Catalyst-Controlled Regiodivergent Synthesis of 1- and 3‑Thiosugars with High Stereoselectivity and Chemoselectivity

An
effective regiodivergent synthesis toward 1- and 3-thiosugars
was achieved through palladium and cobalt catalysis, respectively.
β-1-Thiosugars were obtained from 3,4-O-carbonate-glycals
and various thiols catalyzed by Pd2(dba)3 in
good yields with exclusive stereoselectivity and chemoselectivity,
while only (3S)-3-thiosugars were generated via Co­(BF4)2. Experiments and DFT calculations supported
the proposed mechanism that the thiol moiety preferred to form a hydrogen
bond with C4-oxygen from top-face in Pd conditions but thiols would
coordinate with cobalt from bottom-face in Co conditions. The difference
between Pd/Co–C1 and Pd/Co–C3 bond lengths of key intermediates
provided further insights into the 1,3-regioselectivity.