The Garner aldehyde-derived methylene alkene 5 and the corresponding benzyloxycarbonyl compound 25 undergo hydroboration with 9-BBN-H followed by palladium-catalyzed Suzuki coupling reactions with aryl and vinyl halides. After one-pot hydrolysis-oxidation, a range of known and novel nonproteinogenic amino acids were isolated as their N-protected derivatives. These novel organoborane homoalanine anion equivalents are generated and transformed under mild conditions and with wide functional group tolerance: electron-rich and -poor aromatic iodides and bromides (and a vinyl bromide) all undergo efficient Suzuki coupling. The extension of this methodology to prepare meso-DAP, R,R-DAP, and R,R-DAS is also described.

A rhodium-catalysed 1,4-addition of an arylboron species to an r,β-unsaturated ester was the key chirality-inducing step in the synthesis of an API. We describe herein the development of this chemistry, including optimization of reagent charges, reaction conditions, and metal recovery, in order to allow manufacture at multikilogram scale. A key result was the unexpected discovery that the use of a minimal quantity of an alcohol, rather than water, reduces the extent of rhodium-mediated protodeboronation of the boron species. This allowed the charge of this expensive reagent to be significantly reduced. Furthermore, the use of an alcohol instead of water avoided the agglomeration of the inorganic base present in the reaction, making the process more robust and operationally simpler. To our knowledge this is the first time that this type of C-C bond-forming chemistry has been used in a multikilo manufacture.