Brønsted acid-catalyzed allylboration: short and stereodivergent synthesis of all four eupomatilone diastereomers with crystallographic assignments.
This study describes a novel Brønsted acid-catalyzed allylboration method suitable for the most difficult, electronically deactivated allylboronate and aldehyde substrates. This method circumvents the use of metal ions, and rather employs a simple and cheap catalyst, triflic acid. Its usefulness as a complementary allylboration variant was demonstrated with a four-step, stereodivergent synthesis of all four diastereomers of eupomatilone-6 from a single allylboronate. A thorough proof of stereochemistry supported by as many as five X-ray crystallographic structures brings an end to the ambiguity of the original stereochemical assignments. Further to the TfOH-catalyzed allylboration, the synthetic route featured a number of remarkable observations: the surprising reactivity of 2-bromo-3,4,5-trimethoxybenzaldehyde, the subtle reagent control observed in the hydrogenation of an alpha-exo-methylene lactone intermediate, and the success of a difficult case of Suzuki biaryl coupling using Buchwald's conditions.
Yu
Siu Hong
SH
Department of Chemistry, University of Alberta, Edmonton, AB, T6G 2G2, Canada.
Ferguson
Michael J
MJ
McDonald
Robert
R
Hall
Dennis G
DG
D000143:Acids / Q000737:chemistry*; D001572:Benzofurans / Q000138:chemical synthesis* / Q000737:chemistry; D001896:Boron Compounds / Q000138:chemical synthesis*; D002384:Catalysis; D018360:Crystallography, X-Ray; D008968:Molecular Conformation; D013237:Stereoisomerism
D000143:Acids; D001572:Benzofurans; D001896:Boron Compounds; C485126:eupomatilone-6