The Use and Thermodynamic Origin of the Trityl Buttress Effect in Intramolecular Diels-Alder Cycloaddition Reactions

The triphenylmethyl group can be used as a steric buttress to drive otherwise unfavorable intramolecular Diels-Alder cycloaddition reactions. Using this strategy, we attempted to synthesize the cycloadducts 9-triphenylmethylaza-1,4-oxidobicyclo[4.4.0]dec-2-ene (3b) and 10-triphenylmethylaza-1,4-oxidobicyclo[5.4.0]undec-2-ene (3c) from furfurylamine-based starting materials, and it was found that the presence of the buttress was not sufficient to promote the formation of 3c, even with high temperatures and long reaction times. Density functional theory calculations on the reaction system provide insight into the nature of the trityl group’s effect on the thermodynamics of the cycloaddition reaction, and indicate that the trityl group exhibits equally important kinetic and thermodynamic driving effects.