Orbital Overlap Effects in Sigmatropic Shifts

The above overlap effect generated by the anisotropic influence of a small ring should apply to other concerted pericyclic reactions as well. Let us consider the hydrogen shifts in czs-2-alkenyl-l-alkylcyclopropanes (9 — 10, Eq. 6) and the analogous czs-2-alkenyl-1 -alkylcyclobutanes expressed by the transformation (11 — 12, Eq. 7). These reactions may be considered analogous to the [1,51-hydrogen shift 13 — 14 (Eq. 8) [4, 5]. 

In application of the above to hydrogen shift in the corresponding cyclobutane derivative 22, one will expect 22a as the most reactive conformer on account of better orbital overlap of the breaking cr( n bond and the cyclobutane cjc c bond, as shown by parallel dotted lines in red color required for n bond formation, to generate 23. The conformer 22b will be the least reactive to generate 24. The difference in the geometries of the donor-derived double bonds and also the reversal in the configuration on the acceptor-derived carbon must specifically be noted. 

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