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Synthesis and photochemical transformations of new chloro-substituted butadiene chromophores

Previous publications on the photochemistry of different heteroaryl substituted hexatrienes showed interesting intramolecular cycloaddition reactions and the formation of bicyclo[3.2.1] octadienes as unsaturated structural analogues of the bicyclo[3.2.1]octane skeleton which is found in numerous biologically important active natural products. The photochemical behaviour of mono- and dichloro-substituted butadiene derivatives was studied at low concentrations. These compounds display diverse photochemical behaviour. The α-chloro derivatives photocyclize to give six-membered ring photoproducts. p-Chloro substituted butadienes undergo intramolecular [2 + 2] cycloaddition followed by formation of benzobicyclic structures. The steric hindrance of chlorine(s) in a-position(s) cause a greater deviation from planarity, relative to p-substitution, and shift conformer equilibria and affects the reaction pathways and yields.

butadiene; chlorine; photochemistry; reaction mechanism; spectroscopy

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