Synthesis of Nβ-Protected Amino Sulfenyl Methyl Formamides and Sulfonyl Methyl Formamides: A Simple Protocol.

Chiral amino acid-derived formamides represent one of the most versatile components in multicomponent reactions. Herein, we describe a facile synthesis of Nβ-protected amino sulfenyl methyl formamides and sulfonyl methyl formamides via the Mannich reaction of Nα-protected amino alkyl thiols followed by oxidation using 3-chloroperbenzoic acid (m-CPBA). This protocol is applicable to a wide range of Fmoc- and Cbz-protected amino acids. Notably, the reaction provides high yield and retains the stereochemistry of the chiral center of the starting component.

Introduction

The formamide group has been considered as an important synthon for the synthesis of isocyanides,[1] which has shown diverse applications in isocyanide-based multicomponent reactions (IMCRs).[2] A massive variety of formamides and their corresponding isocyanides[3] have been investigated, thereby greatly expanding the capacity of molecular diversity through IMCRs.[4] Some isocyanides are less stable and possess a foul smell compared to formamides, and therefore, the ability to perform one-pot dehydration/MCR would significantly widen the versatility and scope of formamides.[5] This protocol proved to be particularly valuable when the desired isocyanides were inaccessible. In this context, several groups have made a significant contribution for one-pot dehydration/MCR without the isolation of isocyanides or sensitive aldehydes.[6] It is important to note that amino acid- and peptide-derived formamides and its isocyanides have gained attention in MCRs for medicinal chemistry and drug discovery applications.[7] We also explored the C-terminal modification of amino acids to prepare formamides and isocyanides (Figure ) and employed in MCRs for the construction of a new class of peptidomimetics.[8] With a view toward enabling a new class of formamides as well as isocyanides, we considered that sulfonyl formamides derived from chiral amino acids (Figure b) might be useful for generating a novel class of peptidomimetics. In this context, impressive applications of para-tosylmethylisocyanide (TosMIC)[9] inspired us.

Figure 1

Variations of amino acid-derived formamides.

TosMIC serves as a precursor for the synthesis of several drug intermediates and pharmacologically active compounds.[10] Its utility in the synthesis of nitriles, aldehydes, ketones, alkanes, cyclophanes, and a large number of natural products is known.[11] The solid-phase version of TosMIC has also been utilized for the synthesis of various heterocycles.[12] More importantly, chiral TosMIC analogues have been synthesized and employed for the synthesis of optically active compounds.[13] In addition, the incorporation of sulfur into a given biomolecule can dramatically modify its physical and biological properties, thereby resulting in the enhancement of the stability against proteolysis and improve bioavailability.[14] Certain other properties, such as receptor selectivity or potency, often can be substantially improved. As a result, the development of novel sulfur-based unnatural amino acids and peptidomimetics within a peptide framework constitutes a prime goal of interest in the drug discovery space.

In this regard, we report a novel class of amino acid-derived sulfonyl methyl formamides from the corresponding thiol through acid-catalyzed three-component Mannich type reaction followed by oxidation under mild conditions in a two-step process.

Results and Discussion

We had described an efficient protocol for the synthesis of N-protected amino alkyl thiols and demonstrated their multiple applications for the construction of sulfur-containing peptidomimetics.[15] In the proposed protocol, the sulfonyl methyl formamides were synthesized from corresponding N-protected amino alkyl thiols, which were prepared from our previously reported protocol.[15a] All the N-protected amino alkyl thiols could be stored for a long time under an inert atmosphere.

Initially, the reaction was commenced using Cbz-Phe-ψ[CH2SH] as a model substrate for optimizing the reaction conditions. Cbz-Phe-ψ[CH2SH] was treated with formamide (1.0 equiv) and para formaldehyde (1.0 equiv) in tetrahydrofuran (THF) and stirred at rt. The reaction failed to proceed even after 4 h of stirring, leaving behind the reactant as it is. Further, the investigation of the use of acid catalysts was carried out. Initially, the reaction was performed using PhCOOH and turned to be unproductive, although liquid chromatography–mass spectrometry data provided the formation of 2a in trace amounts. As shown in Table , when 20 mol % of CF3COOH was used, a low conversion (12%) of 2a was observed. Next, without varying the equivalents of formamide and paraformaldehyde, PhSO3H was used wherein a 28% yield of 2a was observed. Thereafter, the reaction was carried out at 90 °C in THF and the equivalents of formamide and paraformaldehyde when increased; the progressive yield of 2a was noticed (Table , entry 7). The preliminary optimization indicated the influence of temperature on reaction; in other words, no increase in yield was recorded when the reaction was carried at rt to 60 °C (Table , entries 2–6). These observations also inferred that the increase in the acidic strength of a catalyst led to a gradual improvement in the yield, as reflected in Table until entry 7. When none of the above acid catalysts was found efficient in conversion to 2a, we turned out to use 98% HCOOH. Using 30 mol % of HCOOH, stoichiometric amounts of both formamide and paraformaldehyde were increased to 4.0 equiv, and the reaction was carried out at reflux in toluene till the completion. The reaction proceeded well to afford 2a in moderate to good yield. Among the reactions carried out with HCOOH, the optimized condition for the efficient conversion was found when 5.0 equiv of formamide and 4.0 equiv of paraformaldehyde were used which led to the highest yield (90%) of 2a without any formation of noticeable impurities (Table , entry 10). Further, when the reaction was carried out with increased equivalents of formamide and formic acid, no improvement in the yield was observed. Alongside, different commonly used solvents, such as dichloromethane, CH3Cl, THF, CH3CN, and toluene, were examined; however, toluene was found beneficial to the reaction to furnish the target molecule 2a in good yield by a one-pot manner and allowing complete solubility of Nα-protected alkyl thiol.

Table 1

Optimization of Reaction Conditions for the Synthesis of Sulfenyl Methyl Formamidea

s. no	catalyst (mol %)	NH2CHO (equiv)	(CH2O)n (equiv)	yield (%)b
1		1.0	1.0
2	PhCOOH (10)	1.0	1.0	trace
3	PhCOOH (20)	1.0	1.0	trace
4	CF3COOH (20)	1.0	1.0	12
5	PhSO3H (20)	1.0	1.0	28
6	PhSO3H (20)	2.0	2.0	36
7	PhSO3H (30)	3.0	3.0	54
8	HCOOH (30)	3.0	3.0	74
9	HCOOH (30)	4.0	4.0	82
10	HCOOH (30)	5.0	4.0	90
11	HCOOH (30)	6.0	4.0	89
12	HCOOH (40)	5.0	4.0	88

Entries 2–6 = reaction carried out at 60 °C. Entries 7–12 = reaction carried out at 90 °C.

Yield obtained after column chromatography.

This protocol could be used for the gram-scale synthesis, as the desired compound 2a was isolated in 84% yield (1.29 g) for 1.3 g of the starting material. All the title compounds were obtained as stable solids and could be stored at room temperature for several months.

To represent the generality and scope of the optimized protocol, various Fmoc- and Cbz-protected amino alkyl thiols were investigated. As shown in Scheme , all the thiols were valid substrates and smoothly proceeded to afford corresponding sulfenyl methyl formamide (2) in good to excellent yields. Even sterically constrained residues such as Phe, Phg, Val, and Ile were well tolerated without demanding additional time for the completion of the reaction.

Scheme 1

List of Nβ-Protected Amino Sulfenyl Methyl Formamides Synthesized

Next, we set out to carry out the oxidation of sulfenyl to sulfone using m-CPBA as an oxidizing agent. Thus, a reaction of sulfenyl methyl formamides 2 with 2.0 equiv of m-CPBA in dry CH2Cl2 at 0 °C afforded the corresponding formamides 3 in excellent yield in an hour (Scheme ).

Scheme 2

List of Nβ-Protected Amino Sulfonyl Methyl Formamides Synthesized

To check the potential loss of chiral integrity during the transformation of the title compounds, chiral reversed-phase high-performance liquid chromatography (RP-HPLC) analysis was carried out employing the Phe residue. Initially, the racemization study was carried out using an enantiomeric mixture of 2a and 2a*, which showed well-separated twin peaks at 23.25 and 28.12 min, respectively (Figure , ii). This indicated the presence of two enantiomers and used the same as the reference racemate. The sample 2a gives a single peak at retention time 24.29 min (Figure , i), which revealed the retention of the stereochemistry of sample 2a with respect to Figure , ii. These results indicated that the protocol was devoid of racemization. Further, the oxidized product sulfonyl methyl formamide was also screened. A single peak appeared for 3a at retention time 19.42 min (Figure , iii), and a mixture of 3a and 3a* showed two distinct peaks with a significant difference at 19.12 and 19.86 min, respectively (Figure , iv). These data inferred that optical homogeneity is maintained throughout the protocol (Figure ).

Figure 2

RP-HPLC profiles of dl & l Cbz-Phe residue sulfenyl and sulfonyl methyl formamides.

A plausible mechanism based on the experimental results obtained during the reaction is depicted in Scheme . The proposed mechanism occurs via a Mannich-type three-component acid-catalyzed fashion. The reaction proceeds through the formation of key intermediate protonated imino formamide (I) by the reaction of paraformaldehyde and formamide in the presence of formic acid. Further, thiol as a Mannich donor attacks the imino carbon of the intermediate (I) to furnish sulfenyl methyl formamide (II). In the next step, sulfonyl methyl formamide (III) forms by the oxidation of sulfenyl methyl formamide by a well-known oxidizing agent m-CPBA.

Scheme 3

Proposed Reaction Pathway

Conclusions

In conclusion, a straightforward procedure has been described for the preparation of a series of Nβ-protected amino sulfonyl methyl formamides by a two-step protocol. The present procedure can afford Nβ-protected amino sulfenyl methyl formamides and Nβ-protected amino sulfonyl methyl formamides in good to excellent yields. These formamides could be employed for MCR for the construction of novel peptidomimetic molecules.

Experimental Section

General Information

All reagents were obtained from commercial suppliers and used without further purification. Thin-layer chromatography (TLC) experiments were carried out using precoated silica gel 60F254. HRMS spectra were recorded on a micromass Q-TOF spectrometer. 1H and 13C NMR spectra were recorded on a Bruker AMX 400 MHz and 100 MHz spectrometer, respectively, in DMSO-d6 and analyzed using MestrNova software and reported as chemical shifts (δ) in parts per million (ppm). The coupling constants were reported in hertz (Hz). Column chromatography for purification of products was performed on silica gel (100–200 mesh). The RP-HPLC experiments were carried out on a Agilent 1260 instrument (chiral column: amylose-2 and cellulose-1, pore size: 5 μm, diameter × length: 4.6 × 250 mm). The melting points of the compounds were determined on a VEEGO (model: VMP-DS) melting point apparatus.

General Procedure for Nβ-Protected Amino Sulfenyl Methyl Formamide

To a stirred solution of Nβ-protected amino thiol (1.0 equiv) in toluene (10 mL), formamide (5.0 equiv), para formaldehyde (4.0 equiv), and formic acid (30 mol %) were added, and the solution was refluxed for 4 h. After the completion of the reaction (monitored by TLC), the mixture was cooled to room temperature, and the solvent was vacuum evaporated and diluted with EtOAc, washed with water (2 × 10 mL) and brine (10 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by column chromatography (silica gel 100–200 mesh) with 4:6 ethyl acetate and hexane to afford the corresponding sulfenyl methyl formamide.

General Procedure for Nβ-Protected Amino Sulfonyl Methyl Formamide

To a stirred solution of Nβ-protected amino sulfenyl methyl formamide (1.0 equiv) in CH2Cl2, m-CPBA (2.0 equiv) was added at 0 °C, and the solution was stirred for 1 h at 0 °C. After completion of the reaction (monitored by TLC), the solvent was vacuum evaporated and diluted with EtOAc, washed with water (2 × 10 mL) and brine (10 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by column chromatography (silica gel 100–200 mesh) with 5:5 ethyl acetate and hexane to afford the corresponding title compounds.