Regio- and Diastereoselectivity in the Reaction Sequence from Secondary 3-Oxa-5-hexen-1-ylamines to Bicyclic Compounds via Nitrones

Abstract

Secondary amines 2 and 8 are oxidized by hydrogen peroxide in the presence of sodium tungstate providing nitrones. If the amino function is substituted by one primary and one secondary alkyl moiety, oxidation proceeds with high regioselectivity at the primary alkyl group affording aldonitrones. Thus, compound 2b gives exclusively 3b that undergoes an intramolecular cycloaddition yielding 4b. From 2c, however, in addition to 4c nitrone 5 is formed as by-product, which affords the bridged compound 6 on heating in toluene. Oxidation of compounds 8 yields nitrones 9 which are converted to bridged compounds 10 as major products along with small amounts of regioisomers 11 at elevated temperatures. The intramolecular cycloaddition of the chiral nitrones 3b and c as well as 9b proceeds with high diastereoselectivity.