Racemization free longer N-terminal peptide hydroxamate synthesis on solid support using ethyl 2-(tert-butoxycarbonyloxyimino)-2-cyanoacetate

Keywords

• Boc-Oxyma;
• Hydroxamic acid;
• Peptide hydroxamic acid;
• Racemization free;
• SAHA

Introduction

Peptide hydroxamates are found in many active pharmaceutical ingredients (APIs) and natural products.¹ They exhibit a wide range of biological activities including, antibacterial, antifungal, anti-inflammatory, anti-asthmatic, anticancer, and antiarthritis.¹ Recently, a series of hydroxamic acid derivatives is discovered to be promising agents for the management of Chagas disease.² Hydroxamic acids are strong bidentate metal-ion-chelating agents, that interact with zinc(II)-containing proteins and thus act as inhibitors of metalloenzymes.³

SAHA (suberoylanilide hydroxamic acid, Fig. 1) is an anticancer agent that works against both hematologic and solid tumors.⁴ Another important hydroxamic acid, Deferoxamine (a siderophore),⁵ is widely used in medicine to remove excess iron from body (Fig. 1). Many N-terminal peptide hydroxamates exhibit highly potent tumor necrosis factor converting enzyme inhibitor (TACE, ADAM17) activity.⁶ Some of the commercially available MMP/ADAM (Matrix metalloproteinase/A disintegrin and metalloproteinase) inhibitors are also shown in Figure 1.

Several methods have been developed for the synthesis of hydroxamic acids directly from carboxylic acids by reacting with hydroxylamine hydrochloride in the presence of various catalysts, including cyanuric chloride,⁷ cyclic phosphonic anhydride (PPAA),⁸ alkylcholoroformate,⁹ and TFFH/PTF (tetramethylfluoroformamidiniumhexafluorophosphate/benzyltriphenyl phosphonium dihydrogen trifluoride).¹⁰

Hydroxamic acids have also been synthesized from esters using sodium methoxide¹¹ and potassium cyanide (KCN).¹² The synthesis of peptide hydroxamic acids on solid supports is also reported.^7 and 13 However, most of these reported solid support based methods are limited to compounds with relatively low molecular mass. Later, Stefanowicz and co-workers developed a new resin for synthesizing longer peptide hydroxamic acids,¹⁴ but the method is limited to only C-terminal peptide hydroxamic acids. Also, many of these methods cause racemization and require resin-bound esters, which are not commercially available. In this Letter, we wish to report an environment friendly method for synthesizing racemization free hydroxamic acids using ethyl 2-(tert-butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma, I, Fig. 2) at room temperature without the use of acid chlorides. Further, we have demonstrated the synthesis of larger N-terminal peptide hydroxamic acids on solid support using this method.

First, we have optimized the reaction conditions using benzoic acid as a model substrate (Scheme 1). Pre-activation of benzoic acid has been achieved using I in THF in the presence of DIPEA (N,N-diisopropylethylamine) for 30 min at 0–5 °C. Next, the addition of hydroxylamine hydrochloride dissolved in DMF to the reaction mixture has generated moderate yield of the desired product in 52% yield (entry 1, Table S1 in Supplementary data) after stirring at room temperature for 2 h. Use of 0.1 equiv of DMAP along with DIPEA has increased the yield to 75% (entry 2, Table S1, Supplementary data). Therefore, we have used 0.1 equiv of DMAP as a catalyst and 2.5 equiv of DIPEA (in total) as a base for further exploration. Next, various solvents and solvent combinations have been examined and THF/DMF solvent mixture produced better yield.