Studies Directed toward the Synthesis of Aspidophytine: Construction of Its Perhydroquinoline Core

Iodine-Mediated δ-Amination of sp3 C-H Bonds

We present a direct δ-amination reaction of sp3 C-H bonds, employing molecular iodine (I2) as the sole oxidant under transition-metal-free conditions. This remote C-H functionalization approach is operationally simple and provides facile, efficient access to pyrrolidines and related heterocyclic derivatives from readily accessible substrates.

Total Synthesis of Strempeliopidine and Non-Natural Stereoisomers through a Convergent Petasis Borono-Mannich Reaction

Strempeliopidine is a member of the monoterpenoid bisindole alkaloid family, a class of natural products that have been shown to elicit an array of biological responses including modulating protein-protein interactions in human cancer cells. Our synthesis of strempeliopidine leverages palladium-catalyzed decarboxylative asymmetric allylic alkylations to install the requisite all-carbon quaternary centers found in each of the two monomeric natural products, aspidospermidine and eburnamine. Initial studies employing Suzuki-Miyaura cross-coupling followed by diastereoselective hydrogenation provided evidence for a structural reassignment of the natural product. Our final synthetic sequence employs a diastereoselective Petasis borono-Mannich reaction to couple eburnamine to a trifluoroborate aspidospermidine derivative. These convergent approaches enabled the synthesis of eight diastereomers of this heterodimer and offer support for the reassignment of the absolute configuration of strempeliopidine.

Formal Total Syntheses of (+)- and (-)-Aspidophytine from a Common, Homochiral Precursor

A formal total synthesis of (-)-aspidophytine (2), a key substructure associated with the heterodimeric indole alkaloid haplophytine (1) and itself a natural product, has been established by employing the homochiral and enzymatically derived cis-1,2-dihydrocatechol 8 as a starting material. Specifically, compound 8 has been converted into the pentacyclic product 26, an advanced intermediate associated with a previously reported synthesis of aspidophytine (2). Simple modifications to the reaction sequence have also allowed for the identification of a synthetic pathway leading from dihydrocatechol 8 to (+)-aspidophytine (ent-2).

Quantitative and convenient real-time reaction monitoring using stopped-flow benchtop NMR

We present a stopped-flow benchtop NMR system (composed of commercially available hardware components) that allows for quantitative reaction monitoring to be completed with relative ease, even with experimentally complex reaction systems.

Two Decades of Progress in the Asymmetric Intramolecular aza-Michael Reaction

The asymmetric intramolecular aza-Michael reaction (IMAMR) is a very convenient strategy for the generation of heterocycles bearing nitrogen-substituted stereocenters. Due to the ubiquitous presence of these skeletons in natural products, the IMAMR has found widespread applications in the total synthesis of alkaloids and biologically relevant compounds. The development of asymmetric versions of the IMAMR are quite recent, most of them reported in this century. The fundamental advances in this field involve the use of organocatalysts. Chiral imidazolidinones, diaryl prolinol derivatives, Cinchone-derived primary amines and quaternary ammonium salts, and BINOL-derived phosphoric acids account for the success of those methodologies. Moreover, the use of N-sulfinyl imines with a dual role, as nitrogen nucleophiles and as chiral auxiliaries, appeared as a versatile mode of performing the asymmetric IMAMR.

Synthetic Studies toward Dimeric Indole Alkaloids Based on Convergent Synthetic Strategy

The total syntheses of dimeric indole alkaloids, haplophytine, and T988s are described. These dimeric compounds comprising two structurally different indole units are ubiquitous in nature, and many possess pharmaceutically important activities. To realize an efficient chemical synthesis of these dimeric indole alkaloids, the establishment of convergent synthetic strategies and development of new coupling methods are indispensable. The linkage of two highly functionalized units at a late stage of the synthesis frequently induces synthetic problems such as chemoselectivity and steric repulsion. Moreover, although transition metal-catalyzed reactions are usually an effective method for the cross-coupling of two units, the application of these cross-coupling reactions to bond formation involving a sterically hindered C(sp³) is often difficult. Thus, even with precise modern synthetic methods, it is currently difficult to realize convergent syntheses of dimeric indole alkaloids possessing a quaternary carbon linking two units. To combat these synthetic problems, we developed a synthetic method to link two indole units using an Ag-mediated nucleophilic substitution reaction. In this review, we provide a detailed discussion of convergent synthetic strategies and coupling methods for dimeric indole alkaloids.

Copper-catalyzed remote (δ) C(sp3)–H bond amination: a practical strategy to construct pyrrolidine derivatives

This work reported a copper-catalyzed remote C(sp³)–H bond amination of primary, secondary and tertiary C–H bonds, which converted acyclic amines to pyrrolidine derivatives.