One-Point Binding Ligands for Asymmetric Gold Catalysis: Phosphoramidites with a TADDOL-Related but Acyclic Backbone

Readily available phosphoramidites incorporating TADDOL-related diols with an acyclic backbone turned out to be excellent ligands for asymmetric gold catalysis, allowing a number of mechanistically different transformations to be performed with good to outstanding enantioselectivities. This includes [2 + 2] and [4 + 2] cycloadditions of ene-allenes, cycloisomerizations of enynes, hydroarylation reactions with formation of indolines, as well as intramolecular hydroaminations and hydroalkoxylations of allenes. Their preparative relevance is underscored by an application to an efficient synthesis of the antidepressive drug candidate (-)-GSK 1360707. The distinctive design element of the new ligands is their acyclic dimethyl ether backbone in lieu of the (isopropylidene) acetal moiety characteristic for traditional TADDOL's. Crystallographic data in combination with computational studies allow the efficiency of the gold complexes endowed with such one-point binding ligands to be rationalized.