Flavonoid epoxides. Part 16. Synthesis and base-catalysed rearrangement of aurone epoxides

Darzens reaction of 2‐bromo‐4,6‐dimethoxy‐3(2H)‐benzofuranone with aromatic aldehydes to form flavonoids

“…Flavonol formation from aurone epoxides in the presence of hydroxide ions has already been observed by Brady et al [11]. The authors have considered an alternative mechanism of an -attack with the opening of the furanone ring prior the epoxide cleavage.…”

“…It is known, that the epoxide oxygen of , -epoxy ketones can be displaced by nucleophilic attack either the -position or, more frequently, at the -position [11]. Thus, the generation of flavonols 3 could be envisaged by a possible mechanism shown in Scheme 1.…”

“…This powerful C-C bond forming process was also applied to the preparation of two flavonoid-derived epoxides. Brady et al [11] have reacted 2-bromo-6-methoxy-3(2H)-benzofuranone with 2-nitrobenzaldehyde to obtain an epoxide III (R = OMe, R' = H, R'' = 2-NO 2 ) in 89% yield of the crude product. By using benzaldehyde, the corresponding epoxide III (R = OMe, R' = R'' = H) was isolated in traces.…”

“…In contrast to the oxidation of 2-benzylidenebenzofuran-3(2H)-one, which easily afforded the unsubstituted III upon treatment with H 2 O 2 and Triton B (benzyl trimethylammonium hydroxide) [10,11], the preparation of the tetramethoxy substituted epoxide III (R = R' = OMe, R'' = 3,4-(OMe) 2 ) turned out to be difficult.…”

Acid-catalysed coupling reactions and conversions of isoflavone epoxides

“…lo), 151 (3), and 91(20). in hexane-benzene-acetone (5:4: 1 v/v; x 2) gave a fraction (40 mg) at R, 0.13 which consisted of the 3hydroxy-2,4',7-trimethoxy-and 2-hydroxy-3,4',7-trimethoxyisoflavanone (9) and(13) respectively. Under conditions similar to the above, the ditosyloxyisoflavone (4) (360 mg) afforded two fractions, RF 0.57 and 0.33, after p.1.c.…”