No abstract

In the interdisciplinary research field of chemical biology and drug discovery, diversity-oriented synthesis (DOS) has become indispensable in the construction of novel small-molecule libraries rich in skeletal and stereochemical diversity. DOS aims to populate the unexplored chemical space with new potential bioactive molecules via forward synthetic analysis. Since the introduction of this concept by Schreiber, DOS has evolved along with many significant breakthroughs. It is therefore important to understand the key DOS strategies to build molecular diversity with maximized biological relevancy. Due to the length limitations of this mini review, we briefly discuss the recent DOS plans using build/couple/pair (B/C/P) and ring-distortion strategies for the synthesis of major biologically relevant target molecules like natural products and their related compounds, macrocycles, and privileged structures.

A facile transition-metal-free oxidative cross-dehydrogenative coupling reaction involving selective formation of a C-S bond leading to the synthesis of arylthiobenzoxazoles, heteroarylthiobenzoxazoles, and arylthiobenzothiazoles has been described. This highly regioselective C-H functionalization reaction with electron-rich aromatic systems including heteroaromatics is achieved by reversing the reactivity of sulfur in the presence of a suitable oxidant and strong acid.

Sulfenylation of β-diketones is challenging as β-diketones undergo deacylation after sulfenylation in the reaction medium. The sulfenylation of β-diketones without deacylation under metal-free conditions at ambient temperature via a cross dehydrogenative coupling (CDC) strategy is reported. The resultant products can be further manipulated to form α,α-disubstituted β-diketones and pyrazoles.

α-Sulfenyl ketones are potential precursors which find a variety of applications in organic synthesis. Their typical synthesis requires pre-functionalized starting materials and two to three step synthetic sequences. In addition, the selective pre-functionalization of unsymmetrical ketones is a challenge, which limits the synthesis of the desired sulfenylated ketones. To overcome these disadvantages, a metal-free, convenient one-step strategy for synthesizing α-sulfenyl ketones at ambient temperature via a cross-dehydrogenative coupling (CDC) strategy has been developed with a broad substrate scope. Therefore, this CDC strategy for C-S bond formation is attractive and may find wide applications in organic synthesis.

A rapid, metal-free and solvent-free (very low-loading of solvent in few cases) reaction conditions for synthesizing thioamides and amides using the Bronsted super acid such as triflic acid has been developed. This method shows a broad substrate scope with a variety of electronrich arenes including thiophene derivatives. The reaction works well for both aromatic as well as aliphatic isothiocyantes. Most of the thioamides are obtained in excellent yields in short reaction duration of time and in most of the examples, a simple work up procedure has been developed which does not require further purification.

Despite the availability of numerous routes to substituted nicotinates based on the Bohlmann–Rahtz pyridine synthesis, the existing methods have several limitations, such as the inevitable ortho-substitutions and the inability to conjugate vitamin B3 to other pharmaceutical agents. Inspired by the biosynthesis of nicotinic acid (a form of vitamin B3) from tryptophan, we herein report the development of a strategy for the synthesis of meta-aminoaryl nicotinates from 3-formyl(aza)indoles. Our strategy is mechanistically different from the reported routes and involves the transformation of (aza)indole scaffolds into substituted meta-aminobiaryl scaffolds via Aldol-type addition and intramolecular cyclization followed by C–N bond cleavage and re-aromatization. Unlike previous synthetic routes, this biomimetic method utilizes propiolates as enamine precursors and thus allows access to ortho-unsubstituted nicotinates. In addition, the synthetic feasibility toward the halo-/boronic ester-substituted aminobiaryls clearly differentiates the present strategy from other cross-coupling strategies. Most importantly, our method enables the late-stage conjugation of bioactive (hetero)arylamines with nicotinates and nicotinamides and allows access to the previously unexplored chemical space for biomedical research.

A non-isothiocyanate route to synthesize trisubstituted thioureas of arylamines using in situ generated dithiocarbamates3