Choice Of Lewis Acid Research Articles

Switchable Reactivity of Cyclopropane Diesters toward (3 + 3) and (3 + 2) Cycloadditions with Benzoquinone Esters.

Herein, we describe an unprecedented (3 + 3) cycloaddition reaction of the donor-acceptor cyclopropanes with quinone esters toward the construction of chroman scaffolds in moderate to good yields. Interestingly, the strategy is also adjustable toward a (3 + 2) cycloaddition by just switching the Lewis acid to furnish benzofuran scaffolds. Based on the choice of Lewis acid used, the same set of precursors has been used to deliver the benzopyran and benzofuran derivatives.

Stereoselective Carbocyclization of Vinyloxiranes Catalyzed by Lewis Acids: Construction of the Musellarin Tricyclic Core

Stereoselective carbocyclizations of vinyloxiranes were efficiently catalyzed by Lewis acids to provide cyclic homoallyl alcohols as single isomers. The choice of Lewis acid, B(C6F5)3 was crucial for the stereoselective transformation in the case of cis vinyloxiranes, whereas BF3∙OEt2 was proven to be an effective catalyst for trans substrates. The method was well implemented in the synthesis of seven‐membered rings and six‐membered rings with functional group tolerance. Utility of the resulting trans‐ and cis homoallyl alcohols was demonstrated to concisely build the tricyclic core of musellarin A and E.

Hydrogenation of Polyesters to Polyether Polyols.

The amount of plastic waste is continuously increasing. Besides conventional recycling, one solution to deal with this problem could be to use this waste as a resource for novel materials. In this study, polyesters are hydrogenated to give polyether polyols by using in situ‐generated Ru‐Triphos catalysts in combination with Lewis acids. The choice of Lewis acid and its concentration relative to the ruthenium catalyst are found to determine the selectivity of the reaction. Monitoring of the molecular weight during the reaction confirms a sequential mechanism in which the diols that are formed by hydrogenation are etherified to the polyethers. To probe the applicability of this tandem hydrogenation etherification approach, a range of polyester substrates is investigated. The oligoether products that form in these reactions have the chain lengths that are appropriate for application in the adhesives and coatings industries. This strategy makes polyether polyols accessible that are otherwise difficult to obtain from conventional fossil‐based feedstocks.

New Di‐, Tri‐, and Tetra‐Core‐Functionalized Naphthalene Diimides from Reactions of Allyl Ethers with Lewis Acids

AbstractCore‐substituted NDIs bearing aryl allyl ethers have a predisposition to deallylation in the presence of Lewis Acids (LA). However, under certain conditions, with appropriate choice of LA, the reaction can be controlled to yield unexpected and highly functionalized dinaphthol systems through a [3,3]‐sigmatropic (Claisen) rearrangement. The major products of the rearrangement study have been characterized by X‐ray crystallographic analysis and the per‐substituted products are shown to undergo ring‐closing metathesis to form heterocyclic core‐extended NDIs. This study highlights a simple method to construct a series of important donor–acceptor building blocks for the preparation of new conjugated materials.

Lewis Acid Catalyzed Cyclizations of Epoxidized Baylis–Hillman Products: A Straightforward Synthesis of Octahydro­benzo[e]azulenes

AbstractTricyclic keto‐diols have been synthesized from 2‐cyclopenten‐1‐one in a three‐step annulation procedure. The importance of aryl ring electronics and steric contributions and the choice of Lewis acid were investigated for the final cyclization step. An unexpected cyclization product was identified, suggesting multiple mechanisms for the cyclization process.

Stereocontrolled synthesis of propionate motifs froml-lactic andl-alanine aldehydes. A DFT study of the hydrogen transfer under endocyclic control

Reported herein is the synthesis of all possible stereotriad propionates derived from L-lactic acid and L-alanine. The approach is based on a stereocontrolled Mukaiyama aldol reaction followed by a free radical-mediated hydrogen transfer, for which the choice of Lewis acid (monodentate or bidentate) dictates the stereochemical outcome in each reaction. The sequential process has been applied iteratively for the synthesis of eight stereopentad motifs derived from β,γ-alkoxyaldehydes. New experimental data and DFT calculations (BHandHLYP/TZVP) of the transition states involved in the radical reduction of aluminate intermediates support the proposed model of the endocyclic effect leading to high 2,3-syn selectivities for the synthesis of propionates and subunits thereof.

Lewis acid-mediated azidolysis of 2,3-three membered heterocyclic amines

The Lewis acid-catalyzed regioselective azidolysis of 2,3-three membered heterocyclic amines has been investigated. The results obtained demonstrated that using TMSN3 as a source of azide, the appropriate choice of Lewis acid allowed to obtain different regio- and stereocontrolled precursors of aminoalcoholic and triaminic sequences. Considering the occurrence of these moieties in the structure of many biologically active compounds, the present methodologies could represent a powerful tool in organic synthesis for the preparation of interesting molecules.

A highly regioselective azidolysis of 2,3-epoxy amines: an unexpected [Ti(O-i-Pr)2(N3)2] mediated C-2 opening

The Lewis acid-catalysed regioselective azidolysis of 2,3-epoxy amines has been investigated. The results obtained demonstrated that using TMSN3 as a source of azide, the appropriate choice of Lewis acid allowed to direct the regiochemistry of the ring opening. The present methodologies provide a powerful tool in organic synthesis for the preparation of diaminoalcohol moieties.

Synthesis of dichloromethyl-substituted salicylates and pyran-4-ones by cyclocondensation of 1,3-bis(silyloxy)-1,3-butadienes with 1,1-dimethoxy-4,4-dichlorobut-1-en-3-one: control of the C,C- and C,O-regioselectivity by the choice of Lewis acid

The TiCl 4-mediated formal [3+3] cyclocondensation of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with 1,1-dimethoxy-4,4-dichlorobut-1-en-3-one afforded a variety of functionalized 6-dichloromethyl-4-methoxysalicylates with very good regioselectivity. Some of the products were transformed into 6-formyl-4-methoxysalicylates. The employment of Me 3SiOTf instead of TiCl 4 resulted in a change of the regioselectivity and in the formation of functionalized 2-(dichloromethyl)pyran-4-ones.

Stereocontrolled Assembly of Tetrasubstituted Tetrahydrofurans: A Concise Synthesis of Virgatusin.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of Silylallene Glycosides and Diene Diglycosides by C‐Glycosidation of D‐Glucal with 1,4‐Bis(trimethylsilyl)‐2‐butyne.

Silylmethylallenyl glycosides, symmetrical and unsymmetrical diene glycosides, were synthesized by C-glycosidation with 1,4-bis(trimethylsilyl)-2-butyne in good yield. The nature of the product is controlled by the choice of Lewis acid, BF3·OEt2, or SnCl4. The efficient construction of unsymmetrical diene glycosides was achieved in one pot on the basis of the order of addition of sugar starting materials.

Stereocontrolled Assembly of Tetrasubstituted Tetrahydrofurans: A Concise Synthesis of Virgatusin

The condensation of substituted allylsiloxanes with aldehydes leads to the highly stereoselective construction of 2,3,4,5-tetrasubstituted tetrahydrofurans. With electron-rich aryl and alpha,beta-unsaturated aldehydes as substrates, the stereochemical outcome at C5 can be dictated by appropriate choice of Lewis acid. The reaction has been applied to a concise (nine step) synthesis of (+)-virgatusin (ent-1). [reaction: see text]

Synthesis of Silylallene Glycosides and Diene Diglycosides by C-Glycosidation of d-Glucal with 1,4-Bis(trimethylsilyl)-2-butyne

[reaction: see text] Silylmethylallenyl glycosides, symmetrical and unsymmetrical diene glycosides, were synthesized by C-glycosidation with 1,4-bis(trimethylsilyl)-2-butyne in good yield. The nature of the product is controlled by the choice of Lewis acid, BF(3).OEt(2), or SnCl(4). The efficient construction of unsymmetrical diene glycosides was achieved in one pot on the basis of the order of addition of sugar starting materials.

1,3-Stereoinduction in Radical Reactions: Radical Additions to Dialkyl 2-Alkyl-4-methyleneglutarates

Tin hydride-mediated radical additions to a series of α-methylene-glutarates 1, furnishing 2,4-dialkyl-substituted glutarates 3 are reported. The diastereoselectivity of hydrogen transfer to the intermediate adduct radicals 2, possessing a stereogenic center in γ-position, was disappointing in the temperature range of −78 to 80 °C. However, the reactions proved to be able to proceed with excellent 1,3-diastereoselectivities under chelation-controlled conditions, depending on the steric impacts of 2- and 4-alkyl substituents as well as on the ester−alkyl moiety and choice of Lewis acid. Using MgBr2·OEt2 as additive, syn-selectivities of 98:2 were achieved upon initial tert-butyl radical addition at −78 °C. High anti-diastereoselectivities were observed in the MgBr2·OEt2-controlled pathway at 70 °C when smaller alkyl radicals such as cyclohexyl, ethyl, and methyl were applied. Interesting and uncommon temperature dependences were observed in the temperature range of −78 to 100 °C, revealing strong entropic ...

The effects of different ester and ketal protecting groups on the reactivity and selectivity of tartrate-derived silylketene acetals

The reaction of tartrate-derived silylketene acetals and benzaldehyde has been investigated and the yields and diastereoselectivities have been found to be dependent upon the nature of the tartrate ester. Utilising the di-tert-butyl tartrate derivatives, high yields were achieved using a variety of aldehyde substrates. The reactions all proceeded with excellent levels of stereoselectivity ( 82∶18); the sense of induction being dependent upon the choice of Lewis acid. BF3·OEt2 and TiCl3(OiPr) furnished complementary products in several cases and a model has been proposed to account for this observation.

Synthesis of isochroman-3-ylacetates and isochromane-γ-lactones through rearrangement of aryldioxolanylacetates

Lewis acid catalysed rearrangement of methyl 4,5-trans-4-aryldioxolan-5-ylacetates 1 provides a convenient route to substituted methyl isochroman-3-ylacetates 2 and isochromane-γ-lactones 3. The choice of Lewis acid is determined by the substitution pattern of the aromatic ring. The two contiguous isochromane stereocentres are transferred unchanged from the parent dioxolanes, while the configuration of the isochromane methyl group is dependent upon the aryl substitution, the reagent and the reaction conditions. Thus treatment of the C-2 epimeric 3′,5′-dimethoxyphenyldioxolanes 4 and 5 with camphorsulfonic acid afforded a mixture of the C-5 epimeric isochromane lactones 26 and 29, the former being favoured at lower acid concentrations, the latter at higher concentrations. Titanium tetrachloride isomerised the analogous 2′-chloro-5′-methoxyphenyldioxolanes 6 and 7 into the methyl isochroman-3-ylacetate 38, which could be lactonised to the isochromane lactone 27. Phosphoric acid converted the 2′,5′-dimethoxyphenyldioxolanes 8 and 9 into a mixture of the isochromane lactones 28 and 31, while similar treatment of the hydroxylactone 22 in the presence of acetaldehyde afforded the isochromane lactone 28 directly and with complete diastereoselectivity. Oxidative demethylation and annulation of this isochromane lactone 28 afforded 5-epi-7-deoxykalafungin 41.

Stereoselective reactions of α,β-epoxy-aldehydes; the formation of “chelation controlled” products

Treatment of the α,β-epoxy-aldehydes (1) and (2) with Lewis acids followed by in situ reaction with carbon nucleophiles produces halohydrins of the products of “chelation controlled” addition to the aldehyde with high diastereoselectivity; The regiochemistry of epoxide opening is determined by choice of Lewis acid.

Observations on the choice of Lewis acid and mode of addition for the Lewis acid mediated reaction of crotyltri-[formula omitted]-butylstannane with aldehydes: convenient and highly selective access to both [formula omitted] and [formula omitted] products

For additions crotyltri-n-butylstannane to simple aldehydes mediated by BF3·Et2O, erythro selectivity may be increased considerably (25:1 vs. 9:1, for example) by simply using 2 eq. of stannane, TiCl4 mediated additions give high erythro or threo selectivity depending simply on the order in which reactants are mixed; and other Lewis acids are also evaluated in such reactions.