A short and efficient synthesis of the title compounds is described, starting from readily available α,β-unsaturated carbonyl compounds 5 and β,γ-unsaturated ketones 4. Their directed aldol condensation yields the unsaturated hydroxy ketones 6 which are dehydrated to the trienones 7. Athough these fail to cyclize on heating, the intramolecular Diels-Alder reaction can be brought about after reduction or alkyllithium addition to the carbonyl group. Alkyl substitution in the positions 1, 2, 3, 5, and 7 has little influence on the ease and yield of cyclization, whereas a methyl group at C-4 hinders it considerably. The title compounds are obtained as mixtures of usually all four stereoisomers, of which one predominates (≥50%) in most cases. O-Alkylation in 8b has little effect on the isomer distribution. The major isomer of the 6-bromo derivative forms a highly crystalline p-nitrobenzoate 27, which permits stereochemical assignment through X-ray crystallography; for most other products assignments can be made by comparison and further evaluation of their 1H NMR spectra. The results are discussed in terms of a simple transition state model. The intramolecular Diels-Alder reaction fails when the length of the tethering chain is reduced by one, or when the diene unit becomes part of a furan ring. Trienones 7 are sensitive to autoxidation, of which some products are described.
ASJC Scopus subject areas
- Organic Chemistry