A Reverse Strategy for synthesis of nucleosides based on n-pentenyl orthoester donors.

Strategically derivatized NPOE glycosyl donors, are able to efficiently glycosylate silylated nucleobases under mild conditions, even as low as -78 °C if necessary. Ensuring trans-1,2 glycosylation, thus permitting, unlike classical procedures, a Reverse Strategy for the synthesis of ribonucleosides, where glycosylation occurs late, rather than early, and convergency is optimized.

Ready preparation of furanosyl n-pentenyl orthoesters from corresponding methyl furanosides.

The 3,5-di-O-benzoyl n-pentenyl orthoesters of the four pentofuranoses have been prepared. The first key intermediate in each case is the methyl pentofuranoside(s), and a user-friendly procedure for the preparation of each, based on the Callam-Lowary precedent, is described, whereby formation of the crucial α/β anomeric mixture is optimized. The mixture is used directly to prepare the corresponding perbenzoylated pentofuranosyl bromide(s) and then the title compounds.

Armed-disarmed effects in carbohydrate chemistry: history, synthetic and mechanistic studies.

This chapter begins with an account of the serendipitous events that led to the development of n-pentenyl glycosides (NPGs) as glycosyl donors, followed by the chance events that laid the foundation for the armed-disarmed strategy for oligosaccharide assembly. A key mechanistic issue for this strategy was that, although both armed and disarmed entities could function independently as glycosyl donors, when one was forced to compete with the other for one equivalent of a halonium ion, the disarmed partner was found to function as a glycosyl acceptor. The phenomenon was undoubtedly based on reactivity, but further insight came unexpectedly.

Efficient chemical synthesis of a dodecasaccharidyl lipomannan component of mycobacterial lipoarabinomannan.

Lipomannan (LM) is one of the domains of lipoarabinomannan (LAM) glycolipids, the latter being one of several cell surface organic molecules that fortify mycobacterial species against external attack. Some members of mycobacterial families are pathogenic, most notably Mycobacterium tuberculosis and Mycobacterium leprae, while others are nonpathogenic, and used in the clinic, such as Mycobacterium smegmatis. Additional biological significance arises from the fact that LM has been implicated in several health disorders outside of those associated with mycobacterial pathogens, notably for treatment of bladder cancer.

Regioselective strategies mediated by lanthanide triflates for efficient assembly of oligomannans.

Readily prepared mannosyl n-pentenylorthoesters (NPOEs) serve as donors in themselves and as convenient intermediates for other glycosyl donors, such as n-pentenyl glycosides (NPGs), thioglycosides, and trichloroacetimidates. These various donors are activated by different reagents, and are therefore amenable to versatile, discriminate use. Scandium and ytterbium triflates respond very differently to these donors, with the result that chemoselective discrimination between NPOEs, NPGs, trichloroacetimidates as well as ethyl and phenyl thioglycosides can be achieved.

A strategy for ready preparation of glycolipids for multivalent presentation.

The olefinic residue of n-pentenyl glycosides serves as the trigger for regioselective construction of higher saccharides and then for elaboration in multivalent glycolipids. [reaction: see text]

One-pot chemo-, regio-, and stereoselective double-differential glycosidation mediated by lanthanide triflates.

Nuanced activation of n-pentenyl, thioglycoside, and trichloroacetimidate donors by lanthanide salts coupled with donor/acceptor matching can simplify oligosaccharide assembly. Thus, a one-pot, double-differential glycosidation process can be designed, in which an n-pentenyl acceptor-diol is chemo- and regioselectively glycosidated by using an n-pentenyl ortho ester under the agency of Yb(OTf)(3)/NIS followed by in situ addition of a 2-O-acylated trichloroacetimidate or ethyl thioglycoside to effect stereoselective glycosidation at the remaining OH.

Preparation, properties, and applications of n-pentenyl arabinofuranosyl donors.

[reaction: see text] The development of n-pentenyl furanosyl donors has been tested using arabinose as a model. The readily prepared ortho ester (NPOE) is converted into disarmed (NPG(AC)) and thence armed (NPG(ALK)) n-pentenyl arabinofuranosides. The reactivities of these furanosyl donors and pyranosyl counterparts have been assessed by allowing pairs of both to compete for an acceptor.

Synthesis of a key Mycobacterium tuberculosis biosynthetic phosphoinositide intermediate.

Regioselective mannosylations of a myoinositol acceptor diol are readily achieved by Lewis acid mediated iodinolysis of n-pentenyl ortho-esters. The procedure affords a psuedotrisaccharide to which the phosphoglyceryl and other lipid residues are added leading to the key biosynthetic intermediate of Mycobacterium species.

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.

Orthoesters versus 2-O-acyl glycosides as glycosyl donors: theorectical and experimental studies.

n-Pentenyl orthoesters (NPOEs) undergo routine acid catalyzed rearrangement into 2-O-acyl n-pentenyl glycosides (NPGs). The reactant and product can both function as glycosyl donors affording 1,2-trans linked glycosides predominantly. However, both donors differ in their rates of reactions, the yields they produce, and the nature of their byproducts, indicating that the NPOE/NPG pair may not be reacting through the same intermediates.

One pot/two donors/one diol give one differentiated trisaccharide: powerful evidence for reciprocal donor-acceptor selectivity (RDAS).

Three component, one-pot reactions involving equimolar amounts of the acceptor diol and both armed and disarmed donors presented simultaneously, produce a single double-differential glycosidation product; this phenomenon provides evidence for Reciprocal Donor Acceptor Selectivity (RDAS).

Unexpected role of O-2 "protecting" groups of glycosyl donors in mediating regioselective glycosidation.

Glycosidation of several vicinal diols reveals that exquisite regioselectivity can be achieved by using 2-O-benzoyl n-pentenyl glycoside donors and/or their cyclic 1,2-ortho ester counterparts. The regioselective preferences for both are the same, although ratios and yields may differ. In stark contrast, glycosidation of the diols with the corresponding 2-O-benzylated donors gives poor, if any, regioselectivity.

A versatile, three-component-reaction route to N-glycosylamines.

[figure: see text] Under the agency of N-bromosuccinimide, n-pentenyl glycosides, acetonitrile, and carboxylic acids participate in three-component-reactions that afford N-acylated glycosylamines. The procedure tolerate diverse donors, and C2-tetrachlorophthalimido and C2-azido groups effectively control anomeric stereoselectivity. Success of the procedure does not appear to depend on the acid's strength, but for an aromatic acid, substitution pattern affects the rate, while the presence of a lone pair on the para substituent inhibits the process.

Targeted glycosyl donor delivery for site-selective glycosylation.

[reaction: see text]n-Pentenyl ortho esters (NPOEs) and n-pentenyl glycosides (NPGs) are interconvertible glycosyl donors which are activated by reaction with halonium ions. In a series of cyclic syn-1,3-diols, NPOEs have been found to specifically glycosylate the equatorial-OH while the NPG glycosylates predominantly, but not exclusively, the axial-OH. When the cyclic diol acceptor is presented with equivalent amounts of an NPOE and an NPG in a three-component-reaction, a single, double-glycosylation product is obtained, which conforms to the foregoing preferences, presenting evidence for site-selective glycosylation.

IMDA/aldol strategy for transforming carbohydrates into functionalized trans-decalins.

L-Rhamnal is readily converted into an allyl 2, 3-unsaturated-C-glycopyranoside. The (S) configuration of the alphaL-anomer defines the stereochemical outcome of the future IMDA reaction, leading to the absolute stereochemistry for the trans-decalin moiety in naturally occurring terpenoids. Selective cleavage of the terminal double bond of the allyl group provides an aldehydo function which serves for an aldol/Claisen addition with ethyl sorbate.

Access to tetrachlorophthalimide-protected ethyl 2-amino-2-deoxy-1-thio-beta-D-glucopyranosides.

Ethyl 2-deoxy-2-tetrachlorophthalimido-1-thio-beta-D-glucopyranoside (7) was prepared from glucosamine hydrochloride in four steps with a 20-25% overall yield. Formation of 1,3,4,6-tetra-O-acetyl-2-deoxy-2-tetrachlorophthalimido-beta-D- glucopyranoside (5) was found to be crucial for this reaction sequence since the corresponding alpha-1-acetate did not react in Lewis-acid-catalyzed ethylthio glycosidations. Formation of the beta-1-acetate (5) was achieved by treatment of 3,4,6-tri-O-acetyl-2-deoxy-2-tetrachlorophthalimido-alpha-D-glucop yranosyl bromide (4) with acetic acid under silver zeolite promotion.