Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 87, Issue 12, Pages 8071-8083Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.joc.2c00730
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Funding
- JSPS, KAKENHI [JP17H05261, JP18K19077, JP18H01964]
- CREST (JST) [JPMJCR2001]
- Cooperative Research Program of Network Joint Research Center for Materials and Devices
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A range of asymmetric photochemical transformations using visible light have recently become considerably attractive. Chiral Lewis acid association to enones for [2 + 2] and ortho photocycloadditions and oxadi-pi-methane rearrangements have shown to be very promising in achieving effective enantiodifferentiation. In this study, we investigated the regio- and enantiodifferentiation of [2 + 2] photocycloaddition reactions of naphthoquinone derivatives mediated by chiral oxazaborolidines. We found that the excited-state decomplexation is a key factor in achieving high regio- and enantioselectivities in the photocycloaddition reactions.
A range of asymmetric photochemical transformations using visible light have recently become considerably attractive. Among the various approaches, chiral Lewis acid association to enones for [2 + 2] and ortho photocycloadditions and oxadi-pi-methane rearrangements have shown to be very promising. Naturally, chiral Lewis acid coordination protects one of the prochiral faces of the C=C double bond, which enables an effective enantiodifferentiation in the following bond-forming process(es). Here, we studied regio- and enantiodifferentiating [2 + 2] photocycloaddition reactions of naphthoquinone derivatives mediated by chiral oxazaborolidines. A stereochemical control was quite challenging for the 2-ene-1,4-dione substrate, as a double coordination of Lewis acid essentially cancels out the face selectivity, and a mono-coordination to each carbonyl group leads to an opposite stereochemical outcome. Furthermore, a stepwise coordination in the ground state of Lewis acid in a 1:1 fashion was practically inaccessible. We found that the excited-state decomplexation is a key to accomplish high regio- and enantioselectivities in the photocycloaddition of an ene-dione.
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