COPE REARRANGEMENTS IN THE BENZO[B]THIOPHENE SERIES

The inability to observe Cope rearrangement products at elevated tempe ratures for diethyl alpha-allyl-2-naphthalenemalonate (1) and diethyl alpha-allyl-9-phenanthrenemalonate (2) does not extend to the analogou s systems resulting from replacement of the aromatic units by 2- and 3 -benzo[b]thiophene nuclei. Thermal rearrangement of diethyl alpha-ally l-3-benzo[b]thiophenemalonate (5) at 215-255-degrees-C for 11 h produc es the expected Cope rearrangement product diethyl 2-allyl-3-benzo[b]t hiophenemalonate (10) (8%) accompanied by trans- and cis-ethyl onyl)-1 H-benzo-[b]cyclopenta[d]thiophene-2-acetate (9a) (10%) and (9b) (5%), respectively. The structure elucidation of 10, 9a, and 9b was done by spectroscopy. The attempted structure verification of 10 by an indepen dent route gave diethyl 2-(1-propenyl)-3-benzo[b]thiophenemalonate (12 ) which when heated at 230-240-degrees-C for 18 h gave 1-carbethoxy-2- hydroxy-3-methyldibenzothiophene (16) as a major product. Similar resu lts were observed with 2-substituted analogues of 5, both diethyl 3-al lyl-2-benzo[b]thiophenemalonate (24) and ethyl onyl)-3H-benzo[b]cyclop enta-[d]thiophene-2-acetate (25) being formed. In this case the struct ure of 24 was verified by synthesis. An extension of this involved a s tudy of thermal rearrangement of analogous compound ethyl alpha-allyl- alpha-cyano-3-benzo[b]thiopheneacetate (30). Thermal rearrangement of 30 at 235-245-degrees-C for 8 h gave the expected Cope rearrangement p roduct ethyl alpha-cyano-2-allyl-3-benzo[b]thiopheneacetate (32) (6%) along with an unexpected diastereomeric mixture of hydro-1H-benzo[b]cy clopenta[d]thiophenes(33)(18%). Speculative mechanistic considerations are offered regarding the mode of transformation of 12 to 16 and 32 t o 33.