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meso-Tetrahexylporphyrin was converted to its corresponding 7,8-dihydroxychlorin using an osmium tetroxide-mediated dihydroxylation strategy. Its diol moiety was shown to be able to undergo a number of subsequent oxidation reactions to form a chlorin dione and porpholactone, the first meso-alkylporphyrin-based porphyrinoid containing a non-pyrrolic building block. Further, the diol chlorin was shown to be susceptible to dehydration, forming the porphyrin enol that is in equilibrium with its keto-chlorin form. The meso-hexylchlorin dione could be reduced and it underwent mono- and bis-methylation reactions using methyl-Grignard reagents, and trifluoromethylation using the Ruppert-Prakash reagent. The optical and spectroscopic properties of the products are discussed and contrasted to their corresponding meso-aryl derivatives (where known). This contribution establishes meso-tetrahexyl-7,8-dihydroxychlorins as a new and versatile class of chlorins that is susceptible to a broad range of conversions to generate functionalized chlorins and a pyrrole-modified chlorin analogue. [ABSTRACT FROM AUTHOR]

A total synthesis of cytospolide D, starting from l-glutamic acid, is described. The critical macrolactonization to the 10-membered lactone containing an (E)-configured double bond was successfully achieved by Shiina esterification. Conversion of cytospolide D to its bicyclic derivatives M, O, and Q was accomplished under mild conditions, lending support to the proposed biosynthetic hypothesis. [ABSTRACT FROM AUTHOR]

A flexible and efficient synthesis of the potent human topoisomerase inhibitor (5 Z,9 Z)-eicosa-5,9-dienoic acid was explored; the presented method represents a generally applicable approach towards demospongic acids and related natural products. Key steps of the synthesis involve chemoselective hydroboration, Corey-Fuchs alkynylation, Z-selective Lindlar reduction, tetrapropylammonium perruthenate catalyzed direct oxidation of a primary alcohol to the corresponding acid, and Arndt-Eistert homologation. Thus, the total synthesis of (5 Z,9 Z)-eicosa-5,9-dienoic acid was achieved in 10 steps in an overall yield of 20 %. [ABSTRACT FROM AUTHOR]

An iterative strategy for the stereodivergent synthesis of unbranched 1,5,9,n-polyenes (and -polyynes) was investigated. Starting from a terminal alkyne the iteration cycle consists of a C3 extension (allylation), a chemoselective hydroboration, an alkyne reduction, and an oxidation of the associated alcohol with subsequent C1 homologation. Double bond geometry is controlled using stereoselective alkyne reductions, employing either the Lindlar hydrogenation protocol or an aluminum hydride reduction. In a model sequence it was demonstrated that the strategy is applicable to the synthesis of 1,5,9,n-polyenes with any possible double bond configuration accessible in equally high efficiency and selectivity. It is worth noting that our approach does not require any protecting group chemistry. Furthermore, using the same strategy, the first total synthesis of chatenaytrienin-4, the proposed unsaturated biosynthetic precursor of the bis-THF acetogenin membranacin, was examined. Thus, the all-cis 1,5,9-triene natural product was prepared in 15 steps from commercially available starting materials in 6% overall yield. [ABSTRACT FROM AUTHOR]

Eine regio ‐ und stereoselektive inverse Prenylierung von Indol ‐ und Tryptophan ‐ Derivaten wird präsentiert. Alle vier möglichen Stereoisomere sind durch diese Iridium ‐ katalysierte Reaktion zugänglich. Die Stereoselektivität wird durch eine Kombination aus chiralem Phosphoramidit ‐ Liganden mit einem achiralen Boran ‐ Additiv gesteuert und kann abhängig von der Art des Borans vollständig umgekehrt werden. Es genügt somit nur ein Enantiomer des Liganden, um alle denkbaren Stereoisomere herzustellen. Das synthetische Potential der Methode wurde in einer kurzen Synthese von Amauromin und seinen zwei natürlichen Diastereomeren demonstriert. Asymmetrische Prenylierung: Eine regio ‐ und stereoselektive inverse Prenylierung von Indol ‐ und Tryptophan ‐ Derivaten wird präsentiert. Alle vier möglichen Stereoisomere sind durch diese Iridium ‐ katalysierte Reaktion zugänglich. Die Stereoselektivität wird durch eine Kombination aus chiralem Phosphoramidit ‐ Liganden und achiralem Boran ‐ Additiv gesteuert und kann abhängig von der Art des Borans vollständig umgekehrt werden. [ABSTRACT FROM AUTHOR]

A regio- and stereoselective reverse prenylation of indole and tryptophan derivatives is presented. All four possible stereoisomers are accessible through this iridium-catalyzed reaction. The stereoselectivity is controlled by a chiral phosphoramidite ligand in combination with an achiral borane additive and can be switched by changing the nature of the borane. One enantiomer of the ligand is thus sufficient to prepare all possible isomers. The synthetic potential of this method was demonstrated by a short total synthesis of amauromine and its two natural diastereomers. [ABSTRACT FROM AUTHOR]

This review focuses on the application of glycol cleavage reactions in the synthesis and derivatization of complex natural products. Reagent classics as well as recent developments are discussed. [ABSTRACT FROM AUTHOR]

The first total synthesisof muricadienin, the unsaturated putativeprecursor in the biosynthesis of trans- and cis-solamin is described. Key steps in the synthesis area chemoselective hydroboration, a Z-selective Wittigreaction, and a Fries rearrangement for introducing the terminal α-substitutedbutenolide. Thus, muricadienin can be synthesized in 11 steps fromcommercially available starting materials in 42% overall yield. [ABSTRACT FROM AUTHOR]

The synthesis of 2-nitro-4-oligoprenyl-substituted pyrrole derivatives relevant to the heronapyrroles and related natural products was investigated. Among numerous approaches, nitration of a 3-farnesyl-substituted unprotected pyrrole using AcONO2 gave the best results, albeit still with unsatisfactory yield and regioselectivity. Therefore, the synthesis of (--)-heronapyrrole C acid, an analogue of the naturally occurring antibiotic heronapyrrole C carrying a bioisosteric carboxylate in place of the nitro group, was examined. In lieu of the unsatisfactory nitration, a regioselective acylation with Cl3CCOCl was carried out (>8:1 regioselectivity, in contrast to the 1:1.3 ratio for the nitration). The trichloromethyl ketone was converted to the desired acid in a haloform reaction at the final stage of the synthesis. Further key steps of the analogue synthesis involved a position- and stereoselective Corey-Noe-Lin dihydroxylation and an organocatalytic double Shi epoxidation. A biomimetic polyepoxide cyclization cascade established the bis-THF backbone. Thus, (--)-heronapyrrole C acid was synthesized in eight steps (14.5% overall yield) from commercially available starting materials. [ABSTRACT FROM AUTHOR]

The stereoselectivity of the reaction is dictated by the combination of an achiral borane additive and one chiral phosphoramidite ligand and can be adjusted by changing the borane. [ABSTRACT FROM AUTHOR]

Review: [141 refs. [ABSTRACT FROM AUTHOR]

The ruthenium tetroxide catalyzed direct oxidative cyclization is a powerful synthetic method for the diastereoselective preparation of THF diols from simple achiral 1,5-diene starting materials with significant relevance to target-oriented and natural product synthesis. We here report that an aged precatalyst solution exhibits an extraordinarily high reactivity. Thus, even at 50 ppm catalyst loading, impressive results were achieved with good to high yields and excellent diastereoselectivities for the four stereogenic centers that were established in this single-step transformation. Turnover numbers of up to 15 000 and turnover frequencies of up to 3660 h−1 were obtained. The ageing effect was analyzed spectroscopically and a preliminary hypothesis to explain the reactivity is given. Finally, the generality of this effect in oxidative cyclization chemistry was demonstrated by using a set of substrates with different substitution patterns and different geometries of the reacting double bonds. [ABSTRACT FROM AUTHOR]

Common photoredox catalysts Ru(bpy)32+ and Ru(bpz)32+ are rapidly converted into Ruthenium(viii)-oxide through continuous visible light irradiation in the presence of NaIO4 or H5IO6. This hitherto unreported photoassisted catalyst oxidation was utilized in the development of tandem catalytic protocols which combine a photoredox reaction with a subsequent RuO4-mediated oxidation. The new concept was demonstrated through one-pot radical cation Diels–Alder (RCDA)/1,5-diene cyclisation sequences. [ABSTRACT FROM AUTHOR]

An asymmetric synthesis of the C11-homoterpenoid (+)-Greek tobacco lactone is developed starting from readily available (R)-linalool. The synthesis is comprised of four operations and features a diastereoablative epoxidation and an oxidative tetrahydropyran formation using vanadium-, palladium-, and selenium-catalyzed cyclizations. [ABSTRACT FROM AUTHOR]

Tetrahydrofuran diols are diastereoselectively prepared by highly efficient oxidative cyclization from simple achiral 1,5-dienes using an aged aqueous solution of ruthenium(III) chloride. [ABSTRACT FROM AUTHOR]