Arylidenes. rearrangement

Boulton-Katritzky rearrangement, 5, 258 Pyrazol-5-one, 4-arylazo-reactions, 5, 262 Pyrazol-5-one, 4-arylidene-configuration, 5, 208... 

Hydrazine-induced rearrangements have been observed with imidazoles, which contain an A -arylcarboxamide function as part of the ring system. 3-Aryl-5-arylidene-2-methylthioimidazolin-4-one (79) reacts with hydrazine to produce 5-arylidene-3-amino-2-arylimino-imidazolidin-4-one... 

With sodium methoxide, arylidene derivatives 209 undergo a rearrangement into photochromic dihetaryl-substituted cyclopentenediones 211. They are easily alkylated to form geminally substituted products 212 in 91-94% yields (07MC301). Their photochemical properties were investigated (07OS975, 08IZV853). 

Due to the rapid rearrangement of the [4,3-a] series into the [1,5-a] series (see Section 4.15.4.5.2) uncontaminated [l,2,4]triazolo[4,3-a]pyrimidines are difficult to prepare. The only useful methods available (Scheme 45) seem to be treatment of an appropriate 2-hydrazinopyrimidine with a low boiling ortho ester (for 3-H, 3-Me, 3-Et and 3-Pr) or oxidative ring closure of an arylidene derivative of 2-hydrazinopyrimidine to form the 3-aryl compounds (77AJC2515). 

Aminonaphtho[l,2-b]pyrans are converted into 5,6-dihydrobenzo[h]quinolines under basic conditions through a Dimroth rearrangement. The oxygen heterocycle is formed by a Michael addition of malononitrile to a 2-arylidene-l-tetralone and the whole sequence provides an... 

Pyrazol-3-one 392 reacts with arylidene malononitriles 393a-c in basic ethanolic medium to yield the 6-(3-oxopyrazol-4-yl)-2-oxo-l,2,3,4-tetrahydropyridine/ 2-hydroxy-6-(3-oxopyrazol-4-yl)-3,4-dihydropyridine adducts 397 398a-c in a 1 1 ratio (87AP140) (Scheme 109). The mechanism is assumed to proceed via intermediate 396 formed from Michael adduct 394 or possible isomer 395. The pyran derivative 396 rearranges to the pyrid-2(l//)-one/2-hydroxypyridine tautomeric mixture 397/398. 

The essential step of this method is the electrocyclization of the azahexatriene system in 5-[(arylmethylene)amino]-6-[(./V,./V-dimethylamino)vinyl]-l, 3-dimethyluracils 5, which rearrange in situ following their formation from 5-(arylidene imino)-1,3,6-trimcthyluracils 4 and dimethylformamide dimethyl acetal. Aromatization then occurs by elimination of dimethyl-amine. The intermediate 6-[(W,Af-dimethylamino)vinyl] compounds 5 may be isolated in low yield.440 441... 

In contrast to product formation by thermal extrusion of molecular nitrogen from the 4-phosphapyrazoIines (446), photoproduct formation results in deep-seated skeletal rearrangements. Irradiation of the 5-alkylidene derivatives yields the azomethineimines (447), which are spectroscopically detectable and thermally stable in solution. Continued irradiation converts (447) into (449) which opens to give (451). In the case of the 5-arylidene derivative, the ring-contracted product (4 ) is converted to primary photoproduct (450) which aromatises on standing to give (452). ... 

A novel fragmentation of iV-arylidene- or Al-(aLkylideneamino)- 8-lactams can be induced by ozone to lead to various enol ethers after a reductive workup with sodium borohydride. The starting 8-lactams can be prepared via [2 + 2] cycloaddition of alkoxy ketenes and an azine and upon treatment with ozone at low temperature, yield the expected secondary ozonides (eq 59). Reduction of the ozonide leads to the corresponding N-nitroso intermediate, which is susceptible to fragmentation of the C4—N1 bond to give a zwitterion intermediate that rearranges to yield the product enol ethers. In the reaction sequence, fra 5- 8-lactams yield predominantly the E-enol ether while the d5- 8-lactams preferentially form the Z-configured enol ethers. 

The action of hydrazine on isorhodanine (364 R = H) or its 5-ethyl homologue (364 R = Et) proceeds in three stages (Scheme 22) The hydrazine addition compound (365) is isolable at 0 but is converted at ambient temperatures, with loss of hydrogen sulphide, into the 4-hydrazonothiazolidin-2-one (366) at higher temperatures, 5-mercapto-methyl(or mercaptopropyl)-l,2,4-triazolin-3-ones (367) are formed as the final products by rearrangement. The action of aromatic aldehydes on (366) yields the expected 4-iV-arylidenehydrazono- (368) and 4-arylidene-hydrazono-5-arylidene analogues (369) successively. Dimerization of (366) to (370) is effected by acids. ... 

---