The regioselectivity of Diels-Alder reaction of myrcene with carbonyl-containing dienophiles catalysed by Lewis acids

The effects of various Lewis acids and reaction temperature on the regiosel ectivity of the Diels-Alder reaction of myrcene (7-methyl-3-methene-1,6-oct adiene) with acrolein or methyl acrylate have been studied. The high regios elective 'para' adducts were obtained when acrolein and methyl acrylate wer e used as dienophiles in the presence of ZnCl2 and AlCl3 catalyst, respecti vely. UV, (HNMR)-H-1 and IR spectroscopic methods, were first used to inves tigate the interactions between acrolein and ZnCl2 or methyl acrylate and A lCl3. The absorbance of the dienophile decreases and the maximum peak of th e dienophile shifts to longer wave region in the UV spectra. All the proton s' chemical shifts of the dienophile moves to downfield in the (HNMR)-H-1 s pectra. The C=O stretching vibration of the dienophile shifts to a lower wa ve number region in the IR spectra. It is attributed to the coordination of acrolein with ZnCl2 and of methyl acrylate with AlCl3. The dienophile acti vated by Lewis acid catalyst is advantageous for improving the 'para/meta' regioselectivity and enhancing the reaction rate. (C) 1999 Elsevier Science B.V. All rights reserved.