Add time:08/31/2019         Source:sciencedirect.com

On irradiation of 4-phenoxybut-1-enes substituted with a carbomethoxy group in the arene, only the ortho compound 1 led to notable ortho photocycloaddition, while the meta regioisomer 2 furnished meta and ortho photocycloadducts albeit in low yield, and the para-substituted compound 3 resulted only in intractable polymers. On irradiation of the carboxymethyl- and 2-carboxyethyl 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 two possible regioisomers 18 and 19, respectively, was formed. The meta-substituted compounds 5 and 8 gave significant amounts of meta photocycloadducts as mixtures of regioisomers 13+16 and 14+17, respectively. The intramolecular meta photocycloaddition was directed exclusively to the 2′,6′-positions of the arene and only 1,5-bridged dihydrosemibullvalenes were formed. On irradiation of the ortho- and para-substituted compounds 4 and 7, and 6 and 9, respectively, considerable amounts of reaction products derived from initial ortho photocycloaddition were detected.

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