This review summarises known sesquiterpenes whose biosyntheses proceed through the intermediate germacrene A. First, the occurrencea nd biosynthesis of germacrene Ai nN ature and its peculiar chemistry will be highlighted , followed by ad iscussion of 6-6 and 5-7 bicyclic compounds and their more complex derivatives. For each compound the absolutec onfiguration, if it is known,a nd the reasoning for its assignment is presented. 2. Germacrene A 2.1. Occurrence in Nature (À)-Germacrene A(1,S cheme 2) was first isolated in 1970 from the gorgonian Eunicea mammosa. [7] Its absolute configuration was established as (S)-(À)-1 through its Cope rearrangement to (+ +)-b-elemene (2)f or which the configurational assignment was performed by chemical correlation of (À)-elemol(3)t o (À)-2. [8, 9] Compound (À)-1 is also believed to occur in the soft coral Lobophytum, [10] andi st he alarm pheromone of the aphid Terioaphis maculata. [11, 12] In the course of this work it was noticed that the optical rotation([ a] D 25 = À26.8, c 1.0, CCl 4)w as significantly highert han initially reported ([a] D 25 = À3.2, c 14.4, CCl 4), [7] which is explainableb yapartial rearrangement of purified (À)-1 to (+ +)-2,o ra lternatively, 1 isolated from E. mammosa was not enantiomerically pure. However,the opticalrotation of (+ +)-2 ([a] D 25 =+15.1, neat) reported in this initial study [7] matches the reported value for (À)-2 ([a] D 25 = À15.8, c 0.50, CHCl 3)o btainedb yC ope rearrangement of (+ +)-1, [13] thus disfa-Scheme1.Te rpene cyclisation modes for FPP. Scheme2.Structure of 1 and its absolute configurationbyc hemicalcorrelation.

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Different mechanisms for the cyclisation of farnesyl pyrophosphate to patchoulol by the patchoulol synthase are discussed in the literature. They are based on isotopic labelling experiments, but the results from these experiments are contradictory. The present work reports on a reinvestigation of patchoulol biosynthesis by isotopic labelling experiments and computational chemistry. The results are in favour of a pathway through the neutral intermediates germacrene A and α-bulnesene that are both reactivated by protonation for further cyclisation steps, while previously discussed intra- and intermolecular hydrogen transfers are not supported. Furthermore, the isolation of the new natural product (2S,3S,7S,10R)-guaia-1,11-dien-10-ol from patchouli oil is reported.

A systematic computational study addressing the entire chemical space of guaianes in conjunction with an analysis of all known compounds shows that 1,3‐hydride shifts are rare events in guaiane biosynthesis. As demonstrated here, 1,3‐hydride shifts towards guaianes can only be realized for two stereochemically well defined out of numerous possible stereoisomeric skeletons. One example is given by the mechanism of guaia‐4(15)‐en‐11‐ol synthase from California poplar, an enzyme that yields guaianes with unusual stereochemical properties. The general results from DFT calculations were experimentally verified through isotopic‐labeling experiments with guaia‐4(15)‐en‐11‐ol synthase.

The product of a terpene synthase from Streptomyces lincolnensis has been identified as the new natural product isoishwarane. The enzyme mechanism was studied by isotopic labelling experiments and site-directed mutagenesis.

The changes of corrosion potential (E corr ) of metals immersed in seawater were investigated with electrochemical technology and epifluoresence microscopy. In natural seawater, changes of E corr were determined by the surface corrosion state of the metal. E corr of passive metals exposed to natural seawater shifted to noble direction for about 150 mV in one day and it didn't change in sterile seawater. The in-situ observation showed that biofilms settled on the surfaces of passive metals when E corr moved in noble direction. The bacteria number increased on the metal surface according to exponential law and it was in the same way with the ennoblement of E corr . The attachment of bacteria during the initial period played an important role in the ennoblement of E corr and it is believed that the carbohydrate and protein in the biofilm are reasons for this phenomenon. The double layer capacitance (C dl ) of passive metals decreased with time when immersed in natural seawater, while remained almost unchanged in sterile seawater. The increased thickness and reduced dielectric constant of C dl may be reasons.

Pathogen infection often leads to the enhanced formation of specialized plant metabolites that act as defensive barriers against microbial attackers. In this study, we investigated the formation of potential defense compounds in roots of the Western balsam poplar (Populus trichocarpa) upon infection with the generalist root pathogen Phytophthora cactorum (Oomycetes). P. cactorum infection led to an induced accumulation of terpenes, aromatic compounds, and fatty acids in poplar roots. Transcriptome analysis of uninfected and P. cactorum-infected roots revealed a terpene synthase gene PtTPS5 that was significantly induced upon pathogen infection. PtTPS5 had been previously reported as a sesquiterpene synthase producing two unidentified sesquiterpene alcohols as major products and hedycaryol as a minor product. Using heterologous expression in Escherichia coli, enzyme assays with deuterium-labeled substrates, and NMR analysis of reaction products, we could identify the major PtTPS5 products as (1S,5S,7R,10R)-guaia-4(15)-en-11-ol and (1S,7R,10R)-guaia-4-en-11-ol, with the former being a novel compound. The transcript accumulation of PtTPS5 in uninfected and P. cactorum-infected poplar roots matched the accumulation of (1S,5S,7R,10R)-guaia-4(15)-en-11-ol, (1S,7R,10R)-guaia-4-en-11-ol, and hedycaryol in this tissue, suggesting that PtTPS5 likely contributes to the pathogen-induced formation of these compounds in planta.

Twenty-one structurally diverse diterpenoids (1–21), wherein 1, 2, and 4 represented unprecedented architectures, were isolated from Isodon pharicus.