Cope Rearrangements in the Benzothiophene Series

Article abstract of DOI:10.1021/jo00068a034

The inability to observe Cope rearrangement products at elevated temperatures for diethyl α-allyl-2-naphthalenemalonate (1) and diethyl α-allyl-9-phenanthrenemalonate (2) does not extend to the analogous systems resulting from replacement of the aromatic units by 2- and 3-benzothiophene nuclei.Thermal rearrangement of diethyl α-allyl-3-benzothiophenemalonate (5) at 215-255 deg C for 11 h produces the expected Cope rearrangement product diethyl 2-allyl-3-benzothiophenemalonate (10) (8percent) accompanied by trans- and cis-ethyl 2,3-dihydro-1-(ethoxycarbonyl)-1H-benzocyclopentathiophene-2-acetate (9a) (10percent) and (9b) (5percent), respectively.The structure elucidation of 10, 9a, and 9b was done by spectroscopy.The attempted structure verification of 10 by an independent route gave diethyl 2-(1-propenyl)-3-benzothiophenemalonate (12) which when heated at 230-240 deg C for 18 h gave 1-carbethoxy-2-hydroxy-3-methyldibenzothiophene (16) as a major product.Similar results were observed with 2-substituted analogues of 5, both diethyl 3-allyl-2-benzothiophenemalonate (24) and ethyl 1,2-dihydro-3-(ethoxycarbonyl)-3H-benzocyclopentathiophene-2-acetate (25) being formed.In this case the structure of 24 was verified by synthesis.An extension of this involved a study of thermal rearrangement of analogous compound ethyl α-allyl-α-cyano-3-benzothiopheneacetate (30).Thermal rearrangement of 30 at 235-245 deg C for 8 h gave the expected Cope rearrangement product ethyl α-cyano-2-allyl-3-benzothiopheneacetate (32) (6percent) along with an unexpected diastereomeric mixture of 1-cyano-1-(ethoxycarbonyl)-2-methyl-2,3-dihydro-1H-benzocyclopentathiophenes (33) (18percent).Speculative mechanistic considerations are offered regarding the mode of transformation of 12 to 16 and 32 to 33.