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 of 1-Deoxynojirimycin: Exploration of Optimised Conditions for Reductive Amidation and Separation of Epimers

1-Deoxynojirimycin (DNJ), which has importance with respect to sugar processing enzymes, is a synthetic target for chemists. A key step in the synthesis of DNJ is the preparation of 2,3,4,6-tetra- O-benzyl-D-glucono-δ-lactam. By varying reaction parameters such as temperature, solvent and reducing reagent, improvements on previous methods are described. A novel approach for the synthesis of 2,3,4,6-tetra- O-benzyl-5-dehydro-5-deoxo-D-gluconamide has been developed by using PCC as an oxidising agent. Separation of epimers permitted DNJ to be obtained in 85% yield after reduction and hydrogenolysis steps.

Synthesis of an Orthogonally Protected Polyhydroxylated Cyclopentene from l-Sorbose

The use of l-sorbose in the synthesis of functionalized cyclopentene derivatives was accomplished. These cyclopentene derivatives are related to those found in naturally occurring jatrophane frameworks and in other bioactive compounds. The formation of allyl α-l-sorbopyranoside was a key synthetic step. Regioselective introduction of protecting groups was followed by the hydrolysis of the allyl glycoside to furnish a fully protected acyclic l-sorbose derivative. This acyclic intermediate was subsequently used to give an orthogonally protected polyhydroxylated cyclopentene, which has potential for further synthesis of bioactive compounds. The protected cyclopentene itself showed a clear cytotoxic activity when tested against a panel of human cancer cell lines (HT29, LS174T, SW620, A549, and HeLa cells).