Facile ring

In aqueous alkaline conditions with chloroacetic acid the pyrido[4,3- f]pyrimidinethione (80) undergoes facile ring opening, attributed to the resonance stabilization of a delocalized covalent hydrate dianion intermediate (81) (82). Pyrido[2,3- f]pyrimidine-4-thiones (and... 

Aziridines represented by the general structure (458 X = 0, S, NR) undergo a facile ring opening and subsequent closure on heating with sodium iodide in acetone or acetonitrile. For (458 X = O) the oxazoline (460) was formed, presumably via the intermediate (459) (66JOC59). 

The chemistry of benzazetidin-2-ones (251) can also be explained in terms of facile ring opening to the iminoketenes (252) which dimerize, rearrange or can be intercepted by nucleophiles or in cycloadditions depending on the conditions. Indeed, this ring opening precludes their isolation in all but exceptional cases (Section 5.09.4.3.5) (76AHC(19)215). 

A characteristic peculiarity of isoxazole derivatives is the relatively facile ring cleavage under suitable conditions, and this is a severe limitation on reactions of substitution in the isoxazole series. The best, though as yet inadequately, studied reactions, are the... 

In general, 1//-azepines are fragile ring systems which undergo facile ring contraction to arene derivatives under acidic, thermal, and photolytic conditions. 

A facile ring closure at the N-l nitrogen of the 1,2,4-triazine ring occurs to afford triazolo[3,4-/][l, 2,4]triazinones 962 when 961 reacted with acids,... 

Functionalized silacyclobutanes 16 result from photochemical decomposition of [azido-, isocya-nato- and isothiocyanato-bis(tert-butyl)silyl]diazoacetates 15. They undergo a remarkably facile ring-expansion reaction to cyclic O-silyl ketene acetals 17 even at 60°C. 

CsF induces the ring-opening cyclization of the thus obtained l-[2-(trimethylsilylmethyl)cyclopropylcarbonyl] imidazole 154 with diethyl fumar-ate or diethyl maleate to give the cyclobutanone 155 [80], The facile ring fission of 154 also occurs in the presence of BF3OEt with the formation of the corresponding y,<5-unsaturated carboxylic acid. (Scheme 55)... 

The lack of stereospecificity in equation (72) (as compared to equations (70) and (71)) is readily accommodated by the long lifetime of the triplet ion pair produced by the sensitized irradiation method. The latter allows the facile ring opening of DBC+ cation radical followed by isomerization of the resulting xylylene cation radical, i.e.,... 

Machiguchi, Nozoe and coworkers have very recently observed that in contrast to chemical reactivity of tropones, the tosylate of tropone oxime undergoes a facile ring-opening to 6-substituted (Z,Z,Z)-1,3,5-hexatriene nitriles on reaction with various nucleophiles305. For example, reaction of phenyl lithium results in the corresponding hexatriene carbonitrile (equation 183). 

Attempts to detect the uncharged intermediates 6-amino-4-phenyl-l,6-dihydro-4-phenylpyrimidine (66), 2-amino-4-phenyl-l, 2-dihydropyrimidine (68), and the aminodiazahexatriene 67, by recording the NMR spectra of the solution during the stepwise addition of the ammonium chloride were not successful. The facile ring opening of the pyrimidine ring in 66 is not unexpected. It has also been observed in the reaction of A-methylpyrimi-dinium salts with liquid ammonia, which also involves as neutral covalent... 

Further syntheses of tetrahydro-l,3-oxazines are individual processes based on the facile ring opening of isoxazolidines and recyclization to oxa-zines. 

Epoxysteroids are important synthetic intermediates since their facile ring opening allows the introduction of various functionalities in a stereospecific manner [85, 86]. In fact, these compounds are often used in synthetic cascades towards several compounds with interesting biological and/or pharmacological activities. 

A second reason of interest in cyclophosphazenes has stemmed from the facile ring opening polymerization of the hexachlorocyclotriphosphazene, N3P3CI6, to the linear macromolecule [NPCl2]x [4, 5] (Eq. 1). 

Much effort has been invested to use oxazolones as intermediates for the synthesis of polymers. Probably the most important group of oxazolones used for this purpose is the 2-alkenyl-4,4-disubstituted-5(4//)-oxazolones. The reactivity of the exocyclic double bond at C-2 can be exploited to prepare new functionalized oxazolones as well as for copolymerization with other monomers. This possibility, together with the facile ring opening with nucleophiles, has opened very interesting routes to new polymers of industrial interest. 

Such disjoint molecules may serve as synthetic intermediates. Thus a [3 + 2] annulation method exploits the facile ring opening of cyclopropanols and the transfer of the donor site to the p carbon of the resulting ketones, the nascent carbanions may... 

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