Preparation of 2-alkenylperhydro-1,3-benzoxazines and application for 2-isoxazolines synthesis

Various 2-substituted-N-benzyl-4,4,7-trimethyl-trans-octahydro-1,3-benzoxazines 2 were prepared from the condensation of (?)-8-benzylaminomenthol 1 derived from (+)-pulegone, with acrolein, crotonaldehyde, cinnamaldehyde, 2(E)-N,N-diisopropyl-4-oxobut-2-enamide, ethyl (2E)-4-oxobut-2-enoate, and 2-furaldehyde in 71–96% yield. The 1,3-dipolar cycloaddition with aceto- and benzonitrile oxide gave the corresponding 2-isoxazoline cycloadducts. The origin of the stereoselectivity (4′S, 5′S-cycloadducts up to 64% de) arises from the cycloaddition of the dipole to the top of the Re,Re-alkene face of dipolarophile 2.