bluuebunny/crossref_metadata_2025_split huggingface dataset:
Synthesis of 2′‐Deoxyribofuranosides of 8‐Aza‐7‐deazaguanine and Related Pyrazolo[3,4‐d]pyrimidines
Authors: Seela Frank , Steker Herbert 
Published: December 2004

AbstractThe N(1)‐ and N(2)‐(2′‐deoxyribofuranosides) 1 and 2, respectively, of 8‐aza‐7‐deazaguanine were prepared via phase‐transfer glycosylation in the presence or absence of Bu4NHSO4 as catalyst of 6‐amino‐4‐methoxy‐lH‐pyrazolo[3,4‐d]pyrimidine (7c) with 2‐deoxy‐3,5‐di‐O‐(p‐toluoyl)‐α‐D‐erythro‐pentofuranosyl chloride (10). On a similar route, but without catalyst and employing THF as organic phase, the 6‐amino‐4‐chloronucleosides 11b and 12b were synthesized from 7a and converted into the N(1)‐and N(2)‐substituted 4‐thioxo analogues 17a and 18a, respectively. The ratio of N(1)‐ to N(2)‐glycosylation was 2:1 for 7c and 1:1 for 7a, viz. depending on the nucleobase structure. The rate of the H+‐catalyzed N‐glycosyl hydrolysis was strongly decreased for the N(2)‐(β‐D‐2′‐deoxyribofuranosides) as compared to the N(1)‐compounds. However, the N(1)‐nucleoside 1, which is an isostere of 2′‐deoxyguanosine, is sufficiently stable to be employed later in solid‐phase oligonucleotide synthesis.
Synthese von 3‐Nitro‐1,1‐bis(triisopropylsilyl)‐1H‐cyclopropabenzol und Azocyclopropabenzol‐Derivaten
Authors: Neidlein Richard , Christen Detlev 
Published: December 2004

Synthesis of 3‐Nitro‐1,1‐bis(triisopropylsilyl)‐1H‐cyclopropabenzene and Derivatives of Azocyclopropabenzene1,1‐Bis(triisopropylsilyl)‐1H‐cyclopropabenzene (4) can be nitrated at C(3) without opening of the three‐membered ring. Further functionalized cyclopropabenzenes are available in this way.
Synthesis of 2,3,4,6,7,8‐Hexahydro‐5H‐1‐benzopyran‐5‐ones and 3‐(6‐Oxo‐1‐cyclohexenyl)alkanoic Acids by Reduction of 4,6,7,8‐Tetrahydro‐2H‐1‐benzopyran‐2,5(3H)‐diones
Authors: Hombrecher Hermann , Margaretha Paul , Tissot Paul 
Published: December 2004

AbstractComparative results on the reduction of 4,6,7,8‐tetrahydro‐7,7‐dimethyl‐2H‐1‐benzopyran‐2,5(3H)‐diones 1 are reported. Hydride reduction (LiAlH4 in Et2O or NaBH4 in i‐PrOH) affords 2,3,4,6,7,8‐hexahydro‐5H‐1‐benzopyran‐5‐ones 5 in 30‐60% isolated yield. Photochemical reduction of 1b and 1d (direct irradiation at λ = 300 or 254 nm in i‐PrOH, or sensitized irradiation in acetone/i‐PrOH or benzene/i‐PrOH) gives 3‐(6‐oxo‐1‐cyclohexenyl)alkanoic acids 6 in 50–80%, while 1c affords the isomeric 3‐(4,4‐dimethyl‐6‐oxo‐1‐cyclohexenyl)‐4‐methyl‐4‐pentenoic acid (9) in 73% isolated yield. Electrochemical reduction (Hg, CH3CN, Bu4N+ClO, −2.6 V vs. Ag/Ag+) requires more than 4 Farad/mol for the consumption of 1 without any major product being detected.
Assignment of the 5,6‐Double‐Bond Configuration in Iloprost and Isoiloprost from 13C‐NMR Shifts Determined by 2D Methods
Authors: Schenker Kurt V. , Von Philipsborn Wolfgang , Anderson Evans C. , Skuballa Werner , Hoyer Georg‐Alexander 
Published: December 2004

AbstractThe configurations of the 5,6‐double bond in the carbacyclins iloprost (3; (E)) and isoiloprost (4; (Z)) are based on a complete assignment of the 13C and 1H resonances determined by 1D and 2D 13C‐NMR and 1H‐NMR methods.
Neue 1,6‐Methano[10]annulen‐Derivate durch Umlagerungsreaktionen der Trimethylsilyl‐Gruppe
Authors: Neidlein Richard , Wirth Wolfgang 
Published: December 2004

New Derivatives of 1,6‐Methano[10]annulene by Rearrangement of Trimethylsilyl GroupsOxidation of derivatives of trimethylsilyl‐substituted bicyclo[4.4.1]undeca‐1,3,5,7‐tetraenes 1–3 leads to 1,6‐methano[10]annulenes. In the case of 2 and 3a, rearrangement of the trimethylsilyl group takes place. This rearrangement can be used to prepare 2,9‐disubstituted 1,6‐methano[10]annulenes.
Zur Photochemie von (Z,Z)‐2,7‐Cyclodecadien‐1‐on und 4,8‐Cyclododecadien‐1‐on. Synthese und Eigenschaften von Tricyclo[5.3.0.02,8]decan‐Systemen
Authors: Gleiter Rolf , Sander Wolfram , Butler‐Ransohoff Irmgard 
Published: December 2004

On the Photochemistry of (Z,Z)‐2,7‐Cyclodecadien‐1‐one and 4,8‐Cyclododecadien‐1‐one. Synthesis and Properties of Tricyclo[5.3.0.02,8]decane SystemsIrradiation of (Z,Z)‐2,7‐cyclodecadien‐1‐one (3) yields (Z,Z)‐3,7‐cyclodecadien‐1‐one (12) or tricyclo‐[5.3.0.02,8]decan‐4‐one (16), depending on the reaction conditions. Irradiation of 4,8‐cyclododecadien‐1‐one (28) results also in a light‐induced transannular [2 + 2] cycloaddition, yielding tetracyclo[7.3.0.02,1003,6]dodecan‐1‐one (30). Starting from 16, the preparation of tricyclo[5.3.0.02,8]dec‐4‐ene (19), tricyclo[5.3.0.02,8]dec‐4‐ene (21) and tricyclo[5.3.0.02,8]deca‐3,5‐diene (24) is described. The 1H‐NMR and 13CNMR spectra of the newly prepared compounds are discussed. In the case of 19, 21, and 24, the electronic structure is discussed on hand of their PE spectra.
Regioselectivity and Deuterium Isotope Effects in Geraniol Hydroxylation by the Cytochrome P‐450 Monooxygenase from Catharanthus roseus (L.) G. DON
Authors: Fretz Heinz , Woggon Wolf‐Dietrich 
Published: December 2004

AbstractThe hydroxylation of geraniol (8) by cytochrome P‐450 (P‐450Cath.) from the subtropical plant Catharanthus roseus (L.) G. DON was optimised to give 8‐hydroxygeraniol (9) as the single product in 35% yield. Incubations of different 13C‐ and 2H‐labelled geraniols revealed that H‐abstraction is completely regioselective in favour of the CH3 group trans to the chain at C(6) of 8. An intramolecular isotope effect kH/kD = 8.0 was determined, suggesting that H‐abstraction is one of the major rate‐contributing steps; however, the intermolecular isotope effect was surprisingly inverse at low conversion kH/kD = 0.50, indicating the existence of rate‐contributing steps preceding the first irreversible, isotope‐sensitive reaction in the sequence.
Triaziridine. 6. Mitteilung. Substituententransformationen an Triaziridinen
Authors: Hilpert Hans , Hoesch Lienhard , Dreiding André S. 
Published: December 2004

Substituent Transformations on TriaziridinesSeveral novel triaziridines were prepared by substituent transformations starting from the known dialkyl‐triaziridine‐carboxylates 1a–c, with the aim to study the influence of the substitution pattern on the properties of the triaziridine ring. The dialkyl‐triaziridines 2a–c were obtained by (t‐BuO−)‐mediated demethylation and decarboxylation and the dialkyl‐triaziridine‐methanols 4a–c by LiAlH4 reduction. Further reduction of the tosylates of 4a, b with LiAlH4 gave the methyl‐dialkyl‐triaziridines 3a, b. The dialkyl‐triaziridines 2a, c could not be N‐methylated directly with CH3I, but the anions 10a, c, obtained from 2a, c with CH3Li, yielded 3a, c. N‐Methylation of 2a with (CH3)3OBF4 did not afford 3a but rather the methyl‐triaziridinium salt 11. The dialkyl‐triaziridine 2c has pKa > 14, its protonated species < 2. A concept that the electron pairs on the triaziridine N‐atoms are more strongly localized than on amine N‐atoms explains (a) that the dialkyl‐triaziridine 2c is hardly basic, (b) that the LiAlH4 reductions of the esters 1 stop at the stage of the methanols 4, and (c) that the methanols 4a, b do not cleave like aminomethanols.
Synthesis and Biological Properties of N‐Acetyl‐4‐deoxy‐D‐neuraminic Acid
Authors: Hagedorn Heinz‐Werner , Brossmer Reinhard 
Published: December 2004

AbstractN‐Acetylneuraminic acid (1) can be transformed into the methyl α‐D‐ketoside 2 which, by reaction with methanesulfonyl chloride, yields the corresponding 4‐O‐mesylate 3 and the 4,7‐di‐O‐mesylate 4 as a by‐product. Compound 3 reacts with Nal giving the 4‐deoxy‐4‐iodo compound 5 with equatorial orientation of the I‐atom. As second product, the dihydrooxazole 6 is produced. Catalytic hydrogenation of 5 is followed by ester cleavage and removal of the isopropylidene group yielding the methyl α‐D‐ketoside 8 which affords the title compound, N‐acetyl‐4‐deoxyneuraminic acid (9), by reaction with fowl plague virus sialidase. Further biochemical activities of 8 and 9 are reported.
New Mechanistic Proposal for Some Heptamolybdate‐Ion‐Catalyzed Isomerizations
Authors: Klaič Branimir , Raza Zlata , Sankovič Mauricio , Šunjič Vitomir 
Published: December 2004

AbstractHeptamolybdate‐ion(HM)‐catalyzed C(2) epimerization of D‐glucose and racemization of D‐glyceraldehyde proceed with comparable rates and activation parameters; HM catalyzes the isomerization of glucal and galactal triacetates as well. For all of the above transformations, a common, new mechanism is proposed that invokes stabilization of 3‐oxa‐allylic cation species by the central Mo(VI) atom.
Coralloidolide A and Coralloidolide B, the First Cembranoids from a Mediterranean Organism, the Alcyonacean Alcyonium coralloides
Authors: D'Ambrosio Michele , Fabbri Daniele , Guerriero Antonio , Pietra Francesco 
Published: December 2004

AbstractThe alcyonacean Alcyonium ( = Parerythropodium) coralloides (PALLAS 1766) is the first Mediterranean organism shown to contain cembranoids. These are of unusual type like coralloidolide A ( = (−)‐(1R*, 2R*, 3R*, 12S*, 5Z, 7Z, 9Z)‐1, 2:7, 10‐diepoxy‐12‐isopropenyl‐5, 9‐dimethylcyclotetradeca‐5, 7, 9‐triene‐1, 3‐carbolactone; (−)‐6) and coralloidolide B ( =(−)‐(1R*,2S*,3R*,7S*,10S*,12S*,5Z,8Z,)‐2, 7:7, 10‐diepoxy‐1, 10‐dihydroxy‐12‐isopropenyl‐5, 9‐dimethylcyclotetradeca‐5,8‐diene‐1,3‐carbolactone; (−)‐8). Structural assignments are mainly based on 1D‐ and 2D‐NMR data and on chemical transformations.
Synthese von ‘D‐Isothreonin’ und ‘L‐Alloisothreonin’ aus L‐Alanin
Authors: Wolf Jean‐Pierre , Pfander Hanspeter 
Published: December 2004

Synthesis of ‘D‐Isothreonine’ and ‘L‐Alloisothreonine’ Starting from L‐AlanineStarting from L‐alanine, ‘D‐isothreonine’ ( = (2R, 3S)‐3‐amino‐2‐hydroxybutanoic acid) and ‘L‐alloisothreonine’ ( = (2S, 3S)‐3‐amino‐2‐hydroxybutanoic acid) were synthesized.
Ringerweiterung unter Lactonisierung von methylierten 1‐(3′‐Hydroxypropyl)‐2‐oxocyclododecan‐1‐carbonitrilen
Authors: Milenkov Branimir , Hesse Manfred 
Published: December 2004

Ring Enlargement by Lactonization of Methylated 1‐(3′‐Hydroxypropy1)‐2‐oxocyclododecane‐1‐carbonitrilesThe title compounds were prepared by Michael reaction of 2‐oxocyclododecane‐1‐carbonitrile (1) and acrylaldehyde and its derivatives followed by NaBH4 reduction or methylation of the aldehyde group with [(CH3)2Ti(i‐PrO)2] (Scheme 1). In all cases, the ring enlargement was performed with Bu4NF/THF to give different methylated derivatives of 12‐cyano‐15‐pentadecanolide (13) in 95–99% yield. The Yields of the rearrangement products are not dependent on the positions and numbers of the CH3 groups in the side chain of 3. The lactonization reaction is of unremarkable stereoselectivity.
Synthese von ω‐(5‐Oxo‐1‐cyclopentenyl)alkansäure‐methylestern aus 2‐Nitrocycloalkanonen
Authors: Stach Hans , Hesse Manfred 
Published: December 2004

Synthesis of Methyl ω‐(5‐Oxo‐1‐cyclopentenyl)alkanoates Starting from 2‐NitrocycloalkanonesA convenient synthesis of methyl ω‐(5‐oxo‐1‐cyclopentenyl)alkanoates 1 is described. 2‐Nitrocycloalkanones 2 are converted to 2‐(3,3‐dimethoxypropy1)‐2‐nitrocycloalkanones 4. Treatment of 4 with MeOH/MeONa led to the ring‐opened nitronates 5 which underwent a Nef reaction to form the corresponding oxo derivatives 6. Partial hydrolysis of followed by base‐catalyzed aldol reaction gave the desired products in high yields.
Cyclobut‐2‐enones from Alkynes via Dichlorocyclobut‐2‐enones
Authors: Ammann Adrian A. , Rey Max , Dreiding André S. 
Published: December 2004

AbstractThe [2 + 2]‐cycloaddition of dichloroketene (prepared in situ from CCl3COCl and Zn(Cu)) with three alkynes 1a–c to form 2,3‐dimethyl‐(2a), 2,3‐diethyl‐(2b) and 3‐butyl‐4,4‐dichlorocyclobut‐2‐enone (2c) proceeds rapidly in the absence of POCl3. The primary products 2a–c rearrange in situ to the 2,4‐dichlorocyclobut‐2‐enones 3a–c under the influence of ZnCL2 produced during the reaction. ZnCl2 converts both 2a and 3a into a 4:6 equilibrium mixture of the two; this isomerization does not occur with LiCl. The Cl‐atoms of both 2a, b and 3a, band of 2c may reductively be removed with Zn(Cu) in AcOH/pyridine to afford the alkylcyclobutenones 4a‐c. Without pyridine, this reduction gives ca. 1:1 mixtures of the double‐bond isomers 4 and 5 in low yields. The cyclobutenones 2c and 4c may be deuterated by CD3COOD in the presence of pyridine. D‐Atom is introduced into 2c at C(4) and at C(γ), and into 4c at C(2) and C(4). A mechanism for this deuteration is considered, which does not involve a cyclobutadienolate 7, but rather a cyclobutenolate of type 8. The reductions of 2 and 3 to 4 might also pass through the same type of intermediate 8.
Photodecarbonylation of 2,5,5‐Trimethyl‐2‐(1‐propynyl)cyclopentanone. 1,5‐ vs. 1,4‐ring Closure of a Propargyl‐Alkyl Biradical
Authors: Rudolph Andreas , Margaretha Paul , Agosta William C. 
Published: December 2004

AbstractPropargyl‐alkyl biradical 12 generated by photodecarbonylation of title ketone 3 undergoes ring closure to cyclobutane 7 and to vinyl carbene 13, this latter intermediate rearranging to the 5‐ethylidenecyclopentenes 8a and 8b.
Synthese von 2‐Hydeoxy‐3‐methyl‐2‐hexen‐4‐olid
Authors: Stach Hans , Huggenberg Walter , Hesse Manfred 
Published: December 2004

Synthesis of 2‐Hydroxy‐3‐methyl‐2‐hexen‐4‐olidThe title compound 13a, a substance used in food‐flavoring, was synthesized in 89% overall yield, starting from methyl 2‐hydeoxy‐3‐butenoate (3a). The key step in this transformation is the isomerization of the CC bond in 3a which yielded methyl 2‐oxobutanoate as an intermediate. The latter underwent a self‐condensation yielding 2‐hydroxy‐4‐(methoxycarbonyl)‐3‐methyl‐2‐hexen‐4‐olid (11a), which, after hydrolysis and decarboxylation, gave 13a. In addition, the syntheses of five other compounds related to 13a are reported.
Synthesis and Placental Binding Potencies of Photosensitive Analogues of Luteinizing Hormone Releasing Hormone (LHRH) with Agonistic and Antagonistic Structures
Authors: Mackiewicz Zbignew , Bélisle Serge , Bellabarba Diego , Gallo‐Payet Nicole , Lehoux Jean‐Guy , Lagacé Ginette , Escher Emanuel 
Published: December 2004

AbstractThe synthesis of new photolabeling analogues of luteinizing hormone releasing hormone is described. Peptides were prepared by the solid‐phase method, and the photolabeling moiety was introduced as the 4′‐nitrophenyl‐alanine precursor. This amino acid, either of the L or D configuration, was placed at different positions and was modified afterwards to 4′‐azidophenylalanine. Structures with agonistic or antagonistic character were prepared, together with a peptide containing simultaneously the photolabeling moiety and a biotin substituent for future receptor isolations. Binding studies on placental and pituitary membranes indicate that these compounds represent promising candidates for receptor labeling studies.
On the Purported Structure of Cyclodeca[1,2,3‐de: 6,7,8‐d′e′]dinaphthalene
Authors: Gleiter Rolf , Schaaff Hans Peter , Rodewald Hans , Jahn Reiner , Irngartinger Hermann 
Published: December 2004

AbstractThe reaction of the bis(sulfonium salt)7 in a solution of Na2CO3 in H2O/EtOH yielded three main products 8–10. The spectroscopic data of 8 were identical to those which led Mithcell and Sondheimer to assign them to cyclodeca[1,2,3‐de: 6,7,8‐d′e′]dinaphthalene (3). Our investigations show, however, that the correct structural assignment leads to the structure of 7,7a‐dihydrodibenzo[de,mn]naphthacene (8).
Complexes of 2,2′,2″‐Nitrilotriphenol. Part 2. Crystal and Molecular Structures of Three Boron Complexes
Authors: Müller Edgar , Bürgi Hans‐Beat 
Published: December 2004

AbstractCrystal and molecular structures of a complex between B(III) and the tripod ligand 2,2′,2″‐nitrilotriphenol as well as of its pyridine and quinuclidine adducts are presented. The 2,2′,2″‐nitrilotriphenyl borate (III) molecule shows a strained, tricyclic chelate system with a central N–B donor‐acceptor bond of 1.681(5) Å. In the adducts with pyridine (IV) and quinuclidine (III‐quin), this bond is broken, the N‐atom inverts and is pushed out of the coordination sphere (B–Nintern = 2.82 Å for Py and 2.85 Å for quin), and a new bond is formed between the boron atom and the external nucleophile (B–Nextern = 1.631 Å for Py and 1.643 Å for quin).
The Microstructure of Selective Palladium Hydrogenation Catalysts Supported on Calcium Carbonate and Modified by Lead (Lindlar Catalysts), Studied by Photoelectron Spectroscopy, Thermogravimetry, X‐Ray Diffraction, and Electron Microscopy
Authors: Schlägl Robert , Noack Klaus , Zbinden Heinz , Reller Armin 
Published: December 2004

AbstractA detailed analysis of the system Pd + Pb on CaCO3 is presented. This system known as Lindlar catalyst is used for selective hydrogenations of CC bonds to CC bonds. It was found to consist of seven distinct chemical phases. Essential ingredients are solid solutions of hydrogen and oxygen in Pd. The latter phase enables the metal particles to be attached to the support via an orientational relationship calcite (113)∥Pd + O (111). The addition of Pb neither changes the micromorphology of the disk‐shaped Pd particles nor does it modify the electronic structure of Pd as studied by chemisorption experiments. The addition of Pb seems to block certain active sites and, thus, to enhance the selectivity. The support was found to be a multi‐domain mixture of calcite and aragonite with domain sites of several hundred Å. The free surface of the pore‐free solid is covered by a calcium‐hydroxide carbonate. This phase as well as several other Pb phases seem to represent only spectator materials. Verification of this as well as finding the correlation between structure and function of the various components requires a kinetic analysis which is outside the scope of this paper. It is demonstrated that a variety of analytical techniques is required to unravel the complex nature of such a ‘simple' catalyst’ as it is Pd on CaCO3.
Synthesis and Copper(I) Complexes of a Series of 9‐ to 13‐Membered N3 Macrocycles
Authors: Briellmann Markus , Kaderli Susan , Meyer Charles J. , Zuberbühler Andreas D. 
Published: December 2004

AbstractEight cyclic triamines with ring sizes between 9 and 13 were synthesized by the p‐toluenesulfonate method. The open‐chain triamines bis(2‐aminoethyl)amine (dien) and bis(3‐aminopropyl)amine (diprop) were used as starting materials. In some cases, the corresponding dimeric cyclic hexaamines have been isolated and characterized as major by‐products. The complexation of Cu(I) by the triamines has been studied potentiometrically in CH3CN/H2O. All ligands L form ternary complexes [Cu(CH3CN)L]+. The corresponding association constants vary between 1011 and 107, decreasing with increasing ring size. In addition, complexes [Cu(CH3CN)yLH]2+, y = 1 or 2, are found as less important species with maximum concentrations of 7 to 50%.
Brominations of Cyclic Acetals from α‐Amino Acids and α‐ or β‐Hydroxy Acids with N‐Bromosucinimide
Authors: Zimmermann Jürg , Seebach Dieter 
Published: December 2004

AbstractThe preparation of novel electrophilic building blocks for the synthesis of enantiomerically pure compounds (EPC) is described. Thus, the 2‐(tert‐butyl)dioxolanones, ‐oxazolidinones, ‐imidazolidinones, and ‐dioxanones obtained by acetalization of pivalaldehyde with 2‐hydroxy‐, 3‐hydroxy‐, or 2‐amino‐carboxylic acids are treated with N‐bromosuccinimide under typical radical‐chain reaction conditions (azoisobuytyronitril/CCl4/reflux). Products of bromination in the α‐position of the carbonyl group of the five‐membered‐ring acetals are isolated or identified (2, 5, and 8; Scheme 1). The dioxanones are converted to 2H, 4H‐dioxinones under these conditions (12, 14, 15, 21, and 22; Schemes 2 and 3). The products can be converted to chiral derivatives of pyruvic acid (methylidene derivatives 3 and 6) or of 3‐oxo‐butanoic and ‐pentanoic acid (16 and 23). The mechanism of the brominations is interpreted. The conversion of serine to enactiomcrically pure dioxanones 26–28 (Scheme 4) is also discussed.
Stereoselektive Alkylierung an C(α) von Serin, Glycerinsäure, Threonin und Weinsäure über heterocyclische Enolate mit exocyclischer Doppelbindung
Authors: Seebach Dieter , Aebi Johannes D. , Gander‐Coquoz Marlyse , Naef Reto 
Published: December 2004

Steroselective Alkylation at C(α) of Serine, Clyceric Acid, Threonine, and Tartaric Acid Involving Heterocyclic Enolates with Evocyelic Double BondsThe chiral, non‐racemic title acids are converted to methyl dioxolane‐(cf.13), oxazoline‐(4) and oxazolidinecarboxylates (cf.9). Deprotonation by Li(i‐Pr)2N at dry‐ice temperature gives solutions of the lithium enolates A–D With exocyclic enolate double bonds. These are stable crough with respect to β‐elimination (Scheme 1) to be alkylated with or without cosolvents such as HMPA or DMPU The products are formed in good to excellent yields and, with the exception of the tartrate‐derived acetonlde (see Scheme 2), with diastereoselectivities above 90%. While the tartrate‐and threonine‐derived enolates (AandB, resp.) are chiral due to the second stereogenic center of the precursors, the serine‐ and glyceric‐acid‐derived enolates (AandB, resp.) are chiral due to the second sterogenic center of the precursors, the serine‐nd glyceric‐acid‐derived enolates are non‐racemic due to atertbutyl‐substituted (pivalaldehyde‐derived) acetal center (CandD, resp.). The products of alkylation can be hydrolyzed to give α‐branched tartaric acid (Scheme 2), allothreonine (Scheme 3), serine (Scheme 4), and glyceric‐acid derivatives (Scheme 5) with quaternary stereogenic centers. The configurations of the products are determined by NOE‐NMR measurements and by chemical correlation. These show that the dioxolane‐derived enolatesAandDare alkylated preferentially from that face of the ring which is already substituted (‘syn’‐attack), while the dihydrooxazol‐and oxazolidine‐derived enolatesBandCare alkylated from the opposite face (‘anti’‐attack). The ‘syn’‐attack is postulated to arise from strong folding of the heterocyclic ring due to electronic repulsion between the enolate π‐system and non‐bonding electron pairs on the heteroatoms (see Scheme 6).
Thermodynamic trans‐Effects of the Nucleotide Base in the B12 Coenzymes
Authors: Kräutler Bernhard 
Published: December 2004

AbstractThe thermodynamic effects of the nucleotide coordination on the Co‐C bond strengths in the B12 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 KI/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 KII/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 5by 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 B12 (1), the nucleotide coordination is found this way to weaken the Co–C bond towards homolysis by ca. 0.7 kcal/mol.