Bioconjugation methods using visible‐light photocatalysis have emerged as powerful synthetic tools for the selective modification of biomolecules under mild reaction conditions. However, the number of photochemical transformations that allow successful protein bioconjugation is still limited because of the need for stringent reaction conditions. Herein, we report that a newly developed water‐compatible fluorescent photosensitizer QPEG can be used for visible‐light‐induced cysteine‐specific bioconjugation for the installation of QPEG by exploiting its intrinsic photosensitizing ability to activate the S−H bond of cysteine. The slightly modified QCAT enables the effective photocatalytic cysteine‐specific conjugation of biologically relevant groups. The superior reactivity and cysteine selectivity of this methodology was further corroborated by traceless bioconjugation with a series of complex peptides and proteins under biocompatible conditions.
A catalytic method for the enantioselective and C4-selective functionalization of pyridine derivatives is yet to be developed. Herein, we report an efficient method for the asymmetric β-pyridylations of enals that involve N-heterocyclic carbene (NHC) catalysis with excellent control over enantioselectivity and pyridyl C4-selectivity. The key strategy for precise stereocontrol involves enhancing interactions between the chiral NHC-bound homoenolate and pyridinium salt in the presence of hexafluorobenzene, which effectively differentiates the two faces of the homoenolate radical. Room temperature is sufficient for this transformation, and reaction efficiency is further accelerated by photo-mediation. This methodology exhibits broad functional group tolerance and enables facile access to a diverse range of enantioenriched β-pyridyl carbonyl compounds under mild and metal-free conditions.
Reported herein is the photochemical activity of quinolinone-containing substrates that directly reach an excited state upon light absorption to trigger radical-based bond-forming processes. The presented transformations allow divergent construction of valuable dihydro-or tetrahydrophenanthridin-6(5H)-ones through the generation of S-or P-centered radicals, subsequent radical addition, cyclization, and a hydrogen atom transfer/electron transfer sequence. This strategy demonstrates the potential generality of quinolinone-tethered substrates to directly participate in the photoexcitation for the development of useful synthetic methods.
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