Synthesis of 1-azagalactofagomine, a potent galactosidase inhibitor

Abstract

1-Azagalactofagomine [(+)-(3R,4S,5R)-4,5-Dihydroxy-3-(hydroxymethyl)hexahydropyridazine, 2] was synthesised from achiral starting materials in a chemoenzymic synthesis. The racemic Diels–Alder adduct (2-hydroxymethyl-8-methyl-1,6,8-triazabicyclo[4.3.0]non-3-ene-7,9-dione, 5) from addition of pentadienol to 4-methyl-1,2,4-triazoline-3,5-dione was resolved using lipase R-catalysed acetylation. The acetate [(S)-2-acetoxymethyl-8-methyl-1,6,8-triazabicyclo[4.3.0]non-3-ene-7,9-dione, 6] was saponified and treated with MCPBA to give a majority of the syn epoxide [(2R,3S,4R)-3,4-epoxy-2-hydroxymethyl-8-methyl-1,6,8-triazabicyclo[4.3.0]nonane-7,9-dione, 7]. This isomer was subjected to epoxide opening with HI followed by a Woodward reaction-like displacement of the iodide with water and peracetylation to give an all-syn triacetate [(2R,3S,4R)-3,4-diacetoxy-2-acetoxymethyl-1,6,8-triazabicyclo[4.3.0]nonane-7,9-dione, 11]. Finally deacetylation and hydrazinolysis gave 2. The pK_a of 2 was determined to be 5.7. 1-Azagalactofagomine was found to be a potent competitive galactosidase inhibitor. The inhibition constants, K_i, were 40, 300 and 7800 nM versus β-galactosidase from Aspergillusoryzae, Eschinchia coli and Saccharomyces fragilis, respectively, and 280 nM vs. α-galactosidase from green coffee beans.