No abstract

The photochemical cycloaddition is highly regioselective. For preparative purposes, the ring junction can be equilibrated to the thermodynamically more stable &-junction. Only the 'endo'/ 'exo' selectivity at the C-atom bearing the olefin substituent cannot be controlled. The photodimerization of 12 is the only side reaction. Using a slight excess of the olefin, the photodimerization can be suppressed. The protecting group at the N-atom of the dihydropyridinone can be varied in order to introduce an internal sensitizer, as shown with 1-acyl-substituted compound 29, which underwent the cycloaddition process even with sunlight.Einleitung. -Die photochemischen [2 + 21-Cycloadditionen an a$-ungesattigte Cy-

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Aus dem Dimethylbrenzcatechin (I) erhält man bei der Oxidation das Dimethyl‐muconsäureanhydrid (II), das photolytisch zum Cyclobuten (III) reagiert, welches katalytisch zum Anhydrid (IV) hydriert wird.