Transition Metal Catalyzed Linchpin-Based Strategies in Natural Product Synthesis: Synthesis of Asteriscunolide D, Aspergillide B and the Core of Labillarides E-H
The construction of complex molecular architectures in a facile and efficient manner remains an overarching goal for the chemical sciences. The development of synthetic linchpins, simple compounds that are used to join complex molecular fragments together, represents a powerful method for improving synthetic efficiency. A major challenge associated with linchpin-based synthetic strategies is the development of chemoselective and atom economic reactions that enable late-stage introduction of the linchpin compound. Chiral propargylic alcohols are versatile synthetic intermediates and their preparation enables entry into a variety of alkyne-based linchpin strategies. Continued optimization of Zn-ProPhenol-catalyzed alkyne addition has led to the development of practical and general conditions for the asymmetric alkynylation of aldehydes. This methodology operates with relatively low catalyst loading and can avoid the use of excess alkyne and dialkylzinc reagents. The total synthesis of asteriscunolide D was completed in just nine steps using a Zn- ProPhenol-catalyzed asymmetric alkynylation and Ru-catalyzed alkene-alkyne coupling. Other highlights of this protecting-group-free synthesis include the use of a thionium-induced cyclization to form the challenging 11-membered humulene ring. At the core of this succinct and selective synthetic strategy is the use of methyl propiolate as a strategic linchpin. In just two steps, three new bonds were formed to atoms derived from methyl propiolate and this ultimately enabled the construction of this structurally challenging natural product. An enantioselective formal total synthesis of aspergillide B has been accomplished using sequential Zn-catalyzed alkyne additions to a masked butane dialdehyde linchpin. This synthesis has led to the development of new conditions for Zn-ProPhenol-catalyzed alkynylation that provide excellent yield and enantioselectivity using just a single equivalent of alkyne. Ru-catalyzed trans-hydrosilylation provides the desired E alkene geometry and also allows chemoselective differentiation of the two double bonds in a subsequent hydrogenation step. This synthetic route provides access to aspergillide B in just 15 steps, using six highly efficient transition metal-catalyzed reactions. Labillarides E-H are a group of diastereomeric oxylipin natural products with very interesting structural features and spectroscopic properties. The development of a Pdcatalyzed allylic alkylation cascade has enabled the rapid construction of the furanopyrone core and alkenyl side chain of these compounds. The use of density functional calculations, in conjunction with spectroscopic data obtained from the truncated labillaride E-H structure, has provided good evidence that labillarides E and G have a 3,6-syn configuration, whereas labillarides F and H have a 3,6-anti configuration. Subsequent development of the Pd-AA cascade methodology has led to the discovery of a highly regioselective Pd-AA cascade with non-symmetric dihydropyran substrates. The combination of allylic carbonate and anomeric siloxy leaving groups in the dihydropyran substrate enables control of the many regiochemical possibilities in this reaction. Ultimately, annulation proceeds stereoconvergently to give a cisfused furopyran from either cis- or trans-substituted starting material. During the course of this research, the assignment of the remote relative stereochemistry of a number of 3,6-dihydro-2Hpyran starting materials and side products was achieved through a novel NMR-based analysis of axial shielding magnitudes.