Theoretical Evidence for Pyramidalized Bicyclic Serine Enolates in Highly Diastereoselective Alkylations

Towards Enantiomerically Pure Unnatural α-Amino Acids via Photoredox Catalytic 1,4-Additions to a Chiral Dehydroalanine

Chemo- and diastereoselective 1,4-conjugate additions of anionic and radical C-nucleophiles to a chiral bicyclic dehydroalanine (Dha) are described. Of particular importance, radical carbon photolysis by a catalytic photoredox process using a simple method with a metal-free photocatalyst provides exceptional yields and selectivities at room temperature. Moreover, these 1,4-conjugate additions offer an excellent starting point for synthesizing enantiomerically pure carbon-β-substituted unnatural α-amino acids (UAAs), which could have a high potential for applications in chemical biology.

Stereoselective α-Deuteration of Serine, Cysteine, Selenocysteine, and 2,3-Diaminopropanoic Acid Derivatives

Efficient methodologies for synthesizing enantiopure α-deuterated derivatives of serine, cysteine, selenocysteine, and 2,3-diaminopropanoic acid have been developed. H/D exchange was achieved by deprotonation of a chiral bicyclic serine equivalent followed by selective deuteration. Additionally, diastereoselective additions of thiols, selenols, and amines to a chiral bicyclic dehydroalanine in deuterated alcohols allowed site-selective deuteration at the Cα atom of cysteine, selenocysteine, and 2,3-diaminopropanoic acid derivatives. A deuterated analogue of carbocysteine, a drug for the treatment of bronchiectasis, was synthesized.

Synthesis of β2,2-Amino Acids by Stereoselective Alkylation of Isoserine Derivatives Followed by Nucleophilic Ring Opening of Quaternary Sulfamidates

Chiral bicyclic N,O-acetal isoserine derivatives have been synthesized by an acid-catalyzed tandem N,O-acetalization/intramolecular transcarbamoylation reaction between conveniently protected l-isoserine and 2,2,3,3-tetramethoxybutane. The delicate balance of the steric interactions between the different functional groups on each possible diastereoisomer controls their thermodynamic stability and hence the experimental product distribution. These chiral isoserine derivatives undergo diastereoselective alkylation at the α position, proceeding with either retention or inversion of the configuration depending on the relative configuration of the stereocenters. Quantum mechanical calculations revealed that a concave-face alkylation is favored due to smaller torsional and steric interactions at the bicyclic scaffold. This synthetic methodology gives access to chiral β^2,2-amino acids, attractive compounds bearing a quaternary stereocenter at the α position with applications in peptidomimetic and medicinal chemistry. Thus, enantiopure α-alkylisoserine derivatives were produced upon acidic hydrolysis of these alkylated scaffolds. In addition, α-benzylisoserine was readily transformed into a five-membered ring cyclic sulfamidate, which was ring opened regioselectively with representative nucleophiles to yield other types of enantiopure β^2,2-amino acids such as α-benzyl-α-heterofunctionalized-β-alanines and α-benzylnorlanthionine derivatives.

Structure-based Design of Anti-cancer Vaccines: The Significance of Antigen Presentation to Boost the Immune Response

Immunotherapy, alone or in combination with other therapies, is widely used against cancer. Glycoprotein Mucin 1 (MUC1), which is overexpressed and aberrantly glycosylated in tumor cells, is one of the most promising candidates to engineer new cancer vaccines. In this context, the development of stable antigens that can elicit a robust immune response is mandatory. Here, we describe the design and in vivo biological evaluation of three vaccine candidates based on MUC1 glycopeptides that comprise unnatural elements in their structure. By placing the Tn antigen (GalNAcα-O-Ser/Thr) at the center of the design, the chemical modifications include changes to the peptide backbone, glycosidic linkage, and at the carbohydrate level. Significantly, the three vaccines elicit robust immune responses in mice and produce antibodies that can be recognized by several human cancer cells. In all cases, a link was stablished between the conformational changes induced by the new elements in the antigen presentation and the immune response induced in mice. According to our data, the development of effective MUC1-based vaccines should use surrogates that mimic the conformational space of aberrantly glycosylated MUC1 glycopeptides found in tumors.

Conformationally Locked Pyramidality Explains the Diastereoselectivity in the Methylation of trans-Fused Butyrolactones

A stereoselectivity model inspired by the total synthesis of stemona alkaloids is developed to explain why enolate-derived 3,4-fused butyrolactones are methylated with a preference for syn alkylation. The model shows how conformational locking present in nonplanar enolate structures favors syn over anti methylation, due to less significant structural distortions in the syn pathway. The developed model was also successfully used to rationalize selectivities of previously documented methylation reactions.

Tn Antigen Mimics by Ring-Opening of Chiral Cyclic Sulfamidates with Carbohydrate C1- S- and C1- O-Nucleophiles

Starting from commercially available ( S)-isoserine and effectively accessible ( S)-α-methylserine, enantiopure cyclic sulfamidates have been prepared as chiral building blocks for the synthesis of various S- and O-glycosylated amino acid derivatives, including unnatural variants of the Tn antigen, through highly chemo-, regio-, and stereoselective nucleophilic ring-opening reactions with carbohydrate C1- S- and C1- O-nucleophiles.

Mucin architecture behind the immune response: design, evaluation and conformational analysis of an antitumor vaccine derived from an unnatural MUC1 fragment

A tripartite cancer vaccine candidate, containing a quaternary amino acid (α-methylserine) in the most immunogenic domain of MUC1, has been synthesized and examined for antigenic properties in transgenic mice. The vaccine which is glycosylated with GalNAc at the unnatural amino acid, was capable of eliciting potent antibody responses recognizing both glycosylated and unglycosylated tumour-associated MUC1 peptides and native MUC1 antigen present on cancer cells. The peptide backbone of the novel vaccine presents the bioactive conformation in solution and is more resistant to enzymatic degradation than the natural counter part. In spite of these features, the immune response elicited by the unnatural vaccine was not improved compared to a vaccine candidate containing natural threonine. These observations were rationalized by conformational studies, indicating that the presentation and dynamics of the sugar moiety displayed by the MUC1 derivative play a critical role in immune recognition. It is clear that engineered MUC1-based vaccines bearing unnatural amino acids have to be able to emulate the conformational properties of the glycosidic linkage between the GalNAc and the threonine residues. The results described here will be helpful to the rational design of efficacious cancer vaccines.

S-Michael additions to chiral dehydroalanines as an entry to glycosylated cysteines and a sulfa-Tn antigen mimic

Stereoselective sulfa-Michael addition of appropriately protected thiocarbohydrates to chiral dehydroalanines has been developed as a key step in the synthesis of biologically important cysteine derivatives, such as S-(β-D-glucopyranosyl)-D-cysteine, which has not been synthesized to date, and S-(2-acetamido-2-deoxy-α-D-galactopyranosyl)-L-cysteine, which could be considered as a mimic of Tn antigen. The corresponding diamide derivative was also synthesized and analyzed from a conformational viewpoint, and its bound state with a lectin was studied.

Synthesis of ApoSense compound [18F]2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-(fluoromethyl)butanoic acid ([18F]NST732) by nucleophilic ring opening of an aziridine precursor

INTRODUCTION

The small molecule 2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-(fluoromethyl)butanoic acid (NST732) is a member of the ApoSense family of compounds, capable of selective targeting, binding and accumulation within cells undergoing apoptotic cell death. It has application in molecular imaging and blood clotting particularly for monitoring antiapoptotic drug treatments. We are investigating a fluorine-18-radiolabeled analog of this compound for positron emission tomography studies.

METHODS

We prepared the tosylate precursor methyl 2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-(tosyloxymethyl)butanoate (4) to synthesize fluorine-18-labeled NST732. Fluorination reaction of the tosylate precursor in 1:1 acetonitrile:dimethylsulfoxide with tetrabutyl ammonium fluoride proceeds through an aziridine intermediate (4A) to afford two regioisomers: 2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-fluorobutanoate (5) and methyl 2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-(fluoromethyl)butanoate (6). Acid hydrolysis of the fluoromethylbutanoate (6) isomer produced NST732. As the fluorination reaction of the tosylate precursor proceeds through an aziridine intermediate (4A) and the fluorination conceivably could be done directly on the aziridine, we have separately prepared an aziridine precursor (4A). Fluorine-18 labeling of the aziridine precursor (4A) was performed with [(18)F]tetrabutyl ammonium fluoride to afford the same two regioisomers (5 and 6). The [18F]2-((5-dimethylamino)naphthalene-1-sulfonamido)methyl)-2-fluorobutanoic acid (NST732) was then obtained by the hydrolysis of corresponding [18F]-labeled ester (6) with 6 N hydrochloric acid.

RESULTS

Two regioisomers obtained from the fluorination reaction of aziridine were easily separated by high-performance liquid chromatography. The total radiochemical yield was 15%±3% (uncorrected, n=18) from the aziridine precursor in a 70-min synthesis time with a radiochemical purity>99%.

CONCLUSION

Fluorine-18-labeled ApoSense compound [18F]NST732 is prepared in moderate yield by direct fluorination of an aziridine precursor.

Conformational effects of the non-natural alpha-methylserine on small peptides and glycopeptides

The synthesis and the conformational analysis in aqueous solution of a peptide and a glycopeptide containing the sequence threonine-alanine-alanine (Thr-Ala-Ala) are reported. Furthermore, the threonine residue has been replaced by the quaternary amino acid alpha-methylserine (MeSer) and their corresponding non-natural peptide and glycopeptide are also studied. The conformational analysis in aqueous solution combines NOEs and coupling constants data with Molecular Dynamics (MD) simulations with time-averaged restraints. The study reveals that the beta-O-glycosylation produces a remarkable and completely different effect on the backbone of the peptide derived from Thr and MeSer. In the former, the beta-O-glycosylation is responsible for the experimentally observed shift from extended conformations (peptide) to folded ones (glycopeptide). In contrast, the beta-O-glycosylation of the MeSer-containing peptide, which clearly shows two main conformations in aqueous solution [extended ones (70%) and beta-turn (30%)], causes a high degree of flexibility for the backbone.

Steric control of alpha- and beta-alkylation of azulenone intermediates in a guanacastepene a synthesis

The origins of different stereoselectivities observed experimentally in the alkylations of azulenone precursors in the guanacastepene A synthesis have been determined through density functional theory investigations. The optimized transition structures of methylation of two different guanacastepene A precursors show that steric effects, rather than torsional factors that often determine such stereoselectivities, dictate the preferred products observed.

Complete mapping of the stereochemical course of retentive deprotonation/alkylation of 1H-benzo[e][1,4]diazepin-2(3H)-ones

Enantiopure amino-acid derived 1H-benzo[e][1,4]diazepin-2(3H)-ones (BZDs) undergo highly retentive deprotonation/alkylation reactions. To confirm the role of stereolabile, axially chiral intermediates in these reactions and to determine the precise stereochemical course of deprotonation and alkylation, a new alanine-derived BZD (S)-1d was prepared. Because of slow diazepine ring inversion of the C3-alkylated derivatives of 1d, it proved possible to determine that electrophiles react at the concave face of the enolate derived from 1d. Furthermore, an enantiopure silyl enol ether derivative of (S)-1d was prepared and characterized by X-ray crystallography, confirming that deprotonation resulted in an (M)-configured axially chiral enolate. Activation parameters for diazepine ring inversion in the potassium enolate of 1d were determined experimentally and are well-matched by density functional calculations. Finally, the factors leading to concave-face alkylation of the enolate derived from 1d are analyzed based on calculated alkylation transition structures. Minimization of torsional effects at the BZD ring fusion and maximization of imine and amide resonance are proposed to favor concave-face alkylation.

Non-natural amino acids as modulating agents of the conformational space of model glycopeptides

The synthesis and conformational analysis in aqueous solution of different alpha-methyl-alpha-amino acid diamides, derived from serine, threonine, beta-hydroxycyclobutane-alpha-amino acids, and their corresponding model beta-O-glucopeptides, are reported. The study reveals that the presence of an alpha-methyl group forces the model peptides to adopt helix-like conformations. These folded conformations are especially significant for cyclobutane derivatives. Interestingly, this feature was also observed in the corresponding model glucopeptides, thus indicating that the alpha-methyl group and not the beta-O-glucosylation process largely determines the conformational preference of the backbone in these structures. On the other hand, atypical conformations of the glycosidic linkage were experimentally determined. Therefore, when a methyl group was located at the Cbeta atom with an R configuration, the glycosidic linkage was rather rigid. Nevertheless, when the S configuration was displayed, a significant degree of flexibility was observed for the glycosidic linkage, thus showing both alternate and eclipsed conformations of the psi(s) dihedral angle. In addition, some derivatives exhibited an unusual value for the phi(s) angle, which was far from a value of -60 degrees expected for a conventional beta-O-glycosidic linkage. In this sense, the different conformations exhibited by these molecules could be a useful tool in obtaining systems with conformational preferences "à la carte".

Fluorine... and pi...alkali metal interactions control in the stereoselective amide enolate alkylation with fluorinated oxazolidines (Fox) as a chiral auxiliary: an experimental and theoretical study

The alpha-alkylation of amide enolates by using a pseudo-C(2) symmetry trans 4-phenyl-2-trifluoromethyloxazolidine (trans-Fox) as a chiral auxiliary occurs with an extremely high diastereoselectivity (>99 % de). The origin of this excellent stereocontrol was investigated by an experimental and theoretical (DFT) study. With this trans chiral auxiliary, both F...metal and pi...metal interactions compete to give the same diastereomer through Re face alkylation of the enolate. A 5.5 kcal mol(-1) energy difference found between the Re face and the Si face attack transition states is consistent with the complete diastereoselectivity that has been experimentally achieved. On the other hand, in the case of the cis chiral auxiliary (cis-Fox) the competition between the F...metal and pi...metal interactions is unfavourable to the diastereoselectivity. In this case, the Re face and the Si face attack transition states were found to be nearly isoenergetic (0.3 kcal mol(-1) difference), which is in good agreement with the very low diastereoselectivity observed.

Role of the Countercation in Diastereoselective Alkylations of Pyramidalized Bicyclic Serine Enolates. An Easy Approach to α-Benzylserine

The use of a chiral serine equivalent as an excellent chiral building block has been demonstrated in the synthesis of alpha-benzylserine through a diastereoselective lithium enolate alkylation reaction and subsequent acid hydrolysis. The role of a coordinating countercation (lithium) in the alkylation reaction has been investigated. Theoretical studies have been performed in order to elucidate the stereochemical outcome of the alkylation process, which occurs with total retention of configuration.