36) have appeared. The intermediates (37) undergo 1,2-addition with isocyanates or carbodi-imides to afford excellent yields of the 3-oxo- (38) or 3-imino-1,2,4-thiadiazolidines (39). Their structure was confirmed by alternative established syntheses. 

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