Recent syntheses and reactions of cyclic imidic esters

Δ²-Oxazolines and 4H-5,6-dihydrooxazines are readily obtainable from carboxylic acids and 2- or 3-amino alcohols by gas-phase dehydration, and 4H-dihydrooxazines can also be obtained by condensation of N-(hydroxymethyl) carboxamides with olefins. These cyclic imidic esters undergo reactions in which the ring is cleaved; these reactions include a new type of polymerization. The behavior of alkyl groups in position 2 and the cyclodditions in the 2,3-position also have been studied. Reactions in side chains on position 2 and addition experiments on the sulfur analogues of the cyclic imidic esters are also reported. A table shows the principal infrared bands of substituted Δ²-oxazolines.     

Transesterification over various zeolites under liquid-phase conditions

Transesterification of β-keto esters with a variety of alcohols to the corresponding β-keto esters has been carried out efficiently over aluminosilicates simply by refluxing in toluene under liquid-phase conditions. Aliphatic β-keto esters exhibit the higher reactivity than the aromatic and/or cyclic β-keto esters, whereas other esters such as -keto esters, ,β-unsaturated esters, normal esters, and -halo esters fail to undergo the transesterification. The reaction proceeds smoothly with primary alcohols than the tertiary-, cyclic-, and allylic alcohols. The large-pore zeolites such as Y, mordenite, and β show higher activity than the medium-pore ZSM-5 and the partial removal of framework aluminum from the large-pore zeolites by dealumination increases their activity. The aluminum containing mesoporous MCM-41 shows poor activity. Among the different solvents used toluene leads to the highest activity and the optimum catalyst concentration found was ca. 20 wt.% of the substrate. The activity increases with increasing reaction temperature. The reaction pathway consists of formation of acyl ketene intermediate by the interaction of β-keto esters with Brønsted acid sites of the catalyst, followed by nucleophilic attack of the alcohol at the electrophilic center and successive elimination of the proton to give the product.     

Study of the secretion from a scale insect (Ceroplastes ceriferus) diterpenoids and sesterterpenoids

Structures of four new minor sesterterpenoids 1–3,18 and three new diterpenoids 12–14 isolated from the secretion of Ceroplastes ceriferus are described. All four sesterterpenoids are 14-membered monocyclic compounds; one of them has been chemically correlated with cericeroic acid¹ of proven absolute stereochemistry. The “cyclic wax” was a complex mixture of esters consisting of fatty acids (C₁₀-C₃₂) and unusual cyclic alcohols (diterpenoids and sesterterpenoids)     

Reactions of oxalyl chloride with 1,2-cycloalkanediols in the presence of triethylamine

The relationship between the product patterns and the configurations of 1,2-cycloheptane- and 1,2-cyclooctanediols 9 in the cyclocondensations with oxalyl chloride in the presence of triethylamine at 0 degrees C has been shown analogous to that obtained for 1,2-disubstituted acyclic ethylene glycols 1: cis-1,2-cyclooctanediol (9f) produced the cyclic oxalate 14f as the major product, while trans-1,2-cycloheptanediol (9e) and trans-1,2-cyclooctanediol (9g) formed the cyclic carbonates 12e, g as the major products. On the other hand, the cyclic oxalates 14a-d were formed as the major products from 1,2-cyclopentane- and 1,2-cyclohexanediols regardless of the configuration. These results can be accounted for by assuming the boat-like transition states for cyclizations of the half esters of comparatively rigid five- and six-membered diols 9a--d. The cyclic oxalates 14a, c may be directly formed through the resulting tetrahedral intermediates from cis-diols (9a,c), and the cyclic carbonates 12a,c as the minor products after ring inversion of the tetrahedral intermediates. The tetrahedral intermediates from the trans-isomers 9b, d cannot undergo ring inversion, producing no traces of the cyclic carbonates 12b, d.     

Preparation, crystal structure and thermal decomposition study of some trimethylsilyl esters of dicarbamic acids

Some new trimethylsilylated dicarbamic acid esters 1–9 and 10 were prepared. Their thermal decomposition was studied in n-alkanes as media. The reactions were monitored by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS), respectively. Some N-silylated cyclic ureas 11–15 were also prepared from the corresponding esters 2–6 by thermolysis. The crystal structures of 2, 5 and 8 are also discussed. The crystal and molecular structures of 2 and 5 have very similar characteristics. Both molecules are sited via their molecular centre of symmetry on crystallographic inversion centres. The Carbamate groups are planar in all three cases as expected. An analysis of the data showed that the force due to Si---O conjugation which constrains the Si atom into the plane of the carbamate group is comparable to the crystal packing forces with a small out-of-plane movement being not unfavourable.     

Notes on Kuhn's ω″ molecular orbital method

(1) It is shown that, contrary to statements sometimes made, it is perfectly possible to obtain consistent sets of charges in Kuhn's ω″ molecular-orbital method, whatever the values of the parameters ω, ω′, ω″.

(2) Particular applications are made to the benzyl cation, and to the non-alternants fulvene, heptafulvene and azulene. It appears that including the ω-terms decreases the magnitude of the largest of the net atomic charges as calculated by the simple Hückel method, and results in an overall smoothing-out process. Further inclusion of the ω′-terms continues this smoothing-out process, but inclusion of the ω″-terms may sometimes slightly reverse this process.

(3) The charge distributions obtained for fulvene and azulene in (1) and (2) lead to molecular dipole moments which are still much too large.

In view of (3) it is not clear that the additional work involved in the inclusion of ω′ and ω″ is justified by the greater accuracy thus obtained.     

磷化氢与环烯烃反应的原位^3^1P NMR研究

本文应用加压原位核磁共振波谱技术, 在反应温度50-70℃、反应压力1.0-2.0MPa, 氘代苯为溶剂, 偶氮二异丁腈为引发剂的条件下, 考察了磷化氢与环烯烃1, 5-环辛二烯、双环戊二烯、1, 3-环己二烯、1, 4-环己二烯及1, 5, 9-环十二三烯的反应。实验结果表明, 磷化氢与1, 5-环辛二烯反应主要生成双环膦杂环壬烷; 与其它环烯烃均不生成膦杂环化合物, 仅为一取代伯膦或二取代仲膦产物。磷化氢与环烯烃反应仍为串行机理。     

Identification of ω-oxocarboxylic acids as acetal esters in aerosols using capillary gas chromatography-mass spectrometry

An homologous series of ω,ω-dimethoxycarboxylic acid methyl esters (acetal esters) has been identified in the most polar methyl ester fraction separated from remote aerosol samples by liquid chromatography. These compounds were separated by silica gel column chromatography and their structures determined by capillary gas chromatography (GC) and GC—mass spectrometry with a synthesized standard. The acetal esters are originally present in the samples as ω-oxocarboxylic acids, which are derivatized to acetal esters during treatment with boron trifluoride in methanol. This method enables the determination of ω-oxocarboxylic acids in environmental samples as their acetal esters.     

Mittlere ringe-XIII Die intramolekulare acylierung der ω-(1-naphthyl)-fettsäuren

The intramolecular Friedel-Crafts cyclisation of ω-arylalkanoic acids, which is in the benzene series a valuable synthetic route to bicyclic ketones with medium-sized and large rings, has been investigated with ω-(1-naphthyl)alkanoyl chlorides. The lower members, including naphthyl-caproic acid, close the ring at position 2 to form IV. The higher homologous acids show a preference for annellation at 7, making available a new type of heteronuclear naphthalene cyclic ketones (VII). During cyclisation of ω-(1-naphthyl) decanoic acid, the 1,4 ring closure competes with the 1,7 type.

Steric hindrance of resonance as a consequence of “medium ring torsion” was studied in different classes of naphthocyclenones.     

The influence of substituents on preparation and tautomerism of open-chain β-thioketoesters Structure determination by NMR and IR spectroscopy

With special regard to the synthesis of β-thioketoesters, the acid catalysed reactions of 36 differently substituted β-keto esters with H₂S have been studied under various conditions in order to determine the influence of the substituents on reaction course. gem-Dithiols may also be obtained in good yields by this reaction. Treatment of T1(I)-salts of β-thioketo esters with alkyl halides results exclusively in S-alkylation. A slow cis-trans isomerization of S-alkylated ,β-unsaturated β-mercapto esters through rotation about the C=C double bond is demonstrated. Ethyl thioacetothionoacetate has been prepared by self-condensation of ethyl thionoacetate.

The tautomerism of the β-thioketo esters has been studied by NMR and IR spectroscopy. In the -H esters, the intramolecularly H-bonded cis-enethiol tautomer is predominant. This tautomer is also important in the -substituted esters but due to steric crowding, H-bonding occurs to the alkoxy O-atom as well as to the carbonyl O-atom; further, the trans-enethiol form appears in percentages comparable with those of the cis-enethiol form. Ethyl -isopropyl thioacetoacetate exists almost exclusively in the thione form.

¹H NMR chemical shifts and couplings of informative value for structure determination are reported and discussed. The application of the chemical shift of potentially chelated enethiolic protons as an indicator of intramolecular H-bonding is confirmed, although a general application of it (and also of corresponding proton chemical shifts in related systems) as a measure on H-bonding strength is questioned. An intramolecular H-bonding involving chlorine as acceptor atom is presented. New long-range “through bondings” couplings as well as a “through space” coupling involving enethiolic protons are also presented.     

Developing the Low-Temperature Oxidation Mechanism of Cyclopentane: An Experimental and Theoretical Study

At present, the reactivity of cyclic alkanes is estimated by comparison with acyclic hydrocarbons. Due to the difference in the structure of cycloalkanes and acycloalkanes, the thermodynamic data obtained by analogy are not applicable. In this study, a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) was applied to study the low-temperature oxidation of cyclopentane (CPT) at a total pressure range from 1–3 atm and low-temperature range between 500 and 800 K. Low-temperature reaction products including cyclic olefins, cyclic ethers, and highly oxygenated intermediates (e. g., ketohydroperoxide KHP, keto-dihydroperoxide KDHP, olefinic hydroperoxides OHP and ketone structure products) were observed. Further investigation of the oxidation of CPT – electronic structure calculations – were carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+ G(d,p) level to explore the reactivity of O₂ molecules adding sequentially to cyclopentyl radicals. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPT radicals with O₂ is HO₂ elimination yielding cyclopentene. The pathways of second and third O₂ addition – the dissociation of hydroperoxide – were further confirmed. The results of this study will develop the low-temperature oxidation mechanism of CPT, which can be used for future research on accurately simulating the combustion process of CPT.     

[3+2] Cycloaddition Approaches to Five-Membered Rings via Trimethylenemethane and Its Equivalents [New Synthetic Methods (55)]

The power of cycloadditions for ring formation derives from the potential for chemo-, regio-, diastereo-, and enantioselectivity. Trimethylenemethane (TMM) and its equivalents offer the possibility of extending such benefits to the synthesis of five-membered rings. Three types of conjunctive reagents appear to be promising candidates: (1) selected 4-alky-lidene-4,5-dihydro-3H-pyrazoles, (2) 2-[(trimethylsilyl)methyl]allyl esters and halides, and (3) alkylidenecyclopropanes. Thermal reactions, especially intramolecular ones, effect the cycloaddition of the pyrazoles to olefins–especially electron-poor olefins. The latter two precursors require a catalyst, notably a palladium complex, to unlock the synthetic potential. With the 2-[(trimethylsilyl)methyl]allyl esters, a general chemo-, regio-, and diastereoselective cycloaddition with typical Diels-Alder dienophiles forms methylenecyclopentanes in both inter- and intramolecular fashion. A tin analogue allows extension of this cycloaddition to aldehydes, and, in a related sequence, to ketones and imines to form 3-methylenete-trahydrofurans and 3-methylenepyrrolidines with excellent diastereoselectivity. The 2-[(tri-methylsilyl)methyl]allyl esters also serve to effect net methylenecyclopentane annulation to cyclic enols. Such adducts can further undergo three-carbon intercalation or combined ring contraction-spiroannulation simply by the use of either a nucleophilic or electrophilic trigger. Alkylidenecyclopropanes undergo cycloadditions to both electron-rich and electron-poor olefins when stimulated by metal catalysts, especially palladium. A complementary regiochemistry to the approach based upon silicon conjunctive reagents is observed. The availability of these cycloadditions provides new strategies for the synthesis of natural products.     

Catalytic intermolecular C-alkylation of 1,2-diketones with simple olefins: a recyclable directing group strategy

We describe the first example of Rh-catalyzed intermolecular C-alkylation of cyclic 1,2-diketones using simple terminal olefins as alkylating agents. Aminopyridine is employed as a recyclable directing group. First, it reacts with ketones to give enamines and delivers Rh to activate the vinyl C-H bonds in the same pot; second, it can be cleaved off and recovered via hydrolysis. A broad range of olefins can be utilized as substrates, including aliphatic, aromatic olefins and vinyl esters. The efficiency of this method is also demonstrated in the synthesis of a natural flavoring compound, 3-ethyl-5-methyl-1,2-cyclopentadione (one-pot 53% yield vs a previous four-step route 16% yield from the same starting material). This work is expected to serve as a seminal study toward catalytic ketone α-alkylation with unactivated olefins.     

Anion radicals and dianions as electron transfer reagents

The catalytic regeneration of depolarizers by certain substrates has been demonstrated by means of cyclic voltammetry. As depolarizers we have employed anion radicals of aromatic hydrocarbons, heteroaromatic compounds, ketones, esters, nitriles, and olefins, and dianions of aromatic hydrocarbons, heteroaromatic compounds, ketones, and esters. As substrates we have investigated simple aromatic and aliphatic halides, 1,2- and 1,3-dihalides, carbon dioxide, and activated olefins. A requirement for the observation of a catalytic wave is that the reduced substrate undergoes a practically irreversible reaction after the reaction with the reduced depolarizer; for simple halides it is a cleavage reaction, for 1,2- and 1,3-dihalides an elimination, for carbon dioxide an addition, and for activated olefins a coupling. Electrochemical investigations of these kinds of reactions may be of interest in connection with the mechanism of several types of reactions in organic chemistry.     

An electrophilic cleavage procedure for the asymmetric dihydroxylation: direct enantioselective synthesis of cyclic boronic esters from olefins

A variation within the osmium-catalysed asymmetric dihydroxylation (AD) of olefins is described that yields cyclic boronic esters from olefins in a straight-forward manner. This process represents the first real product alteration in asymmetric dihydroxylation, since all previous protocols lead to free diols exclusively. A protocol based on the Sharpless AD conditions (for enantioselective oxidation of prochiral olefins) was developed that gives cyclic boronic esters with excellent enantiomeric excesses (ee's). Some of the ee's are higher than those reported for conventional AD. The unprecedented role of phenyl boronic acid on the course of the AD reaction was investigated in detail. PhB(OH)2 does not interfere with the chiral ligand, leaving the enantioselective step of olefin oxidation intact. The main role of the boronic acids-apart from protecting the diol products against potential overoxidation-relies on removing the diol entity in an electrophilic cleavage, which is in contrast to the conventional hydrolytic cleavage of the AD protocols. Thus, a mechanistically new cleavage for enantioselective dihydroxylation reactions is introduced within the present work.     

IPy2BF4-mediated rearrangements of 1,2-difunctionalized compounds and olefins

Acetal derivatives are easily obtained from 1,2-difunctionalized compounds by a new reaction mediated by IPy2BF4 with a mechanism based on a 1,2-migration of aryl or alkyl groups. A new oxidative rearrangement reaction of olefins is also described. Moreover, when this metal-free protocol is applied to cyclic olefins, interesting ring-contraction reactions are observed. The new methodologies described here are a clean and efficient alternative to known strategies that make use of potentially toxic metallic complexes.     

The actual mercurating species in the mercuration of aromatic amines and the aminomercuration of olefins

The reactivity of π- and σ- N-mercurated and C-mercurated amines as electrophiles towards olefins and aromatic amines is studied under different reaction conditions. Depending on the ionic or covalent character of the starting mercury(II) salt, dissociated species or π-complexes 3 respectively, are found to be the most plausible mercurating species in the aminomercuration process. By contrast, complexes 3' do not react with aromatic amines unless the former undergo prior dissociation.     

Kinetic resolution of racemic alcohols with chiral carboxylic acids and dicyclohexylcarbodiimide

The DCC-esterification method has been used to kinetically resolve racemic mixtures of alcohols. With simple chiral carboxylic acids, such as O-aryl lactic acids, in the presence of various basic catalysts, mixtures of the enantiomerically enriched alcohols (e.e. <50%) and the corresponding esters (d.e.<70%) have been obtained.     

Formation of isoprostane bicyclic endoperoxides from the autoxidation of cholesteryl arachidonate

Autoxidation of polyunsaturated fatty acids and esters leads to a complex mixture containing hydroperoxides and cyclic peroxides. Prostaglandin bicyclic endoperoxides have been detected from the autoxidation of cholesteryl arachidonate by LC-MS and GC-MS techniques. All four possible types (I-IV) of bicyclic endoperoxides have been found starting from different regioisomeric hydroperoxides of cholesteryl arachidonate. Furthermore, the stereochemistry of Type IV bicyclic endoperoxides has been determined by conversion to pentafluorobenzyl (PFB) ester, trimethylsilyl (TMS) derivatives, and comparison with synthetic standards by the use of GC-MS. All eight possible diastereomers of the derivatized isoprostanes were observed and were separated by gas chromatography. The bicyclic endoperoxides with the two alkyl chains syn on the cyclopentane ring were formed preferentially to those with anti configuration, a result anticipated from earlier work. Substantial amounts of the anti-substituted isoprostanes, including PGF(2)(alpha), were, however, observed in the product mixture.     

The second and third order non-linear optical properties of liquid crystalline polymers

The second and third order non-linear optical susceptibilities of several donoracceptor substituted side chain polymers, some of which exhibit mesophases, will be reported. The susceptibilities were measured by harmonic generation from thin films, typically 0·5 μm thick, at a fundamental wavelength of 1064 nm and 1579 nm. For second harmonic generation, the spin coated films were contact poled on a glass substrate with an interdigited electrode pattern. Typical values obtained were χ⁽²⁾₃₁ = 1 × 10^-9 esu and χ⁽²⁾₃₃ = 3 × 10^-9 esu, the largest value was χ⁽²⁾₃₃ = 6·3 × 10^-9 esu. The difficulty in deriving reliable second order structure-property relations because of the large effect which the contact poling process has on the non-linear optical susceptibility of the films will be highlighted. The highest χ⁽³⁾ value of 5·8 × 10^-12 esu obtained from an amorphous film of a polymer with an asymmetrically substituted azo side group is comparable with the susceptibility of more typical third order non-linear materials such as the main chain polymers polyphenylacetylene and poly-p-phenylenebenzobisthiazole.