Search

Lignin, a major component of lignocellulose, constitutes the largest source for the production of aromatic building blocks, and offers considerable potential to serve as an entry to biobased products. During the past decades, substantial research activity has been noted in the pursuit of sustainable catalytic methods for accessing lignin-derived platform chemicals. However, the highly cross-linked and irregular structure has hindered the development of efficient and predictable deconstruction strategies from this renewable source. A majority of the developed methods for lignin depolymerization require harsh conditions including high-pressure atmospheres of gases, high temperatures, and prolonged reaction times. Considering the recent renewed interest in photocatalysis as a means for the mild and operationally simple generation of open-shell intermediates, it is not remarkable that such reaction manifolds have also been implemented towards lignin depolymerization. The tremendous potential of visible light photocatalysis for harnessing sustainable energy sources and reducing waste streams will certainly have an enduring impact in future green chemistry innovation., QC 20220915Part of book: ISBN 978-3-030-63712-5

Photo-mediated radical dearomatization involving 5-exo-trig cyclizations has proven to be an important route to accessing spirocyclic compounds, whereas 6-exo-trig spirocyclization has been much less explored. In this work, a dearomative annulation cascade is realized through photoredox-mediated C-O bond activation of aromatic carboxylic acids to produce two kinds of spirocyclic frameworks. Mechanistically, the acyl radical is formed through oxidation of triphenylphosphine and subsequent C-O bond cleavage, followed by a 6-exo-trig cyclization/SET/protonation sequence to generate the spiro-chromanone products in an intramolecular manner. Furthermore, the protocol was extended to more challenging intermolecular tandem sequences consisting of C-O bond cleavage, radical addition to an alkene substrate, and 5-exo-trig cyclization to yield complex spirocyclic lactams. Photo-mediated radical dearomatization involving 5-exo-trig cyclizations has proven to be an important route to accessing spirocyclic compounds, whereas 6-exo-trig spirocyclization has been much less explored. Here, a dearomative annulation cascade is realized through a photoredox-mediated C-O bond activation of aromatic carboxylic acids to produce two kinds of spirocyclic frameworks, whereby the spirocyclizations are triggered by acyl radical formation from benzoic acids leading to spiro-chromanones via a direct intramolecular 6-exo-trig cyclization or spirocyclic lactams via an intermolecular addition/5-exo-trig cyclization cascade., QC 20220818

Photoredox catalysis has recently emerged as a powerful synthetic platform for accessing complex chemical structures through non-traditional bond disconnection strategies that proceed through free-radical intermediates. Such synthetic strategies have been used for a range of organic transformations; however, in carbohydrate chemistry they have primarily been applied to the generation of oxocarbenium ion intermediates in the ubiquitous glycosylation reaction. In this Review, we present more intricate light-induced synthetic strategies to modify native carbohydrates through homolytic C–H and C–C bond cleavage. These strategies allow access to glycans and glycoconjugates with profoundly altered carbohydrate skeletons, which are challenging to obtain through conventional synthetic means. Carbohydrate derivatives with such structural motifs represent a broad class of natural products integral to numerous biochemical processes and can be found in active pharmaceutical substances. Here we present progress made in C–H and C–C bond activation of carbohydrates through photoredox catalysis, focusing on the operational mechanisms and the scope of the described methodologies. [Figure not available: see fulltext.]., QC 20230523

QC 20210331