Stereocomplementary synthesis of casuarine and its 6-epi-, 7-epi-, and 6,7-diepi-stereoisomers

Diastereoselective Synthesis, Glycosidase Inhibition, and Docking Study of C-7-Fluorinated Casuarine and Australine Derivatives

C-7-fluorinated derivatives of two important polyhydroxylated pyrrolizidines, casuarine and australine, were synthesized with organocatalytic stereoselective α-fluorination of aldehydes as the key step. The strategy is extensively applicable to some synthetically challenging fluorinated iminosugars and carbohydrates. The docking studies indicated that the potent inhibitions of trehalase and amyloglucosidase by the fluorinated polyhydroxylated pyrrolizidines are due to the interaction modes dominated by fluorine atoms in these iminosugars with the amino acids' residues of the corresponding enzymes. Steady interactions were established between the C-7 fluoride and a hydrophobic pocket in amyloglucosidase by untypical anion-π interactions. These unexpected docking modes and related structure-activity relationship studies emphasize the value of fluorination in the design of polyhydroxylated pyrrolizidine glycosidase inhibitors.

Stereoselective Total Synthesis of (+)-Casuarine via a Functionalized Pyrrolidine

The stereoselective total synthesis of (+)-casuarine has been achieved via a functionalized pyrrolidine obtained from a chiral 1,3-oxazine. The synthetic strategy includes a stereoselective dihydroxylation reaction using osmium tetroxide and stereoselective Grignard-type vinyl addition reaction as the key steps to generate the two new stereocenters. The dihydroxylation catalyzed by osmium tetroxide formed anti-amino alcohol, and the vinyl addition formed syn-alcohol. (+)-Casuarine was prepared from the functionalized pyrrolidine over 11 steps in an overall yield of 16%.