Construction of Multiple, Contiguous Quaternary Stereocenters in Acyclic Molecules by Lithiation-Borylation

Lithiation
of carbamates followed by borylation provides a powerful
method for the homologation of boron reagents. However, when applied
to hindered systems (secondary carbamates with tBu-boronic
esters) for the construction of two quaternary centers, this methodology
fails. Instead, using mixed boranes (tBuBMe2), the synthesis of adjacent quaternary stereogenic centers with
full stereocontrol was successful. The process can be repeated two
or three times in one pot leading to carbon chains bearing multiple
contiguous quaternary stereogenic centers. The boranes were converted
into tertiary alcohols or C-tertiary amines using chloramine. The
origin of the high selectivity for alkyl over Me group migration was
determined computationally.