Barrier approach capability

A methyl or a f-butyl substituent in 99 in an ortho position dramatically increases the barrier. Thus, for a given model the modification of the barriers can be used to determine a steric scale of substituents in this particular geometrical context provided the electronic contribution of the substituent on the cross-conjugation capability of the heterocycle can be determined. This approach can be applied with success to the steric effect of alkyl groups whose electronic contribution should be very similar. 

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