Fabian Pfrengle

Three Carbons for Complexity! Recent Developments of Palladium-Catalyzed Reactions of Allenes

The three carbon atoms of allene moieties allow unique transformations and rapid generation of complexity. Not surprisingly, allenes became extremely versatile building blocks in organic synthesis. Transition‐metal‐catalyzed reactions of these cumulene π‐systems have been particularly successful, and many applications in the synthesis of complex products have been reported. This review summarizes the palladium‐catalyzed transformation of allenes published during the last decade. Many of the examples presented are impressive multicomponent processes or cascade reactions involving two or more steps leading to molecular complexity in simple one‐pot operations. Consequently, several reactions have been developed with the goal of delivering new synthetic routes to natural products.

Automated Synthesis of Arabinoxylan‐Oligosaccharides Enables Characterization of Antibodies that Recognize Plant Cell Wall Glycans

Monoclonal antibodies that recognize plant cell wall glycans are used for high-resolution imaging, providing important information about the structure and function of cell wall polysaccharides. To characterize the binding epitopes of these powerful molecular probes a library of eleven plant arabinoxylan oligosaccharides was produced by automated solid-phase synthesis. Modular assembly of oligoarabinoxylans from few building blocks was enabled by adding (2-naphthyl)methyl (Nap) to the toolbox of orthogonal protecting groups for solid-phase synthesis. Conjugation-ready oligosaccharides were obtained and the binding specificities of xylan-directed antibodies were determined on microarrays.

Enantiopure Aminopyrans by a Lewis Acid Promoted Rearrangement of 1,2-Oxazines: Versatile Building Blocks for Oligosaccharide and Sugar Amino Acid Mimetics

1,3-Dioxolanyl-substituted 1,2-oxazines, such as syn-1 and anti-1, rearrange under Lewis acidic conditions to provide bicyclic products 2-5. Subsequent reductive transformations afforded enantiopure 3-aminopyran derivatives such as 7 and 9 or their protected diastereomers 16 and 18, which can be regarded as carbohydrate mimetics. An alternative sequence of transformations including selective oxidation of the primary hydroxyl groups in 21 and 24 led to two protected beta-amino acid derivatives with carbohydrate-like backbone (sugar amino acids). Treatment of bicyclic ester 23 with samarium diiodide cleaved the N--O bond and furnished the unusual beta-lactam 27 in excellent yield. Alternatively, gamma-amino acid derivative 29 was efficiently prepared in a few steps. Fairly simple transformations gave azides 32 and 35 or alkyne 30 which are suitable substrates for the construction of oligosaccharide mimetics such as 34 by copper iodide catalyzed cycloadditions. With this report we demonstrate that enantiopure rearrangement products 2-5 are protected precursors of a variety of polyfunctionalized pyran derivatives with great potential for chemical biology.

A Synthetic Glycan Microarray Enables Epitope Mapping of Plant Cell Wall Glycan-Directed Antibodies

Determining exact epitopes for cell wall-directed monoclonal antibodies provides the basis for a detailed elucidation of polysaccharide structures at the cellular level. In the last three decades, more than 200 monoclonal antibodies have been raised against most classes of plant cell wall polysaccharides by different laboratories worldwide. These antibodies are widely used to identify differences in plant cell wall components in mutants, organ and tissue types, and developmental stages. Despite their importance and broad use, the precise binding epitope has been determined for only a few of these antibodies. Here, we use a plant glycan microarray equipped with 88 synthetic oligosaccharides to comprehensively map the epitopes of plant cell wall glycan-directed antibodies. Our results reveal the binding epitopes for 78 arabinogalactan-, rhamnogalacturonan-, xylan-, and xyloglucan-directed antibodies. We demonstrate that, with knowledge of the exact epitopes recognized by individual antibodies, specific glycosyl hydrolases can be implemented into immunological cell wall analyses, providing a framework to obtain structural information on plant cell wall glycans with unprecedented molecular precision.

Automated Glycan Assembly of Oligosaccharides Related to Arabinogalactan Proteins

Arabinogalactan proteins are heavily glycosylated proteoglycans in plants. Their glycan portion consists of type-II arabinogalactan polysaccharides whose heterogeneity hampers the assignment of the arabinogalactan protein function. Synthetic chemistry is key to the procurement of molecular probes for plant biologists. Described is the automated glycan assembly of 14 oligosaccharides from four monosaccharide building blocks. These linear and branched glycans represent key structural features of natural type-II arabinogalactans and will serve as tools for arabinogalactan biology.

A New Ring Closure Approach to Enantiopure 3,6-Dihydro-2H-pyrans: Stereodivergent Access to Carbohydrate Mimetics

A set of enantiopure carbohydrate mimetics has been synthesized via Lewis acid promoted cyclization of 1,3-dioxolanyl-substituted enol ethers as a crucial new step providing highly functionalized 3,6-dihydro-2H-pyran derivatives. The flexible approach starting from glyceraldehyde-derived nitrone is comprised of only six simple steps smoothly allowing synthetic modifications at the different stages of the sequence. All reactions proceeded with good to excellent stereocontrol and can be performed with either of the two enantiomers.

Stereodivergent De Novo Synthesis of Branched Amino Sugars by Lewis Acid Promoted Rearrangement of 1,2‐Oxazines

Well concealed: 1,2-oxazines such as 1 rearrange under Lewis acidic conditions to bicyclic products of type 2, which can be incorporated into oligosaccharides as protected amino sugar equivalents. Subsequent reductive steps provide unusual oligosaccharides 3 having C2-branched 4-amino sugar units. Most of the reactions proceed with excellent stereocontrol and allow the synthesis of a collection of stereoisomers.

Active Site-Mapping of Xylan-Deconstructing Enzymes with Arabinoxylan Oligosaccharides Produced by Automated Glycan Assembly

Xylan-degrading enzymes are crucial for the deconstruction of hemicellulosic biomass, making the hydrolysis products available for various industrial applications such as the production of biofuel. To determine the substrate specificities of these enzymes, we prepared a collection of complex xylan oligosaccharides by automated glycan assembly. Seven differentially protected building blocks provided the basis for the modular assembly of 2-substituted, 3-substituted, and 2-/3-substituted arabino- and glucuronoxylan oligosaccharides. Elongation of the xylan backbone relied on iterative additions of C4-fluorenylmethoxylcarbonyl (Fmoc) protected xylose building blocks to a linker-functionalized resin. Arabinofuranose and glucuronic acid residues have been selectively attached to the backbone using fully orthogonal 2-(methyl)naphthyl (Nap) and 2-(azidomethyl)benzoyl (Azmb) protecting groups at the C2 and C3 hydroxyls of the xylose building blocks. The arabinoxylan oligosaccharides are excellent tools to map the active site of glycosyl hydrolases involved in xylan deconstruction. The substrate specificities of several xylanases and arabinofuranosidases were determined by analyzing the digestion products after incubation of the oligosaccharides with glycosyl hydrolases.

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.

Mixed-Linkage Glucan Oligosaccharides Produced by Automated Glycan Assembly Serve as Tools to Determine the Substrate Specificity of Lichenase

The mixed-linkage (1→3),(1→4)-d-glucan (MLG) specific glycosyl hydrolase lichenase is an important biochemical tool for the structural characterization of MLGs. It holds potential for application in the brewery, animal feed, and biofuel industries. Several defined MLG oligosaccharides obtained by automated glycan assembly are used to analyze the substrate specificities of Bacillus subtilis lichenase. Two glucose building blocks (BBs), equipped with a temporary fluorenylmethyloxycarbonyl chloride (Fmoc) protecting group in the C-3 or C-4 position, served to assemble different oligosaccharides by using an automated oligosaccharide synthesizer. Light-induced cleavage of the glycan products from the solid support followed by global deprotection provided seven MLG oligosaccharides of different length and connectivity. After incubation of the MLG oligosaccharides with lichenase, the digestion products were analyzed by HPLC-MS. These digestion experiments provided insights into the enzymes active site that is in line with other recent evidence suggesting that the substrate specificity of lichenases has to be reconsidered. These results demonstrate that synthetic MLG oligosaccharides are useful tools to analyze mixed-linkage β-glucanases.