Rhodium(II)-Catalyzed Decomposition of 1-Diazo-4-(1- or 2-naphthyl)-2-butanones as a New Route to Rearranged Pimarane and Abietane Skeleta. Synthesis of Umbrosone

Abstract

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 α-diazo ketone 26 with catalytic amount of dirhodium tetraacetate in CH 2 Cl 2 at 0 °C furnished the tetracyclic derivative 27 in good yield. Addition of TFA to 27 (in CH 2 Cl 2 ) resulted in an acid-induced opening of the cyclopropane ring to give the 4a- and 10a-methyldihydrophenanthrenones 28 and 29 in nearly equal amounts. These compounds and their analogs appear to be suitable intermediates for the synthesis of diterpenoids containing aromatic A or C rings. When the diazo ketone 34 was decomposed under Rh(II) catalysis, a 10-methyldihydroanthracenone ( i.e., 36 ) was obtained as the main product, besides minor amounts of the expected tetracyclic ketone 35 . The extension of this result to the preparation of the methoxy-substituted dihydroanthracenone 39 (52% yield) was exploited in a new total synthesis of umbrosone ( 6 ), an unusual diterpenoid possessing a rearranged linear skeleton.