Direct C7 Functionalization of Tryptophan. Synthesis of Methyl (S)-2-((tert-Butoxycarbonyl)amino)-3-(7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-indol-3-yl)propanoate

Unified, Biosynthesis-Inspired, Completely Stereocontrolled Total Synthesis of All Highest-Order [n + 1] Oligocyclotryptamine Alkaloids

We describe the unified enantioselective total synthesis of the polycyclotryptamine natural products (+)-quadrigemine H, (+)-isopsychotridine C, (+)-oleoidine, and (+)-caledonine. Inspired by our hypothesis for the biogenesis of these alkaloids via an iterative concatenative addition of homochiral cyclotryptamines to a meso-chimonanthine headcap, we leverage the modular, diazene-directed assembly of stereodefined cyclotryptamines to introduce successive C3a-C7' quaternary stereocenters on a heterodimeric meso-chimonanthine surrogate with full stereochemical control at each quaternary linkage. We developed a new strategy for iterative aryl-alkyl diazene synthesis using increasingly complex oligomeric hydrazide nucleophiles and a bifunctional cyclotryptamine bearing a C3a leaving group and a pendant C7 pronucleophile. The utility of this strategy is demonstrated by the first total synthesis of heptamer (+)-caledonine and hexamer (+)-oleoidine. Enabled by our completely stereoselective total syntheses and expanded characterization data sets, we provide the first complete stereochemical assignment of pentamer (+)-isopsychotridine C, provide evidence that it is identical to the alkaloid known as (+)-isopsychotridine B, and report that tetramer (+)-quadrigemine H is identical to the alkaloid called (+)-quadrigemine I, resolving longstanding questions about the structures of the highest-order [n + 1] oligocyclotryptamine alkaloids.

Total synthesis of complex 2,5-diketopiperazine alkaloids

The 2,5-diketopiperazine (DKP) motif is present in many biologically relevant, complex natural products. The cyclodipeptide substructure offers structural rigidity and stability to proteolysis that makes these compounds promising candidates for medical applications. Due to their fascinating molecular architecture, synthetic organic chemists have focused significant effort on the total synthesis of these compounds. This review covers many such efforts on the total synthesis of DKP containing complex alkaloid natural products.

Total Synthesis of (-)-Kopsifoline A and (+)-Kopsifoline E

We report the first total synthesis of (-)-kopsifoline A and (+)-kopsifoline E. Our synthetic strategy features a biogenetically inspired regioselective C17-functionalization of a versatile intermediate containing the pentacyclic core of aspidosperma alkaloids. The vinylogous urethane substructure of this intermediate affords (-)-kopsifoline D via C3-C21 bond formation under the Mitsunobu reaction conditions, while it enables selective C17-functionalization en route to (-)-kopsifoline A and (+)-kopsifoline E.

Iridium-Catalyzed Direct C4- and C7-Selective Alkynylation of Indoles Using Sulfur-Directing Groups

Indoles and their analogues have been one of the most ubiquitous heterocycles during the past century, and extensive studies have been conducted to establish practical synthetic methods for their derivatives. In particular, selective functionalization of the poorly reactive benzenoid core over the pyrrole ring has been a great challenge. Reported herein is an iridium-catalyzed direct alkynylation of the indole C4- and C7-positions with the assistance of sulfur directing groups. This transformation shows a wide range of functional-group tolerance with exceptional site selectivity. The directing group can be either easily removed or transformed after catalysis. The synthetic utility of the alkyne fragment is demonstrated by the derivatization into the core structure of natural indole alkaloids.

Enantioselective Synthesis of (-)-Vallesine: Late-Stage C17-Oxidation via Complex Indole Boronation

The first enantioselective total synthesis of (-)-vallesine via a strategy that features a late-stage regioselective C17-oxidation followed by a highly stereoselective transannular cyclization is reported. The versatility of this approach is highlighted by the divergent synthesis of the archetypal alkaloid of this family, (+)-aspidospermidine, and an A-ring-oxygenated derivative, (+)-deacetylaspidospermine, the precursor to (-)-vallesine, from a common intermediate.

Regioselective Diversification of 2,1-Borazaronaphthalenes: Unlocking Isosteric Space via C-H Activation

Methods for the regioselective C–H borylation and subsequent cross-coupling of the 2,1-borazaronaphthalene core are reported. Azaborines are dependent on B–N/C=C isosterism when employed in strategies for developing diverse heterocyclic scaffolds. Although 2,1-borazaronaphthalene is closely related to naphthalene in terms of structure, the argument is made that the former has electronic similarities to indole. Based on that premise, iridium-mediated C–H activation has enabled facile installation of a versatile, nucleophilic coupling handle at a previously inaccessible site of 2,1-borazaronaphthalenes. A variety of substituted 2,1-borazaronaphthalene cores can be successfully borylated and further cross-coupled in a facile manner to yield diverse C(8)-substituted 2,1-borazaronaphthalenes.