Improved studies of cross‐coupling reactions of 5‐(tri‐n‐butystannyl)‐ and 5,5′‐bis(tri‐n‐butylstannyl)‐2,2′‐bithiophene with aryl halides

Stille coupling under standard conditions proceeds in low yield when using hindered organostannanes (1) or (2) and aryl bromide partners. The inclusion of aryl iodide instead of aryl bromide with the same organostannanes, significantly improves the efficiency of the coupling, providing a variety of desired products in good to excellent yield. The yields of Stille coupling are compared to the different reactivity of aryl halides. This study of Stille coupling with different aryl halides are documented and rationalized.     

Microstructure effect in the coupling reactions of polymeric organolithium compounds using chlorosilanes

The most important variable affecting the yield in the coupling reactions of polymeric organolithium compounds with chlorosilane compounds has been investigated through size-exclusion chromatographic (SEC) analysis. The coupling reaction of poly(styryl)lithium with dichlorodimethylsilane as a silane-coupling agent provided 44 wt % of the coupling yield. The coupling yield, depending on the chain end reactivity of active polymers, was not greatly affected. The addition of a Lewis base such as N,N,N′,N′-tetramethylethylenediamine (TMEDA) even after complete polymerization of the dienes in hydrocarbon seems to affect the coupling reaction, resulting in decreasing the yield. The 1,2- or 3,4-enchain contents in the polydiene backbones affected the reduction of the linking efficiency in the coupling of the poly(dienyl)lithiums with chlorosilanes as the linking agent. The linking yields of the active polymers including over 75 mol % of 1,2- or 3,4-enchainment on the polydiene segment were below 20 wt %. The linking yields exhibited a dependence not only on the steric requirement of the chain end, but also the microstructure of the polydiene segment. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1743–1753, 1998     

A total synthesis of (-)-hemiasterlin using N-Bts methodology.

A total synthesis of (-)-hemiasterlin has been accomplished in nine steps from 25(8) (>35% yield overall). An improved enantiocontrolled route to the tetramethyltryptophan subunit 32 was developed using an asymmetric Strecker synthesis (five steps, 50% yield from 25), and the dipeptide 22 was prepared in seven steps, 37% yield from valinol. The synthesis exploits the high reactivity of a Bts-protected amino acid chloride in the difficult peptide coupling of sterically hindered amino acid residues 18 and 20 to form 21 (70%, recrystallized) and also uses N-Bts intermediates for the high-yielding N-methylations of 14 and 31. In addition, the Bts-protected di-tert-butyl N-acylimidodicarbonate 33 is shown to undergo efficient coupling with 22 to form 34 (97% in the coupling step; 79% over the activation; coupling sequence from 32).     

无膦配体氧化钯催化合成树形低聚苯

树形低聚苯;交叉偶联;无膦配体氧化钯催化合成树形低聚苯     

Platinum-catalyzed reductive coupling of activated alkenes under hydrogenation conditions

The Pt complex generated from PtCl₂, PR₃, and SnCl₂ catalyzes the reductive coupling of activated alkenes under environmentally benign hydrogenation conditions. Various bis-enones participated in the intramolecular cyclization, forming the desired cyclization products in moderate to good yield. Intermolecular reductive coupling of the enone and the aldehyde provided the coupling product in good yield. This methodology illustrates the first use of platinum complexes in hydrogen-mediated couplings of activated alkenes.     

A comparison of rigid and flexible crosslinked polymer-supported peptide synthesis

To correlate the coupling difficulty and nature of the support in solid-phase peptide synthesis, a set of difficult peptides were built under identical conditions on rigid and hydrophobic divinyl benzene-crosslinked polystyrene (DVB-PS) and flexible and polar 1,4-butanediol dimethacrylate-crosslinked polystyrene (BDDMA-PS) resins of same functional capacity (∼2 mmol/g). The difficult coupling stages observed on each support were identified by monitoring the acylation time, coupling yield and deprotection efficiency. The yield and purity of the peptides prepared on BDDMA-PS were higher than when DVB-PS support was used.     

Synthesis of selenium-derivatized cytidine and oligonucleotides for X-ray crystallography using MAD

Synthesis of the novel 2'-Se-cytidine phosphoramidite was achieved via transformation of the uridine analogue to the cytidine derivative in high yield. This 2'-Se-cytidine phosphoramidite was used to synthesize selenium-derivatized DNA and RNA oligonucleotides for X-ray crystallography using MAD. The nucleotide coupling yield using this novel phosphoramidite was over 99% when 5-benzylmercaptotetrazole (5-BMT) was used as the coupling reagent.     

Regioselective alkenylation of aromatic ketones with alkenylboronates using a RuH2(CO)(PPh3)3 catalyst via carbon-hydrogen bond cleavage

The ruthenium-catalyzed alkenylation of C-H bonds with alkenylboronates has been explored for a series of aromatic ketones. The coupling reaction of pivalophenone (1) with 2-isopropenyl-5,5-dimethyl[1,3,2]dioxaborinane (2) gave the corresponding isopropenylation product in 73% yield. In the case of the reaction of a sterically congested alkenylboronate, such as 2-methylpropenylboronate (8), the yield was decreased slightly. When beta-styrylboronates were used, the corresponding coupling products were obtained in good yields. The reaction of acetophenone with alpha-styrylboronate afforded the corresponding 1:1 coupling product, exclusively.     

Concise synthesis of quinazoline alkaloids, luotonins A and B, and rutaecarpine

The total synthesis of luotonin A was achieved in excellent yield by using a Pd-assisted biaryl coupling reaction of N-(bromoquinolinyl)methylquinazolinone with Cy₃P and KOAc. The successive treatment of luotonin A with NBS and aq. AgNO₃ gave luotonin B in good yield. Although the Pd-assisted coupling reaction of N-(2-bromoindolyl)ethylquinazolinone with Cy₃P and KOAc yielded rutaecarpine in poor yield, N-acetate under the same reaction conditions yielded the desired rutaecarpine directly in excellent yield.     

Divergent synthesis of fused Benzo-xanthene and oxazine derivatives via Cu-catalyst

Cu-Cu₂O combination showed synergic effects in catalyzing intramolecular Ullmann coupling reaction for halo-Betti bases to afford fused benzo-xanthenes from both electron-rich and electron-deficient aromatic systems in good yield under mild reaction conditions. The sterically hindered halo-Betti bases also provided products in excellent yield. Under optimized condition, a gram scale reaction was also performed to afford the product in excellent yield. However, without ortho-halo substituted Betti-bases failed to afford fused benzo-xanthene derivatives. Exploring the reaction optimization without Cu metal, serendipitously it was produced 1,3-oxazine derivatives in excellent yield via intramolecular cross-dehydrogenative coupling (CDC) reaction. Similarly, electron-rich, electron-deficient and sterically hindered Betti-bases provided the products in good to excellent yield under mild condition.     

Coupling of methane under pulse corona plasma (Ⅰ)——In the absence of oxygen

At normal temperature and pressure, pulse corona plasma was used as a new method for the dehydrogenative coupling of methane in the absence of oxygen. The effects of voltage polarity and input energy on the dehydrogenative coupling of methane were investigated. The parameter "energy efficiency" was introduced to examine the coupling of the input energy and the dehydrogenative coupling of methane. The experimental results show that positive corona gives higher energy efficiency than negative corona. When the positive corona was chosen, C2. yield per pass was 31.6% and acetylene yield per pass was 30.1% with 44.6% methane conversion at an input energy density of 1788kJ/mol and a pulse repetition frequency of 66Hz. The function of input energy density towards methane conversion may be expressed as a formula of -ln(1-X) = k (P/F). In the range of input energy employed, C2 yield is proportional to input energy density, but energy efficiency drops off with increasing input energy density.     

Biaryl synthesis via palladium-catalyzed aryne multicomponent coupling

Aryl iodides have been introduced as electrophiles in the three-component Heck coupling of arynes. Following optimization studies to favor three- versus two-component coupling, the reaction proceeds in good yield to afford a variety of functionalized biaryls.     

The first total synthesis of kwakhurin, a characteristic component of a rejuvenating plant, "kwao keur": toward an efficient synthetic route to phytoestrogenic isoflavones

A convergent synthesis of kwakhurin (5), a characteristic estrogen-like isoflavone of Pueraria mirifica(Leguminosae), is described. Isoflavone skeleton 31 was constructed by Suzuki-Miyaura coupling of 3-bromochromone 26 (AC-ring) and arylboronic acid 30 (B-ring) in the presence of TBAB as an additive. Microwave-assisted coupling was also examined, but did not improve the yield. Baeyer-Villiger oxidation, followed by propargylation and reduction afforded 1,1-dimethylallyl ether 37. 6'-Prenylisoflavone 34 was obtained in high yield by Claisen rearrangement of 37 in N,N-diethylaniline. On the other hand, 1,3-rearrangement of prenyl ether 33 with clay gave 34 in poor yield. Successive methylation of 34 and deprotection yielded the target kwakhurin (5) in 12% overall yield from 2,4-dihydroxybenzaldehyde (23).     

An improved synthesis of resveratrol

In this article we report an improved total synthesis of resveratrol, increasing the overall yield from 22 to 71%. The synthesis reported in our previous publication was made up of two fundamental steps: a Wittig reaction and a Heck coupling. Here we studied these steps separately to increase the individual yields. The yield of the Wittig reaction was increased up to 98%; however, we could not find better reaction conditions for the Heck coupling than those reported in the previous article.     

In situ Raman spectroscopy as a probe for the effect of power on microwave-promoted Suzuki coupling reactions

We report the use of in situ Raman spectroscopy as a probe for the effect of power on microwave-promoted Suzuki coupling reactions. We find that increased initial microwave power leads to greater acceleration of the reaction but that the product yield obtained is essentially independent of initial microwave power. The application of simultaneous cooling lengthens the reaction time but does not alter the relative rates of the Suzuki coupling and deboronation processes. Performing the reaction at an initial microwave power of 5 W leads to an improvement in product yield.     

Efficient coupling of low boiling point alkynes and 5-iodonucleosides

The coupling of unprotected 5-iodonucleosides and low boiling point alkynes has been achieved in a high yield for the first time. The coupling is highly dependent on concentration and can be carried out at low temperature and pressure conditions.     

Highly efficient template-based preparation of shape-persistent macrocyclics

The synthesis of shape-persistent macrocycles via oxidative dimerization or trimerization of rigid bisacetylenes (via Glaser coupling) is described. To enhance the yield of the desired target structures, the precursors are temporarily covalently bound to a template resulting in an intramolecular coupling instead of an intermolecular coupling. As a result, product yields up to over 90 % are observed.     

New approach to phosphinoalkynes based on Pd- and Ni-catalyzed cross-coupling of terminal alkynes with chlorophosphanes

[reaction: see text] The first example of direct phosphination of terminal alkynes with chlorophosphanes catalyzed by Ni or Pd complexes is described. Both aromatic and aliphatic terminal acetylenes undergo the coupling reaction to give corresponding coupling product in high yield.     

Regioselective double Suzuki couplings of 4,5-dibromothiophene-2-carboxaldehyde

Conditions have been identified that enable the one-pot double Suzuki coupling of 4,5-dibromothiophene-2-carboxaldehyde to proceed in good yield. The key to success in these reactions is the use of minimal amounts of water to avoid significant amounts of dehalogenation during the first coupling.     

Total synthesis of leustroducsin B

Leustroducsin B was synthesized via a convergent route based on division of the leustroducsin molecule into three segments A, B, and C. Two coupling reactions (Julia coupling reaction and Nozaki-Hiyama-Kishi (NHK) reaction) were employed for coupling of segments A and B: segment A1 for the Julia coupling reaction was prepared by a combination of Sharpless asymmetric epoxidation and an epoxide-cleavage reaction with an organoaluminum reagent, while segment A2 for the NHK reaction was synthesized from optically active alcohol that had previously been prepared by lipase-catalyzed kinetic resolution. Segment B, whose structure was modified with some functional groups, was synthesized from (R)-malic acid by a combination of Wittig reaction and Sharpless asymmetric dihydroxylation, and segment C, containing a cyclohexane moiety, was prepared by asymmetric Diels-Alder reaction. Segment B was first coupled with segment A1 via the Julia coupling reaction, but the yield was low due to unexpected epimerization. The NHK reaction of segment A2 proceeded to give the coupling product in good yield. This product was coupled with segment C via Wittig and Stille coupling reactions, and finally, phosphorylation was carried out by partial hydrolysis of a cyclic phosphate to give leustroducsin B.