Synthesis of spiro-cyclohexendienone-gamma-lactams via free-radicals and study of thermal regioselective spirocyclization

The spiro-regioisomers 1a and 1b were synthesized from the xanthate-type precursor 2 via a free-radical mediated spirocyclization in different proportions depending on the reaction temperature, from 40:1 at -45 °C to 5:1 at 150 °C. The maximum proportion relationship value was 2:1 at 110-125 °C. We hypothesize that those proportion values are due to inherent stability of the involved free radicals. In the case of the product 1a, the intermediate is a very thermodynamically stable double allyl radical. In the case of the product 1b, the intermediate is an a,b,g,d-unsaturated carbonyl radical. The products 1a-b were fully characterized by ¹H and ¹³C NMR, FT-IR, HRMS, and even by X-ray analysis because adequate crystals were obtained for both products, (1a, CCDC: 1050852) and (1b, CCDC: 1050853) respectively.