The Stereoselective Total Synthesis of Axially Chiral Naphthylisoquinoline Alkaloids

The naphthylisoquinoline (NIQ) alkaloids are a thrilling class of natural biaryls─structurally, biosynthetically, and pharmacologically. A common feature of these metabolites is the biaryl bond between their naphthalene and isoquinoline moieties, which in most cases is rotationally hindered, leading to the phenomenon of axial chirality. Depending on their individual structures, including the respective axial configurations, NIQs show promising bioactivities. Their total synthesis is a challenging but rewarding goal, with the stereocontrolled construction of the biaryl linkage as the key step.The position of the biaryl axis and its configuration determine the overall molecular shape and thus the choice of the best possible method for efficient asymmetric aryl-aryl bond formation. The axis in NIQs can cover a broad range of steric hindrance, from freely rotating to configurationally stable. For dioncophylline B (1) and dioncophylline F (2a/b), with only two ortho-substituents next to the axis, the synthesis is easy to accomplish by direct coupling of the intact naphthalene moiety with the isoquinoline unit, and no atropo-selectivity is required.Naphthylisoquinolines with a configurationally stable biaryl axis are the focus of the present Account. They are more difficult to synthesize because, in addition to the problem of decreased chemical yields with increasing steric hindrance at the axis, the synthesis needs to proceed stereoselectively. Within this class of NIQs, 5,8'-coupled representatives, such as korupensamine A (3a), have received considerable synthetic attention because the rotational barrier is high enough for the existence of atropisomerism without being too excessive, and they show potent bioactivities. Their synthesis, as systematically presented herein, thus occupies a central role in this report. For their aryl-aryl bond formation, both intra- and intermolecular approaches can be successfully applied. Axial stereoinformation is introduced by internal asymmetric induction from stereogenic elements already present in the isoquinoline or its precursors, from chiral auxiliary elements artificially introduced, or by external asymmetric induction using chiral catalysts.To overcome even higher steric hindrance, as in ancistrocladine (4a), innovative approaches were developed. A most successful strategy is the "lactone concept" developed by the Bringmann group, which allows the directed synthesis of any desired atropisomer in high chemical and optical yields, thus permitting the atropo-divergent preparation of the two isomers from a single joint precursor. In this approach, the two formal tasks of stereoselective biaryl synthesis, which are usually done simultaneously─the C-C linkage and the asymmetric induction─are achieved consecutively. The coupling step is performed intramolecularly after prefixation of the coupling partners by an ester bridge. The resulting biaryl lactone already possesses the biaryl axis but is still configurationally unstable; it can then, with internal or external asymmetric induction, be cleaved atropo-divergently with high stereoselectivities. Besides its unique concept, the procedure excels by its broad applicability; among all presented methods, it has been used for the synthesis of the largest number of NIQs, more than 20 representatives, including those with the highest steric hindrance.This Account gives comprehensive insight into the plethora of conceptual approaches for the efficient formation of the hindered biaryl bond of NIQs.