Ring contraction oxidative

A variety of mono- and bi-cyclic /3-lactams have also been prepared by oxidative ring contraction of pyrrolidine-2,3-diones (180) (75JOC2356), and by the photolysis of pyrazolidin-3-ones (181) (78T1731, 75JOC3510, 75JOC3502, 75CC725). 

Benzoselenolo[2,3-ii]benzoselenophenes synthesis, 4, 116, 1078 Benzoselenolopyrylium cations oxidative ring contraction, 4, 969 [l]Benzoselenolo[3,4-ii][l,2,3]selenadiazoles synthesis, 6, 354 Benzo[h]selenophene, 2-acetyl-synthesis, 4, 966 Benzo[h]selenophene, 2-bromo-acylation, 4, 948... 

Ring contraction ofglycals.1 The oxidative ring contraction of cyclic alkenes (4,492-493) can be applied to protected glycals. Thus oxidation of the D-glucal 1 with TTN in CH3CN provides the 2,5-anhydro-D-manose 2, whose structure was established by conversion to the 2,5-anhydro-D-mannitol derivative (3). 

TABLE 6.6. 2-AMINO-5-(CARBOXYALKYL)-4(5ff)-OXAZOLONES FROM OXIDATIVE RING CONTRACTION OE 2-AMINOPYRIMIDINONES ... 

Tee and Endo (76JHC149) reported a novel oxidative ring contraction of the 2-oxopyrimidinium bisulfates 176 to oxazolidindione 177 with excess hydrogen peroxide. The reaction took place only if one of the and R positions was not substituted, but failed with 4,6-dialkylated cations. A completely different reaction was observed when 176 (R = R = H) was oxidized in the presence of potassium iodide (78JHC493), leading to two uracils 178 (X = H and I) and small amounts of 177 (Scheme 30). Similar results were obtained with 179, which yields only 6-oxo derivatives 180 (X = H, I), but no isomeric oxo compounds 181 (Scheme 31). 

The 1,5-benzodiazepine 40 on irradiation in benzene under oxygen undergoes oxidative ring contraction to 2-benzoyl-3-methyl-quinoxaline.42 Similarly, photolysis of 7-chloro-2-methylamino-5-phenyl-3//-l,4-benzodiazepine-4-oxide (41) in benzene yields the N-benzoylquinoxaline 42. Related ring contractions of diazepines to reduced quinoxalines have also been observed.43... 

Treatment of imidazo[2,l-A [l,3,5]benzotriazepines 79 with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) in acetonitrile at ambient temperature resulted in oxidative ring contraction with formation of stable (dihydroimidazo-lybbenzimidazoles 80 (Scheme 15). The reaction was suggested to occur via the initial hydride abstraction from N(6) followed by triazepine ring scission and formation of nitrenium ion <2005FA127>. 

Selenaaromatic rings in selenazolium and selenopyrylium salts are transformed to other rings in reactions other then cycloaddition. Examples are conversion of 1,2,4-selenadiazolium salt (67) to 1,3,5-selenadiazine (68) (Scheme 17), oxidative ring contraction of selenopyrylium salt (81) to selenophene (82), or replacement of selenium in the 2-selenachromylium salts (83) by two nitrogen atoms resulting in formation of benzodiazepines (84) (Scheme 21) [116],... 

A -Isoxazolines exhibit considerable versatility in synthesis and one-pot oxidative ring contraction of stannyl oximes to A -isoxazolines with stereocontrol results from the strong directing effect of the tin group, followed by 1,3-dipolar cycloaddition (Scheme 26). In conjunction with other results, it was suggested that oximes with the stannyl group too far removed from the iminoxyl group (3-electton... 

Oxidative ring contraction. 2-Hydroxy-3,6-di-/-butyl-l,4-bcnzoquinone (1) on treatment with CuCl2-6H20 in hot acetic acid yields 2cyclo-pentene-l,3-dione (2). 

Irradiation of 2,4-disubstituted 1,5-benzodiazepines with a high-pressure mercury arc under an oxygen atmosphere yields 2-acylquinoxalines as oxidative ring-contraction products. A solvent effect is observed in these reactions. ... 

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