Pyridines nitrile ylides

The mechanism of the thermal conversion of 4-cyclopropenyl-4-substituted-5(4f/)-oxazolones to pyridines has been studied. A stepwise cycloaddition of the initially generated nitrile ylide has been proposed to account for the observed products. 

Azirinylcyclopropenes, e.g. 213, are converted quantitatively into the corresponding pyridine derivatives, e.g. 215, on pyrolysis (or photolysis) via nitrile ylides, e.g. 214. The independently generated isomer 216 (equation 72) gives the same pyridine and can be diverted to adduct 217 in the presence of dipolarophile. Insights into the detailed sequence of events involved in the conversion of ylids such as 214 and 216 into pyridine have been... 

Thermolysis or photolysis of 2-cycloprop-2-enyl-2//-azirines, e.g. 1, gives pyridines in essentially quantitative yield in reactions that may be explained in terms of formation of a nitrile ylide, followed by intramolecular dipolar cycloaddition to produce an azabenzvalene, and isomerization of this to the product. ... 

The ylides were also generated by thermolysis of oxazol-5(4//)-ones 2 by 1,3-dipolar reversion and loss of carbon dioxide. In the case of 4, three pyridines were formed,apparently by stepwise addition of an intermediate nitrile ylide to produce a bicyclobutyl zwitterion which can cyclize to an azabenzvalene or via a cyclobutenyl cation which can rearrange to two different aza-Dewar benzenes. ... 

Quaternized pyridines which have an R R CH group on the nitrogen are useful compounds because of the ease with which they react with bases to give reactive ylides the latter add on to some alkynes such as DMAD under mild conditions. The quaternized pyridines form ylides more readily when R is an electron-attracting group such as acyl, 4-nitrophenyl, tosyl, nitrile or carboxylic ester. When both R and R are electrophilic, as in (NQ2CH, ylide formation is very facile [2280, 2284, 2419, 2422, 2970]. The chemistry of iV-substituted pyridines [1870] and the use of azomethine ylides [2509] have been reviewed. 

In earlier reports Sharp and co-workers have described the cyclisation of diene-conjugated nitrile ylides of the general type (3) (Scheme 1), where both A and B are benzene rings, to form benz[c,e]azepines <93JCS(p1)2961>. This work has now been extended to systems where A or B is a heteroaromatic ring, in particular furan, thiophene and pyridine <94JCS(pi)1193>. 

Tire self-chemical ionization reaction of CS2 under chemical ionization conditions (approx. 1 Torr) generated 83, which sulfurized pyridine (97MI1) and nitriles (97JPC6970) to give the corresponding cation radicals 61 and 62, respectively. Ab initio calculations on 83 at the G2 (MP2, 8VP) level revealed that the ylide radical cation form 63 is more stable than the dithiiranethione radical cation form (64) by 42 kJ/mol (97JPC6970). 

Spectroscopically invisible carbenes can be monitored by the ylide method .92 Here, the carbene reacts with a nucleophile Y to form a strongly absorbing and long-lived ylide, competitively with all other routes of decay. Although pyridine (Py) stands out as the most popular probe, nitriles and thiones have also been used. In the presence of an additional quencher, the observed pseudo-first-order rate constant for ylide formation is given by Eq. 2.92,93 A plot of obs vs. [Q] at constant [Y ] will provide kq. With Q = HX, complications can arise from protonation of Y and/or the derived ylides. The available data indicate that alcohols are compatible with the pyridine-ylide probe technique. 

Cyanides can react with 1-aminopyridinium salts to give 2-substituted triazolopyridines, possibly via the pyridinium ylide. With 1-aminopyridinium iodide and cyanide ion the intermediate 4-cyanopyridine reacts with the aminopyridinium salt to give 2-(4-pyridyl)triazolopyridine (45).51 When acetonitrile or benzonitrile are used, 2-methyl- and 2-phenyltriazolo-pyridines are obtained.58 60 The reaction is thought to involve a dipolar cycloaddition of the N- mi nopyridine with the nitrile, as shown in Eq. (4). 

NITRILES l-(N,N-Diethylamino)-propyne. N- lmino-N,N-dimethyl-2-hydroxyptopanaminium ylide. Phos-phorus(lll) chloride. Potassium cyanide. Tosylmethyl isocyanide. Trl-fluoroacetic anhydride-Pyridine. 

Intramolecular cyclisation of nitrile imine ylides occurs when 2-pyridyl- (22 X= CH) or 3-pyridazinylhydrazones (22 X= N) are treated with chloramine-T to yield 1,2,4-triazolo[4,3-a] pyridines (23 X= CH) and l,2,4-triazolo[4,3-l>]pyridazines (23 X= N) in a simple one-pot synthesis [93SC319S]. 

Pyridines have also been used in cyclization reactions. Two noteworthy examples are shown in scheme 3. The reaction of substituted pyridine 32 with a nitrile affords imidazo[l -ajpyridine 33 in excellent yield <01JOC2862>. Oku and co-workers have reported the use of tetrahydroquinolizinium ylides in a 13-dipolar cycloaddition reaction (34- 35) <01JOC1638>. Sieburth has also published an account of the [4+4] photocyclization reaction of pyridones on route to fusicoccin <01S1185>. 

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