Chemoenzymatic Resolutions of myo-Inositols: A synthesis towards PIMs

Abstract

Phosphatidylinositol mannosides (PIMs), isolated from the cell wall of mycobacteria, have been identified as an important class of glycolipids that possess immune-modulating properties. The complex mixture of structurally diverse molecules means that total syntheses of each member of this compound-family is required to determine their individual biological activities. To this end, methodology towards the synthesis of these molecules was developed.

A key step in the synthesis of PIMs from the meso compound myo-inositol is the resolution of racemic mixtures of inositol derivatives. A method of selectively acetylating a single enantiomer of racemic 2,3-O-isopropylidene-myo-inositol-5,6-diols, containing a range of protecting groups at O-1 and O-4, was developed using C. antarctica Lipase B (immobilised on acrylic beads, marketed as Novozym 435). This highly efficient chemoenzymatic kinetic resolution converted one hand of the racemate into the O-5 monoacetate leaving the other hand unreacted allowing for the separation of the enantiomers by simple silica gel chromatography. Zemplén de-esterification of the acetate bearing inositol gave both enantiomers as discrete compounds. This resolution was attempted on a range of differently protected myo-inositols, incorporating alkyl, alkenyl, alkynyl, aromatic and silyl groups, and it was found that only steric factors affected conversion, with selectivity remaining high. Larger protecting groups slowed the conversion, with very bulky groups preventing it entirely.

Using the developed methodology, a total synthesis of an Ac2PIM1 was performed using both products from the kinetic resolution of (±)-1-O-allyl-4-O-benzyl-2,3-O-isopropylidene-myo-inositol. Ac2PIM1 was prepared from this compound in an overall yield of 18% over 19 steps. By designing the synthesis to use both enantiomers of the racemic compound, rather than discarding an undesired one, the overall yield of the synthesis was nearly doubled.

The lipidated backbones of two PIM6 family members were also synthesised using the acetylated product of the same inositol as used for Ac2PIM1. In an initial attempt at the synthesis of an Ac2PIM6, it was found that it was not possible to remove a dimethoxyphenylbenzyl (DMPB) group from a molecule containing (1→2)-α-linked mannosyl residues, so the synthesis was repeated with a MOM group in its stead. However, time restraints meant that this synthesis was not completed. From these advanced intermediates both an Ac1PIM6 and an Ac2PIM6 can readily be synthesised using the methodology exemplified in the synthesis of Ac2PIM1.