Lewis Acid Catalyzed Allylboration: Discovery, Optimization, and Application to the Formation of Stereogenic Quaternary Carbon Centers

A full account of the development of the first catalytic manifold for the additions of allylboronates
to aldehydes is described. The thermal additions (both diastereospecific and enantioselective) of
2-carboxyester 3,3-disubstituted allylboronates 1 to both aromatic and aliphatic aldehydes give
biologically and synthetically important exo-methylene butyrolactones 2 containing a β-quaternary
carbon center. Although the thermal reaction requires 14 d at room temperature to reach completion,
the presence of certain metal salts allows for a 12−16 h reaction while preserving the diastereospecificity observed in the uncatalyzed process. Preliminary mechanistic studies on the origin
of the catalytic effect are described as well as stereoselective transformations of lactones 2 into
cyclic and acyclic stereotriads with potential usefulness as synthetic intermediates.