Spontaneous cyclization and

Condensation of 2-aminothiophenol with the /3-chlorocinnamaldehyde in the presence ofp-toluene sulfonic acid (PISA) gave good yield of benzothia-zole (Scheme 14). The mechanism suggested in this work is beUeved to proceed via a nucleophilic attack of the sulfur atom in an addition-ehmination sequence followed by a spontaneous cyclization and ejection of acetaldehyde [15]. These investigations were performed in a domestic microwave reactor and need 1.5 min for completion (65% yield). Here again, oil bath heating seems to be inferior, providing a maximum conversion of 53% after... 

We have already described the biosynthesis of glutamate and glutamine. Proline is a cyclized derivative of glutamate (Fig. 22-10). In the first step of praline synthesis, ATP reacts with the y-carboxyl group of glutamate to form an acyl phosphate, which is reduced by NADPH or NADH to glutamate y-semialdehyde. This intermediate undergoes rapid spontaneous cyclization and is then reduced further to yield proline. 

However, following its formation, the semialdehyde undergoes a spontaneous cyclization and is converted to a A -pyrroline-5-carboxylic acid (P5C), a proline precursor. 

Alkyl-5-aryl- and 2,5-diaryl-l,3,4-thiadiazoles were prepared by cyclization of II with ethyl imidate hydrochlorides. Holmberg prepared 2-methyl-5-phenyl-l,3,4-thiadiazole by acetylating 11 with carboxymethylthiolacetate, followed by spontaneous cyclization, and by oxidation of acetaldehyde thiobenzhydrazone with ferric chloride. A considerable number of 2-alkyl-5-aryl-l,3,4-thiadiazoles have been prepared by the method of Stolle. ... 

During reduction of the nitro derivatives 261 (with stannous chloride in hydrochloric acid, - with hydrogen over Raney nickel in ethanol, with iron powder in 50% acetic acid, - or with triethyl phosphite - at 170-1 SOX), the resulting amino derivatives 262 sometimes underwent spontaneous cyclization and gave the linearly annelated tricyclic compounds 263. Cyclization of the amino derivatives 262 was effected ther-mally or by the action of phosphoryl chloride. - ... 

Here, the product thiyl radical 3 undergoes spontaneous cyclization and deprotonation (pKa,2 = 5.2) to the radical anion 4, which has a strong UV absorption with Xiaax = 390 run. In this sequence, concentrations can be adjusted such that Reaction (3.22) is rate-determining. The reactivity of carbon-centered radicals with... 

Annelation of pyridinium rings. Protonated nitrogen heterocycles form adducts with methyl vinyl ketone that can be converted into fused pyridinium salts. For example, (1) reacts with the reagent at 120° in acetonitrile or with dimethylacetamide to give the adduct (2), which when heated cyclizes to (3). The corresponding reaction with (4) gives an adduct that undergoes spontaneous cyclization and aromatization to form (5). 

The piperidine was prepared from 2-(2-cyanoethyl)-butryraldehyde (470) via ketal 471, which was reduced with lithium aluminum hydride and reductively alkylated with benzaldehyde over 10% palladium on charcoal to 472. Acid hydrolysis led to spontaneous cyclization and formation of enamine 473. Treatment of this enamine with methyl bromoacetate and reduction of the iminium species with sodium borohydride gave the 1-benzyl-3,3-disubstituted piperidine 474 which was debenzylated with palladium charcoal under acidic conditions to give the desired piperidine, 466 (216). 

In the process of chain-lengthening addition of the third residue (or a peptide) to a dipeptide derivative is an ambiguous step. Alkyl esters of dipeptides undergo spontaneous cyclization and produce 2,5-diketopiperazines ... 

Figure 9, Spontaneous cyclization and dehydration of levulinic acid.

Scheme 19.14 One-pot process based on combination of an aldolase-catalyzed reaction, spontaneous cyclization and a phosphatase-catalyzed reaction.

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