Consider the reaction below.
Give a plausible mechanism for such a synthetic transformation.
In 2009, a group of chemists from South Korea decided to investigate the regioselectivity of this reaction with variations on substituent R2. The results were quite peculiar when tested on pyran-containing substrates with a pendant ketone:
The compound bearing an axial pendant predominantly cyclizes onto the carbon not adjacent to the oxygen atom of the pyran ring, whereas the equatorial isomer only yields the oxygen-adjacent cyclized product. Provide a reasonable interpretation that accounts for the observed regioselectivity.
For reference:
Yun, S. Y., Zheng, J.-C., & Lee, D. (2009).
“Stereoelectronic Effect for the Selectivity in C−H Insertion of Alkylidene Carbenes and Its Application to the Synthesis of Platensimycin.”
Journal of the American Chemical Society, 131(24), 8413–8415.
doi:10.1021/ja903526g