Antarafacial 1,7 H-shift

The complete rules for sigmatropic hydrogen shifts are simple. In thermal reactions, [1,5]H shifts occur suprafacially but [ 1,3]H and [1,7]H shifts must be antarafacial. It is just as well that antara-... 

As you should by now expect (p. 927), all this is reversed in photochemical reactions. Here is an example of a [1,7]H shift that cannot occur antarafacially because the molecule is a rigid ring, but that can and does occur photochemically. 

Since the hydride shift in this problem is a [1,7] H-shift, it must be in an antarafacial manner ... 

The consequences of orbital symmetry for sigmatropic hydrogen shifts are simple. In thermal reactions, [1,5]H shifts occur suprafacially but [1,3]H and [1,7]H shifts must be antarafacial. Antarafacial [1,3] H shifts are impossible, even though they are allowed, because a rigid three-carbon chain is too short to allow the H atom to transfer from the top to the bottom— the H atom just can t reach. This is just as well, as otherwise double bonds would just wander around molecules by repeated [1,3]H shifts. 

When we come to [1,7]H shifts, the situation is different. Now the much longer chain is just flexible enough to allow antarafacial migration. The hydrogen atom leaves the top side of the triene and adds back in on the bottom side. The diagram shows this in orbital terms the LUMO of hexatriene has three nodes. Antarafacial [1,7]H migration is allowed and possible. 

This step happens spontaneously, without the need for light, so the [1,7]H shift must be antarafacial. That s no problem in this triene system—there is enough flexibility for the hydrogen atom to migrate from the top to the bottom face. 

The occurrence of a 1,7-photochemical shift of H in this compound does not, of itself, establish directly that this shift proceeds via a suprafacial pathway. The relatively rigid cyclic structure of (47) must, however, rule out the possibility of the shift having proceeded via the antarafacial route. 

The light-induced ring openings and cyclizations depicted in Scheme 3 abruptly became interpretable once R. B. Woodward and R. Hoffmann had put forward their mles concerning preservation of orbital symmetry during peri-cyclic reactions [19]. Previtamin D (5) and Vitamin D (7) may be converted one into the other via antarafacial 1,7-sigmatropic H-shift. Provitamin D (1)... 

The c ,cw>dipropenylbenzene (605) isomerizes thermally to give an equilibrium mixture with the /ra s,m-isomer (606) no trans,trans- somer is detectable and isomerization is the result of a [1,7]-sigmatropic H-shift (antarafacial) (Scheme 37), as is shown using the dideuteriated starting material (607). ... 

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