A "shortcut" Mosher ester method to assign configurations of stereocenters in nearly symmetric environments. Fluorous mixture synthesis and structure assignment of petrocortyne A

Synthetic studies towards volvalerenol A: access to a fully functionalized cycloheptane framework in an asymmetric fashion through the exploitation of C2-symmetry

The enantioselective synthesis of a fully functionalized cycloheptane core of the naturally occurring triterpenoid volvalerenol A, exhibiting pseudo-C2 symmetry, was achieved. Enantioselective enzymatic desymmetrization (EED), asymmetric methallylation, and reductive ring opening of an cyclopropane overbred intermediate were the key reactions to access the cycloheptanone core. Further synthetic manipulations, via a unique "MPV" (Meerwein-Ponndorf-Verley) type reductive ring-opening of an epoxide and other synthetic transformations, afforded two fully functionalized cycloheptane frameworks of the target molecule.

Expanded Application of Piper nigrum: Guided Isolation of Alkaloids with Inhibitory Activities of AChE/BuChE and Aβ Aggregation

Piper nigrum is a popular crop that can be used as seasoning or as an additive but its active ingredients also have an effect on the nervous system. Nineteen new amide alkaloids (1a/1b, 2-5, 6a/6b, 7, 8a/8b, 9, 10a/10b, 11a-11b, 12-14) were isolated from P. nigrum, guided by inhibitory activity of AChE and LC-MS/MS based on GNPS. The configurations were determined by extensive spectral analysis, Bulkiness rule, and NMR calculations. The inhibitory activities of AChE/BuChE and Aβ aggregation were tested, and the results showed compounds 2, 7, and 12 had significant inhibitory activities. These components were identified in the crude fraction and their relative quantities were tested, which suggested that compound 2 was the index component in the active site from P. nigrum.

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.

On terminal alkynylcarbinols and derivatization thereof

The chemistry of three prototypes of secondary alkynylcarbinols (ACs), recently highlighted as challenging targets in anti-tumoral medicinal chemistry, is further documented by results on n-alkyl, alkynyl and alkenyl representatives. The N-naphthyl carbamate of an n-butyl-AC is thus characterized by X-ray crystallography. A novel dialkynylcarbinol (DAC) with synthetic potential is described, namely the highly dissymmetrical triisopropylsilyl-protected version of diethynylmethanol. The latter is shown to act as a dipolarophile in a selective Huisgen reaction with benzyl azide under CuAAC click conditions, giving an alkenyl-AC, where the alkene unsaturation is embedded in a 1,4-disubstituted 1,2,3-triazole ring, as confirmed by X-ray crystallography.

Systematic comparison of sets of (13)C NMR spectra that are potentially identical. Confirmation of the configuration of a cuticular hydrocarbon from the cane beetle Antitrogus parvulus

A systematic process is introduced to compare (13)C NMR spectra of two (or more) candidate samples of known structure to a natural product sample of unknown structure. The process is designed for the case where the spectra involved can reasonably be expected to be very similar, perhaps even identical. It is first validated by using published (13)C NMR data sets for the natural product 4,6,8,10,16,18-hexamethyldocosane. Then the stereoselective total syntheses of two candidate isomers of the related 4,6,8,10,16-pentamethyldocosane natural product are described, and the process is applied to confidently assign the configuration of the natural product as (4S,6R,8R,10S,16S). This is accomplished even though the chemical shift differences between this isomer and its (16R)-epimer are only ±5-10 ppb (±0.005-0.01 ppm).

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.

Bare-minimum fluorous mixture synthesis of a stereoisomer library of 4,8,12-trimethylnonadecanols and predictions of NMR spectra of saturated oligoisoprenoid stereoisomers

All four diastereomers of a typical saturated oligoisoprenoid, 4,8,12-trimethylnonadecanol, are made by an iterative three-step cycle with the aid of traceless thionocarbonate fluorous tags to encode configurations. The tags have a minimum number of total fluorine atoms, starting at zero and increasing in increments of one. With suitable acquisition and data processing, each diastereomer exhibits characteristic chemical shifts of methyl resonances in its (1)H and (13)C NMR spectra. Together, these shifts provide a basis to predict the appearance of the methyl region of the spectrum of every stereoisomer of higher saturated oligoisoprenoids.

Minimal fluorous tagging strategy that enables the synthesis of the complete stereoisomer library of SCH725674 macrolactones

Four mixtures of four fluorous-tagged quasiisomers have been synthesized, demixed, and detagged to make all 16 stereoisomers of the macrocyclic lactone natural product Sch725674. A new bare-minimum tagging pattern needs only two tags--one fluorous and one nonfluorous--to encode four isomers. The structure of Sch725674 is assigned as (5R,6S,8R,14R,E)-5,6,8-trihydroxy-14-pentyloxacyclotetradec-3-en-2-one. Various comparisons of spectra of 32 lactones (16 with tags, 16 without) and 16 ester precursors (8 with tags, 8 without) provide insights into when and why related compounds have the same or different spectra.

Sponge chemical diversity: from biosynthetic pathways to ecological roles

Since more than 50 years, sponges have raised the interest of natural product chemists due to the presence of structurally original secondary metabolites. While the main objective were first to discover new drugs from the Sea, a large number of interrogations arose along with the isolation and structure elucidations of a wide array of original architectures and new families of natural products not found in the terrestrial environment. In this chapter, we focus on the results obtained during this period on the following questions. A preliminary but still unresolved issue to be addressed will be linked to the role of the microbiota into the biosynthesis of these low-weight compounds. Our knowledge on the biosynthetic pathways leading to plant secondary metabolites is now well established, and this background will influence our comprehension of the biosynthetic events occurring in a sponge. But is the level of similarity between both metabolisms so important? We clearly need more experimental data to better assess this issue. This question is of fundamental interest because sponges have a long evolutionary history, and this will allow a better understanding on the transfer of the genetic information corresponding to the biosynthesis of secondary metabolites. After the how, the why! The question of the ecological role of these metabolites is also of high importance first not only because they can serve as synapomorphic characters but also because they may represent chemical cues in the water environment. Even if most of these compounds are considered as defensive weapons for these sessile invertebrates, they may also be linked to physiological characters as the reproduction. Finally, a metabolomic approach can appear as a complementary tool to give additional information on the sponge fitness. All the new developments in molecular biology and bioanalytical tools will open the way for a better comprehension on the complex field of sponge secondary metabolites.

Long-range shielding effects in the (1)H NMR spectra of Mosher-like ester derivatives

The relative magnitudes of the chemical shift differences (Deltadeltas) in the two diastereomers of menthyl esters of known chiral derivatizing agents (CDAs) were compared to those of the alpha-methoxy-alpha-trifluoromethyl-1-naphthylacetyl (MTN((1))A) analogues I. Discrimination of the terminal diastereotopic methyl resonances in esters of the homologous, symmetrical carbinols II was evaluated. Remarkably, the methyls differed in the MTN((1))A esters III even when n = 15; an unexpected crossover in the sign of the Deltadelta values was also observed.

Assignment of the structure of petrocortyne A by mixture syntheses of four candidate stereoisomers

Two different mixture synthesis routes have been used to make the four stereoisomers of petrocortyne A. A first quick and dirty route provided a mixture of the four isomers in nonselective fashion. Mosher and 2-naphthylmethoxyacetic acid (NMA) ester methods were developed to identify the components, and the mixture was partially resolved on analytical chiral HPLC to give the two pure enantiomers of petrocortyne A and the racemate of its diastereomer. A second fluorous mixture synthesis produced all four isomers of petrocortyne A in individual pure form. Comparison of spectra of Mosher derivatives of the synthetic isomers with two supposedly different natural products showed that both natural samples were instead identical and had the (3S,14S) configuration. Likewise, petrocortynes B, D, and F-H are (3S,14S) and petrocortyne D is (3R,14S). Having access to all possible candidate isomers of both petrocortyne A and its Mosher derivatives provided a secure structure assignment not so much because one of the isomers matched the natural product, but because all of the other isomers did not.

Fluorous Mixture Synthesis of Four Stereoisomers of the C21-C40 Fragment of Tetrafibricin

Four stereoisomers of the C21-C40 fragment are synthesized in a single exercise with the aid of fluorous tagging to encode configurations at C37 and C33. After demixing and detagging, the isomers were found to have substantially identical (1)H NMR spectra. However, there were some small but reliable differences in their (13)C NMR spectra.

Making mixtures to solve structures: structural elucidation via combinatorial synthesis

A domino Horner-Wadsworth-Emmons olefination strategy has been used to prepare homologous series of (polyen)ones, and through combinatorial elaboration, corresponding families of highly branched hydrocarbons. Gas chromatography-mass spectrometry of the mixtures has enabled the rapid and unambiguous identification of several highly branched alkanes of geochemical importance. This is the first example of the use of combinatorial synthesis for the elucidation of structural connectivity.

Olefin metathesis-iodoetherification-dehydroiodination strategy for spiroketal subunits of polyether antibiotics

The convergent synthesis of two pentacyclic analogues of the polyether monensin A is described. Although different with respect to the configuration of the alcohol at the 3 position of the six-membered ring of the spiroketal subunit, the configuration at the acetal center in both structures is unchanged and is consistent with the anomeric effect. The key synthetic steps are the coupling of two complex segments via an olefin metathesis, and the subsequent conversion of a dihydroxyalkene to the spiroketal through an iodoetherification-dehydroiodination sequence. The compatibility of these transformations with a variety of functional groups makes the overall strategy appropriate for highly substituted frameworks.