A-Methoxyindole

Although the isolation of the principal alkaloid, ibogaine, of Taber-nanthe iboga was described at the turn of the present century (1), it was not until the early 1950 s that serious work on its structure was begun. It had been shown to contain a methoxy group and, by means of color reactions (19) and by measurement of its UV-spectrum, to be an indole (20), but it was not recognized to be a methoxyindole until permanganate oxidation was found to afford 5-methoxy-A-oxalylanthranilic acid (21). 

A new alkaloid (7) is the first example of a Gelsemium alkaloid having an A a-methoxyindole moiety in the molecule. This alkaloid may be an early biogenetic intermediate to the A a-methoxyoxindole alkaloids and their related compounds. Full assignments of the IH- and l C-NMR spectra of A7 a-methoxy-19(Z)-anhydrovobasinediol (7) were conducted mainly by CSCM ID (decoupled selective population transfer experiment) (18) and selective INEPT (insensitive nuclei enhanced by polarization tansfer) (19) experiments. The structure was finally determined by single crystal X-ray analysis (11). 

Belley and colleagues demonstrated a general catalytic hydrogenation method for the preparation of N-hydroxyin-doles (Scheme 14, equations 1-6) [90], which can be converted to A-methoxyindoles upon treatment with diazomethane. The reactions in equation 6 afford unstable 1,2-dihydroxyindoles, which are capped with diazomethane. The function of the palladium [(Ph3P) Pd] co-catalyst may... 

The above product (24 g, 0.067 mol) was dissolved in 90 10 dioxane-water (300 ml) and sodium borohydride (92.5 g, 0.067 mol) was added. The mixture was refluxed for 4h. The cooled solution was poured into 0.1 N HCl (1.11). A solid precipitated and was collected by filtration, dried and recrystallized from ether hexane to give 6,7-dibromo-4-methoxyindole (18.5 g, 90%). 

A solution of benzyl 5-methoxyindole-3-propanoate (26Og, 0.084mol) in... 

A solution of l,3-dimethyl-5-methoxyindole (4.5 g, 0.026 mol) in DMSO (27 ml) was maintained at as cone. HCl (23 ml, 0.77 mol) was added dropwise over 15 min. Stirring was continued for 3 h at room temperature and the reaction mixture was then poured into ice-watcr (100 ml). The mixture was neutralized vvith NaHCOj to pH 7 and extracted with EtOAc (100 ml x 2). The EtOAc was removed in vacuo and the residue purified by chromatography on silica using hexane-EtOAc (7 3) for elution. The yield was 4.35 g (88%). 

Another naturally occurring nucleoside antibiotic SF-2140 [93207-27-3] C H2qN20, a 3-cyanomethyl-4-methoxyindole nucleoside (55) is found to... 

Addition of the alcohol 42 to a solution of BF3 Et20/TMSCN in DCM provided the nitrile 43 in 83% yield. Hydrolysis of nitrile 43 then furnished amide 44 in 85% yield. Demethylation of the methoxyindole 44 with BBra in DCM provided the hydroxyindole 45 in 80% yield. This was followed by alkylation of 45 with the bromide 46 under phase transfer conditions to provide the phosphonate ester 47 and subsequent cleavage of the methyl ester by TMS-I furnished trimethylsilyl phosphonic acid 48, which upon alcoholic workup afforded LY311727. 

On the other hand, an electron-donating substituent destabilizes the 1-hydroxy-indole structure, often to the extent that it cannot be isolated. Even in such a case, alkylation of the 1-hydroxy group greatly improves the stability. Among alkylations, methylation is the best choice. This fact explains why every isolated natural product has a 1-methoxyindole structure (91YGK205, 99H1157). 

They have also developed a route to 2-allenylindole derivatives (98T13929). When prop-2-ynyl carbonates (76) are reacted with 73 in the presence of palladium catalyst, a cross-coupling reaction occurs to give 77a (46%) and 77b (45%). Under a pressurized carbon monoxide atmosphere (10 atm), the palladium-catalyzed reaction of 73 with 78 provides 79a (60%) and 79b (60%) (2000H2201). In a similar reaction, when the substrate is changed to aryl halides (80), 2-aryl-1-methoxyindoles such as 81a (70%) and 81b (60%) are prepared (97H2309). 

In fact, iodination of methyl l-methoxyindole-3-carboxylate (109), a wasabi phytoalexin (98P1959), with KI and NaI04 (60LA84, 911OC5903) in TFA-HjO provides methyl 5-iodo-l-methoxyindole-3-carboxylate (110,72%) predominantly... 

Pedras and co-workers (98P1959) isolated a phytoalexin from Wasabi (Wasabia japonica, syn. Eutrema wasabi) and determined its structure to be methyl l-methoxyindole-3-carboxylate (109) (Scheme 38). Compound 109 had already been synthesized by Acheson and co-workers [78JCS(P1)1117] in ten steps from o-nitroaniline. Pedras and co-workers (98P1959) combined our tungstate method and Acheson s work, and synthesized 109 in 9% overall yield but in an impure state. 

Daikon and wasabi phytoalexins are weak fungicidal alkaloids having a stabilized 1-methoxyindole structure. Relying on the expectation that more potent substances can be found among their derivatives, synthetic studies are in progress according to the method developed in Scheme 22 in Section IV.G. 

This method has been applied to a large-scale preparation of 6-bromoindole, which reacts with various arylboronic acids via the Suzuki reaction to afford 6-aryhndoles fEq. 10.50. 6-Bromo-5-methoxyindole for use in the synthesis of marine bromoindole " and 5-amino-7-ethoxycarbonyhndole for use in synthesis of l//-pyrrolo[3,2-g quina2ohne ring system fEq. 10.51 " have been prepared from the appropriate o-nitrotoluene. 

Tegaserod is metabolized mainly via two pathways. The first is a presystemic acid-catalyzed hydrolysis in the stomach followed by oxidation and conjugation, which produces the main metabolite of tegaserod, 5-methoxyindole-3-carboxylic... 

A mixture of 9.5 g pyrrolyl-2-aldehyde, 29.2 g dimethyl-succinate and NaH (9.6 g of 50% suspension in oil) in 100 ml benzene is stirred at room temperature 6 hours, cooled and carefully acidified with glacial acetic acid. Add water and ether and dry, evaporate in vacuum or work up (JACS 72,501 (1950), JCS 1025(1959)) to get ca. 17 g (80%) 3-methoxycarbonyl-4-(2 -pyrrolyl)-3-butenoic acid (I) (recrystallize-acetone-benzene). A mixture of 12 g (I), 7 g sodium acetate and 70 ml acetic anhydride is left overnight at room temperature with occasional shaking. Then gradually raise the temperature to 70-75° over 2 hours, maintain for 4 hours and work up (see JCS 1714(1955), 986( 1958)) to get ca. 8 g (60%) methyl-4-acetoxy-indole-6-carboxylate (II) (recrystallize-petroleum ether). If desired, this can be converted to 4-OH-indole-6-COOH and 4-methoxyindole-COOH as described in the ref. or decarboxylated as described elsewhere here. If the 1-methyl cpd. is used, 1-Me-indole results. 

Other indoles that have been prepared using the Sonogashira coupling and cyclization sequence include 5,7-difluoroindole and 5,6,7-trifluoroindole [219], 4-, 5-, and 7-methoxyindoles and 5-, 6-, and 7-(triisopropylsilyl)oxyindoles [220], the 5,6-dichloroindole SB 242784, a compound in development for the treatment of osteoporosis [221], 5-azaindoles [222], 7-azaindoles [160], 2,2-biindolyls [223,176], 2-octylindole for use in a synthesis of carazostatin [224], chiral indole precursors for syntheses of carbazoquinocins A and D [225], a series of 5,7-disubstituted indoles [226], a pyrrolo[2,3-eJindole [226], an indolo[7,6-g]indole [227], pyrrolo[3,2,l-y]quinolines from 4-arylamino-8-iodoquinolines [228], optically active indol-2-ylarylcarbinols [229], 2-alkynylindoles [176], 7-substituted indoles via the lithiation of the intermediate 2-alkynylaniline derivative [230], and a variety of 2,5,6-trisubstituted indoles [231], This latter study employs tetrabutylammonium fluoride, instead of Cul or alkoxide, to effect the final cyclization of 215 to indoles 216 as summarized here. 

The next three procedures provide useful synthetic intermediates. A stereospecific synthesis of ETHYL (Z)-3-BROMO-2-PROPENOATE affords an alternative vinyl bromide partner for the coupling chemistry in the preceding procedure. A very simple but elegant illustration of the flash vacuum pyrolysis technique is used to prepare BENZOCYCLOBUTENONE from o-toluoyl chloride. Another member of the functionalized indole family of synthetic intermediates is presented in a four-step procedure for 5-METHOXYINDOLE-2-ACETIC ACID METHYL ESTER. 

As a further illustration of the reactivity of the 3 position toward electrophiles, the methoxyindole (25-1) readily undergoes Mannich reaction with formaldehyde and dimethylamine to afford the aminomethylated derivative (25-2). Treatment of that intermediate with potassium cyanide leads to the displacement of dimethylamine and the formation of the nitrile (25-3), possibly by an elimination-addition sequence involving a 3-exomethylene-indolenine intermediate. The protons on the methylene group adjacent to the nitrile are quite acidic and readily removed. Reaction of (25-3) with methyl carbonate in the presence of sodium methoxide gives the carbo-methoxylated derivative (25-4). Catalytic hydrogenation leads to reduction of the nitrile to a primary amine. There is thus obtained the antihypertensive agent indorenate (25-5) [26]. 

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