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The first total synthesis of the unique terpene rippertenol, a molecule with dense stereochemical complexity arrayed on a compact framework largely devoid of functional groups, is described. Key elements include orchestrated and unique applications of aldol condensations, Diels-Alder chemistry, and a ring expansion to advance a chiral starting material containing a single chiral center into the final target in a concise and diastereocontrolled manner. [ABSTRACT FROM AUTHOR]

The biosynthesis of geosmin (1) and (1(10)E,5E)-germacradien-11-ol (2), two volatile terpenoid compounds emitted by the myxobacteria Myxococcus xanthus and Stigmatella aurantiaca, was investigated in feeding experiments with different labeled precursors. In these experiments, the volatiles released by the cell cultures grown on agar plates were collected with a closed-loop stripping apparatus (CLSA) and analyzed by GC-MS. [²H10]Leucine and [4,4,4,5,5,5-²H6]dimethylacrylate were fed to wild-type strains and bkd mutant strains, which are impaired in the degradation of leucine to isovaleryl-CoA. [²H10]Leucine was incorporated into 1 and 2 only by the wild-type strains via the biosynthetic pathway that involves leucine degradation and branching into the mevalonate pathway. Dimethylacrylyl-CoA (DMA-CoA) is an intermediate in the leucine degradation and in the recently discovered pathway from HMG-CoA to isovaleryl-CoA. The corresponding free acid, [4,4,4,5,5,5-²C;H6]dimethylacrylic acid, was incorporated into 1 and 2 only by the mutants impaired in leucine degradation. [4,4,6,6,6-²H5]Mevalonic acid lactone (12) was synthesized and fed to M. xanthus and S. aurantiaca wild-type strains and a double mutant strain of M. xanthus. This strain does not degrade leucine and is impaired in the reduction of 3-hydroxy-3-methylglutaryl-CoA to mevalonic acid. The mass spectral analysis of labeled 1 and 2 obtained in these feeding experiments led to a biosynthetic scheme to 1 with intermediate 2. This pathway differs from that observed in the liverwort Fossombronia pusilla and thus suggests microbial geosmin biosynthesis following a route different from that in liverworts. Our results are supported by a 1,2-hydride shift of the tertiary hydrogen atom at C-4a into the ring opposite to that in F. pusilla. [ABSTRACT FROM AUTHOR]

We describe a novel convergent procedure that has proved useful in the synthesis of a wide range of meroterpenoid-related structures containing a mono-, ses- qui-, or diterpenoid moiety linked to a nonfused aromatic subunit with various substitution patterns. The key steps were the Stille-type coupling of aryl stannanes and allylic carbonates, followed by the titanocene-catalyzed domino cyclization of aryl epoxypolyprenes. The coupling reaction was perfectly compatible with preformed epoxides, while the sequential cyclization, which presumably proceeded via alkyl radicals inert to benzene derivatives, selectively provided exocyclic alkenes. [ABSTRACT FROM AUTHOR]

An asymmetric total synthesis of (+)-chatancin was achieved via a transannular Diels-Alder (TADA) reaction of an in situ generated macrocyclic pyranophane pseudobase. The presented route constitutes the second of two proposed biosynthetic pathways that involves a TADA reaction. It links this diterpene biogenetically to the cembranoids. A set of TADA selection rules that rationalize the formation of (+)-chatancin from a dynamic equilibrium of four 2-hydroxy-2H-pyrane bicycles and their 16 potential TADA transition states are also outlined. Beyond the TADA reaction, highlights of the synthetic work include the assembly of a chiral acyclic macrocyclization substrate from (S)-citronellol and an efficient macrocyclization via a β-ketosulfoxyde/enone Michael addition. [ABSTRACT FROM AUTHOR]

Extension of anion relay chemistry (ARC) beyond the area of dithianes has been achieved by the design of two effective ARC linchpins capable of three- and four-component couplings. To showcase the ARC tactic in natural product synthesis, a cytotoxic gorgonian linear sesquiterpene was constructed, and the absolute configuration assigned via the Kakisawa/Mosher method. The synthesis proceeded in five steps with an overall yield of 22%. [ABSTRACT FROM AUTHOR]