Fluorinated butanolides and butenolides. Part 5. Synthesis and nucleophilic reactions of 3-chloro-2-fluoro-2-buten-4-olide as tetronic acid analogue. Conjugate addition of hard nucleophiles and vinylic halogen displacement with soft phosphorus nucleophile

3-Chloro-2-fluorobut-2-en-4-olide (13) and 2-fluorobut-2-en-4-olide (14) were prepared by a six step synthesis starting from 1,2-dibromo-1,2-dichloro-1,2-difluoroethane (5). UV-initiated addition of 5 to ethylene afforded 1,4-dibromo-1,2-dichloro-1,2-difluorobutane (6a) that was easily dehydrochlorinated to 4-bromo-3,4-dichloro-3,4-difluorobut-1-ene (7) and subsequently by chlorine addition to 1-bromo-1,2,3,4-tetrachloro-1,2-difluorobutane (10). Fuming sulfuric acid affected CH2–Br bond in 6a to afford 4-bromo-3,4-dichloro-3,4-difluorobutan-1-ol (8), while with 10 bearing stable terminal CH2–Cl group the reaction took place at the trihalomethyl group to give 2,3,4-trichloro-2-fluorobutanoic acid (11a). Its sodium salt (11b) cyclized in solution or thermally in the solid state to 2,3-dichloro-2-fluorobutan-4-olide (12) that by dehydrochlorination with tertiary amine gave new butenolide 13 or by didechlorination with zinc 2-fluorobut-2-en-4-olide (14). Hard nucleophiles as methanol and piperidine afforded products of conjugate addition, while soft phosphorus nucleophiles, triethyl phosphite or tributylphosphane, affected vinylic C–Cl and C–F bonds to give products of nucleophilic vinylic substitution.