A partially protected C-5@C-5a unsaturated carbasugar with a-lyxo configuration is synthesised in five
steps and 26% overall yield from a known mannose-derived hemiacetal, using ring-closing metathesis
as a key step. This carbasugar is converted into valienamine derivatives with b-lyxo (i.e., corresponding
to b-manno at C-1–C-4), a-lyxo (i.e., corresponding to a-manno at C-1–C-4) and b-2-acetamido-2-
deoxy-xylo (i.e., corresponding to b-GlcNAc at C-1–C-4) configurations. This is the first report of the synthesis
of the b-lyxo compound, 1,2-bis-epi-valienamine, which was found to inhibit Cellulomonas fimi bmannosidase
(CfMan2A) with Ki 140 lM. We report the crystal structures of three protected C-5@C-5a
unsaturated carbasugars with lyxo configuration.