European Journal of Chemistry ‐ European Journal of Chemistry Journal homepage: www.eurjchem.com Synthesis of ‐hydroxy‐ ‐methoxy‐ ‐pentyl‐ H‐isochromen‐ ‐one from Tessmannia densiflora Aamer Saeed Department of Chemistry, Quaid‐I‐Azam University, Islamabad, PK‐45320, Pakistan Corresponding author at: Department of Chemistry, Quaid‐I‐Azam University, Islamabad, PK‐45320, Pakistan. Tel.: +92.51.90642128; fax: +92.51.90642241. E‐mail address: aamersaeed@yahoo.com (A. Saeed). COMMUN)CAT)ON )NFORMAT)ON Received: July Received in revised form: Accepted: October Online: March ABSTRACT The synthesis of ‐hydroxy‐ ‐methoxy‐ ‐pentyl‐ H‐isochromen‐ ‐one isolated from the stem and root bark extracts of Tessmannia densiflora has been described. The reaction of , ‐ dimethoxyhomophthalic anhydride with hexanoyl chloride in the presence of , , , ‐ tetramethylguanidine and triethylamine afforded , ‐dimethoxy‐ ‐pentylisocoumarin . Regioselective demethylation of the latter using anhydrous aluminum chloride furnished the title isocoumarin . September KEYWORDS )socoumarin Tessmannia densiflora , ‐Dimethoxyhomophthalic acid Antimalarial , ‐Dimethoxyhomophthalic anhydride Mechanism 1. Introduction )socoumarins H‐ ‐benzopyran‐ ‐ones are natural lactones that are isolated from a wide range of natural sources microbes, plant, and insects and possess an array of biological activities including nephratoxic, hepatotoxic, protease inhibitory, antifungal, cytotoxic, immunomodulatory, antiallergic, and antimalarial activities [ ‐ ]. )socoumarins are isomeric to coumarins with an inverted lactone ring. Most of the natural isocoumarins posses a ‐alkyl C ‐C or a substituted ‐phenyl ring and , ‐dioxygenation due to their typical biosynthetic origin [ ]. During investigations for botanical insecticides for the control of malaria‐transmitting Anopheles gambiae mosquitoes, Nkunya and coworkers [ ] isolated a number of compounds from the stem and root bark extracts of Tessmannia densiflora (arms family Caesalpiniaceae that showed mosquito larvicidal activity. The isolated compounds were identified as nor‐halimane diterpenoid tessmannic acid and its methyl, ‐ methylisopropyl and ‐methylbutylesters, ‐pentyl‐ ‐methoxy‐ N‐butylaniline, and two unusual isocoumarins. The structures of the new isocoumarins were established unambiguously as ‐ hydroxy‐ ‐methoxy‐ ‐pentyl‐ H‐isochromen‐ ‐one Figure and ‐chloro‐ ‐hydroxy‐ ‐methoxy‐ ‐pentylisocoumarin, res‐ pectively, by analysis of spectroscopic data [ ]. As a continuance of our focusing on the synthesis, characterization, crystal structure, and bioevaluation of this important class of secondary metabolites [ ‐ ], a simple and efficient synthesis of the title compound was undertaken. The structural simplicity, coupled with the bioactivity associated with this molecule, makes it an attractive target for synthesis. The synthesis not only confirms the structural assignment but also make it accessible for comprehensive evaluation of its bioactivity. )SSN ‐ Figure 1. )socoumarin ‐hydroxy‐ ‐methoxy‐ ‐pentyl‐ H‐isochromen‐ ‐ one from Tessmannia densiflora. 2. Experimental 2.1. Instrumentation Melting points were recorded using a digital Gallenkamp SANYO model MPD BM . apparatus and are uncorrected. ( NMR and C NMR spectra were determined in CDCl solutions at M(z and M(z respectively using a Bruker AM‐ spectrophotometer. FT‐)R spectra were recorded using an FTS MX spectrophotometer, Mass spectra E), eV on a GC‐ MS instrument and elemental analyses with a LECO‐ C(NS analyzer. All compounds were purified by thick layer chromatography using silica gel from Merck Darmstadt, Germany . 2.2. Synthesis 2.2.1. 6,8‐Dimethoxy‐3‐pentylisocoumarin (3) A solution of , ‐dimethoxyhomophthalic anhydride 2, . g, . mmol in acetonitrile mL was added slowly to a solution of , , , ‐tetramethylguanidine TMG . mL, . mmol in acetonitrile mL , while maintaining the internal temperature ≤ °C. Triethylamine . mL, . mmol was European Journal of Chemistry Print / )SSN ‐ Online  DO): . /eurjchem. . . ‐ . EURJC(EM Saeed / European Journal of Chemistry 2 (1) (2011) 117‐119 added in a single portion followed by dropwise addition of hexanoyl chloride . mL, . mmol . The reaction mixture was further stirred for minutes, allowed to warm to ambient temperature and then quenched by addition of M (Cl mL . The organic layer was separated, washed with saturated brine dried and concentrated. The crude compound was purified by thick layer chromatography followed by recrystallization from methanol to yield isocoumarin 3 . g, . mmol, % Scheme . As a colorless oil; )R KBr, ν, cm‐ : , , , , , , , , . ( NMR CDCl , , ppm : . t, J= . (z, (, (‐ ’ , . m, (, (‐ ’ , . quin, J= . (z, (, (‐ ’ , . quin, J= . (z, (, (‐ ’ , . (, t, J= . (z, (‐ ’ , . s, (, MeO , . s, (, MeO , . s, (, ( , . d, J= . (z, (, (‐ , . d, J= . (z, . C=O , . C , (, (‐ . C NMR CDCl , , ppm : . C , . C , . C , . C , . C , . C , . C , . MeO , . MeO , . C ', . C ' , . C ' , . C ' , . C ' . MS m/z : , , , , , , . Anal. Calcd. for C ( O : C, . ; (, . . Found: C, . ; (, . . hexanoyl chloride furnished the , ‐dimethoxy‐ ‐pentyl‐ isocoumarin 3 in % yield. )socoumarin 3 exhibited the characteristic singlet for (‐ olefinic proton at . ppm, the triplet for (‐ ’ at . ppm J= . (z and the carbon signals at . C‐ and . C‐ and . C=O ppm. The δ‐ lactonic carbonyl stretching in the )R spectrum appeared at cm‐ . The construction of isocoumarin 3 from the anhydride 2 may be visualized by the loss of a benzylic proton to base to afford the resonance stabilized anionic species 2a and 2b , which attacks the hexanoyl chloride to give the ‐ hexanoyl‐ , ‐dimethoxyisochroman‐ , ‐dione intermediate ). Loss of proton to the base affords the species )) a, b, which upon intramolecular O‐acylation furnishes the tricyclic intermediate ))), which under basic influence undergoes ring opening to afford )V. Decarboxylation followed by isomerization of the latter provided the isocoumarin 3 Scheme . Scheme 1 2.2.2. 6‐Methoxy‐8‐hydroxy‐3‐pentylisocoumarin (1) Aluminum chloride . g, . mmol was added to a stirred solution of 3 . g, . mmol in freshly distilled dry nitrobenzene mL . The reaction mixture was stirred at ‐ °C for h, then poured into ice water and acidified with . N (Cl. The acidic solution was extracted with diethyl ether x mL and stirred for min. The layers were separated and the aqueous layer extracted with dichloromethane x mL and then the combined organic extracts washed with . M NaO( x mL . The basic solution was extracted with diethyl ether, acidified and again extracted with diethyl ether. The last extract was evaporated and residue purified by thick layer chromatography petroleum ether:ethyl acetate : to afford 1 . g, . mmol, % . M.p.: ‐ °C Lit. [ ]: ‐ °C . ( NMR CDCl , , ppm : . t, J= . (z, (, (‐ ’ , . m, (, (‐ ’ , . quin, J= . (z, (, (‐ ’ , . quin, J= . (z, (, (‐ ’ , . t, J= . (z, (, (‐ ’ , . s, (, MeO , . s, (, ( , . d, J= . (z, (, ( , . d, J= . (z, (, ( , . C=O , . . (, br s, O( . C NMR CDCl , , ppm : C , . C , . C , . C , . C , . C , . C , . C , . MeO , . C ' , . C ' , . C ' , . C ' , . C ' . MS m/z : , , , , , . Anal. Calcd. for C ( O : C, . ; (, . ; found C, . ; (, . . 3. Results and discussion , ‐Dimethoxyhomophthalic acid [ ] was efficiently converted into corresponding anhydride 2 by refluxing with acetic anhydride in dry toluene. The anhydride 2 in acetonitrile was added to a solution of , , , ‐tetramethyl guanidine TMG in the same solvent at °C followed by addition of triethylamine [ ]. Treatment of the reaction mixture with Scheme 2 The C‐ methoxyl in , ‐dimethoxy‐ ‐methylisocoumarin 3 was regioselectively demethylated using anhydrous aluminum chloride in dry nitrobenzene [ ‐ ], due to hydrogen bonding of the resulting hydroxyl with the periplanar lactonic carbonyl to yield the ‐hydroxy‐ ‐methoxy‐ ‐ pentylisocoumarin 1 . )n addition to the disappearance of the C‐ methoxy group at . ppm, the downfield shift of singlet for (‐ and the triplet for (‐ ’ to . and . ppm J= . (z , respectively, was detected. A similar shift for C‐ and C‐ . and . ppm, respectively was also noted. The lactonic carbonyl absorption was also lowered to cm‐ due to chelation with C‐ hydroxyl which appeared at . ppm. 4. Conclusion An efficient synthesis of a natural isocoumarin has been achieved. A likely pathway for formation of isocoumarin 3 from anhydride 2 has also been proposed. Acknowledgement The author gratefully acknowledges a research grant from the (igher Education Commission of Pakistan under project No. ‐Miscel/R&D/ / . References [ ]. [ ]. [ ]. [ ]. Zhang, W.; Krohn, K.; Draeger, S.; Schulz, B. J. Nat. Prod. 2008, 71, ‐ . Devienne, K. F.; Raddi, G.; Coelho, R. G.; Vilegas, W. J. Phytother. Phytopharm. 2005, 12, ‐ . Di Stasi, L. C.; Camuesco, D.; Nieto, A.; Vilegas, W.; Zarzuelo, A.; Galvez, J. Planta Med. 2004, 70, ‐ . Kostova, ). Curr. Med. 2005, 5, ‐ . Saeed / European Journal of Chemistry 2 (1) (2011) 117‐119 [ ]. [ ]. [ ]. [ ]. [ ]. [ ]. [ ]. [ ]. [ ]. [ ]. [ ]. [ [ [ ]. ]. ]. Krohn, K.; Florke, U.; Rao, M. S.; Steingrover, K.; Aust, (.‐J.; Draeger, S.; Schulz, B. 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