A one pot synthesis of [1,3,4]-oxadiazoles mediated by molecular iodine

Abstract

A one-pot synthesis of 3-amino-1,3,4-oxadiazoles has been achieved from the corresponding dithiocarbamate salt, employing the thiophilic property of molecular iodine. The precursor thiosemicarbazides could be derived in situ which underwent an intramolecular cyclodesulfurization in the presence of iodine to afford 3-amino-1,3,4-oxadiazoles exclusively. Apart from being milder and environmentally sustainable, this method involves a simple, reliable approach to give good to excellent yields of the desired products and is compatible with a wide range of functional groups.

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Introduction

The N, O, and S heterocycles are of considerable importance due to their prevalence in biologically active compounds.¹ Amongst these heterocycles, [1,3,4]-oxadiazole scaffold is established as a member of the privileged structures class particularly from a medicinal point of view. Due to their metabolic profile, improved pharmacokinetics and in vivo performance they are recognized as biologically important pharmacophores.^2–5 Their biological activities encompass analgesic, anti-inflammatory, anticonvulsive, diuretic, antineoplastic, antimitotic, antiemetic and muscle relaxant properties.² Some display insecticidal^3a and herbicidal^5b properties as well. Besides, 2,5-diaryl-[1,3,4]-oxadiazoles exhibit inhibition toward platelet aggregation,^4a while derivatives of 2-hydroxyphenyl-[1,3,4]-oxadiazole possess hypnotic and sedative properties.^4b The wide spectrum of biological importance has led to the emergence of a number of marketed medicines bearing a [1,3,4]-oxadiazole core (Fig. 1). Instances of medicinally important compounds containing a [1,3,4]-oxadiazole core are MK-0633 p-toluenesulfonate,^5a nesapidil,^5b,c ABT-751-oxadiazole,^5d and furamizole^5e,f (Fig. 1). Apart from their latent potential in medicinal chemistry they also find applications in the field of material science and have received much attention particularly in the field of organic electronics because of their good electron-transporting and hole-blocking abilities.⁶

Fig. 1 Some pharmaceutical compounds having oxadiazole scaffold.

The widespread applications of 2-amino-[1,3,4]-oxadiazoles have resulted to they being encountered through the development of various protocols. Several methodologies that have been documented in the literature, most commonly employ the cyclodehydration of semicarbazides or the cyclodesulfurization of thiosemicarbazides as two main strategies. The cyclodehydration processes are mediated typically by POCl₃,^7a–d SOCl₂,^7e conc. H₂SO₄,^7f–h or tosyl chloride.⁷ⁱ Alternatively, phosphonium salts,^8a–d and Burgess-type reagents^8e–f are also found to be equally effective in promoting dehydrative cyclization. In the case of thiosemicarbazides as oxadiazole precursors, thiophilic reagents such as salts of mercury^9a,b and lead^2b have been used extensively to accomplish the desired desulfurative cyclization. Alternatively, activation of sulfur using iodine in the presence of strong base (NaOH)^10a–c or tosyl chloride in combination with triethylamine^10d or toxic pyridine,^2d have been performed under refluxing conditions. Coupling reagents such as EDC^11a,b and DCC^11c–e as well as alkylating agents such as methyl iodide^11f,g and ethyl bromoacetate^11h have been exploited to afford various oxadiazoles. The [1,3,4]-oxadiazoles have also been accomplished through a multicomponent reaction involving a Ugi-4CR/aza-Wittig sequence.¹² Very recently our group has also developed a protocol for the synthesis of substituted oxadiazoles through a copper catalyzed imine bond activation/C–O bond formation.¹³ It is noteworthy to mention here that in the latter two protocols the oxadiazole derivatives are devoid of a 2-amino functionality.

Although numerous procedures are enumerated in literature for the synthesis of oxadiazoles, most of them suffer a major set back from the standpoint of environmental concerns as they involve direct or indirect use of strong alkaline or acidic conditions, toxic and corrosive reagents, and emission of odoriferous thiol as a byproduct which are undesirable in pharmaceuticals. Besides expensive reagents and catalysts, elevated temperatures and longer reaction times are also some of the associated drawbacks. In addition, a number of methods seem to have limited substrate scope, resulting in the stoichiometric formation of intractable byproducts which impedes their generality. Thus a more sustainable synthesis for their synthesis is of prime importance.

In recent years, molecular iodine has been identified as an effective reagent to carry out various organic transformations owing to its inexpensive, ready availability and eco-friendly nature.¹⁴ Our interest to explore the thiophilic properties of hypervalent iodine and molecular iodine toward relevant synthetic transformations¹⁵ has led us to envisage a tandem one-pot synthesis of [1,3,4]-oxadiazol-2-yl-amines. Our anticipated tandem synthetic sequence involves the in situ generation of isothiocyanate from the dithiocarbamate salt using molecular iodine. This would then form thiosemicarbazide upon treatment with acyl hydrazide in the same pot and upon reaction with further equivalent of iodine would undergo cyclodesulfurisation to render the corresponding 2-amino-[1,3,4]-oxadiazole.

Results and discussion

In one of our recent achievements^15f we have seen that the dithiocarbamate salt upon treatment with iodine and inorganic base such as K₂CO₃ or Na₂CO₃ in a biphasic medium of ethyl acetate and water afforded the corresponding isothiocyanate. Taking cues from this observation, an initial experiment was performed using freshly prepared phenyl dithiocarbamate salt (1 equiv.) in ethylacetate/water medium, to which was sequentially added K₂CO₃ (3 equiv.) and iodine (1.1 equiv.) pinchwise over a period of 10 min. A complete consumption of salt to isothiocyanate was observed (judged by TLC) along with the precipitation of colloidal sulfur. To the same reaction mixture formic acid hydrazide (1 equiv.) was added and the formation of thiosemicarbazide (1) was detected in the reaction mixture. Addition of a further 1.1 equiv. of iodine and stirring the reaction mixture for 0.5 h resulted in formation of the product (1a) to our delight in a good yield of 77% isolated yield (Table 1, entry 1). It is worth mentioning here that further investigations toward the optimization of the reaction conditions with respect to the appropriate solvent systems suggested that the EtOH/water system was equally effective in promoting the reaction (Table 1, entry 2). Other solvent systems such as acetonitrile/water (Table 1, entry 3) or THF/water (Table 1, entry 4) or simple organic solvents (Table 1, entry 5–8) either failed or gave poorer yields of the product. This substantial decrease in yield could be attributed to the inefficient formation of isothiocyanate or insolubility of base in them. Thus, after subsequent trials we figured out that EtOAc/water was most suitable for this transformation to meet out with the work up procedures in the EtOH/water system. Among the inorganic bases, Na₂CO₃ gave slightly poorer yield while (Table 1, entry 9) K₂CO₃ and Cs₂CO₃ (Table 1, entry 10) were found to be almost equally effective but from cost consideration K₂CO₃ was used. Since we were focusing on the use of milder inorganic carbonates, organic bases were not tried for these reactions.

Table 1 Optimization of the reaction conditions^a

Entry	Solvent(s)	Base (3 equiv.)	Yield (%)^b
a Reactions were monitored by TLC. b Isolated yield.
1	EtOAc/H2O (3 : 1)	K2CO3	77
2	EtOH/H2O (3 : 1)	K2CO3	75
3	CH3CN/H2O (3 : 1)	K2CO3	52
4	THF/H2O (3 : 1)	K2CO3	40
5	EtOAc	K2CO3	10
6	EtOH	K2CO3	15
7	CH3CN	K2CO3	8
8	THF	K2CO3	5
9	EtOAc/H2O (3 : 1)	Na2CO3	63
10	EtOAc/H2O (3 : 1)	Cs2CO3	74

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Thus having a standardized protocol in hand the method was subsequently applied to various in situ generated thiosemicarbazides to examine the generality and scope of the reaction. It was gratifying to observe that all these reactions proceeded smoothly affording the corresponding 2-amino-[1,3,4]-oxadiazole in good to excellent yields (Table 2). Various in situ generated thiosemicarbazides (2, 3, 4, and 5) bearing electron-donating groups all gave satisfactory yields of the respective [1,3,4]-oxadiazol-2-yl-amines (2a), (3a), (4a), and (5a). Structure of the product (4a) has been confirmed by crystal X-ray crystallography as shown in (Fig. 2).

Fig. 2 ORTEP view of compound (4a).

Table 2 Formation of [1,3,4]-oxadiazol-2-yl-amine from thiosemicarbazide^a

Substrate	Product^b	Yield (%)^c
a Reactions were monitored by TLC. b Confirmed by IR, ^1H and ^13C NMR. c Isolated yield.
		77
		80
		75
		66
		82
		72
		76
		82
		92
		90
		74
		78
		72
		88
		75
		86

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This methodology was also equally successful for substrates (6, 7, and 8) bearing moderately electron-withdrawing groups (–Cl and –Br) and for substrates (9 and 10) having strong electron-withdrawing groups, giving corresponding oxadiazoles as shown in Table 2. 1-Naphthyl thiosemicarbazide (11) derived from the corresponding dithiocarbamate salt yielded oxadiazole (11a) in decent yield under the present condition. In addition to formic acid hydrazide (NH₂NHCHO), benzhydrazide (NH₂NH₂COPh) can also be used. Thus the in situ generated thiosemicarbazides (12, 13 and 14) yielded the corresponding N,5-diphenyl-1,3,4-oxadiazole-2-amines, (12a), (13a), and (14a) respectively, in modest yields. Aliphatic thiosemicarbazide (15) and chiral benzylthiosemicarbazide (16) gave their corresponding oxadiazoles (15a) and (16a) respectively as shown in Table 2 in fairly good yields. However in case of aliphatic thiosemicarbazides an excess of K₂CO₃ (5 equiv.) had to be used to effect the transformation.

Our recent study on the formation of isothiocyanate from dithiocarbamate salt and carbodiimide from 1,3-disubstituted thiourea,^15f,g led us to propose the following reaction mechanism for the formation of 2-amino-[1,3,4]-oxadiazoles. Initially this one-pot strategy isothiocyanate (A) can be obtained from dithiocarbamate salt and molecular iodine following our green strategy.^15f Thiosemicarbazide formation (B) can take place in the same pot (Scheme 1) by adding formichydrazide (NH₂NHCHO) to the biphasic reaction mixture containing isothiocyanate (A). Subsequent desulfurization using iodine in the presence of base should lead to the formation of a carbodiimide type intermediate (D) via intermediate (C)^15g to which an intramolecular attack of thioamidic oxygen would generate intermediate (E). Aromatization of the intermediate (E) would yield the target [1,3,4]-oxadiazol-2-yl-amines (F) (Scheme 1).

Scheme 1 Proposed mechanism for the formation of 1,3,4-oxadiazol-2-yl-amine.

Conclusion

In conclusion we have developed a tandem one-pot method for the synthesis of 3-amino-1,3,4-oxadiazoles which have been achieved starting from dithiocarbamate salts. The in situ generated thiosemicarbazides undergo cyclodesulfurization mediated by iodine to give 2-amino-1,3,4-oxadiazoles. This method gives easy access to the preparation of pharmaceutically important heterocyclic scaffold and is compatible with a wide range of functional groups. In comparison to the reported methods, our method perhaps provides the simplest and most convenient approach and thus has potential industrial application.

General procedure

To a freshly prepared dithiocarbamate salt of aniline (540 mg, 2 mmol) in a biphasic medium of water/ethyl acetate (1 : 2, 10 mL) containing K₂CO₃ (828 mg, 6 mmol) was added iodine (554.4 mg, 2.2 mmol) pinchwise over a period of ten minutes whilst stirring. Instant precipitation of colloidal sulfur could be observed with the disappearance of iodine color in the medium. After stirring for further 20 min complete conversion of the dithiocarbamate salt to isothiocyanate was observed (by TLC). Formichydrazide (NH₂NHCHO) (120 mg, 2 mmol) was added to the above reaction mixture. Formation of thiosemicarbazide (1) having lower R_f was detected in the reaction mixture. Further 2.2 mmol of iodine (554.4 mg, 2.2 mmol) was added portion wise over a period of 10 min to the thiosemicarbazide and stirring continued for 0.5 h resulted in the complete disappearance of thiosemicarbazide (1) to yield a product having higher R_f compared to (1). After filtering out colloidal sulphur, the reaction mixture was treated with a 5% hypo solution (5 mL). It was then further extracted with ethyl acetate (2 × 10 mL) and the combined ethyl acetate layer was washed with water (1 × 5 mL), dried over anhydrous Na₂SO₄ and concentrated under reduced pressure. The product was purified over a column of silica gel and eluted with (3 : 1, hexane/ethyl acetate) to give 1a (248 mg, 77% yield).

Acknowledgements

BKP acknowledges the support of this research by the Department of Science and Technology (DST) (SR/S1/OC-79/2009), New Delhi, and the Council of Scientific and Industrial Research (CSIR) (01(2270)/08/EMR-II). SG, SKR, TG and NK thank CSIR for fellowships. Thanks are due to Arghya Basu and Sandeep Dey for crystallographic help. Thanks are due to Central Instruments Facility (CIF) IIT Guwahati for NMR spectra and DST-FIST for XRD facility.

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra and HRMS spectra. CCDC reference numbers 824583. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra00044j

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