HETEROCYCLES

Short Paper | Regular issue | Vol. 85, No. 7, 2012, pp. 1727-1733
Received, 24th May, 2012, Accepted, 29th May, 2012, Published online, 6th June, 2012.
DOI: 10.3987/COM-12-12516

■ Double Intramolecular 1,3-Dipolar Cycloaddition of Diazo-terminal Dialkyne: Synthesis of a New Bis(1,2,3-triazolo-1,4-oxazine)

Mekni Husein Nejib* and Ahmed Baklouti

Department of Chemistry, Faculty of Scince, University of Tunis, Almanar, 2092 Tunis, Tunisia

Abstract

A new bis(1,2,3-triazolo-1,4-oxazine) fused heterocyclic compound was synthesized from terminal meso-1,2,3,4-diepoxybutane via azide ring opening reaction, followed by propargylation and double 1,3-dipolar intramolecular cycloaddition. No effect on the reaction outcome was observed when Cu(I) was used as catalyst. In both cases catalyzed and uncatalyzed reactions, only the exo-tetracyclic two to two fused isomer was obtained.

1,2,3-Triazoles and oxazines, are heterocycles which constitute a large number of synthetic compounds.1 Applications of these heterocycles in various areas have been reported.2,3 For example, they are used as an anti-HIV,4 anti-allergic,5 antibacterial,6 fungicides,7 herbicides,8 paints,9 corrosion inhibitors,10 pesticides11 and agrochemical agents.3,12 For oxazines and their derivatives, a particular interest has emerged since the discovery of Efavirenz (trifluoromethyl-1,3-oxazin-2-one) inhibitor non-nucleoside reverse transcriptase used as a selective anti-HIV drug.13
On the other hand, the 1,3-dipolar azide-alkyne cycloaddition is a well known reaction14 which has been recently applied in the "Click Chemistry".15 A remarkable development is potentiated by the use of catalytic metals such as (Cu+)16 and (Ru2+).17 The majority of azide/alkyne reaction studies deal with intermolecular cycloadducts.18 However those related to the intramolecular processes remain limited.19,20 Furthermore, previous studies have focused on the synthesis of target and specific molecules rather than on a general systematic investigation of its potential application in organic synthesis.19,21
In the present work, we describe the synthesis of one compound having four hetrerocycles, two to two fused, in three steps22 starting from diepoxybutane.23,24
The preparation of the meso-diazido-dialkyne ether 2 was carried out through opening of the meso-diepoxybutane rings 1 by azide ion followed by reaction of propargyl bromide (Scheme 1). The meso-diazido-dialkyne ether 3 was obtained in satisfactory yields (81%).
The meso-diazido-dialkyne 3 is unstable and decomposes on contact with air. Therefore, the mixture obtained from the propargylation reaction was diluted in situ in dry toluene and used in situ without purification in the next step.
The uncatalyzed dipolar [3+2] double intramolecular cycloaddition reaction of product 3 proceeds spontaneously at room temperature, producing exclusively the exo-tetraheterocyclic compound 4. To reduce the reaction time, the mixture was heated at 50 °C in toluene (A Condition25). In an attempt to direct the reaction towards the formation of another polyheterocyclic compound, the reaction was performed in the presence of a catalytic amount of CuCl (5%) (B Condition26).
Either carried out according to A (79%) or B (75%) conditions, the double intramolecular cycloaddition reaction of diazido-dialkyne ether 3 leads always to the formation of the same isomer 4 (Scheme 1).

It is worth to note that several conformers are possible for compound 3, due to the free rotation around the CH-CH central bond. Because of the free rotation around the two CH-O bonds, the stable conformation can exist in three geometric forms (a), (b) and (c). From these forms, two exo-cyclic isomers may be formed 4 and 5 (Scheme 2).
The (a) form has both azide and alkyne dipoles one in face to the other with a favorable orbital overlap flatness which explain the exclusive formation of the exo-tetracyclic isomer 4.
For the (b) form, the α part of favorable flatness cyclizes easily (Scheme 2). So in the β side, the propargylic group must rotate around C-O to cyclize with the second azide group and would lead once again to the isomer 4. The (c) form does not afford a favorable orbital overlap which can explain the absence of the exo-cyclic isomer 5. Therefore, (c) turns into (b) and/or (a) to give also compound 4.

The 1H and 13C NMR spectral data confirm the symmetrical structure proposed. In particular the 1H NMR spectrum shows an AB system corresponding to the allylic protons O-CH2-C=C, appearing into 5.0 ppm (2JAB = 15 Hz). Moreover, the -CH2-N protons show another AB system (2JAB ∼ 12 Hz) which is partially coupled with the proton of the asymmetric carbon.

ANTIBACTERIAL TEST
The preliminary antibacterial activity study of compound 4 was followed by the disk diffusion method against five bacteria (Gram+ and Gram-) of widely encountered references in many human diseases.27 The results are given in Table 1.
As can be seen from Table 1, it appears that compound 4 has no activity against Escherichia coli (DH5α), Pseudomonas aeruginosa (PAO1), Salmonella typhi (ATCC 25922) and Staphylococcus aureus (ATCC 6538). However, it presents a moderate activity against Enterococcus feacium strain ATCC (19,436) for which inhibition diameter is 11 mm. This value is the average of three experimental tries (10 mm, 11 mm and 12 mm).

The exo-tetraheterocyclic isomer 4 was obtained exclusively from simple meso-diepoxybutane, via mild reaction series in which the latter is a double intramolecular cycloaddition.
In addition, the exo-cyclic 1,2,3-triazolo-1,4-oxazine 4, is an aromatic heteropolycyclic compound combining four fused two to two heterocycles that are well known for their wide area of applications. The title compound may present a priori interesting applications28,29 with preliminary antibacterial tests showing a moderate activity towards the Enterococcus feacium bacteria.

EXPERIMENTAL
IR spectra were realized on Perkin Elmer Paragon 1000 PC spectrometer. The NMR 1H and 13C spectra were realized on a Bruker AC 300 spectrometer respectively at 300 and 75 MHz. The TMS was used as a standard reference for 1H and 13C NMR spectra. HRMS spectra was realized on a MAT 95 SBE spectrometer in C. I. mode. The silica gel is of the Merck 7734 type, meso-diepoxybutane, sodium azide, propargyl bromide, CH2Cl2, toluene and Na2SO4 are Fluka commercial products.
Preparation of azido-alcohol 2: A solution of (3.44 g, 40 mmol) of meso-diepoxybutane 1, 10 g of sodium azide, 2.5 g of ammonium chloride in 25 mL of water and 100 mL of MeOH was stirred for 16 h at 80 °C, then filtered and the MeOH was evaporated. The mixture was extracted with Et2O (3 x 100 mL), washed with water and dried on Na2SO4. Et2O was evaporated, the obtained crude product was purified on silica gel chromatographic column, using Et2O as eluant to obtain pure compound 2 as yellowish viscous liquid.
Synthesis of diazido-dialkynes 3: To a mixture of sodium hydroxide (4.80 g, 120 mmol), water (0.4 mL), tetrabutylammonium hydrogen sulfate TBAHS (0.2 g) and 120 mmol of propargyl bromide, vigorously stirred and cooled at 0 °C, (30 mmol, 5.16 g) of azido-alcohol 2 was added slowly. The mixture was stirred for 45 min at 0 °C, then 20 mL of CH2Cl2 was added. The mixture was filtered and the salt washed with CH2Cl2 (3 x 20 mL). After draying on Na2SO4, the mixture was filtered, then 50 mL of dry toluene was added. The CH2Cl2 and the excess of propargyl bromide were evaporated under vacuum. The crude product was conserved in toluene and used in the next step without purification.
Synthesis of bis(1,2,3-triazolo-1,4-oxazine) 4: The crud product of 3 conserved in dry toluene was heated for 2 h at 50 °C (A: without Cu+; B in the presence of: 5% of CuCl, 0.3 g). Toluene was evaporated and the obtained solid was purified by recrystallization in MeOH to give isomer 4 as a "color" solid (A: 79%, 5.88 g; B: 75%, 5.58 g).
6,6’7,7’-Tetrahydro-4H,4’H-6,6’-bi[1,2,3]triazolo[5,1-c][1,4]oxazine (4): mp 221 °C IR (KBr): νC=C = 1453, νN=N = 1495 cm-1; 1H NMR (DMSO): δ 4.63-4.38 (m, 4H, 2CH-CH2-N, 2JAB = 12 Hz), 4.30 (m, 2H, 2CH), 5.13-4.86 (d. d. 4H, 2O-CH2-C=, 2JAB = 15 Hz), 7.58 (s, 2H, C=CH); 13C NMR (DMSO): δ 51.80 (2CH2), 67.20 (2CH2), 75.70 (2CH), 128.60 (2C=), 128.90 (2CH=); HRMS Calculated: 248,10233, Found: 248,10303, Δ(umm): +0.7; Anal. Cacd for C10H12N6O2: C 48.37%, H 4.87%, N 33.86%. Found: C 48.31%, H 4.89%, N 33.78%.

Microorganisms
Antibacterial activity was tested using the disc diffusion method.24 A loop of bacteria from the agar slant stock was cultured in nutrient broth overnight and the inoculates grown were diluted to approximately 2 x 106 CFU/mL in molten nutrient agar. The concentration of the suspension used for inoculation was standardized by adjusting the optical density to 0.5 at 570 nm wavelength (spectrophotometer UV/visible). Sterile filter paper discs (6 mm in diameter) impregnated with the solution were placed on the cultured plates and incubated at 37 °C. The solvent (DMSO) without extracts served as negative control. Standard antibiotic of Gentamicin (10UI) was used as positive controls. After 24 h of incubation, the diameter in mm of the inhibitory or clear zones around the disks was recorded. Each test was repeated three times and the average was reported.

ACNKNOWLEDGEMENTS
The authors wish to thank the Tunisian Ministry of High Education and Scientific Research and Technology for financial support (LR99ES14) of this research, Dr. Feten MEDINI for Antibacterial Tests and Dr. M. A. K. Sanhoury, MRSC from the Department of Chemistry, Faculty of Science of Tunis for technical assistance.

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