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1.
In this paper, we deal with the scope and mechanism of the strong Br?nsted acid-catalyzed intramolecular cyclization reaction of methyl 3-aryl-2-nitropropionates to give 4H-1,2-benzoxazines. This reaction can be regarded as an oxygen functionalization of the aromatic ring wherein the oxygen atom is derived from the nitro group in the molecule, and it is favored by the presence of electron-withdrawing groups on the benzene ring. The reaction rate is strongly influenced by the acidity of the reaction medium, and the methyl ester group on the alpha-carbon atom with respect to the nitro group facilitates deprotonation at the alpha-position to give aci-nitro species in situ. Some correlation was found between the electron-withdrawing ability of the substituents on benzene, represented in terms of Hammett's sigma p value of the substituents, and the rate of disappearance of the starting substrate leading to the product in trifluoromethanesulfonic acid (TFSA)/trifluoroacetic acid (TFA) medium. This would be because the acidity of the alpha-proton with respect to the nitro group is influenced by the substituents on the benzene ring. Experimentally, we excluded the 6pi electrocyclization mechanism involving deprotonation of the benzyl proton of the protonated aci-nitro species. Alternative cyclization mechanisms involving equilibrating monocationic aci-nitro species bearing O-protonated ester carbonyl group and O-protonated aci-nitro species were calculated to be highly energetically unfavorable. Diprotonated or protosolvative species can reduce the activation energy significantly, and this is consistent with the observed acidity-dependent nature of the cyclization. 相似文献
2.
以L-亮氨酸(L-Leu)为手性源, 经酯化和缩合等步骤制备手性单体(NALL), 以该单体为手性识别基团, 在交联剂N,N'-亚甲基双丙烯酰胺和引发剂偶氮二异丁腈的作用下, 与N-异丙基丙烯酰胺(NIPAM)以不同的质量比发生自由基共聚, 制备了一系列新型手性温敏水凝胶P(NIPAM-co-NALL), 其结构经红外光谱确证. 相比于PNIPAM水凝胶, 疏水性单体NALL的引入使 P(NIPAM-co-NALL)凝胶的温敏性下降. 以D, L-苯丙氨酸为模型药物对P(NIPAM-co-NALL)凝胶的手性识别和拆分性能进行研究, 结果表明, 手性温敏凝胶对D型对映体具有选择吸附性, 且吸附量随着手性单体含量的增加而增加; 提高温度(40 ℃)有利于手性温敏凝胶对D, L-苯丙氨酸的手性识别和拆分. P(NIPAM-co-NALL)凝胶在重复使用后依然具有手性识别性能, 但其吸附量随着使用次数的增加而下降, 3次重复使用后吸附量下降到原来的27.6%. 相似文献
3.
[reaction: see text] The synthesis of allocyathin B(3) from an advanced intermediate possessing the ring system and relative stereochemistry but lacking the isopropyl and hydroxymethyl groups is reported. The isopropyl group was introduced by radical cyclization of a methyl propargyl acetal of an alpha-bromo ketone, and the hydroxymethyl group was generated by Pd-catalyzed carbonylation of a vinyl triflate. The route provides functionalized intermediates that could allow access to more complex members of the cyathin family of diterpenes. 相似文献
4.
The sequential coupling and cyclization reactions between aryl halides and methyl propiolate were investigated. The electron-withdrawing groups on the aromatic ring are essential for producing the methyl indole-2-carboxylate derivatives. On the other hand, the presence of an extra methyl propiolate and Pd(PPh3)4 were required to provide an efficient catalytic system for the cyclization reactions. This reaction was used for the total synthesis of duocarmycin SA. 相似文献
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Five new chiral macrocycles, 3a-e, have been prepared by the acylation cyclization of chiral diamine dihydrobromide intermediates 2a-c with 2,6-pyridinedicarbonyl dichloride in highly diluted solution at room temperature. The chiral diesters 1a-c needed for the preparation of the macrocycles were obtained from condensation of corresponding N-(Z)-L-amino acids and 2,6-bishydroxymethyl pyridine in the presence of DCC and DMAP. The enantiomeric recognition of chiral macrocycles 3a-e for D- and L-amino acid methyl ester hydrochlorides has been characterized by fluorescence spectra, which indicate that some of them exhibited significant chiral recognition for the enantiomers of D- and L-amino acid methyl ester hydrochlorides. The stoichiometry and binding constants of 3a-L-Am(2) and 3c-L-Am(2) complexes have been determined. An X-ray analysis of the chiral macrocycle 3b show that the chiral ligand is rather rigid and strained. 相似文献
8.
Milostav Chudík tefan Marchalín Peter Knesl Adam Daich Bernard Decroix 《Journal of heterocyclic chemistry》2000,37(6):1549-1554
Efficient assembly of 6‐substituted 4‐aryl‐5‐oxo‐1,4,5,7‐tetrahydropyrrolo[3,4‐b]pyridines (7a‐f) is described according to a Hantzsch type reaction from formyl‐ester 4 by imination, borohydride reduction and intramolecular thermal amino‐ester cyclization. The starting compound 4 was prepared in three steps from the readily available formyl derivative 1, methyl 4,4‐dimethoxy‐3‐oxobutanoate and methyl 3‐aminocrotonate. 相似文献
9.
Highly enantioselective reduction of five-, six-, seven-, and eight-membered prochiral 1,3-cycloalkanediones possessing a methyl group and a protected hydroxymethyl group at their C2 position with baker's yeast or CBS catalyst and a new efficient and general method for preparing the 1,3-cycloalkanediones have been developed. These baker's yeast mediated reductions were found to produce corresponding ketols with high optical purity (>99% ee) and high yield. All of the prepared ketols and their derivatives, chiral building blocks, have been fully characterized, and their absolute configurations have been determined. These compounds would be useful for the convergent synthesis of complex natural products. 相似文献
10.
Lobach A. V. Leus O. N. Titova N. Yu. Luk'yanenko N. G. 《Russian Journal of Organic Chemistry》2003,39(7):1037-1041
Alkylation of 2-substituted (4S,5S)-4,5-bis(hydroxymethyl)-1,3-dioxolanes with 9-benzyl-1,17-diiodo-3,6,12,15-tetraoxa-9-azaheptadecane afforded new chiral aza and diaza crown ethers as a result of [1+1] and [2+2] additions. Their catalytic debenzylation gave the corresponding derivatives with a secondary amino group. The reaction of diethyl (+)-tartrate and diethyl (4S,5S)-1,3-dioxolane-4,5-diacetates with 1,8-diamino-3,6-dioxaoctane led to formation of chiral macrocyclic lactams which were reduced with lithium aluminum hydride. The resulting diaza crown ethers were tested for enantioselectivity in complex formation with L- and D-valine methyl ester by the potentiometric method. In most cases, the aza crown ethers showed better enantioselectivity than their oxygen analogs. 相似文献
11.
A combination of an amino acid-derived chiral phosphine catalyst and methyl acrylate to catalyze the direct Mannich reaction of 3-substituted oxindoles and imines has been reported to afford 3-tetrasubstituted oxindole derivatives which are key structures for biological activities. The products are formed with a quaternary carbon and featured with two adjacent chiral centers. Various N-EDG(electron-donating group) and N-EWG(electron-withdrawing group) protected oxindoles, including 3-aryl and 3-alkyl substituted ones, have been evaluated with aromatic and aliphatic imines under this catalytic system, smoothly giving desired products in good yields as well as excellent diastereo- and enantioselectivities. 相似文献
12.
Plots of log k(0) vs pH for the cyclization of trifluoroethyl and phenyl 2-aminomethylbenzoate to phthalimidine at 30 degrees C in H(2)O are linear with slopes of 1.0 at pH >3. The values of the second-order rate constants k(OH) for apparent OH(-) catalysis in the cyclization reactions are 1.7 x 10(5) and 5.7 x 10(7) M(-)(1) s(-)(1), respectively. These rate constants are 10(5)- and 10(7)-fold greater than for alkaline hydrolysis of trifluoroethyl and phenyl benzoate. The k(OH) for cyclization of the methyl ester is 7.2 x 10(3) M(-)(1) s(-)(1). Bimolecular general base catalysis occurs in the intramolecular nucleophilic reactions of the neutral species. The value of the Bronsted coefficient beta for the trifluoroethyl ester is 0.7. The rate-limiting step in the general base catalyzed reaction involves proton transfer in concert with leaving group departure. The mechanism involving rate-determining proton transfer exemplified by the methyl ester in this series (beta = 1.0) can then be considered a limiting case of the concerted mechanism. General acid catalysis of the neutral species reaction or a kinetic equivalent also occurs when the leaving group is good (pK(a) = 12.4). That the mechanism and/or rate-determining step of the intramolecular aminolysis reactions is different than in bimolecular reactions or the intramolecular reactions of other esters is attributed to the excellent steric fit of the nucleophile to the reaction center of the 2-aminomethylbenzoate esters. 相似文献
13.
Makoto Shimizu Haruna Katsurayama Isao Mizota Yusong Zhu 《Journal of heterocyclic chemistry》2019,56(9):2479-2486
Synthesis of 2‐methyl‐3‐oxoindoline‐2‐carboxylates is developed using a bis(trimethylsilyl)aluminum chloride‐induced aza‐Brook rearrangement as a crucial step. Regarding the organoaluminum species, use of readily available tris(trimethylsilyl)aluminum·ether complex was not suitable for the present cyclization. After a series of examination of the reaction conditions, the aza‐Brook rearrangement and the subsequent cyclization with 2 equivalens of bis(trimethylsilyl)aluminum chloride were found to be the most effective. An application to the synthesis of a potential intermediate of duocarmycin A is also described, in which ozonolysis of the styrenyl moiety and the Barton–McCombie deoxygenation of the hydroxymethyl group were successfully carried out. 相似文献
14.
Sim TB Kang SH Lee KS Lee WK Yun H Dong Y Ha HJ 《The Journal of organic chemistry》2003,68(1):104-108
5-Funtionalized enantiomerically pure oxazolidin-2-ones were prepared in one pot from commercially available chiral aziridines bearing an electron-withdrawing group at C-2 with retention of the configuration in high yields by regioselective aziridine ring-opening followed by intramolecular cyclization. 相似文献
15.
Chung JY Cvetovich R Amato J McWilliams JC Reamer R DiMichele L 《The Journal of organic chemistry》2005,70(9):3592-3601
[reaction: see text] A practical asymmetric synthesis of N-tert-butyl disubstituted pyrrolidines via a nitrile anion cyclization strategy is described. The five-step chromatography-free synthesis of (3S,4R)-1-tert-butyl-4-(2,4-difluorophenyl)pyrrolidine-3-carboxylic acid (2) from 2-chloro-1-(2,4-difluorophenyl)-ethanone achieved a 71% overall yield. The cyclization substrate was prepared via a catalytic CBS asymmetric reduction, t-butylamine displacement of the chlorohydrin, and a conjugate addition of the hindered secondary amine to acrylonitrile. The key nitrile anion 5-exo-tet cyclization concomitantly formed the pyrrolidine ring with clean inversion of the C-4 center to afford 1,3,4-trisubstituted chiral pyrrolidine in >95% yield and 94-99% ee. Diethyl chlorophosphate and lithium hexamethyldisilazide were shown to be the respective optimum activating group and base in this cyclization. The trans-cis mixture of the pyrrolidine nitrile undergoes a kinetically controlled epimerization/ saponification to afford the pure trans-pyrrolidine carboxylic acid target compound in >99.9% chemical and optical purity. This chemistry was also shown to be applicable to both electronically neutral and rich substituted phenyl substrates. 相似文献
16.
Enantioselective total synthesis of (-)-triptolide, (-)-triptonide, (+)-triptophenolide, and (+)-triptoquinonide 总被引:3,自引:0,他引:3
The first enantioselective total synthesis of (-)-triptolide (1), (-)-triptonide (2), (+)-triptophenolide (3), and (+)-triptoquinonide (4) was completed. The key step involves lanthanide triflate-catalyzed oxidative radical cyclization of (+)-8-phenylmenthyl ester 30 mediated by Mn(OAc)3, providing intermediate 31 with good chemical yield (77%) and excellent diastereoselectivity (dr 38:1). (+)-Triptophenolide methyl ether (5) was then prepared in > 99% enantiomeric excess (> 99% ee), and readily converted to natural products 1-4. In addition, transition state models were proposed to explain the opposite chiral induction observed in the oxidative radical cyclization reactions of chiral beta-keto esters 17 (without an alpha-substituent) and 17a (with an alpha-chloro substituent). 相似文献
17.
Ana Lúcia Cardoso Ana Matos Beja Jesús M. de los Santos Francisco Palacios 《Tetrahedron》2009,65(45):9116-7725
The synthesis of chiral functionalized β-amino esters via the hydride reductive amination of chiral allenes was explored. These compounds can be regarded as β-peptoids building blocks bearing a chiral side chain at the nitrogen and at the same time retaining the β-amino acid side chain. β-Enamino esters were obtained from the nucleophilic addition of α-amino esters (l-Ala, d-Ala, l-Phe, l-Leu, l-Trp and d-Trp methyl esters) to 2,3-allenoates bearing a chiral auxiliary, which determines the stereochemistry outcome of the subsequent reduction reaction. It was also demonstrated that in the reduction of β-enamino esters derived from l-Pro and d-Pro methyl esters the chirality of the new chiral center is controlled by the α-amino ester moiety. 相似文献
18.
Péter Ábrányi-Balogh András Dancsó Dávid Frigyes Balázs Volk György Keglevich Mátyás Milen 《Tetrahedron》2014
In the present work, a practical synthesis of 1-aryl-β-carboline-3-carbaldehydes as versatile building blocks and their application in Biginelli reaction is reported. The starting material of the four-step synthesis is racemic tryptophan methyl ester. The procedure involves a Pictet–Spengler cyclization, a dehydrogenation, an ester reduction, and an alcohol oxidation step. The β-carboline-3-carbaldehydes were further transformed using a Biginelli reaction into derivatives containing a pharmacologically significant dihydropyrimidine ring at position-3. 相似文献
19.