Recueil des travaux chimiques des pays-bas-journal of the royal netherlands chemical society vol:114 issue:11-12 pages:480-&
3-Alkyl-4-phencxybut-1-enes (alkyl: methyl, isopropyl, tert-butyl) On excitation at 254 nm in cyclohexane give rise to 4-alkyl-2-oxatetracyclo[188.8.131.52(1.8).0(5,11)]undec-9-enes via intramolecular meta photocycloaddition. Prevailing stereochemical features are the configuration of the alkyl group at C-4 and the conformation of the tetrahydropyran ring contained in the tetracyclic system. These are revealed by detailed H-1-NMR analysis and interpretation of the coupling constants. Whereas the alkyl group at C-4 invariably occurs in the exo configuration, the conformation of the tetrahydropyran ring depends on the bulkiness of the alkyl substituent. Thus, the C-l methyl group occupies an equatorial position in a chair conformation, while a flattened chair conformation accomodates appropriately the C-4 isopropyl group in the pertaining structure. The required equatorial position of the tert-butyl group at C-4. on the other hand, forces the tetrahydropyran ring into a boat conformation, thereby alleviating steric constraints.