Modification of photochemical reactivity by cyclodextrin complexation: selectivity in photo-Claisen rearrangement

Photorearrangements of meta-alkoxyphenylallyl ethers 1-5. in ethanol, in water and as complexes of a and β-cyclodextrins have been investigated. The ratio of the two ortho rearrangement products was dependent both on the length of the alkoxy substituent and on that of the host cavity suggesting that, subtle and rationale engineering manipulation is required to achieve exclusive obtention of only one ortho isomer. It is speculated that a-cyclodextrin provides a "tight fit" while β-cyclodextrin offers a "loose fit". This tightness of binding is suggested to be responsible for the selectivity among the ortho isomers with a-cyclodextrin as the host. Further, the results indicate that "loose cavity" can be tightened by incorporating a "molecular spacer" such as a long hydrocarbon chain as an intramolecular appendix