Iminosugars from α,β-epoxyamides. Part 1: Synthetic approach to hydroxylated piperidine derivatives

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.

6-Azido-3-O-benzyl-6-de-oxy-N,N-diethyl-1,2-O-isopropyl-idene-d-glycero-α-d-gluco-heptofuran-uronamide

Reaction of 3-O-benzyl-1,2-O-isopropyl-idene-α-xylo-pentodialdo-1,4-furan-ose with N,N-diethyl-2-(dimethyl-sulfuranil-idene)acetamide gave stereoselectively an ep-oxy-amide, which was regioselectively opened by NaN(3) in dimethyl formamide to give the title compound, C(21)H(30)N(4)O(6). X-ray crystallography confirmed the relative stereochemistry of the title compound and the absolute configuration was determined by the use of d-glucose as the starting material. There are two mol-ecules in the asymmetric unit (Z' = 2). The crystal structure consists of two types of chains of O-H⋯O hydrogen-bonded mol-ecules running parallel to the b axis, with each mol-ecule acting as a donor and acceptor of one hydrogen bond.

7-Azido-N,N-diethyl-4,5-O-isopropyl-idene-4-C-methyl-3,6-anhydro-7-de-oxy-d-glycero-d-manno-heptonamide

The reaction of 5-azido-5-de­oxy-2,3-O-isopropyl­idene-2-C-methyl-d-ribose with N,N-diethyl-2-(dimethyl­sulfuranyl­idene)acetamide gave the title compound, C₁₅H₂₆N₄O₅, as the major product arising from initial formation of an epoxide which was subsequently opened by intra­molecular attack of the free 4-hydroxyl group. X-ray crystallography confirmed the relative stereochemistry of the title compound and the absolute configuration was determined by the use of d-ribose as the starting material. The crystal structure contains chains of mol­ecules running parallel to the a axis, being linked by weak bifurcated O—H⋯(N,N) hydrogen bonds.

Short and efficient synthesis of (2S,3R,4R,5R) and (2S,3R,4R,5S)-tetrahydroxyazepanes via the Henry reaction

The Henry reaction with the easily available alpha-d-xylo-pentodialdose afforded a diastereomeric mixture of nitroaldoses with the alpha-d-gluco- and beta-l-ido-configuration, respectively, in good yield. When n-BuLi was used as the base, the reaction afforded the alpha-d-gluco-nitroaldose as the only product. The reduction of the nitro group in the alpha-d-gluco- and beta-l-ido-nitroaldoses, removal of the protecting groups and intramolecular reductive cyclo-amination afforded the corresponding (2S,3R,4R,5R) and (2S,3R,4R,5S) tetrahydroxyazepanes.

Aziridine carboxylate from d-glucose: synthesis of polyhydroxylated piperidine, pyrrolidine alkaloids and study of their glycosidase inhibition

The D-glucose derived aziridine carboxylate 5 was obtained from (E)-ethyl-6-bromo-1,2-O-isopropylidene-3-O-benzyl-5-deoxy-alpha-D-xylo-5-eno-heptofuranuronate 4 through conjugate addition of benzylamine and in situ intramolecular nucleophilic expulsion of bromine. The regioselective aziridine ring-opening, using water as a nucleophile, resulted in the alpha-hydroxy-beta-aminoester 6, which was exploited in the synthesis of six and five membered azasugars 1b/1c and 2b/2c, respectively. The glycosidase inhibitory activity of the title compounds was evaluated.