Sources of error in the analysis of glucosinolates by gas liquid chromatography

A study was made of the accuracy and precision of glucosinolate determination by gas liquid chromatography (GLC) after transformation of the glucosinolates to desulfoglucosinolates and silylation. The condition of the GLC column and the cleanness of the injection port were found to be crucial for quantitative determination of the glucosinolate silyl ethers. The relative stabilities of glucosinolate silyl ethers in pyridine solution remained constant over a period of weeks in air at ambient temperature.     

Cyclic silyl pinacole ethers

Summary Cyclic silyl pinacole ethers are a new class of difunctional free radical initiators which have been prepared and characterized. On the basis of their first order kinetic rates of dissociation and half-lives, these compounds are high temperature initiators useful generally above 120°C. The unusual difunctional character of these initiators suggests some rather interesting applications for their use.The proper IUPAC name for these compounds is 1,3-dioxa-2-siloles; however, for brevity in this paper they will be referred to as cyclic silyl pinacole ethers     

利用Sharpless不对称双羟基化反应制备光学活性的偶姻类化合物

以相应的酮为起始原料,通过烯醇硅醚的中间体,利用Sharpless不对称双羟基化反应制备了5个光学活性的偶姻化合物。4-庚酮、5-壬酮、2,6-二甲基-4-庚酮、5-甲基-3-己酮和4-苯基-2-丁酮在不同碱的作用下与三甲基氯硅烷或三甲基碘硅烷反应得到动力学控制或热力学稳定的烯醇硅醚后,再进行Sharpless不对称双羟基化反应,除5-甲基-3-己酮的烯醇硅醚外,其他烯醇硅醚用AD-mix-α氧化得到S构型的偶姻产物,对映体过量值(ee值)在62%以上,用AD-mix-β氧化得到R构型的偶姻产物,ee值在75%以上。5-甲基-3-己酮的烯醇硅醚分别用AD-mix-α和AD-mix-β氧化都得到R构型的2-羟基-5-甲基-3-己酮,前者ee值只有16%,后者ee值为76%;用D-果糖和L-果糖衍生的史一安催化剂和过硫酸氢钾氧化,则分别得到R构型和S构型产物,ee值分别为75%和73%。     

Synthesis of well-defined graft copolymer with oligodimethylsiloxane and polyvinylalcohol branches

Summary Polyvinylalcohol macromer of well controlled molecular weight and molecular weight distribution, whose hydroxyl groups being protected as silyl ethers, was synthesized by Lewis acid catalyzed sequential silyl aldol condensation. Copolymerization of p-oligodimethylsiloxanylstyrene with the macromer, followed by the removal of the hydroxyl protective groups, gave the well-defined title graft copolymer.     

New enantioselective heterogeneous catalysts

A new enantioselective heterogeneous catalytic system has been made by partly decomposing (hydrogenolyzing) chiral silyl ethers in hydrogen over Pd black and differently dispersed Pd/SiO₂. Preliminary hydrogenations of two prochiral C=C's have demonstrated the feasibility of the method which produced catalysts attaining ee's up to 22%. The chiral silyl ethers appear to hydrolyze off the surface, but in the absence of water, the new catalysts catalyze reproducible ee's. This revised version was published online in July 2006 with corrections to the Cover Date.     

Aqueous Asymmetric Mukaiyama Aldol Reaction Catalyzed by Chiral Gallium Lewis Acid with Trost‐Type Semi‐Crown Ligands

The combination of Ga(OTf)₃ with chiral semi‐crown ligands ( 1a – e ) generates highly effective chiral gallium Lewis acid catalysts for aqueous asymmetric aldol reactions of aromatic silyl enol ethers with aldehydes. A ligand‐acceleration effect was observed. Water is essential for obtaining high diastereoselectivity and enantioselectivity. The p‐phenyl substituent in aromatic silyl enol ether ( 2 h ) plays an important role and increases the enantioselectivity up to 95% ee. Although aliphatic silyl enol ethers provided low enantioselectivities and silylketene acetal is easily hydrolyzed in aqueous alcohol, the aldol reactions of silylketene thioacetal ( 12 ) with aldehydes in the presence of gallium‐Lewis acid catalysts give the β‐hydroxy thioester with reasonable yields and high diastereo‐ (up to 99 : 1) and enantioselectivities (up to 96% ee).     

C?H Functionalization/C?O Bond Cleavage of Benzyl Silyl Ethers with Ynamides for the Chemoselective Synthesis of Skeletally Diverse Compounds

Described here is a practical carbocation‐initiated formal [3+2] or [4+2] cycloaddition reaction for the diversity‐oriented syntheses of 1‐amidoindenes and 2‐amido‐4H‐chromenes. The Lewis acid‐promoted annulation of ynamides and benzyl silyl ethers efficiently constructs 1‐amidoindenes via C H functionalization of benzyl silyl ethers. By utilization of o‐methoxybenzyl silyl ethers, 2‐amido‐4H‐chromenes are also constructed via C O bond cleavage of benzyl silyl ethers.

     

Synthesis of Multisubstituted Silyloxy-based Donor-Acceptor Cyclobutanes by an Acid-Catalyzed [2+2] Cycloaddition

This account summarizes our recent efforts on the acid-catalyzed [2+2] cycloaddition of silyl enol ethers with α,β-unsaturated esters, giving donor-acceptor (D -A) cyclobutanes and their related reactions. The cycloaddition generally produces multisubstituted cyclobutanes in good yields with high diastereoselectivity. We found that aluminum Lewis acids and triflic imide (Tf₂NH) had good catalytic activity. The retro [2+2] cycloaddition proceeded at higher reaction temperatures, and in some cases, diastereoselectivity switching of cycloadducts was observed. A microflow protocol was established for Tf₂NH-catalyzed [2+2] cycloaddition. Although the cycloaddition usually requires cryogenic conditions in a batch reactor, the microreactor system enabled the production of D -A cyclobutanes, even at ambient temperature. [2+2] Cycloaddition of allylsilanes and alkyl enol ethers, instead of silyl enol ethers, afforded the corresponding D -A cyclobutanes.     

二烯醇硅醚在芳环及芳杂环合成中的研究进展

二烯醇硅醚是一类非常重要的有机合成中间体,能通过环化反应合成各种重要的药理活性化食物,这些化合物不易通过其他方法制备.文章对二烯醇硅醚在有机合成中的研究进展进行了综述.     

First Tandem Asymmetric Conjugate Addition of Alkenyl Nucleophiles and Silyl Trapping of the Intermediate Enolates

The tandem asymmetric conjugate addition of alkyl or aryl groups to enones and subsequent silyl trapping has already been achieved and yields valuable silyl enol ethers. Herein, the first method for the respective addition of alkenyl groups is reported, which is based on a rhodium(I)‐catalyzed addition of readily available alkenylzirconocenes. As prerequisite for silyl trapping, the initially formed enolates have to be transmetalated from zirconium to lithium by treatment with methyllithium prior to addition of the silyl chloride. Starting from 5‐ to 7‐membered cycloalkenones, the respective silyl enol ethers were obtained in excellent yields and ≥93% ee; an acyclic substrate furnished a moderate enantioselectivity. Besides trimethylsilyl chloride, the silylation was also performed with tert‐butyldimethylsilyl chloride, and the synthetic scope was evaluated by employing five different alkenyl groups. Moreover, the mechanism of this sequence was elucidated by ¹H NMR studies, and the efficiency of catalyst control was exemplified by synthesis of a cis‐3,5‐disubstituted cyclohexanone which, due to strong substrate control, cannot be obtained by copper‐catalyzed conjugate addition.

     

Cover Picture: (Adv. Synth. Catal. 1/2008)

The cover picture, provided by Shū, Kobayashi, shows an example of chiral Lewis acid catalysis based on Lewis acid–surfactant combined catalysts (LASCs). Chiral hydrophobic environments made up of scandium(III) dodecylsulfate and a chiral bipyridine ligand catalyze the asymmetric aldol-type reactions of silyl enol ethers in water as the sole solvent.     

Improved Synthesis of Pyrroles and Indoles via Lewis Acid‐Catalyzed Mukaiyama–Michael‐Type Addition/Heterocyclization of Enolsilyl Derivatives on 1,2‐Diaza‐1,3‐Butadienes. Role of the Catalyst in the Reaction Mechanism

The Mukaiyama–Michael‐type addition of various silyl ketene acetals or silyl enol ethers on some 1,2‐diaza‐1,3‐butadienes proceeds at room temperature in the presence of catalytic amounts of Lewis acid affording by heterocyclization 1‐aminopyrrol‐2‐ones and 1‐aminopyrroles, respectively. 1‐Aminoindoles have been also obtained by the same addition of 2‐(trimethylsilyloxy)‐1,3‐cyclohexadiene on some 1,2‐diaza‐1,3‐butadienes and subsequent aromatization. Mechanistic investigations indicate the coordination by Lewis acid of the enolsilyl derivative and its 1,4‐addition on the azo‐ene system of 1,2‐diaza‐1,3‐butadienes. The migration of the silyl group from a hydrazonic to an amidic nitrogen, its acidic cleavage and the final internal heterocyclization give the final products. Based on NMR studies and ab initio calculations, a plausible explanation for the migration of the silyl protecting group is presented.     

Calcium catalyzed Mukaiyama–Mannich addition of silyl enol ethers to nitrones

A method for synthesizing β-(silyloxy)amino carbonyls through the addition of ketone-derived silyl enol ethers to N-phenyl nitrones has been developed. The transformation is catalyzed by a commercially available calcium(II) complex affording β-(silyloxy)amino carbonyls in good to excellent yield.     

Sulfines,a unique class of sulfur-centered heterocumulenes†

The chemistry of sulfines (thione S-oxides) is briefly reviewed with focus on highlights and milestones. Sulfines can conveniently be prepared by oxidation of a large variety of thiocarbonyl-containing compounds. Alkylidenation of sulfur dioxide with α -silyl carbanions is an attractive method of preparation as it allows access to sulfines that cannot be obtained by the oxidation method. The reaction of activated methylene compounds with thionyl chloride is a highly versatile method of preparation. The scope of the reaction with thionyl chloride is extended by using silyl enol ethers and silyl ketene acetals as starting materials.

The cycloaddition reaction of sulfines with 1,3-dienes is widely used for trapping purposes. The Diels–Alder type cycloadducts are useful compounds in synthesis. Dipolar cycloaddition reactions are of interest as well as the addition reaction of alkyllithium regents. O-Alkylation of aminosulfines gives iminosulfenates which are of interest for further synthetic elaboration.     

Gold‐Catalyzed 5‐ and 6‐exo‐dig Selective Cyclizations of Alkynyl Silyl Enol Ethers: Efficient Method for [3+2] and [4+2] Annulations onto α,β‐Enones

An efficient method for the annulation of five‐ and six‐membered rings onto α,β‐enones is described via gold‐catalyzed 5‐ and 6‐exo‐dig selective cyclizations of alkynyl silyl enol ethers.     

Asymmetric α‐Amination of Aldehydes Catalyzed by PS‐Diphenylprolinol Silyl Ethers: Remediation of Catalyst Deactivation for Continuous Flow Operation

Polystyrene (PS)‐supported diphenylprolinol silyl ethers have been developed as highly active catalysts for the enantioselective α‐amination of aldehydes. Understanding the mechanism of catalyst deactivation has led to the development of reaction conditions notably extending catalyst life in repeated recycling (10 cycles; accumulated TON of 480) and has allowed the implementation of a continuous flow α‐amination process (6 min residence time, 8 h operation).     

一种新的α-(2-甲基-3-呋喃硫基)酮类香料化合物的制备方法

研究了以4-庚酮、5-壬酮和2,6-二甲基-4-庚酮为原料,通过烯醇化、氧化和亲核取代等反应,制备3-(2-甲基-3-呋喃硫基)-4-庚酮、4-(2-甲基-3-呋喃硫基)-5-壬酮和2,6-二甲基-3-(2-甲基-3-呋喃硫基)-4-庚酮3个香料化合物的方法.首先酮在碱性条件下与三甲基氯硅烷反应得到相应的烯醇硅醚,4-庚酮和5-壬酮形成烯醇硅醚产率均在80%以上,2,6-二甲基-4-庚酮形成烯醇硅醚产率略低,约70%.烯醇硅醚用间氯过氧苯甲酸氧化,得到α-羟基酮,产率均在85%左右.α-羟基酮与甲磺酰氯反应,得到相应的甲磺酸酯,然后在碱性条件下甲磺酸酯与2-甲基-3-呋喃硫醇反应生成α-(2-甲基-3-呋喃硫基)酮类香料化合物,产率在80%左右.最终产物通过1HNMR,13CNMR及MS进行了表征.     

Elusive Dehydroalanine Derivatives with Enhanced Reactivity

For the first time, a simple methodology for the chemical synthesis and use of highly reactive 4-methylenoxazol-5(4H)-ones from serine is presented. These dehydroalanine derivatives, which resemble the natural 4-methylidenimidazole-5-one (MIO) cofactor present in lyases and aminomutases, undergo rapid reaction with carbon nucleophiles such as silyl enol ethers, as well as cycloaddition reactions with diazo compounds and reactive dienes, under very mild conditions and without any need for metal catalysts or ring-strain activation, offering potential for bioconjugation.     

Nickel-catalyzed intermolecular domino reactions

Nickel-catalyzed multiple-component reactions are a promising new type of domino reaction. For instance, a variety of starting materials such as alkenes, alkynes, and unsaturated carbonyl compounds can be connected with high selectivity under reasonably mild conditions. These reactions provide a new route to synthetically important key compounds including dienes, enynes, enol silyl ethers, and carbocycles. Furthermore, an asymmetric version of the reaction is successful. Thus, intermolecular domino reactions should be important reactions for years to come.     

Diversity‐Oriented Synthesis of Functionalized 1‐Aminopyrroles by Regioselective Zinc Chloride‐Catalyzed,One‐Pot ‘Conjugate Addition/Cyclization’ Reactions of 1,3‐Bis(silyl enol ethers) with 1,2‐Diaza‐1,3‐butadienes

In a convenient one‐pot process, the easily accessible 1,2‐diaza‐1,3‐butadienes and 1,3‐bis(silyl enol ethers) are converted into the previously unknown functionalized 1‐aminopyrroles and 1‐amino‐4,5,6,7‐tetrahydroindoles. The domino reaction proceeds through a zinc chloride‐catalyzed ‘conjugate addition/cyclization’ sequence.