7.1',8.3'- and 7.3',8.5'-Connected Bicyclo[3.2.1]octanoids and Oxabicyclo[3.2.2]nonane-Type Neolignans from Ocotea aciphylla: Structures and Absolute Configurations

The phytochemical investigation of the leaves and trunk bark of a specimen of Ocotea aciphylla collected in the southern portion of the Amazon forest led to the isolation of an oxabicyclo[3.2.2]nonane-type neolignan and 15 bicyclo[3.2.1]octanoid neolignans, 14 of which are unreported compounds (2-15), including one with an unusual oxidation pattern of the side chain at C-1' and two rare 7.1',8.3'-connected bicyclo[3.2.1]octanoid derivatives. Their structures and relative configurations were determined by extensive spectrometric analysis based on 1D- and 2D-NMR spectroscopy and HRESIMS data, while their absolute configurations were unambiguously assigned using electronic and vibrational circular dichroism data assisted by density functional theory calculations. Additionally, known sesquiterpenes, phenylpropanoids, and phytosterols were also isolated.

Absolute configuration reassignment of nectamazin A: Implications to related neolignans

The ability of nature to produce structurally complex molecules makes the determination of the absolute configuration of natural products a challenging task. Although extensive NMR analysis generally allows for the reliable assignment of relative configurations, the assignments of absolute stereochemistry are commonly performed by empirical comparisons of optical rotation (OR) and/or electronic circular dichroism (ECD) data obtained for related molecules. Such an approach, however, may lead to misassignments and consequent error propagations. Herein, we present the case of the bicyclo(3.2.1)octane neolignan named (+)-nectamazin A. This compound was first reported in 2009 from Nectandra amazonum Nees. (Lauraceae) and had its absolute configuration determined as 7R,8S,3'S,4'R,5'S by means of experimental ECD spectroscopy. Our chemical studies on Ocotea aciphylla (Lauraceae) led to the isolation of (+)-nectamazin A. The extensive analysis of OR, ECD, and vibrational CD data aided by quantum chemical calculations, however, indicated (+)-nectamazin A to have the 7S,8R,3'R,4'S,5'R absolute configuration, in conflict with the configuration reported in the literature. The cause of the original incorrect assignment of (+)-nectamazin A derives from the direct comparison of experimental OR and ECD data obtained for structurally related molecules with different chromophoric systems. As an alternative, VCD spectroscopy is presented as a more reliable and sensitive technique to stereochemical investigations of bicyclo(3.2.1)octane neolignans.

New bicyclic [3.2.1] octane neolignan derivative from Aniba firmula

bicyclic [3.2.1] octane neolignans have garnered increasing interest, because of their unique structural features and biological activities. This study describes the isolation and identification of a new bicyclic octane neolignan 1 obtained through fractionation of the crude extract of the stem of Aniba firmula (Lauraceae family). The structure of bicyclic octane neolignan 1 was determined through NMR analysis and mass spectrometry data.

Covalent Triazine Framework Nanoparticles via Size-Controllable Confinement Synthesis for Enhanced Visible-Light Photoredox Catalysis

For metal-free, organic conjugated polymer-based photocatalysts, synthesis of defined nanostructures is still highly challenging. Here, we report the formation of covalent triazine framework (CTF) nanoparticles via a size-controllable confined polymerization strategy. The uniform CTF nanoparticles exhibited significantly enhanced activity in the photocatalytic formation of dibenzofurans compared to the irregular bulk material. The optoelectronic properties of the nanometer-sized CTFs could be easily tuned by copolymerizing small amounts of benzothiadiazole into the conjugated molecular network. This optimization of electronic properties led to a further increase in observed photocatalytic efficiency, resulting in total an 18-fold enhancement compared to the bulk material. Full recyclability of the heterogeneous photocatalysts as well as catalytic activity in dehalogenation, hydroxylation and benzoimidazole formation reactions demonstrated the utility of the designed materials.

Chemical Structures of Lignans and Neolignans Isolated from Lauraceae

Lauraceae is a good source of lignans and neolignans, which are the most chemotaxonomic characteristics of many species of the family. This review describes 270 naturally occurring lignans and neolignans isolated from Lauraceae.

Recent advances in research on lignans and neolignans

Covering: 2009 to 2015Lignans and neolignans are a large group of natural products derived from the oxidative coupling of two C6-C3 units. Owing to their biological activities ranging from antioxidant, antitumor, anti-inflammatory to antiviral properties, they have been used for a long time both in ethnic as well as in conventional medicine. This review describes 564 of the latest examples of naturally occurring lignans and neolignans, and their glycosides in some cases, which have been isolated between 2009 and 2015. It comprises the data reported in more than 200 peer-reviewed articles and covers their source, isolation, structure elucidation and bioactivities (where available), and highlights the biosynthesis and total synthesis of some important ones.

Betaine mediated synthesis of annulated dihydrofurans from oxobis(methylthio)ketene acetals and N-butyl-N'-methyl ethane-1,2-diamine as precursors via NHC elimination

First in situ generation of a betaine intermediate has been developed using two new precursors oxobis(methylthio)ketene acetals and N-butyl-N'-methyl ethane-1,2-diamine for the synthesis of annulated dihydrofurans. This protocol adds a new dimension for the formation of annulated dihydrofurans through a series of selective consecutive formation of C-C and C-O bonds after reacting with enone rings. This in situ generated betaine intermediate corresponds to deoxy-Breslow intermediates in the reaction via elimination of NHC.

Photocatalytic synthesis of dihydrobenzofurans by oxidative [3+2] cycloaddition of phenols

We report a protocol for oxidative [3+2] cycloadditions of phenols and alkenes applicable to the modular synthesis of a large family of dihydrobenzofuran natural products. Visible-light-activated transition metal photocatalysis enables the use of ammonium persulfate as an easily handled, benign terminal oxidant. The broad range of organic substrates that are readily oxidized by photoredox catalysis suggests that this strategy may be applicable to a variety of useful oxidative transformations.

2-Arylbenzofuran Neolignans from the Bark of Nectandra purpurascens (Lauraceae)

The toluene extract of the stem bark of N. purpurascens (R&P) Mez. contained a large amount of these 2-arylbenzofurans including methyl 7-methoxy-2-(3,4-methylenedioxyphenyl)benzofuran-5-carboxylate (2), a new benzofuran, along with three known compounds identified as kumatakenin (1), 5-(2-propenyl)-7-methoxy-2-(3,4-methylenedioxyphenyl)benzofuran (3), and egonoic acid (4). The structure of the new compound was established on the basis of spectroscopic analysis.