Iridium-Catalyzed C–H Borylation-Based Synthesis of Natural Indolequinones

From C4 to C7: Innovative Strategies for Site-Selective Functionalization of Indole C-H Bonds

The widespread presence of hydrocarbons makes C-H functionalization an attractive alternative to traditional cross-coupling methods. As indole is an important heteroarene in a plethora of natural products and pharmaceuticals, C-H functionalization of indole moieties has emerged as one of the most important topics in this field. Due to the presence of multiple C-H bonds in indoles, site selectivity is a long-standing challenge. Much effort has been devoted to the C-H functionalization of indoles at the C3 or C2 position, while accessing the benzene core (from C4 to C7) is considerably more challenging.This Account summarizes our recent efforts toward site-selective C-H functionalization of indoles at the benzene core based on innovative strategies. A common method to solve the issue involves the development of directing groups (DGs). Our early studies establish that the installation of the N-P(O)^tBu₂ group at the N position can produce C7 and C6 arylation products using palladium and copper catalysts, respectively. The developed system can also be extended to direct arylation of indoles at the C5 and C4 positions by installing a pivaloyl group at the C3 position. Further investigation of indoles bearing N-P^tBu₂ groups shows a more diverse reactivity for C-H functionalizations at the C7 position, including arylation, olefination, acylation, alkylation, silylation, and carbonylation with different coupling partners. Compared to the P(V) DG, the P(III) group can be easily attached to the indole substrates and detached from the products. However, these attractive reactions rely mostly on precious metal catalysts with ligands; this requirement can be a significant limitation, particularly for large-scale syntheses and the necessity of removal of toxic trace metals in pharmaceutical products. We have also uncovered a general strategy for chelation-assisted aromatic C-H borylation just using simple BBr₃ under mild conditions, in which the installation of pivaloyl groups at the N1 or C3 position of indoles can selectively deliver the boron species to the unfavorable C7 or C4 positions and allow subsequent C-H borylation without any metal. This transition-metal-free strategy can be extended to synthesize C7 and C4 hydroxylated indoles by boron-mediated directed C-H hydroxylation under mild reaction conditions and with broad functional group compatibility.In this Account, we describe our contributions to this topic since 2015. These studies provide efficient and attractive methods for the divergent synthesis of valuable substituted indoles and insights into the exploration of new strategies for the site-selective C-H functionalization and directives for other important heteroarenes.

Indolylboronic Acids: Preparation and Applications

Indole derivatives are associated with a variety of both biological activities and applications in the field of material chemistry. A number of different strategies for synthesizing substituted indoles by means of the reactions of indolylboronic acids with electrophilic compounds are considered the methods of choice for modifying indoles because indolylboronic acids are easily available, stable, non-toxic and new reactions using indolylboronic acids have been described in the literature. Thus, the aim of this review is to summarize the methods available for the preparation of indolylboronic acids as well as their chemical transformations. The review covers the period 2010-2019.

Total synthesis of natural productsviairidium catalysis

An overview of the highlights in total synthesis of natural products using iridium as a catalyst is given.

Evolution of C-H Bond Functionalization from Methane to Methodology

This Perspective presents the fundamental principles, the elementary reactions, the initial catalytic systems, and the contemporary catalysts that have converted C-H bond functionalization from a curiosity to a reality for synthetic chemists. Many classes of elementary reactions involving transition-metal complexes cleave C-H bonds at typically unreactive positions. These reactions, coupled with a separate or simultaneous functionalization process lead to products containing new C-C, C-N, and C-O bonds. Such reactions were initially studied for the conversion of light alkanes to liquid products, but they have been used (and commercialized in some cases) most often for the synthesis of the more complex structures of natural products, medicinally active compounds, and aromatic materials. Such a change in direction of research in C-H bond functionalization is remarkable because the reactions must occur at an unactivated C-H bond over functional groups that are more reactive than the C-H bond toward classical reagents. The scope of reactions that form C-C bonds or install functionality at an unactivated C-H bond will be presented, and the potential future utility of these reactions will be discussed.

A procedure for transforming indoles into indolequinones

A procedure that converts a series of structurally diverse, readily available indole derivatives to their corresponding indolequinones is described. The three-step route commences with an iridium catalyzed C-H borylation to give a 7-borylindole that upon oxidation-hydrolysis affords the 7-hydroxyindole. Subsequent oxidation provides the indolequinone.

Simple indole alkaloids and those with a nonrearranged monoterpenoid unit

This review summarizes the isolation, structure determination, total syntheses and biological activities of simple indole alkaloids and those with a nonrearranged monoterpenoid unit, with literature coverage from 2012 to 2013.

Synthesis and evaluation of 9-deoxy analogues of (-)-thysanone, an inhibitor of HRV 3C protease

9-Deoxy analogues of the HRV 3C protease inhibitor (-)-thysanone display better inhibitory properties than the natural product, inferring the C9-OH hinders binding to the enzyme.