Homochiral carbon branched piperidines from carbon branched sugar lactones: 4-C-methyl-deoxyfuconojirimycin (DFJ) and its enantiomer—removal of glycosidase inhibition

Cyclisations and Strategies for Stereoselective Synthesis of Piperidine Iminosugars

This personal account focuses on synthesis of polyhydroxylated piperidines, a subset of compounds within the iminosugar family. Cyclisations to form the piperidine ring include reductive amination, substitution via amines, iminium ions and cyclic nitrones, transamidification (N-acyl transfer), addition to alkenes, ring contraction and expansion, photoinduced electron transfer, multicomponent Ugi reaction and ring closing metathesis. Enantiomerically pure piperidines are obtained from chiral pool precursors (e. g. sugars, amino acids, Garner's aldehyde) or asymmetric reactions (e. g. epoxidation, dihydroxylation, aminohydroxylation, aldol, biotransformation). Our laboratory have contributed cascades based on reductive amination from glycosyl azide precursors as well as Huisgen azide-alkene cycloaddition. The latter's combination with allylic azide rearrangement has given substituted piperidines, including those with quaternary centres adjacent to nitrogen.

Synthesis and Antiviral Activity of a Series of 2'-C-Methyl-4'-thionucleoside Monophosphate Prodrugs

The NS5B RNA-dependent RNA polymerase of the hepatitis C virus (HCV) is a validated target for nucleoside antiviral drug therapy. We endeavored to synthesize and test a series of 4'-thionucleosides with a monophosphate prodrug moiety for their antiviral activity against HCV and other related viruses in the Flaviviridae family. Nucleoside analogs were prepared via the stereoselective Vorbrüggen glycosylation of various nucleobases with per-acetylated 2-C-methyl-4-thio-d-ribose built in a 10-step synthetic sequence from the corresponding ribonolactone. Conjugation of the thionucleoside to a ProTide phosphoramidate allowed for evaluation of the prodrugs in the cellular HCV replicon assay with anti-HCV activities ranging from single-digit micromolar (μM) to >200 μM. The diminished anti-HCV potency of our best compound compared to its 4'-oxo congener is the subject of ongoing research in our lab and is proposed to stem from changes in sugar geometry imparted by the larger sulfur atom.

Asymmetric Synthesis of 2'-C-Methyl-4'-selenonucleosides as Anti-Hepatitis C Virus Agents

In search of a new template for anti-hepatitis C virus (HCV) agents, we designed and synthesized the 2'-C-methyl-4'-selenopyrimidine and -purine nucleosides and their phosphoramidate prodrugs to replace a furanose oxygen of anti-HCV nucleos(t)ides with a selenium atom on the basis that selenium is a chemical isostere of oxygen. These nucleosides are expected to show different physicochemical properties such as better lipophilicity which might enhance the penetration across cell membranes and the conformational constraint induced by a bulky selenium atom in the sugar ring. The 2'-C-methyl-4'-selenopyrimidine and -purine nucleosides 8 and 9 were synthesized from 2-C-methyl-d-ribono-γ-lactone (14) via construction of 2-C-methyl-d-selenosugar 18 through C-4 epimerization and S_N2 cyclization with Se^2- as key steps. The key 4'-selenosugar was converted to the 2'-C-methyl-4'-selenopyrimidine and -purine nucleosides using Pummerer-type rearrangement and Vorbrüggen glycosylation, respectively. In addition, the ProTide strategy has been applied to synthesize the adenine and uracil phosphoramidate derivatives 10a and 10b to overcome the limitations associated with parent nucleosides such as inefficient conversion to their corresponding 5'-monophosphate form and poor cellular uptake. The regio- and stereochemistry of 4'-selenonucleosides were confirmed by 2D NOESY NMR spectroscopy and X-ray crystallography. None of the final pyrimidine and purine nucleosides and their prodrugs exhibited significant anti-HCV activity up to 100 μM.

Construction of quaternary stereocentres on carbohydrate scaffolds

This review describes a glimpse of the various strategies for constructing stereo-defined quaternary centres in densely functionalised carbohydrates moiety of structurally intriguing and biologically potent natural products and building blocks.

2-N-Benzyl-2,6-dide-oxy-2,6-imino-3,4-O-isopropyl-idene-3-C-methyl-d-allono-nitrile

X-ray crystallography firmly established the relative stereochemistry of the title compound, C₁₇H₂₂N₂O₃. The absolute configuration was determined by use of 2-C-methyl-d-ribonolactone as the starting material. The compound exists as O—H⋯N hydrogen-bonded chains of mol­ecules running parallel to the a-axis.

4-C-Me-DAB and 4-C-Me-LAB - enantiomeric alkyl-branched pyrrolidine iminosugars - are specific and potent α-glucosidase inhibitors; acetone as the sole protecting group

The syntheses of 4-C-Me-DAB [1,4-dideoxy-1,4-imino-4-C-methyl-d-arabinitol] from l-erythronolactone and of 4-C-Me-LAB [from d-erythronolactone] require only a single acetonide protecting group. The effect of pH on the NMR spectra of 4-C-Me-DAB [pK(a) of the salt around 8.4] is discussed and illustrates the need for care in analysis of both coupling constants and chemical shift. 4-C-Me-DAB (for rat intestinal sucrase K(i) 0.89 μM, IC(50) 0.41 μM) is a competitive - whereas 4-C-Me-LAB (for rat intestinal sucrase K(i) 0.95 μM, IC(50) 0.66 μM) is a non-competitive - specific and potent α-glucosidase inhibitor. A rationale for the α-glucosidase inhibition by DAB, LAB, 4-C-Me-DAB, 4-C-Me-LAB, and isoDAB - but not isoLAB - is provided. Both are inhibitors of endoplasmic reticulum (ER) resident α-glucosidase I and II.

2-Azido-2-de-oxy-3,4-O-isopropyl-idene-2-C-methyl-d-talono-1,5-lactone

The relative stereochemistry of the title compound, C(10)H(15)N(3)O(5), was confirmed by the crystal structure determin-ation. The absolute configuration was determined from the use of d-lyxonolactone as the starting material. The six-membered ring adopts a boat conformation with the larger azide group, rather than the methyl group, in the bowsprit position. In the crystal structure, a bifurcated inter-molecular O-H⋯O/O-H⋯N hydrogen bond links mol-ecules into chains running parallel to the b axis.

3-O-Benzhydryl-2,5-dide-oxy-2,5-imino-2-C-methyl-l-lyxono-1,4-lactone

The title bicyclic lactone, C₁₉H₁₉NO₃, is an inter­mediate in the synthesis of chiral α-methyl­prolines and branched C-methyl pyrrolidines; the absolute configuration was determined by the use of d-erythronolactone as the starting material. It exhibits no unusual crystal packing features, and each mol­ecule acts as a donor and acceptor for one C—H⋯O hydrogen bond.