Total synthesis of natural 8- and 9-membered lactones: recent advancements in medium-sized ring formation

Stereoselective Synthesis of Diverse Lactones through a Cascade Reaction of Rhodium Carbenoids with Ketoacids

A convergent cascade approach for the stereoselective synthesis of diverse lactones is described. The Rh₂(TFA)₄-catalyzed cascade reaction proceeds via a carboxylic acid O-H insertion/aldol cyclization with high chemo-, regio-, and diastereoselectivity. The cascade reaction provides quick access to highly functionalized γ-butyro- and δ-valerolactones from readily accessible ketoacid and diazo synthons. To demonstrate the utility of this approach, a thermally induced oxy-Cope ring-expansion strategy has been incorporated in the cascade sequence to access medium-sized lactones, which can undergo a serendipitous rearrangement to form spiro-lactones through an intramolecular aldol/trans-lactonization sequence. The reaction has proven to be general, with a range of ketoacids and diazo carbonyls to provide functionalized lactones of varying ring sizes.

Enantioselective Organocatalytic Four-Atom Ring Expansion of Cyclobutanones: Synthesis of Benzazocinones

An enantioselective Michael addition- four-atom ring expansion cascade reaction involving cyclobutanones activated by a N-aryl secondary amide group and ortho-amino nitrostyrenes has been developed for the preparation of functionalized eight-membered benzolactams using bifunctional aminocatalysts. Taking advantage of the secondary amide activating group, the eight-membered cyclic products could be further rearranged into their six-membered isomers having a glutarimide core under base catalysis conditions without erosion of optical purity, featuring an overall ring expansion- ring contraction strategy.

Rare Medium-Sized Rings Prepared via Hydrolytic Imidazoline Ring Expansion (HIRE)

The hydrolytic imidazoline ring expansion (HIRE) methodology was extended to readily available tetracyclic [1,4]thiazepines as well as sulfoxide and sulfone analogs thereof. The reactions resulted in the facile formation of a rare medium-sized [1,4,7]thiazecine ring system that has an emerging utility in bioactive compound design. Comparing the HIRE rates for representative compounds in the three groups of substrates allowed drawing some generalizations about the substituent effects on the course of the reaction.

Dearomatization of Indole via Intramolecular [3 + 2] Cycloaddition: Access to the Pentacyclic Skeleton of Strychons Alkaloids

An efficient method to build various multisubstituted polycyclic indoline-annulated normal to medium-size rings through dearomatization of indole via a tandem 1,2-acyloxy migration/intramolecular [3 + 2] cycloaddition process is described. The pentacyclic skeleton of strychnine could be synthesized via this tandem cycloaddition and a further Mannich reaction. This approach would provide a novel strategy to the synthesis of strychons alkaloids.

(+)- and (-)-Ascomlactone A: a pair of novel dimeric polyketides from a mangrove endophytic fungus Ascomycota sp. SK2YWS-L

A pair of novel enantiomeric polyketide dimers, (+)- and (-)-ascomlactone A (1a and 1b), were obtained from a mangrove endophytic fungus Ascomycota sp. SK2YWS-L. The structures were elucidated based on spectroscopic methods, and the absolute configurations were determined by X-ray diffraction and electronic circular dichroism (ECD) calculations. Ascomlactone A possessed an unprecedented polymerization system, which constructed an unusual nine-membered lactone ring between the monomers. A possible biogenetic pathway was proposed. Both 1a and 1b exhibited significant inhibitory effects against α-glucosidase with IC₅₀ values of 63.7 and 27.9 μM, respectively. A further docking study provided an inside perspective of the action in α-glucosidase.

Domino Ring Expansion: Regioselective Access to 9-Membered Lactones with a Fused Indole Unit from 2-Nitrophenyl-1,3-cyclohexanediones

The domino anionic fragmentation of 2-nitrophenyl-1,3-cyclohexanediones containing an electrophilic appendage such as aldehyde and epoxide is disclosed. This reaction, initiated by a series of nucleophiles, involves the generation of an intermediate hydroxylate followed by the regioselective formation and fragmentation of an intermediate lactolate into enolate. This strategy, devoid of any protecting group, enlarges the initial ring and provides an original access to decorated 9-membered lactones with a fused indole unit.

Synthesis and evaluation of the antibiotic and adjuvant antibiotic potential of organotin(IV) derivatives

A series of organotin(IV) derivatives was investigated in vitro for their antibiotic and adjuvant antibiotic properties (efflux pump inhibitors) against Staphylococcus aureus strains that overexpress efflux pump proteins for norfloxacin (SA-1199B), erythromycin (RN-4220) and tetracycline (IS-58). Most organotin(IV) compounds showed significant antibacterial activity with small Minimum Inhibitory Concentration (MIC) values, some of which were close to 1.0μg/mL (3.1μM), but this feature was also associated with substantial cytotoxicity. Nevertheless, the cytotoxicity of these organotin(IV) compounds can be overcome when they are used as antibiotic adjuvants. Their remarkable adjuvant antibiotic properties allow potentiation of the action of tetracycline (against IS-58 strain) by up to 128-fold. This likely indicates that they can act as putative inhibitors of bacterial efflux pumps. These results reinforce organotin(IV) complexes as promising antibacterial agents, and many of these complexes, if associated with antibiotics, can act as potential adjuvant antibiotic candidates.

A remote C-C bond cleavage-enabled skeletal reorganization: Access to medium-/large-sized cyclic alkenes

Although great success has been achieved in selective C-C bond cleavage via the intramolecular radical remote migration process of several carbon-based groups, the development of the radical-based remote vinyl migration process remains a formidable challenge because of the energetically unfavorable process. To address this problem, we report here, for the first time, a novel C-C bond reorganization strategy via an unprecedented radical 1,3-, 1,4-, or 1,5-vinyl migration triggered by various types of fluoroalkylation of alkenes for the efficient realization of 1,2-fluoroalkylalkenylation reaction. This strategy provides an expedient and broadly applicable platform to access skeletally and functionally diverse fluoroalkyl-containing medium- and large-sized cyclic alkenes with excellent chemo-, regio-, and stereoselectivity. The broad substrate scope, which covers distinctly electron-neutral or electron-deficient alkenyl migrating groups and various fluoroalkyl radical precursors, the excellent functional group tolerance, the remarkable selectivity, and the operational simplicity, as well as versatile transformations of the products, make this approach practical and attractive.

Asymmetric Total Synthesis of (-)-Astakolactin and Confirmation of Its Stereostructure

The originally proposed structure of astakolactin was revised, and an asymmetric total synthesis of the newly proposed structure was achieved. The key transformations in the synthesis were a Johnson-Claisen rearrangement, an asymmetric Mukaiyama aldol reaction, and a Mitsunobu-type cyclodehydration. The spectroscopic data and specific rotation of the compound obtained matched well with those reported for naturally occurring astakolactin.

Yttrium-Catalyzed Intramolecular Hydroalkoxylation/Claisen Rearrangement Sequence: Efficient Synthesis of Medium-Sized Lactams

An efficient yttrium-catalyzed intramolecular hydroalkoxylation/Claisen rearrangement sequence has been achieved, thus enabling facile access to a diverse array of valuable medium-sized lactams. Furthermore, a mechanistic rationale for this novel cascade reaction is well supported by a variety of control experiments.

Total Synthesis of Thailandepsin B, a Potent HDAC Inhibitor Isolated from a Microorganism

Thailandepsin B, a bicyclic depsipeptide histone deacetylase inhibitor, was efficiently synthesized in 51% overall yield in eight steps, starting from commercially available D-norleucine methyl ester and known (S,E)-3-(4-methoxybenzyloxy)-7-(tritylthio)hept-4-enoic acid. The method features a convergent approach in which the corresponding seco-acid, a key precursor in macrolactonization, is directly assembled through the condensation of a D-allo-isoleucine-D-cysteine-containing segment with a D-norleucine-containing segment.

Intramolecular macrolactonization, photophysical and biological studies of new class of polycyclic pyrrole derivatives

Herein, we report an efficient synthesis of N-substituted pyrrole derivatives and their application to construct macrocyclic oxazocinone.

Structurally diverse arene-fused ten-membered lactams accessed via hydrolytic imidazoline ring expansion

A facile approach to hitherto unknown ten-membered ring systems via a hydrolytic imidazoline ring expansion (HIRE) is described. Medium-sized rings are scarce in the contemporary medicinal chemistry toolbox. The new HIRE strategy broadens the arsenal of synthetic methods to obtain such scaffolds.

Copper-catalyzed construction of eight-membered rings via oxidative ring expansion and intermolecular cyclization sequencing of indoles with amidines: efficient synthesis of benzo[1,3,5]triazocin-6(5H)-ones

Novel and efficient synthesis of eight-membered nitrogen heterocycles in one pot by copper-catalyzed oxidative ring expansion and intermolecular cyclization sequencing of indoles with amidines has been described.

Radical aryl migration enables diversity-oriented synthesis of structurally diverse medium/macro- or bridged-rings

Medium-sized and medium-bridged rings are attractive structural motifs in natural products and therapeutic agents. Due to the unfavourable entropic and/or enthalpic factors with these ring systems, their efficient construction remains a formidable challenge. To address this problem, we herein disclose a radical-based approach for diversity-oriented synthesis of various benzannulated carbon- and heteroatom-containing 8-11(14)-membered ketone libraries. This strategy involves 1,4- or 1,5-aryl migration triggered by radical azidation, trifluoromethylation, phosphonylation, sulfonylation, or perfluoroalkylation of unactivated alkenes followed by intramolecular ring expansion. Demonstration of this method as a highly flexible tool for the construction of 37 synthetically challenging medium-sized and macrocyclic ring scaffolds including bridged rings with diverse functionalities and skeletons is highlighted. Some of these products showed potent inhibitory activity against the cancer cell or derivative of human embryonic kidney line in preliminary biological studies. The mechanism of this novel strategy is investigated by control experiments and DFT calculations.

Enantioselective total synthesis of naturally occurring eushearilide and evaluation of its antifungal activity

The asymmetric total synthesis of a newly proposed structure of (3S,16E,20E,23S)-(+)-eushearilide was achieved primarily through an asymmetric Mukaiyama aldol reaction, Schlosser-modified Wittig reaction and 2-methyl-6-nitrobenzoic anhydride-mediated macrolactonization. Based on detailed spectroscopic analyses, the obtained synthetic compound was found to be identical to natural eushearilide. Therefore, we were able to determine the true structure of eushearilide. Moreover, the synthetic compound was found to exhibit significant in vitro antifungal activity against various fungi and bacteria.

Organoselenium and DMAP co-catalysis: regioselective synthesis of medium-sized halolactones and bromooxepanes from unactivated alkenes

A co-catalytic system has been developed for the synthesis of medium-sized halolactones and bromooxepanes possessing high transannular strain. Mechanistic studies reveal that quaternary selenium intermediate is involved in catalysis.

A novel oxy-oxonia(azonia)-cope reaction: serendipitous discovery and its application to the synthesis of macrocyclic musks

This brief review, including new experimental results, is the summary of a talk at the GDCh conference 'flavors & fragrances 2013' in Leipzig, Germany, 11th-13th September, 2013. Musk odorants are indispensable in perfumery to lend sensuality to fine fragrances, a nourishing effect to cosmetics, and a comforting feeling to laundry. We have recently found serendipitously a new oxy-oxonia-Cope rearrangement. In this account, we review the background of oxonia-sigmatropic rearrangements and the discovery of this novel reaction. Special attention is focused on the versatile lactone and lactam formation reactions via [n+4] ring enlargement and the macrocyclization in the synthesis of new macrocyclic musks. The synthesized structures provide new insights into the structure-odor relationships of musks.