Efficient synthesis of the D-ring fragment of cobyric acid

The synthesis of a highly functionalized 4,5-dihydro-3H-pyrrol, namely, the D-ring fragment 5a of cobyric acid (1), is described in this letter. A very efficient assembly to 5a involves CBS-reduction of 10, a [2,3] Wittig-Still rearrangement, and a stereoselective Michael addition to a nitro olefin.