Structure Elucidation and Synthesis of New Secondary Metabolites from Liverworts and Microorganisms and Investigation of their Biogenesis

Abstract

Chemical investigation of Fossombronia angulosa resulted in the identification of C11 hydrocarbons, dictyopterene A, ectocarpene, and dictyotene, known as brown algal pheromones, along with the new pentylbenzene. Absolute configurations and enantiomeric compositions were assigned upon enantioselective GC using 2,3-Ac-6-TBDMS-beta-cyclodextrin. In addition to the known 7-prenylindole and 6-prenylindole, 3-chloro-7-prenylindole and 3-chloro-6-prenylindole were identified from Riccardia chamedryfolia and their structures were confirmed by partial synthesis. Furthermore, the novel oxazinoindole alkaloid chamedryfolian was identified using NMR techniques and chemical correlation. Investigation of Corsinia coriandrina resulted in the identification of several new 4-methoxystyrenes showing isothiocyanate (coriandrins), isocyanate (corsinians), isocyanide (corsicillins), N-formamide (tuberines), S-methyl thiocarbamate (corsiandrens), N-acetyl-S-methyl thiocarbamate (corsiandrenins), and S,S-dimethyl iminodithiocarbonate (O-methyltridentatols) moieties attached to a (Z)- or (E)-configured 2-(4-methoxyphenyl)ethenyl skeleton, as well as 5-(4-methoxyphenyl)-2-methylthio-1,3-thiazol (O-methyl-tridentatol C). Structures were deduced from NMR, FTIR or EIMS investigations and confirmed by independent synthesis. Furthermore, the synthesis of dehydroniranin A from Glycosmis cyanocarpa was carried out for the first time. In addition, corsifuran A, corsifuran B and corsifuran C with an unusual 2-(4-methoxyphenyl)-5-methoxy-benzo[b]furan skeleton, (E)- and (Z)-corsistilbenes, dimethyllunularin and its 6-hydroxy-derivative were identified as new natural products from Corsinia coriandrina by comparison with synthetic samples. The absolute configuration of (R)-(–)-corsifuran A was established by enantioselective synthesis in cooperation with Dr. Simon Jones. The authentic liverwort origin of 4-methoxystyrenes, corsifurans and corsistilbenes was unambiguously established by their detection in plant material from axenic in vitro cultures of Corsinia coriandrina established in cooperation with Dr. Klaus von Schwartzenberg. A variety of deuterium labelled phenylpropanoid derivatives and aromatic amino acids were synthesized and applied to liquid in vitro cultures of monoclonal haploid C. coriandrina. Incorporation of labelled precursors was investigated by GC-EIMS, 2D NMR and 13C NMR techniques. Application experiments established the L-tyrosine origin of coriandrin and related 4-methoxystyrenes in Corsinia coriandrina. A new pulsed application technique using the temporarily immersion system RITA® was employed to minimize competing reversible transamination mediated by TAT and unambiguously established that the alpha-deuterium of L-[U-D7]-tyrosine is retained in (Z)-[1,2,aryl-D6]-coriandrin biosynthesis. Co-application experiments with different isotopomers indicated that only the 3-pro-S-hydrogen and the carboxyl group are lost during assembly of the (Z)-configured double bond, similar to the biosynthesis of xanthocillin in Ascomycota. The O-methyl group originates from the methylene carbon glycine and is introduced at a later biosynthetic stage, whereas the origin of the isothiocyanate carbon could not be identified. Furthermore, the L-phenylalanine origin of corsifuran A and corsistilbenes via an STS catalyzed phenylpropanoid-polymalonate pathway was unambiguously established. The liverwort specific reduction of the beta-carbonyl group of the trisketo acid intermediate was confirmed for the first time and hydrangenic acid and lunularic acid were excluded as biosynthetic intermediates. Biosynthetic relationships between stilbenoids, bibenzyls and the corresponding 2-arylbenzofurans were investigated, indicating a two-step cyclisation mechanism from (E)-stilbenoid precursors to corsifuran A, and subsequent syn-dehydrogenation to corsifuran C. Furthermore, the MVA pathway to alpha-pinene and (E)-beta-nerolidol was established. In cooperation with Prof. G. Gries a new sesquiterpene hydrocarbon emitted by Ulmus americana upon infection with Ophiostoma novo-ulmi was identified as (1S,2R)-(+)-spiroaxa-5,7-diene by comparison with both enantiomers and both epimers of four double bond isomers, obtained by solid super acid catalyzed rearrangement of (+)- and (–)-ent-aromadendrene and (–)-allo-aroma-dendrene using the solid super acid TiO2/SO4. The unusual (–)-ent-aromadendrene was isolated from the Liverwort Pellia epiphylla. In cooperation with Prof. B. Piechulla a series of new natural products, with an octamethylbicyclo[3.2.1]octadiene skeleton was identified from the rhizobacterium Serratia odorifera. The structure and stereochemistry of odorifen, the major volatile constituent, was deduced from 1H and 13C NMR in combination with 2D NMR spectra, and was unambiguously established to be meso-8-anti-2,4-bisaxial-1,2,4,5,6,7,8-heptamethyl-3-methylenebicyclo[3.2.1]oct-6-ene by comparison with synthetic compounds.