Discovery and investigation of trialkylborane/water mediated radical reactions, and, An enantioselective approach to the Securinega alkaloids: The total synthesis of (+)-norsecurinine and (+)-allonorsecurinine.

摘要

During the course of a total synthesis of phomoidride D, a deoxygenation reaction employing trimethylborane was observed. The absence of an obvious hydrogen atom source in the reaction prompted a deuterium study, which suggested the source of hydrogen atom was water. Experimental and computational studies supported the existence of a Me3B/water complex that was acting as the hydrogen atom donor in the reaction. The trialkylborane/water complex was used for the deoxygenation of xanthates, the dehalogenation of alkyl iodides, and the chemoselective deoxygenation of a xanthate in the presence of alkyl bromides and chlorides. Attempts were made to deliver the hydrogen intramolecularly via a tethered alcohol in order to render the reduction stereoselective.;The Securinega alkaloids are a small family of indolizidine and pyrrolizidine natural products. A flexible route utilizing an enantioselective rhodium carbenoid-initiated O-H insertion/Claisen rearrangement/1,2-allyl migration domino sequence was developed targeting the tertiary alcohol moiety common to many members of this family. The strategy was implemented in the synthesis of (+)-norsecurinine and (+)-allonorsecurinine.