Enantiocontrolled Synthesis of Burseran, Brassilignan, Dehydroxycubebin, and Other Tetrahydrofuran Lignans in Both Enantiomeric Forms. Application of Intermolecular Nitrile Oxide Cycloadditions and Lipase Mediated Kinetic Resolutions

Article abstract of DOI:10.1021/jo00060a037

Several natural and unnatural tetrahydrofuran lignans have been synthesized by a convergent approach.Our methodology utilizes a nitrile oxide cycloaddition to dihydrofuran 8 and an enzymatic resolution of alcohols 11 by lipase PS.The lipase-mediated kinetic resolution of alcohols 11 furnished both enantiomers of the lignan precursors 12 and 14 in high optical purity (>99percent ee).This is followed by a S_N2 displacement of tosylates 15 and 18 by α-lithiobenzyl phenyl sulfides.In this manner, both enantiomers of 3,4-dibenzyltetrahydrofuran (17a, 20a), 3,4-bis(3-methoxybenzyl)tetrahydrofuran (17b, 20b), brassilignan (17c, 20c), dehydroxy cubebin (17d, 20d), and burseran (17e, 20e) were synthesized in overall yields of 6-16percent.