Cyclodehydration acid catalyzed

Under the conditions of the attempted conversion of the indolenine 17 into the quinoline 18 most of the indolenine was recovered, but there was also formed a small amount of a hydrolysis product, o-acetamido-jS-chloro-a-methylstyrene (22), obtainable in good yield with aqueous ethanolic potassium hydroxide. By analogy with a similar sequence of reactions in the carbocyclic series the hydrolysis product 22 might possibly undergo acid-catalyzed cyclodehydration to the quinoline... 

Anthracenes from ort/io-acyl diarylmethanes via acid-catalyzed cyclodehydration. 

The cyclodehydration method used for the synthesis of benzo[a jquinolizinium ion and its congeners (Scheme 97) requires that phenylpyridines (176) (including benzo analogs) be converted into quaternary salts (177) by the action of a -halo ketones or a -halo aldehyde derivatives. The acid-catalyzed cyclization of these salts (177) has provided access to a number of compounds containing the benzo[a]quinolizinium system (178) (Table 8). 

The first synthesis of the benzo[6]quinolizinium ion (Scheme 98, Table 9, example 1) was by hydrobromic acid-catalyzed cyclization of the quaternary salt formed between 2-pyridinecarbaldehyde and benzyl bromide. Aromatic cyclodehydration has continued to the present as almost the only method used for the preparation of the acridizinium ion, its derivatives and benzo analogs. Because of its instability, 2-pyridinecarboxaldehyde has been replaced by more efficient derivatives. The first of these was the oxime (example 2) which not only gave a better overall yield, but also made possible the isolation of a crystalline intermediate (181 Z = NOH). The disadvantages are that it is not suitable for high temperature cyclizations involving polyphosphoric acid, and some products (182) (e.g. example 10, Table 10) may tend to form double salts with hydroxylamine hydrobromide. 

The synthesis of this condensed system by two different approaches was reported in two publications (80JHC1489, 80JHC1665) from the same laboratory. According to the first approach, 1,2,3,4,12,13-hexahydro-l-oxoquino[l,2-c]-quinazolinium perchlorate (461) was prepared (80JH-C1489) from the quinazoline precursors (460) by acid-catalyzed cyclodehydration. 

An efficient general synthesis of a variety of 3(2i/)-furanones has been developed. Aldol condensation of aldehydes with the enolate derived from 3-methyl-3-(trimethylsiloxy)-2-butanone (183) followed by Collins oxidation afforded 1,3-diketones (184). Acid catalyzed cyclodehydration leads to the corresponding 3(2//)-furanones (185) (Scheme 43)... 

A more conventional route is the acid-catalyzed cyclodehydration of amides already containing the appropriate numbers of atoms. For example, carbamates (213) may be cyclized to oxazinediones (214) with concentrated sulfuric acid (76JCS(Pl)1969) and the dihydrobenzoxazin-4-ones and -thiazin-4-ones (216) can be prepared from the appropriate benzamides (215), again by treatment with acidic reagents (53AP437). 

Cyclodehydration of hexane-1,6-diol with tin phosphate as a catalyst (230°C) gives oxepane in 32% yield <2001GC143>. Acid-catalyzed dehydration of 2,2 -bis(hydroxydiphenylmethyl)biphenyl by an ion-exchange resin allows one to prepare a sterically hindered 2,2,7,7-tetraphenyldibenzo[c, ]oxepine in 70% yield under mild conditions (CH2C12, rt) <2002J(P1)2673>. 

Azine approach. Pyridazinium salts (216) can be prepared by acid-catalyzed cyclodehydration of 2-/3-oxoalkyl-3(2//)-pyridazinones (77YZ422). The same cyclization of 3,4-dihydro-... 

Microbial oxidation of D-glucose gives calcium 2,5-diketo-D-gluconate (145) whose hydrazones (146) are converted to the betaines 148 by acid-catalyzed decarboxylation and cyclodehydration (Scheme 6). Derivatives of these stable crystalline compounds have also been prepared by (i) methylation (Me2S04) of 4-hydroxypyridazines (143) and (ii) thermal elimination of methyl iodide from 5-methoxy-l-methylpyridazinium iodides... 

Reaction of diazenes 1144 with 1,3-keto-amides 1145 leads to 2-imidazolin-2-ones 1147 via a two-step procedure. The first step involves a regioselective attack of the active methylene group on the diazene to yield Michael adduct 1146. The adduct 1146 then undergoes an acid-catalyzed cyclodehydration to form imidazolin-2-one 1147 (Scheme 278) <2003TL5965>. 

Bradsher reaction. Acid-catalyzed cyclodehydration of o-acyldiarylalkanes to polycyclic hydrocarbons and their heterocyclic analogs. 

Kinetically controlled acid-catalyzed cyclodehydration of the 2-(p-gluco-or D-manno-pentitol-l-yl)-4,5,6,7-tetrahydroindol-4-one (3) (Section II,-C,l) furnished the 2-(a-D-arabinofuranosyl)-4,5,6,7-tetrahydroindol-4-one... 

During 0-isopropylidenation of the alditolyl chain of the 3-(alditol-l-yl)quinoxalin-2-one 745, the fused furan ring of 746 was formed as a result of acid-catalyzed cyclodehydration of the enolized C2 carbonyl and C2 OH groups. Removal of the ketal group of 746 gave 747 (93MI6) (Scheme 192). 

The only synthetic pathway used so far to prepare these C-nucleosides was the acid-catalyzed cyclodehydration of the alditolyl chains of their acyclo analogs (Section XII,N). Thus, heating l-aryl-3-(D-ara mo-tetritol-l-yl)flavazoles (751) with dilute sulfuric acid gave, stereospecifically, 1-aryl-... 

However, the outcome of the acid-catalyzed cyclodehydration of 1-phenyl-3-(D-/yjco-tetritol-l-yl) flavazole (755) was stereoselective, since it produced a mixture of l-phenyl-3-(/3- and a-D-threofuranosyl)flavazoles (756 and 757) in the ratio 3 1. The a-anomer 757 is stereochemically more stable because the bulky heterocycle and the Cl hydroxyl occupy less crowded positions than in the /3-anomer 756 (94MI8) (Scheme 196). 

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