N,N'-SUBStituted 1,2,5 thiadiazolidine 1,1-dioxides: synthesis, selected chemical and spectral proprieties and antimicrobial evaluation.

The sulfamide functional group is increasingly relevant in both medicinal and bioorganic chemistry. We report here practical access to a series of N2,N5-substituted five-membered cyclosulfamides. The five-membered heterocyclic motif was prepared starting from proteogenic amino acids and chlorosulfonyl isocyanate via the Mitsunobu reaction. Selected chemical and spectral proprieties and the antimicrobial evaluation of these compounds are detailed.

Introduction

The synthesis and reactivity of heterocyclic compounds containing sulfonyl moieties have attracted much interest in recent years because of the interesting chemical and biological proprieties associated with their structural similarities with biomolecules containing carbonyl groups. n class="Chemical">Cyclosulfamides have enjoyed popularity in the field of medicinal chemistry as nonhydrolysable components in peptidomimetics [1,2], agonists of the 5-HTID receptor (regulating serotonin levels) [3], HIV and serine protease inhibitors [4,5,6,7], and constrained di-and tripeptides [8]. The reported strategies for the synthesis of cyclosulfamides are based either on the incorporation of the sulfamoyl moiety by reacting sulfuryl chloride [9] or sulfonyl urea [10] with vicinal diamines, or ring-closing metathesis syntheses [11]. Recently, Johnson and co-workers reported the synthesis of N-Arylcyclosulfamides starting from sulfuryl chloride and chloroethyl-amine [12]. In previous publications [13,14,15], we have described a convenient access to a series of five and n-membered cyclic sulfamides B and N,N’-disubstituted orthogonally protected ones A (Figure 1), starting from natural amino acids, chloroethylamine and chlorosulfonylisocynanate (CSI) followed by 5-exo-tet closure with base. These heterocycles could be useful starting points for the construction of an array of peptidomimetic scaffolds and constrained di and-tripeptides. CSI has been found to be versatile reagent of great interest in synthetic heterocyclic chemistry [16]. In this case, CSI contains the required sulfonyl group and one of the nitrogens of the 1,2,5-thiadiazolidine1,1-dioxides.

Figure 1

Cyclosulfamide stuctures

In continuation of our efforts to design and synthesize new cyclic sulfamides, we have extended our studies to a series of new heterocyclic constrained peptides containing n class="Chemical">sulfamide groups C and D (Figure 1). The derivatization of amino acids allowed the introduction N-C* moieties with a well- defined configuration. Herein, we describe the synthesis and the preliminary results of the biological evaluation of a series of these new heterocycles containing sulfamido groups.

Results and Discussion

As outlined in Scheme 1, the different heterocyles 1b-5b were prepared in a two-step reaction sequence starting from n class="Chemical">(tert-butyloxycarbonylsulfonyl) L-amino acid methyl esters 1-5. These compounds were synthesized by sulfamoylation of aminoester derivatives (Ala, Val, Leu, Asp, Glu) as previously described [17,18,19].

Scheme 1

General synthesis of N2, N5 cyclosulfamides

Reagents and conditions: (a) Chloroethanol (1 equiv.), n class="Chemical">PPh3 (1 equiv.), DEAD (1 equiv.), THF; (b) K2CO3 (1.5 equiv.), DMSO.

In these Boc-sulfamides 1-5, the Boc (t-butyloxycarbonyl) group increases the acidity of the adjacent n class="Chemical">NH group and allows an expedient regiospecific alkylation under Mitsunobu conditions [20,21] using chloroethanol, which provides the N-substituted Boc–sulfamides 1a-5a in good yields. The cyclization reaction of these N,N’-sulfamides 1a-5a under basic conditions in DMSO gives N2,N5- substituted cyclosulfamides 1b-5b in satisfactory yields.

Selective cleavage of the t-butyloxycarbonyl protective group with trifluoroacetic acid gives compounds n class="Chemical">1c-5c in good yield (Scheme 2). N2,N5-Cyclosulfamides 1d-5d were readily prepared in quantitative yield from the cyclosulfamides 1c-5c by treatment with propionyl chloride in the presence of triethylamine. These compounds can be used in asymmetric aldol reactions. Also, attempts to incorporate the amino acid moiety employing the Mitsunobu reaction using an α-hydroxy ester (L-(-)- ethyl lactate) allowed us to obtain two constrained dipeptidal cyclic sulfamides 1e-2e in moderate yields with inversion of the configuration.

Scheme 2

Preparation of constained dipeptide cyclic sulfamides

Reagents and conditions: (a) TFA, n class="Chemical">CH2Cl2; (b) propionyl chloride (1 equiv.), TEA (1 equiv.), CH2Cl2; (c) (L)-(-) ethyl lactate (1 equiv.), PPh3 (1 equiv.), DEAD (1 equiv.), THF.

The structures of all compounds were unambiguously confirmed by the usual spectroscopic methods: 1H- and 13 C-n class="Chemical">NMR spectroscopy, mass spectrometry and IR spectra.

Biological Activity

The bacterial strains used in this study were Staphylococcus aureus and n class="Species">Escherichia coli species. They were isolated from an aquatic medium, followed by successive isolations carried out periodically in specific media in order to obtain strains as pure as possible. The solid media of MacConkey and Chapman have been used for Escherichia coli and Staphylococcus aureus, respectively. Microscopic study, after Gram coloration, was carried out after incubation at 37°C for 24 hours. The biochemical characteristics of each strain have been determined using a classic biochemical gallery. Finally, the pathogenic power of Staphylococcus aureus has been confirmed by showing that the coagulase of this strain was hemolytic, in vitro, towards rabbit or human plasma.

In vitro evaluation of the bacterial sensitivity of the strains to cyclosulfamides and

To draw the antibiogram, the dilution in a liquid medium method was chosen. It is based on putting innoculums of each studied strain in contact with increasing concentrations of the cyclosulfamides 4b and 4d. In a n class="Chemical">glucose medium, each bacterial inoculum (100 µL per suspension) was distributed in a series of tubes (macro-dilution method) containing increasing sulfamide concentrations [22]. The bacterial innoculum corresponding to the two studied strains, was previously prepared from a colony that was collected from a solid medium and then put in suspension in a glucose medium for 18 hours at 37°C. After the incubation of the whole tubes at 37°C for 24 hours, the MIC (minimal inhibited concentration) of each of the two cyclosulfamides with respect to each strain (Escherichia coli and/or Staphylococcus aureus) was measured as indicated by the tube that contained the lower concentration of the product and where no apparent bacterial growth is noticed. The results of the in vitro evaluation of the sensitivity of the bacteria Escherichia coli and Staphylococcus aureus towards cyclosulfamides 4b and 4d are presented in Table 1 and Table 2, respectively.

Table 1

Strain sensitivity towards cyclosulfamide 4b

	µg/mL	2	3	5	8	10	15	20	30	40	50	80	100
strain
E. coli		+	+	+	MIC	-	-	-	-	-	-	-	-
S. aureus		+	+	+	+	+	+	+	+	+	+	+	+

Table 2

Strain sensitivity towards cyclosulfamide 4d

	µg/mL	2	3	5	8	10	15	20	30	40	50	80	100
strain
E. coli		+	MIC	-	-	-	-	-	-	-	-	-	-
S. aureus		+	+	+	+	+	+	+	+	+	+	+	+

No inhibitory effect on the growth of the n class="Species">Staphylococcus aureus strain was observed in the presence of cyclosulfamide 4b. The cyclosulfamide 4d, also tested in this study, similarly showed no significant antimicrobial activity towards S. aureus. It is therefore clear that this species is resistant to these particular compounds. In contrast, however, a significant bacteriostatic effect has been observed towards Escherichia coli, with no growth being observed in tubes containing sulfamide concentrations of 4b equal or greater than 8 µg/mL, while cyclosulfamide 4d also showed a marked antimicrobial activity with regards to Escherichia coli and a MIC of 3µg/mL was obtained for this molecule.

Conclusions

We have established a new synthetic strategy to prepare peptidic structures constrained with a cyclosulfamide moiety. The N2,N-unsymmetric cylic sulfamides can be prepared in three steps (alkylation, cyclization, deprotection). We have also demonstrated the useful application of these cyclic sulfamides in the preparation of pseudopeptides. The preliminary results of antimicrobial activity are encouraging. Further biological evaluation of the resulting compounds and their incorporation into biomolecule analogues are currently in progress.