Dienes electrocydic reactions

Electrocydization is another major mode of reaction utilizing the vinylidene complexes of Group 6 metals. It should be noted that the first example of this type of reaction was reported by Merlic et al. using a ruthenium complex employing nonaromatic diene-yne substrates [26]. 

These reactions, called electrocydic rearrangements,365 take place by pericyclic mechanisms. The evidence comes from stereochemical studies, which show a remarkable stereo-specificity whose direction depends on whether the reaction is induced by heat or light. For example, it was found for the thermal reaction that cw-3,4-dimethylcyclobutene gave only cis,trans-2,4-hexadiene, while the trans isomer gave only the trans-trans diene 366... 

Photochemical electrocycltc ring-closure in a 4-electron system works well for many acyclic dienes (2.17) and related cyclic systems 12.18). The situation with conjugated trienes is more complex, and they can act as 6-electron systems (2.19) leading to cydohexa-1,3-dienes, or as 4-electron systems (2.20) giving cyclobutenes. In addition they can undergo other photochemical reactions such as geometrical isomerization about the central double bond Iwhich must be c/s if a 6-electron electrocydic ring-closure is to take place). 

An important group of conjugated diene/triene systems are those in the vitamin D series. The key reactions in the commercial manufacture of vitamin D (and probably also in its formation in skin exposed to daylight) are a photochemical, conrotatory electrocydic ring-opening in the provitamin, and a thermal 1.7-shift of hydrogen in the previtamin so formed (2.23). High conversions to the vitamin are not normally possible because all three species absorb appreciably at the... 

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