Electrocyclic Reactions of Conjugated Dienes and Trienes

Electrocyclic reactions were first described by Woodward and Hoffmann in their classic series of articles. One very interesting aspect of such reactions is, that for a given conjugated polyene photochemical transformation leads to the opposite stereochemical outcome than the thermal one314). 

The photoequilibrium between 1,3-cyclohexadienes and 1,3,5-hexatrienes 3t5,3i6) s key step jn synthesis of vitamin D, as shown in the formation of vitamin D3 (R = C8H17) via a [l,7]sigmatropic H-shift from previtamin D which is obtained by irradiating provitamin D (3.9) 317). 

The ring closure of 1,3-butadienes to cyclobutenes has been utilized for synthetic purposes as in the preparation of cyclobutene itself318) (3.10), of a bicycloheptenone319) (3.11), of a bicyclic lactam 320) (3.12) or of diphenylbicycloheptene (3.13) 321 

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