Preparation of a Series of 5-Methyl-3-(substituted)-[1,2,4]triazines

Abstract

Procedures for the synthesis of thirty-six 5-methyl-3-(substituted)-[1,2,4]triazines have been described. These compounds were evaluated for antagonism at metabotropic glutamate receptor subtype 5. Two compounds, 5b and 3c, were determined to be low micromolar inhibitors of mGluR5.

There are several studies that show that metabotropic glutamate receptor subtype 5 (mGluR5) plays a role in central nervous system disorders including addiction, pain, fragile X syndrome, and anxiety.^1–5 In a previous study, we discovered that 3-(substituted phenylethynyl)-5-methy[1,2,4]triazines (1) were potent antagonists of glutamate-mediated mobilization of internal calcium in an mGluR5 in vitro efficacy assay.⁶

In 2006, Buttelmann et al.⁷ reported that 2 had K_i of 1 nM in an mGluR5 binding assay and was active in an anxiolytic test after oral administration. We, therefore, considered it would be of interest to synthesize and evaluate triazine analogues of compound 2 (3a–f and 4a–f) for antagonism at mGluR5. In addition, we prepared analogues 5a–f, 6a–f, 7a–f, and 8a–f where the oxygen atom was replaced by a sulfur or amino group.

The phenoxy and benzyloxy substituted methyl-[1,2,4]triazines were synthesized from the corresponding phenols (9a–f) and benzyl alcohols (10a–f), which were first deprotonated with sodium hydride in THF and added to 5-methyl-3-sulfonyl[1,2,4]triazine (11) (Scheme 1). 5-Methyl-3-sulfonyl[1,2,4]triazine (11) was prepared as described previously.⁶ The products 3a–f and 4a–f were isolated in moderate yields after chromatographic separation.

Scheme 1.

Synthesis of substituted phenoxy and benzyloxy methyl[1,2,4]triazines

The thio-linked compounds 5a–f and 6a–f were synthesized after deprotonation of the corresponding thiols (12a–f) and benzyl thiols (13a–f) with sodium hydride in THF followed by displacement of the methylsulfonyl group in 11 (Scheme 2).

Scheme 2.

Synthesis of thiophenol and benzylthiol substituted methyl[1,2,4]triazines

Preparation of either aniline derivatives or benzyl amines did not require basic conditions to facilitate the displacement of the methylsulfonyl group in 11 (Scheme 3). Benzyl amines (14a–f) reacted rapidly in refluxing THF to provide the desired analogues (7a–f) in good yields. Reaction of various substituted anilines (15a–f) with 11 did not occur in refluxing THF but the desired products (8a–f) could be isolated by heating the reactants without solvent at 110 °C under N₂ blanket after chromatographic separation. The low yields (13 – 20%) were as a result of decomposition of 11 under heat.

Scheme 3.

Synthesis of aniline and benzylamine substituted methyl[1,2,4]triazines

Fluorimetric Ca²⁺/flux assay was used to evaluate the compounds for antagonism at mGluR5. Compounds were screened for in vitro efficacy using CHO-K1 cells stably transfected with the human mGluR5 cells. Through the screening process it was found that 5b, 5f, and 3c were weakly to moderately active, producing >35% inhibition of calcium mobilization in CHOK1-mGluR5 cells. Of these three compounds, the two compounds that showed higher levels of inhibition were reevaluated to determine the IC₅₀ values in a calcium efflux assay (Table 1). Compounds 5b and 3c were determined to be low micromolar inhibitors of mGluR5.

Table 1.

Selected Inhibition of Human mGluR5-Mediated Intracellular Calcium Mobilization for Heterocyclic-Linked Analogues

Compound	5b	5f	3c
% inhibition at 10 µM	44	35	51
IC50 (µM)	13.4 ± 2.3	n.d.	15.2 ± 4.0