Simulation of the random scission of C–C bonds in the initial stage of the thermal degradation of polyethylene

We performed molecular dynamics simulations to analyze the initial stage of the thermal degradation of polyethylene, which is dominated by the random scission reaction. The simulations were initiated from structures that were taken from previously equilibrated snapshots of the amorphous polymer and of a free-standing thin film. Isolated chains were also used for comparison. Our systems were coupled to a thermal heat bath, and the effect of different coupling constants was studied. Rate of random scission increases as the strength of the temperature coupling increases. Rates of reaction are almost similar in thin films and the bulk, whereas the rates are much faster in isolated chains. Expansion of the free-standing thin film accompanies degradation, producing fragments of various sizes. Chains of higher molecular weights than the initial chains can be produced due to recombination of fragments during the expansion of thin films. The polydispersity index of the resulting fragments is higher in thin films compared to the bulk. The bonds at the low density portion of the thin films have a higher probability of being broken.     

Synthesis of the C1–C13 fragment of eribulin mesylate

Synthesis of the C1–C13 fragment of eribulin mesylate has been accomplished. It features a highly stereoselective construction of a trans-dihydropyran framework using three key reactions: (1) Sharpless epoxidation, (2) regioselective ring opening, and (3) olefin metathesis.     

Synthesis of the C8–C16 fragment of amphidinolide R

An asymmetric synthesis of the C8–C16 fragment with several stereogenic centers of amphidinolide R, is described. The key reaction, Sharpless asymmetric epoxidation has provided the basis for generation of the required stereocenters. The target fragment was accomplished in a convergent manner in nine steps (longest linear synthesis of the sequence) and 32% overall yield was obtained starting from 2-butene-1,4-diol.     

Synthesis of the C29–C37 segment of spongistatin 1

Starting from racemic 2α-methyl-8-oxabicyclo[3.2.1]oct-6-en-3-one the spongistatin E segment has been prepared in nine steps (2.3 steps per stereogenic center) with umpolung of anomeric reactivity at C37. This 3,5-syn-diol sequence completes our methodology to all stereoisomers of 3,5,7-trihydroxy heptanoic ester building blocks functionalized for α-oxyanion chemistry.     

An improved synthesis of the C42–C52 segment of ciguatoxin 3C

In our previously reported method for the construction of the IJKLM-ring of ciguatoxin 3C (CTX3C), the lengthy synthetic process for the intermediate C42–C52 (L-ring) segment was problematic. Therefore, a new and improved procedure for the C42–C52 segment, having modified protecting groups, was developed. The new route includes a chirality transferring Ireland-Claisen rearrangement for the construction of the vicinal dimethyl branching at C47–48, a one-pot cyclization process for the establishment of the stereocenters at C45 and C46 as well as the γ-hydroxy δ-lactone framework corresponding to the L-ring, and Brown’s asymmetric crotylboration for the installation of the stereocenters at C43 and C44. The new C42–C52 segment was successfully coupled with the previously reported C32–C41 (I-ring) segment to produce the IJKLM-ring.     

Recent strategic advances for the activation of benzylic C–H bonds for the formation of C–C bonds

Alkylarenes, obtained from abundant hydrocarbon feedstock sources, are an attractive starting material for the formation of complex molecular architectures. Conventional activation strategies of the relatively inert sp³-hybridized benzylic C–H bonds usually require relatively harsh conditions and are difficult to apply to the synthesis of fine chemicals. The present review describes recent strategic advances for the activation of benzylic C–H bonds for the catalytic formation of C–C bonds. In particular, two activation methods, i.e., strategies that generate benzylic radicals or benzyl anions, are discussed.     

Synthesis of the C38–C54 spiroketal segment of halichondrin B

Stereoselective synthesis of the C38–C54 segment of a marine metabolite halichondrin B comprising KLM and N rings with a C44 spiroketal including 10 chiral centers is described. The salient features of the synthesis are Carreira’s addition, Sharpless asymmetric dihydroxylation and double alkylation of TosMIC strategy and double vinyl coupling reactions.     

Toward the Total Synthesis of Bryostatin 11: Stereoselective Construction of the C13-Exocyclic Enoate in the C1–C16 Fragment

We have utilized a spiroketal template in an approach to the C1–C16 fragment of bryostatin. The stereoselective construction of an exocyclic enoate at C13 and insertion of a vinyl group on C15 were accomplished by using Peterson–Yamamoto olefination and copper-catalyzed addition of vinyl magnesium bromide, respectively.      

Asymmetric synthesis of the C1–C6 portion of the psymberin using an Evans chiral auxiliary

The C1–C6 region of the potent cytotoxic agent psymberin has been synthesized. The key transformations of the synthesis are an auxiliary-controlled addition of a Sn(II)-glycolate enolate to an aldehyde to yield the anti-aldol product and transforming the primary alcohol into a terminal olefin utilizing organoselenium chemistry.     

Electrochemical and photochemical approaches for the synthesis of the C28–C38 fragment of okadaic acid

Okadaic acid, a potent and selective inhibitor of Protein Phosphatases 1 and 2A (PP1 and PP2A), is widely used as a probe for various biochemical processes. We describe herein two innovative methods for the synthesis of the terminal C28–C38 fragment of the natural polyether. Suárez photochemical oxidative cyclization and electrochemical oxidation of malonates to their ketals equivalents have been successfully applied for the assembly of the key spiroketal core.     

Studies on the syntheses of the aplysiatoxins: Synthesis of a selectively-protected form of the C27 – C30 (dihydroxybutanoate) moiety of oscillatoxin A

A protected form of (R)-3,4-dihydroxybutanoic acid bearing a benzyl protecting group at the C₄ hydroxyl and a dimethylthexylsilyl protecting group at the C₃ hydroxyl was synthesized via a selective Ag(l)-mediated monobenzylation of (R)-methyl 3,4-dihydroxybutanoate. An alternative synthetic route from a chiral allylic ether was successful but problematic. The acid could be cleanly coupled to a model for the C₃ – C₁₁ moiety of the aplysiatoxins.     

Toward the stereoselective synthesis of C1–C23 fragment of spirastrellolide B

A convergent synthesis of the protected C(1)–C(23) fragment 4 of the targeted natural product spirastrellolide B is described. The key step of the synthesis is cross metathesis (CM) and TBAF promoted oxa-Michael to construct tetrahydropyran moiety.     

Application of Brønsted-type LFER in the study of the phospholipase C mechanism

Phosphatidylinositol-specific phospholipase C cleaves the phosphodiester bond of phosphatidylinositol to form inositol 1,2-cyclic phosphate and diacylglycerol. This enzyme also accepts a variety of alkyl and aryl inositol phosphates as substrates, making it a suitable model enzyme for studying mechanism of phosphoryl transfer by probing the linear free-energy relationship (LFER). In this work, we conducted a study of Br?nsted-type relationship (log k = beta(lg) pK(a) + C) to compare mechanisms of enzymatic and nonenzymatic reactions, confirm the earlier proposed mechanism, and assess further the role of hydrophobicity in the leaving group as a general acid-enabling factor. The observation of the high negative Br?nsted coefficients for both nonenzymatic (beta(lg) = -0.65 to -0.73) and enzymatic cleavage of aryl and nonhydrophobic alkyl inositol phosphates (beta(lg) = -0.58) indicates that these reactions involve only weak general acid catalysis. In contrast, the enzymatic cleavage of hydrophobic alkyl inositol phosphates showed low negative Br?nsted coefficient (beta(lg) = -0.12), indicating a small amount of the negative charge on the leaving group and efficient general acid catalysis. Overall, our results firmly support the previously postulated mechanism where hydrophobic interactions between the enzyme and remote parts of the leaving group induce an unprecedented negative-charge stabilization on the leaving group in the transition state.     

On the Reactivity of Phosphaalumenes towards C−C Multiple Bonds

Heterocycles containing group 13 and 15 elements such as borazines are an integral part of organic, biomedical and materials chemistry. Surprisingly, heterocycles containing P and Al are rare. We have now utilized phosphaalumenes in reactions with alkynes, alkenes and conjugated double bond systems. With sterically demanding alkynes 1,2-phosphaalumetes were afforded, whereas the reaction with HCCH or HCCSiMe₃ gave 1,4-phosphaaluminabarrelenes. Using styrene saturated 1,2-phosphaalumates were formed, which reacted further with additional styrene to give different regio-isomers of 1,4-aluminaphosphorinanes. Using ethylene, a 1,4-aluminaphosphorinane is obtained, while with 1,3-butadiene a bicyclic system containing an aluminacyclopentane and a phosphirane unit was synthesized. The experimental work is supported by theoretical studies to shed light on the mechanism governing the formation of these heterocycles.     

Gram scale synthesis of the C(1)–C(9) fragment of amphidinolide C

An allylic cis-epoxide prepared by Sharpless asymmetric epoxidation was transformed in nine steps and 41% overall yield to the cyclization precursor 4 via a key one carbon homologation. Cobalt-catalyzed aerobic oxidative cyclization of 4 gave the trans-THF in 94% yield at gram scale. Subsequent manipulations, including a Still–Gennari olefination, Sharpless asymmetric dihydroxylation, Corey–Fuchs alkynylation, and Kazmaier hydrostannylation provided the fully functionalized C(1)–C(9) fragment 2 suitable for cross-coupling. The sequence is readily scalable and provides gram quantities of 2.     

Kinetics and Mechanism of the Insertion Reaction of Arylisocyanates into C‒C Bond of Phosphabetaine Obtained ont the Basis of Triisopropylphosphine and 2-Cyanoethylacrylate

Abstract

On the basis of kinetic investigation o f previously discovered [I] insertion reaction of arylisocyanates into C-C bond of nvitter-ions, obtained on the basis of trialkylphoshines and Z-cyanoacrylates, the mechanism of this unusual reaction has been suggested. According to this mechanism the nucleophilic attack of nvitter-ion anionic center on the carbon atom of isocyanate group and nucleophilic attack of nitrogen atom on carbethoxylic group, leading to breakup of C-C bond, carry out relatively synchronously within one transition state on the concerted mechanism.     

Efficient Synthesis of the Cl- C7 Fragment of Didemnaketal A

The stereoselective synthesis of the C1-C7 fragment (3R,4S,6R)-3,4-di[(tert-butyl-dimethylsilyl)oxy]-7-hydroxy-6-methylheptan-2-one, which is the crucial intermediate for synthesis of the HIV-1 protease inhibitive didemnaketals, was developed via 12 steps from the natural (+)-pulegone.     

Effect of Temperature on the Yields of Final Products in the γ-Radiolysis of Formaldehyde Solutions in C1–C3 Alkanols

The effect of temperature on the yields of 1,2-alkanediols in the -radiolysis of the methanol(or ethanol)–formaldehyde (0.9 M) systems and on the yields of 1,2-alkanediols, carbonyl compounds, methanol, and ethanol in the -radiolysis of the 1-propanol(or 2-propanol)–formaldehyde (1 M) systems was studied over the range 373–473 K. It was found that the temperature dependence of the yields of 1,2-butanediol and 2-methyl-1,2-propanediol exhibited maximums at 423 and 373 K, respectively.     

On the applicability of the Kasha—Vavilov rule to C60

Via full correction for the instrumental response function, the fluorescence excitation spectrum of C₆₀ in methylcyclohexane between 240 and 580 nm is shown to match the absorption spectrum. This proves that, in contrast to earlier reports, also after UV excitation into higher S_n states, deactivation proceeds virtually completely via the S₁ state, in accordance with the Kasha—Vavilov rule.