High Β-selectivity Research Articles

Practical one-step glucuronidation via biotransformation

We herein report a practical one-step glucuronidation method by biotransformation using Streptomyces sp. SANK 60895. This novel direct method of biotransformation has been shown to be more practical and scalable for glucuronidation than previously reported chemical and enzymatic procedures given its simplicity, high β-selectivity, cost-effectiveness, and reproducibility. We applied the present method to the synthesis of acyl glucuronide and hydroxy-β-glucuronide of mycophenolic acid and compound 4, respectively. This method was also shown to be applicable to the N-glucuronidation of various compounds.

The effect of structure evolution upon heat treatment on the beta-nucleating ability of calcium pimelate in isotactic polypropylene

Calcium pimelate (CaPim) is known as one of the most stable and effective β-nucleating agents for isotactic polypropylene (iPP). To establish the structure-properties relationships of CaPim, the structure and corresponding β-nucleating ability of as-synthesized CaPim and CaPim annealed at temperature (Ta) varying from 130 to 330 °C were investigated. Our results revealed that as-synthesized CaPim and CaPim annealed at a relatively lower Ta were crystalline hydrates. However, the highly ordered crystalline structure would irreversibly destroyed and transformed to a weakly ordered form when treated at about 180 °C, accompanied by the complete deprivation of crystal water. The weakly ordered CaPim still maintained relatively high ordered degree at Ta range of 180–290 °C, but a further increase of Ta leads to the conversion of CaPim to disordered amorphous form. The measurements of relative β-form content (kβ) of iPP samples containing 0.05 wt% of various annealed CaPim suggested that the β-nucleating selectivity was closely correlated to the ordered degree of CaPim. As expected, the crystalline CaPim could induce high kβ values of 0.82–0.91. Interestingly, the weakly ordered CaPim with residual ordered structure still showed a rather high β-selectivity with kβ values of 0.53–0.68, while the amorphous CaPim almost lost its β-nucleating ability. Combined with the epitaxial crystallization theory, a probable mechanism on the correlation between structure and β-nucleating selectivity of CaPim was proposed.

An Improved Approach to the Direct Construction of 2-Deoxy-β-Linked Sugars: Applications to Oligosaccharide Synthesis.

A next-generation reagent-controlled approach for the synthesis of 2,6-dideoxy and 2,3,6-trideoxy sugar donors in good yield and high β-selectivity is reported. The use of p-toluenesulfonyl chloride and potassium hexamethyldisilazide (KHMDS) greatly simplifies deoxy-sugar glycoside construction, and can be used for gram-scale glycosylation reactions. The development of this approach and its application to the construction of β-linked deoxy-sugar oligosaccharides are described.

One step synthesis of unnatural β-arylalanines using mutant phenylalanine aminomutase from Taxus chinensis with high β-regioselectivity.

Phenylalanine aminomutase (TcPAM) from Taxus chinensis catalyzes the regioselective hydroamination of trans-cinnamic acid (t-CA) to yield β-phe. However, the final product mixture consists of both α- and β-phe owing to low regioselectivity, which is still a challenge to synthesize highly pure β-phe. Therefore, a modified TcPAM with high β-selectivity is expected. Based on the catalytic mechanism and structure, two amino acid residues (Asn458 and Leu108) in active sites were identified as the key residues for controlling the regioselective hydroamination of t-CA and as promising candidates for mutagenesis to enhance β-selectivity and decrease α-selectivity. The Asn458 and Leu108 residues were mutated to yield variant TcPAM-Asn458Phe/Leu108Glu, and the β-selectivity was approximately 5.2-fold higher than that of wild-type TcPAM, while α-selectivity decreased to 68%, and the percentage of β-phe in the product mixture increased from 42% to 83%. In addition, the mutant was applied to synthesize β-arylalanines using substituent t-CA as a substrate. The regioselectivity was also affected by the substituent groups at the phenyl ring of t-CA with respect to their electronic properties and position, and the 4-methoxy and methyl substituent t-CA were transferred into β-arylalanines. The conversion rate also exceeded 90%. In summary, the engineered TcPAM proved to be useful for one-step asymmetric amination of t-CA and its derivatives to synthesize highly pure β-arylalanines.

Synthetic Method for 2'-Amino-LNA Bearing Any of the Four Nucleobases via a Transglycosylation Reaction.

A transglycosylation reaction of 2'-amino-locked nucleic acid (LNA) from thymine (T) to other nucleobases adenine (A), guanine (G), and 5-methylcytosine (mC) has been developed. This reaction proceeds in high yield and with high β-selectivity. The mild reaction conditions enable the coexistence of acid-labile protecting groups, including a 4,4'-dimethoxytrytyl (DMTr) group. 2'-Amino-LNAs bearing any nucleobase can now be easily synthesized.

6-O-Picolinyl and 6-O-Picoloyl Building Blocks As Glycosyl Donors with Switchable Stereoselectivity.

Remote 6-O-picolinyl or 6-O-picoloyl substituents often provide high β-selectivity due to H-bond-mediated aglycone delivery (HAD). Herein it has been demonstrated that if the nitrogen atom of the 6-O-picolinyl or picoloyl moiety is temporarily blocked by coordination to a metal center (Pd), it cannot engage in HAD-mediated β-glycosylation. Hence, the stereoselectivity of 6-O-picolinyl/picoloyl-assisted glycosylations can be "switched" to α-selectivity.

Highly stereoselective β-mannopyranosylation via the 1-α-glycosyloxy-isochromenylium-4-gold(I) intermediates.

While the gold(I)-catalyzed glycosylation reaction with 4,6-O-benzylidene tethered mannosyl ortho-alkynylbenzoates as donors falls squarely into the category of the Crich-type β-selective mannosylation when Ph3 PAuOTf is used as the catalyst, in that the mannosyl α-triflates are invoked, replacement of the (-) OTf in the gold(I) complex with less nucleophilic counter anions (i.e., (-) NTf2 , (-) SbF6 , (-) BF4 , and (-) BAr4 (F) ) leads to complete loss of β-selectivity with the mannosyl ortho-alkynylbenzoate β-donors. Nevertheless, with the α-donors, the mannosylation reactions under the catalysis of Ph3 PAuBAr4 (F) (BAr4 (F) =tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) are especially highly β-selective and accommodate a broad scope of substrates; these include glycosylation with mannosyl donors installed with a bulky TBS group at O3, donors bearing 4,6-di-O-benzoyl groups, and acceptors known as sterically unmatched or hindered. For the ortho-alkynylbenzoate β-donors, an anomerization and glycosylation sequence can also ensure the highly β-selective mannosylation. The 1-α-mannosyloxy-isochromenylium-4-gold(I) complex (Cα), readily generated upon activation of the α-mannosyl ortho-alkynylbenzoate (1 α) with Ph3 PAuBAr4 (F) at -35 °C, was well characterized by NMR spectroscopy; the occurrence of this species accounts for the high β-selectivity in the present mannosylation.

ChemInform Abstract: A Stereoselective Ring‐Closing Glycosylation via Nonglycosylating Pathway.

Two glycosyl partners were first coupled with as ester linkage, which upon reductive acetylation produced an α-acetoxy ether group. The subsequent activation with TfOH triggered the ring-closing process and provided the corresponding glycosidic bond in high β-selectivity without relying on neighboring group participation.

ChemInform Abstract: GaCl3‐Catalyzed Activation of Alkynyl Glycosides for the Synthesis of O‐Glycosides.

A catalytic amount of GaCl3 (5 mol%) was found to activate alkynyl glycosides effectively under mild reaction conditions to furnish a variety of O-glycosides in good yields with high β-selectivity.

A stereoselective ring-closing glycosylation via nonglycosylating pathway.

Two glycosyl partners were first coupled with as ester linkage, which upon reductive acetylation produced an α-acetoxy ether group. The subsequent activation with TfOH triggered the ring-closing process and provided the corresponding glycosidic bond in high β-selectivity without relying on neighboring group participation.

Synthesis of benzyl protected β-d-GlcA-(1 → 2)-α-d-Man thioglycoside building blocks for construction of Cryptococcus neoformans capsular polysaccharide structures

In a project targeting the synthesis of large oligosaccharide structures corresponding to the Cryptococcus neoformans GXM capsular polysaccharide, an easy access to thiodisaccharide building blocks comprising a β-linked glucuronic acid moiety and a 6-O-acetyl group was required. Several pathways to such building blocks have been investigated, addressing the problem of constructing a β-linked glucuronic acid residue protected with groups that are orthogonal to a primary acetyl group. Two efficient routes have been developed, one using benzoylated glucosyl donors to form the β-linkage followed by a change of protecting groups to benzyls and subsequent introduction of the carboxyl function and the acetyl group. The second route explored the possibility to achieve β-selectivity using glucuronyl donors without acyl protecting groups. BF3-etherate promoted glycosylations with benzyl (2,3,4-tri-O-benzyl-α-d-glucupyranosyl)uronate trichloroacetimidate in the presence of nitrile solvents and at low temperatures reproducibly gave good yields of disaccharides with high β-selectivity. Furthermore, the use of recently reported glucuronyl thioglycoside donors protected with a cyclic 2,4-silylene acetal was found to represent another efficient and completely β-selective way to desired disaccharide building blocks.

GaCl3-catalyzed activation of alkynyl glycosides for the synthesis of O-glycosides

A catalytic amount of GaCl3 (5 mol%) was found to activate alkynyl glycosides effectively under mild reaction conditions to furnish a variety of O-glycosides in good yields with high β-selectivity.

Synthesis of Lipid-Linked Arabinofuranose Donors for Glycosyltransferases

Mycobacteria and corynebacteria use decaprenylphosphoryl-β-D-arabinofuranose (DPA) as a critical cell wall building block. Arabinofuranosyltransferases that process this substrate to mediate cell wall assembly have served as drug targets, but little is known about the substrate specificity of any of these enzymes. To probe substrate recognition of DPA, we developed a general and efficient synthetic route to β-D-arabinofuranosyl phosphodiesters. In this approach, the key glycosyl phosphodiester bond-forming reaction proceeds with high β-selectivity. In addition to its stereoselectivity, our route provides the means to readily access a variety of different lipid analogues, including aliphatic and polyprenyl substrates.

ChemInform Abstract: Regiocontrolled CuI‐Catalyzed Borylation of Propargylic‐Functionalized Internal Alkynes.

Good to excellent reactivity and regiocontrol have been achieved in the CuI-catalyzed borylation of dialkyl internal alkynes with bis(pinacolato)diboron. The presence of a propargylic polar group (OH, OR, SAr, SO2Ar, or NHTs), in combination with PCy3 as ligand, allowed maximizing the reactivity and site-selectivity (β to the propargylic function). DFT calculations suggest a subtle orbitalic influence from the propargylic group, matched with ligand and substrate size effects, as key factors involved in the high β-selectivity. The vinylboronates allowed the stereoselective synthesis of trisubstituted olefins, while allylic substitution of the SO2Py group without affecting the boronate group provided access to formal hydroboration products of unbiased dialkylalkynes.

Regiocontrolled CuI-Catalyzed Borylation of Propargylic-Functionalized Internal Alkynes

Good to excellent reactivity and regiocontrol have been achieved in the Cu(I)-catalyzed borylation of dialkyl internal alkynes with bis(pinacolato)diboron. The presence of a propargylic polar group (OH, OR, SAr, SO(2)Ar, or NHTs), in combination with PCy(3) as ligand, allowed maximizing the reactivity and site-selectivity (β to the propargylic function). DFT calculations suggest a subtle orbitalic influence from the propargylic group, matched with ligand and substrate size effects, as key factors involved in the high β-selectivity. The vinylboronates allowed the stereoselective synthesis of trisubstituted olefins, while allylic substitution of the SO(2)Py group without affecting the boronate group provided access to formal hydroboration products of unbiased dialkylalkynes.

Studies on Selective Hydroxylation of Aliphatic C-H Bonds Using Tridentate NHC-Amidate-Alkoxide Pd(II) Complexes

Catalytic functionalization of unreactive sp C-H bonds under mild conditions is a highly desirable prospect that has generated a considerable amount of interest in recent years. Numerous studies have been carried out in an effort to develop efficient methods; nonetheless, controlling selectivities in these reactions remains a challenge. The chemo-, stereo-, and/or enantioselectivity of these reactions have been particularly difficult to control, especially in those intended to produce hydroxylated products, as overoxidation to ketones and even C-C bond cleavage products are prevalent. Under relatively mild conditions, overoxidation can be reduced significantly to offer hydroxylation products somewhat selectively while some starting material usually remains. As shown in Scheme 1, Sen developed relatively mild conditions using oxygen instead of other reactive oxidants, which converted 59% of the starting acid 1 to produce an equimolar mixture of βand γ-hydroxylated products. Surprisingly, α-hydroxylation was not observed whereas remote C-H functionalization was efficient. Recently, we introduced a unique tridentate NHC-amidate Pd catalyst which showed surprising reactivity toward unreactive C-H bonds under mild conditions. We wondered if our new Pd catalysts with a unique architecture of tridentated ligands would be able to effect improved regioselectivity and furthermore promote asymmetric functionalization. This report describes initial results on regioselectivity and enantioselectivity using new catalysts 5 and 6 depicted in Figure 1. We examined hydroxylation of butyric acid using catalyst 5 and hydrogen peroxide (Scheme 2). Contrary to Sen’s conditions, we observed hydroxylation on all three possible centers with poor regioselectivity, which was also observed from n-hexane (7) in a very similar way. Using cyclic substrates, we found that cyclopentane (8) and cyclohexane (9) furnished mixed selectivities, which might imply relocation of reaction centers during the course of the reactions. Catalyst 6 gave rise to similar results on acyclic and cyclic substrates, 1 and 8, respectively (Scheme 3). We also learned that this enantiopure catalyst induced high enantioselection on the β-hydroxylation product 3 which could be due to direct C-H activation by Pd. Although we did not pursue identification of the absolute configuration, we conducted an NMR study with a chiral shift reagent to confirm the optical purity of 83% ee. It is evident that this type of catalyst can facilitate chiral induction at an unreactive C-H site. In addition, we found that ketone 10 provided high β-selectivity in the beginning of the oxidation, thus we hypothesized that the incipient reaction center would be β-center to the carbonyl,

Synthesis of an aryl 2-deoxy-β-glycosyl tetrasaccharide to probe retaining endo-glycosidase mechanism

The synthesis of the 1,3–1,4-β-glucanase substrate analogue 4-nitrophenyl O-β- d-glucopyranosyl-(1→4)- O-β- d-glucopyranosyl-(1→4)- O-β- d-glucopyranosyl-(1→3)-2-desoxi-β- d-glucopyranoside 2 is reported. Starting from the main tetrasaccharide obtained by enzymatic depolymerization of barley β-glucan, the synthetic scheme involves preparation of the corresponding 3- O-substituted glycal which was converted into a 2-deoxy-α-glycosyl iodide as a glycosyl donor. The key glycosylation step was successfully achieved by nucleophilic substitution of the iodide donor with 4-nitrophenolate with high β-selectivity.

Anomeric selectivity in the synthesis of galloyl esters of d-glucose

The anomeric selectivity of the ester formation between d-glucopyranose and gallic acid was investigated under a variety of conditions. A new protocol was established that allows performing the reaction under conditions where mutarotation is very slow. Highly α- or β-selective transformations are possible when starting with α- or β- d-glucopyranose, respectively. Due to the kinetic anomeric effect, a high α-selectivity is more difficult to achieve than a high β-selectivity. The new methodology presented in this article was compared with established procedures for the synthesis of gallotannins. In addition to the advantages of a high yield and an easy purification protocol, the new procedure uniquely allowed for a highly selective synthesis of α-products.

Direct stereocontrolled synthesis of 3-amino-3-deoxy-beta-mannopyranosides: importance of the nitrogen protecting group on stereoselectivity.

The highly stereocontrolled synthesis of the 3-amino-3-deoxy-beta-mannopyranosides is achieved by means of thioglycoside donors protected with a 4,6-O-benzylidene or alkylidene acetal and a benzylidene imine group. Among the various nitrogen protecting groups investigated only the Schiff's base was found to give high beta-selectivity. N-Phthalimido and N-acetamido protected donors were found to be highly alpha-selective, whereas 3-azido-3-deoxy glycosyl donors gave intermediate selectivity. The reasons for the protecting group dependency are discussed in terms of the change in the O2-C2-C3-N3 torsional interaction on conversion of the covalent glycosyl triflates to the transient oxacarbenium ions.

N-Glycoside neoglycotrimers from 2,3,4,6-tetra- O-acetyl-β- d-glucopyranosyl azide

2,3,4,6-Tetra- O-acetyl-β- d-glucopyranosyl azide is available on large scale from d-glucose by means of a three-step sequence involving acetylation, activation as the glycosyl bromide, and stereospecific displacement with azide anion. The azide functionality then serves as a convenient anchor upon which to introduce new functionality, usually with retention of the β-stereochemistry. Here we report the synthesis of an amide-linked N-glycosyl trimer, by employing a Staudinger–aza-Wittig process on the azide, as well as a hybrid N-glycosyl triazole–amide-linked trimer in which the sugars are separated by 1,2,3-triazole heterocycles. Both of these neoglycotrimers are isolated in good yield with high β-selectivity in each case.