Lewis acid complexes of 1,2-diketones and their derivatives. The synthesis of seven-membered rings

The structures and charge distributions of Lewis and complexes of butane-2,3-dione, 1, cyclohexane-1,2-dione, 2, and 2-methoxycyclohex-2-en-1-one, 5, have been investigated by 1 H nmr spectroscopy. A 1:1 complex was formed between 1 and SnCl 4 which appeared to have a bridged structure. In the case of 2 and 5 both oxygens were again involved in complex formation with TiCl 4 or SnCl 4 . In each case the large downfield shift of the vinyl proton resonance, H3, indicated extensive charge delocalization within the complexes and that these systems can be regarded as complexed α,β-unsaturated enones. The reactions of Lewis acid complexes of 2, 5, and the silyl ethers 2-(trimethylsiloxy)cyclohex-2-en-1-one, and 3-(trimethylsiloxy)-but-3-en-2-one, 11, with butadiene have been investigated. The acyclic enol ether 11 reacted with butadiene in the presence of either TiCl 4 or SnCl 4 to give products corresponding to both 4C + 3C and 4C + 2C cycloaddition reactions; in all other cases 4C + 2C products only could be isolated.