Synthetic approaches to 4,8‐dimethyl‐5′‐(N‐pyridiniummethyl)‐ 4′,5′‐dihydropsoralens and 4,8‐dimethyl‐5′‐ (N‐aminomethyl)‐ 4′,5′‐dihydropsoralens

New synthetic approaches to 4,8‐dimethyl‐5′‐(N‐pyridiniummethyl)‐4′,5′‐dihydropsoralens and 4,8‐dimemyl‐5′‐(N‐aminomethyl)‐4′,5′‐dihydropsoralens are described. The 5′‐halomethyl‐4′,5′‐dihydro‐psoralen precursors are formed by electrophilic ring closures of 4,8‐dimethyl‐6‐allyl‐7‐hydroxycoumarin. The ring‐closure reactions may also be applied to the synthesis of 5′‐halomethyl‐4‐methyl‐4′,5′‐dihydroangelicins. The compounds are potential therapeutic agents for improved psoralen ultraviolet A radiation treatment.     

Synthesis and Hybridization Properties of an Oligonucleotide Consisting of 1′,4′‐Anhydro‐2′,5′‐dideoxy‐2′‐(thymin‐1‐yl)‐D‐altritol

The novel oligonucleotide analogue 7 , consisting of 1′,4′‐anhydro‐2′,5′‐dideoxy‐2′‐(thymin‐1‐yl)‐D ‐altritol ( 4 ), residues was synthesized by the phosphoramidite approach on an automated DNA synthesizer. The phosphoramidite building block 6 was obtained by phosphitylation of the corresponding isonucleoside 5 . Oligoisonucleotide 7 contains an extended phosphodiester linkage with a higher flexibility. Oligoisonucleotide 7 was studied with respect to hybridization properties, enzymatic stability, and CD spectra. It exhibits a high stability towards snake‐venom phosphodiesterase and an acceptable hybridization to complementary single‐stranded DNA and RNA.     

Building the “Phosphoindigo” Backbone by Oxidative Coupling of Phosphindolin‐3‐ones with Selenium Dioxide

Oxidative coupling of racemic 1‐ethoxy‐1‐oxophosphindolin‐3‐one ( 1 ) and its 5‐CF₃‐derivative 6 with SeO₂ furnishes 1,1′‐diphosphaindigo derivatives 5 and 7 as bis‐phosphinic esters, i. e. as P^V‐compounds. Like indigo and thioindigo, 5 and 7 exist in the E‐configuration; the crude products of 5 and 7 are mixtures of isomers that are trans‐ and cis‐configurated with respect to the relative orientation of the ester groups oat phosphorus. The structure of the centrosymmetric E‐P(R)P′(S) isomer [(E)‐trans‐isomer] of 5 was determined by X‐ray crystallography. Ester cleavage of 5 , followed by addition of triethylamine to bis‐phosphinic acid 9 (the 1,1,1′,1′‐tetroxide of “phosphoindigo”), furnishes the related bis‐triethylammonium salt 10 as a crystalline hydrate that exhibits an extended hydrogen bonding network.     

Geometry‐Retentive C‐Alkenylation of Lithiated α‐Aminonitriles: Quaternary α‐Alkenyl Amino Acids and Hydantoins

α‐Amino nitriles tethered to alkenes through a urea linkage undergo intramolecular C‐alkenylation on treatment with base by attack of the lithionitrile derivatives on the N′‐alkenyl group. A geometry‐retentive alkene shift affords stereospecifically the E or Z isomer of the 5‐alkenyl‐4‐iminohydantoin products from the corresponding starting E ‐ or Z ‐N ′‐alkenyl urea, each of which may be formed from the same N ‐allyl precursor by stereodivergent alkene isomerization. The reaction, formally a nucleophilic substitution at an sp² carbon atom, allows the direct regioselective incorporation of mono‐, di‐, tri‐, and tetrasubstituted olefins at the α‐carbon of amino acid derivatives. The initially formed 5‐alkenyl iminohydantoins may be hydrolyzed and oxidatively deprotected to yield hydantoins and unsaturated α‐quaternary amino acids.     

3′‐Deoxyribopyranose (4′→2′)‐Oligonucleotide Duplexes (=p‐DNA Duplexes) as Substitutes of RNA Hairpin Structures

By automated synthesis, we prepared hybrid oligonucleotides consisting of covalently linked RNA and p‐DNA sequences (p‐DNA=3′‐deoxyribopyranose (4′→2′)‐oligonucleotides) (see Table 1). The pairing properties of corresponding hybrid duplexes, formed from fully complementary single strands were investigated. An uninterrupted π‐π‐stacking at the p‐DNA/RNA interface and cooperative pairing between the two systems was achieved by connecting them via a 4′‐p‐DNA‐2′→5′‐RNA‐3′ and 5′‐RNA‐2′→4′‐p‐DNA‐2′ phosphodiester linkage, respectively (see Fig. 4). The RNA 2′‐phosphoramidites 9 – 12 , required for the formation of the RNA‐2′→4′‐p‐DNA phosphodiester linkage were synthesized from the corresponding, 3′‐O‐tom‐protected ribonucleosides (tom=[(triisopropylsilyl)oxy]methyl; Scheme 1). Analogues of the flavin mononucleotide (=FMN) binding aptamer 22 and the hammerhead ribozyme 25 were prepared. Each of these analogues consisted of two p‐DNA/RNA hybrid single strands with complementary p‐DNA sequences, designed to substitute stem/loop and stem motifs within the parent compounds. By comparative binding and cleavage studies, it was found that mixing of the two complementary p‐DNA/RNA hybrid sequences resulted in the formation of the fully functional analogues 23 ⋅ 24 and 27 ⋅ 28 of the FMN‐binding aptamer and of the hammerhead ribozyme, respectively.     

Unifying the Current Data on the Mechanism of Cleavage–Transesterification of RNA

The various mechanisms by which RNA is hydrolyzed are currently under intense investigation. The first step in the hydrolysis pathway is a cleavage–transesterification in which a 2′-OH group attacks a 3′,5′-phosphodiester linkage with departure of the 5′ group. The second step involves the opening of a 2′,3′-cyclic phosphodiester. Complications in these steps arise from multiple possible pathways involving specific acid and base as well as general acid and base catalysis. In addition, controversy exists concerning the protonation states of the phosphodiesters and any intermediate phosphoranes under various experimental conditions. A summary of mechanistic studies involving general and specific acid/base catalysis of RNA hydrolysis and the hydrolysis of RNA analogs is presented herein, along with the interpretations given by the original authors. Included are theoretical calculations, kinetic studies, pK_a determinations, isotope effects, Hammett and Brønsted correlations, and model studies. Recent analyses of the mechanism of RNase A are also briefly reviewed. Two limiting mechanisms for the cleavage–transesterification step that unify the data in the literature and differ only in the role of the phosphorane and its protonation state are given at the outset. An analysis of the literature studies supporting these mechanisms is then provided.     

Nucleic acid related compounds. 114. Synthesis of 2,6‐(disubstituted)purine 2′,3′‐dideoxynucleosides and selected cytotoxic,anti‐hepatitis b,and adenosine deaminase substrate activities

Selected 2,6‐(disubstituted)purine 2′,3′‐didehydro‐2′,3′‐dideoxynucleosides and 2′,3′‐dideoxynucleosides were prepared and evaluated. Treatment of 5′‐protected ribonucleosides with phenoxythiocarbonyl chloride and 4‐(dimethylamino)pyridine, or under Schotten‐Baumann conditions, gave high yields of 2′,3′‐O‐thiono‐carbonates that underwent Corey‐Winter elimination. Treatment of unprotected ribonucleosides with α‐ace‐toxyisobutyryl bromide in “moist” acetonitrile gave trans 2′,3′‐bromohydrin acetate mixtures that underwent reductive elimination with zinc‐copper couple or zinc/acetic acid. Catalytic hydrogenation of the resulting 2′,3′‐enes gave 2′,3′‐dideoxynucleosides. Treatment of the 2‐amino‐6‐chloropurine and 6‐amino‐2‐fluoro‐purine derivatives with nucleophiles gave 2,6‐(disubstituted)purine 2′,3′‐dideoxynucleosides. 2′,3′‐Dideoxyguanosine and the 2‐amino‐6‐[amino ( 16d ), methoxy ( 16b ), ethoxy ( 16c ), and methylamino ( 16j )]purine 2′,3′‐dideoxynucleosides showed good anti‐hepatitis B activity with infected primary duck hepatocytes. Cytotoxic effects with selected analogues were evaluated in human T‐lymphoblastic and promyelocytic leukemia cell lines. The 2‐amino‐6‐fluoro derivative 16m was the most cytotoxic of the 2‐amino‐6‐(substituted)purine 2′,3′‐dideoxynucleosides, and 2‐fluoro‐2′,3′‐dideoxyadenosine ( 21a ) was the most cytotoxic compound. The order of efficiency of hydrolysis of the 6‐substituent from 2‐amino‐6‐(sub‐stituted)purine 2′,3′‐dideoxynucleosides (V_max/K_m) with adenosine deaminase from calf intestine was: 2‐amino‐6‐[amino ( 16d ) > methoxy ( 16b ) > ethoxy ( 16c )], all of which were ≤3% of the efficiency with adenosine. The 6‐methylamino derivative 16j , as well as 16b , 16c , and 16d were readily converted into 2′,3′‐dideoxyguanosine by duck cell supernatants.     

Efficient Solid Phase Synthesis of Cleavable Oligodeoxynucleotides Based on a Novel Strategy for the Synthesis of 5′‐S‐(4,4′‐Dimethoxytrityl)‐2′‐deoxy‐5′‐thionucleoside Phosphoramidites

The incorporation of a specific cleavage site into an oligodeoxynucleotide can be achieved by utilizing the four 5′‐S‐(4,4′‐dimethoxytrityl)‐2′‐deoxy‐5′‐thionucleoside 3′‐(2‐cyanoethyl diisopropylphosphoramidites) 5 and 15a – c (Fig. 1). Based on the silver ion assisted cleavage of P? S and C? S bonds, we synthesized oligodeoxynucleotides with an achiral 5′‐phosphorothioate linkage 3′–O–P–S–5′ by the solid‐phase phosphoramidite procedure. The efficient cleavage of these modified oligodeoxynucleotides can be detected by HPLC, PAGE, and surface plasmon resonance (SPR) spectrometry. The liberated 5′‐thiol moiety can be used directly for post‐reaction labeling with appropriately functionalized reporter groups.     

Synthesis and properties of novel polyimides derived from 2,2′,3,3′‐benzophenonetetracarboxylic dianhydride

A new synthetic route to 2,2′,3,3′‐BTDA (where BTDA is benzophenonetetracarboxylic dianhydride), an isomer of 2,3′,3′,4′‐BTDA and 3,3′,4,4′‐BTDA, is described. Single‐crystal X‐ray diffraction analysis of 2,2′,3,3′‐BTDA has shown that this dianhydride has a bent and noncoplanar structure. The polymerizations of 2,2′,3,3′‐BTDA with 4,4′‐oxydianiline (ODA) and 4,4′‐bis(4‐aminophenoxy)benzene (TPEQ) have been investigated with a conventional two‐step process. A trend of cyclic oligomers forming in the reaction of 2,2′,3,3′‐BTDA and ODA has been found and characterized with IR, NMR, matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry, and elemental analyses. Films based on 2,2′,3,3′‐BTDA/TPEQ can only be obtained from corresponding polyimide (PI) solutions prepared by chemical imidization because those from their polyamic acids by thermal imidization are brittle. PIs from 2,2′,3,3′‐BTDA have lower inherent viscosities and worse thermal and mechanical properties than the corresponding 2,3′,3′,4′‐BTDA‐ and 3,3′,4,4′‐BTDA‐based PIs. PIs from 2,2′,3,3′‐BTDA and 2,3′,3′,4′‐BTDA are amorphous, whereas those from 3,3′,4,4′‐BTDA have some crystallinity, according to wide‐angle X‐ray diffraction. Furthermore, PIs from 2,2′,3,3′‐BTDA have better solubility, higher glass‐transition temperatures, and higher melt viscosity than those from 2,3′,3′,4′‐BTDA and 3,3′,4,4′‐BTDA. Model compounds have been prepared to explain the order of the glass‐transition temperatures found in the isomeric PI series. The isomer effects on the PI properties are discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2130–2144, 2004     

Syntheses of some new azolopyrido‐[4′,3′:4,5]thieno[2,3‐d]pyrimidines

Diethyl 2‐[(ethoxythioxomethyl)amino]‐4,5,6,7‐tetrahydrothieno[2,3‐c]‐pyridine‐3,6‐dicarboxylate 2 , prepared from diethyl 2‐isothiocyanato‐4,5,6,7‐tetrahydrothieno[2,3‐c]pyridine‐3,6‐dicarboxylate 1 by boiling in anhydrous ethanol, was converted into pyrido[4′,3′:4,5]thieno[2,3‐d]pyrimidine derivatives 3, 4 by treatment with hydrazine hydrate. The tetracyclic systems imidazo[1,2‐a]pyrido‐[4′,3′:4,5]thieno[2,3‐d]pyrimidine 9 and pyrido[4′,3′:4,5]thieno[2,3‐d][1,3]thiazolo‐[3,2‐a]pyrimidine 10 were synthesized by the reaction of 2 with 1,2‐diaminoethane and aminoethanethiol, respectively. The hydrazino derivative 4 underwent cyclization reactions with orthoesters and nitrous acid to give the corresponding pyrido[4′,3′:4,5]thieno[2,3‐d][1,2,4]triazolo[1,5‐a]pyrimidines 5, 6 and pyrido[4′,3′:4,5]thieno[3,2‐e][1,2,3,4]tetrazolo[1,5‐a]pyrimidine 8 , respectively. Moreover, reactions of 3 with cyanogen bromide, N‐carbethoxyhydrazine, carbon disulfide, and ethylchloroformate resulted in the formation of the new pyrido[4′,3′:4,5]thieno[2,3‐d][1,3,4]thiadiazolo[3,2‐a]pyrimidine derivatives 12–15 . © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:280–286, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10030     

Synthesis,structure and some reactions of 4a',5′,6′,7′,8′,8a'‐hexahydro‐4′H‐spiro[cyclohexane‐1,9′‐[1,2,4]triazolo[5,1‐b]‐quinazolines]

2′‐Substituted 5′,6′,7′,8′‐tetrahydro‐4′H‐spiro[cyclohexane‐1,9′‐[1,2,4]triazolo[5,1‐b]quinazolines] 3a‐d were synthesized by condensation of 3‐substituted 5‐amino‐1,2,4‐triazoles 1a‐d with 2‐cyclohexylidene cyclohexanone 2 in DMF. The compounds 3 were hydrogenated with sodium borohydride in ethanol to give 2′‐substituted cis‐4a',5′,6′,7′,8′,8a'‐hexahydro‐4′H‐spiro[cyclohexane‐1,9′‐[1,2,4]triazolo[5,1‐b]quinazolines] 4a‐d in high yields. The reactions of alkylation, acylation and sulfonylation of the compounds 4 were studied. The structure of the synthesized compounds was determined on the basis of NMR measurements including HSQC, HMBC, NOESY techniques and confirmed by the X‐ray analysis of 6 and 11b . The described synthetic protocols provide rapid access to novel and diversely substituted hydrogenated [1,2,4]triazolo[5,1‐b]quinazolines.     

The Thermal N9/N7 Isomerization of N2‐Acylated 2′‐Deoxyguanosine Derivatives in the Melt and in Solution

The protected 2′‐deoxyguanosine derivatives 5a – c undergo N⁹→N⁷ isomerization in the melt and in solution. The rate of isomerization is much faster than in the case of the corresponding ribonucleosides and occurs even in the absence of a catalyst. In the melt (195°, 2 min), the N²,3′‐O,5′‐O‐tris(4‐toluoyl) derivative 5b and the N²‐acetyl‐3′,5′‐bis‐O‐[(tert‐butyl)dimethylsilyl] derivative 5c gave anomeric mixtures of the N⁷‐isomers 9b / 10b (43%) and 9c / 10c (55%), respectively. In addition, the N⁹‐α‐D ‐anomers 8b and 8c were obtained. Different from 5b , the isomerization of peracetylated 5a resulted in low yields. Compound 5b was also prone to isomerization performed in solution (toluene, 100°, 5 min; chlorobenzene, 120°, 5 min), furnishing the N⁷‐regioisomers in 24–53% yield. The highest yield of the N⁹→N⁷ isomerization occurred in the presence of 2‐deoxy‐3,5‐di‐O‐(4‐toluoyl)‐α‐D ‐erythro‐pentofuranosyl chloride.     

(E/Z)‐Isomerization of 3′‐Epilutein

3′‐Epilutein (=(all‐E,3R,3′S,6′R)‐4′,5′‐didehydro‐5′,6′‐dihydro‐β,β‐carotene‐3,3′‐diol; 1 ), isolated from the flowers of Caltha palustris, was submitted to both thermal isomerization and I₂‐catalyzed photoisomerization. The structures of the main products (9Z)‐ 1 , (9′Z)‐ 1 , (13Z)‐ 1 , (13′Z)‐ 1 , (15Z)‐ 1 , and (9Z,9′Z)‐ 1 were determined based on UV/VIS, CD, ¹H‐NMR, and MS data.     

An Efficient Synthesis of 1′,7′,8′,9′‐Tetrahydrospiro[indoline‐3,4′‐pyrazolo[3,4‐b]quinoline]‐2,5′(6′H)‐dione Derivatives in Aqueous Medium

An efficient and green reactions of isatins, 3‐amine‐1H‐pyrazole (5‐methyl‐1H‐pyrazol‐3‐amine) and 1,3‐diketone in aqueous medium for the synthesis of novel 1′,7′,8′,9′‐tetrahydrospiro[indoline‐3,4′‐pyrazolo[3,4‐b]quinoline]‐2,5′(6′H)‐dione derivatives were reported in this research. The advantages of this reaction are simple operation, mild‐reaction conditions, wide scope substrate, high yields, and friendly environment. The products were confirmed by IR, ¹H NMR, ¹³C NMR, and HRMS.     

2‐[(Acetyloxy)methyl]‐4‐(acetylsulfanyl)‐2‐(ethoxycarbonyl)‐3‐oxobutyl Group: A Thermolabile Protecting Group for Phosphodiesters

The phosphodiester linkage of 3′‐O‐levulinoylthymidine 5′‐methylphosphate ( 5 ) has been protected with 2‐[(acetyloxy)methyl]‐4‐(acetylsulfanyl)‐2‐(ethoxycarbonyl)‐3‐oxobutyl group (to give 1 ) to study the potential of this group as an esterase‐ and thermolabile protecting group. The group turned out to be unexpectedly thermolabile, being removed as ethyl 3‐(acetyloxy)‐4‐(acetylsulfanyl)‐2‐methylidenebut‐3‐enoate ( 10 ) without accumulation of any intermediates. The half‐life of this reaction at pH 7.5 and 37° is 14 min. Hog liver esterase (HLE), in turn, removes the protecting group as ethyl 4‐(acetylsulfanyl)‐2‐methylidene‐3‐oxobutanoate ( 12 ). On using 2.6 units of HLE in 1 ml, the rate of the enzymatic deprotection was still only one third of that of the nonenzymatic reaction. The mechanisms of both reactions have been studied and discussed. The crucial step seems to be removal of the O‐bound Ac group, either by esterase or by migration to the neighboring 3‐oxo group (nonenzymatic removal). This triggers the removal by retro‐aldol condensation/elimination mechanism. No alkylation of glutathione (GSH) upon the deprotection of 1 could be detected.     

DNA with 3′‐5′‐Disulfide Links—Rapid Chemical Ligation through Isosteric Replacement

Efforts to chemically ligate oligonucleotides, without resorting to biochemical enzymes, have led to a multitude of synthetic analogues, and have extended oligomer ligation to reactions of novel oligonucleotides, peptides, and hybrids such as PNA. 1 Key requirements for potential diagnostic tools not based on PCR include a fast templated chemical DNA ligation method that exhibits high pairing selectivity, and a sensitive detection method. Here we report on a solid‐phase synthesis of oligonucleotides containing 5′‐ or 3′‐mercapto‐dideoxynucleotides and their chemical ligations, yielding 3′‐5′‐disulfide bonds as a replacement for 3′‐5′‐phosphodiester units. Employing a system designed for fluorescence monitoring, we demonstrate one of the fastest ligation reactions with half‐lives on the order of seconds. The nontemplated ligation reaction is efficiently suppressed by the choice of DNA modification and the 3′‐5′ orientation of the activation site. The influence of temperature on the templated reaction is shown.     

Nucleosides XII.1 Synthesis of 5‐Modified Isoguanosines and Reinvestigation of 5′‐Deoxy‐N3,5′‐cycloisoguanosine

Isoguanosine ( 3 ) underwent a coupling reaction with diaryl disulfides in the presence of tri‐n‐butylphosphine when its 6‐amino group was protected by N,N‐dimethylaminomethylidene. The synthesis of 5′‐deoxy‐N³,5′‐cycloisoguanosine ( 6 ) and its 2′,3′‐O‐isopropylidene derivative ( 11 ) were accomplished in excellent yields from isoguanosines ( 3 & 10 ) in the presence of triphenylphospine and carbon tetrachloride in pyridine. Chlorination at the 5′‐position of isoguanosine ( 3 ) with thionyl chloride followed by the aqueous base‐promoted cyclization afforded the same product 6 . The structures were elucidated by spectroscopic analysis including IR, UV, 1‐D and 2‐D NMR.     

Polycyclic N‐Heterocyclic Compounds. Part 84: Reaction of N‐(pyrido[3′,2′:4,5]thieno[3,2‐d]pyrimidin‐4‐yl)amidines or N‐(pyrido[2′,3′:4,5]furo[3,2‐d]pyrimidin‐4‐yl)amidines with Hydroxylamine Hydrochloride

The reactions of nine N‐(pyrido[3′,2′:4,5]thieno[3,2‐d]pyrimidin‐4‐yl)amidines ( 3 ) with hydroxylamine hydrochloride produced new cyclization products. These were formed via ring cleavage of the pyrimidine component followed by a 1,2,4‐oxadiazole‐forming ring closure to give N‐[2‐([1,2,4]oxadiazol‐5‐yl)thieno[2,3‐b]pyridin‐3‐yl]formamide oximes ( 11 ). Reaction of six N‐(pyrido[2′,3′:4,5]furo[3,2‐d]pyrimidin‐4‐yl)amidines ( 12 ) with hydroxylamine hydrochloride gave similar results. Effects of the newly synthesized compounds on pentosidine formation were also evaluated.     

Experimental Study of Hydrogen Bonding Potentially Stabilizing the 5′‐Deoxyadenosyl Radical from Coenzyme B12

Coenzyme B₁₂ can assist radical enzymes that accomplish the vicinal interchange of a hydrogen atom with a functional group. It has been proposed that the Co? C bond homolysis of coenzyme B₁₂ to cob(II)alamin and the 5′‐deoxyadenosyl radical is aided by hydrogen bonding of the corrin C19? H to the 3′‐O of the ribose moiety of the incipient 5′‐deoxyadenosyl radical, which is stabilized by 30 kJ mol^?1 (B. Durbeej et al., Chem. Eur. J. 2009 , 15, 8578–8585). The diastereoisomers (R)‐ and (S)‐2,3‐dihydroxypropylcobalamin were used as models for coenzyme B₁₂. A downfield shift of the NMR signal for the C19? H proton was observed for the (R)‐isomer (δ=4.45 versus 4.01 ppm for the (S)‐isomer) and can be ascribed to an intramolecular hydrogen bond between the C19? H and the oxygen of CHOH. Crystal structures of (R)‐ and (S)‐2,3‐dihydroxypropylcobalamin showed C19? H???O distances of 3.214(7) Å (R‐isomer) and 3.281(11) Å (S‐isomer), which suggest weak hydrogen‐bond interactions (?ΔG<6 kJ mol^?1) between the CHOH of the dihydroxypropyl ligand and the C19? H. Exchange of the C19? H, which is dependent on the cobalt redox state, was investigated with cob(I)alamin, cob(II)alamin, and cob(III)alamin by using NMR spectroscopy to monitor the uptake of deuterium from deuterated water in the pH range 3–11. No exchange was found for any of the cobalt oxidation states. 3′,5′‐Dideoxyadenosylcobalamin, but not the 2′,5′‐isomer, was found to act as a coenzyme for glutamate mutase, with a 15‐fold lower k_cat/K_M than 5′‐deoxyadenosylcobalamin. This indicates that stabilization of the 5′‐deoxyadenosyl radical by a hydrogen bond that involves the C19? H and the 3′‐OH group of the cofactor is, at most, 7 kJ mol^?1 (?ΔG). Examination of the crystal structure of glutamate mutase revealed additional stabilizing factors: hydrogen bonds between both the 2′‐OH and 3′‐OH groups and glutamate 330. The actual strength of a hydrogen bond between the C19? H and the 3′‐O of the ribose moiety of the 5′‐deoxyadenosyl group is concluded not to exceed 6 kJ mol^?1 (?ΔG).     

An intermediate in a new synthesis approach to β‐substituted β‐hydroxy­aspartame

The crystal and molecular structure of 1‐tert‐butyl 4‐ethyl (2′R,3′R,5′R,2S,3S)‐3‐bromo­methyl‐3‐hydroxy‐2‐[(2′‐hydroxy‐2′,6′,6′‐tri­methyl­bi­cyclo­[3.1.1]­hept‐3′‐yl­idene)­amino]­succinate, C₂₁H₃₄BrNO₆, is presented. This compound is an intermediate in the new synthetic route to β‐substituted β‐hydroxy­aspartates, which are blockers of glutamate transport.