THE C(1)-C(10) BOND CLEAVAGE AND B RING AROMATIZATION OF SOME 6-HYDROXY-5βH-ABIETA-6,8,11,13-TETRAEN-18-OIC ACID 18,6-LACTONES

Fusion of 6-hydroxy-5βH-abieta-6,8,11,13-tetraen-18-oic acid 18,6-lactone (2) with potassium hydrogensulfate at 220 °C afforded bis[3-(2,3-dihydro-7-isopropyl-3,4-dimethyl-2-oxonaphtho[2,3-b]furan-3-yl)propyl] ether (12) in 37% yield, together with two minor products in 12% yield: 2,3-dihydro-7-isopropyl-3,4-dimethyl-3-propylnaphtho[2,3-b]furan-2-one and 3-allyl-2,3-dihydro-7-isopropyl-3,4-dimethylnaphtho[2,3-b]furan-2-one. Treatment of 12 with p-toluenesulfonic acid at 160 °C gave 2,3-dihydro-7-isopropyl-3,4-dimethyl-3-(3-tosyloxypropyl)naphtho[2,3-b]furan-2-one, which was converted into 2,3-dihydro-3-(3-hydroxypropyl)-7-isopropyl-3,4-dimethylnaphtho[2,3-b]furan-2-one (16). The alcohol 16 was further characterized as its acetate (17). After several attempts, the above C(1)–C(10) bond cleavage and B ring aromatization reaction were improved by use of concentrated sulfuric acid in refluxing acetic acid. Under these conditions, 2 gave 17 in 47% yield. Hydrolysis of 17 with hydrochloric acid afforded 16. Sub...