Preparation of S‐ and N‐Linked Glycosylated Amino Acid Building Blocks for Solid‐phase Glycopeptide Library Synthesis*

A general route for the preparation of 1,2‐trans‐linked S‐glycosylated amino acid building blocks by a Lewis‐acid‐promoted condensation of peracetylated glycosyl donors and N ^α‐Fmoc‐Cys‐OH, in good overall yield, is described. In addition, a short and time‐efficient route was applied for the synthesis of N‐glycosylated amino acid building blocks in good overall yields by coupling unprotected glycosylamines and N ^α‐Fmoc‐Asp(OH)‐O ^t Bu using TBTU activation.     

Diastereoselective Synthesis of C-Glycosylated Amino Acids with Lactams as Peptide Building Blocks

Readily available by lipase-catalyzed kinetic resolution or from a chiral pool, beta-, gamma-, and delta-lactams can be used as peptide building blocks for the synthesis of C-glycosylated amino acids 1. By reaction with glycosyl dianions, metabolic stable glycosylated amino acids can be prepared diastereoselectively. Ac=acetyl; Bn=benzyl; Boc=tert-butoxycarbonyl; R=Et, Bn; R'=H, alkyl; n=1-3.     

Synthesis and antifungal activities of N-glycosylated derivatives of Tunicyclin D,an antifungal octacyclopeptide

A series of glycosylated derivatives of Tunicyclin D were synthesized through a highly efficient and versatile synthetic method. The method is based on solid-phase peptide synthesis using 2-chlorotrityl resin as the solid-phase support and glycosyl amino acids as building blocks. Biological studies of the synthetic Tunicyclin D derivatives showed monosaccharide-containing compounds exhibit improved or similar antifungal activities, whereas the compounds carrying disaccharide glycans, showed much weaker antifungal activities.     

A Unified Strategy for the Synthesis of Mucin Cores 1–4 Saccharides and the Assembled Multivalent Glycopeptides

By displaying different O‐glycans in a multivalent mode, mucin and mucin‐like glycoproteins are involved in a plethora of protein binding events. The understanding of the roles of the glycans and the identification of potential glycan binding proteins are major challenges. To enable future binding studies of mucin glycan and glycopeptide probes, a method that gives flexible and efficient access to all common mucin core‐glycosylated amino acids was developed. Based on a convergent synthesis strategy starting from a shared early stage intermediate by differentiation in the glycoside acceptor reactivity, a common disaccharide building block allows for the creation of extended glycosylated amino acids carrying the mucin type‐2 cores 1–4 saccharides. Formation of a phenyl‐sulfenyl‐N‐Troc (Troc=trichloroethoxycarbonyl) byproduct during N‐iodosuccinimide‐promoted thioglycoside couplings was further characterized and a new methodology for the removal of the Troc group is described. The obtained glycosylated 9‐fluorenylmethoxycarbonyl (Fmoc)‐protected amino acid building blocks are incorporated into peptides for multivalent glycan display.     

A Convergent Strategy for the Synthesis of Type‐1 Elongated Mucin Cores 1–3 and the Corresponding Glycopeptides

Mucins are a class of highly O‐glycosylated proteins found on the surface of cells in epithelial tissues. O‐Glycosylation is crucial for the functionality of mucins and changes therein can have severe consequences for an organism. With that in mind, the elucidation of interactions of carbohydrate binding proteins with mucins, whether in morbidly altered or unaltered conditions, continue to shed light on mechanisms involved in diseases like chronic inflammations and cancer. Despite the known importance of type‐1 and type‐2 elongated mucin cores 1–4 in glycobiology, the corresponding type‐1 structures are much less well studied. Here, the first chemical synthesis of extended mucin type‐1 O‐glycan core 1–3 amino acid structures based on a convergent approach is presented. By utilizing differentiation in acceptor reactivity, shared early stage Tn‐ and T‐acceptor intermediates were elongated with a common type‐1 [β‐D ‐Gal‐1,3‐β‐D ‐GlcNAc] disaccharide, which allows for straightforward preparation of diverse glycosylated amino acids carrying the type‐1 mucin core 1–3 saccharides. The obtained glycosylated 9‐fluorenylmethoxycarbonyl (Fmoc)‐protected amino acid building blocks were employed in synthesis of type‐1 mucin glycopeptides, which are useful in biological applications.     

Internally Protected Amino Sugar Equivalents from Enantiopure 1,2‐Oxazines: Synthesis of Variably Configured Carbohydrates with C‐Branched Amino Sugar Units

A stereodivergent synthesis of differently configured C2‐branched 4‐amino sugar derivatives was accomplished. The Lewis acid mediated rearrangement of phenylthio‐substituted 1,2‐oxazines delivered glycosyl donor equivalents that can directly be employed in glycosidation reactions. Treatment with methanol provided internally protected amino sugar equivalents that have been transformed into the stereoisomeric methyl glycosides 28 , ent‐ 28 , 29 , ent‐ 29 and 34 in two simple reductive steps. Reaction with natural carbohydrates or bicyclic amino sugar precursors allowed the synthesis of homo‐oligomeric di‐ and trisaccharides 44 , 46 and 47 or a hybrid trisaccharide 51 with natural carbohydrates. Access to a bivalent amino sugar derivative 54 was accomplished by reaction of rearrangement product 10 with 1,5‐pentanediol. Alternatively, when a protected L ‐serine derivative was employed as glycosyl acceptor, the glycosylated amino acid 60 was efficiently prepared in few steps. In this report we describe the synthesis of unusual amino sugar building blocks from enantiopure 1,2‐oxazines that can be attached to natural carbohydrates or natural product aglycons to produce new natural product analogues with potential applications in medicinal chemistry.     

Efficient synthesis of complex glycopeptides based on unprotected oligosaccharides

N-Glycopeptides containing 1 to 4 trisaccharide chains, with the carbohydrates vicinal to each other in the multivalent glycopeptides, were efficiently synthesized by using the glycosylated Fmoc-asparagine as a key building block. While the couplings of amino acids with glycopeptides could be achieved in the homogeneous solutions in N-methylpyrrolidinone (NMP) to give excellent yields, all products were conveniently isolated from the reaction mixtures through a precipitation method by using the free carbohydrate chains as phase tags. Commercially available pentafluorophenyl (Pfp) esters of amino acids were employed for the glycopeptide elongation. Longer glycopeptides were constructed by means of a highly convergent synthetic design that is based on the coupling of glycopeptide/peptide fragments. Hydrogen bond interactions between free oligosaccharides were proposed to explain the exceptionally high efficiency of the couplings between two glycosylated building blocks.     

Development of building blocks for the synthesis of N-heterocyclic carbene ligands

[reaction: see text] The synthesis of a series of NHC building blocks that can then be incorporated into more complicated structures by palladium catalysis is reported. This approach is used for the synthesis of three amino acids containing NHC side chains. The ability to use the amino acids in solid-phase peptide synthesis to make NHC-containing peptides is also demonstrated. Additionally, the NHC side chain can be deprotected and coordinated to a catalytically active transition metal. Finally, it is illustrated that the building blocks participate in Suzuki coupling to provide access to substituted NHC ligands.     

Synthesis of a new conformationally constrained glycoamino acid building block

The synthesis of a suitably protected β-d-glucopyranosyl-(S)-α-methylserine derivative—a new conformationally constrained glycosylated quaternary amino acid analogue of β-d-glucopyranosyl-l-serine—is described. This compound can be used as an attractive building block for the synthesis of new, constrained glycopeptides.     

Stereoselective assembly of complex oligosaccharides using anomeric sulfonium ions as glycosyl donors

The development of selectively protected monosaccharide building blocks that can reliably be glycosylated with a wide variety of acceptors is expected to make oligosaccharide synthesis a more routine operation. In particular, there is an urgent need for the development of modular building blocks that can readily be converted into glycosyl donors for glycosylations that give reliably high 1,2-cis-anomeric selectivity. We report here that 1,2-oxathiane ethers are stable under acidic, basic, and reductive conditions making it possible to conduct a wide range of protecting group manipulations and install selectively removable protecting groups such as levulinoyl (Lev) ester, fluorenylmethyloxy (Fmoc)- and allyloxy (Alloc)-carbonates, and 2-methyl naphthyl ethers (Nap). The 1,2-oxathiane ethers could easily be converted into bicyclic anomeric sulfonium ions by oxidization to sulfoxides and arylated with 1,3,5-trimethoxybenzene. The resulting sulfonium ions gave high 1,2-cis-anomeric selectivity when glycosylated with a wide variety of glycosyl acceptors including properly protected amino acids, primary and secondary sugar alcohols and partially protected thioglycosides. The selective protected 1,2-oxathianes were successfully employed in the preparation of a branched glucoside derived from a glycogen-like polysaccharide isolated form the fungus Pseudallescheria boydii , which is involved in fungal phagocytosis and activation of innate immune responses. The compound was assembled by a latent-active glycosylation strategy in which an oxathiane was employed as an acceptor in a glycosylation with a sulfoxide donor. The product of such a glycosylation was oxidized to a sulfoxide for a subsequent glycosylation. The use of Nap and Fmoc as temporary protecting groups made it possible to install branching points.     

Sulfinimine-derived 2,3-diamino esters in the asymmetric synthesis of piperidine (2S,3S)-(+)-CP-99,994

Sulfinimine-derived, differentially protected, 2,3-diamino esters are useful building blocks for the asymmetric synthesis of heterocycles and is illustrated by an efficient synthesis of amino piperidine (+)-CP-99,994.     

Synthesis of Spirocyclic and Fused Isoxazoline Building Blocks

A highly efficient multigram synthesis of spirocyclic and fused isoxazoline building blocks is described. Isoxazoline-3-carboxylates were synthesized via a regioselective 1,3-dipolar cycloaddition reaction of 2-chloro-2-(hydroxyimino)acetate and carbo- or heterocyclic alkenes bearing endo- or exocyclic C=C double bonds, resulting in fused or spirocyclic isoxazolines, respectively. The preparation of up to 300 g of these compounds was achieved in a single run. The ester group of isoxazolines was then subjected to common synthetic transformations for the synthesis of corresponding building blocks, including alcohols, chlorides, azides, amines, aldehydes, carboxylic acids, amino acids and their derivatives, difluoromethyl-substituted compounds, and bicyclic γ-lactones. Additionally, a direct cycloaddition-based approach to the synthesis of aminoalkyl- and chloromethyl-substituted isoxazolines was proposed to improve their preparation. The described isoxazoline building blocks are expected to be valuable tools for drug discovery.     

Synthesis and applications of masked oxo-sulfinamides in asymmetric synthesis

This short perspective reports on the synthesis and applications of a class of chiral amino carbonyl compounds, masked oxo-sulfinamides where the amine is protected with an N-sulfinyl moiety and the carbonyl group is protected as the ketal or 1,3-dithiane. These polyfunctionalized chiral building blocks are prepared by addition of organometallic reagents to masked oxo-sulfinimines (N-sulfinyl imines) or the addition of oxo-organometallic reagents and lithio-1,3-dithianes to sulfinimines. Because unmasking of the amino and carbonyl groups results in cyclic imines, these chiral building blocks are particularly useful for the asymmetric synthesis of functionalized nitrogen heterocycles, including prolines, pipecolic acids, pyrrolidines, homotropinones, tropinones, and tropane alkaloids such as cocaine and C-1 cocaine analogues.     

N,O-Benzylidene Acetal Dipeptides (NBDs) Enable the Synthesis of Difficult Peptides via a Kinked Backbone Strategy

Proteins with highly hydrophobic regions or aggregation-prone sequences are typically difficult targets for chemical synthesis at the current stage, as obtaining such type of peptides via solid-phase peptide synthesis requires sophisticated operations. Herein, we report N,O-benzylidene acetal dipeptides (NBDs) as robust and effective building blocks to allow the direct synthesis of difficult peptides and proteins via a kinked backbone strategy. The effectiveness and easy accessibility of NBDs have been well demonstrated in our chemical syntheses of various challenging peptides and proteins, including chemokine, therapeutic hormones, histone, and glycosylated erythropoietin.     

Differently Glycosidated 2‐Amino‐2‐deoxy‐d‐glucopyranosiduronic Acids as Building Blocks in Peptide Synthesis

Seven differently glycosidated sugar amino acids (SSAs) derived from glucosamine have been prepared. Following standard solution‐phase peptide‐coupling procedures, the glycosidated 2‐amino‐2‐deoxy‐D ‐glucopyranosiduronic acids were condensed with natural amino acids to furnish useful heterodi‐ and ‐trimeric building blocks to be used in peptide synthesis. Combinations of these building blocks yielded hetero‐oligomeric peptides with two sugar amino acid units in different distances to each other. These were prepared to evaluate the influence of glycosidic side chains on the peptide backbone. Conformations of selected examples were examined by means of ROESY spectroscopy in combination with molecular dynamics (MD) simulations and circular‐dichroism (CD) studies.     

Expanding the genetic code

Although chemists can synthesize virtually any small organic molecule, our ability to rationally manipulate the structures of proteins is quite limited, despite their involvement in virtually every life process. For most proteins, modifications are largely restricted to substitutions among the common 20 amino acids. Herein we describe recent advances that make it possible to add new building blocks to the genetic codes of both prokaryotic and eukaryotic organisms. Over 30 novel amino acids have been genetically encoded in response to unique triplet and quadruplet codons including fluorescent, photoreactive, and redox-active amino acids, glycosylated amino acids, and amino acids with keto, azido, acetylenic, and heavy-atom-containing side chains. By removing the limitations imposed by the existing 20 amino acid code, it should be possible to generate proteins and perhaps entire organisms with new or enhanced properties.     

Light-directed maskless synthesis of peptide arrays using photolabile amino acid monomers

Novel photolabile amino acid monomers for photolithographic solid-phase peptide synthesis has been developed and a method for the maskless synthesis of individually addressable peptide microarrays using new building blocks has been described; these peptide microarrays are suitable for repetitive epitope-binding assays.     

Enantiocontrolled synthesis of alpha-methyl amino acids via Bn2N-alpha-methylserine-beta-lactone

[Reaction: see text] Enantiocontrolled synthesis of alpha-methyl amino acids proceeds via the regioselective organocuprate opening of Bn2N-alpha-methylserine-beta-lactone. From this chiral intermediate, a wide variety of alpha-methyl amino acids and building blocks were synthesized in excellent yields.     

A Unified Approach to Phytosiderophore Natural Products

This work reports on the concise total synthesis of eight natural products of the mugineic acid and avenic acid families (phytosiderophores). An innovative „east-to-west“ assembly of the trimeric products resulted in a high degree of divergence enabling the formation of the final products in just 10 or 11 steps each with a minimum of overall synthetic effort. Chiral pool starting materials (l -malic acid, threonines) were employed for the outer building blocks while the middle building blocks were accessed by diastereo- and enantioselective methods. A highlight of this work consists in the straightforward preparation of epimeric hydroxyazetidine amino acids, useful building blocks on their own, enabling the first synthesis of 3’’-hydroxymugineic acid and 3’’-hydroxy-2’-deoxymugineic acid.     

Expanding the Genetic Code

Although chemists can synthesize virtually any small organic molecule, our ability to rationally manipulate the structures of proteins is quite limited, despite their involvement in virtually every life process. For most proteins, modifications are largely restricted to substitutions among the common 20 amino acids. Herein we describe recent advances that make it possible to add new building blocks to the genetic codes of both prokaryotic and eukaryotic organisms. Over 30 novel amino acids have been genetically encoded in response to unique triplet and quadruplet codons including fluorescent, photoreactive, and redox‐active amino acids, glycosylated amino acids, and amino acids with keto, azido, acetylenic, and heavy‐atom‐containing side chains. By removing the limitations imposed by the existing 20 amino acid code, it should be possible to generate proteins and perhaps entire organisms with new or enhanced properties.