Ring-cleavage reactions

HaNNHC(0)NPhC(0)NHN=CHCH2Ar. Treatment of the substituted 5-aryl-idene-thiazolidin-2-ones (105 X = NH or S, Y = O) with hydrazines has been reported to give l,2,4-triazolin-3-one derivatives. With compounds (105 X = O, Y = S) and hydrazine, however, 2-amino-5-arylidene-AMhiazolin-4-ones are formed.  

With acetic anhydride or dicyclohexylcarbodi-imide, 5-hydroxy-thiazolidine-2-thiones give 2-keto-isothiocyanates R C(0)CR R NCS, which are useful starting materials for the synthesis of imidazolines, oxazolidines, and other heterocyclic systems.  

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Unlike the 2-aLkyl-2-imida2olines, this stmcture is stable and resistant to hydrolysis. After ring cleavage, reaction with sodium chloroacetate yields linear products ... 

Unsubstituted 3-alkyl- or 3-aryl-isoxazoles undergo ring cleavage reactions under more vigorous conditions. In these substrates the deprotonation of the H-5 proton is concurrent with fission of the N—O and C(3)—-C(4) bonds, giving a nitrile and an ethynolate anion. The latter is usually hydrolyzed on work-up to a carboxylic acid, but can be trapped at low temperature. As shown by Scheme 33, such reactions could provide useful syntheses of ketenes and /3-lactones (79LA219). 

Thermolysis of the t]-coordinated zirconium complex of thiophene Cp2Zr(SiMc3) (C4H3S) is a thienyl ring cleavage reaction (920M1646). 

On the other hand, pathway c is initiated by a hydroxylation at the C4 position of the substrate. The second step, a monooxygenation of the product 2 at C3, is followed by 2,4-dioxygenolytic cleavage of lH-3-hydroxy-4-oxoquinaldine. This rather unique ring cleavage reaction yields carbon monoxide and N-acetylanthranilate. The hydrolysis... 

Most enzymes that catalyze the cleavage of heterocyclic compounds are scantily understood and most of them have not even been identified. Different types of enzymes are involved in various heterocyclic ring cleavage reactions (see schemes in Fig. 33). 

Furoxans and benzofuroxans undergo thermal and photochemical ring cleavage, reactions with nucleophiles, Boulton-Katritzky rearrangement, reduction and deoxygenation, ring transformation, etc. (see also Section 5.05.6.2). 

Recent developments are ring-cleavage reactions of the heterocycles [R2M-SbR2]w with 4-(dimethylamino)pyridine leading to base-stabilized monomers, [L — R M-SbR, (R = Me, SiMe3 R = Me, Et, i-Bu M = A1, Ga).83,84 Reaction of [L-Al(Me2)-Sb(SiMe3)2] [L = 4-(dimethylamino)pyr-idine] with [Ni(CO)4] leads to the corresponding tricarbonyl nickel complex (Equation 4).85... 

Beckmann fragmentation is frequently applied to cyclic oximes resulting in a ring-cleavage reaction. Normally, a nitrile-alkene compound is obtained from the oxime and further transformation is usual. 

Analysis of excreta samples indicated that both psoralen and Isopsoralen are metabolized extensively by polyxenes caterpillars, primarily by the same furan ring cleavage reactions observed in our earlier studies with xanthotoxin (Figure 3). 

Ring-cleavage reactions of some l,3,4-dioxazol-2-one derivatives, having an a- or a )3-hydroxyl... 

Isoxazoline Ring Cleavage Reactions of the Aldol Type... 

Schnur and co-workers " summarized typical reactions that can be performed on functional groups of substituted 2,4-oxazolidinediones without ring opening. These reactions include reduction with iron-acetic acid, chlorosulfonation, nucleophilic displacements of aromatic fluorides, and acid hydrolysis with HCl/formic acid. Nonetheless, there are examples of useful ring cleavage reactions involving 2,4-oxazolidinediones. 

Thieno[3,4-3]thiophenes 500 were prepared from thiophen-3-yl-substituted [l,3]dithiol-2-ones 496 in good yields <2003JOG7115>. The photochemical irradiation of 496 leads to thieno[3,4-f]dithiines 497 via a unique ring cleavage reaction and subsequent reductive treatment of dithiines 497 proceeds to give 2,3-dihydrothieno[3,4-/ ]thiophenes 498 and 499. Heating of 499 in the presence of further iodomethane leads to the formation of the thieno[3,4-/ ]-thiophenes 500 (Scheme 63). 

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