Showing papers in "Journal of Organic Chemistry in 1997"
TL;DR: 1H and 13C chemical shifts of what are, in the authors' experience, the most popular “extra peaks” in a variety of commonly used NMR solvents are collected, in the hope that this will be of assistance to the practicing chemist.
Abstract: In the course of the routine use of NMR as an aid for organic chemistry, a day-to-day problem is the identification of signals deriving from common contaminants (water, solvents, stabilizers, oils) in less-than-analytically-pure samples. This data may be available in the literature, but the time involved in searching for it may be considerable. Another issue is the concentration dependence of chemical shifts (especially 1H); results obtained two or three decades ago usually refer to much more concentrated samples, and run at lower magnetic fields, than today’s practice. We therefore decided to collect 1H and 13C chemical shifts of what are, in our experience, the most popular “extra peaks” in a variety of commonly used NMR solvents, in the hope that this will be of assistance to the practicing chemist.
3,225 citations
TL;DR: Catalytic amounts of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) are used in combination with [bis(acetoxy)iodo]benzene (BAIB) as a stoichiometric oxidant in the conversion of primary and secondary alcohols to carbonyl compounds.
Abstract: Catalytic amounts of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) are used in combination with [bis(acetoxy)iodo]benzene (BAIB) as a stoichiometric oxidant in the conversion of primary and secondary alcohols to carbonyl compounds. This procedure works efficiently at room temperature in almost all common solvents and neat in some cases. This process exhibits a very high degree of selectivity for the oxidation of primary alcohols to aldehydes, without any noticeable overoxidation to carboxyl compounds, and a high chemoselectivity in the presence of either secondary alcohols or of other oxidizable moieties. This procedure allows an easy, convenient, high-yielding method for the oxidation of alcohols starting from commercially available compounds.
671 citations
TL;DR: All experimental evidence points to a mechanism involving an N-acyliminium ion as the key intermediate, formed by acid-catalyzed condensation of benzaldehyde and urea (or N-methylurea) formed by the Biginelli dihydropyrimidines.
Abstract: The mechansim of the three-component Biginelli dihydropyrimidine synthesis was reinvestigated using 1H and 13C NMR spectroscopy. Condensation of benzaldehyde, ethyl acetoacetate, and urea (or N-methylurea) in CD3OH according to the procedure described by Biginelli produced the expected 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carboxylates. According to NMR measurements, there is no evidence that the first step in the Biginelli reaction is an acid-catalyzed aldol reaction of ethyl acetoacetate and benzaldehyde leading to a carbenium ion intermediate, as has been suggested previously. In contrast, all experimental evidence points to a mechanism involving an N-acyliminium ion as the key intermediate, formed by acid-catalyzed condensation of benzaldehyde and urea (or N-methylurea). Interception of this iminium ion by ethyl acetoacetate produces open-chain ureides which subsequently cyclize to the Biginelli dihydropyrimidines.
383 citations
330 citations
TL;DR: Several novel fluorinated fluoresceins (Oregon green dyes) were prepared by the reaction of fluororesorcinols with phthalic anhydride and its derivatives.
Abstract: Several novel fluorinated fluoresceins (Oregon Green dyes) were prepared by the reaction of fluororesorcinols with phthalic anhydride and its derivatives. A novel regiospecific synthesis of fluororesorcinols was key to the successful synthesis of these new fluorophores. (Polyfluoro)nitrobenzenes were reacted with 2 equiv of sodium methoxide followed by reduction, hydrodediazoniation, and demethylation, giving the first straightforward synthesis of 2-fluororesorcinol, 4-fluororesorcinol, 2,4-difluororesorcinol, and 2,4,5-trifluororesorcinol. These fluorinated fluoresceins have higher photostability and ionize at a lower pH (pKa = 3.3−6.1) than fluorescein (pKa = 6.5). Some of the fluorinated fluoresceins have very high quantum yields (0.85−0.97), which, in combination with their lower pKas and high photostability, makes them superior fluorescent dyes for use as reporter molecules in biological systems.
288 citations
276 citations
TL;DR: In this paper, a combined catalytic system consisting of N-hydroxyphthalimide (NHPI) and Co(OAc)2 was developed to convert alkylbenzenes into the corresponding carboxylic acids under atmospheric dioxygen at ambient temperature.
Abstract: A practical catalytic method to convert alkylbenzenes into the corresponding carboxylic acids under atmospheric dioxygen at ambient temperature using a combined catalytic system consisting of N-hydroxyphthalimide (NHPI) and Co(OAc)2 was developed. For instance, the oxidation of toluene was completed by NHPI combined with Co(OAc)2 under an oxygen atmosphere at room temperature to give benzoic acid in 81% yield. Under these conditions, o- and p-xylenes were selectively converted into the corresponding monocarboxylic acids without the formation of the dicarboxylic acids. ESR measurements showed that Co(II) species assists in the formation of phthalimide-N-oxyl (PINO), which is a key species in this oxidation, from NHPI.
274 citations
TL;DR: New Horner-Emmons reagents, ethyl (diarylphosphono)acetates 1, were prepared from triethyl phosphonoacetate, PCl(5), and the corresponding phenols and were found to be the most effective, giving Z-unsaturated esters with 93-99% selectivity.
Abstract: New Horner-Emmons reagents, ethyl (diarylphosphono)acetates 1, were prepared from triethyl phosphonoacetate, PCl(5), and the corresponding phenols. The reaction of 1 with several kinds of aldehydes in the presence of Triton B or NaH in THF solvent revealed that these reagents are useful for the synthesis of Z-unsaturated esters. Among the reagents examined, ethyl(di-o-tolylphosphono)-, [bis(o-ethylphenyl)phosphono]-, and [bis(o-isopropylphenyl)phosphono]acetates (1k-m) were found to be the most effective, giving Z-unsaturated esters with 93-99% selectivity.
266 citations
TL;DR: The stability of the linker and subsequent cleavage upon photolysis from the support of an isotopically enriched 4-thiazolidinone was demonstrated by gel phase (13)C NMR and the kinetics of their photolytic cleavage examined in solution.
Abstract: Both a model phenacyl and o-nitrobenzyl photolabile linker from the literature along with four new o-nitrobenzyl linkers were prepared and the kinetics of their photolytic cleavage examined in solution. The linkers were prepared by amidation of the carboxylic acid anchoring tether with benzylamine, and the cleavable benzylic substituent was chosen to be either acetic acid or acetamide. Irradiation of the linkers in four solvents (methanol, p-dioxane, and aqueous buffer ± dithiothreitol) at 365 nm and analysis via HPLC afforded kinetic rates of cleavage suitable for comparative purposes. The phenacyl linker was found to cleave slowly under aqueous conditions with no detectable cleavage being observed in the organic solvents. Known o-nitrobenzyl linker 4 showed modest rates of cleavage in aqueous and organic solvents. Incorporation of two alkoxy groups in the benzene ring to generate the veratryl-based linker 13a increased the rate of cleavage dramatically, and introduction of an additional benzylic methyl ...
262 citations
249 citations
TL;DR: These methods provide a practical approach to the rational design of multichelating ligands for coo-tritopic and hetero-ditopic ligands in fair to excellent yields.
Abstract: Rigid preorganized multitopic ligands have been designed and synthesized. The Pd(0)-catalyzed cross-coupling reactions between ethynylated derivatives of bipyridine or terpyridine and the corresponding bipyridine or terpyridine halides or triflates provide access to various homo-ditopic, hetero-ditopic, homo-tritopic, and hetero-tritopic ligands bearing acetylene or diphenylacetylene central units in fair to excellent yields. Optimal conditions were found with [Pd(PPh3)2Cl2], CuI in THF and diisopropylamine at rt, or with [Pd(PPh3)4] in benzene and diisopropylamine at 80 °C. When a phenylethynyl group is present in the molecule, the relevant conditions involve [Pd(PPh3)4] in n-propylamine at 60 °C. Oxidative dimerization of the ethynylated derivatives in the presence of cupric salts and oxygen gives the corresponding homo-ditopic ligands bearing diacetylene or diphenyldiacetylene as a spacer in good yields. These methods provide a practical approach to the rational design of multichelating ligands for coo...
TL;DR: Compar pattern-recognition analyses of the NCI 60-cell mean graph screening profiles of 1 did not reveal any significant correlations to the profiles of known antitumor compounds in theNCI's "standard agent database", thus supporting the conclusion that the salicylihalamides represent a potentially important new class for antitumors lead optimization and in vivo investigations.
Abstract: Two novel, highly potent, cytotoxic macrolides, salicylihalamides A (1) and B (2), were isolated from the sponge Haliclona sp. This new macrolide class incorporates salicylic acid, a 12-membered lactone ring, and an enamide side chain. COMPARE pattern-recognition analyses of the NCI 60-cell mean graph screening profiles of 1 did not reveal any significant correlations to the profiles of known antitumor compounds in the NCI's “standard agent database”, thus supporting the conclusion that the salicylihalamides represent a potentially important new class for antitumor lead optimization and in vivo investigations.
TL;DR: The "ligandless" palladium acetate-catalyzed Suzuki cross-coupling reaction of ArX with aryl- and vinylboronic acids in water without organic cosolvent in the presence of tetrabutylammonium bromide is reported.
Abstract: The “ligandless” palladium acetate-catalyzed Suzuki cross-coupling reaction of ArX with aryl- and vinylboronic acids in water without organic cosolvent in the presence of tetrabutylammonium bromide is reported. Aryl bromides give high yields and considerably accelerate the coupling. A wide variety of functional groups can be tolerated. Aryl iodides, however, give incomplete conversion and aryl triflate coupling shows no improvement over reported conditions.
TL;DR: Dienes with allylic functional groups yielded functionalized cyclic olefins when treated with 1, and those with sterically demanding and/or electron-withdrawing substituents were cyclized successfully with 2, but did not cyclize with 1.
Abstract: Ruthenium alkylidene 1 and molybdenum alkylidene 2 have been utilized in the ring-closing metathesis (RCM) of dienes containing gem-disubstituted olefins to yield tri- and tetrasubstituted cyclic olefins. Dienes with sterically demanding and/or electron-withdrawing substituents such as Ph, CO(2)Me, and (t)()Bu were cyclized successfully with 2, but did not cyclize with 1. Tetrasubstituted cyclic olefins could be formed with 2, but not using alkylidene 1. Dienes with allylic functional groups yielded functionalized cyclic olefins when treated with 1.
TL;DR: An intriguing relationship between substitution on the oxazoline ring, particularly the commonly-derivatized 4-position, and catalyst efficiency was discovered and the results are rationalized by either partial or complete bis(oxazoline) decomplexation during the course of the reaction.
Abstract: Studies toward the development of an enantioselective diazomethane-based cyclopropanation reagent derived from bis(oxazoline)palladium(II) complexes are reported. Several simple palladium chelates, 2 and 7, in addition to the novel carbon-bound complexes 15 were synthesized and evaluated in the cyclopropanation of various electron-deficient olefins. The X-ray crystal structure of aryl−bis(oxazoline)palladium complex 15c is described. Although all catalysts efficiently affected cyclopropanation, all products were racemic. An intriguing relationship between substitution on the oxazoline ring, particularly the commonly-derivatized 4-position, and catalyst efficiency was discovered. The results are rationalized by either partial or complete bis(oxazoline) decomplexation during the course of the reaction.
TL;DR: The diversity of reactions indicates that high-temperature aqueous media could play an increasingly important role in the development of new preparative processes.
Abstract: Preparative organic synthesis was investigated in aqueous media at temperatures up to 300 °C. Experiments were conducted with a recently disclosed pressurized microwave batch reactor (MBR) or in conventionally heated autoclaves. Thirty-six examples are presented. Among these, methods were developed for a Fischer synthesis, an intramolecular aldol condensation that was scaled up, decarboxylation of indole-2-carboxylic acid, Rupe rearrangement of 1-ethynyl-1-cyclohexanol, isomerization of carvone to carvacrol, and conversion of phenylacetylene to acetophenone. The applicability of high-temperature water was also demonstrated for biomimetic processes important in food, flavor, and aroma chemistry and for tandem reactions such as formation of 2-methyl-2,3-dihydrobenzofuran from allyl phenyl ether. When addition of acid or base was necessary, less agent was usually required for high-temperature processes than for those at and below boiling, and the reactions often proceeded more selectively. In some instances ...
TL;DR: Ferrocenecarboxaldehyde is readily transformed into the acetal of (S)-1,2,4-butanetriol (1,3-dioxane structure) and further methylated to give acetal 15. This can then be ortho-lithiated using t-BuLi with very high diastereoselectivity as mentioned in this paper.
Abstract: Ferrocenecarboxaldehyde is readily transformed into the acetal of (S)-1,2,4-butanetriol (1,3-dioxane structure) and further methylated to give acetal 15. This can then be ortho-lithiated using t-BuLi with very high diastereoselectivity (98% de). Electrophilic quenching provided a large array of compounds of established stereochemistry. Controlled hydrolysis leads to many ortho-substituted ferrocenecarboxaldehydes (98% ee) which themselves are starting materials in the synthesis of various classes of enantiopure ferrocene derivatives with planar chirality of predictable absolute configuration.
TL;DR: In this paper, the authors reported the synthesis of molecularly imprinted polymers (MIPs) using an amide as the hydrogen-bonding functional group, which showed good enantiomeric recognition of the imprinted species when evaluated by high-performance liquid chromatography (HPLC).
Abstract: In this paper we report the synthesis of molecularly imprinted polymers (MIPs) using an amide as the hydrogen-bonding functional group. All the amide MIPs made in this investigation exhibited good enantiomeric recognition of the imprinted species when evaluated by high-performance liquid chromatography (HPLC). Most of the amide MIPs were synthesized in a polar organic solvent (acetonitrile). By contrast, when a carboxyl group was used as the hydrogen-bonding functional group, MIPs made in acetonitrile exhibited only very weak recognition and in some cases no recognition at all with the print molecules tested. The amide MIPs also showed much improved load capacity. This suggests that for a number of cases the protocol described could be employed to reduce background, nonspecific binding compared to polymers using carboxyl groups, by avoiding an excess of negatively charged functionalities. It was shown that the tailing and peak asymmetry problems often associated with the use of MIPs in HPLC could be overc...
TL;DR: In this paper, a combination of BINAP and Pd(dba)2 catalyzes the amination of aryl triflates, a reaction that allows for the conversion of phenols to arylamines.
Abstract: We report that a combination of DPPF (1,1-bis(diphenylphosphino)ferrocene) and Pd(dba)2 leads to the amination of aryl triflates, a reaction that allows for the conversion of phenols to arylamines. A combination of BINAP and Pd(dba)2 also catalyzes the amination of aryl triflates, but P(o-tolyl)3 complexes were not effective catalysts. In some cases, slow addition of the aryl triflate was necessary to prevent cleavage of the triflate and generation of phenol. We found that added halide, necessary in some cross-coupling chemistry of aryl sulfonates, was an unnecessary additive and even inhibited the amination chemistry.
TL;DR: In this article, the synthesis of soluble oligo(2-alkyl-1,4-phenylene ethynylene)s via an iterative divergent/convergent approach starting from 1-(diethyltriazyl)-3-alkynyl-4-[(trimethylsilyl)ethynyl]benzenes is described.
Abstract: The syntheses of soluble oligo(2-alkyl-1,4-phenylene ethynylene)s via an iterative divergent/convergent approach starting from 1-(diethyltriazyl)-3-alkyl-4-[(trimethylsilyl)ethynyl]benzenes are described. When the solublizing alkyl group is an ethyl substituent, the monomer, dimer, tetramer, and octamer can be synthesized. The octamer, however, is only minimally soluble. When the alkyl substituent is 3-ethylheptyl or dodecyl, the compounds are easily dissolved even at the 16-mer stage. The 16-mer is 128 A long in its near-linear extended conformation. At each stage in the iteration, the length of the framework doubles. Only three sets of reaction conditions are needed for the entire iterative synthetic sequence: an iodination, a protodesilylation, and a Pd/Cu-catalyzed cross coupling. Synthesis of the dodecyl-containing 16-mer was also achieved on Merrifield's resin using the iterative divergent/convergent approach. The oligomers were characterized spectroscopically and by mass spectrometry. The optical ...
TL;DR: Vicinal diarylphosphinites derived from carbohydrates are excellent ligands for the Rh(I)-catalyzed enantioselective asymmetric hydrogenation of dehydroamino acid derivatives.
Abstract: Vicinal diarylphosphinites derived from carbohydrates are excellent ligands for the Rh(I)-catalyzed enantioselective asymmetric hydrogenation of dehydroamino acid derivatives, producing the highest enantioselectivity of any ligands directly prepared from natural products. The enantioselectivity can be enhanced by the appropriate choice of substituents on the aromatic rings of the phosphinites. For example, the use of phosphinites with electron-donating bis(3,5-dimethylphenyl) groups on phosphorus provides ee's up to 99% for a wide range of amino acids including some with easily removable N-protecting groups. Electron-withdrawing aryl substituents, on the other hand, decrease the enantioselectivity. Sense of chiral induction in the amino acid product depends on the relative juxtaposition of the vicinal diphosphinites on a given sugar backbone. When readily available d-glucopyranosides are used as the starting sugars, 2,3-phosphinites give the S-amino acids and 3,4-phosphinites give the R-amino acids. In th...
TL;DR: The first demonstration of the suitability of fluorous phase methods for combinatorial synthesis of "druglike" organic molecules is described, using the Ugi and Biginelli reactions.
Abstract: A new protocol for multicomponent condensation reactions that uses fluorous (highly fluorinated) substrates is introduced. This method takes advantage of the ease of purification of fluorous compounds by liquid-liquid extractions between fluorous and organic solvents. The application of this method to the Ugi and Biginelli reactions is described. The condensation products of these two reactions, amino acid amides and dihydropyrimidines, are easily obtained without chromatography in high purities, even though the other reagents are used in very large excesses. This is the first demonstration of the suitability of fluorous phase methods for combinatorial synthesis of "druglike" organic molecules.
TL;DR: The levels of racemization in the model peptide were in the entirely unacceptable range of 5-33%.
Abstract: Contrary to the conventional wisdom of the peptide synthesis field, N,S-protected derivatives of cysteine can undergo substantial levels of racemization with widely-used reagents and protocols for stepwise incorporation. A systematic study of this problem has been carried out as a function of coupling conditions and beta-thiol protecting groups, i.e., S-acetamidomethyl (Acm), S-triphenylmethyl (trityl or Trt), S-2,4,6-trimethoxybenzyl (Tmob), and S-9H-xanthen-9-yl (Xan), taking advantage of a convenient and quantitative model system assay involving HPLC resolution of H-Gly-L-Cys-Phe-NH(2) from H-Gly-D-Cys-Phe-NH(2). For example, standard protocols for couplings mediated by phosphonium and aminium salts, e.g., (benzotriazolyloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), N-[(1H-benzotriazol-1-yl)(dimethylamino)methylene]-N-methylmethanaminium hexafluorophosphate N-oxide (HBTU), N-[[(dimethylamino)-1H-1,2,3-triazolo[4,5-b]pyridin-1-yl]methylene]-N-methylmethanaminium hexafluorophosphate N-oxide (HATU), and (7-azabenzotriazol-1-yloxy)tris(pyrrolidino)phosphonium hexafluorophosphate (PyAOP), typically involve 5-min preactivation times and are conducted in the presence of suitable additives such as 1-hydroxybenzotriazole (HOBt) or 7-aza-1-hydroxybenzotriazole (HOAt) plus a tertiary amine base such as N,N-diisopropylethylamine (DIEA) or N-methylmorpholine (NMM). Under such conditions, the levels of racemization in the model peptide, expressed as the ratio of D:L peptide formed, were in the entirely unacceptable range of 5-33%. However, these levels were in general reduced by a factor of 6- or 7-fold by avoiding the preactivation step. Additional strategies to reduce racemization involved change to a weaker base, with 2,4,6-trimethylpyridine (TMP, collidine) being substantially better than DIEA or NMM; 2-fold reduction in the amount of base; and change in solvent from neat N,N-dimethylformamide (DMF) to the less polar CH(2)Cl(2)-DMF (1:1). Coupling methods for the safe incorporation of cysteine with minimal racemization (<1% per step) in 9-fluorenylmethyloxycarbonyl (Fmoc) solid-phase peptide synthesis include BOP (or HBTU or HATU)/HOBt (or HOAt)/TMP (4:4:4) without preactivation in CH(2)Cl(2)-DMF (1:1), DIPCDI/HOBt (or HOAt) (4:4) with 5-min preactivation, and preformed pentafluorophenyl (Pfp) esters in CH(2)Cl(2)-DMF (1:1).
TL;DR: Under mild conditions, an interesting selectivity was observed with functional groups that are known to react either in palladium-catalyzed reactions or reactions catalyzed by other transition-metals, and aryl halides, terminal alkynes, 1,6-enynes, and alpha-allenic alcohols were tolerated.
Abstract: The scope of the palladium-catalyzed cyclization/dimerization of terminal allenyl ketones 1 to the 2,4-disubstituted furans 3 has been investigated. Simplified and improved conditions almost exclusively provided the dimer 3, accompanied by only traces of the easily separable monomer 2. The formation of an isomer of 3, the unconjugated ketone 4, was completeley suppressed. Under these mild conditions, besides the normal functional group tolerance known for palladium-catalyzed reactions, an interesting selectivity was observed with functional groups that are known to react either in palladium-catalyzed reactions or reactions catalyzed by other transition-metals. Thus aryl halides, terminal alkynes, 1,6-enynes, and α-allenic alcohols were tolerated. In the latter example the selective reaction of only one out of two different allenes was achieved. Mechanistic investigation indicated a Pd(II)/Pd(IV)-cycle involving palladium(II)-γ-alkoxyvinylcarbene and furylpalladium(IV) hydride intermediates, although a sec...
TL;DR: A modified cyclodextrin containing a dansyldiethylenetriamine metal-binding group (6-deoxy-6-N-(N‘-dansylethylenediamino)-β-cyclodextrin, CD-dien-DNS) was synthesized in this paper.
Abstract: A modified cyclodextrin containing a dansyldiethylenetriamine metal-binding group (6-deoxy-6-N-(N‘‘-dansyldiethylenetriamino)-β-cyclodextrin, CD-dien-DNS) was synthesized. The conformation of CD-dien-DNS was studied by 2D NMR (ROESY spectra) in D2O, by circular dichroism, and by fluorescence. The results were compared with those previously obtained with the analogous 6-deoxy-6-N-(N‘-dansylethylenediamino)-β-cyclodextrin (CD-en-DNS) and were consistent with the self-inclusion of the dansyl group within the macrocycle cavity. However, the orientation of the dansyl group for CD-dien-DNS was found to be equatorial, whereas for CD-en-DNS it was axial, suggesting a dependence of the orientation of the dansyl group upon the length of the linker. In the presence of lipophilic organic molecules, CD-dien-DNS showed sensing properties similar to those observed for CD-en-DNS, suggesting a similar “in-out” movement of the dansyl group, due to competitive inclusion of the guest. Unlike CD-en-DNS, CD-dien-DNS was found ...