Acid Promoted Syntheses of New Enantiopure Amino Sugar Derivatives from 3,6-Dihydro-2H-1,2-oxazines

Abstract
The acid promoted rearrangement of syn-1 with HCl led to bicyclic acetal 2 from which enantiopure furan derivative 4 was obtained by hydrogenolysis. The same sequence was applied to anti-1 leading to bicyclus 3 and diastereomeric tetrahydrofuran 5. Treatment of syn-6 with pyridinium hydrogen fluoride provided either semiacetal 7 or bicyclic acetal 8, depending on the ratio of HF and pyridine. Lewis acid mediated intramolecular aldol reaction of syn-6 afforded bicyclic 1,2-oxazine 9. Protection and subsequent stereoselective reduction of 9 led to 10, which was hydrogenated to yield enantiopure amino substituted pyran derivative 11.