Synthesis of ß2,2-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 ß2,2-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 ß2,2-amino acids such as α-benzyl-α-heterofunctionalized-ß-alanines and α-benzylnorlanthionine derivatives.

Synthesis of Nß-Substituted α,ß-Diamino Acids via Stereoselective N-Michael Additions to a Chiral Bicyclic Dehydroalanine.

The highly diastereoselective 1,4-conjugate additions of several nitrogen nucleophiles to chiral bicyclic dehydroalanines have been assessed effectively at room temperature in good to excellent yields without needing any catalyst or additional base. This methodology is general, simple, oxygen and moisture tolerant, high-yielding, totally chemo- and stereoselective. This procedure offers an efficient and practical approach for the synthesis of Nß-substituted α,ß-diamino acids, such as 1-isohistidine, τ-histidinoalanine, ß-benzylaminoalanine, ß-(piperidin-1-yl)alanine, ß-(azepan-1-yl)alanine, and fluorescent and ciprofloxacin-containing amino acid derivatives.

Cell-Penetrating Peptides Containing Fluorescent d-Cysteines.

A series of fluorescent d-cysteines (Cys) has been synthesized and their optical properties were studied. The key synthetic step is the highly diastereoselective 1,4-conjugate addition of aryl thiols to a chiral bicyclic dehydroalanine recently developed by our group. This reaction is fast at room temperature and proceeds with total chemo- and stereoselectivity. The Michael adducts were easily transformed into the corresponding amino acids to study their optical properties and, in some selected cases, into the corresponding N-Fmoc-d-cysteine derivatives to be used in solid-phase peptide synthesis (SPPS). To further demonstrate the utility of these non-natural Cys-derived fluorescent amino acids, the coumaryl and dansyl derivatives were incorporated into cell-penetrating peptide sequences through standard SPPS and their optical properties were studied in different cell lines. The internalization of these fluorescent peptides was monitored by fluorescence microscopy.

Bifunctional Chiral Dehydroalanines for Peptide Coupling and Stereoselective S-Michael Addition.

A second generation of chiral bicyclic dehydroalanines easily accessible from serine has been developed. These scaffolds behaved as excellent S-Michael acceptors when tri-O-acetyl-2-acetamido-2-deoxy-1-thio-α-d-galactopyranose (abbreviated as GalNAc-α-SH) was used as a nucleophile. This addition proceeds with total chemo- and stereoselectivity, complete atom economy, quickly, and at room temperature, making it a true click reaction. The Michael adducts were easily transformed into S-(2-acetamido-2-deoxy-α-d-galactopyranosyl)-l- and -d-cysteines, which can be regarded as mimics of the Tn antigen derived from l-Ser (α-d-GalNAc-l-Ser) and d-Ser (α-d-GalNAc-d-Ser), respectively.

Design of α-S-Neoglycopeptides Derived from MUC1 with a Flexible and Solvent-Exposed Sugar Moiety.

The use of vaccines based on MUC1 glycopeptides is a promising approach to treat cancer. We present herein several sulfa-Tn antigens incorporated in MUC1 sequences that possess a variable linker between the carbohydrate (GalNAc) and the peptide backbone. The main conformations of these molecules in solution have been evaluated by combining NMR experiments and molecular dynamics simulations. The linker plays a key role in the modulation of the conformation of these compounds at different levels, blocking a direct contact between the sugar moiety and the backbone, promoting a helix-like conformation for the glycosylated residue and favoring the proper presentation of the sugar unit for molecular recognition events. The feasibility of these novel compounds as mimics of MUC1 antigens has been validated by the X-ray diffraction structure of one of these unnatural derivatives complexed to an anti-MUC1 monoclonal antibody. These features, together with potential lack of immune suppression, render these unnatural glycopeptides promising candidates for designing alternative therapeutic vaccines against cancer.

Influence of amino acid stereocenters on the formation of bicyclic N,O-acetals.

In recent years, our group has reported the highly diastereoselective acid-catalyzed N,O-acetalization/intramolecular transcarbamoylation cascade of reactions between protected α-amino acid derivatives (Ser and Thr) and tetramethoxyalkanes. The resulting oligocyclic N,O-acetals have been used as excellent chiral building blocks for asymmetric transformations such as diastereoselective alkylation of the α-position. We now evaluate the scope of the reaction with related non-natural α-amino acid derivatives. A combined experimental and theoretical study reveals the key influence of the α-carbon substitution (serine versus α-methylserine) and the relative configuration of α-/ß-carbons (threonine versus allo-threonine) in the thermodynamic stability of the products and, as a consequence, the stereochemical outcome of the reaction. Notably, the complete diastereoselectivity achieved with natural amino acid precursors is completely lost with their non-natural analogues.

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-(ß-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.

A double diastereoselective Michael-type addition as an entry to conformationally restricted tn antigen mimics.

A totally stereocontrolled C-Michael addition of serine-equivalent C-nucleophiles to tri-O-benzyl-2-nitro-d-galactal was used as the key step to synthesize several pyrano[3,2-b]pyrrole structures. These scaffolds could be regarded as conformationally restricted Tn antigen mimics, as we have demonstrated by biological assays. The pyranose rings retain their (4)C1 chair conformation, as shown by molecular modeling and NMR spectroscopy. The expected bioactivity was established by a competition-tailored enzyme-linked lectin assay using both soybean and Vicia villosa agglutinins as model lectins. The facile described synthetic route and the strategic combination of computational and experimental techniques to reveal conformational features and bioactivity demonstrate the prepared glycomimics to be promising candidates for further exploitation of this scaffold to give glycans for lectin blocking and vaccination.

A biomimetic approach to lanthionines.

The asymmetric sulfa-Michael additions of appropriately protected L- and D-cysteine derivatives to new chiral dehydroamino acid derivatives have been developed as key steps in the synthesis of biologically important cysteine derivatives, such as lanthionine (Lan) and ß-methyllanthionine (MeLan), which are unusual bis-α-amino acids found in the emerging lantibiotics such as nisin.

A domino Michael/Dieckmann process as an entry to α-(hydroxymethyl)glutamic acid.

A domino process that involves a Michael-type addition followed by a Dieckmann reaction mediated by the participation of the cyclic carbamate group is the key step in the synthesis of both enantiomers of α-(hydroxymethyl)glutamic acid (HMG).

Role of the countercation in diastereoselective alkylations of pyramidalized bicyclic serine enolates. An easy approach to alpha-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.

Theoretical evidence for pyramidalized bicyclic serine enolates in highly diastereoselective alkylations.

A new chiral serine equivalent and its enantiomer have been synthesized from (S)- and (R)-N-Boc-serine methyl esters (Boc: tert-butyloxycarbonyl). The use of these compounds as chiral building blocks has been demonstrated in the synthesis of alpha-alkyl alpha-amino acids by diastereoselective potassium enolate alkylation reactions and subsequent acid hydrolyses. Theoretical studies were performed to elucidate the stereochemical outcome of both the formation of five-membered cyclic N,O-acetals and the subsequent alkylation process, which occurs with total retention of configuration. This feature could be explained in terms of the high degree of pyramidalization of enolate intermediates.

Asymmetric hetero Diels-Alder as an access to carbacephams.

A short and efficient asymmetric synthesis of the (6R,7S)-7-tert-butoxycarbonylamino-2-ketocarbacepham is described. The key step involves the hetero Diels-Alder reaction of the benzylimine derived from the enantiomer of Garner's aldehyde with Danishefsky's diene.

Preparation and Synthetic Applications of (S)- and (R)-N-Boc-N,O-isopropylidene-alpha-methylserinals: Asymmetric Synthesis of (S)- and (R)-2-Amino-2-methylbutanoic Acids (Iva).

This report describes an efficient and convenient large-scale synthesis procedure for (S)- and (R)-N-Boc-alpha-methylserinal acetonides (3 and 4) starting from (R)-2-methylglycidol 5. The application of both of these compounds as valuable chiral building blocks in the asymmetric synthesis of alpha-methylamino acids is also demonstrated by the synthesis of (S)- and (R)-isovalines (Iva) (6 and 7).