New experimental evidence for the mechanism of the Paal-Knorr reaction involving the acidcatalyzed cyclization of al, &diketone to form a furan is reported. In aqueous or alcoholic solutions containing hydrochloric acid and in chloroform containing boron trifluoride- etherate d, l-and meso-3, 4-diethyl-2, 5-hexanediones (2r and 2m) cyclize at unequal rates; the stereochemical configuration of the unchanged dione is preserved during the reaction. ...
