Enantioselective Synthesis of Protected Nitrocyclohexitols with Five Stereocenters. Total Synthesis of (+)-Pancratistatin

2-Methoxymethylpyrrolidine best performed, among several
other
proline derivatives, to control the enantioselective [3+3] annulation
of β-(hetero)­aryl-α-nitro-α,β-enals with commercial
2,2-dimethyl-1,3-dioxan-5-one, a procedure that renders highly oxygenated
nitrocyclohexanes endowed with five new stereocenters. Use of this
reaction allowed the development of a total synthesis of the antitumoral
natural product (+)-pancratistatin; it also converted our previous
racemic route to tetrodotoxin into an enantioselective one.