Galacturonic acid lactones in the synthesis of all trisaccharide repeating units of the zwitterionic polysaccharide Sp1

A modular approach toward the synthesis of all possible trimer repeating units of the type 1 capsular polysaccharide of Streptococcus pneumonia, Sp1, is described. This zwitterionic polysaccharide is built up from trisaccharide repeats, which in turn are composed of two galacturonic acid monomers and a 2,4,6-trideoxy-4-amino-2-acetamido-D-galactose moiety. All monomeric constituents are linked through cis-glycosidic bonds. To overcome the difficulty associated with the efficient stereoselective introduction of the α-galacturonic acid bonds, we have employed galacturonic acid-[3,6]-lactone building blocks. Not only did these building blocks perform well when used as donor galactosides, they were also shown to be reactive acceptor glycosides when equipped with a free hydroxyl function. All three frame-shifted trimer repeats were constructed via highly stereoselective glycosylation reactions, with one exception. The epimeric mixture of trisaccharides, formed in the nonselective glycosylation event, could be readily separated after global deprotection using high performance anion exchange chromatography (HPEAC).     

First synthesis of C. difficile PS-II cell wall polysaccharide repeating unit

Clostridium difficile is the most commonly diagnosed cause of nosocomial diarrhea with increasing incidence and mortality among elderly and hospitalized patients. We report the first synthesis of the surface polysaccharide PS-II repeating unit and its nonphosphorylated analogue, with a linker for conjugation, via a (4 + 2) convergent approach from a common AB(D)C tetrasaccharide intermediate.     

An expedient synthesis of the repeating unit of the acidic polysaccharide of the bacteriolytic complex of lysoamidase

The first synthesis of the trisaccharide repeating unit of the acidic polysaccharide of the bacteriolytic complex of lysoamidase is presented. The construction is based on a linear glycosylation strategy that starts from the reducing end and employs thio- and selenoglycosides in a highly stereoselective manner by a single set of activation conditions. The thus-formed trisaccharide is selectively deprotected and oxidised, after which a final deprotection step furnishes the desired repeating unit.     

A synthetic study towards the PSA1 tetrasaccharide repeating unit

A synthetic study to the protected tetrasaccharide repeating unit of zwitterionic polysaccharide PSA1 using 1-thio, 1-seleno and 1-hydroxyl functionalized donor glycosides is presented. The ABC trisaccharide part was successfully assembled using an iterative dehydrative glycosylation protocol.     

Convergent synthesis of the tetrasaccharide repeating unit related to the O-antigenic polysaccharide of Escherichia coli 78

A convergent synthesis of the tetrasaccharide repeating unit of the O-antigenic cell wall polysaccharide of Escherichia coli 78, as the corresponding methyl glycoside (I), is being reported. It involved stereoselective glycosidation of a β-linked mannodisaccharide acceptor with a β-linked glucosamine based disaccharide thioglycoside donor, which were prepared from the corresponding functionalised monosaccharide based glycosyl donors and acceptors. The resulting tetrasaccharide derivative was finally converted to (I) by selective deprotection and also by global protection and deprotection techniques.     

First synthesis of a pentasaccharide repeating unit of the O-antigenic polysaccharide from enterohaemorrhagic Escherichia coli O48:H21

A concise synthesis of a pentasaccharide as its 4-methoxyphenyl glycoside, found in the O-antigenic polysaccharide of enterohaemorrhagic Escherichia coli O48:H21 has been achieved for the first time in excellent yield. Most of the intermediate steps are high yielding and the stereooutcome of each glycosylation step was excellent. Stereoselective glycosylation and removal of the 4-methoxybenzyl group were achieved in one-pot by tuning the reaction conditions. A late-stage TEMPO-mediated oxidation strategy has been adopted for the oxidation of a primary hydroxyl group to carboxylic acid.     

Total synthesis of the heptasaccharide repeating unit of the iron-binding exopolysaccharide secreted by Klebsiella oxytoca BAS-10

The first total synthesis of a heptasaccharide found in the iron-binding exopolysaccharide produced by Klebsiella oxytoca BAS-10 has been achieved in excellent yield using a block synthetic strategy. A trisaccharide glycosyl donor was stereoselectively coupled with a tetrasaccharide glycosyl acceptor using the trichloroacetimidate activation procedure. The yields and stereo outcome were excellent in each step of glycosylation. A late stage oxidation protocol was adopted for the oxidation of the primary hydroxyl group to the carboxylic functionality while keeping a secondary hydroxyl group unaffected.     

Progress toward developing a carbohydrate-conjugate vaccine against Clostridium difficile ribotype 027: synthesis of the cell-surface polysaccharide PS-I repeating unit

Clostridium difficile strain ribotype 027 is a hypervirulent pathogen that is responsible for recent, severe outbreaks of serious nosocomial infections. As a foundation for the development of a preventative carbohydrate-based vaccine, we have synthesized a pentasaccharide cell wall repeating unit from PS-I unique to this strain, by the linear assembly of four monosaccharide building blocks.     

Synthesis of the trisaccharide repeating unit of capsular polysaccharide from Klebsiella pneumoniae

The incidences of primary, pyrogenic liver abscess complicated by meningitis and septic endophthalmitis caused by Klebsiella pneumoniae has increased globally. Earlier studies have shown that the capsular polysaccharide (CPS) of Klebsiella pneumoniae plays an important role in the resistance to phagocytosis and related pathogenicity. The first chemical synthesis of the trisaccharide repeating unit of this CPS has been achieved via stereoselective glycosylation with orthogonal and appropriate protecting groups. A new method for the pyruvylation of trans diol was developed.     

Synthesis of the pentasaccharide repeating unit of the O-specific polysaccharide from salmonella strasbourg

The pentasaccharide α - Tyv - (1→3) - β - d - Man - (1→4) - α - l - Rha - (1→3) - d - Gal - (4←1) -α - d - Glc 1, the repeating unit of the O-specific polysaccharide chain of the lipopolysaccharide from S. Strasbourg, was obtained by glycosylation of benzyl - 2,6 - di - O - benzyl - 4 - O - (2,3,4 - tri - O - benzyl - 6 - O - benzoyl - α - d - glucopyranosyl) - β - d - galactopyranoside with 1,2 - methylorthoacetyl - 3 - O - acetyl - 4- O - [3 - O - (2,4 - di - O - acetyl - 3, 6 - dideoxy,- α - d - arabino - hexopyranosyl) - 2,4,6 - tri - O - acetyl - β - d - mannopyranosyl] - β - l - rhamnopyranose 3 followed by removal of protecting groups. The structure of the synthetic pentasaccharide was proved by methylation analysis and ¹³C NMR.     

Efficient multigram synthesis of the repeating unit of gallic acid-triethylene glycol dendrimers

A multigram synthesis of the repeating unit of GATG (gallic acid-triethylene glycol) dendrimers is described through an efficient and cost-effective route. These conditions overcome major problems precluding scaling up and afford product in excellent overall yield and purity. Special attention has been paid in this process to green chemistry principles: atom economy, safety, and waste reduction. This scheme could be easily adapted for the preparation of similar dendritic systems.     

Synthesis of the tetrasaccharide repeating unit of the o-specific polysaccharide from salmonella muenster and salmonella minneapolis

The tetrasaccharide β-D-Man-(1→4)-α-L-Rha-(1→3)-D-Gal-(4←1)-α-D-Glc the repeating unit of the O-specific polysaccharide chain of the lipopolysaccharides from Salmonella muenster and S, minneapolis was obtained by glycosylation of benzyl 2,6-di-O-benzyl-4-0(2,3,4-tri-O-benzyl-6-O-benzoyl-α-D-glucopyranosyl)-β-D-galactopyranoside with 3-O-acetyl-4-0-(2,3,4,6-tetra-O-acetyl-β-D-mannopyranosyl)- β-L-rhamnopyranose-1,2-(methylorthoacetate), followed by removal of protecting groups. The structure of the synthetic tetrasaccharide was confirmed by methylation analysis, CrO₃-AcOH oxidation and ¹³C-NMR.     

Total synthesis of GEX1A

An efficient and readily modifiable synthesis of GEX1A/herboxidiene/TAN-1609 ( 1) was developed. This modular synthesis featured a Suzuki coupling to install the conjugated diene and a Ru-catalyzed lactonization and Roush crotylation to construct the functionalized tetrahydropyran moiety. Myers' alkylation, cross-metathesis, and Keck crotylation were employed for assembly of the biologically essential side-chain domain.     

Total synthesis of bafilomycin A1

A convergent synthesis of bafilomycin A(1), a potent inhibitor of V-type ATPases, is presented. The synthesis relies on the zinc triflate mediated diastereoselective addition of a complex enyne to a sensitive aldehyde as the key fragment coupling. A ruthenium-catalyzed trans-reduction of the resulting propargylic enyne efficiently installs the required C10-C13 trans,trans-diene subunit, implementing an alternative strategy to traditional palladium-catalyzed cross-coupling strategies. A highly selective oxidation of a secondary hydroxyl group in a triol sets the stage for the completion of the synthesis.     

Total synthesis of myxovirescin A1

A convergent total synthesis of the antibiotic macrolide myxovirescin A1 (1) is described that is largely based on reagent- and catalyst-controlled transformations. This includes a highly regioselective Negishi reaction of dibromo-alkene 48 with an alkynylzinc reagent, and a palladium catalyzed alkyl-Suzuki coupling of the resulting enyne derivative 12 with the 9-BBN-adduct derived from alkene 61. The latter was obtained via an asymmetric hydrogenation of the chlorinated beta-ketoester 49 and an anti-selective oxyallylation of the functionalized aldehyde 53 as the key steps. The preparation of the bis-borylated allyl-donor 57 used in the oxyallylation step, however, required careful optimization and led to important insights into the nature of the classical hydroborating agent "di(isopinocampheyl)borane (Ipc2BH)". It was unambiguously shown by X-ray crystallography that in the solid state this compound is dimeric, but it is prone to undergo an essentially quantitative mono-deborylation when dissolved in CH2Cl2 or benzene; its composition in ethereal solvents is even more complex as evident from 11B NMR data. Product 71 derived from 12 and 61 was elaborated into the enyne-yne derivative 75, which served as the substrate for an exquisitely selective ring closing alkyne metathesis reaction (RCAM) catalyzed by the molybdenum tris-amido complex 20 activated in situ with CH2Cl2. The resulting cyclic enyne 76 was subjected to a ruthenium catalyzed trans-hydrosilylation/proto-desilylation tandem. Although [Cp*Ru(MeCN)3]PF6 had previously been recommended as catalyst of choice for trans-hydrosilylation reactions of internal alkynes, this complex failed to afford the desired product, whereas its sterically less hindered congener [CpRu(MeCN)3]PF6 permitted the reaction to be performed in appreciable yield, but at the expense of a lower stereoselectivity. AgF-mediated proto-desilylation of the isomeric silanes 79 and 80 followed by cleavage of the remaining acetal protecting groups afforded myxovirescin A1 and its hitherto unknown 14Z-isomer 81, respectively.     

Total synthesis of Cinncassin A1

In this communication, we report the first total synthesis of the phenolic natural product Cinncassin A₁, which was recently isolated from the twigs of the cinnamon plant (Cinnamomum cassia). The novel and concise synthesis strategy involves a highly diastereoselective titanium enolate homocoupling and a permanganate-mediated oxidative lactonization reaction as key steps. The structure of the compound was verified by single-crystal X-ray crystallography.     

Total synthesis and properties of the crambescidin core zwitterionic acid and crambescidin 359

The total synthesis of the crambescidin core acid 9, crambescidins 359 (8) and 431 (7), and the properties of the crambescidin core are described. A key step of the synthetic route to guanidinium carboxylate 9 is Pd(0) catalyzed cleavage of the ester side chain of pentacyclic cinnamyl ester 15. This ester is also employed to prepare a small library of crambescidin alkaloid analogues that differ in their C14 side chain. The zwitterionic guanidinium carboxylate 9 was shown to readily decarboxylate to form crambescidin 359 (8). Decarboxylation of crambescidin core acid 9 was fastest under basic conditions. In the presence of base, up to eight deuterium atoms can be incorporated into the pentacyclic crambescidin core. Both deuterium incorporation and decarboxylation of crambescidin core acid 9 are the result of facile ring opening of the spirocyclic ether rings of the pentacyclic guanidinium moiety.