Synthesis of New Indolecarboxylic Acids Related to the Plant Hormone Indoleacetic Acid IAA

A síntese dos ácidos 5,6-metilenodioxi-indol-3-il-metanóico 8 e 5,6-metilenodioxi-indol-3-ilacético 13 é descrita. Piperonal foi empregado como material de partida, sendo a construção do heterociclo altamente regioespecífica e está fundamentada na reação de Hemetsberger do correspondente β-azidoestireno. O composto 8 foi obtido como intermediário pivotal na preparação de 13, tendo-se conduzida a reação de Mannich para a introdução da cadeia lateral alquílica. A rota sintética empregada englobou oito etapas e conduziu a formação de 13 com rendimento total de 26%. A formação do heterociclo indólico via ciclização redutiva de o, β-dinitroestireno é também apresentada.


Introduction
Plant growth regulators comprise a large number of structurally diverse compounds capable of regulating many biological processes, including cell division, differentiation and enlargement, chloroplast development and senescence.][5][6] However, the methylenedioxy group frequently found in many secondary metabolities has not received much attention.At this point, it is worth mentioning the work of Barreiro et al. 7 which focus on the preparation of indolecarboxylic acid analogue to the anti-inflammatory indomethacin from the methylenedioxyarene safrole.
Therefore, in the search for potential plant growth regulators from abundant natural products and their derivatives, presently is described the synthesis of new methylenedioxyindolecarboxylic acids structurally related to IAA.

Results and Discussion
For the synthesis of compounds with the general structure 4, Scheme 1, we considered two complementary main disconnections, D1 and D2, where a vinyl azide and o-β-dinitrostyrene would be the pivotal intermediates in the construction of the heterocyclic ring, respectively.Such compounds can be readily accessed by condensation reactions of the appropriate nucleophile and the commercially available piperonal 5, a derivative of safrole.
For the first synthetic strategy (D1) the Hemetsberger reaction 8 was employed (see Scheme 2), and this was initiated with the preparation of the vinyl azide 9 6 which, upon heating in refluxing xylene, generated the highly electrophilic singlet nitrene species. 102][13] Hydrolysis of indol 7 under typical reaction conditions provided the new indole 8, which could be regarded as a potential plant growth regulator since such a property has been associated to some aryl homocyclic carboxylic acids. 14For the synthesis of the IAA analogue, the indole unsubstituted heterocyclic ring 9 was required.This was achieved by decarboxylation of indole 8 in solid phase at high temperature, in the presence of barium hydroxide, with the product being obtained as an analytically pure compound, since it was separated from the reaction mixture by sublimation.
Before conducting the reactions for the preparation of the acetic acid derivative, it was decided to investigate the disconnection D2 as a means of accessing 9 without employing precursors substituted at the heterocyclic ring.Therefore, the use of the o, β-dinitrostyrene 11 was examined. 15,16This compound can be readily obtained by the condensation reaction of nitromethane and piperonal to give 10, followed by nitration (Scheme 2).Although 9 had already been prepared by Yang and Chen 17 in very high yield (94%), under conditions of catalytic hydrogenation, two other methods were considered.Palladium on carbon with cyclohexene as a source of hydrogen, 18 a procedure that had not been previously applied to this system, gave the desired compound in a poor 37% yield.Furthermore, the method has a strong drawback because it requires stoichiometric amount of palladium catalyst.The method of choice, following the literature procedure was the known reductive cyclisation 19 Scheme 1.

Scheme 2.
of 11 assisted by silicagel in a mixture of 1:3benzene:cyclohexane leading to compound 9, as shown in Scheme 2 in 72% yield (single experiment).The compound prepared in this way showed identical ( 1 H and 13 C) NMR spectra as, the compound obtained by decarboxylation of 8. Attempts to prepare 9 using toluene instead of the hydrocarbon mixture above afforded the desired product in very poor yield, different to that claimed in the literature. 20or the introduction of the alkyl side chain, 9 was submitted to a Mannich reaction 21 to give the expected tertiary amine 12a.In situ quaternization of 12a to provide a better leaving group followed by cyanide nucleophilyc displacement gave 12b (ν = 2240 cm -1 ) in very good yield.Finally, basic hydrolysis of 12b and subsequent acidic workup produced the desired indoleacetic acid 13. 22,23 he total synthesis of the target molecule 13 was accomplished in eight steps via the vinyl azide 6, in a significant overall yield of 26%.On the other hand, the reductive cyclisation of o, β-dinitrostyrene gave 13 in six steps, in an overall yield of 16%.
The plant growth regulatory properties of compounds 7, 8 and 13 are currently under investigation by means of in vitro and in vivo assays. 24,25perimental General Melting points were determined on Kofler melting point apparatus (Microquímica APF-301) and values were uncorrected.IR spectra were recorded with a Perkin-Elmer 781 Spectrophotometer in KBr. 1 H and 13 C NMR spectra were recorded using Brüker Ac 200 and 300 Spectrometers in solvents as indicated with Me 4 Si (TMS) as the internal standard.The mass spectra were obtained on a Shimadzu CGMS-QP-2000-A Spectrometer adapted with an EI source.The elemental analyses were obtained on a Carlo Erba-EA 1110 CHNS-O.Column chromatography was performed using silica gel (70-230 mesh), and the reactions were monitored by TLC (the plates were coated with Merck Kiesegel 60GF 254 silica gel).The visualization of the compounds on the chromatograph plates was achieved under ultraviolet light and exposure to iodine vapour.

Conclusions
The synthesis of 2 new indolecarboxylic acids incorporating the methylenedioxy subunit has been successfully achieved from the commercially available piperonal, an important derivative of the natural product safrole.Both nitrene insertion reaction from vinylazide and reductive cyclization from o-β-dinitrostyrene were efficient in the construction of the indole heterocyclic ring.The regulatory properties of compounds 7, 8 and 13 are currently under investigation to establish their potential as plant growth regulators.