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.

Stereo- and Regiospecific SN2' Reaction of MBH Adducts with Isocyanoacetates: en Route to Transition-Metal-Free α-Allylation of Isocyanoacetates

Herein, we report that under mild and transition-metal-free conditions an unprecedented and practical S_N2' reaction of Morita-Baylis-Hillman adducts with isocyanoacetates takes place in a stereo- and regiospecific manner. This reaction which tolerates a wide variety of functionalities delivers transformable α-allylated isocyanoacetates in high efficiencies. Preliminary studies on the asymmetric version of this reaction indicate that ZnEt₂/chiral amino alcohol combinations are an asymmetric catalytic system for this transformation, giving an enantioenriched α-allylated isocyanoacetate with a chiral quaternary carbon in a high yield.

I2-mediated Csp2–P bond formation via tandem cyclization of o-alkynylphenyl isothiocyanates with organophosphorus esters

A highly efficient molecular-iodine-catalyzed cascade cyclization reaction has been developed, creating a series of 4H-benzo[d][1,3]thiazin-2-yl phosphonates in moderate to excellent yields. This approach benefits from metal-free catalysts and available raw materials.

Copper(ii)-catalyzed tandem cyclization for the synthesis of benzo[d][1,3]thiazin-2-yl phosphonates involving C-P and C-S bond formation

A copper(ii)-catalyzed, high-efficiency and atom-economical synthesis of valuable organophosphorus compounds via tandem cyclization of o-alkynylphenyl isothiocyanates with phosphites is described. This protocol, having a good functional-group compatibility, provides a simple and direct pathway to organophosphorus heterocycles in good yields under mild conditions. The method could be efficiently scaled up to gram scale, thus providing a potential application of this cascade cyclization strategy in synthesis.

A simple and highly efficient cascade cyclization of o-alkynylphenyl isothiocyanates with phosphites has been developed, affording a series of 4H-benzo[d][1,3]thiazin-2-yl phosphonates in moderate to good yields.

Synthesis of thiazolo[2,3-b]quinazoline derivatives via base-promoted cascade bicyclization of o-alkenylphenyl isothiocyanates with propargylamines

A highly efficient cascade bicyclization reaction of o-alkenylphenyl isothiocyanates with propargylamines has been developed, which affords a series of thiazolo[2,3-b]quinazolines in good to excellent yields by using K₂CO₃ as a base in MeCN at 80 °C. This method is transition-metal-free and operationally simple with broad functional group tolerance. Mechanistically, 6-exo-trig hydroamination followed by 5-exo-dig hydrothiolation was involved in this transformation.

A metal- and base-free domino protocol for the synthesis of 1,3-benzoselenazines, 1,3-benzothiazines and related scaffolds

Efficient protocols have been described for the synthesis of 1,3-benzoselenazines, 1,3-benzothiazines, 2-aryl thiazin-4-ones and diaryl[b,f][1,5]diazocine-6,12(5H,11H)-diones. These transformations were successfully driven towards the product formation under mild acid catalyzed reaction conditions at room temperature using 2-amino aryl/hetero-aryl alkyl alcohols and amides as substrates. The merits of the present methods also rely on the easy access of rarely explored bioactive scaffolds like 1,3-benzoselenazine derivatives, for which well-documented methods are rarely known in the literature. A broad range of substrates with both electron-rich and electron-deficient groups were well-tolerated under the developed conditions to furnish the desired products in yields up to 98%. The scope of the devised method is not only restricted to the synthesis of 1,3-benzoselenazines, but it was also further extended towards the synthesis of 1,3-benzothiazines, 1,3-benzothiazinones and the corresponding eight membered N-heterocycles such as diaryl[b,f][1,5]diazocine-6,12(5H,11H)-diones.

A silver(i)-catalyzed cascade bicyclization strategy for synthesis of 5H-benzo[d]tetrazolo[5,1-b][1,3]thiazines

A simple and efficient protocol for silver(i)-catalyzed tandem reaction of o-alkynylphenyl isothiocyanates with sodium azide has been developed, affording a series of 5H-benzo[d]tetrazolo[5,1-b][1,3]thiazines in moderate to good yields. In this transformation, a [3 + 2] cycloaddition reaction mechanism was involved and two new rings were formed in one pot.

Synthesis of thiazolidine-thiones, imino-thiazolidines and oxazolidines via the base promoted cyclisation of epoxy-sulfonamides and heterocumulenes

Epoxy-sulfonamides react with heterocumulenes (CS₂/RNCS/RNCO) in the presence of a base to afford ring expansion products in good to high yields with excellent regioselectivity.

One-Pot Synthesis of Benzo[4,5]imidazo[2,1-a]isoquinolines and Isoquinolino[3,4-b]quinoxalines via Tandem Cyclization Strategies

Two operationally simple one-pot protocols have been developed for the synthesis of amino-functionalized benzo[4,5]imidazo[2,1-a]isoquinolines and isoquinolino[3,4-b]quinoxalines. Optimization data and substrate scope for these atom-economical transformations, which engage commercially available o-phenylenediamines and o-cyanobenzaldehydes, are discussed.

A copper-catalyzed cascade reaction of o-bromoarylisothiocyanates with isocyanides leading to benzo[d]imidazo[5,1-b]thiazoles under ligand-free conditions

A copper-catalyzed method has been realized for the synthesis of benzo[d]imidazo[5,1-b]thiazoles via intermolecular [3 + 2] cycloaddition and intramolecular C–S bond formation.