Synthetic studies towards complex diterpenoids. Part VIII. Stereo-controlled synthesis of (±)-2-methoxy-11β-methyl-4bαH-gibba-1(10a),2,4-trien-8-one and related compounds through intramolecular alkylations of γδ-unsaturated α′-diazomethyl ketones

Abstract

Like the corresponding 1-demethyl analogues, the α′-diazomethyl ketones 2β-diazoacetyl-7-methoxy-1β-methyl-1,2,3,4-tetrahydrofluorene and -1,2,3,4,9,10-hexahydrophenanthrene (1a and b) undergo smooth intramolecular oxocarbenoid addition and boron trifluoride–ether-catalysed cyclisation, leading to the cyclopropyl ketones 2-methoxy-11β-methyl-4bβ,9-cyclogibba-1(10a),2,4-trien-8-one (2a) and 3-methoxy-14β-methyl-9β,15-cyclo-17,18,19,20-tetranorphylloclada-1(10),2,4,-trien-16-one (2b) and the tetracyclic ketones 2-methoxy-11β-methylgibba-1(10a),2,4,4b-tetraen-8-one (3a) and 3-methoxy-14β-methyl-17,18,19,20-tetranorphylloclada-1(10),2,4,9(11)-tetraen-16-one (3b), respectively. Catalytic hydrogenation of the cyclopropyl ketones (2a and b) and of the unsaturated ketones (3a and b) produced 2-methoxy-11β-methyl-4bαH-gibba-1(10a),2,4-trien-8-one (4a) and 3-methoxy-14β-methyl-9αH-17,18,19,20-tetranorphylloclada-1(10),2,4-trien-16-one (4b).