Total Synthesis of +-alexine by utilizing a highly stereoselective [3+2] annulation reaction of an N-tosyl-alpha-amino aldehyde and a 1,3-bis(silyl)propene.

A novel route towards the polyhydroxylated pyrrolizidine alkaloid (+)-alexine has been developed. A key step in this synthesis is a highly stereoselective [3+2] annulation reaction of N-Ts-alpha-amino aldehyde 7 a (Ts=tosyl) and 1,3-bis(silyl)propene 8 a for the construction of the polyhydroxylated pyrrolidine subunit of the target molecule. Previous synthetic strategies rely on carbohydrates that require several protecting-group manipulations, thereby making the total number of steps relatively high. The [3+2] annulation strategy compares favorably with carbohydrate-based syntheses and constitutes a highly efficient entry to polyhydroxylated alkaloids.