Ring cleavage electrocyclic thermal

D-Ribonic acid, esters D-psicosides from, 272 Ribonucleic acid. See RNA Ring cleavage electrocyclic (cycloreversion) [l+l-i- 2]- double CO elimination, 78, 339 [1 + 4]- CO elimination, 337 S02 elimination, 80,153 [2 + 2]- benzene elimination, 331, 332-333 prismane opening, 330-331 rhodium( 1 + )-catalysed, 332 thermal, 79-80... 

Heteroaromatic N-imines and IV-aminoazonium salts show a variety of reactivities, depending on the nature of the heteroaromatic ring and the substituents on the imino or amino nitrogen. The most important types of the reactions are (i) reactions with electrophiles at the imino or amino nitrogen, (ii) reactions with nucleophiles on the heteroaromatic ring, (iii) 1,3-dipolar cycloaddition, (in) 671-electrocyclic reaction of 1,5-dipoles (mainly thermal reaction), (n) 47t-electrocyclic reaction (mainly photochemical reaction), and (vi) N—N bond cleavage (by thermolysis, photolysis, oxidation, and reduction). 

Attempts to form a penicillin sulfilimine by reaction of the penicillin (378) with chloramine T resulted in a rearranged product (379) (Campbell and Johnson, 1974). This product presumably arose from the initially formed sulfilimine (380) which, on C-5—S bond cleavage and subsequent ring closure, resulted in the cyclic sulfilimine (381). Reaction with excess chloramine afforded the observed product. Thermal treatment of 379 resulted in an electrocyclic rearrangement to the azetidinone (382) (Campbell and Johnson, 1975). 

A computational study of the mechanism of cyclohexyne insertion into a C(0)-Ca bond of cyclic ketones has been reported to proceed through a stepwise 2 + 2-cycloaddition of cyclohexyne to the enolate, followed by thermally allowed conrotatory electrocyclic ring opening, thermally forbidden disrotatory electrocyclic ring opening, or nonpericyclic C-C bond cleavage (Scheme 34)... 

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