Hoffmann-like rearrangement

Whether for these reasons or not, the reactivity indices excited little interest among organic chemists, and it was only after Woodward and Hoffmann s early papers that perturbational methods were widely invoked again. Fukui and Fujimoto17-90 adopted Fukui s frontier orbital theory to explain the Woodward-Hoffmann rules, and both they91 and Hoffmann and Woodward92 were able to explain stereoselective effects [such as the preference in the Cope rearrangement (1) (2) for the chair-like intermediate (16) rather than the boat-like one (17), and the tendency for Diels-... 

According to the Woodward-Hoffmann rales, five concerted transition states are possible for the Claisen as well as the closely related Cope rearrangements chair, boat, twist, cross and plane (Table 6). Only the chair and boat TS have to be considered, as twist, cross and plane are antarafacial-anta-rafacial processes and require highly elevated temperatures. - For the correct prediction of product stereochemistry it is nevertheless crucial to know the preference for chair- or boat-like transition state in the actual 3,3-sigmatropic shift. 

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