6-Siloxy-Substituted 5,6-Dihydro-4H-1,2-oxazines as Key Building Blocks for Natural Products

Methyl (E)‐5‐bromo‐4‐oxo‐2‐pentenoate (6) and the corresponding oxime 5 were prepared in high yields by starting from methyl levulinate (3). The novel nitrosoalkene 8, generated in situ from oxime 5, smoothly added to silyl enol ether 7 to give 1,2‐oxazine 9 and nitrone 10 as byproduct. Methods are described transforming intermediate 9 into unsaturated diketo ester 11, or oximes 13, 15, and 18, which may serve as precursors of pyrenophorin. Reductive ring cleavage of 1,2‐oxazine 9 with Raney nickel efficiently afforded the saturated diketo ester 19. Its intramolecular aldol reaction gave cyclopentenone derivative 20, which is a known precursor of (Z)‐jasmone. Cycloaddition of methyl α‐nitrosoacrylate to silyl enol ether 22 provided 1,2‐oxazine 23. This compound was directly converted into the ant‐trail pheromone 24 by hexacarbonylmolybdenum‐promoted ring contraction.