RING-CLOSURE OF ENAMINONES VIA INTRA-MOLECULAR ELECTROPHILIC ARYLATION INVOLVING BENZYNE INTERMEDIATES AND ITS USE IN THE SYNTHESIS OF GAMMA-LYCORANE AND RELATED COMPOUNDS

Reaction of a series of halo enaminones with lithium amide was investigated. Treatment of the (V-phenyl enaminones with the ortho bromine atom (4 and 5) or the meta bromine atom (8) with phenyllithium and secondary amines resulted in halogen-metal exchange or benzyne formation followed by amination reaction, respectively. Upon similar treatment with lithium diethylamide, 3-(2-bromo-4, 5-methylenedioxybenzyl)aminocyclohex-2-en-1-one (17) and its 5, 5-dimethyl analogue 18 underwent intramolecular cyclization via benzyne intermediates to give the 3, 4-dihydro-l(2H)-phenanthridone 20 and 21, respectively. When this reaction was carried out with the N-ethyl derivative of 18 (i.e., 22), the 3, 4, 5, 6-tetrahydro-l(2H)-phenanthridone (23) was obtained. In the competing formation between five- and seven-membered rings on the benzyne cyclization using the N-phenethyl analogues 27 and 31 as substrates, N-arylation leading to the five-membered ring predominantly occurred to give the indoline derivatives 28 and 32, respectively, rather than the azepine ring formation. The same result was obtained even if the N-ethyl derivative of 27 (i.e., 29) was used, that being the simple indoline, l-ethyl-5, 6-dimethoxy-2, 3-trimethylsilyl (30). In order to extend the synthetic utility of the benzyne cyclization, appropriate enaminones, l-(2-halo-4, 5-methylenedioxybenzyl)-l, 2, 3, 3a, 4, 5-hexahydro-6H-indol-6-ones (42a and 42b), were conveniently synthesized by the reaction of 3, 3a, 4, 5-tetrahydro-6-methoxy-2H-indole (35) with respective 2-bromo- or 2-iodo-4, 5-methylenedioxybenzyl chlorides. Upon treatment with lithium diethylamide, these halo enaminones were efficiently cyclized to 3, 3a, 4, 5, 6, 7-hexahydro-9, 10-methylenedioxypyrrolo[3, 2, l-de]phenanthridin-l(2/7)-one (45). Air oxidation of 45 provided 3, 3a, 4, 5-tetrahydro-9, 10-methylenedioxypyrrolo[3, 2, l-de]phenanthridine-l, 7(2H)-dione (46), which has been appreciated as a key intermediate to a-anhydrodihydrocaranine and 7-lycorane. On the other hand, reduction of 45 with lithium aluminum hydride afforded (±)-α-dihydrocaranone (47) and (±)-l-epi-γ-dihydrocaranine (48). © 1979, American Chemical Society. All rights reserved.