The Implications of (2S,4S)‐Hydroxyproline 4‐O‐Glycosylation for Prolyl Amide Isomerization

The conformations of peptides and proteins are often influenced by glycans O‐linked to serine (Ser) or threonine (Thr). (2S,4R)‐4‐Hydroxyproline (Hyp), together with L ‐proline (Pro), are interesting targets for O‐glycosylation because they have a unique influence on peptide and protein conformation. In previous work we found that glycosylation of Hyp does not affect the N‐terminal amide trans/cis ratios (K_trans/cis) or the rates of amide isomerization in model amides. The stereoisomer of Hyp—(2S,4S)‐4‐hydroxyproline (hyp)—is rarely found in nature, and has a different influence both on the conformation of the pyrrolidine ring and on K_trans/cis. Glycans attached to hyp would be expected to be projected from the opposite face of the prolyl side chain relative to Hyp; the impact this would have on K_trans/cis was unknown. Measurements of ³J coupling constants indicate that the glycan has little impact on the C^γ‐endo conformation produced by hyp. As a result, it was found that the D ‐galactose residue extending from a C^γ‐endo pucker affects both K_trans/cis and the rate of isomerization, which is not found to occur when it is projected from a C^γ‐exo pucker; this reflects the different environments delineated by the proline side chain. The enthalpic contributions to the stabilization of the trans amide isomer may be due to disruption of intramolecular interactions present in hyp; the change in enthalpy is balanced by a decrease in entropy incurred upon glycosylation. Because the different stereoisomers—Hyp and hyp—project the O‐linked carbohydrates in opposite spatial orientations, these glycosylated amino acids may be useful for understanding of how the projection of a glycan from the peptide or protein backbone exerts its influence.     

A Novel Synthesis of the Macrocyclic Spermidine Alkaloid (+)‐(S)‐Dihydroperiphylline

A novel, short, and highly stereoselective synthesis of the macrocyclic spermidine alkaloid (+)‐(S)‐dihydroperiphylline ( 15 ) is described. The key synthetic steps were the stereoselective addition of the chiral amine 1 to the cinnamate 2 and cyclization of the bis[toluene‐4‐sulfonamide] precursor 12 in the presence of Cs₂CO₃ as a template. Unambiguous assignments of the signals in both the ¹H‐ and ¹³C‐NMR spectra of 15 were achieved by 2D NMR spectra.     

Total Synthesis of (−)‐Daphenylline

A concise and highly stereoselective total synthesis of the Daphniphyllum alkaloids (?)‐daphenylline has been accomplished. The synthesis was started from (S)‐carvone and proceeded via a stereoselective Mg(ClO₄)₂‐catalyzed intramolecular amide addition cyclization, an intramolecular Diels–Alder reaction to construct the ABCD tetracyclic core architecture, and a Robinson annulation coupled with an oxidative aromatization sequence. Finally, the DF ring system was installed through an intramolecular Friedel–Crafts cyclization. The total synthesis of (?)‐daphenylline is achieved in 19 steps in the longest reaction sequence and in 7.6 % overall yield.     

Synthesis of (2′S)‐2′‐Deoxy‐2′‐C‐methylpurine Nucleosides and Their Phosphoramidites

We describe the stereoselective synthesis of (2′S)‐2′‐deoxy‐2′‐C‐methyladenosine ( 12 ) and (2′S)‐2′‐deoxy‐2′‐C‐methylinosine ( 14 ) as well as their corresponding cyanoethyl phosphoramidites 16 and 19 from 6‐O‐(2,6‐dichlorophenyl)inosine as starting material. The methyl group at the 2′‐position was introduced via a Wittig reaction (→ 3 , Scheme 1) followed by a stereoselective oxidation with OsO₄ (→ 4 , Scheme 2). The primary‐alcohol moiety of 4 was tosylated (→ 5 ) and regioselectively reduced with NaBH₄ (→ 6 ). Subsequent reduction of the 2′‐alcohol moiety with Bu₃SnH yielded stereoselectively the corresponding (2′S)‐2′‐deoxy‐2′‐C‐methylnucleoside (→ 8a ).     

Total Chemical Synthesis and Biological Activities of Glycosylated and Non‐Glycosylated Forms of the Chemokines CCL1 and Ser‐CCL1

CCL1 is a naturally glycosylated chemokine protein that is secreted by activated T‐cells and acts as a chemoattractant for monocytes. 1 Originally, CCL1 was identified as a 73 amino acid protein having one N‐glycosylation site, 1 and a variant 74 residue non‐glycosylated form, Ser‐CCL1, has also been described. 2 There are no systematic studies of the effect of glycosylation on the biological activities of either CCL1 or Ser‐CCL1. Here we report the total chemical syntheses of both N‐glycosylated and non‐glycosylated forms of (Ser‐)CCL1, by convergent native chemical ligation. We used an N‐glycan isolated from hen egg yolk together with the Nbz linker for Fmoc chemistry solid phase synthesis of the glycopeptide‐^αthioester building block. 3 Chemotaxis assays of these glycoproteins and the corresponding non‐glycosylated proteins were carried out. The results were correlated with the chemical structures of the (glyco)protein molecules. To the best of our knowledge, these are the first investigations of the effect of glycosylation on the chemotactic activity of the chemokine (Ser‐)CCL1 using homogeneous N‐glycosylated protein molecules of defined covalent structure.     

A Novel 2H‐Azirin‐3‐amine as a Dipeptide (Aib‐Hyp) Synthon

The synthesis of methyl (2S,4R)‐4‐(benzyloxy)‐N‐(2,2‐dimethyl‐2H‐azirin‐3‐yl)prolinate ( 10 ), a novel 2H‐azirin‐3‐amine (`3‐amino‐2H‐azirine'), is described (Scheme 1). The reaction of methyl (2S,4R)‐N‐(2‐methylpropanoyl)‐4‐(benzyloxy)prolinate ( 7 ) with Lawesson reagent gave methyl (2S,4R)‐4‐(benzyloxy)‐N‐[2‐(methylthio)propanoyl]prolinate ( 8 ) and consecutive treatment with COCl₂, 1,4‐diazabicyclo[2.2.2]octane (DABCO), and NaN₃ led to 10 . The use of 10 as a building block of the dipeptide Aib‐Hyp (Aib=2‐aminoisobutyric acid, Hyp=(2S,4R)‐4‐hydroxyproline) is demonstrated by the syntheses of several model peptides (Scheme 2 and Table). The benzyl protecting group of the 4‐OH function in Hyp in the model peptides has been removed in good yields.     

Thermosensitive gel formation of novel polypeptides containing a collagen‐derived Pro‐Hyp‐Gly sequence and an elastin‐derived Val‐Pro‐Gly‐Val‐Gly sequence

A triple‐helix‐forming collagen model peptide, (prolyl‐trans‐4‐hydroxyprolyl‐glycyl)₁₀ [(Pro‐Hyp‐Gly)₁₀], and a thermosensitive elastin‐derived pentapeptide, valyl‐prolyl‐glycyl‐valyl‐glycyl (Val‐Pro‐Gly‐Val‐Gly), were copolymerized in various mole ratios using 1‐ethyl‐3‐(3‐dimethylaminopropyl)‐carbodiimide hydrochloride and 1‐hydroxybenzotriazole in dimethyl sulfoxide at 20 °C. All of the obtained polypeptides have molecular weight higher than 10³ and contain a triple‐helical structure, and showed an inverse phase transition from transparent solution to turbid suspension in response to a rise in temperature. The lower critical solution temperature of the polypeptide solution decreased upon increasing the content of Val‐Pro‐Gly‐Val‐Gly. Furthermore, polypeptides containing 82–86 mol % of Val‐Pro‐Gly‐Val‐Gly in composition showed reversible gel formation, suggesting that (Pro‐Hyp‐Gly)₁₀ acts as a hydrated unit and Val‐Pro‐Gly‐Val‐Gly acts as a thermosensitive crosslinking point. These biodegradable thermosensitive polypeptides may be useful for biomedical applications, including, as a scaffold for tissue regeneration. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6048–6056, 2005     

(Quasi‐)Racemic X‐ray Structures of Glycosylated and Non‐Glycosylated Forms of the Chemokine Ser‐CCL1 Prepared by Total Chemical Synthesis

Our goal was to obtain the X‐ray crystal structure of the glycosylated chemokine Ser‐CCL1. Glycoproteins can be hard to crystallize because of the heterogeneity of the oligosaccharide (glycan) moiety. We used glycosylated Ser‐CCL1 that had been prepared by total chemical synthesis as a homogeneous compound containing an N‐linked asialo biantennary nonasaccharide glycan moiety of defined covalent structure. Facile crystal formation occurred from a quasi‐racemic mixture consisting of glycosylated L ‐protein and non‐glycosylated‐D ‐protein, while no crystals were obtained from the glycosylated L ‐protein alone. The structure was solved at a resolution of 2.6–2.1 Å. However, the glycan moiety was disordered: only the N‐linked GlcNAc sugar was well‐defined in the electron density map. A racemic mixture of the protein enantiomers L ‐Ser‐CCL1 and D ‐Ser‐CCL1 was also crystallized, and the structure of the true racemate was solved at a resolution of 2.7–2.15 Å. Superimposition of the structures of the protein moieties of L ‐Ser‐CCL1 and glycosylated‐L ‐Ser‐CCL1 revealed there was no significant alteration of the protein structure by N‐glycosylation.     

Total Synthesis of Siomycin A: Construction of Synthetic Segments

The five practical segments for the total synthesis of siomycin A, that is, the dehydropiperidine segment A ( 5 ), the pentapeptide segment B ( 3 ), the dihydroquinoline segment C ( 6 ), and the β‐phenylselenoalanine dipeptide segments D ( 7 ) and E ( 4 ), were synthesized. Segment A ( 5 ) was constructed by the coupling of the azomethine ylide and the chiral sulfinimine, followed by the stereoselective reduction of the six‐membered imine function. Segment B ( 3 ) was synthesized by the phenylselenylation of the β‐lactone, stereoselective vinylzinc addition to the chiral sulfinimine, and oxazoline–thioamide conversion. Segment C ( 6 ) was prepared by the one‐pot olefination of the tetrahydroquinoline N‐oxide using triflic anhydride and triethylamine, stereoselective reduction of the methyl ketone function, and regioselective Yb(OTf)₃‐catalyzed epoxide opening by the amino group. Segments D ( 7 ) and E ( 4 ) were synthesized by coupling of the properly protected β‐phenylselenoalanines.     

The Largest Metalloid Group 14 Cluster,Ge18[Si(SiMe3)3]6: An Intermediate on the Way to Elemental Germanium

The oxidation of [Ge₉(Hyp)₃]^? (Hyp=Si(SiMe₃)₃) with an Fe^II salt leads to Ge₁₈(Hyp)₆ ( 1 ), the largest Group 14 metalloid cluster that has been structurally characterized to date. The arrangement of the 18 germanium atoms in 1 shows similarities to that found in the solid‐state structure Ge(cF136). Furthermore, 1 can be described as a macropolyhedral cluster of two Ge₉ units. Quantum‐chemical calculations further hint at a strained arrangement so that 1 can be considered as a first trapped intermediate on the way from Ge₉ units to elemental germanium with the clathrate‐II structure (Ge(cF136)).     

Synthetic Studies on Coenzyme Q10

A practical, highly stereoselective ten‐step synthesis of coenzyme Q₁₀ ( 1 ) has been accomplished (overall yield ca. 28%), starting from commercially available 2,3‐dimethoxy‐5‐methylbenzoquinone (Scheme). The introduction of the first side‐chain isoprenyl group with (E)‐configuration (compound 6 ) was realized by means of a coupling reaction of the aromatic system 3 with oxirane, followed by Swern oxidation and Wittig olefination. The tosyl (Ts) group in the sulfone 9 was selectively removed with sodium naphthalenide in THF to afford 1 .     

Mild and practical stereoselective synthesis of (Z)- and (E)-allyl bromides from Baylis-Hillman adducts using Appel agents (PPh3/CBr4): a facile synthesis of semiplenamides C and E

Appel agents (PPh₃/CBr₄) have been utilized for high-yielding stereoselective synthesis of (Z)- and (E)-allyl bromides from Baylis-Hillman adducts at room temperature. The method has been applied for the synthesis of naturally occurring bioactive fatty acid amides, semiplenamides C and E.     

A Stereoselective Aldol Approach for the Total Syntheses of Two 6‐Alkylated 2H‐Pyran‐2‐ones

A simple and highly efficient stereoselective total synthesis of the 6‐alkylated pyranones (6R)‐6‐[(1E,4R,6R)‐4,6‐dihydroxy‐10‐phenyldec‐1‐en‐1‐yl]‐5,6‐dihydro‐2H‐pyran‐2‐one ( 1 ) and (6S)‐5,6‐dihydro‐6‐[(2R)‐2‐hydroxy‐6‐phenylhexyl]‐2H‐pyran‐2‐one ( 2 ) was developed using Crimmins' aldol reaction, SmI₂ reduction, Grubbs‐II‐catalyzed olefin cross‐metathesis, and Still's modified Horner? Wadsworth? Emmons reaction.     

Efficient and scalable process for the synthesis of antihypercholesterolemic drug ezetimibe

An efficient and scalable process for the synthesis of antihypercholesterolemic drug ezetimibe through chiral Evans auxiliary (S)-4-phenyl-2-oxazolidinone is described. The key steps in this process are the condensation of (S)-3-(5-(4-fluorophenyl)-5,5-dimethoxypentanoyl)-4-phenyloxazolidin-2-one and N-(4-((tert-butyldimethylsilyl)oxy)benzylidene)-4-fluoroaniline, and the stereoselective reduction of ezetimibe-ketone with NaBH₄/I₂, which is first applied in the synthesis of ezetimibe. The process is concise, mild, easy to operate, and highly stereoselective (99.6% of de value of ezetimibe). In addition, three diastereomers of ezetimibe are synthesized and served as the references in quality control of the product.     

Chemical Synthesis of a Glycopeptide Derived from Skp1 for Probing Protein Specific Glycosylation

Skp1 is a cytoplasmic and nuclear protein, best known as an adaptor of the SCF family of E3‐ubiquitin ligases that label proteins for their degradation. Skp1 in Dictyostelium is posttranslationally modified on a specific hydroxyproline (Hyp) residue by a pentasaccharide, which consists of a Fucα1,2‐Galβ‐1,3‐GlcNAcα core, decorated with two α‐linked Gal residues. A glycopeptide derived form Skp1 was prepared to characterize the α‐galactosyltransferase (AgtA) that mediates the addition of the α‐Gal moieties, and to develop antibodies suitable for tracking the trisaccharide isoform of Skp1 in cells. A strategy was developed for the synthesis of the core trisaccharide‐Hyp based on the use of 2‐naphthylmethyl (Nap) ethers as permanent protecting groups to allow late stage installation of the Hyp moiety. Tuning of glycosyl donor and acceptor reactivities was critical for achieving high yields and anomeric selectivities of glycosylations. The trisaccharide‐Hyp moiety was employed for the preparation of the glycopeptide using microwave‐assisted solid phase peptide synthesis. Enzyme kinetic studies revealed that trisaccharide‐Hyp and trisaccharide‐peptide are poorly recognized by AgtA, indicating the importance of context provided by the native Skp1 protein for engagement with the active site. The trisaccharide‐peptide was a potent immunogen capable of generating a rabbit antiserum that was highly selective toward the trisaccharide isoform of full‐length Skp1.     

Synthesis of the C1‐C10 Fragment of Muamvatin

This report delineates our efforts towards the synthesis of a stereochemically well‐defined ketone, the C₁?C₁₀ fragment of muamvatin, the first example of a 2, 4, 6‐trioxaadamantane ring skeletal polypropionate marine natural product, using two non‐aldol variants. i) The Shimizu reaction, a Pd(0) mediated stereoselective epoxy‐ring opening of alkenyl oxiranes, was employed for the stereoselective installation of methyl groups in syn‐fashion and ii) Bode's protocol, a NHC‐mediated reaction on β‐epoxy aldehydes, was utilized for stereoselective construction of methyl and hydroxyl groups in anti‐fashion.     

An easy and stereoselective synthesis of N-Boc-dolaproine via the Baylis-Hillman reaction

In this communication we report a stereoselective total synthesis of N-Boc-dolaproine (Dap), an amino acid residue of the antineoplastic pentapeptide Dolastatin 10. Our strategy is based on a Baylis-Hillman reaction between N-Boc-prolinal and methyl acrylate, followed by a diastereoselective double bond hydrogenation and hydrolysis of the ester function.     

Novel Route to L‐Hexoses from L‐Ascorbic Acid: Asymmetric Synthesis of L‐Galactopyranose and L‐Talopyranose Preliminary Communication

A novel route with L ‐ascorbic acid as a single common starting material to asymmetric synthesis of all eight diastereomers of L ‐hexoses is described. Assessment of this new approach is demonstrated by the expedient synthesis of L ‐galactopyranose and L ‐talopyranose derivatives. Key steps involve stereoselective preparation of chiral (E)‐ and (Z)‐γ‐hydroxy‐α,β‐unsaturated esters and their stereo‐controlled dihydroxylation by OsO₄.     

A Mild and Efficient Method for Chlorination of Baylis–Hillman Adducts Using PPh3/Cl3CCONH2

A new and convenient stereoselective synthesis of (Z)‐2‐(chloromethyl)alk‐2‐enoates has been achieved from Baylis–Hillman adducts by treatment with PPh₃/Cl₃CCONH₂ at room temperature. The synthesis can proceed under mild and acid‐free conditions to form the products in high yields.     

Concise Stereoselective Total Synthesis of Leiocarpin C

A simple and highly concise strategy has been developed for the stereoselective total synthesis of leiocarpin C starting from commercially available mandelic ester. The strategy utilizes the OsO₄‐catalyzed cis‐hydroxylation and selective reduction with K‐Selectride as key steps.