Disrotatory ring opening/closur

The molecular mechanisms for the ring openings of various cyclopropanone systems in the gas phase have been studied at the PM3 semiempirical level and shown to be disrotatory processes, while an experimental study of the stereomutation of 1,1-difluoro-2-ethyl-3-methylcyclopropane has confirmed the predicted preference for disrotatory ring opening and ring closure for this system. 

On the other hand, it is conceivable that, if 7a undergoes disrotatory ring opening to 8a, energy remains in the disrotatory mode of methylene rotations for the short time that 8a takes to cross the TS for disrotatory closure to 7a. In this dynamical model, disrotatory ring opening to 8a, rather than resulting in preferential... 

The initial photochemical reaction is intramolecular ortho cycloaddition leading to bicyclo[4.2.0]octa-2,4-diene derivatives. These normally are not detected because they undergo rapid thermal disrotatory ring opening to all-cis cyclooctatrienes which have large extinction coefficients and absorb a second photon to undergo electrocyclic ring closure to the photostable bicyclo[4.2.0] octa-2,7-dienes. 

The stereochemical significance of ring opening or ring closure becomes marked by taking substituted reactants. The identification of the product will indicate whether the reaction has taken place in con or disrotatory manner. 

In the simple four-electron systems, a route for cis-trans isomerisation of a diene is made available by the photochemical reaction usually being a disrotatory ring closure and the thermal reaction being a conrotatory ring opening ... 

In every case, Ri and R2 are trans to each other in the major product. R2 is always cis to the cyclobutene ring in the final product. The initial cycloaddition to the benzene ring occurs syn ring opening proceeds disrotatory and yields a boat-shaped all-cis cyclooctatriene. The photochemical 4-rr ring closure is also disrotatory it occurs in such a manner that the five-membered ring is trans to the... 

Irradiation of the substituted cyclohexadiene resulted in the formation of the Dewar benzene skeleton by a disrotatory ring closure. Reaction with lead tetraacetate (a reaction that is not covered in this book) was used to remove the anhydride group and introduce the final double bond of Dewar benzene. Again, because of the forbidden nature of the conrotatory opening to benzene, Dewar benzene has an appreciable lifetime. At 25°C the half-life for its conversion to benzene is 2 days, and at 90°C its half-life is 30 min. 

For the 1,2-dimethylenecyclobutane rearrangement, Gajewski and Shih 53> have demonstrated preferred conrotatory ring opening and closing. For the allene-allene cycloaddition, then, the sequence would be disrotatory motion as two allenes approach to form the perpendicular biallylene intermediate, followed by conrotatory closure of that species. 

It turns out that there is an alternating relationship between the number of electron pairs (double bonds) undei going bond leorganization and the stereochemistry of ring opening or closure. Polyenes with an even number of electron pairs undergo thermal electrocyclic reactions in a conrotatory sense, whereas polyenes with an odd number of electron pairs undergo the same reactions In a disrotatory sense. 

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