Use Of Lewis Acids Research Articles

Synthesis and characterisation of optically active spiro[4.5]decanes

The synthesis of spiro[4.5]decanes through a Diels–Alder cycloaddition between 5-methylenefuran-2(5H)-one (protoanemonin) 8, and chiral dienes 1-[(–)-menthyloxy]-3-trimethylsilyloxybuta-1,3-diene, 5, 1-[(–)-bornyloxy]-3-trimethylsilyloxybuta-1,3-diene, 6, and 1-[(+)-isopinocampheyloxy]-3-trimethylsilyloxybuta-1,3-diene, 7, is reported for the first time. Diastereofacial selectivity together with the preference for an endo/exo attack are examined. The effect on the diastereofacial selectivity of substituting dienophile 8 by 3-acetoxy-5-methylenefuran-2(5H)-one 18 is also discussed. The use of Lewis acids as catalysts to enhance selectivity has been attempted without success.

Total Syntheses of Epothilones A and B

Convergent, stereocontrolled total syntheses of the microtubule-stabilizing macrolides epothilones A (2) and B (3) have been achieved. Four distinct ring-forming strategies were pursued (see Scheme 1). Of these four, three were reduced to practice. In one approach, the action of a base on a substance possessing an acetate ester and a nonenolizable aldehyde brought about a remarkably effective macroaldolization see (89 → 90 + 91; 99 → 100 + 101), simultaneously creating the C2−C3 bond and the hydroxyl-bearing stereocenter at C-3. Alternatively, the 16-membered macrolide of the epothilones could be fashioned through a C12−C13 ring-closing olefin metathesis (e.g. see 111 → 90 + 117; 122 → 105 + 123) and through macrolactonization of the appropriate hydroxy acid (e.g. see 88 → 93). The application of a stereospecific B-alkyl Suzuki coupling strategy permitted the establishment of a cis C12−C13 olefin, thus setting the stage for an eventual site- and diastereoselective epoxidation reaction (see 96 → 2; 106 → 3). The development of a novel cyclopropane solvolysis strategy for incorporating the geminal methyl groups of the epothilones (see 39 → 40 → 41), and the use of Lewis acid catalyzed diene−aldehyde cyclocondensation (LACDAC) (see 35 + 36 → 37) and asymmetric allylation (see 10 → 76) methodology are also noteworthy.

Aldehydes vs Aldimines. Unprecedented Aldimine-Selective Nucleophilic Additions in the Coexistence of Aldehydes Using a Lanthanide Salt as a Lewis Acid Catalyst

It is well-recognized that aldimines are less reactive than aldehydes toward nucleophilic additions. In this paper, an unprecedented change in the reactivity is described: preferential reactions of aldimines over aldehydes in nucleophilic additions using a lanthanide salt as a Lewis acid catalyst. In the presence of a catalytic amount of ytterbium triflate (Yb(OTf)3), only aldimines reacted with silyl enol ethers, ketene silyl acetals, allyltributylstannane, or cyanotrimethylsilane to afford the corresponding adducts in high yields, even in the coexistence of aldehydes. Selective formation of an aldimine−Yb(OTf)3 complex rather than an aldehyde−Yb(OTf)3 complex was indicated by 13C NMR analyses. While this report demonstrates the effective use of Lewis acids in organic synthesis, the basic idea of changing reactivity as shown here will be widely applied to many other nucleophilic additions.

ChemInform Abstract: Use of Lewis Acid in the Reaction of Zinc Salts of Indoles and Acyl Chloride.

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Thermal glycosylations: Reactions of β-cellobiosyl fluoride heptaacetate with tigogenin and its trimethylsilyl ether

Thermal reaction of β-cellobiosyl fluoride heptaacetate with ticogenin in toluene or o-xylene at 112–142 °C gave ticogenyl β-cellobioside heptaacetate in 69–75% yields with good stereoselectivity. The procedure avoids the use of Lewis acid or traditional glycosidation promotors. The deacetylated product is a hypocholesterolemic and anti-atherosclerosis agent.

The Use of Lewis Acid in the Reaction of Zinc Salts of Indoles and Acyl Chloride

3-Acylindoles have been prepared in 62 – 92% isolated yield from the corresponding indoles and acyl chlorides in the presence of AlCl3.

Heteroatom-Substituted Radicals: 1,2-Asymmetric Induction

Radical reactions became during the last decade a very useful tool in organic synthesis. Spectacular progress has been made in the control of the stereoselectivity of these reactions. This contribution presents our recent results with 1- and 2-heteroatom substituted radicals in cyclic and acyclic systems. Several examples dealing with the use of Lewis acids to achieve high stereochemical control are presented.

Lewis acids in diastereoselective processes involving acyclic radicals

Abstract

Studies on the transannular Diels–Alder reaction of 15-membered macrocyclic trienes containing a trans-trans diene. Part II: Evidence for unsymmetrical transition states in the formation of A.B.C.[6.6.7] tricyclic products

In the preceding paper (Part I) we described the preparation of 10 model TTT and TTC cyclopentadecatrienes. Their conversion to A.B.C.[6.6.7] tricyclic products via a transannular Diels–Alder (TADA) reaction and the analysis of the resulting diastereoselection along with proofs of structures are described in the present article (Part II). The required temperature of reaction was significantly higher (~100 °C) in comparison to that required for the 14-membered lower homologs, indicating that proximity effects are playing a key role in the TADA reaction. The presence of ester groups in the chain was found to favor either of the two possible transition states (endo or exo) on the basis of steric effects, thus controlling the TST/CSC ratio of tricycles from TTC macrocycles, and the TAC/CAT ratio from TTT macrocycles. A formyl substituent on the dienophile can further influence the diastereoselection through the hypothesis of unsymmetrical transition states and allowed, for the first time, the use of Lewis acids as catalysts in the TADA reaction. This investigation demonstrates a general methodology for the stereocontrolled synthesis of A.B.C.[6.6.7] tricycle structures related to several polycyclic natural products. Keywords: transannular, Diels–Alder, 15-membered ring, A.B.C.[6.6.7] tricycles, diterpene.

Diels-Alder reaction of N-allylic enamides and lactam derivatives through iodine mediated activation

The Diels-Alder reaction of N-allylic enamide and α,β-unsaturated lactam derivatives proceeded in the presence of I 2 at low temperature through a cationic iodolactonization intermediate. With some substrates, this method of activation was proved to be more effective than by use of Lewis acids.

Electrophilic activation of carbon dioxide. Preparation of vinylacetic acid from CO2 and allyltrimethylsilane

The search for fundamentally new methods to activate the carbon dioxide molecule and involve it in various reactions attracts special attention. In this respect, the problem of the possible use of Lewis acids and protic superacids has been repeatedly discussed in the literature. However, no actual results in this field have been obtained until recently. Now the authors can report the first example of direct electrophilic activation of the CO{sub 2} molecule by aprotic Lewis acids under mild conditions.

Stereocontrolled addition of 2-lithiothiazole to the nitrone derived from d-glyceraldehyde acetonide. A revision and extension

The sense of diastereoselective addition of 2-lithiothiazole 1a to the nitrone 2 derived from d-glyceraldehyde is reversed by the use of Lewis acids (Et 2AlCl, TiCl 4) as complexing agents; the configuration of the hydroxylamine adduct obtained from 1a and 2 in the absence of the above chelating agents, previously reported from these laboratories was revised.

Growth of II–VI compounds by metal-organic chemical vapour deposition. Structural characterisation of dimethylcadmium-N,N,N′,N′-tetramethylethane-1,2-diamine by gas-phase electron diffraction and its use in the growth of epitaxial layers of CdS and CdSe upon GaAs

Aspects of the use of Lewis acid–base adducts of dimethylcadmium in the growth of CdY (Y = S or Se) by metal-organic chemical vapour deposition (MOCVD) have been investigated. The adduct dimethylcadmium-N,N,N′,N′-tetramethylethane-1,2-diamine, Me2Cd·Me2NCH2CH2NMe2(1), was made and its structure determined by gas-phase electron diffraction. In 1 the cadmium atom is four-co-ordinate, being bound to two methyl groups [rα(Cd–C)= 2.11(2)A] and two nitrogen atoms [rα(Cd—N)= 2.47(5)A] with ∠C—Cd—C and ∠N—Cd—N being 132(11) and 84(3)°, respectively. The electron diffraction study provides the first unequivocal evidence for the existence of an adduct of dimethylcadmium being transported in the gas phase. Further evidence for the existence of 1 in the gas phase is provided by mass spectrometry and gas-phase infrared spectroscopy. Compound 1 was subsequently employed in the MOCVD process to grow visibly good-quality layers of CdY (Y = S or Se) on gallium arsenide substrates, but without overcoming the problem of pre-reaction experienced in the growth of CdY (Y = S or Se) by MOCVD.

The Use of Lewis Acids During Rapid Steam Hydrolysis

Abstract Mixed hardwood chips were treated with various concentrations of aluminum chloride hexahydrate, aluminum sulfate hydrate, and ferric chloride and were subjected to rapid steam hydrolysis pretreatment (RASH). The three levels included 0.01, 0.03, and 0.05 moles of catalyst per 1000 grams of wood. Rapid steam hydrolysis (RASH) was done from 180° to 260°C at 20°C intervals for one minute. The Lewis acid catalyst affected the overall recovery yield of solids, the recovery values of the individual components, the enzymatic rates, and the methanol and water solubility. Overall recovery of pretreated solids generally decreased with the increase in levels of the catalyst. The one exception was AlCl3 ·6H2O where the minimum recovery levels were reached at 0.03 moles per 1000 grams of wood and increased at the higher level of catalyst. Cellulose degradation occurred in the temperature range of 250° to 260°C for the control and at the two lower concentrations of the catalyst. At the higher levels of catalyst, appreciable amounts of cellulose degradation occurred at lower temperatures. Hemicellulose solubilization and degradation were extremely sensitive to the RASH temperature and to the levels of catalyst. Almost all hemicellulose was lost at high temperatures or at high levels of catalyst. Losses of lignin appeared to be affected mainly by the RASH temperature and not by the changes in the levels of catalyst. In contrast, both the temperature and the level of catalyst strongly affected the rates of enzymatic hydrolysis. Generally, the intermediate level of catalyst seemed to give the highest rates of enzymatic hydrolysis at the lowest temperature. Methanol and water solubles increased in the presence of the catalyst and reached a maximum at levels of 0.03 moles between 230° to 250°C.

3-Methoxy-1-(tributylstannyl)-1-propene, a versatile vinyltin synthon

3-Methoxy-1-(tributylstannyl)-1-propene ( 1) can be readily transformed into the silylated analogue ( 2), which can react both as a vinyltin and as an allylsilane. Thus, transmetallation of 2 with n-butyllithium, followed by quenching with various electrophiles, gives functionalized allylsilanes. Although reagent 2 can react with benzaldehyde as an allylsilane, the diastereoisomeric mixture of monoprotected 1,2-diols ( 14) can be best obtained from 1 by direct lithiation and quenching with benzaldehyde. The use of Lewis acids allows the control of the stereochemistry of the condensations.

Assessment of butene-1,4-diols as starting materials for the preparation of π-Allyltricarbonyliron complexes

This work assesses the use of butene-1,4-diols as starting materials for the synthesis of π-allyltricarbonyliron lactone complexes. It has been shown that substituents, and the use of Lewis acids, mainly ZnBr 2, in reactions with Fe 2(co) 9 in benzene under ultrasonic conditions or in THF, greatly influence the yield and the product distribution.

ChemInform Abstract: The Use of Lewis Acids in the Diels‐Alder Reaction with Organophosphorus Dienophile Compounds. Asymmetric Approach with (RP, SP)‐o‐Anisylphenylvinylphosphine Oxide.

AbstractThe vinylic phosphine oxides or phosphonates (V) are subjected to cycloaddition reaction with cyclopentadiene (VI) in the presence of equimolar or catalytic amounts of various Lewis acids.

Use of Lewis acids and high pressure to carry out extremely hindered Diels-Alder reactions

Use of Lewis acids and high pressure to carry out extremely hindered Diels-Alder reactions

Selectivity in reactions involving α-alkoxyallyltributyltins

The behaviour of α-alkoxyallyltributyltins has been studied in terms of chemo-, regio- and stereoselectivity. Chemoselectivity is readily controlled by the experimental conditions, as exemplified by the reaction of p-bromobenzaldehyde with α-ethoxycrotyltributyltin. Cross-coupling products are the sole products when the reaction is catalyzed by Pd(PPh 3) 4 while the use of Lewis acids, or even simple heating of the reagents, cause addition to the aldehyde group. The regio and stereochemistries of the addition reactions can also be controlled in some circumstances. For instance, the BF 3. Et 2 O promoted reactions (CH 2 Cl 2, −78°C) lead regio- and stereoselectively to the α -glycol monoether 16E (93 % syn E). On the other hand, the (E)-α-ethoxycrotyltributyltin has been shown to be much more reactive than the (Z)- isomer. It allows highly regio and stereoselective reaction affording 19Z ( anti Z) on heating (100°C, 6 h). These results are discussed and an interpretation of the stereochemical trends is given on the basis of the interactions occurring in usually accepted transition states.

ChemInform Abstract: Selective Synthesis by Use of Lewis Acids in the Presence of Organocopper and Related Reagents

ChemInformVolume 18, Issue 13 Reviews ChemInform Abstract: Selective Synthesis by Use of Lewis Acids in the Presence of Organocopper and Related Reagents Y. YAMAMOTO, Y. YAMAMOTO Dep. Chem., Fac. Sci., Tohoku Univ., Sendai 980, Jap.Search for more papers by this author Y. YAMAMOTO, Y. YAMAMOTO Dep. Chem., Fac. Sci., Tohoku Univ., Sendai 980, Jap.Search for more papers by this author First published: March 31, 1987 https://doi.org/10.1002/chin.198713385Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume18, Issue13March 31, 1987 RelatedInformation