An updated asymmetric total synthesis of (+)-tronocarpine

Synthetic Studies on Enantioselective Total Syntheses of Tabercarpamines and Voacangine-Type Indole Alkaloids

The proposed post-iboga natural products, (+)-tabercarpamines G and I, along with their corresponding C20 stereoisomers and C10-methoxy-regioisomers were synthesized. An extensive NMR spectroscopic study showed that the spectral data of these synthetic compounds were not consistent with the proposed (+)-tabercarpamines G and I in isolation report, indicating that the proposed structures were misassigned. Subsequently, from the advanced intermediates for the synthesis of the proposed (+)-tabercarpamines G and I, the unified enantioselective syntheses of six iboga‒type natural products including (-)-isovoacangine and (-)-voacangine, (-)-3-oxo-isovoacangine and (-)-3-oxo-voacangine, and (-)-(3S)-cyanoisovoacangine and (-)-(3S)-cyanovoacangine were accomplished via a divergent manner. The synthetic route provided the first catalytic asymmetric approach to access these three types of iboga‒type natural products and achieved the first enantioselective syntheses of (3S)-cyano(iso)voacangines. Our synthesis hinged on a catalytic asymmetric Michael/aldol reaction for the construction of chiral aza-[3.3.1]-bridged bicyclic framework at the early stage and a ring reorganization process from [3.3.1]- to [2.2.2]-bicyclic ring system-a pathway opposite to the proposed biogenic synthesis-at the late stage.

The Phosphinamide-Based Catalysts: Discovery, Methodology Development, and Applications in Natural Product Synthesis

In the total synthesis of natural products, synthetic efficiency has been an important driver for designing and developing new synthetic strategies and methodologies. To this end, the step, atom, and time economy and the overall yield are major factors to be considered. On the other hand, developing unified routes that can be used for synthesizing multiple molecules, specifically skeletally different classes of molecules, are also important aspects with which to be concerned. In the efforts toward efficient and flexible synthesis of structurally unique terpenoid and indole alkaloid natural products, we have designed and developed several phosphinamide-based new catalysts and reaction methodologies that have been compellingly demonstrated to be widely useful as strategic protocols for the diverse synthesis of various complex terpenoids and indole alkaloids. The important progress of these results will be summarized in this Account.In the first part, we present the stories of successful design and establishment of a novel method for the synthesis of P-stereogenic phosphinamides (P-SPhos) via a Pd-catalyzed C-H desymmetric enantioselective arylation, as well as the flexible derivatization of the P-stereogenic phosphinamides into various types of skeletally unique tricyclic and N,P-bidentate P-stereogenic compounds. Subsequently, the discovery of P-stereogenic phosphinamides as chiral organocatalysts for the desymmetric enantioselective reduction of cyclic 1,3-diketones and of phosphinamide-based cyclopalladium complex (C-Pd) as precatalysts for highly efficient Suzuki-Miyaura cross-coupling reaction of sterically congested nonactivated enolates is introduced. The notable features of the P-stereogenic phosphinamide-catalyzed desymmetric enantioselective reduction are highlighted by the broad substrate compatibility and excellent stereoselectivity, as well as most significantly, the good recoverability and reusability of catalysts. With regard to the sterically congested nonactivated enolates, such substrates are challenging for Suzuki cross-coupling reactions. We demonstrate that the phosphinamide-based cyclopalladium is a type of highly active precatalyst that allows the reaction to proceed under mild conditions and to be easily scaled up. Following the methodology development, the practical applications of these methods serving as strategic transformations are highlighted by the unified synthesis of four cyathane-type and two hamigeran-type terpenoids.In the second part, we describe the development of a robust method for oxidative Heck cross-coupling of indolyl amides by using the phosphinamide-based cyclopalladium as catalyst or phosphinamide as coligand. The method provides a general and straightforward method for diverse synthesis of indolyl δ-lactam derivatives, which present as a common core in a variety of Aspidosperma-derived indole alkaloids. The successful demonstration of this protocol for a concise and divergent synthesis of leuconodine-type indole alkaloids is also presented. We believe the results presented in this Account would have significant implications beyond our results and would find further applications in the field of synthetic methodology and natural product synthesis.

Recent applications of asymmetric organocatalytic annulation reactions in natural product synthesis

The past two decades have witnessed remarkable growth of asymmetric organocatalysis, which is now a firmly established synthetic tool, serving as a powerful platform for the production of chiral molecules. Ring structures are ubiquitous in organic compounds, and, in the context of natural product synthesis, strategic construction of ring motifs is often crucial, fundamentally impacting the eventual fate of the whole synthetic plan. In this review, we provide a comprehensive and updated summary of asymmetric organocatalytic annulation reactions; in particular, the application of these annulation strategies in natural product synthesis will be highlighted.