Diels–Alder reactivity of oxygenated dienes and furans. Synthesis of oxygenated biphenyls

Abstract

2,3-Dimethoxybutadiene (4)(prepared by a new route from biacetyl) and 3,4-dimethoxyfuran (17) were heated with a series of styrenes to assess their potential for the synthesis of oxygenated biphenyls. Both dienes proved to be remarkably unreactive and only adducts (15) and (18) were obtained. Under comparable conditions 1-acetoxybutadiene (19) reacted in an efficient and regioselective manner with a variety of styrenes to give the trisubstituted cyclohexenes (21)–(25). 2,3-Dimethoxyfuran (28), generated in situ, reacted with phenylpropiolic ester in a non-regioselective manner to give a ca. 1 : 1 mixture of the highly functionalised biphenyls (42) and (43). The oxabicycloheptadiene (18)[from (4)] could not be aromatised but under varying acidic conditions it was hydrolysed to the oxabicycloheptenone (46) and gave the addition products (48) and (47)via cis-exo-addition.