The direct and highly diastereoselective synthesis of 3,4-epoxy-2-piperidones. Application to the total synthesis and absolute configurational assignment of 3α,4α-epoxy-5β-pipermethystine

The substrate-controlled asymmetric total synthesis and absolute configurational assignment of biologically active 3 alpha, 4 alpha-epoxy-5 beta-pipermethystine, a minor component in the aerial parts of kava, has been achieved by featuring, as a key step, the environmentally friendly and direct synthesis of 2,3-epoxyamides from allyl amines. By using the chiron approach, first a carbohydrate-derived dehydropiperidine was prepared and subjected to a stereoselective tandem C-H/C=C oxidation reaction. In this attempt, the required alpha,alpha-trans-epoxy-2-piperidone skeleton of the kava metabolite precursor was not achieved, although the tandem oxidation was highly stereoselective. However, starting from non-carbohydrate 3-hydroxy-4,5-dehydropiperidine, and using the same tandem oxidation, the target intermediate was obtained in high yield and complete unprecedented anti-stereoselectivity. Since the proposed mechanistic course of this tandem oxidation implies the transient formation of an alpha,beta-unsaturated amide followed by the subsequent epoxidation reaction, this second approach supports the previously established biotransformation proposal of (-)-pipermethystine to (-)-3 alpha,4 alpha-epoxy-5 beta-pipermethystine.