Double Morita–Baylis–Hillman (MBH) strategy; an intermolecular and a chemo selective intramolecular MBH reactions for 5/6 substituted, functionalized piperidine unit

A Facile Synthesis of 3-Substituted Coumarins and Investigation of Their 3CLpro Inhibition Activity Against SARS-CoV-2

The major threat to public health due to the outbreak of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection has been recognised as a global issue. The increase in morbidity is primarily due to the lack of SARS-CoV-2 specific drugs. One of the major strategies to combat this threat is to deactivate the enzymes responsible for the replication of corona virus. To this end, 3-arylidene/3-hydroxycoumarin induced deactivation of 3-chymotrypsin like protease (3CLpro) enzyme, which takes the pivotal role in the replication and maturation, was investigated. For ready availability of the compounds for the above investigation, we have developed a user-friendly protocol for the synthesis 3-hydroxycoumarin derivatives from cheap and readily available starting materials in two steps; i) Bronsted acid catalysed Friedel-Crafts alkylation of phenols with Morita-Baylis-Hillman adducts followed by intramolecular lactonization to trans-3-arylidenechroman-2-ones in one-pot and ii) ozonolysis in reasonably good yields. Pharmacokinetic assessments of coumarin derivatives revealed drug-like characteristics with moderate or low toxicity values. Notably, these hydroxycoumarins exhibited enhanced binding affinity against the 3CL protease of SARS-CoV-2, fitting well into the binding pocket akin to the previously studied inhibitor N3. Furthermore, a molecular dynamics study elucidated the dynamic behaviour of these small molecules when bound to the protein, showcasing intriguing complexities within the active site. Despite backbone variations and residual fluctuations, compounds 3 d-f and 6 a exhibited a consistent behaviour, instilling confidence in the therapeutic potential of these coumarins for combating SARS-CoV-2.

Domino Sequence of Ketimization and Electrophilic Amination for an Inverse Aza Intramolecular Morita-Baylis-Hillman Adduct

Morita-Baylis-Hillman (MBH) reaction, typically catalyzed by a Lewis base, is a popular and useful method for C-C bond formation. Unfortunately, it is limited by a slow reaction rate and has sensitivity toward steric and electronic parameters. Despite tremendous efforts, the versatility of the reaction keeps the quest open for new mechanistic and catalytic pathways. Here, we have reported a Bro̷nsted acid-catalyzed, electrophilic amination (Umpolung of imine) as a method for an inverse Aza Intramolecular MBH adduct in the form of 2-acylindole. Umpolung of imine with nitrogen acting as an electrophilic center has been achieved. Interestingly, the reaction was also shown to occur under catalyst-free conditions also. The expected products of ketimine formation, 6π electrocyclization, or quinoline formation were least/not observed. A large number of examples have demonstrated the reaction strength. β-aryl-substituted acrylate and acrylamide (cinnamates and cinnamides), which are extremely sluggish in conventional MBH chemistry, are the highlights of the developed methodology. The annulated product exhibited keto-enol tautomerism, which was proven by 1H NMR integrals. As an application, another tandem reaction in the form of Michael addition on a highly complex amine was carried out to provide spiro-annulated indole.

N-Benzyloxyacrylamides as Bisnucleophiles in an Organocatalyzed Domino aza-Michael/Morita-Baylis-Hillman Sequence

We herein describe a stereoselective organocatalyzed aza-Michael/Morita-Baylis-Hillman domino reaction between readily accessible acrylamides and α,β-unsaturated carbonyls. This novel, PPh3-promoted atom economic one-pot process features medium to good yields and good stereoselectivity leading to variously substituted piperidin-2-ones bearing an exocyclic olefinic bond, which was shown to be an excellent anchor for further chemical diversification.

Chemoselective Intramolecular Morita-Baylis-Hillman Reaction; Acrylamide and Ketone as Sluggish Reacting Partners on a Labile Framework

Chemoselectivity is an important issue frequently encountered while working over labile precursors. Carbonyl compounds with a heteroatom at the β carbon are sensitive precursors because they are prone to elimination under different conditions. Morita-Baylis-Hillman (MBH) reaction, although a widespread method for C-C bond formation, has its own limitations. Acrylamide and ketone are such limitations of the MBH reaction. Using them together for an intramolecular MBH (IMBH) reaction on a labile framework prone to elimination is a significant 2-fold synthetic challenge. A highly chemoselective IMBH reaction on such precursors has been established using 1,4-diazabicyclo[2.2.2]octane (DABCO) as a promoter. The protocol leads to quick access to a diversely substituted and functionalized piperidone framework in high yields. Various substitution patterns in the form of 34 successful examples have been studied. A diastereoselective version and tolerance to various functional and protecting groups are the added advantages of the developed methodology. A tertiary carbon at the β position of ketone, however, led to complete reversal of selectivity and gave only the elimination product. Control experiments toward a better understanding of the substitution pattern, role of catalyst, and mechanistic study have been carried out. As an application of the IMBH adduct, a one-step allylic rearrangement for the dihydropyridone framework has also been demonstrated.

Chemoselective and Highly Rate Accelerated Intramolecular Aza-Morita-Baylis-Hillman Reaction

Despite being a very useful C-C bond forming and highly applicative reaction, Morita-Baylis-Hillman (MBH) reaction has been limited by its excessive slow reaction rate, including its intramolecular version. In certain cases, reaction time may even go to weeks and months. A highly chemoselective and rate accelerated intramolecular MBH reaction of just 15 min has been developed. The product dihydroquinoline, being unstable, was converted to an important quinoline framework. In some cases IMBH adducts were isolable, thus confirming the reaction path. Control experiments toward mechanism investigation have been carried out. Use of sodium sulfide has emerged as a rate accelerating catalyst in DMF-EtOH solvent system. Reaction intermediate for IMBH pathway was isolated and characterized. Other aspects such as the application of IMBH adduct for Michael addition and amidation have also been carried out.

Magnetically recoverable Cu0/Fe3O4catalyzed highly regioselective synthesis of 2,3,4-trisubstituted pyrroles from unactivated terminal alkynes and isocyanides

An efficient, one pot tandem, Cu⁰/Fe₃O₄catalyzed highly regioselective synthesis of 2,3,4-trisubstituted pyrroles from unactivated terminal alkynes and isocyanides.

Magnetically separable nano-copper catalyzed unprecedented stereoselective synthesis of E-vinyl sulfones from tosylmethyl isocyanide and alkynes: TosMIC as a source of the sulfonyl group

Cu⁰/Fe₃O₄ catalyzed stereoselective synthesis of E-vinyl sulfones from tosylmethyl isocyanide and alkynes. TosMIC act as sulfonyl source.