Synthesis of monosaccharide-derived spirocyclic cyclopropylamines and their evaluation as glycosidase inhibitors

The glucose-, mannose-, and galactose-derived spirocyclic cyclopropylammonium, chlorides la-1d, 2a-2d and 3a-3d were prepared as potential glycosidase inhibitors. Cyclopropanation of the diazirine 5 with ethyl acrylate led in 71% yield to a 4:5:1:20 mixture of the ethyl cyclopropanecarboxylates 7a-7d, while the Cu-catalysed cycloaddition of ethyl diazoacetate to the exo-glycal 6 afforded 7a-7d (6:2:5 :3) in 93-98% yield (Scheme 1). Saponification, Curtius degradation, and subsequent addition of BnOH or t-BuOH led in 60 - 80% overall yield to the Z- or Boc-carbamates 11a-11d and 12a-112d, respectively. Hydrogenolysis of 11a-11d afforded 1a-1d, while 12a-12d was debenzylated to 13a-13d prior to acidic cleavage of the N-Boc group. The manno- and galacto-isomers 2a-2d and 3a-3d, respectively, were similarly obtained in comparable yields (Schemes 2 and 4). Also prepared were the differentially protected manno-configured esters 24a-24d; they are intermediates for the synthesis of analogous N-acetylglucosamine-derived cyclopropanes (Scheme 3). The cyclopropylammonium chlorides 1a-1d, 2a-2d and 3a-3d are very weak inhibitors of several glycosidases (Tables I and 2). Traces of Pd compounds, however, generated upon catalytic debenzylation, proved to be strong inhibitors. PdCl42- is, indeed, a reversible, micromolar inhibitor for the beta-glucosidases from C saccharolyticum and sweet almonds (non-competitive), the beta-galactosidases from bovine liver and from E. coli (both non-competitive), the a-galactosidase from Aspergillus niger (competitive), and an irreversible inhibitor of the alpha-glucosidase from yeast and the alpha-galactosidase from coffee beans. The cyclopropylamines derived from 1a-1d or 3a-3d significantly enhance the inhibition of the beta-glucosidase from C saccharolyticum by PdCl42-, lowering the K-i value from 40 mum (PdCl42-) to 0.5 mum for a 1 : 1 mixture of PdCl42- and 1d. A similar effect is shown by cyclopropylamine, but not by several other amines.