Hetero Diels-Alder reactions of nitroso alkenes with alkoxyallene derivatives bearing carbohydrate auxiliaries:: Asymmetric synthesis of 6H-1,2-oxazines and subsequent reductive transformations

Alkoxyallene derivatives 1a-f bearing carbohydrate auxiliaries at the oxygen were examined in asymmetric hetero Diels-Alder reactions with nitroso alkenes. Diacetoneglucose derived compound la turned out to be the best precursor furnishing the primary cycloadducts 3a-c with a diastereomeric ratio of approximately 90:10. Isomerization provided the thermodynamically more stable 6H-1,2-oxazines 4a-c, Similarly, diacetonefructose derived allene If gave compounds 4h-j with good efficiency. Gratifyingly, it turned Out that la and If were complementary with respect to the preferential absolute configuration at C-6 of 6H-1,2-oxazines 4a-c and 4h-j. respectively. Cycloadducts derived from la have 6S configuration in excess whereas those derived from If are predominantly 6R configured. Exhaustive hydrogenolysis of 6H-1,2-oxazines 4a and 4h in the presence of palladium on charcoal furnished the expected primary amine 5 in an enantioenriched form, If this reduction was performed under addition of hydrochloric acid, pyrrolidine derivative 6 together with secondary amine 7 as side product were isolated.