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Synthetic studies in dihydroindole and indole alkaloids

University of British Columbia

A synthetic approach toward the synthesis of vindoline (3) and a reinvestigation of the total synthesis of vincaminoridine (4) and epivincaminoridine (4a) is described. The synthetic sequence involves alkylation with benzyl chloride of the monosodium salt of propane-l,3-diol to give y-benzyloxypropanol (197). Treatment of 197 with thionyl chloride afforded benzyl-y-chloropropyl ether (198). Alkylation of ethyl diethyl malonate with 198 provided diethyl Y~DenzyloxyProPyletnyl malonate (134). Basic hydrolysis of 134 gave y-benzyloxypropylethyl malonic acid (199), which upon decarboxylation provided 2-(y-benzyloxypropyl)-butanoic acid (200). The monoacid (200) was esterified with ethanol to provide ethyl tx-(y-benzyloxypropyl)-butanoate (135). Alkylation of 135 with allyl bromide gave ethyl-a-(y-benzyloxypropyl)-a-allyl-butanoate (201), which upon treatment with osmium tetroxide and sodium periodate gave ethyl a(y-benzyloxypropyl)-a-(a-formylmethyl)-butanoate (140). Condensation of 140 with 6-methoxy tryptamine afforded the tetracyclic lactam (150) . Lithium aluminum hydride reduction of the latter, followed by hydrogenolysis of the benzyl group gave two isomeric tetracyclic alcohols (204) . These intermediates were converted via their mesylate derivatives to the quaternary salts (205), which upon treatment with potassium cyanide gave the isomeric cyanides (216). Acid hydrolysis of 216 gave the corresponding carbomethoxy derivative (151). Alkylation of 151 with methyl iodide provided dl-vincaminoridine (4) and dl-epivincaminoridine (4a) . Transannular cyclization of the latter substances gave the pentacyclic aspidosperma-type system (195) . The degradation sequence involved acid hydrolysis of vindoline (3) to provide desacetyl vindoline (224), which upon catalytic hydrogenation gave desacetyldihydrovindoline (225) . Pyrolysis of 225 afforded the ketone (86), which upon treatment with dimethyl carbonate provided the g-ketoester (226) . Treatment of the sodium enolate of 226 with oxygen-hydrogen peroxide gave the hydroxy ketoester (227). Treatment of desacetyldihydrovindoline (225) with N,N-thiocarbonyldiimidazole gave the thiocarbonate derivative (230), which upon desulfurization with Raney nickel afforded the unsaturated ester (231) . Catalytic hydrogenation of 231 gave the saturated ester (232) , which upon treatment with lithium diisopropyl amide and oxygen-hydrogen peroxide provided the hydroxyester (234). The saturated ester 232 was converted to the alcohol derivative (237) by reduction with aluminum hydride. Oppenauer oxidation of 237 gave the aldehyde (238). Finally potassium permanganate oxidation of the unsaturated ester (231) gave 5-membered lactam (240), 6-membered lactam (241), N -formyl-5-membered lactam (242), ct and NQ-formyl-6-membered lactam (243) .

Text

2010-01-22

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http://hdl.handle.net/2429/19047

Chemistry

University of British Columbia

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