Antarafacial motion

Let s set about finding an experiment to test the requirement of the theory for suprafacial [1,5] motion. What will we know at the end of the experiment If we find that the [1,5] shift is indeed strictly suprafacial, the theory will be supported (not proved ), and we will certainly feel better about the mechanistic hypothesis. If we find antarafacial motion, or both suprafacial and antarafacial motions, the theory will be proved (yes, proved) wrong. There is no way our hypothesis can accommodate antarafacial motion there is an absolute demand for suprafaciality, which nicely illustrates the precarious life of a theory. It can always be disproved by the next experiment, and it can never become free of this state of affairs. 

Antarafacial motion (Section 20.5) Migration of a group from one side of a Jl system to the other in a sigmatropic shift. 

The linear cheletropic reactions in which the polyene is a suprafacial component (i.e., involving disrotatory motion of the termini) will be allowed if it has a total of (4n + 2) electrons. But linear cheletropic reactions in which the polyene is an antarafacial component (i.e., involving conrotatory movement of the termini) are allowed if it has a system of 4n electrons. 

The conrotatory motion involves an antarafacial interaction between the termini, and disrotation involves a suprafacial interaction between these centres. There are two distinct possibilities for each mode of rotation, and therefore four possible products in aU. In many cases, however, the inherent mmetry of the system may not allow such distinction to be made. Even in cases where the two forms of one mode are distinguishable, it does not necessarily follow that both will occur the geometry of the system and the steric factors can be decisive. For example, the conrotatory ting-opening of trans-3,4-dimethylcyclobut-l-ene should, in principle, yield a 1 1 mixture of trans, /nms-hexa-2,4-diene and as, cis-hexa-2,4-diene (Equation 3.17). However, the inward conrotation of the two methyl groups is unfavourable because of the rapid increase in steric compression between these two substituents, and the 78... 

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