AN EFFICIENT AND SIMPLE SYNTHESIS OF 3,4,5,6-TETRAHYDRO-2H-1,2-OXAZINES BY SODIUM CYANOBOROHYDRIDE REDUCTION OF 5,6-DIHYDRO-4H-1,2-OXAZINES

3,4,5,6-Tetrahydro-2H-1,2-oxazines are prepared by reduction of the co rresponding 5,6-dihydro-4H-1,2-oxazines with sodium cyanoborohydride a s the reducing agent in acetic acid. This reaction gives the 3,5-disub stituted compounds 2a-c, 2f-g, and 5a,b with good to excellent cis sel ectivities, while a 3,6-disubstituted 1,2-oxazine leads to a trans con figurated product as the major isomer. Under the same reaction conditi ons the bicyclic heterocycle 14 affords two products, the expected com pound 15 and the cyclopentenol derivative 16 as a byproduct. Also, the formation of trifluoromethylated ketoximes 18 and 21 starting from th e precursors 17 and 19 is described.