Cyclizing reagents acetal

A mixture of 2.6 g of-tetralone, 4.0 g of-naphthylhydrazine hydrochloride, and 3.0 g sodium acetate was refluxed with 50 mL ethanol for 2 h. After cooling, an excess of water was added the precipitate of the crude naphthylhydrazone was collected by suction, washed with water, and dissolved in 20 mL of the cyclization reagent (acetic acid and hydrogen chloride). After 5 min of heating on a water bath, the mixture was poured into water and the precipitate was washed thoroughly with water, dried, and crystallized from the mixture of benzene and ligroin to afford 2.5 g 3,4-dihydro-1,2,7,8-dibenzocarbazole, m.p., 178°C. 

A recent report describes the conversion of A-formyl- and N-acetyl-L-leucine into optically active azlactones with dicyclohexyl-carbodiimide (DCC) [Eq. (29)]. Other cyclization reagents, e.g. acetic anhydride, POCI3, SOCI2, and polyphosphoric acid, cause racemiza-tion. These azlactones react with optically active amino acid esters to give esters of dipeptides with retention of activity. 

Purine (6) itself is a colorless crystalline weak base which was first prepared by Fischer " the first synthesis using the Traubc condensation was reported by Isay. The Traube protocol is probably the best method to prepare purine, using pyrimidine-4,5-diamine as precursor and formic acid in an atmosphere of carbon dioxide or diethoxymethyl acetate as cyclizing reagents. 

To a solution of 4 g A-nitroso-A-piperonylglycine in 150 mL dry benzene at TO C was added a 50 mL dry benzene solution (also 70°C) containing enough N,N-dicyclohexylcarbodiimide (DCC), and A,A-dicyclohexylurea immediately precipitated. The mixture was stirred at 50-60° C for 2 h and filtered while hot. The filtrate was evaporated to dryness in vacuo, and the residue was recrystallized from 75 mL toluene 2.7 g 3-piperonylsydnone (shiny needles) was obtained, in a yield of 72.4%, m.p. 158-159°C. A parallel run was also carried out using acetic anhydride as a cyclizing reagent, and a comparable yield was obtained. 

The reactions of ketenes or ketene equivalents with imines, discussed above, all involve the imine acting as nucleophile. Azetidin-2-ones can also be produced by nucleophilic attack of enolate anions derived from the acetic acid derivative on the electrophilic carbon of the imine followed by cyclization. The reaction of Reformatsky reagents, for example... 

By using various trapping reagents, it has been deduced that the transannular fragmentation is rapidly reversible. The cyclization of the fragmented radical C is less favorable, and it is trapped at rates which exceed that for recyclization under most circumstances. " Radicals derived from ethers and acetals by hydrogen abstraction are subject to fragmentation, with formation of a ketone or ester, respectively. 

Aqueous hydrofluoric acid dissolved in acetonitrile is a good catalyst for intramolecular Diels-Alder reactions [9] This reagent promotes highly stereoselective cyclizations of different triene esters (equation 8) The use of other acids, such as hydrochloric, acetic, and trifluoroacetic acid, results in complete polymerization of the starting trienes [9] (equation 8)... 

Kelly applied this chemistry to the synthesis of cyclosexipyridine 66. This is an example of an intramolecular variation to this method. Masked enal 65 was prepared and treated with the standard reagents. The acidic medium liberated the aldehyde from its acetal protection. This in situ formation of the reactive species, similar to the above example, then undergoes cyclization to the expected pyridine derivative 66. 

Ullmann condensation of the sodium salt of p-chlorothiophe-nol (31) with 2-iodobenzoic (32) acid gives 33. Cyclization by means of sulfuric acid affords the thioxanthone, 34. Reaction with the Grignard reagent from 3-dimethylaminopropyl chloride affords the tertiary carbinol (35). Dehydration by means of acetic anhydride affords chlorprothixene as a mixture of geometric isomers, 36. (Subsequent work showed the Z isomer-chlorine and amine on the same side—to be the more potent compound.) Chlorprothixene is said to cause less sedation than the phenothiazines. ... 

Manganese(III)-promoted radical cyclization of arylthioformanilides and a-benzoylthio-formanilides is a recently described microwave-assisted example for the synthesis of 2-arylbenzothiazoles and 2-benzoylbenzothiazoles. In this study, manganese triacetate is introduced as a new reagent to replace potassium ferricyanide or bromide. The 2-substituted benzothiazoles are generated in 6 min at 110°C imder microwave irradiation (300 W) in a domestic oven with no real control of the temperature (reflux of acetic acid) (Scheme 15). Conventional heating (oil bath) of the reaction at 110 °C for 6 h gave similar yields [16]. 

To be of maximum synthetic value, the generation of the cationic site that initiates cyclization must involve mild reaction conditions. Formic acid and stannic chloride are effective reagents for cyclization of polyunsaturated allylic alcohols. Acetals generate oxonium ions in acidic solution and can also be used to initiate the cyclization of... 

---