Biao Yu

Glycosyl trifluoroacetimidates. Part 1: Preparation and application as new glycosyl donors

Abstract Glycosyl ( N -phenyl)trifluoroacetimidates, readily prepared from 1-hydroxyl sugars by treatment with ( N -phenyl)trifluoroacetimidoyl chloride in the presence of K 2 CO 3 , were demonstrated to be effective glycosyl donors.

Gold(I)-Catalyzed Glycosylation with Glycosyl ortho-Alkynylbenzoates as Donors: General Scope and Application in the Synthesis of a Cyclic Triterpene Saponin

Glycosyl ortho-alkynylbenzoates have emerged as a new generation of donors for glycosidation under the catalysis of gold(I) complexes such as Ph(3)PAuOTf and Ph(3)PAuNTf(2) (Tf = trifluoromethanesulfonate). A wide variety of these donors, including 2-deoxy sugar and sialyl donors, are easily prepared and shelf stable. The glycosidic coupling yields with alcohols are generally excellent; even direct coupling with the poorly nucleophilic amides gives satisfactory yields. Moreover, excellent alpha-selective glycosylation with a 2-deoxy sugar donor and beta-selective sialylation have been realized. Application of the present glycosylation protocol in the efficient synthesis of a cyclic triterpene tetrasaccharide have further demonstrated the versatility and efficacy of this new method, in that a novel chemoselective glycosylation of the carboxylic acid and a new one-pot sequential glycosylation sequence have been implemented.

Effects of polyphyllin D, a steroidal saponin in Paris Polyphylla, in growth inhibition of human breast cancer cells and in xenograft

Paris Polyphylla is a traditional Chinese Medical herb that has been used in treating cancer for thousands of year. Without studies on the anticancer effects of Paris Polyphylla being initiated before, we have first studied the component of Paris Polyphylla and have spotted out a steroidal saponin, polyphyllin D. As long as the chemical structure and the improved synthesis of polyphyllin D were ascertained, both in vitro to in vivo studies were performed. It was found that treatment of MCF-7 and MDA-MB-231 cells with polyphyllin D resulted in the inhibition of viability and induction of apoptosis in a dose-dependent manner, with an IC50 of 5?M and 2.5?M, respectively, after 48 hours of incubations. Apoptosis of MCF-7 and MDA-MB-231 cells by polyphyllin D was evidenced by the occurrence of DNA fragmentation, formation of a hypodiploid peak in the cell cycle analysis, phosphatidyl-serine externalization and a late loss of membrane integrity. Mechanistically, polyphyllin D dissipates the mitochondrial membrane potential, induces a down-regulation of anti-apoptotic Bcl-2 expression and an up-regulation of pro-apoptotic Bax expression, and activates caspase-9. These results suggest that polyphyllin D elicits apoptosis through mitochondria dysfunction. In vivo study demonstrated that daily administration of polyphyllin D (2.73 mg/kg body weight) through intravenous injection for ten days in nude mice bearing MCF-7 cells effectively reduced tumor growth for 50% in terms of tumor weight and size, given no significant toxicity in heart and liver to the host. All these findings provide novel insights that polyphyllin D could serve as a candidate in breast cancer treatment.

Characterization of the Isochromen‐4‐yl‐gold(I) Intermediate in the Gold(I)‐Catalyzed Glycosidation of Glycosyl ortho‐Alkynylbenzoates and Enhancement of the Catalytic Efficiency Thereof

We have recently developed a new glycosylation protocol with glycosyl ortho-alkynylbenzoates as donors and a gold(I) complex (e.g., [Ph3PAuOTf], OTf = O3SCF3) as catalyst. [1] The power and versatility of this gold(I)-catalyzed glycosylation method have been demonstrated in the effective construction of a wide variety of glycosidic linkages and the total synthesis of complex oligosaccharides and glycoconjugates. Moreover, the unprecedented activation mechanism has endowed this protocol with unique merits, including 1) the absence of competitive nucleophilic species (which usually occur in the leaving entity or promoter in classical glycosylation reactions), which enables glycosylation-initiated polymerization of tetrahydrofuran to proceed smoothly; 2) the lack of deteriorative electrophilic species (such as the soft Lewis acidic species used as promoters in classical glycosylation reactions), which enables flavonol 3-OH derivatives vulnerable toward electrophiles to be glycosylated efficiently; and 3) the mild and nearly neutral conditions, which allow the extremely acid-labile aglycones, such as the N-Boc-protected purine derivatives (Boc = tert-butoxycarbonyl) and dammarane derivatives, to be glycosylated effectively. 6] This glycosylation protocol has been developed on the basis of mechanistic rational as depicted in Scheme 1. Activation of the C C triple bond positioned in the oalkynylbenzoate moiety in donor A with a gold(I) complex (e.g., [Ph3PAuOTf]) led to isochromen-4-yl-gold(I) complex D and sugar oxocarbenium ion B. Capture of the putative sugar oxocarbenium species B or related intermediates by the nucleophilic acceptor HNu provided glycoside C. The H released from HNu then protodeaurated the vinyl gold(I) complex D to give isocoumarin E with regeneration of the active Au species to complete the catalytic cycle. Activation of a C C triple bond with gold(I) species toward nucleophilic attack has been reported for numerous gold(I)-catalyzed transformations. Recently, a few of the proposed vinyl gold(I) intermediates in these transformations were characterized. Herein we report the isolation and characterization of the isochromen-4-yl-gold(I) intermediate D, which has enabled us to gain insight into the detailed catalytic cycle so as to provide a solution to enhance the catalytic efficiency of the gold(I)-catalyzed glycosylation reaction. Unexpected but easily understandable results were obtained when we attempted to glycosylate with 3,4,6-tri-Oacetyl-2-deoxy-2-p-methoxybenzylideneamino-b-d-glucopyranosyl-o-hexynylbenzoate (1) as donor (Scheme 2). Under the normal conditions (0.1 equiv [Ph3PAuOTf], toluene, 4 molecular sieves (M.S.), RT), the coupling reaction of donor 1 with n-pentenol (2) was hardly observable; upon raising the loading of the gold(I) catalyst to 0.5 equiv, the reaction proceeded smoothly, albeit leading to the b-glycoside 3 and aglycoside 4 in 37 and 47% yield, respectively (within 4 h). The a-glycoside 4 was assumed to be derived from the corresponding 2-p-methoxybenzylideneamino-a-glucoside by hydrolysis of the N-p-methoxybenzylidene group. It has been shown that the 2-N-substituent in a-d-glucosamine derivatives is much more labile than the corresponding 2-Nsubstituent in the b-counterpart. Hydrolysis of the imine consumed H generated in situ; thus, protodeauration of the isochromen-4-yl-gold(I) complex 5 (i.e., intermediate D in Scheme 1) to regenerate the active [Ph3PAu] + would be hampered, leading to a stop of the activation of the donor and accumulation of the gold(I) complex 5. Indeed, we managed after many attempts to isolate the desired gold(I) complex 5 in a high 91 % yield (based on the amount of starting [Ph3PAuOTf]) by flash chromatography on silica gel. Complex 5 was unambiguously characterized by spectroscopic (H, C, and P NMR and MS) and X-ray diffraction analysis (Figure 1). Scheme 1. The proposed mechanism for the gold(I)-catalyzed glycosidation of glycosyl o-alkynylbenzoates.

Mechanistic insights into the gold(I)-catalyzed activation of glycosyl ortho-alkynylbenzoates for glycosidation.

Anomerization, which involves cleavage and formation of the anomeric C-O bond, is of fundamental importance in the carbohydrate chemistry. Herein, the unexpected gold(I)-catalyzed anomerization of glycosyl ortho-alkynylbenzoates has been studied in detail. Especially, crossover experiments in the presence of an exogenous isochromen-4-yl gold(I) complex confirm that the anomerization proceeds via the exocleavage mechanism, involving (surprisingly) the addition of the isochromen-4-yl gold(I) complex onto a sugar oxocarbenium (or dioxolenium) and an elimination of LAu(+) from the vinyl gold(I) complex. The inhibitory effect of the exogenous isochromen-4-yl gold(I) complex when in stoichiometric amount on the anomerization has guided us to disclose an isochromen-4-yl gem-gold(I) complex, which is inactive in catalysis but in equilibrium with the monogold(I) complex and the LAu(+) catalyst. The proposed key intermediate in the anomerization, a transient glycosyloxypyrylium species, is successfully trapped via a cycloaddition reaction with n-butyl vinyl ether as a dienophile. SN2-like substitution of the initially formed glycosyloxypyrylium intermediate has then been achieved to a large extent via charging with acceptors in an excess amount to lead to the corresponding glycosides in a stereoselective manner.

Total Synthesis and Structural Revision of TMG-chitotriomycin, a Specific Inhibitor of Insect and Fungal β-N-Acetylglucosaminidases

TMG-chitotriomycin, a potent and selective inhibitor of the beta-N-acetylglucosaminidases that possesses an unique N,N,N-trimethyl-d-glucosamine (TMG) residue, is revised to be the TMG-beta-(1-->4)-chitotriose instead of the originally proposed alpha-anomer via its total synthesis, for which a highly convergent approach was developed in which the sterically demanding (1-->4)-glycosidic linkages are efficiently constructed by the Au(I)-catalyzed glycosylation protocol with glycosyl o-hexynylbenzoates as donors.

An in Vitro Selection System for TNA

(3‘-2‘)-α-l-Threose nucleic acid (TNA) is an unnatural polymer that possesses the rare ability to base-pair with RNA, DNA, and itself. This feature, coupled with its chemical simplicity, makes TNA of interest as a possible progenitor of RNA during the early history of life. To evaluate the functional potential of TNA, we have developed a system for the in vitro selection of TNA. We identified the Therminator DNA polymerase as a remarkably efficient DNA-dependent TNA polymerase capable of polymerizing more than 50 tNTPs. We have also developed a method of covalently linking a DNA template to the TNA strand that it encodes, thus obviating the need for a TNA-dependent DNA polymerase during cycles of selection.

Kinetic Analysis of an Efficient DNA-Dependent TNA Polymerase

α-l-Threofuranosyl nucleoside triphosphates (tNTPs) are tetrafuranose nucleoside derivatives and potential progenitors of present-day β-d-2‘-deoxyribofuranosyl nucleoside triphosphates (dNTPs). Therminator DNA polymerase, a variant of the 9°N DNA polymerase, is an efficient DNA-directed threosyl nucleic acid (TNA) polymerase. Here we report a detailed kinetic comparison of Therminator-catalyzed TNA and DNA syntheses. We examined the rate of single-nucleotide incorporation for all four tNTPs and dNTPs from a DNA primer−template complex and carried out parallel experiments with a chimeric DNA−TNA primer−DNA template containing five TNA residues at the primer 3‘-terminus. Remarkably, no drop in the rate of TNA incorporation was observed in comparing the DNA−TNA primer to the all-DNA primer, suggesting that few primer-enzyme contacts are lost with a TNA primer. Moreover, comparison of the catalytic efficiency of TNA synthesis relative to DNA synthesis at the downstream positions reveals a difference of no greater than 5-fold in favor of the natural DNA substrate. This disparity becomes negligible when the TNA synthesis reaction mixture is supplemented with 1.25 mM MnCl2. These results indicate that Therminator DNA polymerase can recognize both a TNA primer and tNTP substrates and is an effective catalyst of TNA polymerization despite changes in the geometry of the reactants.

Synthesis of three diosgenyl saponins: dioscin, polyphyllin D, and balanitin 7

Dioscin, polyphyllin D, and balanitin 7, which belong to a group of structurally similar diosgenyl saponins with promising bioactivities, were synthesized by stepwise glycosylation.

An improved synthesis of the saponin, polyphyllin D

Polyphyllin D, namely diosgenyl alpha-L-rhamnopyranosyl-(1 --> 2)- [(alpha-L-arabinofuranosyl)-(1 --> 4)]-[beta-D-glucopyranoside, was synthesized from diosgenyl-beta-D-glucopyranoside in four steps and in 30% overall yield, taking advantage of regioselective pivaloylation and alpha-L-rhamnopyranosylation reactions.