Constitution and synthesis of naturally occurring isopentenylated kaempferol derivatives, noranhydroicaritin and isoanhydroicaritin and related flavonolsincluding Di-O-methylicaritin

Abstract

Kaempferol (20) reacted with 2-methylbut-3-en-2-ol in the presence of boron trifluoride etherate to yield a mixture of the 6,8-di(3- methylbut-2-enyl) derivative (21), 3,5-dihydroxy-2-(4?-hydroxyphenyl)- 8,8-dimethyl-9,10-dihydro-4H,8H-benzo[1,2-b:3,4-b?]dipyran-4-one (19), and natural noranhydroicaritin(5). The orientation of the 3-methylbut- 2-enyl unit in the 8-position in the flavonol(5) has been unambiguously established by synthetic experiments. Complete acetylation of the flavonol (5) followed by reaction of the product (22) with one molar proportion of methyl iodide in the presence of dry potassium carbonate and acetone afforded 8-(3-methylbut-2-enyl)rhamnocitrin triacetate (23) which, on deacetylation, gave natural isoanhydroicaritin3 (4). 3- Methylbut-2-enylation of kaempferol 3,4?-dimethyl ether (7) was effected in two ways and the products [(8), (9) and (10)]were oxidatively cyclized with 2,3-dichloro-5,6-dicyano-p-benzoquinone to make available the corresponding monopyrano derivatives (26)-(29) to act as reference compounds for investigation of natural products. The 3-methoxyflavone (10) was also converted into di-O-methylicaritin (24) via the partial methyl ether (17a).