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The kinetics of cyclization for 3-ethyl-, 3-isopropyl-and 3-hexyl-substituted hydantoates 2-UE to the corresponding hydantoins 2-Hyd were studied in an attempt to examine the gem-dimethyl effect (GDME) in a case where the tetrahedral center of the intermediate T is not fully screened. However, in these congested compounds, the increase in strain in the open chains paralleled that of the intermediates resulting in meager rate enhancements – an interpretation confirmed by MM3 calculations. In the case of the largest substituent 2d-UE a new reaction appeared in the rate profile, together with substantial changes concerning the points of transition from r.d. formation to breakdown of T in the OH and general base catalysis. All these agree with the concept that the GDME tends to shift the transition state to an earlier step of the cyclization reaction.

The gem-dimethyl effect in reactions through tetrahedral intermediates: cyclizations of some ethyl 2,3-disubstituted-5-(p-nitrophenyl) hydantoates