A New Access to Pyrrolizidine Derivatives: Ring Contraction of Methyl (<i>E</i>)-[1,2-Oxazin-3-yl]propenoates

Abstract

Nitrosoalkene 2 generated in situ from oxime 3 underwent smooth hetero Diels-Alder reaction with enol ethers 1 to afford 1,2-oxazine derivatives 4 bearing an exocyclic C=C bond. Methoxyallene 8 and 2 provided 6H-1,2-oxazine 10 in good overall yield. The exocyclic double bond of this type of 1,2-oxazines can be employed for addition reactions as demonstrated by dihydroxylation of 4a with potassium permanganate, smoothly delivering 1,2-diol 11. A reductive cascade reaction involving ring cleavage at the N-O bond followed by cyclization steps furnished pyrrolizidinone derivatives 12 in good yields. In the case of 12b this transformation proceeded with excellent stereoselectivity. Finally, the lactam moiety of 12 could be reduced with borane to provide the corresponding pyrrolizidine derivatives 19 in good yield.