(1R,5S,6R)-Ethyl 5-(Pentan-3-yloxy-d5)-7-oxabicyclo[4.1.0]hept-3-ene-3-carboxylate is the labeled analogue of (1R,5S,6R)-Ethyl 5-(Pentan-3-yloxy)-7-oxabicyclo[4.1.0]hept-3-ene-3-carboxylate (E917990), the enantiomer of Ethyl (3R,4S,5S)-4,5-Epoxy-3-(1-ethylpropoxy)cyclohex-1-ene-1-carboxylate (E917820), an intermediate of Oseltamivir.