We hypothesized that selectivity towards dehydropiperidine formation might be improved by introducing electron-withdrawing groups (e.g. -CO2Me) at the β-carbon of the diazoester precursor (e.g. C3 in 2a). This should lead to greater delocalization of the negative charge of the ylide on the β carbon, which could increase the preference for formation of the six-membered ring. To test this hypothesis, the carbene precursor was altered from 2a (Scheme 1) to the CO2Me-containing vinyl diazoester 2c (Scheme 3A). Indeed, an increase in the 4ac:3ac selectivity to 3:1 was observed in a 78% NMR yield. To provide experimental support for these divergent mechanisms dependent on the carbene precursor, a series of stereochemical probe experiments were carried out (Scheme 4). Nonracemic 1b was obtained in >99% ee from the corresponding enantioenriched propargyl alcohol. Treatment of 1b with 2c showed that some of the stereochemical information is lost in the ring expansion to 4bc (26% ee), suggesting that a competing stepwise pathway may be operating in the reaction. Unfortunately, 3bc could not be isolated and the ee could not be determined. However, while treatment of non-racemic 1b with 2b also gave reduced ee in the dehydropiperidine product 4bb (81% ee), interestingly, complete chirality transfer was noted in 3bb (>99% ee). Finally, use of 2f as the carbene precursor also resulted in complete transfer of chirality to the azetidine 3bf (>99% ee); no formation of the dehydropiperidine was noted. These experiments support the likelihood that the mechanism of the ring expansion differs depending on the carbene precursor. These experiments also suggest that ring closure to the azetidine occurs faster than the formation of any zwitterionic or other achiral intermediates.