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Synthesis of an advanced welwitindolinone intermediate via an alkylation/cyclization reaction is reported. The key step involves a one pot Lewis acid mediated alkylation of a silylketene aminal with a furan alcohol followed by an intramolecular cyclization. The reaction is stereoselective and takes place at low temperature. The cycloadduct is highly functionalized and contains the welwitindolinone core structure.
Multiple-drug-resistance (MDR) is known to operate via several pathways to diminish the effectiveness of anti-cancer medicines used in chemotherapy.1 Drug efflux via upregulated production of P-glycoprotein, a membrane transport protein for lipophilic molecules, is the most commonly observed MDR mechanism.2 Agents that inhibit P-glycoprotein and/or are active against MDR cells have significant value in cancer treatment due to their potential application as single and combination therapies in the treatment of MDR tumors. 3
In studies directed toward the discovery of biologically active natural products of blue-green algae (cyanobacteria), Moore isolated the welwitindolinone alkaloids (Figure 1) from Hapalosiphon welwitschii and Westiella intricata.4 The welwitindolinone family is divided into three subgroups: welwitindolinone A (1), welwitindolinones B (2-4), and welwitindolinones C (5-7). N-Methylwelwitindolinone C isothiocyanate (7), the major component of the lipophilic extract, was found to possess anti-fungal activity and reverse MDR in vinblastine resistant cell lines through inhibition of the drug-efflux ability of P-glycoprotein.5 Welwitindolinone C 6 was later identified as a tubulin-interacting agent demonstrating activity against drug resistant tumors.6 Structurally, these molecules are comprised of an oxindole ring linked to a terpenoid ring system containing three quarternary centers, chlorine substitution, and either an isonitrile or isothiocyanate moiety. In contrast to the other B and C welwitindolinones, 4 is epimeric at C-3.
Elegant syntheses of welwitindolinone A isonitrile (1) have been completed in the laboratories of Wood7 and Baran.8 However, in spite of many strategies,9,10 no total syntheses of the B and C welwitindolinones have been reported.
In an earlier report our laboratory disclosed a method for the synthesis of the [4.3.1] welwitindolinone core using a type 2 intramolecular Diels–Alder (IMDA) reaction.10i Although this route established proof of concept by providing short access to the welwitindolinone skeleton, we sought to develop a more expedient strategy for preparing the welwitindolinones.
The retrosynthetic strategy for the total synthesis of N-methylwelwitindolinone C isothiocyanate (7) constructs the bridged tetracyclic system using an intramolecular cycloaddition reaction (Scheme 1). Compound 8 possesses the welwitindolinone core structure, several functional handles, and could be accessed from either the bridged pentacycle 9 or 10. The key transformation in our strategy is the formation of pentacyclo[4.3.1] oxindole 9 or 10 via cycloaddition of intermediate 11 or 12, respectively, which in turn was envisioned to arise from alkylation of silyl ketene aminal 15 with substituted furan 13 or 14. Compound 15 would arise from vinyl oxindole 16, which would be derived from N-Boc-4-bromoindole (17).11
The synthesis began from the known indole 17 prepared from 4-bromo-indole12 (Scheme 2). N-Boc-4-bromo-indole (17) was oxidized to afford N-Boc-4-bromo-oxindole (18) via 2-indolyl boronic acid.13 4-Bromo-oxindole (19),14 prepared by removal of the Boc group of oxindole 18, was N-methylated to yield 4-bromo-N-methyl-oxindole (20). Vinylation of oxindole 20 with diethylfumarate via a Heck coupling15 furnished oxindole 16 as a mixture of olefin isomers. Vinyl oxindole 16 was treated with TBSOTf to form silyl ketene aminal 15 in situ. Compound 15 subsequently underwent alkylation upon treatment with furan alcohol 13 (R = H)16 to yield Diels–Alder precursor 11. Experimental evidence suggests that the acid byproduct of the silylation reaction facilitates ionization of the furan alcohol to produce the reactive electrophilic species.17 At this low temperature cyclization of intermediate 12 produced oxa-bicyclo[4.3.1] adduct 9 as a single diastereomer in 62% yield from 16 in a one pot cascade reaction. Cycloadduct 9 contains two all–carbon quaternary centers, and all but two carbon atoms of the B and C welwitindolinones. The relative configuration of the stereogenic carbons of cycloadduct 9 were confirmed by X-ray crystallography. The esters, oxygen bridge and C-3 hydrogen are positioned on the same face of the molecule. Hydrogenation of cycloadduct 9 produced oxo-bridged 21 as a single diastereomer via addition exclusively to the exo face of the bicyclic system.
Based on an analogy to a related system using LiHMDS,18 we envisioned a base-induced ring opening of 21 to afford 22 via an elimination mechanism. Unfortunately, the desired product was not formed. Treatment of 21 with 1.3 equivalents of LIHMDS followed by quenching with ethanol-d6 resulted in complete and exclusive incorporation of deuterium at C-3 to yield 23. This deuterium labeling experiment indicated that the C-3 proton of the oxindole was removed selectively over the C-12 proton. Treatment of 21 with up to four equivalents of base did not result in the formation of 22.
An alternative synthetic route employing silyloxyfuran 14 (R = OTIPS) for the alkylation of silylketene aminal 16 was pursued (Scheme 1). This sequence accesses ketone intermediate 8 in fewer steps and circumvents the problematic base mediated oxo-bridge opening reaction through the use of a labile silyl ketal.
Silyloxyfuran 14 was prepared by from furan 2419 via metal–halogen exchange followed by 1,2–addition to acetone (Scheme 3). Due to the sensitivity of 14 to the acidic conditions used in the alkylation of dessilyloxyfuran 13, an alternative alkylation strategy was devised. Silylketene aminal 15 was isolated as the Z-isomer exclusively from a 7:1 (Z:E) mixture of 16.20 Lewis acids and solvents were evaluated for their ability to effect the desired alkylation reaction. ZnI2 in acetonitrile enabled the alkylation of 15 with furan alcohol 14 to prepare furan 12 which underwent intramolecular cyclization to yield cycloadduct 10 as single diastereomer in 41% yield. The relative stereochemistry of 10 was verified by X-ray crystallography (Figure 2). This cascade reaction prepares a highly functionalized welwitindolinone core structure containing two all–carbon quaternary centers, sets two stereocenters, and assembles all but two carbon atoms of the B and C welwitindolinone skeleton from simple materials in short fashion. Desilylation of 10 with HF yielded a 25:1 mixture of the desired alcohol 8 to lactone 25. Compound 8 was observed to isomerize to lactone 25 under basic solutions or during silica gel flash chromatography.21
Although the configuration of compound 8 at C-3 matches 4, it is epimeric relative to the other welwitindolinones. Molecular modeling studies were pursued to gain insight concerning the relative stabilities about the epimerizable center at C-3 (Figure 3). Compound 26 with C-3 hydrogen beta was calculated to be 4.4 kcal/mol lower in energy than the C-3 epimer (epi-26) (DFT PBEPBE 6-311G*). At this same level of theory, the natural diastereomer of N-methylwelwitindolinone C isothiocyanate (7) is calculated to be 3.9 kcal/mol lower in energy than its C-3 epimer (DFT PBEPBE 6-311G*). This result suggests that an advanced synthetic intermediate can adopt the α-H configuration at C-3.
In conclusion, a highly functionalized welwitindolinone core structure 8 has been prepared in 8 steps from N-Boc-4-bromoindole (17) using an alkylation/cyclization cascade reaction. The relative stereochemistry of 9 and 10 has been confirmed by X-ray crystallography. Future work will focus on installation of the remaining quaternary center and functional group manipulations to prepare N-methylwelwitindolinone C iosthiocyanate (7) and the related welwitindolinones.
This research was supported by a grant from the National Institutes of Health. We also wish to thank Dr. Joseph W. Ziller (UCI) for X-ray crystallography, and Dr. John Greaves (UCI) for mass spectral data.
Supporting Information Available: Experimental procedures, spectroscopic data (1H, 13C) for new compounds, and CIF files for compounds 9 and 10. This material is available free of charge via the Internet at http://pubs.acs.org.