Development of an Efficient Synthesis of rac -3-Demethoxyerythratidinone via a Titanium(III) Catalyzed Imine-Nitrile Coupling

Recent Advances in the Total Synthesis of the Tetrahydroisoquinoline Alkaloids (2002-2020)

The tetrahydroisoquinoline (THIQ) natural products constitute one of the largest families of alkaloids and exhibit a wide range of structural diversity and biological activity. Ranging from simple THIQ natural products to complex trisTHIQ alkaloids such as the ecteinascidins, the chemical syntheses of these alkaloids and their analogs have been thoroughly investigated due to their intricate structural features and functionalities, as well as their high therapeutic potential. This review describes the general structure and biosynthesis of each family of THIQ alkaloids as well as recent advancements of the total synthesis of these natural products from 2002 to 2020. Recent chemical syntheses that have emerged harnessing novel, creative synthetic design, and modern chemical methodology will be highlighted. This review will hopefully serve as a guide for the unique strategies and tools used in the total synthesis of THIQ alkaloids, as well as address the longstanding challenges in their chemical and biosynthesis.

Reductive Umpolung and Defunctionalization Reactions through Higher-Order Titanium(III) Catalysis

The single-electron transfer from an in situ formed titanium(III) catalyst to ketones, imines, nitriles, Michael acceptors, and many other functions has enabled a large number of intra- and intermolecular reductive umpolung reactions. Likewise, it allows the homolytic cleavage of functional groups for selective defunctionalizations. These reactions often take place with the participation of two titanium(III) species, avoiding free-radical pathways and enabling high catalyst control of the reaction selectivity. This account discusses the development of the individual reactions together with the fundamental mechanistic discoveries that led to a better understanding of such titanium(III)-catalyzed processes in general.

1 Introduction

2 Active Titanium(III) Species and Additives

3 Ketone-Nitrile Couplings

4 Further Reductive Umpolung Reactions

5 Catalytic Homolytic C–CN and C–SO2R Cleavage

6 Conclusion

Multi-Photocatalyst Cascades: Merging Singlet Oxygen Photooxygenations with Photoredox Catalysis for the Synthesis of Alkaloid Frameworks

The development of photocascades that rapidly transform simple and readily accessible furan substrates into polycyclic alkaloid frameworks or erythrina natural products is described. Each of the sequences developed makes use of photocatalyzed energy transfer processes, which generate singlet oxygen, to set up the substrates for the second photocatalyzed reaction, wherein electron transfer generates carbon-centered radicals for the cyclizations that give the final complex frameworks. A chemical switch has been developed that can "switch off" one photocatalyst; thus, allowing a second photocatalyst to take over control of the sequence. As a corollary, this strategy represents the first time it has been possible to use multiple photocatalysts in photocascades, and, as such, it expands significantly the reactions that can be included in such cascades and the order in which they can be initiated.

Lewis acid-catalyzed domino generation/[2,3]-sigmatropic rearrangement of ammonium ylides to access chiral azabicycles

[2,3]-Sigmatropic rearrangement of ammonium ylides represents a fundamental reaction for stereoselective synthesis of nitrogenous compounds. However, its applicability is limited by the scarcity of efficient, catalytic, and mild methods for generating ammonium ylides. Here, we report silver-catalyzed domino generation/[2,3]-sigmatropic rearrangement of ammonium ylides, furnishing chiral azabicycles with bridgehead quaternary stereogenic centers in high enantiomeric purity (up to 99% ee). A combination of density functional theory calculations and experimental studies revealed that residual water in the reaction system is crucial for the mild reaction conditions by functioning as a proton shuttle to assist carbon-silver bond protonation and C2─H deprotonation to generate the ammonium ylide. This reaction has a broad application scope. Besides the diverse substituents, N-fused azabicycles of various ring sizes are also easily accessed. In addition to silver salts, this strategy has also been successfully implemented by using a stoichiometric amount of nonmetallic I2.

The Proven Versatility of Cp2TiCl

In the last two decades, titanocene monochloride has been postulated as a monoelectronic transfer reagent capable of catalyzing an important variety of chemical transformations. In this Perspective, our contributions to this growing field of research are summarized and analyzed. Especially known have been our contributions in C-C bond formation reactions, hydrogen-atom transfer from water to radicals, and isomerization reactions, as well as the development of a catalytic cycle that has subsequently allowed the preparation of a great variety of natural terpenes. It is also worth mentioning our contribution in the postulation of this single-electron transfer agent (SET) as a new green catalyst with a broad range of applications in organic and organometallic chemistry. The most significant catalytic processes developed by other research groups are also briefly described, with special emphasis on the reaction mechanisms involved. Finally, a reflection is made on the future trends in the research of this SET, aimed at consolidating this chemical as a new green reagent that will be widely used in fine chemistry, green chemistry, and industrial chemical processes.

Titanium catalysis for the synthesis of fine chemicals – development and trends

Atlas as a Titan(ium) is holding the earth-abundant chemistry world. Titanium is the second most abundant transition metal, is a key player in important industrial processes (e.g. polyethylene) and shows much promise for diverse applications in the future.