Regiochemistry of the Intramolecular [2 + 2] Photocycloaddition of Enones to Vinyl Ethers as a Function of Chain Length

The intramolecular [2 + 2] cycloaddition of a cyclohexenone moiety bound to a vinyl ether fragment has been explored. The regiochemistry and the quantum yield of the reaction have been investigated as a function of the chain length n and the position of the methoxy group. It has been found that in those cases where the chain consists of three and four members the position of the methoxy group has no influence on the regiochemistry but on the quantum yield. Only head‐to‐head cycloaddition is observed. In the case of n = 2 both the regiochemistry and the quantum yield depend strongly on the position of the methoxy group. It is concluded that the main reason for the different behavior of n = 2 is due to a through‐bond effect between the two olefinic units mediated by a C 2 H 4 bridge.