6-O-Picolinyl and 6-O-Picoloyl Building Blocks As Glycosyl Donors with Switchable Stereoselectivity.

Remote 6-O-picolinyl or 6-O-picoloyl substituents often provide high β-selectivity due to H-bond-mediated aglycone delivery (HAD). Herein it has been demonstrated that if the nitrogen atom of the 6-O-picolinyl or picoloyl moiety is temporarily blocked by coordination to a metal center (Pd), it cannot engage in HAD-mediated β-glycosylation. Hence, the stereoselectivity of 6-O-picolinyl/picoloyl-assisted glycosylations can be "switched" to α-selectivity.