Photochemical electrocyclic ring-opening

The cyclohexadiene-hexatriene system seems to be less complicated than the cyclobutene-butadiene system. Cyclohexadiene undergoes photochemical electrocyclic ring opening ... 

The other commonly quoted industrial photochemical process is the production of vitamin D3 involving a photochemical electrocyclic ring opening followed by a thermal 1,7-hydride shift (Scheme 7.2). This is a further example of a successful low quantum yield process in this case there is no viable thermal alternative. Vitamin A acetate has also been produced commercially using a photochemical isomerization process to convert a mixed tetra-alkene precursor to the all-trans form. 

Photochemistry is used commercially in the synthesis of Vitamin A and Vitamin D3, Figure 3.14. Vitamin A is formed via a photochemical isomerisation process and the synthesis of Vitamin D3 involves a photochemical electrocyclic ring-opening step. The industrial relevance of this process is due to the lack of a viable thermal alternative and therefore, if unusual and useful chemistry is only possible via photochemistry, an industrial process may result, due to the high intrinsic value of the chemical product. 

We can now go back to the reaction that introduced this section—the photochemical electrocyclic ring opening of ergosterol to give provitamin D2. By looking at the starting material and product we can deduce whether the reaction is conrotatory or disrotatory. 

The cyclohexadiene-hexatriene photochemical interconversion is predicted by orbital symmetry considerations to involve conrotatory motion. Cyclohexadiene derivatives undergo photochemical electrocyclic ring opening. The photostationary state... 

Mention has already been made in Section III of the photochemical formation of sulfene from thietanone 1,1-dioxide19, presumably by a [2 + 2] cycloreversion. Lan-gendries and DeSchryver have also observed photochemical electrocyclic ring opening of thiete 1,1-dioxides, e.g. 42, forming either the products of desulfinylation or sulfene trapping122, as in equation 31. 

Show that the photochemical electrocyclic ring opening of 1,3-cyclohexadiene to cis-l,3,5-hexatriene should occur by conrotatory motion. 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

A similar result is obtained for the thermal electrocyclic ring-opening of 3,4-dimethylcyclobutene. The trans isomer yields only (2 ,4 )-2,4-hexadiene when heated, and the cis isomer yields only (2 ,4Z)-2,4-hexadiene. On UV irradiation, however, the results are opposite. Cyclization of the 2E,4 isomer under photochemical conditions yields cis product (Figure 30.4). 

Pyrans and napthopyrans (chromenes) are photochromic compounds that undergo photochemically induced electrocyclic ring opening to give colored ortho-quinone methides.95-98 For example, chromene 153 opens on irradiation to give 154 (Eq. 1.41). 

Photolysis of the 1,2,3,5-oxathiadiazine (132) in the presence of ethanol leads to the quinazolone (133). The product is not formed in the absence of ethanol, and the proposed mechanism involves electrocyclic ring opening, addition of ethanol, photochemical diradical formation and closure on to the C-6 phenyl ring followed by aromatization as shown in Scheme 8 (79JOC4435). 

Figure 5.23 Chemical structures of 7-dehydrocholesterol and vitamin D. The photochemical reaction is an electrocyclic ring opening...

The formation of the oxepin is reasonably explained by an electrocyclic ring opening of rearranged epoxide 299 in a thermal reaction. As mentioned above, two routes to 299 are possible. If the rearrangement is concerted, a 1,5-sigmatropic reaction with inversion of the reaction center (oxygen) in 299 is photochemically allowed. It is possible to separate a nonconcerted process... 

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