Photochemical Synthesis of 2,2′‐Biflavanones from Flavone

Abstract

Photolysis of flavone (4) with the electron-donating amines including triethylamine or 2-(N,N-dimethylamino)ethanol in acetonitrile, benzene or methylene dichloride can easily afford two hydrodimers of 2,2¢-biflavanone(racemate) (5a) and 2,2¢-biflavanone(meso) (5b) and one reductive product of flavanone (6). Their yields were dependent on the molar ratios of substrate to amine, the kinds of amines, the solvents used and the irradiation sources. Higher yields were afforded 2,2¢-biflavanone(racemate) (5a), 2,2¢-biflavanone(meso) (5b) and flavanone (6) (30.0%, 20.9% and 15.8%, respectively) in the reaction condition of 1/2 molar ratio of flavone (4) to triethylamine in acetontrile with fourteen hours of irradiation.

Photochemistry of Flavonoids

“…Single electron transfer (SET) [ 46 ] is a well-known photoreaction between amines and α,β-unsaturated carbonyl compounds. The amine donates an electron to form an exciplex or a contact ion radical pair [ 47 ] (CIP) that undergoes hydrogen transfer to yield the radical responsible for dimerisation.…”

“…Chen and co-workers [ 47 ] reinvestigated the photoinduced electron transfer reactions of flavone 38 with amines that was studied by his namesake in 1995. They used triethylamine or 2-( N , N -dimethylamino)ethanol at 250 and 300 nm in benzene, dichloromethane or acetonitrile, respectively, and isolated meso -2,2'-biflavone 39 , (+/-)- meso -2,2'-biflavone 40 and flavanone 42 ( Scheme 18 ).…”

Synthesis of Biologically Relevant Biflavanoids – A Review

“…The yield of 11a is dependent on the molar ratio of substrate to amine, the type of amine and solvent, and the irradiation source [109] (Scheme 20).…”

“…After escaping out of the cage, radical 107 follows a process described above in case of electrochemical reduction to yield racemic or meso-11a. The photolytic process also affords reduced by-product flavanone 104 through enol 112 (Scheme 21) [109].…”

A Simple Biomimetic Synthesis of dl-Chamaejasmine, a Unique 3,3‘-Biflavanone

“…The intriguing structures of these highly oxygenated plant polyphenol substances have emerged as attractive synthetic targets. − Despite considerable synthetic efforts in the past two decades, including oxidative dimerization 11 of naringenin ( 3 ) or analogous flavanone derivatives, various reductive (chemical, photochemical, or electrochemical) dimerizations 12 of apigenin ( 4 ) or analogous flavone derivatives, hydrogenation 13 of 3,3‘-biflavone derivatives, and other attempts, chemical synthesis of 1 is still an unanswered challenge to date . Herein we report the first synthesis of dl - 1 by a simple biomimetic approach, which would be generally applicable to the synthesis of 3,3‘-biflavanones.…”