Pyridinium carbinols

The well-known triphenyltetrazolium chloride (TTC) reaction for the detection of a-ketolsteroids, pyridinium carbinols and pyridinium glycols can also be included here [20-23]. The chromophore system of the red-colored formazan dye produced by reduction of the TTC is composed of highly conjugated double bonds resulting from the combination of a phenylhydrazone group with an azo group ... 

Purpuric acid lb 174 Pyrazolidine derivatives la 426 3,5-Pyrazolidindione derivatives lb 20 Pyrazolinone derivatives lb 277 Pyrazolin-5-one derivatives lb 327,329 Pyrazone lb 332 Pyrene lb 379 PyrethrinI lb 18 Pyrethrin II lb 18 Pyrethroids lb 86,87 Pyrethroid insecticides la 359 Pyridine alkaloids la 66 lb 279 Pyridine derivatives lb 119,244 Pyridinium carbinols lb 65 Pyridinium glycols lb 65 Pyridoxal la 157,158,253 Pyridoxamine la 253 Pyridoxine la 253... 

The most intensively studied oxidizing system is that developed by Pfitzner and Moflatt in which the oxidation is carried out at room temperature in the presence of dicyclohexylcarbodiimide (DCC) and a weak acid such as pyridinium trifluoroacetate or phosphoric acid. The DCC activates the DMSO which in turn reacts with the carbinol to give an oxysulfonium intermediate. This breaks down under mild base catalysis to give the desired ketone and dimethyl sulfide. 

Prior to our original report7 on this method, acceptable and general preparative routes to a-iodocycloalkenones had not been described. Treatment of a p-substituted cycloalkenone with trimethylsilyl azide and a mixture of iodine and pyridine sequentially in dichloromethane has now been reported as a method for the preparation of p-substituted-a-iodocycloalkenones.8 The combination of iodine and pyridinium dichromate has also been reported to provide a-iodoenones from enones as well as from ethynyl carbinols.9 10 Some successes have also been achieved with enones and iodine azide (IN3)11 and iodine/ceric ammonium nitrate.12-14 The submitters first variant5 of the present procedure used carbon tetrachloride as a solvent. In this procedure this solvent has been replaced with the more benign diethyl ether. 

Diazinium salts resemble pyridinium salts in their behavior. They form pseudo-bases with hydroxide ions which can disproportionate (e.g. 2-methylphthalazinium ion (199) — 2-methylphthalaz-l-one + 2-methyl-l,2-dihydrophthalazine) or undergo ring fission (e.g. 3-methylquinazolinium ion — (200). Aqueous acid converts (201) into (202), presumably by attack of a water molecule on a protonated species with subsequent intramolecular oxidative-reductive rearrangement of an intermediate carbinol base (201a) as shown. 

A simpler large scale method to obtain pyrantel, morantel (10a,b) and oxantel (11) involves condensation of 23 with an aryl aldehyde in presence of a base. Water is removed by azeotropic distillation or by using a chemical scavenger like methyl/ethyl formate, which reacts with water to push the reaction in the forward direction (Scheme 3). Other methods to prepare pyrantel and its derivatives are also reported [5,6,13-17]. The l-(2-arylvinyl)pyridium salts (18,19), structural congeners of pyrantel/moratel, are prepared by quaternisation of pyridine with the appropriate bromomethylaryl ketones (27) to afford 28. The latter is reduced with sodium boro-hydride to give the carbinol 29, which on dehydration leads to the formation of l-(2-arylvinyl)pyridinium bromides (18,19) [8]. 

Tamura and colleagues generated an IQD (16) from 3-carboxy-l-methylindole-2-acetic anhydride by treatment with sodium hydride. Subsequent trapping with acetylenic dienophiles and a chloroquinone afforded the expected cycloadducts (Scheme 9, equation 1, and 17, 18) [65]. Ila, Junjappa, and coworkers also employed basic conditions to generate and trap a bright red IQD (19) from 1,2-dime-thyl-3-carboxyaldehyde (equation 2) [66]. A large number of carbazoles (20-22) and 1,2-dihydrocarbazoles (23-25) were synthesized in very good yields. Either silica gel or pyridinium tosylate was required to dehydrate/aromatize the initially formed carbinol intermediate. 

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