An Enantioselective Synthesis of (+)-Picrasin B, (+)-A²-Picrasin B, and (+)-Quassin from the R-(-) Enantiomer of the Wieland-Miescher Ketone

An enantioselective total synthesis of (+)-picrasin B (1), (+)-A²-picrasin B (11), and (+)-quassin (12) from the R-(-) enantiomer of the Wieland—Miescher ketone (3) employed an A-AB-ABC-ABCD sequence to assemble the tetracyclic skeleton. The crucial steps in this sequence relied upon a Diels-Alder reaction of a bicyclic AB dienophile 15 with l-methoxy-2-methyl-3-((trimethylsilyl)oxy)-1,3-butadiene to obtain a tricyclic diol 16, an a’-oxidation of a tricyclic enone intermediate 20 using manganese(III) acetate in order to introduce the C-11 substituent needed to invert the C-9β stereochemistry, and a free-radical cyclization of an a-bromo acetal 23 in order to introduce a protected δ-lactol as a progenitor of the quassinoid D ring.