A FACILE ENTRY INTO NAPHTHOPYRAN QUINONES VIA AN ANNELATION REACTION OF LEVOGLUCOSENONE - THE TOTAL SYNTHESIS OF (-)-HONGCONIN

The annelation reactions of levoglucosenone, prepared by pyrolysis of paper, with 3-cyano-1(3H)-isobenzofuranone and 3-cyano-5-methoxy-1(3H) -isobenzofuranone have been studied. Reductive ring-opening of the ann elation products with zinc/copper couple and subsequent chemical trans formations provide a facile entry into the naphthopyran quinone ring s ystem. Standard chemical transformations of the annelation product fro m 3-cyano-5-methoxy-1(3H)-isobenzofuranone and levoglucosenone afforde d ethoxy-1-methyl-1H-naphtho[2,3,-c]pyran-4(3H)-one. The lithium enola te of this compound undergoes an interesting and potentially useful re action with oxygen to afford -trimethoxy-3-methylnaphtho[2,3-c]furan-1 (SH)-one. When oxygen is rigorously excluded, methylation of the enola te could be performed in good yield using 10 equiv of methyl iodide in the presence of 10 equiv of DMPU. This chemistry culminated in a tota l synthesis of(-)-hongconin in six steps from levoglucosenone.