The synthesis and biological evaluation of novel N-acetylhexosaminidase inhibitors
Keywords: lysosomal storage disorders
hexosaminidase
iminosugar
carbohydrate

Iminosugars are known to behave as carbohydrate mimics in biological systems by virtue of their similar structures. However as the ring nitrogen prevents metabolism it means that iminosugars have the potential to become inhibitors of these systems. It is known, for example, that iminosugars can behave as mimics in the hydrolysis mechanism. This leads to possible medicinal applications of iminosugars. One such case is lysosomal storage disorders which arise as a result of a genetic defect which causes missense mutations coding for the N-acetylhexosaminidase enzymatic protein. N-Acetylhexosaminidases are a sub-member of the class of glycosidase enzymes. They are responsible for the cleavage of N-acetylhexosamine residues from glycoconjugates in the lysosome. Mutations in the gene coding for this protein lead to a deficiency in the enzymatic activity resulting in accumulation of unhydrolysed substrate in the lysosome. Lysosomal storage disorders have a phenotype of poor motor development and neurological problems. The infantile form usually leads to death before the age of five. An iminosugar mimic could give rise to a possible treatment for lysosomal storage disorders by acting as a molecular chaperone during protein folding, promoting correct folding by its intrinsic affinity for the native fold of the enzyme. Likewise in the treatment of cancer, the inhibitory ability of iminosugars has potential applications. In cancer, extracellular hydrolysis occurs which favours cancer cell survival. Macrophages, which attack and eliminate cancer cells, can be activated by macrophage activating factor (MAF) which displays an α-N-acetylgalactosamine residue that appears essential for the activation cascade. Cancer cells secrete an α-N-acetylgalactosaminidase enzyme that acts to decrease the potency of MAF, thus promoting cancer cell survival. Inhibition of cancer cell α-N-acetylgalactosaminidase may restore macrophage activation and generate potential therapeutics.
Chapter 1 of this thesis contains extended discussion of the aforementioned, and related, diseases and the therapeutic applications of iminosugars. Some historically and biologically important iminosugars are described along with some current iminosugar drugs.
Chapter 2 describes the synthetic strategies explored in an attempt to synthesise all the members of the 2-acetamido pyrrolidine iminosugars. An overview of the compounds synthesised towards this end by a past group member is given along with the work performed as part of this thesis to complete this goal. Both enantiomers with arabino- and ribo- stereochemistry and D-lyxo- were previously synthesised. The syntheses of both enantiomers with xylo- stereochemistry along with the L-lyxo- compound were completed as part of this thesis, from either D- or L-glucuronolactone and D-ribose, respectively.
Chapter 3 details the synthetic strategy adopted to synthesise the enantiomer of D-DNJNAc, the first potent α-N-acetylgalactosaminidase inhibitor to be found. The synthesis towards another piperidine iminosugar, 6-deoxy DGJNAc, is presented in the second half of this chapter, along with two related compounds.