Asymmetric synthesis of cytotoxic sponge metabolites R-strongylodiols A and B and an analogue

Polyacetylene glucosides from the florets of Carthamus tinctorius and their anti-inflammatory activity

Five previously undescribed polyacetylene glucosides, namely, four C₁₀- and one C₁₄-acetylenes, together with three known analogues, were isolated from the florets of Carthamus tinctorius L. The structures of these novel compounds were elucidated to be (5R)-5-acetoxy-8,10,12-tetradecatriyne-1-O-β-D-glucopyranoside, (2Z)-decaene-4,6,8-triyne-1-O-β-D-glucopyranoside, (8Z)-1-[(3-O-β-D-glucosyl)-isovaleroyloxy]-8-decaene-4,6-diyne, (8Z)-decaene-1-isovaleroyloxy-4,6-diyne-10-O-β-D-glucopyranoside, and (2E,8E)-decadiene-4,6-diyne-1-O-β-D-glucopyranoside via spectroscopic and chemical methods. All of the isolated compounds were tested for cytotoxicity against cancer cell lines, antibacterial activity, and inhibitory effects on the lipopolysaccharide (LPS)-induced nitric oxide (NO) production. The results showed that (5R)-5-acetoxy-8,10,12-tetradecatriyne-1-O-β-D-glucopyranoside significantly inhibited LPS-induced NO production in RAW264.7 cells in a dose-dependent manner.

Stereoselective total synthesis and structural revision of the diacetylenic diol natural products strongylodiols H and I

The stereoselective total synthesis of strongylodiol H and I has been accomplished. The synthetic procedure comprised the stereoselective reduction of a ketone functionality in an ene-yne-one employing CBS as a catalyst and a Cadiot-Chodkiewicz coupling reaction as the key reaction steps. A common aldehyde intermediate has been used for the synthesis of both strongylodiols.

Efficient Access to Multifunctional Trifluoromethyl Alcohols through Base-Free Catalytic Asymmetric C-C Bond Formation with Terminal Ynamides

The asymmetric addition of terminal ynamides to trifluoromethyl ketones with a readily available chiral zinc catalyst gives CF3 -substituted tertiary propargylic alcohols in up to 99 % yield and 96 % ee. The exclusion of organozinc additives and base as well as the general synthetic utility of the products are key features of this reaction. The value of the β-hydroxy-β-trifluoromethyl ynamides is exemplified by selective transformations to chiral Z- and E-enamides, an amide, and N,O-ketene acetals. The highly regioselective hydration, stereoselective reduction, and hydroacyloxylation reactions proceed with high yields and without erosion of the ee value of the parent β-hydroxy ynamides.

Asymmetric total synthesis of (+)-O-methylasparvenone, a rare nitrogen-free serotonin 2C receptor antagonist

The first enantioselective synthesis of the fungal metabolite (+)-O-methylasparvenone is reported.

Chiral alkynylcarbinols from marine sponges: asymmetric synthesis and biological relevance

Covering: up to March 2014. Previous review on the topic: B. W. Gung, C. R. Chim., 2009, 12, 489-505. Chiral α-functional lipidic propargylic alcohols extracted from marine sponges, in particular of the pacific genus Petrosia, constitute a class of acetylenic natural products exhibiting remarkable in vitro biological activities, especially anti-tumoral cytotoxicity. These properties, associated to functionalities that are uncommon among natural products, have prompted recent projects on asymmetric total synthesis. On the basis of a three-sector structural typology, three main sub-types of secondary alkynylcarbinols (with either alkyl, alkenyl, or alkynyl as the second substituent) can be identified as the minimal pharmacophoric units. Selected natural products containing these functionalities have been targeted using previously known or on purpose-designed procedures, where the stereo-determining step can be: (i) a C-C bond forming reaction (e.g. the Zn-mediated addition of alkynyl nucleophiles to aldehydes in the presence of chiral aminoalcohols), (ii) a functional layout (e.g. the asymmetric organo- or metallo-catalytic reduction of ynones), or (iii) an enantiomeric resolution (e.g. a lipase-mediated kinetic resolution via acetylation). The promising medicinal importance of these targets is finally surveyed, and future investigation prospects are proposed, such as: (i) further total synthesis of known or future extraction products; (ii) the synthesis of non-natural analogues, with simpler lipophilic environments of the alkynylcarbinol-based pharmacophoric units; (iii) the variation and optimization of both the pharmacophoric units and their lipophilic environment; and (iv) investigations into the biological mode of action of these unique structures.

Asymmetric reduction of diynones and the total synthesis of (S)-panaxjapyne A

The asymmetric transfer hydrogenation of a series of diynones has been achieved in high conversion and enantiomeric induction. When R(1) is a phenyl group, a competing alkyne reduction takes place; however, when R(1) is an alkyl group, this side-reaction is not observed. The application of the reduction to the total synthesis of the natural product (S)-panaxjapyne A in high enantiomeric excess is described.

Identification of chiral alkenyl- and alkynylcarbinols as pharmacophores for potent cytotoxicity

Illumination by acetylene: Systematic structural variations in a series of archetypal acetylenic lipids derived from the naturally occurring (S,E)-icos-4-en-1-yn-3-ol allowed the discovery of a series of 3R-like 1,4-di-unsaturated carbinol units with a significant and systematic enantiomeric effect on cytotoxicity.

Enantioselective ProPhenol-catalyzed addition of 1,3-diynes to aldehydes to generate synthetically versatile building blocks and diyne natural products

A highly enantioselective method for the catalytic addition of terminal 1,3-diynes to aldehydes was developed using our dinuclear zinc ProPhenol (1) system. Furthermore, triphenylphosphine oxide was found to interact synergistically with the catalyst to substantially enhance the chiral recognition. The generality of this catalytic transformation was demonstrated with aryl, alpha,beta-unsaturated and saturated aldehydes, of which the latter were previously limited in alkynyl zinc additions. The chiral diynol products are also versatile building blocks that can be readily elaborated; this was illustrated through highly selective trans-hydrosilylations, which enabled the synthesis of a beta-hydroxyketone and enyne. Additionally, the development of this method allowed for the rapid total syntheses of several biologically important diynol-containing natural products.

Spirodiepoxides: synthesis of epoxomicinoids

Three closely related analogues of epoxomicin have been synthesized. Allene-derived spirodiepoxides were key intermediates. Spirodiepoxide formation and stereochemical dependence on solvent, oxidant, and allene structure were cataloged. The facial selectivity of the first epoxidation of 1,3-disubstituted and trisubstituted allenes was found to be >20:1 with dimethyldioxirane in chloroform. For stereogenic allenes, the facial selectivity of the second oxidation is dependent primarily on allene structure and secondarily on solvent and oxidant. For the acyclic systems evaluated this ratio was as high as 8:1. A conformational model is advanced to account for these observations.

The Enantioselective Addition of Alkyne Nucleophiles to Carbonyl Groups

Over the past decade, large strides have been achieved in the invention of methods for the direct enantioselective addition of alkynes and metal alkynylide nucleophiles into prochiral aldehydes, ketones, and imines. This review highlights and compares the available methods for these transformations.

Addition of alkynes to aldehydes and activated ketones catalyzed by rhodium-phosphine complexes

A mixture of 2-(di-tert-butylphosphino)biphenyl and dicarbonylacetonato rhodium(I) provides an effective catalyst system for the addition of alkynes to aldehydes and activated ketones. In contrast to the more common zinc-catalyzed processes, enolizable 1,2-dicarbonyls are excellent substrates for these rhodium-catalyzed additions. This reaction allows for the formation of propargylic alcohols under mild conditions, tolerating many functional groups (such as carboxylic acids) that are incompatible with other methods. Little selectivity was observed in cases of unsymmetrical 1,2-diketones. Addition of alkynes to aldehydes with an adjacent chirality center usually provides the Felkin addition product with excellent selectivity in some cases. Studies on the catalyst structure show that both the beta-diketonate and a carbon monoxide ligand appear to be bound to the active catalyst. The use of chiral phosphines to induce asymmetry in the propargyl alcohol products provided low enantioselectivity, which may be due to the phosphine having a distal relationship to the reacting centers. Modification of other ligands, such as the beta-diketonate, appears to be a more promising avenue for the development of an enantioselective variant.

Dimethylzinc-Mediated, Enantioselective Synthesis of Propargylic Amines

A one-pot, enantioselective synthesis of N-aryl propargylic amines, using alkynylation reagents obtained from dimethylzinc and terminal acetylenes in combination with various aldehydes and o-methoxyaniline as starting materials, has been developed. Enantiopure beta-amino alcohols derived from norephedrine were used as non-covalent chiral auxiliaries, both in stoichiometric or substoichiometric amount. After optimization, propargylic amines were obtained in good to high yields (up to 93%) and with moderate to high enantiomeric excesses (up to 97% ee). The possibility to recover the chiral auxiliary after the reaction was demonstrated.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2005 is provided. In a similar vein to earlier installments in this series, the emphasis is on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments.

Advances in Lewis Acid Controlled Carbon−Carbon Bond-Forming Reactions Enable a Concise and Convergent Total Synthesis of Bullatacin

Lewis acids control regioselectivity in the alkylation of epi-chlorohydrin and the stereochemistry of an alkyne addition to set the C24/C23 anti relationship. These advances facilitate an efficient total synthesis of bullatacin in 13.3% overall yield from commercial starting materials.

Synthesis of Naturally Occurring Polyynes

Over the past fifty years, hundreds of polyyne compounds have been isolated from nature. These often unstable molecules are found in sources as common as garden vegetables and as obscure as bacterial cultures. Naturally occurring polyynes feature a wide range of structural diversity and display an equally broad array of biological properties. Early synthetic efforts relied primarily on Cu-catalyzed, oxidative acetylenic homo- and heterocoupling reactions to assemble the polyyne framework. The past 25 years, however, have witnessed a renaissance in the field of polyyne natural product synthesis: transition-metal-catalyzed alkynylation reactions and asymmetric transformations have combined to substantially expand access to natural polyynes. This Review recounts these synthetic achievements and also highlights both the natural source(s) and biological relevance for many of these compounds.