Antitubulin sulfonamides: The successful combination of an established drug class and a multifaceted target

Tubulin, the microtubules and their dynamic behavior are amongst the most successful antitumor, antifungal, antiparasitic, and herbicidal drug targets. Sulfonamides are exemplary drugs with applications in the clinic, in veterinary and in the agrochemical industry. This review summarizes the actual state and recent progress of both fields looking from the double point of view of the target and its drugs, with special focus onto the structural aspects. The article starts with a brief description of tubulin structure and its dynamic assembly and disassembly into microtubules and other polymers. Posttranslational modifications and the many cellular means of regulating and modulating tubulin's biology are briefly presented in the tubulin code. Next, the structurally characterized drug binding sites, their occupying drugs and the effects they induce are described, emphasizing on the structural requirements for high potency, selectivity, and low toxicity. The second part starts with a summary of the favorable and highly tunable combination of physical-chemical and biological properties that render sulfonamides a prototypical example of privileged scaffolds with representatives in many therapeutic areas. A complete description of tubulin-binding sulfonamides is provided, covering the different species and drug sites. Some of the antimitotic sulfonamides have met with very successful applications and others less so, thus illustrating the advances, limitations, and future perspectives of the field. All of them combine in a mechanism of action and a clinical outcome that conform efficient drugs.

Reversal of H-bonding direction by N-sulfonation in a synthetic reverse-turn peptide motif

This communication depicts an intriguing example of hydrogen-bonding reversal upon introduction of a sulfonamide linkage at the N-terminus of a synthetic reverse-turn peptide motif. The ready availability of two sulfonyl oxygen atoms, as hydrogen-bonding acceptors, combined with the inherent twisted conformation of sulfonamides are seen to act as switches that engage/disengage the hydrogen-bond at the sticky ends/termini.

Synthesis of taurine-containing peptides, sulfonopeptides, and N- and O-conjugates

Taurine-containing water-soluble peptidomimetics were designed and synthesized. N-terminal taurine acylations allowed synthesis of a number of taurine-containing peptides. N-protection of taurine with Cbz and SO2-activation with benzotriazole followed by coupling with various amino esters, dipeptides and nucleophiles provided taurine N- and O-conjugates and sulfonopeptides.

Sulfonamide-related conformational effects and their importance in structure-based design

Structure-based design (SBD) is a challenging endeavour since even localised SAR can hardly ever be explained by the variation of just one dominating factor. Here, we present a rare example where structural information combined with ab initio calculations clearly indicate that the observed difference in biological activity is dominated by conformational effects. The learnings discussed are successfully put to the test and have the potential to be of general use as a qualitative guide in SBD efforts.

Synthesis and Structural Characterization of Trivalent Amino Acid Derived Chiral Phosphorus Compounds

Reaction of the N-toluenesulfonyl derivatives of (S)-alanine, phenylalanine, and valine (4-6) with PhPCl(2) gave in high yield the 4-methyl, benzyl, and isopropyl derivatives (7-9) of 2-phenyl-1-p-toluenesulfonyl-1,3,2-oxazaphospholidin-5-one. The ratios of the (2S,4S)/(2R,4S) diastereomers (cis/trans isomers) were 1:1, 2:1, and 10:1 for the methyl, benzyl, and isopropyl derivatives 7a,b, 8a,b, and 9a,b, respectively. For 7a,b, both isomers could be crystallized, but for the others only the major isomers were isolable. The X-ray crystal structure of 9a shows that the isopropyl and phenyl groups are mutually cis and that the tolyl moiety is oriented s-trans to both the isopropyl and phenyl groups. Reaction of 6 with Cl(2)PCH(2)CH(2)PCl(2) (10) gave a 56:38:7 mixture of the cis/cis, cis/trans, and trans/trans diphosphorus heterocycles 11a-c. The major isomer could be crystallized and isolated free of the other diastereomers. Reaction of 6 with EtPCl(2) gave a 6:1 mixture of cis/trans isomers of the ethyl-substituted heterocycles 12a,b as an inseparable oil but allowed confirmation of the structure of 11a. Slow epimerization at phosphorus may occur by inversion but more likely by ring opening/closure, since 7b, 9a, and 11a give rise upon standing in solution to mixtures containing starting material and 7a, 9b, and 11b, respectively, along with the free amino acid derivatives 4 and 6. The NMR spectra, and in particular the coupling constants between the alpha-hydrogen atom of the amino acid moiety and phosphorus, were used to establish the identities of the cis and trans isomers. Reaction of 9a with (THF)W(CO)(5) gave the phosphorus-ligated adduct (9a)W(CO)(5) (13), and the IR spectrum of this complex shows that 9a is a strongly electron-withdrawing ligand. The geometry of the sulfonamide moiety is discussed in detail, as are the (1)H NMR coupling constants. The data are consistent with the presence of little steric interaction between the cis isopropyl and phosphorus substituent in 9a, 11a, and 12a and orientation of the tolyl moiety s-cis to the isopropyl group in 9b, 12b, and 13.

The stability of pseudopeptides bearing sulfoximines as chiral backbone modifying element towards proteinase K

Incorporation of sulfoximines as backbone modifying element results in two new pseudopeptide bonds which display enhanced (bond A) and strongly reduced reactivity (bond B) towards hydrolysis by Proteinase K.

Synthesis of pseudopeptides with sulfoximines as chiral backbone modifying elements

The synthesis of pseudopeptides with a chiral alpha-sulfonimidoylcarboxy moiety in the backbone is described. Starting from readily available (Ss)-S-methyl S-phenyl sulfoximine and various cyclic and acyclic alpha-amino acids the desired products are obtained in good yields with peptide coupling methodology. Specific secondary structures caused by intramolecular hydrogen bonds may be adopted. Results of NMR studies to reveal conformational preferences will be discussed.

Peptides containing the sulfonamide junction. 2. Structure and conformation of Z-Tau-Pro-D-Phe-NHiPr

The taurine (Tau) containing N-protected pseudotripeptide isopropylamide Z-Tau-Pro-D-Phe-NHiPr (1) has been specifically designed and synthesized as suitable model to test the ability of the sulfonamido group to participate as H-bond acceptor to a type II beta-turn and to get information on the preferred rotameric conformation around the S-N bond and the hybridization state of the nitrogen atom. The present structural investigation reveals that, although the sulfonamide junction is invariably folded in a gauche mode, the beta-turn structure, stabilized by the 4 --> 1 hydrogen bond, is not found in the crystal and the sulfonamido oxygen atoms are not involved in any intra- or intermolecular hydrogen-bond interaction. More than one conformer populates the CDCl(3) solution with only a minor contribution by the expected beta-turn. The Pro nitrogen is significantly pyramidalized and the nitrogen lone pair points in opposite direction to that of the Pro C(alpha)H bond thus adopting R chirality, in an arrangement practically identical to that found in the previously studied homochiral analogue Z-Tau-Pro-Phe-NHiPr.

Metal-catalyzed asymmetric oxidations

Recent advances in enantioselective metal-catalyzed oxidations are presented. Two transformations are discussed in detail: (a) asymmetric Baeyer-Villiger reactions of racemic and prochiral cyclic ketones giving optically active lactones, and (b) sulfide oxidations affording enantiomerically enriched sulfoxides. For both reactions, new metal/ligand combinations have been introduced which allow catalysis of these reactions with high enantioselectivities. For some substrates, the ee-values are above 90%. As previously demonstrated, sulfoxides can be iminated to give the corresponding sulfoximines. Under appropriate reaction conditions, this transformation is stereospecific and enantiopure sulfoximines are obtained. These highly oxidized sulfur reagents have now been modified to give beta-amino acid analogues which may be incorporated into peptides. Two variants have been realized: either the sulfoximine-containing pseudo-amino acids are connected with standard alpha-amino acids to give mixed pseudopeptides, or the sulfoxime units are mutually attached to give homooligomers, which can be regarded as beta-peptide analogues.

Sequence-specific polypeptoids: a diverse family of heteropolymers with stable secondary structure

We have synthesized and characterized a family of structured oligo-N-substituted-glycines (peptoids) up to 36 residues in length by using an efficient solid-phase protocol to incorporate chemically diverse side chains in a sequence-specific fashion. We investigated polypeptoids containing side chains with a chiral center adjacent to the main chain nitrogen. Some of these sequences have stable secondary structure, despite the achirality of the polymer backbone and its lack of hydrogen bond donors. In both aqueous and organic solvents, peptoid oligomers as short as five residues give rise to CD spectra that strongly resemble those of peptide alpha-helices. Differential scanning calorimetry and CD measurements show that polypeptoid secondary structure is highly stable and that unfolding is reversible and cooperative. Thermodynamic parameters obtained for unfolding are similar to those obtained for the alpha-helix to coil transitions of peptides. This class of biomimetic polymers may enable the design of self-assembling macromolecules with novel structures and functions.