Enantioenriched Cyanohydrins and Acetoxyphosphonates – Synthesis and Applications

Abstract: In this thesis, the synthesis of enantioenriched compounds using novel methodologies that employ metal- and biocatalysis is described.In the first part, the synthesis of enantioenriched cyanohydrins, which are highly versatile synthetic intermediates, is described. A minor enantiomer recycling methodology, which uses a catalytic system consisting of a titanium salen dimer and a lipase, was highly successful in yielding the desired products, often in essentially enantiopure form. Alternatively, when the minor enantiomer recycling method gave unsatisfactory results, the same two catalysts were used in a sequential two-step process. The minor enantiomer recycling procedure was used to synthesize three different ?-adrenergic antagonists with very high enantiomeric excesses via the corresponding O-acetylated cyanohydrins. With the same cyclic process, O-(?-bromoacyl) cyanohydrins were synthesized and subsequently transformed to aminofuranones via an intramolecular Blaise reaction. In addition, substitution of the bromide in the O-(?-bromoacyl) cyanohydrins with different nitrogen nucleophiles followed by reduction gave N-substituted ?-amino alcohols. This reaction sequence was applied to the synthesis of the ?3-adrenergic receptor agonist solabegron. Finally, the O-(?-bromoacyl) cyanohydrins were subjected to a palladium catalyzed cross-coupling with a range of boronic acids. This reaction proceeded with high yields, and was performed with enantiopure substrates with no or only minor racemization of the resulting products.In the second part, the first asymmetric direct addition of acylphosphonates to aldehydes is described. This transformation is catalyzed by a tridentate Schiff base aluminum(III) Lewis acidic complex, a Lewis base, and a Brønstedt base. Several aromatic and aliphatic acetoxyphosphonates were isolated, in most cases in high yields. Unfortunately, the enantioselectivity in the reaction was only moderate. Therefore, an investigation to develop a minor enantiomer recycling system for the synthesis of acetoxyphosphonates was initiated, but a working cyclic process could not be found in this work.