Electrocyclic reactions electronic factors

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

Scheme 13 may look unfavorable on the face of it, but in fact the second two reactions are thermally allowed 10- and 14-electron electrocyclic reactions, respectively. The aromatic character of the transition states for these reactions is the major reason why the benzidine rearrangement is so fast in the first place.261 The second bimolecular reaction is faster than the first rearrangement (bi-molecular kinetics were not observed) it is downhill energetically because the reaction products are all aromatic, and formation of three molecules from two overcomes the entropy factor involved in orienting the two species for reaction. 

Subsequent investigations showed that D vitamins were involved, and eventually it became known that one of several D vitamins, called vitamin D3, is the curative factor. Vitamin D3 is formed in the skin from 7-dehydrocholesterol by two reactions. In the first reaction (below), ultraviolet light in the UV-B range (280-320 nm, which can penetrate the epidermal layer) brings about a 6-electron conrotatory electrocyclic reaction (see Special Topic H, WileyPLUS) to produce pre-vitamin D3. Following this event, a spontaneous isomerization (by way of a [1,7] sigmatropic hydride shift) produces vitamin D3 itself. 

As has been shown by an initio calculations [45] and experimental studies [51-53], the electronic factors also determine stereoselectivity of reactions of opening of substituted cyclobutenes. Donor substituents at C3, such as OR, display considerable preference for outward rotation in the electrocyclizations XXIV XXV. Such an effect is observed for the compound with R = t-Bu. In spite of the large steric effect, tcrt-butyl rotates inward due to the electronic effect of the small hydroxy group. 

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