First synthesis of a malarial prototype: a fully lipidated and phosphorylated GPI membrane anchor

A strategy is described for syntheses of a fully lipidated and phosphorylated prototype of the GPI of Plasmodium falciparum, the causative agent of lethal cerebral, drug-resistant malaria. Orthoesters, prepared in four steps from d-mannose, and methyl α-d-glucopyranoside are the key starting materials. The latter furnishes the inositol moiety using Bender’s procedure, while the former gives the other four units of the pseudo-pentasaccharide. The strategy for installing the three biologically important acyl units of the phosphoinositide has been worked out. The critical, biosynthetically important C2-O-acyl group of the inositol is exceptionally stable, showing no tendency to migrate to the cis-related C3-OH in several test substrates.     

Synthesis of the fully phosphorylated GPI anchor pseudohexasaccharide of Toxoplasma gondii.

Retrosynthesis of the fully phosphorylated glycosylphosphatidyl inositol (GPI) anchor pseudohexasaccharide 1a led to building blocks 2-6, of which 5 and 6 are known. The formation of pseudodisaccharide building block 2 is based on readily available building block 7, which gave, via derivative 11 and its glycosylation with known donor 12, the desired compound 2. Building block 3, with the required access to all hydroxy groups being permitted, was prepared from mannose in five steps. From a readily available precursor, building block 4 was obtained, which on reaction with 3 gave disaccharide 23. The synthesis of the decisive pseudohexasaccharide intermediate 32 was based on the reaction of 23 with 5, then with 6, and finally with 2. To obtain high stereoselectivity and good yields in the glycosylation reactions, anchimeric assistance was employed. To enable regioselective attachment of the two different phosphorus esters, the 6f-O-silyl group of 32 was first removed and the aminoethyl phosphate residue was attached. Then the MPM group was oxidatively removed, and the second phosphate residue was introduced. Unprotected 1a was then liberated in two steps: treatment with sodium methanolate removed the acetyl protecting groups, and finally, catalytic hydrogenation afforded the desired target molecule, which could be fully structurally assigned.     

Total synthesis of a glycosylphosphatidylinositol anchor of the human lymphocyte CD52 antigen

The first total synthesis of a glycosylphosphatidylinositol (GPI) anchor bearing a polyunsaturated arachidonoyl fatty acid is reported. This lipid is found in mammalian GPIs that do not undergo lipid remodeling, a process that has important implications in the localization and function of GPI-anchored proteins. Incorporation of the oxidation- and reduction-sensitive arachidonoyl lipid in the target GPI was accomplished by using the para-methoxybenzyl (PMB) group for permanent hydroxyl group protection, which featured a selective, rapid, and efficient global deprotection protocol. The flexibility of this synthetic strategy was further highlighted by the inclusion of two additional GPI core structural modifications present in the GPI anchor of the human lymphocyte CD52 antigen.     

Convergent synthesis of a fully lipidated glycosylphosphatidylinositol anchor of Plasmodium falciparum

A highly convergent strategy for the synthesis of fully lipidated GPI anchors of malarial origin is reported. This strategy utilized three orthogonal protecting groups, which can be chemoselectively deprotected and functionalized in the late stage of the synthesis. Rapid access to the target GPIs in a highly efficient manner in sufficient quantities for the biological studies has been achieved.     

Convergent synthesis of a GPI containing an acylated inositol

A GPI of sperm CD52 was synthesized by a highly convergent procedure, representing the first chemical synthesis of a complex GPI having an acylated inositol. The presence of a large acyl group resulted in unusual properties and reactions of the relevant intermediates, which gave rise to a number of problems. To overcome the problems and achieve the target molecule, a new synthetic strategy was developed. First, the pseudodisaccharide of 2-O-palmitoylinositol was phospholipidated, and then the trimannose segment and the phosphoethanolamine group were sequentially attached. Global deprotection eventually afforded the sperm CD52 GPI. The method may be useful for the synthesis of other GPIs having an acylated inositol.     

Studies directed toward the synthesis of protein-bound GPI anchor

Mannobioside-linked phosphoethanolamine 10, a prototype model of the GPI anchor, was synthesized via glycosidation of the monosaccharide donor and acceptor, and subsequent phosphorylation. In order to test the reactivity of the amino group involved in 10 against the activated amino acid esters, 10 was reacted with N-protected amino acid pentafluorophenyl esters in the presence of HOBt. The reactions gave the aminoacylated products in moderate yields. When Fmoc-Ser-OPfp 12 and Fmoc-Cys(SBu^t)-OPfp 14 were reacted with 10, byproducts 19, 20 and 21 derived from N- and O-acylation were produced. In contrast, reactions of 10 and N-protected amino acid thioesters were promoted with AgNO₃, HOSu, and DIEA to afford the coupling products without the undesired O-acylation. Peptidylation of 10 with the synthesized oligopeptide thioesters 24 and 27 was also successful under the segment coupling conditions of the peptide thioester method as well as those of the native chemical ligation.     

Total synthesis of the fully lipidated glycosylphosphatidylinositol (GPI) anchor of malarial parasite Plasmodium falciparum

We report a new and convergent strategy for the total synthesis of fully lipidated glycosylphosphatidylinositol (GPI) anchor, the major pro-inflammatory factor of malarial parasite (Plasmodium falciparum). The key features of our approach include, the access to the key glucosamine-inositol intermediate by a novel route without a priori resolution of myo-inositol, convergent assembly of the tetramannose glycan domain, flexibility for the placement of the three fatty acids in the desired order in the final steps, and the opportunity to construct GPI analogues/mimics to probe the biosynthesis, immunology and cell biology of the GPI anchor pathway in the malaria parasite.     

Stereoselective synthesis of myo-inositol via ring-closing metathesis: a building block for glycosylphosphatidylinositol (GPI) anchor synthesis

Here we report a concise stereoselective synthesis of myo-inositol via ring-closing metathesis. A readily available bis-Weinreb amide of D-tartrate served as a key intermediate. [reaction: see text]     

Synthesis of a malaria candidate glycosylphosphatidylinositol (GPI) structure: a strategy for fully inositol acylated and phosphorylated GPIs

A congener of the glycosylphosphatidylinositol (GPI) membrane anchor present on the cell surface of the malaria pathogen Plasmodium falciparum has been synthesized. This GPI is an example of a small number of such membrane anchors that carry a fatty acyl group at O-2 of the inositol. Although the acyl group plays crucial roles in GPI biosynthesis, it rarely persits in mature molecules. Other notable examples are the mammalian GPIs CD52 and AchE. The presence of bulky functionalities at three contiguous positions of the inositol moiety creates a very crowded environment that poses difficulties for carrying out selective chemical manipulations. Thus installations of the axial long-chain acyl group and neighboring phosphoglyceryl complex were fraught with obstacles. The key solution to these obstacles in the successful synthesis of the malarial candidate and prototype structures involved stereoelectronically controlled opening of a cyclic ortho ester. The reaction proceeds in very good yields, the desired axial diastereomer being formed predominantly, even more so in the case of long-chain acyl derivatives. The myoinositol precursor was prepared from methyl alpha-d-glucopyranoside by the biomimetic procedure of Bender and Budhu. For the glycan array, advantage was taken of the fact that (a). n-pentenyl ortho ester donors are rapidly and chemospecifically activated upon treatment with ytterbium triflate and N-iodosuccinimide and (b). coupling to an acceptor affords alpha-coupled product exclusively. A strategy for obtaining the GPI's alpha-glucosaminide component from the corresponding alpha-mannoside employed Deshong's novel azide displacement procedure. Thus all units of the glycan array were obtained from a beta-d-manno-n-pentenyl ortho ester, this being readily prepared from d-mannose in three easy, high-yielding steps. The "crowded environment" at positions 1 and 2, noted above, could conceivably be relieved by migration of the acyl group to the neighboring cis-O-3-hydroxyl in the natural product. However, study of our synthetic intermediates and prototypes indicate that the O-2 acyl group is quite stable, and that such migration does not occur readily.     

A Suzuki-Miyaura coupling mediated deprotection as key to the synthesis of a fully lipidated malarial GPI disaccharide

Ligandless palladium-catalyzed Suzuki-Miyaura coupling converted an inert p-bromobenzyl ether to a DDQ-labile p-(3,4-dimethoxyphenyl) benzyl ether in the presence of azide functionality and this strategy serves as a key step for the convergent synthesis of a fully lipidated malaria GPI disaccharide.     

Convergent synthesis of fully functionalized ring C allocolchicinoids. Benzannulation approach

[reaction: see text]. A novel convergent approach to fully functionalized ring C allocolchicinoids is developed which is based on the benzannulation reaction of Fischer carbene complexes with alkynes. The efficacy of this strategy was established with the conversion of bromide 1a (R1 = Me, R2 = H) to the biaryl phenol 3a (R = Me, R(L) = Pr, R(S) = H) via the carbene complex 2a. Bromide 1b (R1 = t-Bu, R2 = OMe) was then used for the analogous preparation of the diastereomeric allocolchicinoids 3b (R = Me, R(L) = Pr, R(S) = H).     

Chemoselective ligation applied to the synthesis of a biantennary N-linked glycoform of CD52

We report here a strategy for the synthesis of N-linked glycopeptide analogues that replace the glycosidic linkages extending from the core pentasaccharide with thioethers amenable to construction by chemoselective ligation. The key building block, a pentasaccharide-Asn analogue containing two thiol residues, was incorporated into CD52 by 9-fluorenylmethoxycarbonyl (Fmoc)-based solid-phase peptide synthesis. An undecasaccharide mimetic was then readily generated by alkylation of this glycopeptide with an N-bromoacetamido trisaccharide. The rapid assembly of a complex type N-linked glycopeptide mimetic was accomplished using this technique.     

A new synthesis of fully phosphorylated flavones as potent pancreatic cholesterol esterase inhibitors

Five flavones possessing one to four phenolic groups were fully phosphorylated efficiently and the obtained compounds showed excellent pancreatic cholesterol esterase (CEase) inhibitory activities with IC(50) in the nanomolar range, which were much more potent than their parent compounds. The inhibition mechanism and kinetic characterization studies indicate that they are irreversible competitive inhibitors.     

Chemical synthesis of CD52 glycopeptides containing the acid-labile fucosyl linkage

Glycopeptide 1 with the fucosylated trisaccharide, beta-d-GlcNAc(1-->4)[alpha-l-Fuc(1-->6)]-beta-d-GlcNAc, linked to the Asn of CD52 peptide was prepared by two methods, both of which used the free glycosyl Asn 12 and glycotripeptide 21 as key intermediates. Thus, after the trisaccharide was prepared and linked to Asn, the carbohydrate moiety was deprotected to give 12. From 12, 21 was constructed in homogeneous NMP solutions by elongating the peptide chain alone the N-terminus. Though the glycopeptides were easily soluble in NMP, they were barely soluble in diethyl ether, because of the free trisaccharide. Consequently, addition of diethyl ether to the reaction mixtures could precipitate the glycopeptides, and the products were conveniently isolated and purified in the solid form. The coupling of 21 with a free nonapeptide 24 in NMP afforded 1. 1 was also prepared by solid-phase synthesis, using the acid-sensitive 2-chlorotrityl resin. In this case, 21 was attached to the nonapeptide on the resin, and the resulting glycopeptide was then released with dilute acetic acid. Deprotection of the peptide under moderate acidic conditions gave 1. The acid-labile alpha-fucose was not affected in these syntheses.     

Convergent synthesis of leukotriene a methyl ester

A convergent total synthesis of methyl 5,6-oxido-7,9,11,14-eicosapentaenoate from oct-2-yn-l-ol and metyl 4-formylbutyrate is described.     

Semisynthetic murine prion protein equipped with a GPI anchor mimic incorporates into cellular membranes

Conversion of cellular prion protein (PrP(C)) into the pathological conformer (PrP(Sc)) has been studied extensively by using recombinantly expressed PrP (rPrP). However, due to inherent difficulties of expressing and purifying posttranslationally modified rPrP variants, only a limited amount of data is available for membrane-associated PrP and its behavior in vitro and in vivo. Here, we present an alternative route to access lipidated mouse rPrP (rPrP(Palm)) via two semisynthetic strategies. These rPrP variants studied by a variety of in vitro methods exhibited a high affinity for liposomes and a lower tendency for aggregation than rPrP. In vivo studies demonstrated that double-lipidated rPrP is efficiently taken up into the membranes of mouse neuronal and human epithelial kidney cells. These latter results enable experiments on the cellular level to elucidate the mechanism and site of PrP-PrP(Sc) conversion.     

Convergent synthesis of pyragonicin

This paper describes a second-generation synthesis of an antitumor tetrahydropyran (THP) acetogenin, pyragonicin. The key step involved an olefin cross-metathesis between the THP segment and the terminal gamma-lactone residue. The coupling reaction in the presence of Grubbs' second-generation catalyst resulted in an unseparable mixture of a desired coupling product and its one-carbon eliminated product while the use of Grubbs' first-generation catalyst afforded the former exclusively. A novel MOM-migrating reaction found in a cyclization reaction is also discussed.     

膦酰肽和膦酰酯肽的汇聚式合成

本文首先介绍了膦酰肽的分类、结构特征以及膦酰肽的主要合成方法,然后详细介绍了类Mannich反应在合成氨基膦酸衍生物中的应用,并介绍了以类Mannich反应为关键步骤通过汇聚式的方法合成膦酰肽、膦酰酯肽、次膦酰肽、次膦酰酯肽、磺酰膦酰杂交肽的研究进展.汇聚式的合成方法具有原料简单、合成步骤少、原子经济性好等优点.