Intramolecular ortho and meta photocycloadditions of 4-phenoxybut-1-enes substituted in the arene residue with carbomethoxy, carbomethoxymethyl, and 2-carbomethoxyethyl groups

On irradiation of 4-phenoxybut-1-enes substituted with a carbomethoxy group in the arene, only the ortho compound 1 led to notable ortho photocycloadd ition, while the meta regioisomer 2 furnished meta and ortho photocycloaddu cts albeit in low yield, and the para-substituted compound 3 resulted only in intractable polymers. On irradiation of the carboxymethyl- and 2-carboxy ethyl homologues 4-9, the photoreactivity altered dramatically. The highest yields of meta photocycloadducts were observed on irradiation of the ortho -substituted compounds 4 and 7. Due to steric hindrance, only one of the tw o possible regioisomers 18 and 19, respectively, was formed. The meta-subst ituted compounds 5 and 8 gave significant amounts of meta photocycloadducts as mixtures of regioisomers 13+16 and 14+17, respectively. The intramolecu lar meta photocycloaddition was directed exclusively to the 2',6'-positions of the arene and only 1,5-bridged dihydrosemibullvalenes were formed. On i rradiation of the ortho- and para-substituted compounds 4 and 7, and 6 and 9, respectively, considerable amounts of reaction products derived from ini tial ortho photocycloaddition were detected. (C) 2000 Elsevier Science S.A. All rights reserved.