<jats:title>Abstract</jats:title><jats:p>Two novel secondary metabolites, namely cruentaren A (<jats:bold>1</jats:bold>) and B (<jats:bold>2</jats:bold>), have been isolated from the myxobacterium <jats:italic>Byssovorax cruenta</jats:italic>. Their structures have been elucidated by detailed NMR spectroscopic analysis. Both compounds are isomers of a C<jats:sub>22</jats:sub> polyketide with a 2‐hydroxy‐4‐methoxybenzoic acid terminus, which forms a 12‐membered lactone in <jats:bold>1</jats:bold> and a six‐membered lactone in <jats:bold>2</jats:bold>. An amino group at C‐22 is acylated by a 3‐hydroxy‐2‐methylhexanoic acid group whose absolute configuration has been determined by GC analysis after hydrolysis and stereoselective synthesis. Degradation of <jats:bold>2</jats:bold> by olefin cross‐metathesis in the presence of ethylene yields the C‐12 to C‐21 fragment <jats:bold>8</jats:bold>, whose relative configuration has been predicted applying Kishi’s <jats:sup>13</jats:sup>C NMR spectroscopic data base approach, and whose absolute configuration has been proven by comparison with two synthetic stereoisomers. The configuration of C‐9 and C‐10 has been determined by Mosher’s method and NMR spectroscopy. Degradation of cruentaren A (<jats:bold>1</jats:bold>) by olefin cross‐metathesis in the presence of ethylene gives crystalline 21,22‐<jats:italic>seco</jats:italic>‐cruentaren (<jats:bold>14</jats:bold>). Its crystal structure analysis reveals the relative configuration and conformation of the macrocycle and enables solution conformation analysis. The high cytotoxic activity of <jats:bold>1</jats:bold> is essentially lost on derivatisation or rearrangement to <jats:bold>2</jats:bold>.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)