Synthesis and Molecular Structure of 6-Amino-3-benzylmercapto-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one

Abstract

The title compound 6-amino-3-benzylmercapto-1,2,4-triazolo[3,4-f][1,2,4]- triazin-8(7H)-one (4), molecular formula C₁₁H₁₀N₆OS, was obtained by the reaction of 3-amino-2-benzyl-6-hydrazino-1,2,4-triazin-5(2H)-one (3) with carbon disulfide in a water/pyridine mixture. Compound 4 can also be synthesized by reacting 6-amino-3(2H)mercapto-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one (7) with benzyl bromide in methanolic ammonia water. The compound crystallizes in the monoclinic space group P2₁/c with a = 7.2926(15), b = 14.456(2), c = 11.436(2) Å, β = 105.30(2)°, V = 1162.9(4) ų and Z = 4, resulting in a density D_calc of 1.567 g/cm³. Molecules of 4 are linked by extensive intermolecular N-H···N and N-H···O hydrogen bonding [graph set ${\text{R}}_2^2$(9)]. The structure is further stabilized by π-π stacking interactions.

Introduction

1,2,4-Triazines and their condensed derivatives form an important class of heteroaromatic compounds with various biochemically interesting properties and pharmacologically significant activities [1,2]. We have recently explored the mechanism of the cyclization of the monocyclic compound 3-amino-6-hydrazino-1,2,4-triazin-5(2H)-one (1) to form the heterobicyclic 6-amino-3-substituted-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-ones 2 [3]. This convenient reaction of 1 to afford 2 (Scheme 1) had been previously described by Lovelette [4]. As a part of our ongoing program, the compound 3-amino-2-benzyl-6-hydrazino-1,2,4-triazin-5(2H)-one (3) was treated with a variety of one-carbon fragment reagents (formic acid, glacial acetic acid or cyanogen bromide) and no cyclization was observed [5]. In this paper we report an unexpected ring closed product 4 obtained when compound 3 is treated with carbon disulfide. As we know, many aza/deaza analogues of purine have attracted considerable interest due to their biological activities. Compound 4 contains a 6-amino-1,2,4-triazolo- [3,4-f][1,2,4]triazin-8(7H)-one moiety (4,8-diaza-9-deazaguanine), which is an isosteric isomer of guanine. In the last decade numerous fused 1,2,4-triazines have been synthesized and screened in vitro, revealing their anti-HIV and anti-cancer activities [6,7,8,9,10,11,12,13]. Meanwhile, fused 1,2,4-triazoles have acquired considerable importance because of their CNS depressant, anti-allergy, anti-inflammatory and antimicrobial properties [14,15,16,17]. However, the 3-substituted derivatives of 6-amino-1,2,4-triazolo- [3,4-f][1,2,4]triazin-8(7H)-one have seldom been reported.

Scheme 1.

Scheme 1.

Results and Discussion

When compound 3 was refluxed with carbon disulfide in a 1:1 water/pyridine mixture an unexpected ring closed product 4 was obtained (Scheme 2). An intense IR absorption at 1735 cm^-1 suggested the carbonyl absorption of fused triazines and confirmed the occurrence of ring closure [3,4,18]. The ¹H-NMR spectrum showed two singlets at δ_H 4.50 and 6.50 ppm, corresponding to the benzyl group methylene protons and the 6-amino group, respectively. The remaining multiplets at δ_H 7.30 to 7.37 ppm were attributed to the aromatic protons of the benzyl group, while the broad singlet at δ_H 11.61 ppm corresponded to the NH proton of the 1,2,4-triazinone ring amide. The location of the 3-benzylmercapto group of 4 at C-3 was established by 2D-NMR. The long-range ¹H-¹³C heteronuclear correlation (HETCOR) NMR revealed the connectivity of the methylene protons (δ_H 4.50 ppm) on the benzyl group to C-3 (δ_C 144.86 ppm) in the 1,2,4-triazolo ring. Additional supporting evidence for 4 was found in the high-resolution mass spectrum showing the molecular ion at m/z 274.0635, confirming the precise molecular formula as C₁₁H₁₀N₆OS (calcd. 274.0637).

Scheme 2.

Scheme 2.

Further confirmation of the structure of product 4 was obtained from its synthesis via an alternative approach, by reacting 6-amino-3(2H)mercapto-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one (7) with benzyl bromide in methanolic ammonia (Scheme 2). The resulting nucleophilic substitution product was a material identical in every respect with compound 4. Finally, the structure of 4 was unambiguously confirmed by X-ray crystallography, revealing the structural framework as a 1,2,4-triazole five-membered ring fused at C(9)-N(4) with a 1,2,4-triazine six-membered ring (Figure 1). The most contributing prototropic tautomerism of 4 is the amino-oxo form 7H-tautomer (not the 5H-tautomer), similar to the tautomerism of 6-amino-3-ethyl-1,2,4-triazolo[3,4-f][1,2,4]-triazin- 8(7H)-one, as previously reported by us [19]. The benzylmercapto group was substituted at C-3 showing an exoextensibility. Taken together, 4 was assigned as 6-amino-3-benzylmercapto-1,2,4- triazolo[3,4-f][1,2,4]-triazin-8(7H)-one. The ring closed compound 5 and the hydrazinecarbodithioic acid compound 6 were not observed (Scheme 2).

Figure 1. ORTEP drawing and atom labelling scheme of the compound 4 with thermal ellipsoids drawn at the 50% probability level.

Figure 1. ORTEP drawing and atom labelling scheme of the compound 4 with thermal ellipsoids drawn at the 50% probability level.

The ORTEP drawing for the title compound is depicted in Figure 1. The molecular packing is shown in Figure 2. In the structure of 4 the short bonds 1.310(2) Å (N(5)-C(6)), 1.320(2) Å (N(1)-C(9)) and 1.325(2) Å (N(2)-C(3)) have an appreciable double-bond character. The bond lengths 1.732(2) Å of C(3)-S(1) and 1.214(2) Å of C(8)-O(1) are shorter than the bond lengths of Car-S(2) (1.773 Å) and Csp²=O(1) (1.240 Å) in δ-lactams [20], respectively. These may be attributed to the electron abstraction by the π-deficient heterobicyclic ring. The bond length 1.340(2) Å between C(6)-N(17) is shorter than 1.355 Å of Car-NH₂ (Nsp²: planar) [20]. Meanwhile, the 119.15(15) value of the N(5)-C(6)-N(17) angle, close to 120°, confirms the sp² hybridization of the nitrogen atom, which suggests that the 6-amino group strongly donates the unpaired electrons and resonates with the [1,2,4]triazolo[3,4-f][1,2,4]triazinone ring. The mean plane of 1,2,4-triazolo[3,4-f][1,2,4]triazine ring forms a dihedral angle of 20.27° with the benzyl group benzene ring.

Figure 2. A perspective drawing of the packing arrangement of compound 4. Dashed lines are intermolecular N-H···N and N-H···O hydrogen bonds.

Figure 2. A perspective drawing of the packing arrangement of compound 4. Dashed lines are intermolecular N-H···N and N-H···O hydrogen bonds.

Analysis of the molecular packing in the unit cell reveals that each molecule is linked with two other molecules by intermolecular hydrogen bonds (Figure 3 and Table 1). Each title molecule is linked into two ${\text{R}}_2^2$(9) graph set associations (Figure 3, notation [a], [d] and [b], [c]), each via one N-H···N hydrogen bond and one N-H···O hydrogen bond. Assignment of the H-bond descriptors is based on the graph-set theory [21]. The structure is further stabilized by π-π stacking (Figure 3 notation [e]) interactions, which results in the centroid···centroid distance (3.695 Å) being that between the benzene ring of the molecule at x, y, z and the 1,2,4-triazine ring of the molecule at 1-x, 0.5+y, 1.5–z (iii) ([e]). Molecular graphics and hydrogen bond geometry were obtained using the program Mercury (version 1.4, CCDC, Cambridge, UK).

Figure 3. A part of the crystal structure of the title compound, showing the molecule stacking (notation [e]) direction along the a-view. Broken lines indicate the intermolecular hydrogen bonding patterns. For notation and symmetry codes see Table 1.

Figure 3. A part of the crystal structure of the title compound, showing the molecule stacking (notation [e]) direction along the a-view. Broken lines indicate the intermolecular hydrogen bonding patterns. For notation and symmetry codes see Table 1.

Table 1. Hydrogen bond geometry in compound 4

Notation	D-H···A	D-H (Å)	H···A (Å)	D···A (Å)	Length-VdW	D-H···A(°)
a	N(7)-H(7A)i···N(1)	0.860	2.104	2.961	–0.646	173.94
b	N(17)-H(17B)···O(1)ii	0.860	2.311	2.976	–0.409	134.34
c	N(7)-H(7A)···N(1)ii	0.860	2.104	2.961	–0.646	173.94
d	N(17)-H(17B)i···O(1)	0.860	2.311	2.976	–0.409	134.34

*Note. symmetry codes: (i) x, –0.5–y, –0.5+z; (ii) x, –0.5–y, 0.5+z; (iii) 1-x, 0.5+y, 1.5–z.

Table 1. Hydrogen bond geometry in compound 4

Notation	D-H···A	D-H (Å)	H···A (Å)	D···A (Å)	Length-VdW	D-H···A(°)
a	N(7)-H(7A)i···N(1)	0.860	2.104	2.961	–0.646	173.94
b	N(17)-H(17B)···O(1)ii	0.860	2.311	2.976	–0.409	134.34
c	N(7)-H(7A)···N(1)ii	0.860	2.104	2.961	–0.646	173.94
d	N(17)-H(17B)i···O(1)	0.860	2.311	2.976	–0.409	134.34

*Note. symmetry codes: (i) x, –0.5–y, –0.5+z; (ii) x, –0.5–y, 0.5+z; (iii) 1-x, 0.5+y, 1.5–z.

Conclusions

We have synthesized the title compound 4, an unexpected ring closed product resulting from a reaction of a heterocyclic precursor (compound 3). The structure of 4 was clearly identified by 2D-NMR spectroscopy and unambiguously confirmed by X-ray crystallography. A reasonable mechanism for the ring cyclization of 3 to 4 is now being explored as a part of our ongoing research programs.

Experimental

General

Melting points were determined on a YANACO micromelting point apparatus and are uncorrected. ¹H- and ¹³C-NMR spectra (in DMSO-d₆, internal standard TMS) were taken on a Varian Unity 400 (400 MHz) spectrophotometer. For the assignments of signals, standard and long-range ¹H-¹³C heteronuclear chemical shift correlation (HETCOR) 2D NMR experiments were used. Chemical shifts are given in the δ-scale (ppm). Infrared spectra (IR) were recorded on a Perkin-Elmer FTIR 1650 spectrophotometer with KBr discs. MS and HRMS spectra were obtained on a Quattro VG-5022 spectrometer and VG 70-250S GC/MS, respectively, with an ionization potential of 70 eV. Elemental analyses (EA) were performed on a Heraeus CHN-O-Rapid elemental analyzer. 3-Amino-2-benzyl- 6-hydrazino-1,2,4-triazin-5(2H)-one (3) and 6-amino-3-thio(2H)-1,2,4-triazolo- [3,4-f][1,2,4]triazin- 8(7H)-one (7) were prepared according to the methods described by Hwang et al. [3] and Lovelette [4], respectively.

Syntheses of 6-Amino-3-benzylmercapto-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one (4)

Method A: Compound 3 (0.35 g, 1.5 mmol) was suspended in a 1:1 pyridine/water mixture (15 mL), then carbon disulfide (0.16 g, 2.1 mmol) was added and the mixture was refluxed for 12 hours. Evaporation of the solvent under vacuum afforded a crude solid which was recrystallized from water to give 4 (0.16 g, 44%), m.p. 229-230 °C; IR (ν_max, cm^–1): 3431, 3330, 3016, 1735 (C=O), 1630, 1404, 1099; ¹H-NMR: δ 4.50 (s, 2H, S-CH₂-C₆H₅), 6.50 (s, 2H, NH₂), 7.30-7.37 (m, 5H, C₆H₅), 11.61 (br s, 1H, NH); ¹³C-NMR: δ 34.79 (S-CH₂-C₆H₅), 127.39, 128.42, 128.89, 137.24 (C₆H₅), 141.03 (C-8a), 144.86 (C-3), 150.67 (C-6), 151.59 (C-8); MS [EI]: m/z 274 (M⁺, 1), 214 (19), 197 (3), 165 (1), 106 (40), 91 (100), 77 (7), 70 (6), 65 (31); HRMS: m/z Calcd for C₁₁H₁₀N₆OS: 274.0637. Found: 274.0635; Anal. Calcd for C₁₁H₁₀N₆OS: C, 48.16; H, 3.67; N, 30.64. Found: C, 48.25; H, 3.77; N, 30.83.

Method B: A solution of 7 (0.27 g, 1.5 mmol) and benzyl bromide (0.26 g, 1.5 mmol) in methanolic ammonia (previously saturated at 0°C; 25 mL) was stirred at room temperature in a sealed flask for 24 hours. Evaporation of the solvent on a vacuum evaporator and recrystallization of the residue from water afforded 251 mg (61%) of a material identical in every respect with compound 4 prepared by Method A.

X-ray techniques

X-ray quality crystals of the colorless title compound 4 were obtained by crystallization from dimethyl sulfoxide. Crystal and experimental data are summarized in Table 2. The data were collected with a NONIUS CAD4 automated diffractometer equipped with a graphite-monochromatized Mo K_α radiation (λ= 0.71073Å) at 295(2) K. The crystal structure data have been deposited at the Cambridge Crystallographic Data Centre [22]. The unit-cell parameters were determined from 25 reflections with 8.75° ≤ 2θ ≤ 14.60°. Intensity data with 2θ ≤ 27.50° were collected with the ω-2θ scan technique at 2676 reflections. All reflections were corrected for Lorentz and polarization effects. Absorption corrections were made with the psi-scan method. The crystal structure was resolved by direct methods using SHELXS-86 [23] and refined by full-matrix least-squares methods on F² using SHELXL-93 [24]. All non-H atoms were refined anisotropically. Hydrogen atoms were allowed as riding atoms with isotropic displacement parameters related to the non-H atoms on which they were riding.

Table 2. Crystal and experimental data for compound 4

Formula	C11H10N6OS
Formula weight	274.31
Crystal system	monoclinic
Space group	P21/c
Unit-cell dimensions (Å)	a = 7.2926(15)
	b = 14.456(2)
	c = 11.436(2)
	β = 105.30(2)°
Unit-cell volume, V (Å3)	1162.9(4)
Formula per unit cell, Z	4
Dcalcd (g/cm3)	1.567
Absorption coefficient, μ (mm-1)	0.28
F(000)	568
Crystal size (mm)	0.50 × 0.25 × 0.20
Index ranges	– 9 ≤ h ≤ 9
	0 ≤ k ≤ 18
	0 ≤ l ≤ 14
Max. and min. transmission	0.9769 and 0.8844
Independent reflections	2676 (Rint = 0.0000)
Reflections/restraints/parameters	2676 /0 /173
Final R indices [I > 2σ(I)]	R1 = 0.0363, wR2 = 0.1041
R indices (all data)	R1 = 0.0482, wR2 = 0.1108
Goodness-of-fit on F2	1.062
Max. shift/error	–0.001
Largest difference peak and hole (e/Å3)	0.377 and – 0.304

Table 2. Crystal and experimental data for compound 4

Formula	C11H10N6OS
Formula weight	274.31
Crystal system	monoclinic
Space group	P21/c
Unit-cell dimensions (Å)	a = 7.2926(15)
	b = 14.456(2)
	c = 11.436(2)
	β = 105.30(2)°
Unit-cell volume, V (Å3)	1162.9(4)
Formula per unit cell, Z	4
Dcalcd (g/cm3)	1.567
Absorption coefficient, μ (mm-1)	0.28
F(000)	568
Crystal size (mm)	0.50 × 0.25 × 0.20
Index ranges	– 9 ≤ h ≤ 9
	0 ≤ k ≤ 18
	0 ≤ l ≤ 14
Max. and min. transmission	0.9769 and 0.8844
Independent reflections	2676 (Rint = 0.0000)
Reflections/restraints/parameters	2676 /0 /173
Final R indices [I > 2σ(I)]	R1 = 0.0363, wR2 = 0.1041
R indices (all data)	R1 = 0.0482, wR2 = 0.1108
Goodness-of-fit on F2	1.062
Max. shift/error	–0.001
Largest difference peak and hole (e/Å3)	0.377 and – 0.304