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Rhodium(I)/Binap complexes catalyze highly enantioselective additions of methyl- and arylaluminum reagents to cyclic α,β-unsaturated N-tosyl ketimines. Depending on the solvent and substituents at the ring, the reaction occurs either in a 1,2-manner to deliver α-tertiary allylic amines or in a 1,4-manner to yield, after subsequent reduction, 3-substituted cycloalkyl amines. Well known in the case of the respective cycloalkenones, these first transformations of the aza-analogues enable the synthesis of amine structures of pharmaceutical and biochemical interest.

The first synthesis of N-tosyl imines from various cyclopent-2-enones and cyclohex-2-enones was achieved by direct condensation with tosyl amide in the presence of TiCl(OEt)3 and Et3N. In addition, N-tert-butylsulfonyl imines from five- to seven-membered cycloalk-2-enones were obtained through formation of the ­respective oximes and subsequent Hudson reaction. These compounds are easy to handle solids and they are interesting starting materials for a variety of transformations.

(R)-Sarkomycin was prepared using a five-step total synthesis. Key steps in the enantioselective construction of the targeted scaffold were a rhodium-catalyzed asymmetric conjugate alkenyl addition with subsequent silyl trapping and a Mukaiyama aldol reaction with aqueous formaldehyde. Protection of the hydroxy group as a THP acetal and oxidative cleavage of the C,C-double bond provided a stable direct precursor to the natural product. The final liberation was carried out under slightly acidic conditions in a microwave-assisted reaction, resulting in a high yield of the "deceptive" sarkomycin. This represents the shortest enantioselective synthesis of this rather unstable compound to date and the first to employ asymmetric catalysis to introduce the stereogenic center.

The feasibility of kinetic resolutions of racemic monosubstituted cyclohex-2-enones by Rh/binap-catalyzed reactions was investigated. 1,2-Addition of AlMe(3) to the 5-substituted derivatives furnished allylic alcohols in the matched case, while the less reactive enantiomers were either left over or transformed into trans-3,5-disubstituted cyclohexanones in parallel or sequential 1,4-additions. Altogether, these represent regiodivergent reactions on racemic mixtures. In contrast, 1,4-addition of aryl groups led to inferior results since either catalyst or substrate control dominated.

(Trifluoromethyl)diazirines are well established photoaffinity labels (PAL) used in biochemical investigations to create covalent ligand-receptor bonds. Two new diazirinylbenzoic acids 8b,c with perfluorobutyl and perfluorooctyl chains (F-PAL) were efficiently prepared from p-bromobenzaldehyde and attached to the highly potent and specific V-ATPase inhibitors 21-deoxyconcanolide A (2) and bafilomycin A 1 (5), deriving from the natural product pool from Streptomyces producer strains. The labelled derivatives 17 and 18 were efficiently purified by fluorous solid-phase extraction. Functional biochemical assays with the V-ATPase holoenzymes proofed strong inhibition activities. So far, radioactive isotopes or biotin-tags have mainly been used for tracing compounds in photoaffinity studies. The C 4 F 9 - and C 8 F 17 -fluorous tags aim to enable advantageous separation and identification of labelled peptide fragments by fluorous chromatography followed by MS analysis. Therefore, F-PAL represents an innovative new concept for binding site determination and should significantly accelerate and simplify such biochemical investigations.

The tunicate-associated strain Streptomyces sp. JP90 produces the unprecedented metabolite cinnamoylphosphoramide (1) among several other compounds. Structure elucidation was accomplished by NMR spectroscopic studies and efficient total synthesis. The absolute configuration at phosphorus was determined by synthesis of both enantiomers of 1 performing a resolution of the corresponding diastereomeric phosphoramides of L-phenylalanine ethyl ester. Unusual cinnamic acid derivative 1 represents the first bacterial organophosphoramide. As it matches the Schrader's formula for insecticidal organophosphates, its biological activity was investigated. A weak inhibition of acetylcholinesterase was observed in in vitro tests, and water-soluble analogues of 1 were prepared. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Paultheo von Zezschwitz Gut verfugbare Metallkomplexe ermoglichen hochselektive Nitren-Insertionen in C-H-Bindungen. Dabei bilden sich intermolekular sekundare Amide, intramolekular dagegen verschiedenartige Heterocyclen. Ausgehend von Aziden wurden kurzlich Heteroaromaten synthetisiert.

Als starke Bronsted-Sauren protonieren chirale Phosphorsaurediester C-N-Doppelbindungen und erlauben so enantioselektive organokatalytische Umsetzungen. Neben Additionen an Imine sind nun auch andere Reaktionstypen moglich.

Aluminum organyls are valuable reagents for carbon-carbon bond formation as they can either be purchased at low prices or conveniently be prepared, for example, by hydro- or carboalumination of alkenes and alkynes. Although their application in reactions with α,β-unsaturated carbonyl compounds is rather limited, it creates a diverse picture concerning the selectivity of product formation which depends on several factors such as the type of organic residue, the conformation of the enone, the presence of additional functional groups and the donor ability of the solvent. This review describes issues of selectivity of such uncatalyzed transformations as well as conjugate additions catalyzed by achiral copper and nickel catalysts in which alanes can act similarly or even superiorly to the more common lithium or magnesium organyls. Moreover, the rapidly growing field of transition-metal-catalyzed asymmetric 1,2- and 1,4-additions is reviewed, in which aluminum reagents can not only lead to enantioselective alkyl additions, thus substituting zinc organyls, but also afford introduction of aryl, alkenyl and alkynyl groups. 1 Introduction 2 Uncatalyzed Additions to α,β-Unsaturated Carbonyl Compounds- 2.1 Regioselectivity with Plain Enones 2.2 Selective Conjugate Additions to Cyclic Enones 2.3 Conjugate Additions to Carboxylic Compounds 3 Racemic Copper- and Nickel-Catalyzed 1,4-Additions 4 Asymmetric Addition to Carbonyl Compounds 4.1 Enantioselective Conjugate Additions to Enones 4.2 Enantioselective 1,2-Additions 5 Conclusion

Die Enzymgruppe der ATPasen katalysiert grundlegende zellulare Vorgange wie die ATP-Synthese oder Transportprozesse unter Verbrauch von ATP. Sie ist damit ein wichtiges pharmakologisches Target. Zwei potente Inhibitoren wurden kurzlich erstmals synthetisiert.

Die Addition von Metallorganylen an Ketone, etwa bei der Umsetzung von Grignard- Reagenzien oder Lithiumorganylen mit Carbonylverbindungen zu sekundaren oder tertiaren Alkoholen, ist ein Standardverfahren der organischen Chemie. Die enantioselektive Variante dieser Reaktion ist dagegen immer noch eine Herausforderung.

To elucidate the structure of the new natural product spirodionic acid, which has three stereogenic centers of hitherto unknown configuration, two 4-ethenylspiro[4.5]dec-6-en-1,8-diones with opposite relative configuration at C4 and C5 were prepared. The first access involved the synthesis of 3-ethenyl-2-formylcyclopentanone (8) using a three-component coupling process. A sequence of Michael addition to penten-3-one (7) and intramolecular aldol condensation then led to the highly selective formation of the 4S*,5S*-configured spirocycle 5. In contrast, a selective spiroannelation of a cyclohexenone ring was accomplished by a novel type of twofold Michael addition to the dialkenyl ketone 11 with subsequent dehydrogenation to furnish the 4S*,5R*-configured spirocycle 25. Diastereoselective methylation and oxidative degradation then completed a highly efficient synthesis of the natural product as prerequisite for the assignment of its absolute configuration.

Diazirine-tagged systems are considered reliable compounds for photoaffinity labeling (PAL) in biochemical studies as they enable investigation and understanding of biological mechanisms through covalent bonding to the target and subsequent detection. 14 C-labeled 4-(3-trifluoromethyl-3H-diazirin-3-yl)benzoic acid (11) was prepared by a lithium-bromide exchange on the bis-silylated 4-bromophenyldiaziridine 19 with subsequent transformations with electrophiles as key steps of the synthesis. Using 14 CO2, which was generated from rather inexpensive Ba 14 CO3, the desired diaziridinylbenzoic acid 21 was obtained in 78 % yield based on the

The total synthesis of iromycin A (1a), a microbial metabolite combining a novel structure with an interesting biological activity as a NO synthase inhibitor, was accomplished using a flexible and highly convergent approach. Thus, the ring fragment was prepared as 6-bromomethylpyrone 27 by acylation of the respective beta-ketoester 13 and subsequent lactonization of the thus-obtained beta,delta-diketoester 11, followed by bromination of the 6-methyl group. In addition, the unsaturated side chain was efficiently prepared as terminal alkyne 34 which was then carboaluminated to furnish the alkenyldimethylalane 35. The assembly of these two fragments was thoroughly studied using nickel, palladium, and copper catalysts yet only succeeded in the absence of any transition metal after formation of the respective lithium alkenyltrialkylalanate. Treatment of the coupled product 41 with liquid ammonia then completed the total synthesis which furnished an 18% overall yield over the nine steps of the longest linear sequence.

The enantioselective 1,4-addition of dialkylzinc to cyclic N-sulfonyl imines in the presence of Ferringa′s phosphoramidite catalyst is presented.

On attempting photochemically induced electrocyclizations of the previously reported 1,6-bismethoxycarbonyl- or 1,6-bistrimethylsilyl-substituted ring-attached (E,Z,E)-1,3,5-hexatrienes 4 b, 4 c and 5 b, 5 c, equilibrium mixtures of the starting materials and their diastereomers, the corresponding (E,Z,Z)-hexatrienes 4 b, 4 c and 5 b, 5 c were obtained. The desired trans-disubstituted ring-annelated cyclohexadienes 9 b and 10 b were formed by subsequent thermal 6pi-electrocyclization of the (E,Z,Z)-hexatrienes 4 b and 5 b in good yields (77-83 %). Upon treating the bissilyl-substituted hexatrienes (E,Z,E)-5 c or (E,Z,Z)-5 c under the same conditions, 6pi-electrocyclizations did occur, but the primary products immediately isomerized to a large extent, and mixtures of the cyclohexane-annelated cyclohexadienes 10 c-12 c along with the dehydrogenation products 13, 14 c were formed. When the bismethoxycarbonyl-substituted hexatriene (E,Z,E)- 5 b was irradiated for an extended period of time (4.5 h), the gradual formation of the oxabicyclo[3.2.1]octa-2,6-diene 17 b by a formal intramolecular hetero-Diels-Alder reaction was observed and 17 b could be isolated in up to 69 % yield. To explore the scope of this new photochemical reaction, the new ring-attached (E,Z,E)-hexatrienes 4 a, 5 a and 6 b were synthesized by twofold Heck reactions from 1,2-dibromocycloalkenes 1-3 (59-66 %). While irridiation of the cyclopentene-attached 1,3,5-hexatrienes only led to decomposition, the cyclohexene- and cycloheptene-attached hexatrienes gave the hetero-Diels-Alder products or other photoproducts depending on the size of the cycloalkene moiety and the nature of the alkoxycarbonyl substituents at the vinyl termini. The photoreaction products 17 b and 18 b which are bicyclic acetals, underwent cleavage upon treatment with a Lewis acid such as Me(3)SiOTf to give the ring-annelated methoxycarbonyl-substituted troponecarboxylates 21 b and 22 b.

Multiple palladium-catalyzed cross-coupling reactions of the Heck, Suzuki and Stille types on oligofunctionalized cycloalkenes and arenes can be performed in very high yields. While Heck type reactions on 1,2-dibromocycloalkenes or 2-bromocyclohexenyl triflates lead only to the twofold coupling products, the Stille coupling of the latter substrates occurs chemoselectively at the site of the triflate leaving group to give bromobutadienes which readily undergo subsequent Heck reactions. The thus obtained 1,3,5-hexatrienes undergo thermal 6π-electrocyclizations to furnish bicyclic skeletons, among others cyclohexenone-annelated systems after acidic work-up. Cascades of carbopalladation, rearrangement and [4+2] cycloaddition with bicyclopropylidene, aryl iodides and dienophiles constitute a new class of three-component reactions with a remarkable combinatorial potential, especially since they can be performed on the solid phase using the triazene linker. With nucleophiles instead of dienophiles yet another domino reaction consisting of carbopalladation, rearrangement and nucleophilic substitution can be performed with bicyclopropylidene, aryl or alkenyl iodides and C-, O- and N-centered nucleophiles to give a wide variety of interesting compounds. Under properly adjusted conditions even sixfold coupling reactions on hexabromoarenes can be achieved. When using Heck type reactions the initially formed intermediates en route to hexaalkenylarenes undergo intramolecular ring closures to yield multi-component mixtures of isomers of the expected products, but sixfold Stille and Suzuki couplings give pure hexaalkenylarenes in up to 73% yield. Crystal structure analyses reveal that depending on the nature of the alkenyl groups the six arms are positioned either on the same side of the central ring making it an interesting cup-shaped molecule or alternating above and below the central plane.

Tosylation of cyclohept-2-enone occurs with complete conversion, but yields an inseparable mixture of the desired imine and its tautomer cyclohept-1,3-dienyltosyl amide.

The first enantioselective 1,2-addition of methyl- and arylaluminum reagents to cycloalkenone-derived N-tosyl imines, yielding α-tertiary allylamides, is developed.

In copper(I)/phosphoramidite-catalyzed asymmetric 1,4-additions of dialkylzinc, N-sulfonyl imines are more reactive and furnish higher enantiomeric excesses than the respective cycloalk-2-enones. This enables formation of a quaternary stereocenter as well as a cis-selective addition to an imine derived from 5-methylcyclohex-2-enone. The 1,4-adducts can be transformed in stereodivergent reductions yielding cis- or trans-3-alkylcycloalkyl amides.

The development of an improved protocol for the enantioselective Rh(I)/BINAP-catalyzed 1,2-addition of trimethylaluminum to cyclic enones is reported.

(R)-Sarkomycin was prepared using a five-step total synthesis. Key steps in the enantioselective construction of the targeted scaffold were a rhodium-catalyzed asymmetric conjugate alkenyl addition with subsequent silyl trapping and a Mukaiyama aldol reaction with aqueous formaldehyde. Protection of the hydroxy group as a THP acetal and oxidative cleavage of the C,C-double bond provided a stable direct precursor to the natural product. The final liberation was carried out under slightly acidic conditions in a microwave-assisted reaction, resulting in a high yield of the “deceptive” sarkomycin. This represents the shortest enantioselective synthesis of this rather unstable compound to date and the first to employ asymmetric catalysis to introduce the stereogenic center.

Palladium-catalyzed cross-coupling reactions of 2-bromocyclohex-1-enyl triflates 7 and 11 with a variety of alkenylstannanes occurred chemoselectively at the site of the triflate leaving group to give bromobutadienes which readily underwent Heck reactions with acrylates and styrene. Both steps could be performed in the same flask to give differentially functionalized hexatrienes in up to 88% overall yield. With simple stannanes, the same catalyst precursor could be used for both coupling steps making it possible to perform the whole sequence with only one portion of catalyst. For some of the functionally substituted stannanes, specifically adjusted catalyst systems had to be used. The 1,3,5-hexatrienes obtained were further transformed, in particular the methoxy-substituted compounds 14a-c were converted to bicyclo[4.4.0]decenones 30 (71-97%), bicyclo[4.3.0]nonenones 35 (74-93%), cyclodecynone 37a (47%), and cyclononynone 39a (15%). Thermal electrocyclizations of the other hexatrienes gave tetrahydronaphthalines 31 (60-61%), the tricyclic lactone 32 (72-75%) and decahydrophenanthrene 33 (75 %) in good yields.

1,6-Disubstituted (E,Z,E)-1,3,5-hexatrienes (1a−c, 2a−e), obtained in good yields (43−81%) by 2-fold Heck couplings of 1,2-dibromocyclopentene and 1,2-dibromocyclohexene, respectively, with alkenes, can be oxidized regioselectively at the central double bond with various oxidants such as dimethyldioxirane (DMDO), m-CPBA, or trifluoroperacetic acid in moderate to good yields (45−87%). The resulting 1,2-dialkenylcyclohexene epoxides 4a−e undergo thermal Cope rearrangements to furnish 1,6-oxygen-bridged cyclodeca-1,5-dienes 8a−e, which are 2-fold anti-Bredt bridgehead alkenes. The structure of compound 8a was established by X-ray analysis. The corresponding 1,2-dialkenylcyclopentene epoxides 3a−c give only 2,2-dialkenylcyclopentanones 9a,b by an acid-catalyzed 1,2-alkenyl shift (71−100% yield) or indan derivatives 12b,c (46−82%) resulting from elimination, 6π-electrocyclization, and aromatization.

The development of an improved protocol for the enantioselective Rh(I) /binap-catalysed 1,2-addition of AlMe3 to cyclic enones is reported. (31)P NMR analysis of the reaction revealed that the catalyst in its resting state is a chloride-bridged dimer. This insight led to the use of AgBF4 as an additive for in situ activation of the dimeric precatalyst. Thus, the catalyst loading can now be reduced to only 1 mol% with respect to rhodium. Various 5-7-membered cyclic enones can be transformed into tertiary allylic alcohols with excellent levels of enantioselectivity and high yields. The obtained products are versatile synthetic building blocks, shown by a highly enantioselective formal total synthesis of the pheromone (-)-frontalin as well as formation of a bicyclic lactone that has the core structure of the natural flavour component "wine lactone".

For the first time the transformation in both cis- and trans-thiadiazolines is achieved via diastereoselective hydride addition [(II) (III)] and subsequent novel isomerization [(III) (IV)].

Both, cis- and trans-3,4-disubstituted thiadiazolidines 5 and 6 can enantioselectively be obtained from thiadiazoles 2 which, in turn, are efficiently prepared from the respective 1,2-diketone by an improved protocol. An asymmetric ruthenium-catalyzed transfer hydrogenation followed by a diastereoselective hydride addition furnishes exclusively the cis-isomers 5 which, under acidic conditions, undergo a novel isomerization into the trans-isomers 6. These cyclic sulfamides can be transformed into 1,2-diamines as well as 2,3-diamino acids.

The investigation of V-ATPases as potential therapeutic drug targets and hence of their specific inhibitors is a promising approach in osteoporosis and cancer treatment because the occurrence of these diseases is interrelated to the function of the V-ATPase. Apicularen belongs to the novel inhibitor family of the benzolactone enamides, which are highly potent but feature the unique characteristic of not inhibiting V-ATPases from fungal sources. In this study we specify, for the first time, the binding site of apicularen within the membrane spanning V(O) complex. By photoaffinity labeling using derivatives of apicularen and of the plecomacrolides bafilomycin and concanamycin, each coupled to (14)C-labeled 4-(3-trifluoromethyldiazirin-3-yl)benzoic acid, we verified that apicularen binds at the interface of the V(O) subunits a and c. The binding site is in the vicinity to those of the plecomacrolides and of the archazolids, a third family of V-ATPase inhibitors. Expression of subunit c homologues from Homo sapiens and Manduca sexta, both species sensitive to benzolactone enamides, in a Saccharomyces cerevisiae strain lacking the corresponding intrinsic gene did not transfer this sensitivity to yeast. Therefore, the binding site of benzolactone enamides cannot be formed exclusively by subunit c. Apparently, subunit a substantially contributes to the binding of the benzolactone enamides.

While the stereoselective addition of zinc organyls to carbonyl compounds is nowadays an established synthetic method, the use of aluminum reagents is less common, even though they offer distinct advantages. This chapter presents an overview of the current status of catalytic asymmetric additions to aldehydes, ketones, and imines, as well as the difficulties and the limitations of such transformations, respectively. Certain combinations of substrate types and carbon nucleophiles were so far only achieved using stoichiometric systems under substrate or auxiliary control. These examples are also included, as well as aspects of cross-coupling reactions of aluminum organyls with organic halides.

The macrolactone archazolid is a novel, highly specific V-ATPase inhibitor with an IC(50) value in the low nanomolar range. The binding site of archazolid is presumed to overlap with the binding site of the established plecomacrolide V-ATPase inhibitors bafilomycin and concanamycin in subunit c of the membrane-integral V(O) complex. Using a semi-synthetic derivative of archazolid for photoaffinity labeling of the V(1)V(O) holoenzyme we confirmed binding of archazolid to the V(O) subunit c. For the plecomacrolide binding site a model has been published based on mutagenesis studies of the c subunit of Neurospora crassa, revealing 11 amino acids that are part of the binding pocket at the interface of two adjacent c subunits (Bowman, B. J., McCall, M. E., Baertsch, R., and Bowman, E. J. (2006) J. Biol. Chem. 281, 31885-31893). To investigate the contribution of these amino acids to the binding of archazolid, we established in Saccharomyces cerevisiae mutations that in N. crassa had changed the IC(50) value for bafilomycin 10-fold or more and showed that out of the amino acids forming the plecomacrolide binding pocket only one amino acid (tyrosine 142) contributes to the binding of archazolid. Using a fluorescent derivative of N,N'-dicyclohexylcarbodiimide, we found that the binding site for archazolid comprises the essential glutamate within helix 4 of subunit c. In conclusion the archazolid binding site resides within the equatorial region of the V(O) rotor subunit c. This hypothesis was supported by an additional subset of mutations within helix 4 that revealed that leucine 144 plays a role in archazolid binding.

Palladium-catalyzed cross-coupling reactions of 2-bromocyclohex-1-enyl triflates 7 and 11 with a variety of alkenylstannanes occurred chemoselectively at the site of the triflate leaving group to give bromobutadienes which readily underwent Heck reactions with acrylates and styrene. Both steps could be performed in the same flask to give differentially functionalized hexatrienes in up to 88% overall yield. With simple stannanes, the same catalyst precursor could be used for both coupling steps making it possible to perform the whole sequence with only one portion of catalyst. For some of the functionally substituted stannanes, specifically adjusted catalyst systems had to be used. The 1,3,5-hexatrienes obtained were further transformed, in particular the methoxy-substituted compounds 14a-c were converted to bicyclo[4.4.0]decenones 30 (71-97%), bicyclo[4.3.0]nonenones 35 (74-93%), cyclodecynone 37a (47%), and cyclononynone 39a (15%). Thermal electrocyclizations of the other hexatrienes gave tetrahydronaphthalines 31 (60-61%), the tricyclic lactone 32 (72-75%) and decahydrophenanthrene 33 (75 %) in good yields.

The structure of a new secondary metabolite from Streptomyces sp. was determined as 4-acetyl-1,3-dihydroimidazo[4,5-c]pyridin-2-one by synthesis of the natural product itself and of the regioisomeric 7-acetylimidazo[4,5-b]pyridine derivative. The former compound was prepared, in 28% overall yield, in a sequence of nitration, reduction, condensation, and Stille reaction of 4-aminopyridine, while the regioisomer was obtained in 5% overall yield by amination, nitration, reduction, condensation, and oxidation of 4-ethylpyridine.

Aluminum organyls are valuable reagents for carbon-carbon bond formation as they can either be purchased at low prices or conveniently be prepared, for example, by hydro- or carboalumination of alkenes and alkynes. Although their application in reactions with α,β-unsaturated carbonyl compounds is rather limited, it creates a diverse picture concerning the selectivity of product formation which depends on several factors such as the type of organic residue, the conformation of the enone, the presence of additional functional groups and the donor ability of the solvent. This review describes issues of selectivity of such uncatalyzed transformations as well as conjugate additions catalyzed by achiral copper and nickel catalysts in which alanes can act similarly or even superiorly to the more common lithium or magnesium organyls. Moreover, the rapidly growing field of transition-metal-catalyzed asymmetric 1,2- and 1,4-additions is reviewed, in which aluminum reagents can not only lead to enantioselective alkyl additions, thus substituting zinc organyls, but also afford introduction of aryl, alkenyl and alkynyl groups. 1 Introduction 2 Uncatalyzed Additions to α,β-Unsaturated Carbonyl Compounds- 2.1 Regioselectivity with Plain Enones 2.2 Selective Conjugate Additions to Cyclic Enones 2.3 Conjugate Additions to Carboxylic Compounds 3 Racemic Copper- and Nickel-Catalyzed 1,4-Additions 4 Asymmetric Addition to Carbonyl Compounds 4.1 Enantioselective Conjugate Additions to Enones 4.2 Enantioselective 1,2-Additions 5 Conclusion

Two new alpha-pyridone metabolites, iromycins E and F, were isolated from cultures of strain Streptomyces sp. Dra 17, thus expanding the recently discovered iromycin family. The inhibitory potential on the mitochondrial respiratory chain was examined and revealed that iromycin metabolites block NADH oxidation in beef heart submitochondrial particles with different efficacy, yet remarkably show only very low cytotoxicity. Difference spectroscopic studies indicated that iromycins inhibit the electron transport at the site of complex I (NADH-ubiquinone oxidoreductase). Derivatives of the natural products were semisynthetically prepared and provided detailed insights into structure-activity relationships. Drawn from these results, there are strong similarities with the piericidins, which are among the most potent complex I inhibitors of the mitochondrial electron transport chain. Furthermore, total synthesis afforded new analogues, and the non-natural iromycin S (IC50 = 58 ng/mL) emerged as the most active compound, thus opening avenues of future studies with the iromycins as new valuable biochemical tools.

Spirodionic acid (1), a novel microbial metabolite with a spiro[4.5]decene skeleton, the 6-ethyl-2H-pyrone 5, dihydrosarkomycin (6), and other metabolites were isolated from the strain Streptomyces sp. Tu 6077. Structural elucidation was accomplished by NMR spectroscopic and mass-spectrometric studies, and the biosyntheses of compounds 1, 5, and 6 were investigated by feeding experiments with (13)C-labeled precursors. All results indicate a biogenetic sequence with metabolite 5 and sarkomycin (7) as precursors in the formation of spirocyclus 1 through an intermolecular Diels-Alder-type reaction.