Synthesis of C-5a-substituted derivatives of 4-epi-isofagomine: notable β-galactosidase inhibitors and activity promotors of GM1-gangliosidosis related human lysosomal β-galactosidase mutant R201C.

•The first synthesis of (5aS)-chain extended 4-epi-isofagomines.•New compounds are powerful β-galactosidase inhibitors.•Residual human β-galactosidase activity of mutant R201C is enhanced up to 10-fold (at 2.5 µM).