Improved synthesis of hexahydrodibenzopyranones

The current application relates to an improved process for the synthesis of a 6a, 10a-cis-hexahydrodibenzo[b,d]pyran-9-one derivative, such as nabilone, by condensing a 5-substituted resorcmol with a 1-alkoxy-4-(1 -hydroxy-1-methylethyl)-1,4-cyclohexadiene in the presence of a Lewis acid selected from triflic anhydride, triflic acid, and sulfuric acid. The starting 1,4-cyclohexadiene derivative is prepared by an improved Birch reduction process in which a benzene derivative is reacted with gaseous ammonia, an alcohol, and an alkaline metal such as lithium, sodium, or potassium.