Hetero Diels-Alder Reactions of Nitroso Alkenes with Alkoxyallene
Derivatives Bearing Carbohydrate Auxiliaries: Asymmetric Synthesis
of 6H-1,2-Oxazines and Subsequent Reductive Transformations

Abstract

Alkoxyallene derivatives 1a-f bearing carbohydrate auxiliaries at the oxygen were examined in asymmetric hetero Diels­-Alder reactions with nitroso alkenes. Diacetoneglucose derived compound 1a 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 1f gave compounds 4h-j with good efficiency. Gratifyingly, it turned out that 1a and 1f 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 1a have 6S configuration in excess whereas those derived from 1f 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.

allenes - carbohydrates - asymmetric synthesis - hetero Diels-Alder reaction - 6H-1,2-oxazines - reduction - pyrrolidines

References

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