SOLVENT AND SALT EFFECT IN PERICYCLIC-REACTIONS .10. INTRAMOLECULAR ENE REACTIONS VS COMPETITIVE PERICYCLIC PROCESSES - THE EFFECT OF SOLVENTS AND SALTS ON THE REACTION OF 5-BROMO-2-(3-METHYL-2-BUTENYLOXY)BENZYLIDENE-MALONIC ACID DIMETHYL ESTER

The reactivity of 5-bromo-2-(3-methyl-2-butenyloxy)benzylidene-malonic acid dimethyl ester (3) was studied in several reaction media. In aprotic solvents intramolecular ene reaction (IER) occurred to give cis and trans chroman derivatives (4) and (5), and some kinetic determinations were performed. In protic solvents, besides IER adducts, the products of intramolecular hetero-Diels-Alder reaction and Claisen rearrangement (10 and 12 respectively) were obtained. In strong protic media, deallylation occurs to give the chromen-2-one derivative (6). Finally, the reaction was investigated in acetonic solutions of inorganic perchlorate (barium, lithium, magnesium, and sodium), and these conditions enhanced chemo- and stereo-selectivity and increased the rate. The kinetic effect was dramatic for magnesium perchlorate.