(1S,4R,5R,6R)-6-methoxycarbonyl-4-pivaloyloxy-2-(pivaloyloxymethyl)- bicyclo[3.1.0]hex-2-ene-1-carboxylic acid.

The stereochemistry of the title compound, C20H28O8, a key step in the preparation of analogues of mannostatins, potent inhibitors of α-mannosidase, has been established. The carboxyl­ic acid group at C1 unexpectedly eclipses the C1—C2 bond. The cyclo­propane ring makes a dihedral angle of 109.4 (2)° with the cyclo­pentene ring.