Diastereoselective Access to 3-Nitro-4-vinylidenetetrahydrofurans and 3-Nitro-4-vinylidenetetrahydropyrans and Their Conversion into 3,6-Dihydro-1,2-oxazines by Reverse Cope Elimination of Hydroxylamine Precursors

A new approach to the synthesis of unsaturated nitro-allenyl-tetrahydrofurans 3 and -tetrahydropyrans 5 has been developed, involving oxa-Michael addition/SN2′ substitution of propargyl and homopropargyl alcohol derivatives 2a and 4h on nitroalkenes 1a−g. Complete allylic 1,3-strain control during the cyclization accounts for the observed diastereoselectivity. Reduction of nitro compounds 3 and 5 with SmI2 provided new α−allenylhydroxylamines 7 and 8, which were easily isomerized into 3,6-dihydro-1,2-oxazines 9 and 10 by reverse Cope elimination.