Production and Reactions of Substituted Dihydrofurans and Thiophenes

摘要

Results of Section I not only unambiguously support the previous proposed two-step mechanism for the dimerization of 2,3-dimethylene-2,3-dihydrofuran (1), they also provide two of the most unreactive o-quinodimethanes, namely, 2-methylene-3-t-butylmethylene-2,3-dihydrofuran (2) and 2,3-di-t-butylmethylene-2,3-dihydrofuran. In Section II, the preparation and the dimerization of a series of alkyl ring-substituted and carbomethoxy ring-substituted derivatives of 1 are presented. This study concludes that bulky alkyl substituents on the ring of furan monomers do not promote the (4+2) dimerization process. In Section III, a Diels-Alder reaction of 1 and its two deuterated analogues with methyl acrylate reveals the presence of a secondary deuterium isotope effect which strongly supports a stepwise mechanism. This conclusion is further strengthened by the fact that the Diels-Alder reaction of 1 with cis dienophiles proceeds non-stereo-specifically. A comparison study of o-xylylene with dienophiles reaches the same conclusion. Section IV presents a convenient synthesis of cyclooctadecane from the thiophene trimer of 2,5-dimethylene-2,5-dihydrothiophene. (ERA citation 12:037397)