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].     

Electrofugal Fragmentation of Alkylcobalamin Derivatives Using Cob(I)Alamin and Heptamethyl Cob (I)yrinate as Catalysts

The cob (I)alamin- ( 1(I) ) and the heptamethyl cob(I)ynnate- ( 2(I) ) catalyzed transformation of an epoxide to the corresponding saturated hydrocarbon 3→4→5 is examined (see Schemes 1 and 3–5). Under the reaction conditions, the epoxyalkyl acetate 3 is opened by the catalysts with formation of appropriate (b?-hydroxyalkyl)-corrinoid derivatives ( 13 , 14 , 17 , 18 , see Schemes 12 and 14). Triggered by a transfer of electrons to the Co-corrin-π system, the Co, C-bond of the intermediates is broken, generating the alkenyl acetate 4 (cf. Schemes 12 and 14) following an electrofugal fragmentation (cf. Schemes 2 and 12). The double bond of 4 is also attacked by the catalysts, leading to the corresponding alkylcorrinoids ( 15 , 19 , see Schemes 12 and 14) which in turn are reduced by electrons from metallic zinc, the electron source in the system, inducing a reductive cleavage of the Co, C-bond with production of the saturated monoacetate 5 (see Schemes 2, 5 and 12). In the cascade of steps involved, the transfer of electrons to the intermediate alkylcorrinoids ( 13–15 , 17–19 , see Schemes 12 and 14) is shown to be rate-limiting. Comparing the two catalytic species 1(I) and 2(I) , it is shown that the ribonucleotide loop protects intermediate alkylcobalamins to some extent from an attack by electrons. The protective function of the ribonucleotide side-chain is shown to be present in alkylcobalamins existing in the base-on form (cf. Chap. 4 and see Scheme 14).     

Cob(I)alamin als Katalysator, 5. Mitteilung [1]. Enantioselektive Reduktion α,β-ungesättigter Carbonylderivate

Cob(I)alamin as Catalyst. 5. Communication [1]. Enantioselective Reduction of α,β-Unsaturated Carbonyl Derivatives The cob(I)alamin-catalyzed reduction of an α,β-unsaturated ethyl ester in aqueous acetic acid produced the (S)-configurated saturated derivative 2 with an enantiomeric excess of 21%. The starting material 1 is not reduced at pH = 7.0 in the presence of catalytic amounts of cob(I)alamin (see Scheme 2). It is shown that the attack of cob(I)alamin and not of cob(II)alamin, also present in Zn/CH₃COOH/H₂O, accounts for the enantioselective reduction observed. All the (Z)-configurated starting materials 1 , 3 , 5 , 7 , 9 and 11 have been transformed to the corresponding (S)-configurated saturated derivatives 2 , 4 , 6 , 8 , 10 and 12 , respectively. The highest enantiomeric excess revealed to be present in the saturated product 12 (32,7%, S) derived from the (Z)-configurated methyl ketone 11 (see Scheme 3 and Table 1). The reduction of the (E)-configurated starting materials led mainly to racemic products. A saturated product having the (R)-configuration with a rather weak enantiomeric excess (5.9%) has been obtained starting from the (E)-configurated methyl ketone 23 (see Scheme 5 and Table 2). The allylic alcohols 16 and 24 have been reduced to the saturated racemic derivative 17 .     

Cob(I)alamin Differentiating Alkenes During Saturation

The olefins 2, 7, 11 , and 19 , have been reduced using catalytic amounts of cob(I)alamin( I(I) ). During a slow saturation, the catalyst is able to differentiate the two diastereotopic faces of the endocyclic double bonds in 11 (t_1/2 4 h,. cf. Scheme 3) are reduced much faster. A rationalization of the data can be obtained formulating tertiary alkylcobalamins as intermediates. Of the oxime 6 (cf. Scheme 2) and the p- bromobenzoate 23 (cf. Scheme 5) the structures have been determined by X-ray analysis.     

Aspects of the Reduction of Double Bonds Using Cob (I)alamin as Catalys

The olefinic system in 3β-methoxy-4-cholesten-6 a-ol ( 2 ) is reduced using cob (I)alamin ( 1 ( I ); see Scheme 1) as catalyst, aqueous acetic acid as solvent and metallic zinc as electron source (cf. Schemes 2 and 3). Experimental evidence for an attack of 1 ( I ) on both faces of the double bond is presented. By the same catalyst (1 R)-10, 10-dimethyl-2-pinene- 10-carbonitrile ( 9 ) is first transformed to the menthene derivative 11 (see Schemes 4 and 5). The ring opening is then followed by a fast saturation of the disubstituted olefinic system in 11 , and ultimately the remaining double bond is reduced in a slow reaction. The cis-configurated saturated menthane derivative 16 is the main final product ( 16 / 17 ≈ 10:1).     

Cob(I)alamin as Catalyst. 8th Communication. Cob(I)alamin and Heptamethyl Cob(I)yrinate During the Reduction of α,β-Unsaturated Carbonyl Derivatives

Hydrogen bonds as presented in Figure 2 cannot account for the enantioselective attack of cob(I)alamin ( 4 ( I )) or heptamethyl cob(I)yrinate ( 5 ( I )) on one of the two enantiotopic faces of the substrates. The attack of the strongly nucleophilic 3d orbital is preferentially directed to the re-side of the starting materials with (Z)-configuration and leads, after the highly stereoselective reductive cleavage of the Co, C bond, to saturated products with (S)-configuration in varying enantiomeric excesses (see Schemes 1, 3 and Table 1).     

Thermodynamic trans-Effects of the Nucleotide Base in the B12 Coenzymes

The thermodynamic effects of the nucleotide coordination on the Co-C bond strengths in the B ₁₂ coenzymes were analyzed. Methyl group transfer reactions from methylcob( III )inamides to cob( II )inamides and cob( I )inamides in neutral aqueous solution were used in equilibration experiments to determine the effect fo the intramolecular coordination of the nucleotide function on the Co-C bond dissociation energies of methylcob( III )alamin ( 4 ). In the equilibrium between 4 , cob( I )inamide ( 11 ), cob( I )alamin ( 10 ) and methylcob( III )inamide 6 (Scheme 2), 4 and 11 were found to predominate ( 4 + 11 ? 10 + 6 , equilibrium constant K_I/III≈0.004), while the equilibrium between 4 , cob( II )inamide 9 , cob( II )alamin ( 5 ), and 6 (Scheme 1) proved to be well balanced ( 4 + 9 ? 5 + 6 , equilibrium constant K_II/III=0.60). These equilibrium values indicate the nucleotide coordination to stabilize the Co–C bond in 4 both against homolysis (slight effect) and against nucleophilic heterolysis (considerable effect). They reflect a stabilization of the complete corrins 4 and 5 by the nucleotide coordination, which is also indicated for 4 and 5 by their (nucleotide) basicity. The latter information, where available for other organocobalamins, allows the analysis of the thermodynamicnucleotide trans effect there as well: e.g. in coenzyme B ₁₂ ( 1 ), the nucleotide coordination is found this way to weaken the Co–C bond towards homolysis by ca. 0.7 kcal/mol.     

Cob (I)alamin als Katalysator 4. Mitteilung. Reduktion von α,β-ungesättigten Nitrilen

Cob(I)alamin as Catalyst. 4. Communication. Reduction of α,β-Unsaturated Nitriles Using catalytic amounts of cob (I)alamin and an excess of metallic zinc as source of electrons 1-naphthonitril ( 5 ) has been reduced to (1-naphthyl)methylamin ( 6 ) and in small amounts to (1-naphthyl)methanol ( 7 ) and (1,2,3,4-tetrahydro-1-naphthyl)methanol ( 8 ) (5 ½ h, CH₃COOH/H₂O; s. Scheme 3). Starting from cyclododecylideneacetonitrile ( 15 ) similar conditions (68 h, CH₃COOH/H₂O) produced the amines 16–19 as well as the nitrogen free saturated aldehyde 20 , the corresponding allylic alcohol 21 and the saturated derivative 22 (s. Scheme 6). It is deduced that the first attack of cob (I)alamin on an α,β-unsaturated nitrile might occur on both the nitrile dipole as well as on the carbon atom in β-position. Cob (I)alamin in aqueous acetic acid saturates the isolated double bonds in allylic alcohols and amines. In a slow reaction the two different aromatic rings of (1-naphthyl)methanol ( 7 ) have been reduced giving the corresponding tetrahydronaphthalene derivatives 8 and 12 , and in one case the production of the octahydroderivative 14 has been observed in a low yield (s. Scheme 5).     

Skeletal Migrations Observed during the Cob(I)alamin-Catalyzed Reduction of 4β-(tert-Butyl)-1β-(1-methylvinyl)cyclohexanecarbaldehyde

During the cob(I)alamin( 1(I) )-catalyzed reduction of 3 , intermediate formation of 2 and final generation of 4–10 was observed (see Scheme 1, cf. Tables 1 and 2). Identical products in similar ratios were generated starting from either 2 or 3 . Accepting the intermediate formation of six interconnected cobalt complexes, i.e. A–F (cf. Scheme 2), the generation of all the products observed can be explained.     

Carbocyclische Verbindungen aus Monosacchariden. I. Umsetzungen in der glucosereihe

Carbocyclic Compounds from Monosaccharides. 1. Transformations in the Glucose Series A method for the preparation of pentasubstituted cyclopentanes from monosaccharides is presented, involving two crucial steps, viz. the reductive fragmentation of 5-bromo-5-deoxyglucosides (such as 10, 17 and 23 , see Scheme 3) with Zn or butyl lithium yielding 5,6-dideoxy-hex-5-enoses (such as 11 and 24 , see Schemes 3 and 4), and the subsequent cyclization of these hexenoses with N-methyl- or N-(alkoxyalkyl)hydroxylamines (via the corresponding nitrones) to form cyclopentano-isoxazolidines (see Scheme 2). Thus, the glucosides 17 and 23 were converted diastereoselectively and in good yields into the cyclopentano-isoxazolidines 27 and 45 (Schemes 5 and 7), which were characterized by their transformation into various derivatives. 27 and 45 were correlated through the common derivative 62 . The configuration of the cyclization products were established by pyrolysis of the N-oxide 65 to the enol ether 67 (Scheme 10).     

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

The irreversible-reduction potentials of 26 alkylcob(III)alamins (RCbl^III 1a – z ) and 26 alkylcob(III)yrinates (R‘Cby’^III; 2a – z ) (E_p 1a – z and E_p 2a – z , resp.) have been measured in situ by single-scan voltammetry of hydroxocob(III)alamin hydrochloride (vitamin B_12b- HCl; 1 ) or heptamethyl cob(II)yrinate perchlorate (ClO₄‘Cby’^II; 2 ) in presence of the corresponding alkyl halides (RX; 3a – z ) in DMF. The reduction potentials of alkylcobalt complexes exhibiting half-life times as short as a few seconds become measurable by this technique. Thermodynamic cycles prove that the observed reduction potentials are closely related to the standard reduction potentials E°(R? Co^III + e^??R? + Co^I). Electron-withdrawing groups and/or an increased degree of substitution at the Co-bound C-atom in RCbl^III and, R‘Cby’^III shift E_p( 1a – z ) and E_p ( 2a – z ) towards positive potentials. Linear correlations have been found between E_p( 1a – z ) (E_p( 2a – z )) of RCbl^III (R‘Cby’^III) and the pK_a of RH (or the Taft σ*- or the Hammett σ-values of R) within each class of R, i. e. MeCbl^III (Me‘Cby’^III), primary RCbl^III (R‘Cby’^III) and secondary RCbl^III (R‘Cby’^III). The correlations allow to distinguish between electronic effects of the Co-bound alkyl residues and their steric interactions with the corrin side chains. The correlations have further been used to visualize the light-induced formal insertion of an olefin into the Co, C-bond of an alkylcobalamin (Scheme 2, 1a → 1u ), a key step in the vitamin-B₁₂-catalized C, C-bond formation.     

Thermal Methyl-Group Transfer between Methylcobalt(III) Corrinates and Cobalt(II) Corrinates. Equilibration Experiments with Heptamethyl Cobyrinates and Cobalamins

Separate neutral aqueous solutions of either (a) methylcob(III)alamin ( 2 ) and (heptamethyl cob(II)yrinate) perchlorate ( 3 ) or of b) cob(II)alamin ( = vitamin B_12r; ( 4 ) and [Coβ-methyl(heptamethyl cob(III)yrinate)] perchlorate ( 5 ) equilibrated thermally at r.t. according to 2 + 3 ? 4 + 5 . The corresponding equilibrium constant K_e was determined (K_e = 0.63 ± 0.15). This equilibration experiment indicates that the coordination of the nucleotide function in methylcob(II)alamin ( 2 ) hardly affects the thermodynamics of the Co? C bond homolysis in aqeous solution when compared to nucleotide-free methylcorrinoids such as 5 .     

Protolytic Opening of Two Diastereoisomeric Cyclopropanols

Cob(I) alamin ( 1(I) )-catalyzed reduction of the aldehyde 2 led to the two crystalline cyclopropanols 3 and 4 (see Scheme 2). The protolytic ring-opening starting from 3 produced the saturated aldehydes 6 and 7;8 was formed in traces only (see Scheme 3). The protolysis starting from 3 led, therefore, mainly to retention of configuration at the spiro C-atom ( 7 ); ring-opening with inversion was observed in traces only ( 8 ). Starting from 4 , the protolysis produced 9 and 7 ; the absence of 8 showed this protolysis to proceed 9 and 7 ; the absence of 8 showed this protolysis to proceed exclusively with inversion of configuration at the spiro center. Of the p-bromobenzoate 5 (cf. Scheme 2) the structure has been determined by X-ray analysis.     

the Mechanism of the Oxidation of Cob(I)alamins

Abstract

Spectroelectrochemical investigations of the reoxidation sequence of the reduced cob(I)alamin to the oxidized cob(III)alamin show that two different cob(II)alamin intermediates are formed during the processes which appear to correlate to base-on and base-off cob(II)-alamin species.     

The chemistry of coumarin derivatives. Part 3. Synthesis of 3-alkyl-4-hydroxycoumarins by reductive fragmentation of 3,3′-alkyiidene-4,4′-dihydroxybis[coumarins]

Treatment of 3,3′-alkylidene-4,4′-dihydroxybis[coumarins] 1 with NaBH₃CN in refluxing MeOH affords 3-alkyl-4-hydroxycoumarins 2 and 4-hydroxycoumarin ( 3 ; Scheme 1). The reaction might take place via hydride trapping of alkylidenechromandiones C formed from 1 in a retro-Michael reaction. Such a retro-Michael reaction of 1 might be biologically relevant. The presence of C during the reductive fragmentation 1 → 2 is suggested by Diels-Alder and nucleophilic trapping of the alkylidenechromandiones C as well as from cross-over experiments with coumarins other than 3 (see Scheme 2). The reductive fragmentation of 1 allows the chemo- and regioselective synthesis of a variety of 3-alkyl-4-hydroxycoumarins 2 (see Table).     

Lacton-Bildung durch ringerweiternde Fragmentierung von Epoxycyclodecanon-Derivaten

Lactones from Epoxycyclodecanone Derivatives by Ring Enlargement Involving Fragmentation Reactions A stereospecific ring-enlargement reaction of alkyl esters of 2,3-epoxy-1-(3-hydroxypropyl)-10-oxocyclo-decanecarboxylic-acid derivatives is described, involving Grob fragmentation of in situ formed hemiacetals. The assignment of the relative configuration of the starting materials was accomplished on the basis of ¹H-NMR data. The rearrangement of the epoxides 9 and 10 (with cis-orientation of the ester group and the epoxide ring, Scheme 1) gives the lactone 15 as the single and as the major product, respectively, with (Z)-configuration of the newly formed C?C bond. A concerted reaction mechanism is assumed. The formation of a small amount of 12 from 10 is probably due to a competitive two-step carbanion pathway. The reaction of the diastereoisomers 7 and 8 leads to the lactones 11 and 12 , respectively, as the only ring-enlargement products (Scheme 1), with (E)-configuration of the newly formed C?C bond. On the basis of our results, we cannot distinguish in this case between a concerted and a two-step carbanion mechanism. This type of reaction takes place only in the presence of an ester group; no ring enlargement was detected in case of compound 20 (Scheme 3), which is the de(alkoxycarbonyl) derivative of 9 . The eliminative opening of the epoxide ring in the epoxylactone 17 affords 11 as the single product (Scheme 2). A carbanion mechanism was assumed for this reaction.     

Zur Photochemie von 1H- und 2H-Indazolen in saurer Lösung

On the Photochemistry of 1H- and 2H-Indazoles in Acidic Solution It is shown that 1H- and 2H-indazoles (cf. Scheme 2) on protonation (0, 1N H₂SO₄ in water or alcoholic solution) give analogous indazolium ions (see Fig. 1 and 2) which on irradiation undergo heterolytic cleavage of the N (1), N (2) bond whereby aromatic nitrenium ions in the singlet ground state are formed (cf. Scheme 13). If the para position of these nitrenium ions is not occupied by a substituent (e.g. a methyl group) they are readily trapped by nucleophiles present (e.g. water, alcohols, chloride ions) to yield the corresponding 5-substituted 2-amino-benzaldehydes or acetophenones (cf. Schemes 4–10). Photolysis of indazole ( 4 ) and 3-methyl-indazole ( 5 ) in 0,75N H₂SO₄ in alcoholic solutions gives in addition minor amounts of the corresponding 3-substituted 2-amino-benzaldehydes and acetophenones, respectively (cf. Schemes 6 and 8 and Table 2). Phenylnitrenium ions carrying a methyl group in the para position give in aqueous sulfuric acid mainly the reduction products, i.e. 2-amino-5-methyl-benzaldehydes (cf. Schemes 11 and 12 and Table 3). In methanolic sulfuric acid, in addition to the reduction products, 6-methoxy substituted benzaldehydes are found (cf. Schemes 11 and 12 and Table 3) which are presumably formed by an addition-elimination mechanism (cf. Scheme 18). It is assumed that precursors of the reduction products are the corresponding nitrenium ions in the triplet ground state. Singlet-triplet conversion of the nitrenium ions may become efficient when addition of nucleophiles to the singlet nitrenium ions is reversible (cf. Scheme 22) thus, enhancing the probability of conversion or when conjugation in the singlet nitrenium ions is disturbed by steric effects (cf. Scheme 20) thus, destabilizing the singlet state relative to the triplet state.     

Reduction‐Labile Organo‐cob(III)alamins via Cob(II)alamin: Efficient Synthesis and Solution and Crystal Structures of [(Methoxycarbonyl)methyl]cob(III)alamin

An efficient synthesis of Coβ‐[(methoxycarbonyl)methyl]cob(III)alamin ( 6 ) is reported as an example of a new method for the preparation of some easily reducible organo‐cob(III)alamins via the alkylation of cob(II)alamin. The procedure represents a considerable improvement compared to earlier methods that were based on an alkylation of cob(I)alamin. Thus, aquacob(III)alamin chloride ( 5 ⁺?Cl) was reduced to cob(II)alamin ( 4 ), either by controlled potential electrolytic reduction or with an excess of sodium formate as reducing agent. The solution of 4 was then treated with an excess of methyl bromoacetate while being reductively poised potentiostatically or kept reduced by the formate, to give crystalline 6 in a yield of up to 91%. The structure of 6 in aqueous solution was mainly established by the completely assigned ¹H‐ and ¹³CNMR spectra (Table 1). The NOE data (Table 2) were best rationalized by the presence of a single main conformation of the (methoxycarbonyl)methyl ligand. Single crystals of 6 were obtained by crystallization from an aqueous solution, and the crystal structure was determined by X‐ray analysis at cryotemperatures. The NMR and crystallographic data of 6 indicated similar structures in aqueous solution and in the crystal with the (methoxycarbonyl)methyl ligand preferring a ‘southern' orientation in each case.     

Determination of hydroxyalkyl derivatives of cobalamin (vitamin B12) using reversed phase high performance liquid chromatography with electrospray tandem mass spectrometry and ultraviolet diode array detection.

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) and ultraviolet diode array detection (UV-DAD), coupled on-line to reversed phase high performance liquid chromatography (HPLC), was used for the characterization of hydroxyalkyl derivatives of cob(I)alamin. The reduced form of vitamin B12, cob(I)alamin, denoted a supernucleophile due to its high nucleophilic strength, has shown promise as an analytical tool in studies of electrophilically reactive compounds in vitro and in vivo. A method for analysis of DNA-phosphate adducts was developed earlier utilizing the supernucleophilicity of cob(I)alamin to transfer alkyl groups from the phosphotriester configuration in DNA, with the formation of a Co-substituted alkyl-cobalamin (alkyl-Cbl) complex. For the purpose of identification and quantification of alkyl-Cbls at high sensitivity, an MS/MS method has been developed with application to a number of 2-hydroxyalkyl-cobalamins (OHalkyl-Cbls). The precursor oxiranes were reacted with cob(I)alamin, followed by clean-up and mass spectrometric analysis of the resulting OHalkyl-Cbls. It was found that ionization was highly dependent on solvent composition. By using acetonitrile/water/trifluoroacetic acid (TFA) (eluent I), the base peak was the doubly protonated molecule [M + 2H](2+), whereas acetonitrile/water/1-methylpiperidine (eluent II) yielded the singly protonated molecule [M + H](+) as the base peak. Excellent separation was obtained with eluent II, with good separation between stereoisomers, thus enabling the characterization of these by means of UV spectra. Limits of quantitation for 2-hydroxypropyl-cobalamin (OHPr-Cbl) were 0.2 and 2 pg/microL (or 0.1 and 1 fmol/microL) using selected ion recording (SIR) with eluent I and II, respectively. The obtained detection level should be sufficient for analysis of alkyl-Cbls from a wide range of toxicological applications.     

Interaction between super-reduced cobalamin and selenite

The kinetics of the reaction between the two-electron reduced form of cobalamin (super-reduced cobalamin, cob(I)alamin, or Cbl(I)) and sodium selenite in an alkaline medium is studied spectrophotometrically. It is shown that the selenite rapidly oxidizes Cbl(I) to cob(II)alamin (Cbl(II)). It is established that the active form of the oxidant is the protonated selenite anion (HSeO₃^-), which receives six electrons during the reaction and transforms into HSe^–. The reactions of cob(I)alamin oxidation by selenite and sulfite are compared.