STEREOCHEMICAL FEATURES OF META PHOTOCYCLOADDUCTS DERIVED FROM 3-ALKYL-4-PHENOXYBUT-1-ENES

3-Alkyl-4-phencxybut-1-enes (alkyl: methyl, isopropyl, tert-butyl) On excitation at 254 nm in cyclohexane give rise to -2-oxatetracyclo[5.4. 0.0(1.8).0(5,11)]undec-9-enes via intramolecular meta photocycloadditi on. Prevailing stereochemical features are the configuration of the al kyl group at C-4 and the conformation of the tetrahydropyran ring cont ained in the tetracyclic system. These are revealed by detailed H-1-NM R analysis and interpretation of the coupling constants. Whereas the a lkyl group at C-4 invariably occurs in the exo configuration, the conf ormation of the tetrahydropyran ring depends on the bulkiness of the a lkyl substituent. Thus, the C-l methyl group occupies an equatorial po sition in a chair conformation, while a flattened chair conformation a ccomodates appropriately the C-4 isopropyl group in the pertaining str ucture. The required equatorial position of the tert-butyl group at C- 4. on the other hand, forces the tetrahydropyran ring into a boat conf ormation, thereby alleviating steric constraints.