Analysis and Formation Mechanism of Trace Impurity in Darunavir Key Intermediate: (3aS,6aR)-9-cis-Tetrahydrogen-4-Methoxyfuran［3,4b］Furan-2(3H)-One

The trace impurity in (3aS,6aR)-9-cis-tetrahydrogen-4-methoxyfuran［3,4b］furan-2(3H)-one（MFO）is enriched and purified. The impurity was identified by high resolution MS, elemental analysis, NMR, and monocrystal X-ray diffraction, and its structrue is a new dimer of MFO(β-DIMFO), which is an epimer of known impurity, α-DIMFO. Formation mechanism of the impurity is discussed.