Cyclohexadiene ring opening, thermal

On the positive side, there are photochemical reactions that are essential for human health. One of these is the formation of vitamin D (the antirachitic vitamin) by irradiation of ergosterol. This photochemical reaction is an electro-cyclic ring opening of the cyclohexadiene ring of ergosterol of the type described in Section 28-2D. The product, previtamin D2, subsequently rearranges thermally to vitamin D2 ... 

In the formation of tetraenes from bicyclo[4.2.0]octa-2,4-dienes, two bonds are broken. This may occur in one concerted reaction which can be regarded as a retro [2 + 2] cycloaddition. It is also possible that the central bond, being part of a cyclohexadiene system, is the first one to break in a thermal, concerted disrota-tory process that leads to a 1,3,5-cyclooctatriene derivative. Ring opening of the cyclooctatriene then might take place photochemically, again disrotatory, to produce a tetraene. This two-step sequence was first observed by Mirbach et al. [114] in their study of the photocycloaddition of the two parent molecules benzene and ethene. The same explanation for the formation of a tetraene was given by Nuss et al. [160] in their report on the intramolecular ortho photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile (see Scheme 40). 

The electrocyclic reactions of phenyl fulgides and l,8a-DHNs can be photo-induced by a conrotatory process. The electrocyclic ring-opening reaction of cyclohexadiene systems (l,8a-DHNs) can also be induced thermally via a disrota-tory process. 

The thermal or chemically induced ring-opening of gem-dihalocyclopropanes, readily available from the addition of dihalocarbene to olefins, has been widely studied ". The reaction is dependent not only on the kind of halogen but also on the molecular geometry of the substrate. For instance, when en6 o-6-chloro-exo-6-fluorobicyclo[3.1.0]hexane (84) is treated with quinoline at 110°C and 140°C, 3-chloro-2-fluorocyclohexene (85) and 2-fluoro-l,3-cyclohexadiene (86) are obtained, respectively (equation 54) . ... 

The hexatriene/cyclohexadiene isomerization has been extensively studied and has been the topic of numerous reviews and monographs this section will attempt to deal only with applications of these reactions to synthesis, and in particular the use of these reactions for the synthesis of natural products. Much of the early work in this area was done by Havinga and coworkers during the course of their detailed work on the stereochemical consequences of the thermal and photochemical conversions in the vitamin D field this work provided much of the impetus for the development and elaboration of the Woodward-Hoffman mles (Scheme 7). The reversible photochemical ring opening of provitamin 30 to precalciferol (31) and the photochemical ring closure of 31 to lumisterol 32 can be explained by consecutive photochemically allowed conrotatory processes. ... 

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