The chemoenzymatic synthesis of a series of C6-modified GDP-d-Man sugar nucleotides is described. This provides the first structure-function tools for the GDP-d-ManA producing GDP-d-mannose dehydrogenase (GMD) from Pseudomonas aeruginosa. Using a common C6 aldehyde functionalization strategy, chemical synthesis introduces deuterium enrichment, alongside one-carbon homologation at C6 for a series of mannose 1-phosphates. These materials are shown to be substrates for the GDP-mannose pyrophosphorylase from Salmonella enterica, delivering the required toolbox of modified GDP-d-Mans. C6-CH3 modified sugar-nucleotides are capable of reversibly preventing GDP-ManA production by GMD. The ketone product from oxidation of a C6-CH3 modified analogue is identified by high-resolution mass spectrometry.