Tautomer-dependent Bergman cyclization of novel uracil-enediyne chimeras

Uracil-enediyne chimeras 4, 7, and 8 were prepared and examined for their propensity to undergo Bergman cyclization. Kinetic experiments showed lactam tautomers 7 and 8 reacted up to 25 times faster than lactim ether 4. Determination of the activation energy for each cycloaromatization reaction, along with radical trapping agent dependent studies, indicate the rate differences result from different ground state energies of the starting enediynes.