Site-Selective and Stereospecific Functionalization of Organoboronate Compounds

Abstract

This dissertation presents three projects focusing on catalytic, site-selective and stereospecific functionalization of organoboronate compounds. First, we have developed a Cu-catalyzed regioselective coupling of 1,2-bis(boronic esters). This transformation provided an effecient route to a range of enantiomerically enriched secondary boronic esters. Mechanistic studies indicated that a chelated cyclic boron “ate” complex may promote transmetallation to Cu catalyst. We have also developed a Cu-catalyzed sterespecific cross-coupling of alkylboronic esters. Boron “ate” complexes derived from alkylboronic pinacol esters and tert-butyllithium underwent stereospecific transmetallation to copper cyanide. The so-formed organocopper species engaged alkynyl bromides, allyl halides, propargylic halides, β-haloenones, hydroxylamine esters, and acyl chlorides in coupling reactions. Finally, we have developed non-directed site-selective activation of bis(boronic esters), followed by Cu-catalyzed cross-coupling. A bulky activator was shown to selectively activate the less hindered boronic ester, enabling it to undergo stereospecific cross-coupling to a variety of electrophiles. This steric-based site-selectivity offered a simple and efficient route to prepare propionate derivatives as well as other valuable difunctional molecules. The appendix describes an ongoing study towards the synthesis of a novel analog of amphidinolide H. In an effort to elucidate the relationship between biological activity and structure of amphidinolide H, we designed a simplified analog that is likely to resemble amphidinolide H in shape. Development of its synthesis is currently underway. With an anticipated streamlined route to the analog, we expect to carry out rapid interrogation of its biological activity in collaboration with other research groups.