Synthetic Studies in the Diterpene Series. Part-XI. A Synthesis of 7, 16-Epoxy-20-norcleistantha-5(10),6,8, 11,13-pentaen-3-one, a Naphthalenic Norditerpene from Velloziaceae

Department of Chemistry, Indian Institute of Technology, Kharagpur-721 302

A synthesis of the title compound (4), a naphthalenic norditerpene recently isolated from Velloziaceae by Brazilian workers, has been achieved using 4,5-dihydro-3-methyl­naphtho[1,8-bc]pyran (6) as the key intermediate. The latter has been prepared from 4-chromanone through a series of reactions involving Knoevanagel condensation with ethyl cyanoacetate, elaboration of the side chain to 3-substituted-butyric acid, its cyclisation to a tricyclic ketone and aromatisation of the newly formed ring. The dihydronaphthopyran is succinoylated, the keto-acid (8) reduced and the substituted-butyric acid cyclised to 4,5,10,11-tetrahydro-3-methylphenanthro[10,1- bc] pyran-8(9H)- one (10). The corresponding alcohol is dehydrated, the resultant olefinic product oxidised with N-methylmorpholine N-oxide (with O_SO₄ as catalyst) to the cis-diol and the latter submitted to acid catalysed dehydration. Transposition of the ketonic function takes place smoothly leading to 4,5,10,11-tetrahydro-3-methylphenanthro­-[10,1-bc]pyran-9(8H)-one (15) which on methylation affords 7,16-epoxy-20-norcleis­tantha-5(10),6,8,11,13-pentaen-3-one (4), identical with the natural product.