Zur Kenntnis des Faktors F430 aus methanogenen Bakterien: Struktur des proteinfreien Faktors

Factor F430 from Methanogenic Bacteria: Structure of the Protein-free Factor Factor F430, the porphinoid nickel-containing coenzyme of the methylcoenzyme-M reductase of metanogenic bacteria is shown to be the 3³,8³,12²,13³,18²-pentaacid derivative of the pentamethylester F430M, the structure of which had been determined previously (see structural formulae 1 and 2 ). The structure assignment rests on chromatographic, UV/VIS-, CD-, IR-, and ¹³C-NMR-spectroscopic as well as FAB-mass spectral comparision of F430 with F430M and the pentaacid prepared by acid-catalyzed hydrolysis of F430M. In the cells of Methanobacterium thermoautotrophicum, factor F430 is present in a ‘bound’ and also, depending on the growth conditions, in ‘free’ form, the latter being defined as the part of total F430 that can be extracted from the cells under extremely mild conditions (80% EtOH at 0–4°). From the (protein)-‘bound’ form, F430 is extracted by subsequently treating the cells at 0–4° with 80% EtOH containing (e.g.), 2m LiCi. From both sources, the extracted factor is the same pentaacid, and there is no indication for the existence of a protein-free F430 species that would contain additional (covalently bound) structural elements.     

Coenzyme F430 from Methanogenic Bacteria: Complete Assignment of Configuration Based on an X-Ray Analysis of 12,13-Diepi-F430 Pentamethyl Ester and on NMR Spectroscopy

The structure of a derivative of coenzyme F430 from methanogenic bacteria, the bromide salt of 12,13-diepi-F430 pentamethyl ester ( 5 , X = Br), was determined by X-ray structure analysis. It reveals a more pronounced saddle-shaped out-of-plane deformation of the macrocycle than any hydroporphinoid Ni complex investigated so far. The crystal structure confirms the constitution proposed for coenzyme F430 ( 2 ) and shows that in the epimer 5 , the three stereogenic centers in ring D, C(17), C(18), and C(19), have the (17S)-, (18S)-, and (19R)-configuration, respectively. Deuteration and 2D-NMR studies independently demonstrate that native coenzyme F430 (2) has the same configuration in ring D as the epimer 5 . Therefore, our original tentative assignment of configuration at C(19) and C(18) [1] has to be reversed. This completes the assignment of configuration for all stereogenic centers in coenzyme F430, which has the structure shown in Formula 2 .     

Zur Kenntnis des Faktors F430 aus methanogenen Bakterien: Absolute Konfiguration

Factor F430 from Methanogenic Bacteria: Absolute Configuration Experiments on F430M ( 2 ) aiming at a potentially biomimetic, reductive reconstruction of the F430 ( 1 ) chromophore from corresponding pyrrocorphinate intermediates provided us with F430 derivatives which contain an isobacteriochlorinate chromophore system similar to the one occurring in sirohydrochlorin ( 3 ) (cf. the Scheme). Comparison of their CD spectra with the-CD spectrum of nickel( II )-sirohydrochlorinate octamethyl ester demonstrates that the absolute configurations of factor F430 and sirohydrochlorin in the region of rings A and B are the same.     

Zur Kenntnis des Faktors F430 aus methanogenen Bakterien: Struktur des porphinoiden Ligandsystems

Factor F430 from Methanogenic Bacteria: Structure of the Porphninoid Ligand System A structure is proposed for F430M, a non-cristalline methanolysis product of isolates of the nickel-containing, porphinoid factor F430 from Methanobacterium thermoautotrophicum. Crucial to the structure determination are five incorporation experiments with M. thermoautotrophicum (strain Marburg) in which the specifically mono-¹³C-labeled biosynthetic precursors (2-¹³C), (3-¹³C), (4-¹³C)-, (5-¹³C) ALA (ALA = δ-amino-levulinic acid) and L-(methyl-¹³C)methionine were incorporated into F430 with high efficiency. The ¹³C-NMR,-spectra of the specifically labeled F430M samples derived therefrom, together with the UV./VIS. spectral data of F430M, contain all the information necessary for the deduction of the constitution of the F430M chromophore, assuming the established pattern of porphinoid biosynthesis to be operative in F430 biosynthesis. ¹H-NMR. spectroscopy and, in particular, ¹H-NMR.-NOE-difference spectroscopy corroborates and completes the constitutional assignments and, furthermore, makes possible an almost complete derivation of the molecule's relative configuration. Schemes 3 and 4 summarize the results of ¹H-NMR. spectroscopy, presenting them within the context of the proposed structure for F430M. The assignment of absolute configuration implied in the formula is given preference because of F430M's very close structural and (assumed) biosynthetic relationship to sirohydrochlorin and vitamin B₁₂ (with respect to ring C, the assignment is based on degradative evidence). According to the proposed structure, the nickel complex F430M possesses an uroporphinoid (Type III) ligand skeleton with an additional carbocyclic ring and a chromophore system not previously encountered among natural porphinoids. It can be considered to be a (tetrahydro) derivative of the corphin system, combining structural elements of both porphyrins and corrins.     

Coenzyme F430 from Methanogenic Bacteria: Mechanistic studies on the reductive cleavage of sulfonium ions catalyzed by F430 pentamethyl ester

Mechanistic questions regarding the reductive cleavage of sulfonium ions by the Ni^I form of coenzyme F430 pentamethyl ester (F430M) were addressed in a series of kinetic studies and isotope labeling experiments. In neat DMF, methane formation from dialkyl(methyl)sulfonium ions consistently showed a delay time of ca. 1 h. In the presence of excess propanethiol, no delay was observed and methane formation followed pseudo-first-order kinetics with a logarithmic dependence of the initial rate on the concentration of propanethiol. From the temperature dependence of the reaction rate, an estimate for the activation parameters of ΔH^# = 49 kJ mol^?1 and (apparent) ΔS# = –114 J K^?1 mol^?1 was derived. The observation of deuterium incorporation into methane from (CH₃)₂CHOD, but not from (CH₃)₂CDOH, indicates that the fourth H-entity is introduced into CH₄ as a proton, and that free CH₃ radicals are not involved. In contrast to the reaction with the homogeneous one-electron reductant sodium naphthalide, the F430M-catalyzed reduction of mixed dialkyl(methyl)sulfonium ions showed a pronounced selectivity for the cleavage of Me? S over that of alkyl-S (alkyl ≠ Me) bonds. Mechanisms that are consistent with these results, as well as possible explanations for the time delay and the apparent highly negative entropy of activation, are discussed.     

Derivatives of Coenzyme F430 with a Covalently Attached α‐Axial Ligand. Part II

Coenzyme F430 pentamethyl ester 2 was partially hydrolyzed to a mixture of the five F430 tetramethyl esters 7 – 11 , which were separated by HPLC and identified by means of a full NMR characterization. The tetramethyl ester with a free COOH group at the side chain at C(3) of F430 was coupled to the N‐terminus of the peptidic spacer? ligand construct 12 selected and studied as described before. The UV/VIS and NMR spectra in CH₂Cl₂/3,3,3‐trifluoroethanol 6 : 1 show that the new derivative, the Ni^II(3³‐dehydroxy‐8³,12²,13³,18²‐tetra‐O‐methyl‐F430‐3³‐yl)‐L ‐prolyl‐L ‐prolyl‐N^π‐methyl‐L ‐histidine methyl ester ( 13 ), is an intramolecular, pentacoordinate, paramagnetic complex. In the same solvent system, the parent 3³,8³,12²,13³,18²‐penta‐O‐methyl‐F430 ( 2 ) is four coordinate and diamagnetic even in the presence of equimolar 1H‐imidazole. Protonation of the axially coordinating histidine residue of 13 gave the diamagnetic tetracoordinate base‐off form, which allowed us to establish the constitution of 13 by NMR.     

Coenzyme F430 from Methanogenic Bacteria: Oxidation of F430 Pentamethyl Ester to the Ni(III) Form

F430M, the pentamethyl ester of coenzyme F430, can be oxidized reversibly by one electron. The oxidation potential has been determined, and the electrolytically prepared oxidation product was characterized by its UV/VIS and ESR spectrum. The strongly anisotropic and nearly axial ESR spectrum is consistent with a S = ½ species with the unpaired-electron spin density predominantly in a d-type orbital of the central nickel ion. The properties of Ni(III)F430M are discussed in the context of two hypothetical mechanisms for the catalytic role of coenzyme F430 in methyl coenzyme M reductase, which catalyses the last step of methane formation in methanogenic bacteria.     

Coenzyme F430 from Methanogenic Bacteria: Detection of a Paramagnetic Methylnickel(II) Derivative of the Pentamethyl Ester by 2H-NMR Spectroscopy

A methylnickel(II) derivative of coenzyme F430 ( 1 ) was proposed as an intermediate in the enzymic process catalyzed by methyl-CoM reductasc. Indirect evidence points to formation of CH₃–F430M^II in the reaction of F30M¹ (obtained from F430M^II ( 2 )) with eleclrophilic methyl donors. The results presented here show, that such a compound does exist. A paramagnetic CD₃–Ni^II derivative 5b of the pentamethyl ester 2 (F430M) of coenzyme F430 was prepared by in situ methylation with (CD₃)₂Mg and characterized by its isotropically shifted ²H-NMR spectrum. At ?40°, the very broad D-signal of the axially coordinated CD₃ group is found at ?490 ppm. Comparison with the ²H- and ¹H-NMR spectra of mcthyl(tetramethylcyclam)nickel(II) derivatives 4 ([Ni^II(CH₃))(tmc)]CF₃SO₃ ( 4a ) is the only isolated CH₃–Ni derivative of a N₄macrocyclic Ni^II complex' shows that the large isotropic shift to high field is characteristic for a Me group axially bound to the Ni center. The temperature dependence of the isotropic shift of the CD₃–Ni group in both 4b and 5b follows Curie's law and yields ²H hyperfine coupling constants of ?0.65 ( 4b ) and ?0.85 MHz ( 5b ), respectively. The ¹H-NMR spectrum indicates that, in contrast to the five-coordinate monochloro complex [Ni^IICl(tmc)]⁺, intermolecular exchange of the axial ligand in [Ni^II(CH₃)(tmc)]⁺ 4a is either slow at the NMR time scale or does not occur at all.     

Cob(I)alamin als Katalysator. Retention der Konfiguration bei der reduktiv-protonenübertragenden Spaltung der Co,C-Bindung eines Alkylcobalamins. 7. Mitteilung [1]

Cob(I)alamin as Catalyst. 7. Communication [1]. Retention of Configuration during the Reductive Cleavage of the Co, C-Bond of an Alkylcobalamin Using catalytic amounts of cob(I)alamin (see Scheme 1) in aqueous acetic acid (?)-α-pinen ( 1 ) and (?)-β-pinen ( 2 ; s. Scheme 3) have been reduced. A large excess of metallic zinc served as electron source. The saturated products 5–8 (see Scheme 3) and the mechanistic aspects of their generation are discussed. The relative amounts of cis- ( 5 ) and trans-pinane ( 6 ) lead to the conclusion that the reductive cleavage of the Co, C-bond accompanied by H⁺ transfer in an alkylcobalamin occurs with retention of configuration. This result is in agreement with the corresponding cleavage of the Co,C-bond of an alkyl[hydroxy-diazaoctahydroporphinato]cobalt complex [9].     

Der massenspektrometrische Zerfall isomerer Diacetamido-cyclo-hexane,ihrer N-Phenäthyl-Derivate und Bis (acetamidomethyl)cyclohexane. 32. Mitteilung über massenspektrometrische Untersuchungen,,

The Mass Spectral Decomposition of Isomeric Diacetamido-cyclohexanes, their N-Phenethyl-Derivatives and Bis(acetamidomethyl)cyclohexanes In the mass spectra of the six isomeric diacetamidocyclohexanes 2--4 (cis and trans each, Scheme 2) as well as of the six isomeric bis(acetamidomethyl)cyclohexanes 6--8 (cis and trans each, Scheme 5) are clear differences between the constitutional isomers, whereas cis/trans isomers show very similar spectra. The lack of stereospecific fragmentations is explained by loss of configurational integrity of the molecular ion before fragmentation. However, the mass spectral fragmentation of epimeric diamidocyclohexanes becomes very stereospecific by the introduction of a phenethyl group on one of the nitrogen atoms: this group avoids epimerization of the molecular ion prior to fragmentation. In the N-phenethyl derivatives 10, 11, 13 and 14 (Scheme 8) the typical fragmentations of the cis-isomer after loss of ·C₇H₇ from the molecular ion are the elimination of CH₂CO by formation of cyclic ions, and the loss of p-toluenesulfonic acid or benzoic acid, respectively, with subsequent elimination of CH₃CN (Scheme 9). In the trans-isomer the typical fragmentations are the loss of the side chain bearing a tertiary nitrogen atom, and the elimination of the tosyl or benzoyl radical, respectively, with subsequent loss of CH₃CONH₂ (Scheme 10).     

Moderating influence of proteins on nonplanar tetrapyrrole deformations: coenzyme F430 in methyl-coenzyme-M reductase

Normal-coordinate structural decomposition, cluster analysis, and molecular mechanics calculations were undertaken to examine the effect of methyl-coenzyme-M reductase (MCR) on the nonplanar deformations of coenzyme F430. Although free 12,13-diepi-F430 has a lower energy conformation than free F430, the protein restraints exerted by MCR are responsible for F430 having a lower energy conformation than the 12,13-diepimer in MCR. According to the NSD analysis, the crystal structure of free diepimerized F430M is highly distorted. In MCR the protein prevents 12,13-diepi-F430 from undergoing nonplanar deformations; therefore, MCR favors F430 over the 12,13-diepimeric form. The strain imposed on 12,13-diepi-F430 in the protein is so large that although 88% of free F430 is found in the diepimeric form, none of the diepimeric form is found in MCR. This is of significance since the two forms have different chemistries. MCR also moderates the nonplanar deformations of coenzyme F430, which are known to affect redox potentials and axial ligand affinities in tetrapyrroles, suggesting that the protein environment (MCR) is responsible for tuning the chemistry of the active site nickel ion. F430 is bound to MCR by hydrogen bonds between the protein and the F430 carboxylate groups. Conformational searches have shown that F430 has very little rotational and translational freedom within MCR.     

Structure of an F430 variant from archaea associated with anaerobic oxidation of methane

Microbial mats collected at cold methane seeps in the Black Sea carry out anaerobic oxidation of methane (AOM) to carbon dioxide using sulfate as the electron acceptor. These mats, which predominantly consist of sulfate-reducing bacteria and archaea of the ANME-1 and ANME-2 type, contain large amounts of proteins very similar to methyl-coenzyme M reductase from methanogenic archaea. Mass spectrometry of mat samples revealed the presence of two nickel-containing cofactors in comparable amounts, one with the same mass as coenzyme F430 from methanogens (m/z = 905) and one with a mass that is 46 Da higher (m/z = 951). The two cofactors were isolated and purified, and their constitution and absolute configuration were determined. The cofactor with m/z = 905 was proven to be identical to coenzyme F430 from methanogens. For the m/z = 951 species, high resolution ICP-MS pointed to F430 + CH2S as the molecular formula, and LA-ICP-SF MS finally confirmed the presence of one sulfur atom per nickel. Esterification gave two stereoisomeric pentamethyl esters with m/z = 1021, which could be purified by reverse phase HPLC and were subjected to comprehensive NMR analysis, allowing determination of their constitution and configuration as (17(2)S)-17(2)-methylthio-F430 pentamethyl ester and (17(2)R)-17(2)-methylthio-F430 pentamethyl ester. The corresponding diastereoisomeric pentaacids could also be separated by HPLC and were correlated to the esters via mild hydrolysis of the latter. Equilibration of the pentaacids under acid catalysis showed that the (17(2)S) isomer is the naturally occurring albeit thermodynamically less stable one. The more stable (17(2)R) isomer (80% at equilibrium) is an isolation artifact generated under the acidic conditions necessary for the isolation of the cofactors from the calcium carbonate-encrusted mats.     

Carbocyclen aus Monosacchariden. III. Zur Diastereoselektivität der Bildung von Cyclopentanderivaten. Umsetzungen in der Galactosereihe

Carbocycles from monosaccharides. III. Concerning the diastereoselective formation of cyclopentane derivatives. Transformations in the galactose series. The diastereoselectivity of the intramolecular nitrone-olefine cycloaddition of 1 , 3 and 4 (Scheme 1), yielding only 2 , 5 and 6 but none of the isomers 8 , 9 and 10 is explained by assuming a kinetic control and postulating that the relative activation energies of the two relevant transition states in the cyclization of e.g. 1 can be estimated from the conformers A and B , the latter being destabilized by a synperiplanar arrangement of the nitrone function and the 2-alkoxy-group (Scheme 2). It is further postulated, that this destabilization is responsible for the formation of (2,3)-trans configurated products. Since 2 , 5 and 6 are presumably thermodynamically more stable than 8 , 9 and 10 , a case was investigated, where the cycloaddition can either give thermodynamically less stable (2,3)-trans-product such as 12 or a thermodynamically more stable (2,3)-cis-product such as 13. 12 and 13 could both be formed from the aldehyde 25 via the nitrone 11 (Schemes 3 and 5). Treatment of the galactoside 16 first with Zn in aqueous butanol (forming among other products 25 and its 2-debenzyl-oxy-derivative) and then with N-Methyldroxylamine yielded the isoxazolidines 12 (72%), 13 (2%) and 27 (7%) (Schemes 4 and 6). Similarily, the anomeric silylated galactosides 17 and 23 gave 29 (78% from 17 , 77% from 23 ) and 27 (5% and 3%). Upon desilylation, 29 gave 32 , which was converted into 12 . The structure of the isoxazolidines was unambiguously deduced from their NMR. spectra and those of their derivatives 33 and 34 . Compound 32 was further transformed into its deoxyderivative 36 . The high diastereoselectivity of the cycloaddition restricts the number of diastereomeric, pentasubstituted cyclopentanes available by this method. However, cyclization of the 2-Hydroxy-aldehyde 37 (Scheme 8) gave the kinetically less favoured isomer 40 in a higher proportion, showing the differential influence of hydrogen-bonds on the relevant activation energies. Thermolysis of 32 gave 40 (79%) and 41 (11%). The structure of 41 was deduced from its NMR. spectra and those of its derivatives 42 and 43 . Thermolysis of 29 gave, after desilylation, 41 (42%), 40 (22%) and 32 (13%) and thermolysis of 6 lead to a 25 : 75 equilibrium with 44 (combined yield 90%). These transformations illustrate means leading to additional isomers and are in agreement with the proposed explanation of the diastereoselectivity in question.     

Über Umlagerungen bei der Cyclialkylierung von Arylpentanolen zu 2,3‐Dihydro‐1H‐inden‐Derivaten, 5. Mitteilung

On Rearrangements by Cyclialkylations of Arylpentanols to 2,3‐Dihydro‐1 H ‐indene Derivatives. Part 5. The Acid‐Catalyzed Cyclialkylation of 2‐(2‐Chlorophenyl)‐2,4‐dimethylpentan‐3‐ol The mechanism proposed in [1] to explain the surprising result of the cyclialkylation of 4‐(2‐chlorophenyl)‐2,4‐dimethylpentan‐2‐ol ( 3 , R=Me), which gives not only the ‘normal' product, i.e., the 4‐chloro‐2,3‐dihydro‐1,1,3,3‐tetramethyl‐ ( 4 ), but also the isomer trans‐4‐chloro‐2,3‐dihydro‐1,1,2,3‐tetramethyl‐1H‐inden ( 5 ), could be differentiated in two sections (cf. Scheme 2): the first from 3 to the intermediary ion IIa ⇌ IIb , and the second from the latter ions to the final product 5 . For the first section, a sufficiently satisfactory explanation has been given in [1]; the second section has received important support from the mechanisms of the cyclialkylation of 2,4‐dimethyl‐2‐phenylpentan‐3‐ol ( 6 ), the precursor of II′a , the ion IIa without the o‐Cl substituent (cf. Schemes 2, 3 and 5 and [4]). The present communication gives an explanation of the influence of the o‐Cl substituent: a mechanism is proposed for the very complex cyclialkylation of 2‐(2‐chlorophenyl)‐2,4‐dimethylpentan‐3‐ol ( 11 ; cf. Scheme 9). Both mechanism may be considered as definitive. It is very surprising that, by the cyclialkylation of the compounds 1, 3, 8, 11, 15 , and 17 , only compound 1 gives the ‘normal' product; the cyclialkylation of all other phenylpentanols follows complex pathways including Et, i‐Pr, and Ph migrations, which could not be expected. In addition, it has been established that the transformation of 21 to 22 (cf. Scheme 12) and that of 23 to 24 (cf. Scheme 13) occur through two consecutive 1,2‐ and not through a single 1,3‐hydride migration or through an elimination‐addition process (cf. Scheme 13). It can be assumed that the transformation of ion IV (the 2‐(2‐chlorophenyl)‐3,4‐dimethylpent‐2‐ylium ion) to the ion V (the 4‐(2‐chlorophenyl)‐3,4‐dimethylpent‐2‐ylium ion (both shown in Scheme 9 as D‐isomers) occurs through the same pathway.     

Über Umlagerungen bei der Cyclialkylierung von Arylpentanolen zu 2,3‐Dihydro‐1H‐inden‐Derivaten, 2. Mitteilung

On Rearrangements by Cyclialkylations of Arylpentanols to 2,3‐Dihydro‐1 H ‐indene Derivatives. Part 2. An Unexpected Rearrangement by the Acid‐Catalyzed Cyclialkylation of 2,4‐Dimethyl‐2‐phenylpentan‐3‐ol under Formation of trans ‐2,3‐Dihydro‐1,1,2,3‐tetramethyl‐1 H ‐indene The acid catalyzed‐cyclialkylation of 4‐(2‐chloro‐phenyl)‐2,4‐dimethylpentan‐2‐ol ( 1 ) gave two products: 4‐chloro‐2,3‐dihydro‐1,1,3,3‐tetramethyl‐1H‐indene ( 2 ) and also trans‐4‐chloro‐2,3‐dihydro‐1,1,2,3‐tetramethyl‐1H‐indene ( 3 ). A mechanism was proposed in Part 1 (cf. Scheme 1) for this unexpected rearrangement. This mechanism would mainly be supported by the result of the cyclialkylation of 2,4‐dimethyl‐2‐phenylpentan‐3‐ol ( 4 ), which, with respect to the similarity of ion II in Scheme 1 and ion V in Scheme 2, should give only product 5 . This was indeed the experimental result of this cyclialkylation. But the result of the cyclialkylation of 1,1,1,2′,2′,2′‐hexadeuterated isomer [²H₆]‐ 4 of 4 (cf. Scheme 3) requires a different mechanism as for the cyclialkylation of 1 . Such a mechanism is proposed in Schemes 5 and 6. It gives a satisfactory explanation of the experimental results and is supported by the result of the cyclialkylation of 2,4‐dimethyl‐3‐phenylpentan‐3‐ol ( 9 ; Scheme 7). The alternative migration of a Ph or of an i‐Pr group (cf. Scheme 6) is under further investigation.     

Photolyse von 3-Methyl-2, 1-benzisoxazol (3-Methylanthranil) und 2-Azido-acetophenon in Gegenwart von Schwefelsäure und Benzolderivaten

Photolysis of 3-Methyl-2, 1-benzisoxazole (3-Methylanthranil) and 2-Azido-acetophenone in the Presence of Sulfuric Acid and Benzene Derivatives Irradiation of 3-methylanthranil ( 1 ) in acetonitrile in the presence of sulfuric acid and benzene, toluene, p-xylene, mesitylene or anisole with a mercury high-pressure lamp through a pyrex filter yields beside varying amounts of 2-amino-acetophenone ( 3 ) and 2-amino-5-hydroxy- ( 4a ) and 2-amino-3-hydroxy-acetophenone ( 4b ) the corresponding diphenylamine derivatives 5 (see Table 1). In the case of toluene and anisole mixtures of the corresponding ortho- and para-substituted isomers ( 5b, 5d or 5g, 5i respectively), but no meta-substituted isomers ( 5c or 5h ) are obtained. In addition to these products, the irradiation of 1 in the presence of anisole yields also 2-amino-5-(4′-methoxyphenyl)-acetophenone ( 7 ), 2-amino-3-(4′-methoxyphenyl)-acetophenone ( 8 ) and 2-methoxy-9-methyl-acridine ( 6 ; see Scheme 1). The latter product is also formed thermally by acid catalysis from the diphenylamine derivative 5i . Irradiation of 2-azido-acetophenone ( 2 ) in acetonitrile solution in the presence of sulfuric acid and benzene leads to the formation of 1, 3, 4a, 4b, 5a and 9 (see Table 2). Compounds 3, 4a, 4b and 5a are also obtained after acid catalyzed decomposition of 2 in the presence of benzene. Thus, it is concluded that irradiation of 1 or 2 in the presence of sulfuric acid yields 2-acetyl-phenylnitrenium ions 10 in the singlet ground state which will undergo electrophilic substitution of the aromatic compounds, perhaps via the π-complex 11 (see Scheme 2).     

Beitrag zum Reaktionsmechanismus der Synthese von Seychellen [1]

New Mechanistical Details Concerning the Synthesis of Seychellen [1] In the last step of our synthesis of Seychellen ( 2 ) [1], the solvolysis of 1 , only one side-product was formed, namely 3 (Scheme 1). Now the structure of 3 has been elucidated, mainly by spectroscopic studies of its derivatives 7 and 9 (Scheme 2). In order to differentiate between two different solvolytic pathways from 1 to 3 (see Scheme 1 and 3) d₃- 1 was prepared. Solvolysis of d₃- 1 proved the mechanism shown in Scheme 1. Solvolysis of 1 and of 2-epi- 1 , respectively, furnished the same product distribution, which makes a common intermediate a very probable. In both cases 10 is an intermediate, which is slowly converted into 2 and 3 . 2-epi- 1 was prepared from 1 (Scheme 5). Kinetic measurements with 1 , d₃- 1 and 2-epi- 1 are also in agreement with the mechanism drawn in Scheme 4: k₁(72°) = (5,2±0,5) · 10^?5 sec^?1, k₁(H)/k₁(D)(72°) = 1,4±0,15; k₂(H)/k₄(H) = 0,66 and k₂(H)/k₂(D) = 2,2 if k₄(H) ≈ k₄(D) is assumed.     

Photochemische Umsetzung von optisch aktiven 2-(1′-Methylallyl)anilinen mit Methanol

Photochemical Reaction of Optically Active 2-(1′-Methylallyl)anilines with Methanol It is shown that (?)-(S)-2-(1′-methylallyl)aniline ((?)-(S)- 4 ) on irradiation in methanol yields (?)-(2S, 3R)-2, 3-dimethylindoline ((?)-trans- 8 ), (?)-(1′R, 2′R)-2-(2′-methoxy-1′-methylpropyl)aniline ((?)-erythro- 9 ) as well as racemic (1′RS, 2′SR)-2-(2′-methoxy-1′-methylpropyl) aniline ((±)-threo- 9 ) in 27.1, 36.4 and 15.7% yield, respectively (see Scheme 3). By deamination and chemical correlation with (+)-(2R, 3R)-3-phenyl-2-butanol ((+)-erythro- 13 ; see Scheme 4) it was found that (?)-erythro- 9 has the same absolute configuration and optical purity as the starting material (?)-(S)- 4 . Comparable results are obtained when (?)-(S)-N-methyl-2-(1′-methylallyl)aniline ((?)-(S)- 7 ) is irradiated in methanol, i.e. the optically active indoline (+)-trans- 10 and the methanol addition product (?)-erythro- 11 along with its racemic threo-isomer are formed (cf. Scheme 3). These findings demonstrate that the methanol addition products arise from stereospecific, methanol-induced ring opening of intermediate, chiral trans, -(→(?)-erythro-compounds) and achiral cis-spiro [2.5]octa-4,6-dien-8-imines (→(±)-threo-compounds; see Schemes 1 and 2).     

Direct determination of the number of electrons needed to reduce coenzyme F430 pentamethyl ester to the Ni(I) species exhibiting the electron paramagnetic resonance and ultraviolet-visible spectra characteristic for the MCR(red1) state of methyl-coenzyme M reductase

The UV-visible and electron paramagnetic resonance (EPR) spectra of MCR(red1), the catalytically active state of methyl-coenzyme M reductase, are almost identical to those observed when free coenzyme F430 or its pentamethyl ester (F430M) are reduced to the Ni(I) valence state. Investigations and proposals concerning the catalytic mechanism of MCR were therefore based on MCR(red1) containing Ni(I)F430 until, in a recent report, Tang et al. (J. Am. Chem. Soc. 2002, 124, 13242) interpreted their resonance Raman data and titration experiments as indicating that, in MCR(red1), coenzyme F430 is not only reduced at the nickel center but at one of the C=N double bonds of the hydrocorphinoid macrocycle as well. To resolve this contradiction, we have investigated the stoichiometry of the reduction of coenzyme F430 pentamethyl ester (F430M) by three independent methods. Spectroelectrochemistry showed clean reduction to a single product that exhibits the UV-vis spectrum typical for MCR(red1). In three bulk electrolysis experiments, 0.96 +/- 0.1 F/mol was required to generate the reduced species. Reduction with decamethylcobaltocene in tetrahydrofuran (THF) consumed 1 mol of (Cp)(2)Co/mol of F430M, and the stoichiometry of the reoxidation of the reduced form with the two-electron oxidant methylene blue was 0.46 +/- 0.05 mol of methylene blue/mol of reduced F430M. These experiments demonstrate that the reduction of coenzyme F430M to the species having almost identical UV-vis and EPR spectra as MCR(red1) is a one-electron process and therefore inconsistent with a reduction of the macrocycle chromophore.     

Anwendung der α-Alkinon-Cyclisierung: Synthese von rac-Modhephen

Application of the α-Alkynone Cyclization: Synthesis of rac-Modhephene rac-Modhephene 1 , the first sesquiterpene with a propellane C-skeleton and its epimer rac-epi-modhephene 27 , were synthesized starting from bicyclo[3.3.0]oct-1(5)-en-2-one ( 2 ). The key step in the construction of the [3.3.3]-propellane system is an application of the α-alkynone cyclization, namely 3 → 4 and 11 → 14 . The preferred formation of the propellanes 4 and 14 in this step shows that the insertion of the postulated alkylidene carbene intermediate into tertiary C,H-bonds outweighs the one into the secondary ring-C,H-bonds leading to 12/13 and 15/16 , respectively. The two starting materials for the α-alkynone cyclization, 3 and 11 , were prepared from 2 by the reactions shown in Scheme 3. The further elaboration and separation of the cyclization products 4 and 14 to rac-modhephene 1 and its epimer 27 are outlined in Scheme 5.