1‐Methoxycarbonyl‐substituiertes 2,3‐Dihydropyridin‐4(1H)‐on (= Methyl‐1,2,3,4‐tetrahydro‐4‐oxopyridin‐1‐carboxylat) als Chromophor für die photochemische [2 + 2]‐Cycloaddition

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-

Enantioselective Lewis Acid Catalysis in Intramolecular [2 + 2] Photocycloaddition Reactions: A Mechanistic Comparison between Representative Coumarin and Enone Substrates

“…The intermolecular [2 + 2] photocycloaddition of 2,3-dihydropyridin-4­(l H )-ones has been studied by the group of Neier, and there is a detailed study on the N -methoxycarbonyl analogue of compound 3a as chromophor in these reactions . However, Lewis acid coordination of this compound class has not been previously investigated.…”

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“…The intermolecular [2 + 2] photocycloaddition reaction of 2,3-dihydropyridin-4­(l H )-one was discovered by Neier and co-workers in the 1980s. − The intramolecular variant of this reaction was used by the Comins group for the synthesis of various alkaloids. − They observed a high facial diastereoselectivity in the intramolecular [2 + 2] photocycloaddition if a stereogenic center was present within the dihydropyridone ring. The reaction of substrates rac - 261 delivered for example exclusively products rac - 262 in good yields (Scheme ).…”

Chromophore Activation of α,β‐Unsaturated Carbonyl Compounds and Its Application to Enantioselective Photochemical Reactions

“…Despite having been mentioned in previous publications, [24] the Lewis acid mediated bathochromic shift as atool for enantioselective synthesis was not employed until this decade.I ndeed, one would not expect aL ewis acid to exert ac atalytic effect as direct excitation of III,w hich subsequently undergoes rapid ISC to T 1 ,l eads to highly efficient photoreactions. [25] However,in2013, our group reasoned that at ac ertain wavelength, the larger molar extinction coefficient e' of complex IV could lead to ah igher photon absorption by IV than by complex III.Indeed, at l = 360 nm, the absorbance cross-section of dihydropyridone 13 a was very small, [26] whereas its EtAlCl 2 complex strongly absorbed at this wavelength (Scheme 9).…”