Total Synthesis of Calothrixin B via an Intramolecular Baylis‐Hillman Cyclization/6π Electrocyclization/Dehydro‐aromatization Sequence and a Specific Oxidative Quinone Formation

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.

A rapid approach towards a specific carbazolelactam system related to calothrixin B

Reported here is a rapid and versatile synthetic approach towards the specific pentacyclic carbazolelactams related to calothrixin B. The crucial transformation is an acid-promoted cascade involving a dehydration/electrocyclization/C-N bond cleavage/lactamization sequence.

Synthesis of Cytotoxic Quino[4,3-b]carbazole Frameworks through an Intramolecular Diels-Alder Reaction

An intramolecular Diels-Alder reaction of positionally isomeric indole-2/3-phenylvinyl-N-alkynylated (N-phenylsulfonyl)amines has been successfully exploited for the synthesis of quino[4,3-b]carbazole and its analogues. This reaction proceeds through a [4 + 2] cycloaddition followed by elimination and deprotection of phenylsulfonyl units to afford the quinocarbazoles in moderate to good yields. The reaction features a broad substrate scope and remarkable functional group forbearance. A preliminary in vitro cytotoxicity evaluation of representative quino[4,3-b]carbazoles was performed against NCI-H460 human cancer cell culture. Among the quino[4,3-b]carbazoles evaluated, five of the fluorine-containing quinocarbazoles displayed nano molar range (0.8-2.0 nm) GI50 values. The UV-vis and fluorescence spectral studies of representative quinocarbazoles were also performed. Like ellipticine, four of the quinocarbazoles displayed dual emissions confirming the existence of p-quinonoid like tautomeric forms in a polar protic solvent.

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.

Trends in carbazole synthesis - an update (2013-2023)

The interest of scientists in the carbazole core has risen steadily over the last 30 years, particularly over the last decade given its presence in several active pharmaceutical ingredients, functional materials and a wide range of biologically active natural products. The continuous development of more efficient, more (regio-)selective and "greener" methodologies to access the carbazole core is thus imperative. This review compares and evaluates synthetic strategies towards the carbazole core that have been reported since 2013, with a focus on their applicability towards the total synthesis of carbazole-containing natural products.

Total Synthesis and Structural Revision of the 6,11-Epoxyisodaucane Natural Sesquiterpene Using an Anionic 8π Electrocyclic Reaction

A new synthetic strategy that forms a seven-membered carbocycle using an anionic 8π electrocyclic reaction facilitated the first total synthesis of the 6,11-epoxyisodaucane natural sesquiterpene in 9.0% yield over 10 steps in the longest linear sequence. The misassigned proposed stereochemistry was corrected by the synthesis of both the proposed structure and its C6 epimer. In addition, the 5-7-fused ring system was concisely constructed by tandem decyanation/five-membered-ring formation from an epoxynitrile.

Exploiting an intramolecular Diels-Alder cyclization/dehydro-aromatization sequence for the total syntheses of ellipticines and calothrixin B

The tetracyclic and pentacyclic skeletons of pyrido and quinolinocarbazole alkaloids have been synthesized via a unified strategy. The prominent key step involved a Diels-Alder intramolecular cyclization/dehydro-aromatization sequence. From these carbazole-lactam cores, linear syntheses have been developed for ellipticines and calothrixin B.