The Synthesis of (-)-Robustadial A and Some Analogues.

The naturally occurring chroman derivative, robustadial A, and several analogues have been prepared by a four-step synthesis starting from phloroglucinol. The synthetic scheme involves as the first step a Friedel-Crafts condensation of an alpha,beta-unsaturated acid derivative with phloroglucinol to the corresponding chromanone. The isobutyl group at the 4-position was introduced with methallylzine bromide followed by hydrogenation, carried out as a one-pot reaction. Finally, both aldehyde functions were attached using dichloromethyl methyl ether and titanium tetrachloride. Optically pure 6,6-dimethylbicyclo[3.1.1]heptan-2-ylidene-acetic acid, needed for the synthesis of robustadial, was prepared from (-)-nopol by two consecutive oxidations in 77% overall yield.