Steric constraints against [3,3]-sigmatropic rearrangement of allylic azides. A convenient approach to β-L-4-aminopent-2-enoglyceropyranosides

Starting from alkyl α-D-4-0-methanesulphonylpent-2-enoglyceropyranosides 13a-c, nucleophilic substitution carried out with polymer-supported azide ion led to regioisomeric mixtures of the azides 14a-c and 15a-c. An analogous result, due to a [3,3]-sigmatropic rearrangement, was observed starting from methyl α-D-hex-2-enoerythropyranosides derivatives 6a and 6b. On the contrary, starting from alkyl β-D-4-0-methanesulphonylpent-2-enoglyceropyranosides 21a-c, azides 22a-c were exclusively obtained, and subsequently converted into the corresponding amino derivatives 23a-c. The behaviour of β-anomers 21a-c was ascribed to steric interactions in the cyclic transition state, as supported by ab initio calculations.