Differential Reactivity of Purified Bioactive Coffee Furans, Cafestol and Kahweol, with Acidic Nitrite: Product Characterization and Factors Controlling Nitrosation Versus Ring-Opening Pathways

cafestol and kahweol, coffee-specific furan diterpenes, are believed to cause various physiol. effects in human subjects, including an increase in cholesterol and plasma triacylglycerol levels as well as cancer chemopreventive effects. Despite the increasing interest in these compds. raised by the diverse range of biol. activities, their reaction behavior and degrdn. pathways under physiol. relevant conditions remain uncharted. Herein, we report a detailed investigation of the structural modifications suffered by cafestol and kahweol in the presence of acidic nitrite under conditions mimicking those occurring in the stomach during digestion as well as by action of other oxidants. Prior to the chem. study, an isolation procedure for kahweol from green coffee beans was developed based on Soxhlet extn. followed by preparative HPLC. Preliminary expts. showed that kahweol is much more reactive than cafestol toward nitrite at pH 3, as evidenced by inhibition expts. with the 2,3-diaminonaphthalene assay as well as by product anal. in coffee exts. When exposed to equimolar nitrite in phosphate buffer, pH 3, kahweol gave as a main product the ring-opened dicarbonyl deriv. 1. Under more forcing conditions, cafestol reacted as well to give a main nitrogenous product identified as the 1-hydroxy-2-pyrrolinone 2. It is concluded that the conjugated double bond in kahweol is a crit. structural element, increasing the susceptibility of the furan ring to protonation rather than nitrosation and favoring ring-opening routes driven by the irreversible oxidn. steps. These results offer a useful background to assess the effects of coffee-specific lipids in assocn. with abnormally high nitrite levels from the diet.