Isothiocyanates: happy-go-lucky reagents in organic synthesis

Owing to their unique structural features, isothiocyanates (ITCs) are a class of highly useful and inimitable reagents as the -NCS group serves both as electrophile and nucleophile in organic synthesis. ITCs share a rich legacy in organic, medicinal, and combinatorial chemistry. Compared to their oxygen equivalents, isocyanates, ITCs are easily available, less unpleasant, and somewhat less harmful to work with (mild conditions) which makes them happy-go-lucky reagents. Functionalized ITCs can finely tune the reactivity of the -NCS group and thus can be exploited in the late-stage functionalization processes. This review's primary aim is to outline ITC chemistry in the construction and derivatization of heterocycles through the lens of sustainability. For ease and brevity, the sections are divided based on reactive centers present in functionalized ITCs and modes of cyclisation. Scrutinizing their probable unexplored directions for future research studies is also addressed.

Construction of α,β-Diamino Diacid Derivatives with Adjacent Acyclic Tetrasubstituted Stereocenters

A convenient strategy for the diastereoselective synthesis of α,β-diamino diacid derivatives bearing congested vicinal acyclic tetrasubstituted stereocenters via catalytic Mannich-type reactions of azlactones and 2-aminoacrylates was established. A diverse set of α,β-diamino diacid derivatives were synthesized in good to excellent yields and diastereoselectivities. Good enantioselectivity (up to 98:2 er) was achieved by employing the catalyst (DHQD)₂PHAL in the subsequent asymmetric study.

Advances of α-activated cyclic isothiocyanate for the enantioselective construction of spirocycles

The efficient and enantioselective synthesis of pharmaceutically important spirocycles has attracted the focus of organic and medicinal chemists. In this context, with the excellent reactivity of α-activated isothiocyanate as formal 1,3-dipoles in the (3 + 2) cyclization process, the cyclic isothiocyanates featuring important pharmacophores, such as oxindole, pyrazolone, and indanone moieties, have emerged as powerful precursors to access a variety of spirocycles with highly structural diversities. In addition, the facile transformations of these spirocycles have shown potential applications in drug design. This review will cover the recent advances of α-activated cyclic isothiocyanates in the enantioselective construction of spirocycles since 2015, and the applications of corresponding products in organic and medicinal chemistry.

Double Spirocyclization of Arylidene-Δ2-Pyrrolin-4-Ones with 3-Isothiocyanato Oxindoles

Arylidene-Δ2-pyrrolin-4-ones undergo organocatalyzed double spirocyclization with 3-isothiocianato oxindoles in a domino 1,4/1,2-addition sequence. The products contain three contiguous stereocenters (ee up to 98%, dr up to 99:1, 12 examples). The absolute configuration of the major diastereomer was determined by single crystal X-ray analysis. Along with heterocyclic Michael acceptors based on oxazolone, isoxazolone, thiazolidinone, pyrazolone, and pyrimidinedione, the reported results display the applicability of unsaturated Δ2-pyrrolin-4-ones (pyrrolones) for the organocatalyzed construction of 3D-rich pyrrolone-containing heterocycles.

Asymmetric Synthesis of Oxindole-Derived Vicinal Tetrasubstituted Acyclic Amino Acid Derivatives by the Mannich-Type Reaction

The catalytic asymmetric Mannich-type reaction of 3-hydroxy/3-aminooxindoles with 2-aminoacrylates to afford oxindole-derived acyclic amino acid derivatives bearing vicinal tetrasubstituted stereocenters is reported. (DHQ)₂PHAL (4g) and quinine-derived squaramide (4d) were identified as efficient catalysts. Transformations of the Mannich-type reaction products highlight the utility of this synthetic strategy.

Phosphine-catalyzed [3 + 2] annulation of 2-aminoacrylates with allenoates and mechanistic studies

A phosphine catalyzed formal [3 + 2] annulation was disclosed, affording 3-pyrrolines containing an amino quaternary stereogenic center in good to excellent yields. The catalytic mechanism was investigated by DFT and kinetic studies.