Synthesis of a boron-containing amidoxime reagent and its application to synthesize functionalized oxadiazole and quinazolinone derivatives

Abstract

Herein, we report the design, synthesis and application of a borylated amidoxime reagent for the direct synthesis of functionalized oxadiazole and quinazolinone derivatives. This reagent exhibits broad synthetic utility to obtain a variety of biologically relevant drug-like molecules. It can be easily prepared at large scale from relatively inexpensive reagents, and can undergo facile transformations to obtain target compounds. The developed amidoxime reagent was synthesized from 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile and hydroxyl amine hydrochloride using N,N-diisopropylethylamine as a base in ethanol under reflux conditions. Overall advantages include a metal-free route to boronated oxadiazoles, quinazolinone derivatives, and restriction of the multistep sequences. Importantly, the boron-rich pharmacophore derived compounds were obtained through an efficient and inexpensive strategy.

Introduction

Boron chemicals in various forms have emerged as potent drug candidates in recent years.^1–7 For example (Figure 1), Velcade is an aliphatic boronic acid that inhibits protease enzymes, and has been in the market since 2007 as a licensed drug for treating multiple myeloma.^{3, 8} The benzoxaborole scaffold led to the discovery of AN2690, an antifungal drug developed to treat onychomycosis, an infection of the toenail.^{9, 10} GSK2251052, a benzoxaborole moiety tethered with a functional aliphatic alcohol has profound activity against gram-negative bacteria, with a potential to be the first new-class antibacterial in 30 years to treat serious hospital gram-negative infections.¹ Robust clinical activity of Ixazomib against multiple myeloma led to the approval of the drug as the first orally bioavailable human proteasome inhibitor.^{11, 12} The importance of boron to activity of the Crisaborole compound as an inflammatory drug that inhibits phosphodiesterase 4B (PDE4B) was demonstrated by replacing boron with carbon, which eliminated activity.¹³ Due to empty porbitals, boron compounds have a high tendency to accept electrons from heteroatoms and thereby form strong interactions at enzyme action sites. Boron compounds exhibit several forms such as sp² (trigonal) to sp³ (tetragonal) with different ligands affecting their binding properties. The extended stability obtained under various alcohol and water conditions provides a platform for the boron compounds to sustain and conformationally survive during synthesis. Furthermore, the boronic compounds form cyclic hydrogen bonding networks with controllable abilities affecting various biological phenomena. Subsequently the benzoxaboroles are generally advantageous over free boronic acids due to cyclic rigidity and enhanced solubility. In addition, boronic acids also have shown good selectivity towards H₂O₂, and could be potential therapeutic agent to develop new anticancer drugs.^{14, 15}

Figure 1:

Boron containing clinical approved drugs

Amidoximes provide extensive pathways to reach certain bioactive heterocycles, including oxadiazoles and quinazolinone¹⁶. Oxadiazoles are highly important pharmacologically, and are well-studied core structural units of various muscarinic agonists, benzodiazepine receptor partial agonists,¹⁷ dopamine transporters,¹⁸ antirhinovirals,¹⁹ a growth hormone secretagogue,²⁰ and 5-HT agonists,²¹ as well as a urea bioisostere in β3- adrenergic receptor agonists.²²

In our ongoing chemical biology studies, we focused on synthesizing biology-oriented boron chemicals (BioBCs) as potential probes to study developmental biology and pharmacological agents for neurodegenerative and other diseases.^23–25

Previously we and others have reported the synthesis of Combretastatin A-4 (CA-4) (Scheme 1a) as an antimitotic agent, which binds to the colchicine site on tubulin causing inhibition of tubulin polymerization, microtubule depolymerization, and mitotic block.^{25,26, 27} CA-4 exhibits potent cytotoxicity against a broad spectrum of human cancer lines, including multidrug resistance (MDR) lines, and also acts as a vascular disrupting agent (VDA) Scheme 1a. With an objective to increase the cytotoxicity and anti-tubulin activity of CA-4, we envisioned developing a set of boron containing, 3, 5-disubstituted, 1, 2, 4-oxadiazole derivatives as potential new analogs of CA-4.

Scheme 1.

3,5-disubstituted boron-containing 1,2,4-oxadiazoles as combretastatin A-4 (CA-4) analogues.

In order to limit the steps needed to generate the target compound 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)phenyl)-5-(3,4,5-trimethoxyphenyl)-1,2,4-oxadiazole (4), we strategized a simple and efficient protocol. The plan included a new anticipated boronate pre-installed amidoxime moiety 5, which could be easily transformed into the product oxadiazole in a single step method (Scheme 2). Toward this goal, we focused on synthesizing amidoximes via easily obtained chemicals and based on published methods.³⁶ Generally, the synthetic methods available employ aromatic as well as aliphatic nitriles with hydroxylamine under basic conditions to furnish the corresponding amidoximes.^{37, 38} However, a recent report employed aryl halide to convert directly into amidoxime via a palladium catalysed process in a one-pot cyanation and amidoximation³⁷.

Scheme 2.

Retro-synthetic analysis of 1,2,4-oxadiazole framework

To achieve the amidoxime (5), we initiated the study with 4-cyanophenyl boronic acid pinacol ester (6) as a prototype substrate with hydroxylamine hydrochloride salt as the reagent. Initially, our approach to synthesize the target compound failed using published protocols³⁹. This can be explained due to predominant formation of product (Z)-N’,4dihydroxybenzimidamide (7) (Table 1, entry 1–2). This has been a well-studied reaction in the literature.⁴⁰ To avoid the formation of product 7 and obtain the desired product 5 we screened amine bases as replacement for oxygen bases. No significant formation of the expected product was observed with trimethylamine (Table 1, entry 3). Subsequently, we examined conditions with different temperatures and bases. Surprisingly, under N,N-diisopropylethylamine as base exclusively in ethanol the corresponding amidoxime product was produced with good yield (Table 1, entry 4, 46%) and target product might be lost during the recrystallization process. ³⁶

Table 1.

Optimization conditions for amidoxime formation^a

Entry	Base	Solvent	Temp	Yield (%)b
				9	10
1	Na2CO3	Ethanol	reflux	nd	61
2	NaHCO3	Ethanol	reflux	trace	50
3	Et3N	Ethanol	Reflux	10	45
4	IPr 2 NEt	Ethanol	reflux	46	nd
5	IPr2NEt	Ethanol	rt	nd	nd

^aReaction conditions: substrate 6 (0.5 mmol), NH₂OH.HCl (1 mmol), base (1 mmol), under reflux conditions.

^bIsolated yields.

Our protocol was further tested under room temperature conditions, which resulted in no product formation (entry 5). In brief, (Z)-N’-hydroxy-4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)benzimidamide (5) was successfully synthesized from 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)benzonitrile (6) by using N,N-diisopropylethylamine as base in ethanol under refluxed conditions. The structure of 5 was ascertained by NMR and mass spectrometry data: ¹H NMR (500 MHz, DMSO-d₆) δ 9.75 (s, 1H), 7.72 – 7.64 (m, 4H), 5.85 (s, 2H), 1.30 (s, 12H). ¹³C NMR (125 MHz, DMSO) δ 150.5, 136.0, 134.1, 126.8, 124.7, 83.7, 24.6. HRMS (EI) Calcd. for C₁₃H₁₉BN₂O₃ [M+H]⁺ requires 263.1561, found 263.1566.

To the best our knowledge, this is the first report to synthesize this class of boron-containing amidoxime, which further leads to a variety of boron-containing pharmacophore groups. As boron-based compounds are gaining high importance in both academia and the pharmaceutical industry, this type of reagent is highly significant for further development of boron-based biological agents.

In our attempt to synthesize the target compound 4 with the new class of boronated amidoxime compound (5), we were successful under the established conditions. The target compound 4 was synthesized in a single step with good yield (45%, Scheme 3), the method proves cost effective as it avoids the metal catalyzed protocols and limits the two-step process. Moreover, compounds 8 and 9 were also synthesized by using previously reported method with good yields.^41–44

Scheme 3.

Synthesis of target compounds 4, 8 and 9.

Having achieved proof of concept, we tried to expand the scope and explore the reagent with different substrates. In this regard, for our ongoing program to develop potential anti-angiogenic agents through zebrafish phenotypic screening, we planned to synthesize 6-chloro-3-(3-(4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)phenyl)-1,2,4-oxadiazol-5-yl)-2H-chromen-2one (12) and 6-chloro-3-(3-(4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)phenyl)-1,2,4-oxadiazol-5-yl)quinolin-2(1H)one (13).

Interestingly, under the standard reaction conditions 2-oxochromene carboxylic acid (10) and quinolone carboxylic acid (11) were reacted smoothly with boronated amidoximes (5) and provided the respective products (12, 13)⁵⁵ with good yields, (12-44%) and (13-39%) respectively (Scheme 4). We found that the developed protocol produces the expected products in a single step by using easily available commodities. This procedure overcomes the difficult and costly palladium catalyzed borylation reaction. This methodology will open new avenues to synthesize boron-containing oxadiazoles as potential pharmacological agents.

Scheme 4.

Synthesis of compounds 12 and 13. ^aReaction conditions: substrate 1 (0.5 mmol), amidoxime (0.5 mmol), CDI (0.5 mmol), at 80 °C. ^b Isolated yields.

We further expanded our protocol to synthesize imidazo[1,2a]pyridine based biological active compounds. Imidazo[1,2a]pyridine is an important pharmacophore group in medicinal chemistry and exhibits a broad range of biological activities such as antifungal, anti-inflammatory, anticancer, antipyretic, antiapoptotic, analgesic, antimicrobial, antiprotozoal and hypnoselective activities.^45–50 In addition, the imidazo[1,2a]pyridine motif is present in various marketed drugs including zolpidem, alpidem, saripidem, zolimidine, necopidem, GSK812397 and GABA receptor.

We next focused on synthesizing boron based imidazo[1,2a]pyridine compounds. We found that 7-chloroimidazo[1,2a]pyridine-3-carboxylic acid (14a) reacted smoothly with boronated amidoxime (5) in the presence of 1,1’-carbonyldiimidazole (CDI) in DMF at 70 °C and afforded the respective product (15a) in 51% yield (Scheme 5).⁵⁶ Furthermore, the developed protocol was successfully utilized for the synthesis of other boron based imidazo[1,2-a]pyridine derivatives (15b-d) in 45–55% yields.

Scheme 5.

Synthesis of compound 15a-d. ^aReaction conditions: substrate 14a-d (0.5 mmol), amidoxime (0.5 mmol), CDI (0.5 mmol), at 70 °C. ^b Isolated yields.

Quinazolinones have received significant attention in medicinal chemistry due to their wide range of biological applications such as antibacterial, anticonvulsant, antifungal, anticancer, antiinflammatory, anti-HIV and analgesic activities.^51–53 Due to high importance of quinazolinone in drug discovery, we utilized our newly synthesized amidoxime reagent for the synthesis of quinazolinone under reported reaction condition.⁵⁴ For this strategy, we reacted compound 5 with isatoic anhydride (16) using FeCl₃ (10 mol%) as catalyst in 1,4-dioxane at 80°C for 12 h. The desired product (18) was observed with 41% yield, which reduces the two step synthesis of quinazolinone in a single step and avoids the costly palladium catalyzed borylation reaction (Scheme 6).⁵⁷ The structure of 18 was ascertained by NMR and mass spectrometry data: ¹H NMR (400 MHz, DMSO) δ 12.61 (s, 1H), 8.21–8.15 (m, 3H), 7.85–7.76 (m, 3H), 7.56–7.54 (m, 1H), 7.52 (s, 1H), 1.30 (s, 1H). ¹³C NMR (100 MHz, DMSO) δ 162.6, 152.5, 149.1, 135.1, 134.9, 132.1, 130.3, 128.0, 127.6, 127.2, 126.3, 121.5, 84.5, 25.1. HRMS (ESI) calcd for C₂₀H₂₂BN₂O₃ 349.1718 found 349.1945 [M+H]⁺.

Scheme 6.

Synthesis of quinazolinone compound 18

Further to explore the utility of our developed protocol, we applied our approach for the synthesis of 3-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-5-(3,4,5-trimethoxyphenyl)-1,2,4-oxadiazole from 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile under optimal reaction conditions. To our delight, target product (21) was observed with 40% yield, which will open the new window for medicinal chemist for the synthesis of diversely substituted oxadiazole derivatives.

Conclusions

In conclusion, we report here for the first time, the design and synthesis of novel boron-containing amidoxime reagent (5). We used this reagent to synthesize potential protease inhibitors and antimicrobial agents. This reagent significantly simplifies synthesis of various pharmacophore groups. An important advantage of this advancement is that it can display different boron-containing prodrugs in a direct transformation as well as avoiding the pre-functionalized substrates. The protocols showed high synthetic utility by synthesizing oxadiazoles and quinazolinone derivatives. The developed reagent also showed a wide range of tolerance with Cl, Br and CF₃ and other functional groups under developed reaction conditions. The detail biological studies of these pharmacological agents as potential biological agents is currently undergoing in our laboratory.

Scheme 7.

Synthesis of 3-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-5-(3,4,5-trimethoxyphenyl)-1,2,4-oxadiazole