Ring cleavage electrocyclic

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... 

Relatively few examples of photochemically induced 6n -electrocyclizations of azahexa-l,3,5-trienes to azacyclohexa-1,3-dienes have been documented the reverse process involving ring cleavage, however, is more common. The formation of azalumazin-7-ones 19 from triazahexatrienes 20 under oxidative conditions can be regarded as an example of such a process,18 but... 

Photochemically induced electrocyclic reactions in 2/f-pyrans have been discussed in preceding sections. Ring cleavage has been reported on irradiation of cannabinol in ethanol to give the alkene (238)189 details of the... 

The small inverse dependence of the first-order rate constant on solvent polarity is in agreement with a concerted electrocyclic ring cleavage through an isopolar activated complex to vinylketene, which is converted into the corresponding ester in alcoholic solvents [158]. 

The reverse process, the electrocyclic ring cleavage of azacyclohexadienes, can also be effected photochemically. Evidence that ring opening of N-substituted 1,2-dihydroquinolines proceeds from the lowest triplet excited state has been published.The formation of homoadamantano[4,5-d]imidazoles (29) to (31) from the homoadamantano[4,5-b]-5, 6 -dihydropyrazines (32)... 

A theoretical study of the sequential addition of a chiral alkyl-substituted cyclopen-tenyl lithium and then an achiral cyclopentenyl lithium to a squarate ester has shown that the second carbonyl addition initiates a domino process whereby the dialkox-ide intermediates undergo electrocyclic four-membered ring cleavage to yield endo-and exo-octatetraene rotamers which are interconvertible these then form isomeric cyclooctatriene products by electrocyclization. ... 

Figure 3.10 (b) - (e) illustrates the topology of the interactions between cr-bond and rr-system for the electrocyclic ring opening of a cyclobutene (cf. Equation 3.3). We have the usual four possible modessupra-supra. supra-antara, antara supra, and antara-antara (cf. Fig. 3.2), and these modes are related to the conrotatory and disrotatory ways of ring cleavage. There are in fact four possible pairs of interactions in Fig, 3.10(c) and (d) the alternatives which, in so far as the orbital-orbital interactions are concerned are enantiomeric with those shown, and are therefore omitted. 

The photocycloaddition of aliphatic and aromatic aldehydes with 2,4,5-trimethyloxazole (131) gave bicyclic oxetanes 132 in almost quantitative yields hydrolitic cleavage led selectively to erytro a-amino-P-hydroxy methyl ketones 133 <00CC589>. The oxazolium salt 134 was converted to the azomethine ylide 136 via electrocyclic ring opening of the oxazoline 135. Intramolecular cycloaddition afforded 137 in 66% overall yield which was transformed into the aziridinomitosene derivative 138 . 

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