Partially fluorinated heterocyclic compounds. Part 12. Novel internal Diels–Alder adducts via Claisen rearrangement intermediates from tetrafluoro-4- and -3-pyridyl prop-2-enyl ethers. Syntheses of tetrafluoro-3-aza- and -4-aza-tricyclo[3.3.1.0]non-3-en-6-one