Novel pyrazines from the myxobacterium Chondromyces crocatus and marine bacteria

The volatiles released by two strains of the myxobacterium Chondromyces crocatus and seven strains of marine Alpha-proteobacteria from the North Sea were collected using the CLSA or SPME headspace methods and analysed by GC-MS. In the extracts of C. crocatus 27 pyrazines were identified, belonging to different classes. 2,5-Dialkylpyrazines and related 3-methoxy-2,5-dialkylpyrazines dominated. Several pyrazines like 2-(1-methylethenyl)-5-(l-methylethyl)pyrazine (7) and 3-methoxy-2,5-dialkylpyrazines with methyl, isopropyl, isobutyl or sec-butyl side-chains were obtained from natural sources for the first time. It was essential for the identification to rely on synthetic reference materials, which were obtained using Furstner's iron-catalysed coupling of chloropyrazines with Grignard reagents or condensation of azido ketones as key steps. The synthetic material allowed the identification of two previously unknown attractants of bacterial origin for the pineapple beetle Carpophilus humeralis, namely 3-methoxy-2-(1-methylpropyl)-5-(2-methylpropyl)pyrazine (17) and 3-methoxy-2,5-bis(1-methylpropyl)pyrazine (52). Several 2,5-dialkylpyrazines were identified in the extracts of the marine Alphaproteobacteria. The unique 2,5-dimethyl-3-(methylsulfanyl)pyrazine (67) represents a new type of natural pyrazine. Our results, together with literature reports, show that pyrazines are an important class of bacterial volatiles which might be more widespread than previously thought. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)