Fluorinated acetylenes. Part 15. The reaction of ethyl 4,4,4-trifluorobut-2-ynoate with dienes and cycloheptatriene

Abstract

Thermal reaction of the acetylenic ester CF 3CCCO 2Et ( 1) with cyclopentadiene ( 4), buta-1,3-diene ( 5) and 1,4-diphenylbuta-1,3-diene ( 6) gave the corresponding Diels-Alder 1:1 cycloadducts 10, 11 and 12, respectively in high yield (73%–97%). With norbornadiene ( 7), the homo Diels-Alder adduct ( 13) was formed. The reaction with 3-carboethoxyfuran gave a mixture (55%) of the isomeric Diels-Alder 1:1 adducts 14 and 15 (ratio 55:45), together with four 2:1 adducts, i.e., the exo,endo compounds 16a (10%) and 16b (9%) and the exo,exo compounds 17a (8%) and 17b (9%), resulting from Diels-Alder cycloaddition of the furan to the least substituted double bond in the 1:1 adducts 14 and 15. The cycloadditions to the 1:1 adducts were regioselective with the two CO 2Et groups on the unsaturated bonds involved in the cycloadditions being 1,4 to each other in the 2:1 adducts. The products from reaction of ester 1 with cycloheptatriene ( 9) were formed via two competing reaction paths; (a) cycloaddition of ester 1 to norcaradiene ( 23) which gave the Diels-Alder adduct 18 (74%) and (b) an initial regioselective ‘ene’ reaction followed by isomerisation of the resulting 1:1 adduct 24 to the corresponding norcaradiene 25, which then underwent both rearrangement to afford 1:1 adduct 19 (15%) and Diels-Alder cycloaddition with ester 1 to give the 2:1 adduct 20 (6%).