Pyridines electrocyclic ring opening

As a continuation of earlier studies on electrocyclic ring opening of the pyridine moiety of 3-aryltetrazolo[l,5-tfjpyridinium salts 34 <1996CHEC-II(8)409> further extension of this type of transformation was published by a... 

In the above cases, the photoreactivity of indole, coumarin, pyridine, and so forth, does not compete effectively with PFR, which becomes the major reaction. However, in the case of the chromene depicted in Scheme 28, the electrocyclic ring opening prevails over PFR. By contrast, the analogous chromane undergoes clearly acyl migration [86]. 

The concept of electrocyclic ring opening of a 1,2-dihydro six-membered heterocycles is familiar from the numerous examples found after nucleophilic attack, especially on cationic rings. Similar reactions occur with isolated 1,2-dihydro derivatives. Dihydropyridines can undergo isomerization by electrocyclic ring opening (see Equation 10). 1 -Vinyl- 1,2-dihydropyridines in a somewhat similar sequence yield pyridines via azacycloocatrienes (Equation 11). 

On refluxing in benzene with excess pyridine, the monocycloadduct of diphenylnitrile imine and pyridine smoothly underwent a [l,5]-sigmatropic shift and subsequent electrocyclic ring opening to afford 1,2,4-triazole derivative (44) (Scheme 33) <95H(40)515>. 

Irradiation of crude 1,2-dihydropyridine 365, obtained by the reaction of pyridine and a Grignard reagent prepared from chloromethyldimethylphenylsilane, at 300 nm in acetone gave the substituted 2-azabicyclo[2.2.0]hex-5-ene 366 in 30% overall yield (Equation 46). The low yield in the photocyclization step could be due to the competitive aromatization and/or electrocyclic ring opening of the 1,2-dihydropyridine <2005JOC590>. 

Practically all the reactions of quinolizinium ions are similar to those of pyridinium salts, thus they are resistant to electrophilic attack, but readily undergo nucleophilic addition, the initial adducts undergoing spontaneous electrocyclic ring opening to afford, finally, 2-substituted pyridines however the susceptibility of the cations to nucleophiles is not extreme - Uke simpler pyridinium salts they are stable to boiling water. 

The recently developed dealkylation of phenol ethers with trimethylsilyl iodide has been shown to work well for OO -dialkyl-uracils. A number of ring transformations of uracils have been reported. The [2 + 2]cyclo-adducts (232) can be transformed into pyridones (233) by treatment with base overall yields of pyridines from the starting uracils (231) can be quite reasonable. The reaction is thought to proceed via electrocyclic ring-opening of the dianion of (232) (Scheme 92). 

Primary products of this photoreaction are bicyclic diaziridines 13, which yield the diazepines 14 in a thermal electrocyclic ring-opening of the three-membered ring. The diaziridines 13 can be thermally reconverted into the pyridine N-ylides 12. 

A remarkably simple two-stage synthesis of 2-aryl-pyridines from cyclo-pentadienes has been reported. The 4a,7a-dihydrocyclopenta[ ][l,2]oxazines (10), available from cyclopentadiene and a-halogeno-ketoximes, at 200 °C undergo successive electrocyclic ring-opening and ring-closure to form the acyl-dihydropyridines (11), which aromatize by loss of acetaldehyde as illustrated in Scheme 2. 

The Zincke reaction is an overall amine exchange process that converts N- 2,A-dinitrophenyl)pyridinium salts (e.g, 1), known as Zincke salts, to iV-aryl or iV-alkyl pyridiniums 2 upon treatment with the appropriate aniline or alkyl amine. The Zincke salts are produced by reaction of pyridine or its derivatives with 2,4-dinitrochlorobenzene. This venerable reaction, first reported in 1904 and independently explored by Konig, proceeds via nucleophilic addition, ring opening, amine exchange, and electrocyclic reclosure, a sequence that also requires a series of proton transfers. By... 

The reaction of 2-(phenylamino)- and 2-(dimethylamino)-thiazoles (38) with DMAD produced dimethyl 6-(phenylamino)- and 6-(dimethylamino)-3,4-pyridinedicarboxyl-ates exclusively. The intermediate 2 + 2-cycloadduct (39) undergoes ring opening, 6jr-electrocyclization with the extrusion of sulfur to produce the final pyridine derivatives (40) (Scheme ll).38... 

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