Development of a flexible strategy towards FR900482 and the mitomycins

Total synthesis of 14-membered ring β-resorcylic acid lactone (+)-monocillin II

This study outlines the total synthesis of (+)-monocillin II, wherein a cis-isomer selectively produces a trans-isomer during the ring-closing metathesis. The Mitsunobu reaction conducted at -60 °C, facilitating the formation of an ester bond, was the key to completing the total synthesis, which was accomplished in the longest linear sequence of 10 steps with an overall yield of 9.3%.

Crowned Macrocycles Containing Two Pyrrolo[1,2-a] Indoles Created By Intramolecular Fusion

Heterocyclic fused-ring systems are of utmost importance because of their presence in many natural products with biological activity. Pyrroloindoles are tricyclic heterocycles that are present in various bioactive and medicinally valuable compounds. Herein, we report the synthesis of phenylene-bridged bis-pyrrolo[1,2-a]indole crowned macrocycles 1-3 in which the pyrrolo[1,2-a]indole moieties were formed via intramolecular fusion. The macrocycles were thoroughly characterized by 1D and 2D NMR, HRMS and X-ray crystallographic studies. The X-ray structure revealed that the two pyrrolo[1,2-a]indole moieties were parallel to each other, and one pyrrolo[1,2-a]indole unit was deviated by an angle of 9.54° while the other pyrrolo[1,2-a]indole unit was deviated by an angle of 12.0° from the mean plane defined by 28 core atoms. The macrocycles 1-3 absorb in the visible region and readily undergo oxidations because of their electron rich nature. The macrocycles 1-3 upon treatment with trifluoroacetic acid (TFA) generates the corresponding cation radicals 1-3^.+ which were stable in the open air for a week. The cation radicals 1-3^.+ absorb strongly in the NIR region and the experimental observations on crowned macrocycles 1-3 were corroborated by DFT and TD-DFT studies.

A Convenient Formal [4+2] Heterocylization Route to Bis(triflyl)tetrahydroquinolines

We report the sustainable and efficient synthesis of a new type of quinoline derivatives bearing one or two SO2 CF3 groups. The protocol is metal-, catalyst- and irradiation-free, involves the use of readily available and stable precursors, and avoids the formation of side products. Also, the mild conditions of the process allow the tolerance of a wide range of functional groups.

PIII/PV═O-Catalyzed Intermolecular N-N Bond Formation: Cross-Selective Reductive Coupling of Nitroarenes and Anilines

An organophosphorus-catalyzed method for the synthesis of unsymmetrical hydrazines by cross-selective intermolecular N-N reductive coupling is reported. This method employs a small ring phosphacycle (phosphetane) catalyst together with hydrosilane as the terminal reductant to drive reductive coupling of nitroarenes and anilines with good chemoselectivity and functional group tolerance. Mechanistic investigations support an autotandem catalytic reaction cascade in which the organophosphorus catalyst drives two sequential and mechanistically distinct reduction events via P^III/P^V═O cycling in order to furnish the target N-N bond.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

Chemoselective Intramolecular Formal Insertion Reaction of Rh-Nitrenes into an Amide Bond Over C-H Insertion

The past few decades have witnessed extensive efforts to disclose the unique reactivity of metal-nitrenes, because they could be a powerful synthetic tool for introducing the amine functionality into unactivated chemical bonds. The reactivity of metal-nitrenes, however, is currently mainly confined to aziridination (an insertion into a C=C bond) and C-H amination (an insertion into a C-H bond). Nitrene insertion into an amide C-N bond, however, has not been reported so far. In this work we have developed a rhodium-catalyzed one-nitrogen insertion into amide C-N and sulfonamide S-N bonds. Experimental and theoretical analyses based on density functional theory indicate that the formal amide insertion proceeds via a rhodium-coordinated ammonium ylide formed between the nitrene and the amide nitrogen, followed by acyl group transfer concomitant with C-N bond cleavage. Mechanistic studies have allowed rationalization of the origin of the chemoselectivity observed between the C-H and amide insertion reactions. The methodology presented herein is the first example of an insertion of nitrene into amide bonds and provides facile access to unique diazacyclic systems with an N-N bond linkage.

Dirhodium(ii)-catalyzed ortho C-H amination of sterically congested N,N-dialkylanilines

Dirhodium(ii)-catalyzed ortho C-H amination of N,N-dialkylanilines has been developed. Sterically congested 1,2-diaminobenzenes were obtained in high yields with excellent chemo- and regioselectivities from para-substituted anilines and even from para-unsubstituted anilines. The ortho selectivity observed was rationalized in terms of the interaction between the dialkylamino group and a Rh(ii)-nitrene intermediate.

Enantioselective Synthesis of (+)-Mitomycin K by a Palladium-Catalyzed Oxidative Tandem Cyclization

The mitomycins, a family of bioactive natural products, feature a compact 6/5/5-fused polycyclic ring structure densely decorated with highly reactive and/or fragile quinone, amino ketal, and aziridine as well as carbamate moieties. It is this striking feature that has defeated numerous synthetic attempts towards these apparently small molecules, rendering them one of the most formidable targets for total synthesis. We herein report the first enantioselective synthesis of (+)-mitomycin K, a representative of G series mitomycins. The key step of this synthesis is an enantioselective oxidative cyclization catalyzed by a palladium/(+)-sparteine system that had previously been developed by our group. The robustness of this method bodes well for further applications in the asymmetric total synthesis of natural products, particularly those with characteristic 6/5/5-fused pyrroloindole skeletons.

Catalytic C–H amination at its limits: challenges and solutions

Pushing C–H amination to its limits fosters innovative synthetic solutions and offers a deeper understanding of the reaction mechanism and scope.

Pushing the limits of catalytic C-H amination in polyoxygenated cyclobutanes

A synthetic route to a new class of conformationally constrained iminosugars based on a 5-azaspiro[3.4]octane skeleton has been developed by way of Rh(ii)-catalyzed C(sp(3))-H amination. The pivotal stereocontrolled formation of the quaternary C-N bond by insertion into the C-H bonds of the cyclobutane ring was explored with a series of polyoxygenated substrates. In addition to anticipated regioselective issues induced by the high density of activated α-ethereal C-H bonds, this systematic study showed that cyclobutane C-H bonds were, in general, poorly reactive towards catalytic C-H amination. This was demonstrated inter alia by the unexpected formation of a oxathiazonane derivative, which constitutes a very rare example of the formation of a 9-membered ring by way of catalyzed C(sp(3))-H amination. A complete stereocontrol could be however achieved by activating the key insertion position as an allylic C-H bond in combination with reducing the electron density at the undesired C-H insertion sites by using electron-withdrawing protecting groups. Preliminary biological evaluations of the synthesized spiro-iminosugars were performed, which led to the identification of a new class of correctors of the defective F508del-CFTR gating involved in cystic fibrosis.

Synthesis of a new class of iminosugars based on constrained azaspirocyclic scaffolds by way of catalytic C-H amination

The synthesis of the first examples of a new class of iminosugars based on constrained spirocyclic scaffolds has been achieved via Rh-catalyzed C(sp(3))-H amination. In this process, the needed electronic control in securing high regioselectivity from substrates with a high density of activated C-H bonds was achieved by using a combination of activating and electron-withdrawing groups.

Synthesis of 7a-phenyl-1a,7a-dihydro-benzopyrano[2,3-b]azirin-7-ones via photoisomerization reaction

An environmentally friendly and novel protocol has been developed for the synthesis of 7a-phenyl-1a,7a-phenyl-benzopyrano[2,3-b]azirin-7-ones in moderate to high yieldsviathe photoisomerization of 4-phenyl-5-(2-hydroxyphenyl)isoxazoles.

Palladium-catalyzed asymmetric allylic amination of racemic butadiene monoxide with isatin derivatives

Pd-catalyzed asymmetric aminations of butadiene monoxide with isatin derivatives using P/alkene ligands gave high yields, ees and regioselectivity ratios.

Photoassisted diversity-oriented synthesis: accessing 2,6-epoxyazocane (oxamorphan) cores

The modular synthesis of photoprecursors and their photoinduced cyclization into substituted 1-benzazocanes of two distinct topologies is described. The key step producing an extended polyheterocyclic system involves the photogeneration of azaxylylenes and their subsequent intramolecular cycloaddition with furan-containing pendants tethered either via the aniline nitrogen or through the carbonyl group containing arm. The primary photoproducts-secondary or tertiary anilines which are not acylated at the nitrogen atom-undergo facile acid-catalyzed or spontaneous ring-opening-ring-closing rearrangement to yield fused polyheterocyclic structures possessing a 2,6-epoxyazocane (or oxamorphan) core.

Application of diazene-directed fragment assembly to the total synthesis and stereochemical assignment of (+)-desmethyl-meso-chimonanthine and related heterodimeric alkaloids

We describe the first application of our methodology for heterodimerization via diazene fragmentation towards the total synthesis of (-)-calycanthidine, meso-chimonanthine, and (+)-desmethyl-meso-chimonanthine. Our syntheses of these alkaloids feature an improved route to C3a-aminocyclotryptamines, an enhanced method for sulfamide synthesis and oxidation, in addition to a late-stage diversification leading to the first enantioselective total synthesis of (+)-desmethyl-meso-chimonanthine and its unambiguous stereochemical assignment. This versatile strategy for directed assembly of heterodimeric cyclotryptamine alkaloids has broad implications for the controlled synthesis of higher order derivatives with related substructures.

Simultaneous structure-activity studies and arming of natural products by C-H amination reveal cellular targets of eupalmerin acetate

Natural products have a venerable history of, and enduring potential for the discovery of useful biological activity. To fully exploit this, the development of chemical methodology that can functionalize unique sites within these complex structures is highly desirable. Here, we describe the use of rhodium(II)-catalysed C-H amination reactions developed by Du Bois to carry out simultaneous structure-activity relationship studies and arming (alkynylation) of natural products at 'unfunctionalized' positions. Allylic and benzylic C-H bonds in the natural products undergo amination while olefins undergo aziridination, and tertiary amine-containing natural products are converted to amidines by a C-H amination-oxidation sequence or to hydrazine sulfamate zwitterions by an unusual N-amination. The alkynylated derivatives are ready for conversion into cellular probes that can be used for mechanism-of-action studies. Chemo- and site-selectivity was studied with a diverse library of natural products. For one of these-the marine-derived anticancer diterpene, eupalmerin acetate-quantitative proteome profiling led to the identification of several protein targets in HL-60 cells, suggesting a polypharmacological mode of action.

Reactivity of aziridinomitosene derivatives related to FK317 in the presence of protic nucleophiles

The syntheses and reactivity of N-TBDPS and N-trityl protected derivatives of an aziridinomitosene corresponding to FK317 are described. New reactivity patterns were observed for these highly sensitive and functionally dense heterocycles under mild nucleophilic conditions approaching the threshold for degradation. Thus, the silyl or trityl protected aziridinomitosene reacted with Cs(2)CO(3)/CD(3)OD to give isomeric products where substitution occurred at C(10) and C(9a) (mitomycin numbering) providing a CD(3) ether and a CD(3) hemiaminal, respectively. These findings show that heterolysis at C(10) is faster than at aziridine C(1), in contrast to the behavior of typical aziridinomitosenes in the mitomycin series. The labile N-TBDPS hemiaminal and the more stable N-trityl hemiaminal resemble the mitomycin K substitution pattern. A reagent consisting of CsF in CF(3)CH(2)OH/CH(3)CN desilylated a simple N-TBDPS aziridine but caused nucleophilic cleavage at C(1) as well as C(10) without cleavage of the N-TBPDS group in the fully functionalized penultimate aziridinomitosene. The high reactivity of the C(10) carbamate with nucleophiles precludes the use of deprotection methodology that requires N-protonation for fully functionalized aziridinomitosenes in the FK317 series.

Nickel-Catalyzed Regiodivergent Approach to Macrolide Motifs

A strategy for regiochemical reversal of reductive macrocyclizations of aldehydes and terminal alkynes has been developed. Using an advanced synthetic intermediate directed towards the methymycin/neomethymycin class of macrolides, selective endocyclization provides the natural twelve-membered ring series, whereas ligand alteration enables selective exocyclization to provide access to the unnatural eleven-membered ring series. The twelve-membered ring adduct was converted to 10-deoxymethynolide, completing an efficient total synthesis of this natural product.