Synthesis and Biological Activity of Some Novel Derivatives of

To discover new 1,2,4-triazole derivatives which may possess significant biological activities, we synthesized a series of novel 6-aryl-3-(D-galactopentitol-1-yl)-7H-1,2,4-triazolo[3,4-b][1,3,4]thiadiazines and 4-(arylmethylidene)amino-5-(D-galactopentitol-1-yl)-3-mercapto-4H-1,2,4-triazoles from 4-amino-3-(D-galactopentitol-1-yl)-5-mercapto-1,2,4-triazole. All the title compounds were characterized by elemental analysis, IR, 1H- and 13C-NMR. Plant growth-regulating activity tests showed that these compounds have remarkable effects on the growth of radish and wheat.


Introduction
Various substituted 1,2,4-triazolo [3,4-b]-1,3,4-thiadiazines and Schiff's bases are associated with diverse pharmacological activities, such as analgesic, anthelmintic, antitubercular, plant growth regulating, antiviral, antifungal and anticancer properties [1][2][3][4][5][6][7][8].3-Substituted-4-amino-5-mercapto-1,2,4-triazole is the key intermediate in the formation of these heterocyclic compounds.Based on the principle of activity addition, we expected to obtain reinforcement of biological activities by means of substitutions at different positions of 3-substituted-4-amino-5-mercapto-1,2,4-triazole derivatives.Unfortunately, the water-solubility of most these compounds was too poor for use in a clinical trial as medicines.Attachment of a galactose residue containing five hydroxyl groups to the 3-position of the fused heterocycle could bring about changes in its solubility; in addition, we expected that there might be a strong affinity for the the galactose residue in organisms, resulting in special activity.
The syntheses of thiocarbohydrazide and 4-amino-3-(D-galactopentitol-1-yl)-5-mercapto-1,2,4triazole (1) were carried out according to the literature methods with some improvements [9~11].First, we quickly raised the reaction temperature to 100~110°C for half an hour, in order to maximize the yield of thiocarbohydrazide.Second, some water was added to the mixture when D(-)-galactonic acid γ-lactone was reacted with thiocarbohydrazide in pyridine.Finally, the solubility of the triazole intermediate in hot water differed noticeably from that in cold water, so the crude product was recrystallized from water.
The reaction conditions of the two steps are mild and highly efficient.The rates of ring-closure are closely related to the structures of the substituents R. When R contains electron-withdrawing groups, the rates of ring-closure are increased because the electron-cloud densities of the carbonyl carbons are lowered.
In an attempt to prepare ring closed 3,6-disubstituted-5,6-dihydrogen-1,2,4-triazolo [3,4-b][1,3,4]thiadiazole derivatives, we have applied a previously reported procedure [12], and treated the triazole 4 with the appropriate benzaldehyde maintaining the pH values during the reaction at 5-6, since the acidity of the reaction medium is crucial.However, the desired intramolecular Mannich reaction did nor occur, and we only obtained open-chain hydrazones 3a~3b, as in our previous research [13].Consequeltly we did not adjust the pH values in the subsequent reactions, and thus obtained 3c~3h more expediently.
Because of the successful attachment of a galactose residue to the 3-position of 1,2,4-triazole, the fused heterocycles display increased solubility and most of the title compounds can be dissolved in ethanol.Moreover, compounds 2e (possessing a phenyl ring without any substituent) and 3a (having a phenyl ring with a hydroxyl group) dissolve in water, whereas compound 3e (having two galactose residues but too large a relative molecular mass) dissolves somewhat in water and ethanol.Scheme 1. Syntheses of the title compounds 2a~2h, 3a~3h.

IR spectra of the title compounds
The absence of N-H, S-H and C=O absorptions in the IR spectra confirms that the cyclocondensation to form the title compounds 2a~2h has taken place.The characteristic stretching vibrations of the products are at 3413~3440 cm -1 (O-H, strong), 1615~1634 cm -1 (C=N).The C-S-C bending vibrations are in the fingerprint region, around 670 cm -1 .The CH 2 stretching vibration bands are observed at about 2921~2978 cm -1 .
The absence of N-H and C=O absorption bands in the IR spectra also confirmed that the title compounds 3a~3h have been obtained via condensation.The stretching vibration bands of OH group are at 3261~3533 cm -1 .The C-H stretching vibration bands of the CH 2 groups are at 2873~2975 cm -1 .The characteristic stretching vibrations of the products are at 1582~1631cm -1 (C=N), 1234~1292 cm -1 (C=S).The absorbance band of S-H stretching vibration (2550-2650 cm -1 ) was not observed because compounds 3a~3f display tautomerism between the thiol and thioketone forms and exist mainly in the latter form [14~19].

H-NMR spectra of the title compounds
In the 1 H-NMR spectra of triazolothiadiazines 2a~2h, the characteristic downfield signal at δ 13.50 ppm attributed to the -N=C-SH (-NH-C=S of the tautomer) in compound 1 is absent, as is a sharp signal at δ 5.53 ppm attributable to the N-NH 2 group in the parent triazole.In the spectra of the title compounds, the signal at δ 3.33~6.03ppm is attributed to the CH 2 and OH of the galactopentitol-1-yl group.Resonance peaks at δ 7.30~8.42ppm are attributed to the aromatic ring protons.
In all the title compounds 3a~3h, additional resonances assigned to the -CH=N-(δ 9.56─10.41ppm) were observed, which confirmed the condensation between the amino group and the carbonyl group.A downfield signal appearing at δ 13.51-13.92ppm is attributed to the -NH-C=S moiety.whereas the chemical shifts (δ values) of the SH protons are at less than 4.0 ppm, in general.All these show the compounds 3a~3h have 4-arylmethylideneamino-5-aryl-2,4-dihydro-3H-1,2,4-triazole-3-thione structures (D).The remaining protons resonated as multiplets in the aromatic region (δ 6.91-8.41ppm).In typical methylideneamino groups, the methylidene hydrogen atoms display chemical shifts in the region above δ 8.8 ppm, but in the title compounds the corresponding δ values were in the region between 9.56 and 10.41 ppm.The triazole NH protons also showed downfield δ values (13 ppm or so).We are confident that compounds 3a~3h exist only in an E-configuration, which results from the deshielding effects of the keto-thione (C=S) groups' anisotropy on the N=CH hydrogen.The chemical shifts of N=CH hydrogen and triazole N-H hydrogen appear downfield.We cconclude that the mechanism of this type of reaction involves an elimination to give imines, followed by a nucleophilic substitution, and in the transition state an E-configuration would posses a lower energy.
The 13   [20].All the benzene carbons of the title compounds exhibit chemical shifts between 105.23~151.50,whereas the peaks with δ values near 22.86~25.83ppm are attributed to the S-CH 2 in the thiadiazine rings because of the influence from the ortho and para substituents of the benzene rings.The title compounds 3a~3h show peaks at δ 151.99~165.65 ppm due to N-C=N, N-C=S and Ph-CH=N group.The δ values differ among the title compounds resulting from the presence of phenyl groups with different substituents.

Biological evaluation
The effects of the title compounds 1, 2a~2h, 3a~3h on sprouting of wheat and radish seeds have been investigated.After treating with solutions of 100 μg/mL and 10 μg/mL of the title compounds for 7 days (25 °C), the germination percentages have been determined, and from the difference in length between stems and radicles of seedlings treated with the title compounds and those treated only with distilled water, the plant growth regulating activities have been calculated.A positive result represents a growth increase, whereas a negative result implies an inhibition: effect = the length of sample's stem/radiclethe reference's length × 100％ the length of reference's stem/radicle The results are shown in Table 1.From the data it is apparent that most of the title compounds show a more pronounced inhibiting effect than 4-amino-3-(D-galactopentitol-1-yl)-5-mercapto-1,2,4-triazole (1) on the growth of wheat and radish.It was interesting to note that compound 2e showed inhibitory activities towards the growth of the dicotyledon (radish) at two concentration levels, but under the same conditions it expressed stimulative activities towards the growth of the monocotyledon (wheat).It has a good level of activity and is worthy of further study to establish a relationship between structure and activity.

General
All melting points were determined on an XT-4A melting point apparatus.The IR spectra (KBr disks) were recorded in the 4000─400 cm -1 range on a Bruker Quinox 55 spectrophotometer.The 1 H-NMR and 13 C-NMR spectra were measured at 25°C on a Bruker Advance 300 spectrometer operating at 300 and 75 MHz, respectively (DMSO-d 6 solutions using TMS as internal reference).Elemental analyses were carried out with a Flash-EA 1112 elemental analyzer.All the reagents used were AR grade.

thiadiazines 2a~2h
The appropriate ω-haloacetophenone (1 mmol) was added to a solution of compound 1 (1 mmol) in 75% C 2 H 5 OH (20 mL).The mixture was refluxed for 4 h.The solid obtained on concentration of the solvent was filtered, dried and recrystallized from 75% C 2 H 5 OH to give the title compounds.The physical and spectral data of the title compounds 2a~2h are given in Tables 2~4.