Total synthesis of the Galbulimima alkaloid (-)-GB17

Total synthesis of (±)-3-demethoxyerythratidinone via Tf2O-promoted cascade reaction of enaminone

The tetracyclic rings with chiral quaternary center represent a formidable synthetic challenge for Erythrina alkaloids. We present a 6-step synthesis of the Erythrina alkaloid 3-demethoxyerythratidinone with a 16% overall yield. Our synthesis highlights a cascade reaction initiated by Tf2O-induced activation of an enaminone substrate, yielding an iminium species and an enol triflate, followed by a Pictet-Spengler reaction. This method efficiently constructs the tetracyclic core skeleton, featuring an N-substituted quaternary center. It exhibits versatility across diverse (hetero)arenes and enaminone structures, providing a general strategy for the rapid synthesis of fused or spiro ring systems including the core structure of homoerythrina alkaloids.

Bifunctional amino sulfonamide-catalyzed asymmetric conjugate addition to alkenyl alkynyl ketimines as enone surrogates

A bifunctional amino sulfonamide-catalyzed asymmetric conjugate addition of aldehydes to alkenyl alkynyl ketimines as reactive surrogates for enones has been developed. Use of a phenylcyclopropane-based amino sulfonamide catalyst, which can activate and orient the ketimines through hydrogen bonding, affords the desired conjugate adducts with high chemo-, diastereo- and enantioselectivity.

In Pursuit of Synthetic Efficiency: Convergent Approaches

The field of total synthesis has reached a stage in which emphasis has been increasingly focused on synthetic efficiency rather than merely achieving the synthesis of a target molecule. The pursuit of synthetic efficiency, typically represented by step count and overall yield, is a rich source of inspiration and motivation for synthetic chemists to invent innovative strategies and methods. Among them, convergent strategy has been well recognized as an effective approach to improve efficiency. This strategy generally involves coupling of fragments with similar complexity to furnish the target molecule via subsequent cyclization or late-stage functionalization. Thus, methodologies that enable effective connection of fragments are critical to devising a convergent plan. In our laboratory, convergent strategy has served as a long-standing principle for pursuing efficient synthesis during the course of planning and implementing synthetic projects. In this Account, we summarize our endeavors in the convergent synthesis of natural products over the last ten years. We show how we identify reasonable bond disconnections and employ enabling synthetic methodologies to maximize convergency, leading to the efficient syntheses of over two-dozen highly complex molecules from eight disparate families.In detail, we categorize our work into three parts based on the diverse reaction types for fragment assembly. First, we demonstrate the application of a powerful single-electron reducing agent, SmI₂, in a late-stage cyclization step, forging the polycyclic skeletons of structurally fascinating Galbulimima alkaloids and Leucosceptrum sesterterpenoids. Next, we showcase how three different types of cycloaddition reactions can simultaneously construct two challenging C-C bonds in a single step, providing concise entries to three distinct families, namely, spiroquinazoline alkaloids, gracilamine, and kaurane diterpenoids. In the third part, we describe convergent assembly of ent-kaurane diterpenoids, gelsedine-type alkaloids, and several drug molecules via employing some bifunctional synthons. To access highly oxidized ent-kaurane diterpenoids, we introduce the hallmark bicyclo[3.2.1]octane ring system at an early stage, and then execute coupling and cyclization by means of a Hoppe's homoaldol reaction and a Mukaiyama-Michael-type addition, respectively. Furthermore, we showcase how the orchestrated combination of an asymmetric Michael addition, a tandem oxidation-aldol reaction and a pinacol rearrangement can dramatically improve the efficiency in synthesizing gelsedine-type alkaloids, with nary a protecting group. Finally, to address the supply issue of several drugs, including anti-influenza drug zanamivir and antitumor agent Et-743, we exploit scalable and practical approaches to provide advantages over current routes in terms of cost, ease of execution, and efficiency.

The Future of Retrosynthesis and Synthetic Planning: Algorithmic, Humanistic or the Interplay?

The practice of deploying and teaching retrosynthesis is on the cusp of considerable change, which in turn forces practitioners and educators to contemplate whether this impending change will advance or erode the efficiency and elegance of organic synthesis in the future. A short treatise is presented herein that covers the concept of retrosynthesis, along with exemplified methods and theories, and an attempt to comprehend the impact of artificial intelligence in an era when freely and commercially available retrosynthetic and forward synthesis planning programs are increasingly prevalent. Will the computer ever compete with human retrosynthetic design and the art of organic synthesis?

Structures of New Alkaloids from Rain Forest Trees Galbulimima belgraveana and Galbulimima baccata in Papua New Guinea, Indonesia, and Northern Australia

Following on our 60-year research on the chemical constituents of the rain forest trees Galbulimima belgraveana and Galbulimima baccata, we report the isolation of seven new alkaloids: GB14 (14), GB22 (15), GB25 (16), GB21 (17), GB23 (18), GB24 (19), and GB26 (20). Their structures were elucidated by a combination of spectroscopic analyses and single-crystal X-ray crystallography, as well as structure degradation and interconversion. The newly isolated alkaloids are precursors or derivatives of the known family members from our early studies and could be intermediates in the biosynthesis of the Galbulimima alkaloids. Therefore, the present study has expanded the range of structures in this family of alkaloids and provided some missing links in the biosynthetic sequences.

Galbulimima Alkaloids

This chapter provides a comprehensive overview of recent achievements in the area of Galbulimima alkaloids. Following a discussion of the isolation of Galbulimima alkaloids and structural features of members of this fascinating family of secondary metabolites, biological properties of selected compounds are briefly discussed. Furthermore, the proposed biosynthetic routes toward Galbulimima alkaloids are outlined. The main section of the chapter is devoted to a detailed discussion and comparison of all total syntheses of Galbulimima alkaloids published to date.

Concise Enantioselective Synthesis of Oxygenated Steroids via Sequential Copper(II)-Catalyzed Michael Addition/Intramolecular Aldol Cyclization Reactions

A new scalable enantioselective approach to functionalized oxygenated steroids is described. This strategy is based on chiral bis(oxazoline) copper(II) complex-catalyzed enantioselective and diastereoselective Michael reactions of cyclic ketoesters and enones to install vicinal quaternary and tertiary stereocenters. In addition, the utility of copper(II) salts as highly active catalysts for the Michael reactions of traditionally unreactive β,β'-enones and substituted β,β'-ketoesters that results in unprecedented Michael adducts containing vicinal all-carbon quaternary centers is also demonstrated. The Michael adducts subsequently undergo base-promoted diastereoselective aldol cascade reactions resulting in the natural or unnatural steroid skeletons. The experimental and computational studies suggest that the torsional strain effects arising from the presence of the Δ(5)-unsaturation are key controlling elements for the formation of the natural cardenolide scaffold. The described method enables expedient generation of polycyclic molecules including modified steroidal scaffolds as well as challenging-to-synthesize Hajos-Parrish and Wieland-Miescher ketones.

Total Synthesis of the Galbulimima Alkaloids Himandravine and GB17 Using Biomimetic Diels-Alder Reactions of Double Diene Precursors

The enantioselective total syntheses of himandravine and GB17 were completed through a common biomimetic strategy involving Diels-Alder reactions of unusual double diene containing linear precursors. The double diene precursors, containing or lacking a C12 substituent as required to produce GB17 or himandravine, respectively, were found to undergo Diels-Alder reactions to afford mixtures of regioisomeric cycloadducts that map onto the alternative carbocyclic frameworks of both himandravine and GB17. Computational investigations revealed that these Diels-Alder reactions proceed via transition state structures of similar energy that have a high degree of bispericyclic character and that the low levels of regioselectivity observed in the reactions are a consequence of competing orbital interaction and distortion energies. The combined experimental and computational results provide valuable insights into the biosynthesis of the Galbulimima alkaloids.