6‐Siloxy‐Substituted 5,6‐Dihydro‐4 H ‐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.