Design, synthesis, and biological evaluation of novel C14-C3'BzN-linked macrocyclic taxoids

Novel macrocyclic paclitaxel congeners were designed to mimic the bioactive conformation of paclitaxel. Computational analysis of the "REDOR-Taxol" structure revealed that this structure could be rigidified by connecting the C14 position of the baccatin moiety and the ortho position of C3'N-benzoyl group (C3'BzN), which are ca. 7.5 A apart, with a short linker (4-6 atoms). 7-TES-14beta-allyloxybaccatin III and (3R,4S)-1-(2-alkenylbenzoyl)-beta-lactams were selected as key components, and the Ojima-Holton coupling afforded the corresponding paclitaxel-dienes. The Ru-catalyzed ring-closing metathesis (RCM) of paclitaxel-dienes gave the designed 15- and 16-membered macrocyclic taxoids. However, the RCM reaction to form the designed 14-membered macrocyclic taxoid did not proceed as planned. Instead, the attempted RCM reaction led to the occurrence of an unprecedented novel Ru-catalyzed diene-coupling process, giving the corresponding 15-membered macrocyclic taxoid (SB-T-2054). The biological activities of the novel macrocyclic taxoids were evaluated by tumor cell growth inhibition (i.e., cytotoxicity) and tubulin-polymerization assays. Those assays revealed high sensitivity of cytotoxicity to subtle conformational changes. Among the novel macrocyclic taxoids evaluated, SB-T-2054 is the most active compound, which possesses virtually the same potency as that of paclitaxel. The result may also indicate that SB-T-2054 structure is an excellent mimic of the bioactive conformation of paclitaxel. Computational analysis for the observed structure-activity relationships is also performed and discussed.     

镍催化氮杂环丁酮和丁二烯环加成反应机制的理论研究

采用密度泛函理论(DFT)方法对镍催化1-Boc-3-氮杂环丁酮和2,3-二甲基-1,3-丁二烯的环加成反应进行了理论研究. 计算结果表明, 该反应采用氧化加成机制而非实验推测的β-碳消除机制. 氧化加成机制主要由3个基元反应步骤组成, 分别为氮杂环丁酮底物中C—C(=O)键的氧化加成、 二烯顺式插入Ni—C(=O)键、 以及还原消除生成八元氮杂环产物, 其中烯烃插入是整个反应的决速步骤, 反应能垒为86.74 kJ/mol. 通过探讨烯烃分别插入到Ni—C(=O) 键和Ni—C(sp³) 键的2种反应途径分析了烯烃插入步骤的区域选择性, 得到了与实验数据基本一致的结果.     

Formation of C(sp3)–C(sp3) Bonds through Nickel‐Catalyzed Decarboxylative Olefin Hydroalkylation Reactions

Olefins and carboxylic acids are among the most important feedstock compounds. They are commonly found in natural products and drug molecules. We report a new reaction of nickel‐catalyzed decarboxylative olefin hydroalkylation, which provides a novel practical strategy for the construction of C(sp³)?C(sp³) bonds. This reaction can tolerate a variety of synthetically relevant functional groups and shows good chemo‐ and regioselectivity. It enables cross‐coupling of complex organic molecules containing olefin groups and carboxylic acid groups in a convergent fashion.     

聚硅氧烷负载富勒烯铂配合物的合成及其催化性能

对 C60 及其衍生物的金属化合物催化性能的研究表明 ,以富勒烯为基体的催化剂对许多化学反应具有独特的催化活性 ,这方面的研究是富勒烯科学的一个重要发展方向 ,并显示出良好的应用前景 .但在已有的工作中 ,研究的主要是 C60 的小分子金属配合物、有机金属复合物以及纳米级金属吸附在C60 的表面所形成的金属 - C60 混合物等的催化性能 ,而有关高分子负载富勒烯金属配合物在催化方面的应用较少报道 [1～ 5] .富勒烯是一个具有独特球形 π电子体系的弱电子给体 ,且体积庞大 ,它的引入势必会改变高分子负载催化剂金属活性中心的局部电荷密度…     

Chemistry and chemical biology of taxane anticancer agents

Taxol (paclitaxel) and Taxotere (docetaxel) are currently considered to be among the most important anticancer drugs in cancer chemotherapy. The anticancer activity of these drugs is ascribed to their unique mechanism of action, i.e., causing mitotic arrest in cancer cells, leading to apoptosis through inhibition of the depolymerization of microtubules. Although both paclitaxel and docetaxel possess potent antitumor activity, treatment with these drugs often results in a number of undesirable side effects, as well as multidrug resistance (MDR). Therefore, it has become essential to develop new anticancer agents with superior pharmacological properties, improved activity against various classes of tumors, and fewer side effects. This paper describes an account of our research on the chemistry of paclitaxel and taxoid anticancer agents at the biomedical interface, including: 1. The structure-activity relationship (SAR) study of taxoids leading to the development of the "second-generation" taxoids, which possess exceptional activity against drug-resistant cancer cells expressing the MDR phenotype. 2. Development of fluorinated taxoids to study the bioactive conformation of paclitaxel and photoaffinity labeling taxoids for mapping of the drug-binding domain on both microtubules and P-glycoprotein. 3. The synthesis of novel macrocyclic taxoids for the investigation into the common pharmacophore for microtubule stabilizing anticancer agents.     

Expeditious synthesis of 3,4-dihydroisocoumarins and phthalides using the Heck-Matsuda reaction

Several 3,4-dihydroisocoumarins and phthalides were synthesized by an effective Heck-Matsuda reaction involving an ortho carboxybenzenediazonium salt with a series of styrenes bearing electron donating and electron withdrawing groups, methylvinyl ketone, and methyl acrylate. The reaction was carried out in an open-flask with 1% mol of palladium acetate in aqueous ethanol at ∼80 °C, giving the correspondent 3-aryl-3,4-dihydroisocoumarins and phthalides with good overall yields. The electronic nature of the group attached to the olefin is a key feature for the regioselectivity of the cyclization step.     

Efficient synthesis of 40- and 48-membered tetraether macrocyclic bisphosphocholines

An efficient route toward the synthesis of unsaturated (bis-diacetylenic) and saturated 40- and 48-membered macrocyclic biphosphocholines has been developed using 2-phenyl-5-hydroxy-1,3-dioxane as a common glycerol synthon. Ring closure was accomplished using either high-dilution Glaser oxidation of [(Cy3P)2RU==CHPh]Cl2-catalyzed olefin metathesis conditions. Deprotection of benzyl ethers using trimethylsilyl iodide (TMS-I) in the presence of diacetylenic moieties has also been demonstrated for the first time.     

Phenol-containing macrocyclic diamides as new catalysts in the highly regioselective conversion of epoxides to beta-hydroxy thiocyanates.

The regioselective ring-opening reactions of some epoxides with ammonium thiocyanate in the presence of a series of new phenol-containing macrocyclic diamides and also dibenzo-18-crown-6-, 18-crown-6-, benzo-15-crown-5-, and pyridine-containing macrocyclic diamide have been studied. The epoxides were subject to cleavage by NH(4)SCN in the presence of these catalysts under mild reaction conditions in various aprotic solvents. In this study, reagents and conditions have been discovered with which the individual beta-hydroxy thiocyanates can be synthesized in high yield and with more than 90% regioselectivity. The results can be discussed in terms of a four-step mechanism: (1) formation of complex between catalyst and NH(4)SCN, (2) release of SCN(-) nucleophile from the complex, (3) reaction of the active nucleophile at the less sterically hindered site in the epoxide, and (4) regeneration of catalyst. The major advantages of this method are as follows: (1) high regioselectivity, (2) simple regeneration of catalyst, (3) its reuse through several cycles without a decrease in activity, and (4) ease of workup of the reaction.     

Theoretical investigation on regioselectivity of aromatic ketones in the addition with olefin catalyzed by RuH2(CO)(PPh3)3

Ab initio method is employed to study the structures of twelve aromatic ketones at HF/3-21G, HF/6-31G and HF/6-31G levels, respectively. A theoretical analysis is also carried out to study the regioselectivity and reactivity of aromatic ketones in the addition with olefin catalyzed by RuH2(CO)(PPh3)3. The results indicate that a U shape LUMO conjugation of aromatic ketones in a plane plays an important role in regioselectivity on the cleavage of p C-H bond and is a nec-essary factor to success of addition with olefin, and that sterle effect is an indispensable factor in forming additional ortho-product. Meanwhile, electronic effect may influence the rate of addition for the structures alike which only have different replacements in the same site of aromatic ring, such as furan, thiophene and pyrole. A possible catalytic reaction mechanism is proposed that the addition of C-H bond may be carried out by a coordination of aromatic ketones with Ru complex.     

A highly regioselective approach to multiple adducts of C60 governed by strain minimization of macrocyclic malonate addends

New macrocyclic malonates 2-5 have been prepared by reaction of malonyl dichloride with alkanediols. Reactions of these cyclo-[n]-alkylmalonates with C60 are highly regioselective. The macrocycles containing identical alkyl spacers selectively form bis- and trisadducts of C60 with rotational symmetry. The addition pattern of the regioselectively formed oligoadducts is determined by the size of the alkyl spacer within the macrocyclic malonate. A variety of bis-, tris-, tetra-, and hexaadducts have been synthesized to show the scope of this approach. "Exotic" addition patterns such as trans-4,trans-4,trans-4, which has been synthesized and completely characterized for the first time, are also accessible by this method. The regioselectivity is ruled by the even distribution of the strain within the macrocyclic malonates containing spacer alkane chains of identical lengths: addition patterns with rotational symmetry provide exactly identical distances of the malonate oxygen atoms and are thus exclusively formed by this method. In contrast, when macrocycles with two different alkyl spacer lengths are used, such as 9 and 10, the reaction exclusively yields C(s)-symmetric bisadducts.     

Alkyne-aldehyde reductive C-C coupling through ruthenium-catalyzed transfer hydrogenation: direct regio- and stereoselective carbonyl vinylation to form trisubstituted allylic alcohols in the absence of premetallated reagents

Nonsymmetric 1,2-disubstituted alkynes engage in reductive coupling to a variety of aldehydes under the conditions of ruthenium-catalyzed transfer hydrogenation by employing formic acid as the terminal reductant and delivering the products of carbonyl vinylation with good to excellent levels of regioselectivity and with complete control of olefin stereochemistry. As revealed in an assessment of the ruthenium counterion, iodide plays an essential role in directing the regioselectivity of C-C bond formation. Isotopic labeling studies corroborate reversible catalytic propargyl C-H oxidative addition in advance of the C-C coupling, and demonstrate that the C-C coupling products do not experience reversible dehydrogenation by way of enone intermediates. This transfer hydrogenation protocol enables carbonyl vinylation in the absence of stoichiometric metallic reagents.     

C−H Activation and Olefin Insertion in d8 and d0 Complexes: Same Elementary Steps,Different Electronics

Metallocene alkyl complexes with d⁰ electron configuration and d⁸-configured square planar α-diimine late transition metal alkyl complexes show activity in both C−H activation through σ-bond metathesis and olefin insertion. Herein, we show by analysis of their M−CH₃ ¹³C chemical shift tensors that these reactions involve a π(M−C) interaction in the horizontal plane of the complex for both d⁰ and d⁸ systems. While in the case of d⁰ systems the interaction of an empty metal d-orbital and a filled carbon p-orbital causes partial alkylidene character of the M−C bond, the corresponding metal d-orbital is filled in d⁸ systems, thus generating a filled π*(M−C) orbital that increases the anionic character of the methyl group. This entails fundamentally different reaction mechanisms for d⁰ and d⁸ systems, which are reflected in the structures of the transition states: While d⁰ olefin insertion can be viewed as a [2+2] cycloaddition reaction, d⁸ olefin insertion rather resembles methyl group migration onto a positively polarized olefin, thus explaining the observed differences in regioselectivity. These findings are translated to σ-bond metathesis, a reaction which is isolobal to olefin insertion for both early and late transition metals.     

Theoretical investigation on regioselectivity of aromatic ketones in the addition with olefin catalyzed by RuH2(CO)(PPh3)3

Ab initio method is employed to study the structures of twelve aromatic ketones at HF/3-21G, HF/6-31G and HF/6-31G* levels, respectively. A theoretical analysis is also carried out to study the regioselectivity and reactivity of aromatic ketones in the addition with olefin catalyzed by RuH₂(CO)(PPh₃)₃. The results indicate that a U shape LUMO conjugation of aromatic ketones in a plane plays an important role in regioselectivity on the cleavage of β C-H bond and is a necessary factor to success of addition with olefin, and that steric effect is an indispensable factor in forming additional ortho-product. Meanwhile, electronic effect may influence the rate of addition for the structures alike which only have different replacements in the same site of aromatic ring, such as furan, thiophene and pyrole. A possible catalytic reaction mechanism is proposed that the addition of C-H bond may be carried out by a coordination of aromatic ketones with Ru complex.     

Highly Selective Construction of Seven‐Membered Carbocycles by Olefin‐Assisted Palladium‐Catalyzed Oxidative Carbocyclization–Alkoxycarbonylation of Bisallenes

An olefin‐assisted palladium‐catalyzed oxidative carbocyclization–alkoxycarbonylation of bisallenes to afford seven‐membered carbocycles has been established. This dehydrogenative coupling reaction showed excellent substrate scope and functional group compatibility. The reaction exhibited high chemo‐ and regioselectivity, and ester 3 was the only product obtained. The olefin unit has been proven to be indispensable during the reaction. Moreover, intramolecular oxidative coupling suggests that the reaction proceeds via a (π‐allyl)palladium intermediate.     

A practical access into conveniently functionalized,homochiral C-ring system of taxuyunnanine C

The preparation of a homochiral 7-nor taxoid C-ring framework, to be used in an A+C approach for the synthesis of low oxygenation pattern taxoids with an exocyclic olefin at C(4)–(20) and oxygenation at C(14), is described. Retrosynthetic analysis associated with the preparation of 3 has been designed around the targets 11 and 15 which could be produced by functional group manipulation starting from the known hydrindenone 4, obtained with a moderate e.e. (ca. 70%). A lipase was used to perform clean and high yield saponification on its corresponding acetoxy enone, while chemical resolution of the resulting acyloins allowed straightforward access to diastereomerically pure material.     

Co-oligomerization of ethylene and higher linear alpha olefins. II. Olefin reactivities in various reaction steps of co-oligomerization with nickel ylide-based system

Part I described co-oligomerization reactions of ethylene and various linear α-olefins (propylene, 1-butene, 1-hexene, 1-heptene, 1-octene, and 1-decene) in the presence of the homogeneous catalyst consisting of sulfonated nickel ylide and diethylaluminum ethoxide. The present article analyzes olefin reactivities in various reaction steps of the co-oligomerization reactions as well as reactivities of various catalytic species. Chain propagation reactions (insertion into the Ni? C bonds) with participation of α-olefins exhibit poor regioselectivity, primary insertion being ca. 60% more probable than the secondary insertion. Ethylene is significantly more reactive in chain propagation reactions: 50–70 times compared to olefin primary insertion and 100–120 times compared to olefin secondary insertion. Reactivities of α-olefins in chain propagation reactions decrease slightly for higher olefin alkyl groups. Reactivities of Ni? C bonds in chain propagation and chain termination reactions strongly depend on the structure of the alkyl group attached to the nickel atom. The Ni? CHR? CH₂? R bond has very low reactivity in ethylene insertion reaction and usually decomposes in the α-hydrogen elimination process. Kinetic analysis of olefin co-oligomerization reactions provides numerous analogies with olefin copolymerization reactions in the presence of Ziegler–Natta catalysts.     

Transannular macrocyclization via intramolecular B-alkyl Suzuki reaction

[reaction: see text]Transannular macrocyclizations via intramolecular B-alkyl Suzuki reactions are described. Regioselective terminal olefin hydroboration with 9-BBN followed by Pd(0)-catalyzed Suzuki reaction in the presence of a base such as TlOEt at high dilution generates macrocycles with a high degree of control over olefin geometry with isomerically pure E or Z vinyl iodide substrates. These reactions are complementary to ring closing metathesis (RCM) macrocyclizations and may prove superior in cases where control of olefin geometry is required.     

The vinylogous intramolecular Morita-Baylis-Hillman reaction: synthesis of functionalized cyclopentenes and cyclohexenes with trialkylphosphines as nucleophilic catalysts

The development of the vinylogous intramolecular Morita-Baylis-Hillman reaction for the synthesis of functionalized cyclopentenes and cyclohexenes is described. The reaction involves the trialkyphosphine-catalyzed cyclization of 1,6- or 1,7-diactivated 1,5-hexadienes or 1,6-heptadienes, containing carboxyaldehyde, methyl ketone, or methoxycarbonyl as the olefin activating groups. A representative example of this reaction is the Me(3)P-catalyzed cyclization of 1a in tert-amyl alcohol, which provides the substituted cyclopentene 2a in 95% yield and with 97:3 regioselectivity.     

On the regioselectivity of the intramolecular [2 + 2]-photocycloaddition of alk-3-enyl tetronates

The simple diastereoselectivity and the regioselectivity of the [2 + 2]-photocycloaddition of alk-3-enyl tetronates were studied, depending on the nature of the substituent R in the α-position. Fourteen tetronates were synthesized and their intramolecular photocycloaddition reactions performed. If R was an alkoxycarbonyl group and if the olefin, which underwent the intramolecular addition, was sterically congested, crossed photoproducts prevailed and were formed in yields between 35 and 65%. In all other cases, the straight photoproducts were obtained as the main reaction products in yields ranging from 39 to 84%. The structures of the photochemical products were thoroughly investigated by one- and two-dimensional NMR experiments. In all cases, the diastereoselectivitiy was excellent and only a single diastereoisomer was formed. A mechanistic model is proposed, which explains the regioselectivity of the [2 + 2]-photocycloaddition reaction.     

Part 1. modular approach to obtaining diverse tetrahydroquinoline-derived polycyclic skeletons for use in high-throughput generation of natural-product-like chemical probes

A practical synthesis of a tetrahydroaminoquinoline scaffold (12) was developed that used a stereocontrolled aza Michael as the key reaction. Three tetrahydroquinoline alkaloid-like, tricyclic derivatives 16, 18, and 19 with different medium to macrocyclic ring skeletons were obtained, using this scaffold as the starting material, in a modular manner. The macrocyclic compounds with an isolated olefin and an electron-deficient olefin were obtained by ring-closing metathesis approaches. Compounds 16 and 18 are unique and contain bridged 10- and 12-membered functionalized rings. The NMR studies of these compounds revealed interesting information on the conformation of the bicyclic scaffolds that was dependent on the nature and the size of the macrocyclic rings. Finally, this modular methodology, using compound 21 anchored onto the solid support, successfully led to the generation of different macrocyclic derivatives, 23, 25, and 27 in solid-phase synthesis. The solid-phase synthesis approach outlined in this article has the potential to generate tetrahydroquinoline-based tricyclic compounds containing different medium to macrocyclic architectures.