Condensed cyclic and bridged-ring systems. Part 6. Stereochemically defined synthesis of (±)-3-methoxy-17,18,19,20-tetranor-B-homophylloclada-1(10),2,4-triene-16-one through intramolecular alkylation of a γδ-unsaturated α′-diazomethyl ketone

Abstract

A Stereochemically defined synthesis of (±)-3-methoxy-17,18,19,20-tetranor-B-homophylloclada-1 (10),2,4-triene-16-one (7) is described. The key intermediate, 1,2,3,4,6,7-hexahydro-9-methoxy-5H-dibenzo[a,c]cyclo-heptene-3-carboxylic acid (3), is converted into the corresponding 3-diazomethyl ketone (4) and subjected to intramolecular oxo-carbenoid addition and boron trifluoride–ether-catalysed cyclisation, leading to the cyclopropyl ketone, (±)-3-methoxy-9β,15-cyclo-17,18,19,20-tetranor-B-homophylloclada-1 (10),2,4-trien-16-one (5) and the tetracyclic unsaturated ketone, (±)-3-methoxy-17,18,19,20-tetranor-B-homophylloclada-1(10),2,4,9(11)-tetraen-16-one (6), respectively. Catalytic hydrogenation of the cyclopropyl ketone (5) and of the unsaturated ketone (6) gave the product (7) in high yield.