Unsaturated Acetals Research Articles

Bi(OTf)3-catalysed intramolecular cyclisation of unsaturated acetals.

A series of highly functionalized carbocycles was efficiently prepared via the selective cyclisation of unsaturated acetals and ketals in the presence of only 1 mol% of Bi(iii) or Fe(iii) triflates as the catalysts at room temperature, with yields ranging from 60 to 90%. With Bi(OTf)3 catalysis, α,β-unsaturated ether carbocycles are formed selectively, whereas with the Fe(OTf)3 system, a cycloisomerisation to carbocyclic diethers is mainly obtained. This acetal/olefin cyclisation could be run at a multi-gram scale and compound 2c could be obtained on a 300 gram-scale with a yield of 69% after precipitation in hexane.

ChemInform Abstract: Enantioselective Synthesis of Piperidines Through the Formation of Chiral Mixed Phosphoric Acid Acetals: Experimental and Theoretical Studies.

AbstractA highly enantioselective intramolecular cyclization of unsaturated acetals (I) and (III) is utilized for the synthesis of chiral piperidines (II) and (IV), respectively.

Intramolecular oxonium ylide formation-[2,3] sigmatropic rearrangement of diazocarbonyl-substituted cyclic unsaturated acetals: a formal synthesis of hyperolactone C.

Rh(II)-catalyzed oxonium ylide formation-[2,3] sigmatropic rearrangement of α-diazo-β-ketoesters possessing γ-cyclic unsaturated acetal substitution, followed by acid-catalyzed elimination-lactonization, provides a concise approach to 1,7-dioxaspiro[4.4]non-2-ene-4,6-diones. The process creates adjacent quaternary stereocenters with full control of the relative stereochemistry. An unsymmetrical monomethylated cyclic unsaturated acetal leads to hyperolactone C, where ylide formation-rearrangement proceeds with high selectivity between subtly nonequivalent acetal oxygen atoms.

Enantioselective synthesis of piperidines through the formation of chiral mixed phosphoric acid acetals: experimental and theoretical studies.

An enantioselective intramolecular chiral phosphoric acid-catalyzed cyclization of unsaturated acetals has been utilized for the synthesis of functionalized chiral piperidines. The chiral enol ether products of these cyclizations undergo subsequent in situ enantioenrichment through acetalization of the minor enantiomer. A new computational reaction exploration method was utilized to elucidate the mechanism and stereoselectivity of this transformation. Rather than confirming the originally postulated cyclization proceeding directly through a vinyl oxocarbenium ion, simulations identified an alternative two-step mechanism involving the formation of a mixed chiral phosphate acetal, which undergoes a concerted, asynchronous S(N)2'-like displacement to yield the product with stereoselectivity in agreement with experimental observations.

Enantioselective Synthesis of Piperidines through the Formation of Chiral Mixed Phosphoric Acid Acetals: Experimental and Theoretical Studies

AbstractAn enantioselective intramolecular chiral phosphoric acid‐catalyzed cyclization of unsaturated acetals has been utilized for the synthesis of functionalized chiral piperidines. The chiral enol ether products of these cyclizations undergo subsequent in situ enantioenrichment through acetalization of the minor enantiomer. A new computational reaction exploration method was utilized to elucidate the mechanism and stereoselectivity of this transformation. Rather than confirming the originally postulated cyclization proceeding directly through a vinyl oxocarbenium ion, simulations identified an alternative two‐step mechanism involving the formation of a mixed chiral phosphate acetal, which undergoes a concerted, asynchronous SN2′‐like displacement to yield the product with stereoselectivity in agreement with experimental observations.

ChemInform Abstract: Chiral Broensted Acid‐Catalyzed Enantioselective Ionic [2 + 4] Cycloadditions.

AbstractThe first asymmetric N‐triflylphosphoramide‐catalyzed ionic [2 + 4] cycloaddition reaction of unsaturated acetals is described.

Chiral Brønsted acid-catalyzed enantioselective ionic [2+4] cycloadditions

The first asymmetric chiral N-triflylphosphoramide-catalyzed ionic [2+4] cycloaddition reaction of unsaturated acetals is described. This reaction proceeds through the intermediacy of a vinyl oxocarbenium/chiral anion pair, and the chiral N-triflylphosphoramide anion controls the stereoselectivity of the cycloaddition step. Moderate enantioselectivities (up to 80:20 e.r.) have been obtained when α,β-unsaturated dioxolanes were employed as the dienophiles. These reactions demonstrate strong dependence on the counterion coordinating properties and solvent polarity, a behavior characteristic of oxocarbenium ions.

ChemInform Abstract: Unsaturated Carbohydrates. Part 31. Trichothecene-Related and Other Branched C-Pyranoside Compounds.

The C-glycosidic alkene 21, on treatment with tributyltin hydride and a radical initiator, underwent intramolecular radical cyclisation to give the 2-oxabicyclo[3.2.1]octane derivative 23 which is structurally related to the sesquiterpenoid trichothecenes. The unsaturated acetals 27 and 29, formed using methods encountered in the first part of the work, permitted branch-point substituents to be introduced regio- and stereo-specifically at C-3 and at C-4 (carbohydrate numbering) of compounds of this type. Methods are thus available for the synthesis of pyranoid derivatives bearing several C-substituents.

Flavouring Group Evaluation 10, Revision 1 (FGE10 Rev1) - Aliphatic primary and secondary saturated and unsaturated alcohols, aldehydes, acetals, carboxylic acids and esters containing an additional oxygenated functional group and lactones from chemical g

Flavouring Group Evaluation 10, Revision 1 (FGE10 Rev1) - Aliphatic primary and secondary saturated and unsaturated alcohols, aldehydes, acetals, carboxylic acids and esters containing an additional oxygenated functional group and lactones from chemical g

Stereoselective cyclopropanation of unsaturated acetals, using carbohydrates with d- gluco, l- rhamno and d- xylo configurations as chiral auxiliaries

The stereoselective synthesis of (2-phenylcyclopropyl)methylidene acetals of sugar derivatives from methyl d-glucopyranoside, dodecyl N-acetyl-2-amino-2-deoxy- d-glucopyranoside, 1,2- O-isopropylidene- d-glucofuranose, methyl l-rhamnopyranoside and 1,2- O-isopropylidene- d-xylofuranose, as a chiral auxiliary, is described. The cyclopropanation reaction of the corresponding alkenylidene derivatives with CH 2I 2/ZnEt 2 took place with different stereoselectivity, depending on the configuration on the acetal carbon, the size of the acetal ring, the sugar configuration, the protecting group of the hydroxyl groups of the sugar and the substitution of the unsaturated system. The stereochemistry of the new stereogenic centres was then determined by acid hydrolysis of the cyclopropane moiety of the chiral auxiliary, which was also recovered.

Palladium‐Catalyzed C—C Bond Formation: Synthesis of 1,1‐Dialkylbuta‐1,3‐dienes and β‐Phenylstyrenes via Organoboron Intermediates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Palladium‐Catalyzed C−C Bond Formation: Synthesis of 1,1‐Dialkylbuta‐1,3‐dienes and β‐Phenylstyrenes via Organoboron Intermediates

Abstractα,β‐Unsaturated and α‐phenyl acetals show different reactivity when treated with LIC‐KOR superbase and trialkylboranes, in the presence of the reagent system PdL4‐ArX (Suzuki−Miyaura cross‐coupling conditions). In particular, unsaturated acetals yield gem‐dialkylbutadienes via the formation of an intermediate π‐allyl complex that facilitates an anionotropic alkyl migration; in the case of α‐phenyl derivatives, the corresponding cross‐coupling products are recovered. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

α,β-Unsaturated Acetals in Synthesis

In this review some results are reported that allow the acetal group of α,β-unsaturated carbonyl derivatives to be considered not simply a protective function but also a synthetic tool for transforming aldehydes (and ketones) into a cheap and easily accessible starting material for the construction of various and more complex structures. The reported results can be grouped into three main sections: α,β-unsaturated as masked acyl anions, as precursor of dienophilic moieties, and as reagent in organometallic syntheses. In many reported examples the key reagents are polar organometallic reagents, and in particular, the bimetallic (lithium-potassium) mixed base LICKOR, which is also known as Schlossers superbase. We are not acquainted with previous reviews on these topics, and in our survey we have considered the literature later than the eighties. Keywords: unsaturated acetals, masked acyl anions, dienophilic moieties, polar organometallic reagents, (lithium-potassium) mixed base, Schlossers superbase

17O, 13C, and 1H NMR Studies of p–π Conjugation in 2-Substituted 4-Methylene-1,3-dioxolanes and 4-Methyl-1,3-dioxoles

The 17O NMR spectra of a number of unsaturated 5-membered cyclic acetals, 2-substituted 4-methylene-1,3-dioxolanes and their endocyclic isomers, 4-methyl-1,3-dioxoles, have been recorded. The 17O NMR chemical shifts, in comparison with those of similarly 2-substituted 1,3-dioxolanes, were used to explore the variation of the strength of p–π conjugation in the unsaturated acetals as a function of the nature of substitution at C2. The 17O NMR shift data reveal that alkoxy substituents have a significantly more favorable effect on the strength of p–π conjugation in 4-methyl-1,3-dioxoles than in 4-methylene-1,3-dioxolanes. This fact appears to be responsible for the previously observed unexpectedly large effect of alkoxy substitution on the relative thermodynamic stabilities of these two classes of isomeric compounds. Additional information of the unexpected charge distribution in 4-methyl-1,3-dioxoles is provided by their 1H and 13C NMR spectra.

Quantum Chemical Study of the Preferred Conformations, Molecular Geometries, and Relative Thermodynamic Stabilities of 2-Substituted 4-Methylene-1,3-Dioxolanes and 4-Methyl-1,3-Dioxoles

The preferred conformations, molecular geometries, and relative stabilities of carbon–carbon double-bond exo–endo isomeric 2-substituted 4-methylene-1,3-dioxolanes (a) and 4-methyl-1,3-dioxoles (b) have been studied by DFT calculations at the B3LYP/6-31G* level of theory. The main interest of this work was devoted to the contribution of alkoxy substituents on the relative thermodynamic stabilities of these isomeric unsaturated acetals. Comparison of the computational data with previous experimental findings shows both the enthalpies and entropies of the a → b isomerization to be accurately predictable by the DFT calculations. Most importantly, the good agreement between experiment and theory proved also to be applicable to the previously observed unexpectedly large effect of 2-alkoxy substitution on the isomer equilibria.

ChemInform Abstract: Synthesis of 3‐Hydroperoxy (or Hydroxy)‐Substituted 1,2‐Dioxanes and 1,2‐Dioxepanes by the Ozonolysis of Unsaturated Hydroperoxy Acetals.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Radical‐Chain Cyclization of Unsaturated Acetals and Thioacetals in the Presence of Thiols as Polarity‐Reversal Catalysts.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Radical-chain cyclisation of unsaturated acetals and thioacetals in the presence of thiols as polarity-reversal catalysts

Polarity-reversal catalysis by thiols has been applied to promote the radical-chain cyclisation of 2-(pent-4-enyl)-substituted 1,3-dioxolanes, 1,3-dithianes and 1,3-dithiolanes to give spirocyclic products.

Functional transformation of aldehydes and ketones via homolytic induced decomposition of unsaturated peroxy acetals and peroxy ketals

Induced decomposition of unsaturated peroxy acetals prepared from trimethyl orthoformate, dodecanal or 2-methylundecanal and 2,3-dimethyl-2-hydroperoxybut-3-ene, in the presence of ethyl iodoacetate, CCl4 or dodecanethiol, allowed respectively their iodo-, chloro- and hydro- decarbonylation with yields of over 70%; the same reaction applied to the monoperoxy ketal or diperoxy ketal of cyclohexanone in the presence of ethyl iodoacetate resulted in its functional transformation in methyl 6-iodohexanoate or 1,5-diiodopentane with respective yields of 65 and 40%.

Iron(II)-mediated fragmentation of unsaturated hydroperoxy acetals: a rapid synthetic route to 13-membered macrolides

Treatment of (1-hydroperoxy)cyclohexyloxyethyl acrylate with either iron(II) sulfate or iron(II) sulfate/copper(II) chloride affords novel 13-membered macrolides via tandem radical scission-cyclisation reactions.