Uracils ring contraction

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). 

Although hydrazinolysis of uracil derivatives into pyrazoles is well known (see Scheme 133), reaction of 5-formyluracils with hydrazines in the presence of acetic acid causes another type of ring contraction... 

Ozonolysis of cellular substances, such as uracil and thymine derivatives, is of special interest. Ozonolysis of aqueous solutions of uracils leads to ring cleavage and subsequent ring contraction forming 5-hydroxy-hydantoins (89CL723 90JOC1396) (Scheme 158). 

Moss et al. succeeded in producing imidazolidine 517 by a one-pot procedure involving alkylation of N,A -dimethylethylenediamine with 1-bromo-l-phenyldiazirine, carbene generation, and subsequent cyclization (84TL1023). Rh2(OAc)4-catalyzed composition of diazo uracils 518 affords good yields of imidazolidinones 519 via carbenoid ring contraction... 

An efficient two-step procedure for the introduction of an acetic acid substituent into the 4-posltion of 2-methyl-l(2H)-isoquinollnone Involves reaction with ethyl diazoacetate in the presence of a copper catalyst to give a mixture of exo and endo adducts, which are then isomerlzed t g4-carboethoxymethyl-N-methyl-2(lH)-isoqulnollnog0 with ethanolic HCl. Uracil 5-acetlc acids could be prepared analogously. The cyclopropane adduct from 2-methyl-l(2H)-lsoquinolinone and dichlorocarbene can be ring-contracted to an isolndole, or ring-expanded to a benzazeplnone P The reaction of carbenes with other heterocyclic enamides has been reviewed. 

Photochemically induced ring contraction of novel 4-iminodi-hydro-1,2,3-triazoles to the corresponding ( )- and (Z)-aziridin-imines has also been reported. A different pathway is followed in the 6-(l -triazolyl)uracils (24) which on irradiation in acetonitrile were converted into the pyrrolo[2,3-d]pyrimidines (25) via triplet biradical intermediates a novel cyclisation leading to the formation of pyrimido[4,5-c]isoquinolines is observed, however, when C-5 phenyl substituents are present on the triazole ring. Rate constants for the cycloaddition f diarylnitrilimines, generated by photoelimination of nitrogen from diaryl 2H-tetra-zoles, to various dipolarophiles have been determined experiment-... 

Although compounds 60-64 were not designed for anti-HIV activity, their structures make them potential candidates. Deamination of l-(3-amino-2,3-dideoxy-P-D- r femo-hexopyranosil)uracil produced ring contraction to afford... 

In a previous section (p. 194), the thermal electrocyclization of 5-aryIazo-6-ethoxymethyieneamino-uracils to 8-arylamino-theophyllines was noted. Related are the 6-dimethylaminomethyleneamino-derivatives (353), which on irradiation under anaerobic conditions also cyclize to 8-arylamino-theophyllines (242). However, in the presence of oxygen, photocyclization of (353) affords 6-aryl-6,7-dihydro-6-azalumazin-7-ones (355). A mechanistic rationale has been proposed (Scheme 81) involving the initial formation of the 5,6-dihydroazalumazine (354) followed either by formation of hydroperoxide (path a) or ring contraction (path b), depending on the reaction conditions. Thermolysis of (353) produces a mixture of the 5,6-dihydro-azalumazine (354) and theophylline (242). 

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