Enantioselective synthesis of martinelline chiral core and its diastereomer using asymmetric tandem Michael–aldol reaction

Enantioselective Synthesis of Sealutomicin C

The sealutomicins are a family of anthraquinone antibiotics featuring an enediyne (sealutomicin A) or Bergman-cyclized aromatic ring (sealutomicins B-D). Herein we report the development of an enantioselective organocatalytic method for the synthesis of dihydroquinolines and the use of the developed method in the total synthesis of sealutomicin C which features a transannular cyclization of an aryllithium onto a γ-lactone as a second key step.

Synthesis of the Hexahydropyrrolo-[3,2-c]-quinoline Core Structure and Strategies for Further Elaboration to Martinelline, Martinellic Acid, Incargranine B, and Seneciobipyrrolidine

Natural products are rich sources of interesting scaffolds possessing a plethora of biological activity. With the isolation of the martinella alkaloids in 1995, namely martinelline and martinellic acid, the pyrrolo[3,2-c]quinoline scaffold was discovered. Since then, this scaffold has been found in two additional natural products, viz. incargranine B and seneciobipyrrolidine. These natural products have attracted attention from synthetic chemists both due to the interesting scaffold they contain, but also due to the biological activity they possess. This review highlights the synthetic efforts made for the preparation of these alkaloids and formation of analogues with interesting biological activity.

Heterogeneous Synergistic Catalysis for Promoting Aza-Michael–Henry Tandem Reaction for the Synthesis of Chiral 3-Nitro-1,2-Dihydroquinoline

Heterogeneous synergistic catalysis by SBA-15 immobilized chiral amines catalysts has promoted efficient aza-Michael–Henry tandem reaction for the synthesis of chiral 3-Nitro-1,2-Dihydroquinoline. Final products in the asymmetric aza-Michael–Henry cascade reactions between 2-aminobenzaldehyde and β-nitrostyrolene were afforded in a yield of 85% and an enantiomeric excess (ee) value of 98% on (S)-(–)-2-aminomethyl-1-ethylpyrrolidine immobilized SBA-15. SBA-15-AEP catalyst has been also extended to the asymmetric aza-Michael–Henry cascade reaction of substituted R1-2-aminobenzaldehyde and R2-substituted nitroolefin. The heterogeneous synergistic mechanism for both tertiary amine and secondary amine immobilized mesoporous has been proposed in detail including the geometrical constraints in the ee promotion.

Utilization of Cyclic Amides as Masked Aldehyde Equivalents in Reductive Amination Reactions

An operationally simple protocol has been discovered that couples primary or secondary amines with N-aryl-substituted lactams to deliver differentiated diamines in moderate to high yields. The process allows for the partial reduction of a lactam in the presence of Cp₂ZrHCl (Schwartz's reagent), followed by a reductive amination between the resulting hemiaminal and primary or secondary amine. These reactions can be telescoped in a one-pot fashion to significantly simplify the operation. The scope of amines and substituted lactams of various ring sizes was demonstrated through the formation of a range of differentiated diamine products. Furthermore, this methodology was expanded to include N-aryl pyrrolidinone substrates with an enantiopure ester group at the 5-position, and α-amino piperidinones were prepared with complete retention of stereochemical information. The development of this chemistry has enabled the consideration of lactams as useful synthons.

Recent advances in the asymmetric synthesis of pharmacology-relevant nitrogen heterocycles via stereoselective aza-Michael reactions

In this review, recent applications of a stereoselective aza-Michael reaction for asymmetric synthesis of naturally occurring N-containing heterocyclic scaffolds and their usefulness to pharmacology are summarized.

Synthesis of 3-Methylidene-1-tosyl-2,3-dihydroquinolin-4(1H)-ones as Potent Cytotoxic Agents

An efficient synthetic strategy to 3-methylidene-2,3-dihydroquinolin-4(1H)-ones variously substituted in position 2 has been developed. The title compounds were synthesized in the reaction sequence involving reaction of diethyl methylphosphonate with methyl 2-(tosylamino)benzoate, condensation of thus formed diethyl 2-oxo-2-(2-N-tosylphenyl)ethylphosphonate with various aldehydes followed by successful application of the obtained 3-(diethoxyphosphoryl)-1,2-dihydroquinolin-4-ols as Horner-Wadsworth-Emmons reagents for the olefination of formaldehyde. Also, enantioselective approach to the target compounds has been evaluated using 3-dimenthoxyphosphoryl group as a chiral auxiliary. Single X-ray crystal analysis of (2S)-3-(dimenthoxyphosphoryl)-2-phenyl-1-tosyldihydroquinolin-4-ol revealed the presence of strong resonance-assisted hydrogen bond (RAHB). The obtained 3-methylidene-2,3-dihydroquinolin-4(1H)-ones were then tested for their cytotoxic activity against two leukemia cell lines NALM-6 and HL-60 and a breast cancer MCF-7 cell line. All compounds showed very high cytotoxic activity with the IC₅₀ values mostly below 1 μm in all three cancer cell lines. The selected analogs were also tested on human umbilical vein endothelial cells (HUVEC) and on human mammary gland/breast cells (MCF-10A) to evaluate their influence on normal cells. Since one of the most serious problems in cancer chemotherapy is the development of drug resistance, the mRNA levels and activity of ABCB1 transporter considered to be the most important factor engaged in drug resistance, were evaluated in MCF-7 cells treated with two selected analogs. Both compounds were strong ABCB1 transporter inhibitors that could prevent efflux of anticancer drugs from cancer cells.

Construction of tetrahydropyranoquinoline derivativesviaan asymmetric organocatalytic aza-Michael-IED/HAD cascade reaction

A highly efficient organocatalytic asymmetric aza-Michael-IED/HAD cascade reaction of (E)-ethyl 4-(2-(4-methylphenylsulfonamido)phenyl)-2-oxobut-3-enoate and enals is reported.

Enantioselective Total Synthesis of (-)-Martinellic Acid

An enantioselective total synthesis of martinellic acid is described. The pyrroloquinoline alkaloid core is efficiently prepared from a quinoline, employing a method which relies on a newly developed Cu-catalyzed enantioselective alkynylation using the chiral imidazole-based biaryl P,N ligand StackPhos to establish the absolute stereochemistry. The remaining carbon atoms are then installed by means of a diastereoselective Pd-catalyzed decarboxylative allylation and the synthesis is completed after straightforward functional-group manipulation. This new synthetic method enables the most concise enantioselective synthesis of this important class of molecules to date.

A silver-promoted solid-phase guanidylation process enables the first total synthesis of stictamide A

First total synthesis of stictamide A, a structurally unique peptide with a statine motif and a N-prenyl modified arginine in the side chain, is disclosed with a novel silver-promoted solid-phase strategy for the first time.

Enantioselective approach to quinolizidines: total synthesis of cermizine D and formal syntheses of senepodine G and cermizine C

The formal syntheses of C5-epi-senepodine G and C5-epi-cermizine C have been accomplished through a novel diastereoselective, intramolecular amide Michael addition process. The total synthesis of cermizine D has been achieved through use of an organocatalyzed, heteroatom Michael addition to access a common intermediate. Additional key steps of this sequence include a matched, diastereoselective alkylation with an iodomethylphenyl sulfide and sulfone-aldehyde coupling/reductive desulfurization sequence to combine the major subunits. The utility of a Hartwig-style C-N coupling has been explored on functionally dense coupling partners. Diastereoselective conjugate additions to α,β-unsaturated sulfones have been investigated, which provided the key sulfone intermediate in just six steps from commercially available starting materials. The formal syntheses of senepodine G and cermizine C have been accomplished through an intramolecular cyclization process of a N-Boc-protected piperidine sulfone.

Asymmetric synthesis of (-)-martinellic acid

A high-yielding total asymmetric synthesis of (-)-martinellic acid is reported. The conjugate addition of lithium (R)-N-allyl-N-(α-methyl-4-methoxybenzyl)amide to tert-butyl (E)-3-[2'-(N,N-diallylamino)-5'-bromophenyl]propenoate and alkylation of the resultant β-amino ester have been used as the key steps to install the C(9b) and C(3a) stereogenic centers, respectively, and a highly diastereoselective Wittig reaction/intramolecular Michael addition was then used to create the C(4) stereogenic center within this tricyclic molecular architecture.

Organocatalytic tandem Michael addition reactions: A powerful access to the enantioselective synthesis of functionalized chromenes, thiochromenes and 1,2-dihydroquinolines

Enantioselective organocatalysis has become a field of central importance within asymmetric chemical synthesis and appears to be efficient approach toward the construction of complex chiral molecules from simple achiral materials in one-pot transformations under mild conditions with high stereocontrol. This review addresses the most significant synthetic methods reported on chiral-amine-catalyzed tandem Michael conjugate addition of heteroatom-centered nucleophiles to α,β-unsaturated compounds followed by cyclization reactions for the enantioselective construction of functionalized chiral chromenes, thiochromenes and 1,2-dihydroquinolines in optically enriched forms found in a myriad of bioactive natural products and synthetic compounds.

Chiral phosphoric acid-catalyzed enantioselective three-component Povarov reaction using cyclic enethioureas as dienophiles: stereocontrolled access to enantioenriched hexahydropyrroloquinolines

Three become one: Phosphoric acid-catalyzed enantioselective three-component Povarov reactions of aldehydes, anilines, and endocyclic enethioureas have been developed (see scheme). This process afforded hexahydropyrroloquinolines in high yields with excellent diastereo- and enantioselectivities. The presence of the thiourea functionality is crucial for the enantioselectivity of the reaction.

Organocatalytic cascade reactions as a new tool in total synthesis

The total synthesis of natural products and biologically active compounds, such as pharmaceuticals and agrochemicals, has reached an extraordinary level of sophistication. We are, however, still far away from the 'ideal synthesis' and the state of the art is still frequently hampered by lengthy protecting-group strategies and costly purification procedures derived from the step-by-step protocols. In recent years several new criteria have been brought forward to solve these problems and to improve total synthesis: atom, step and redox economy or protecting-group-free synthesis. Over the past decade the research area of organocatalysis has rapidly grown to become a third pillar of asymmetric catalysis standing next to metal and biocatalysis, thus paving the way for a new and powerful strategy that can help to address these issues - organocatalytic cascade reactions. In this Review we present the first applications of such asymmetric organocascade reactions to the total synthesis of natural products.