Glycosyl alkoxythioimidates as complementary building blocks for chemical glycosylation

It is reported that S-glycosyl O-methyl phenylcarbamothioates (SNea carbamothioates) have a fully orthogonal character in comparison to S-benzoxazolyl (SBox) glycosides. This complete orthogonality was revealed by performing competitive glycosylation experiments in the presence of various promoters. The results obtained indicate that SNea carbamothioates have a very similar reactivity profile to that of glycosyl thiocyanates, yet are significantly more stable and tolerate selected protecting group manipulations. These features make the SNea carbamothioates new promising building blocks for further utilization in oligosaccharide synthesis.     

On orthogonal and selective activation of glycosyl thioimidates and thioglycosides: application to oligosaccharide assembly

Discrimination among S-thiazolinyl (STaz), S-benzoxazolyl (SBox), and S-ethyl anomeric leaving groups was achieved by fine-tuning activation conditions. Preferential glycosidation of a certain leaving group is determined neither by the strength of the activating reagent nor by the stability of the leaving group itself; instead, the type of activation plays the key role. The activation conditions established herein were applied to a sequential five-step synthesis of a hexasaccharide using six monosaccharide building blocks equipped with six different leaving groups.     

Hydrogen bonding in labdane diterpenoids, labda-7,12(E),14-triene-17-oic acid and labda-12(Z),14,17-triene-18-oic acid

Two labdane diterpenoids, labda-7,12(E),14-triene-17-oic acid (1) and labda-12(Z),14,17-triene-18-oic acid (2), C₂₀H₃₀O₂, have been isolated from Croton oblongifolius. Both 1 and 2 crystallized in the monoclinic system, space group C2, with cell dimensions of a = 21.912(1) Å, b = 7.4002(4) Å, c = 11.5079(7) Å, = 101.999(1)^o and a = 21.308(2) Å, b = 11.9067(9) Å, c = 7.5606(6) Å, = 100.763(1)^o, respectively. Compound 1, a rare example of carboxylic group bound to a cyclohexene ring, forms an infinite intermolecular hydrogen-bonded polymer [O1 O2(–x + 1/2, y + 1/2, –z + 2) 2.697(2) Å], whereas molecules of compound 2 are linked to form an asymmetric hydrogen-bonded dimer [O1 O2(–x, y, –z) 2.657(3) Å].     

<Emphasis Type="Italic">seco</Emphasis>-Kaurane skeleton diterpenoids from <Emphasis Type="Italic">Croton oblongifolius</Emphasis>

A new seco-kaurane type diterpenoid, ent-3,4-seco-17-oxo-kaur-4(19),15(16)-dien-3-oic acid, and a known compound, ent-3,4-seco-kaur-4(19),16(17)-dien-3-oic acid, were isolated from the stem bark of Croton oblongifolius. The structures of these compounds were established on the basis of spectroscopic data.     

Silver(I) tetrafluoroborate as a potent promoter for chemical glycosylation

We have identified silver tetrafluoroborate (AgBF₄) as an excellent promoter for the activation of various glycosyl donors including glycosyl halides, trichloroacetimidates, and thioimidates. Easy handling and no requirement for azeotropic dehydration prior to application makes AgBF₄ especially beneficial in comparison to the commonly used AgOTf. Selective activation of glycosyl halides or thioimidates over thioglycosides or n-pentenyl glycosides, including simple sequential one-pot syntheses, has also been demonstrated. Versatility of glycosyl thioimidates was further explored by converting these intermediates into a variety of other classes of glycosyl donors.     

Diaporthichalasin, a novel CYP3A4 inhibitor from an endophytic Diaporthe sp.

A novel CYP3A4 inhibitor, diaporthichalasin, together with pycnidione were isolated from an endophytic fungus, Diaporthe sp. Their structures were elucidated on the basis of spectral data and the structure of diaporthichalasin was confirmed by X-ray crystallographic analysis. Diaporthichalasin exhibited significantly potent inhibition of CYP3A4 with an IC₅₀ value of 0.626 μM, while the IC₅₀ value of pycnidione was 465 μM.     

Synthesis of the Highly Glycosylated Hydrophilic Motif of Extensins

Extensin, the structural motif of plant extracellular matrix proteins, possesses a unique highly glycosylated, hydrophilic, and repeating Ser₁Hyp₄ pentapeptide unit, and has been proposed to include post‐translational hydroxylation at proline residue and subsequent oligo‐L ‐arabinosylations at all of the resultant hydroxyprolines as well as galactosylation at serine residue. Reported herein is the stereoselective synthesis of one of the highly glycosylated motifs, Ser(Galp₁)‐Hyp(Araf₄)‐Hyp(Araf₄)‐Hyp(Araf₃)‐Hyp(Araf₁). The synthesis has been completed by the application of 2‐(naphthyl)methylether‐mediated intramolecular aglycon delivery to the stereoselective construction of the Ser(Galp₁) and Hyp(Araf_n) fragments as the key step, as well as Fmoc solid‐phase peptide synthesis for the backbone pentapeptide.     

Fungicidal activity of tributyltin alkylsulfate esters

Tributyltin alkyl sulfate esters, e.g. tributyltin dodecyl sulfate, tributyltin hexadecyl sulfate and tributyltin octadecyl sulfate, were synthesized from the reaction of bis(tributyltin) oxide and appropriate alkyl hydrogen sulfates. The resulting tributyltin alkyl sulfate esters were found to exhibit fungicidal activity on Penicillium, Aspergillus and Syncephalustrum species.