RHODIUM(II)-CATALYZED DECOMPOSITION OF 1-DIAZO-4-1-NAPHTHYL-2-BUTANONES OR 1-DIAZO-4-2-NAPHTHYL-2-BUTANONES AS A NEW ROUTE TO REARRANGED PIMARANE AND ABIETANE SKELETA - SYNTHESIS OF UMBROSONE

The Rh-2(OAc)(4)-catalyzed intramolecular Buchner reaction of 1-diazo- 4-(1- or 2-naphthyl)butan-2-ones was examined as a potential route to abietane and rearranged abietane derivatives. Treatment of the alpha-d iazo ketone 26 with catalytic amount of dirhodium tetraacetate in CH2C l2 at 0 degrees C furnished the tetracyclic derivative 27 in good yiel d. Addition of TFA to 27 (in CH2Cl2) resulted in an acid-induced openi ng of the cyclopropane ring to give the 4a- and 10a-methyldihydrophena nthrenones 28 and 29 in nearly equal amounts. These compounds and thei r analogs appear to be suitable intermediates for the synthesis of dit erpenoids containing aromatic A or C rings. When the diazo ketone 34 w as decomposed under Rh(II) catalysis, a 10-methyldihydroanthracenone ( i.e., 36) was obtained as the main product, besides minor amounts of t he expected tetracyclic ketone 35. The extension of this result to the preparation of the methoxy-substituted dihydroanthracenone 39 (52% yi eld) was exploited in a new total synthesis of umbrosone (6), an unusu al diterpenoid possessing a rearranged linear skeleton.