Enantioselective Crossed Photocycloadditions of Styrenic Olefins by Lewis Acid Catalyzed Triplet Sensitization

Abstract

The synthesis of unsymmetrical cyclobutanes by controlled heterodimerization of olefins remains as ubstantial challenge,particularly in an enantiocontrolled fashion. Shown herein is that chiral Lewis acid catalyzedt riplet sensitization enables the synthesis of highly enantioenriched diarylcyclobutanes by photocycloaddition of structurally varied 2'-hydroxychalcones with arange of styrene coupling partners.The utility of this reaction is demonstrated through the direct synthesis of arepresentative norlignan cyclobutane natural product.

Cage-confined photocatalysis for wide-scope unusually selective [2 + 2] cycloaddition through visible-light triplet sensitization

“…value and good isolated yield of product (8). In addition, pyridine-, thiophene-, and furan-containing products (19)(20)(21)(22)(23)(24)(27)(28)(29), which are of interest to medical researchers, are generated smoothly and selectively by applying this photocatalytic protocol. Not only mono-substituted substrates, but also di-substituted ones (25)(26)(27)(28)(29), are suitable candidates for this photoreaction.…”

“…Noteworthily, good diastereoselectivity can also be obtained from other functionalized chalcones (12,18,(19)(20)(21)(27)(28)(29) without limitation to the substitution positions, and some others present relatively low but satisfactory d.r. values in moderate to excellent isolated yields (11,(13)(14)(15)(16)(17)(22)(23)(24)(25)(26). Excellent tolerance of diversified functional groups represents another remarkable merit of the photoreaction.…”

“…As a green and clean procedure for chemical process and production that are contemporarily demanded by sustainable development, the visible-light sensitized intra-and intermolecular [2 + 2] cycloaddition of olefins has drawn great attention of chemical society in past decades [10][11][12][13][14][15] , leading to fast progress in [2 + 2] photocycloaddition of nonrigid olefins with excellent regio-and stereoselectivities [16][17][18][19][20] . Nevertheless, owing to steric preference and relative radical stability of excited-state intermediates, hitherto, the stereochemical control is mainly achieved for the thermodynamic favored anti-diastereomers from acyclic olefins, e.g., styrenes, chalcones, and cinnamates 16,[21][22][23][24] . A general synthetic strategy for thermodynamic unfavored syn-diastereomers, which present as the important backbones of many biological active natural products [1][2][3] , remains underdeveloped and desired for a long time 25,26 .…”

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

“…This strategy was applied in the total synthesis of norlignan, a diaryl cyclobutane natural product ( Scheme 395 ). 472 …”

Chiral Brønsted acid-controlled intermolecular asymmetric [2 + 2] photocycloadditions

“…In this approach, chiral Lewis acid coordination lowers the triplet energy of α,β-unsaturated carbonyls, allowing for their selective activation by a triplet sensitizer over unbound substrate molecules (Fig. 1b) [11][12][13] . Although fundamentally distinct, both strategies use chiral Lewis acids to modify the excitedstate properties of the substrate and preferentially activate the catalyst-bound complex, thereby minimizing racemic background reactivity.…”