Total syntheses of natural furan derivatives rehmanones A, B, and C

The synthesis of naturally occurring furan derivatives rehmanones A, B, and C is described. Diverse alternative synthetic strategies were developed for the preparation of these natural products. Conjugate addition of a furan to captodative alkene 1-acetylvinyl 4-nitrobenzoate was carried out under Lewis acid catalysis to give the corresponding adduct, which was transformed into rehmanones B and C in three and two steps, respectively. They were also prepared by condensation of acetone with the key furan-2-carbaldehyde intermediates, which could be readily obtained from D-fructose. The bis-furan rehmanone A was not only obtained in a one-step procedure by a double condensation of acetone with 5-(methoxymethyl)furan-2-carbaldehyde, but by condensing the latter with rehmanone B in high overall yields.