ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. An unexpected effect of acetal stereochemistry on the course of its reductive cleavage. Allylsilanes attack cyclohexanone ketals axially (93:7)73 and attack 2-phenylpropionaldehyde acetal with a degree of Cram selectivity that is influenced by the choice of Lewis acid, tin(IV) chloride giving the highest ratio (3.5:1).80 With β-alkoxyacetals, chelation control is not possible as it is with the corresponding aldehydes; stereoselectivity is not high, it is in the opposite sense (Scheme 41, compare Scheme 16), and it depends upon the choice of Lewis acid.80. document.write("   ") It appears that the better the leaving group, the more polar the solvent and the more powerful the Lewis acid, the better the stereoselectivity, the combination of p-nitrobenzoate and boron trifluoride etherate being notably effective. In the hydrolysis of O-(2-hydroxypropyl)cellulose, residues of d-glucose substituted with a single O-(2-hydroxypropyl) substituent at O-2 (irrespective of the pattern of additional substitution at O-3 or O-6) form 1,2-O-(1-methyl-1,2-ethanediyl)-α-d-glucose acetals. No votes so far! By using our site, you acknowledge that you have read and understand our Cookie Policy, Privacy Policy, and our Terms of Service. trans‐Alkenyl ethers always gave adducts with a single structure stereospecifically, indicating that the intermediate carbocation attacks a trans‐alkenyl ether from a definite direction independent of the bulkiness of substituents. These cookies do not store any personal information. By continuing you agree to the use of cookies. Follow ChemTube3D on Kudos Number of times cited according to CrossRef: Stereochemistry of the acetal addition of a β‐deuterated vinyl ether as a model reaction for the polymerization of vinyl ethers. The acetals of optically active 1,2- and 1,3-glycols show striking stereoselectivity, giving homoallylic ethers in high diastereoisomeric excess, especially when titanium tetrachloride-titanium tetraisopropoxide mixtures are used as the Lewis acid (Scheme 44).85 Even non-cyclic acetals made from aldehydes and enantiomerically pure 1-phenylethanol give homoallylic ethers with high diastereoselectivity.86, Allylstannanes react with acetals in the presence of bis(diethylaluminum) sulfate (Scheme 45)87 and the reaction has been used in a synthesis of lavandulol (Scheme 46). Possible mechanisms for these various acid-catalyzed transformations are discussed. The terms 'acetal' and 'ketal' have had varied use over the years. Kinetic control of acetal stereochemistry. The two stages in the formation of acetal are shown. Organic Chemistry Animations Introduction, Acid Chloride Formation – Thionyl Chloride, Acid chloride formation-Phosphorus Pentachloride, Addition to C=O - loss of carbonyl oxygen, Molecules with a Plane of Symmetry – Feist’s Acid, Chiral Allenes Without Stereogenic Centres, Conformations of ethane – Newman projection, Conformational Analysis – Pea Moth Pheromone, Substrate structure controls substitution mechanism S, E2 Regioselective Elimination to Menthenes A, E2 Regioselective Elimination to Menthenes B, Formation of Diazonium Salt – Diazotization, Benzyne formation – Diazotization-decarboxylation, Enolisation and formation of syn aldol product, Enolisation and formation of anti aldol product, Simple Diastereoselectivity - cis gives syn aldol, Simple Diastereoselectivity - trans gives anti aldol, Conjugate Addition of MeSH to an Unsaturated Aldehyde, Conjugate Addition of Diethylamine to an Unsaturated Nitrile (Acrylonitrile), Conjugate Addition of Diethylamine to an Unsaturated Ester, Conjugate Addition of Enamine to Unsaturated Imine, Conjugate addition of peroxide to form epoxides, Regioselectivity 2-methoxybuta-1,3-diene and acrylonitrile, Regioselectivity 1,1-dimethylbutadiene and methyl acrylate, Stereochemistry of the dienophile - diesters, Stereochemistry of the dienophile - dinitrile, The Woodward Hoffman description of the Diels-Alder, Intramolecular Diels-Alder (E)-3-Methyldeca-1,3,9-triene, Intramolecular Diels-Alder – 1,3,9-decatrien-8-one, 2,3-Dimethylbutadiene and Acrolein(propenal), Quinone as Dienophile – Steroid Framework, Intramolecular Diels-Alder – Regioselectivity reversal, 8-Phenylmenthol auxiliary-controlled Diels-Alder, Paal-Knorr pyrrole synthesis via hemiaminal, Pyridine N-Oxide – Nucleophilic Substitution, Pyridine N-Oxide – Remote Oxidation And Rearrangement, 1,3-Dipolar Cycloaddition Isoxazole from nitrile oxide, Electrocyclic reactions are stereospecific, Conrotatory ring closure/opening - cyclobutene, Disrotatory ring closure/opening - hextriene, Semipinacol rearrangements of diazonium salts, Rearrangements with different nucleophiles, Retention of stereochemistry can indicate neighbouring group participation, Neighbouring group participation: alpha-lactone formation, Fragmentations are controlled by stereochemistry, Controlled by stereochemistry (Cis isomer), Controlled by stereochemistry (Trans – Less severe interactions), Controlled by stereochemistry (Trans – Severe interactions), Fragmentation of diastereoisomers (Trans-decalin I), Fragmentation of diastereoisomers (No ring fragmentation), Photolysis of diazomethane to produce a carbene, Methylation of carboxylic acid using diazomethane, Cyclopropanation of an Alkene by a Carbenoid, Stereoselective Aldol Reaction – Cis gives Syn, Stereoselective Aldol Reaction - Trans gives Anti, Endo-trig reactions (5-endo-trig orbital overlap), Hydroboration (Addition of boron hydride to alkenes), Pd-Carbonylative Kosugi-Migita-Stille Coupling Reaction, Pd-Butenolide Formation From Carbonylation Of A Vinyl Bromide, Pd-catalysed nucleophilic allylic substitution of functionalised compounds, Hydroboration of cyclopentadiene Ipc-borane, Acetylenic Ketone Reduction – Alpine Borane, Intermolecular aldol -proline – hydroxyacetone, BISCO Bismuth Strontium Calcium Copper Oxide – BSCCO, Chalcogenides, Intercalation Compounds and Metal-rich phases, Compare shape and size of 1s, 2s and 2p orbitals, Orbital-orbital Interactions and Symmetry Adapted Linear Combinations, Distortions of a octahedral complex with chelating ligands, Ligand Substitution Square Planar Complex, Possible morphologies of Au Nanoparticles, Electrophilic Addition Addition of bromine to an alkene, Electrophilic addition to alkenes – Symmetrical and Unsymmetrical, Nucleophilic Addition Addition of Hydride, Cyanohydrin Formation – Nucleophilic addition to the carbonyl group, Nucleophilic Substitution at Saturated Carbon, Nucleophilic Substitution Cyanide + Ethyl Bromide, Elimination – E2 Stereoselective for E alkenes, Radical Reactions Synthesis of Chloroalkanes, Radical Reactions CFCs and the Ozone Layer, Polyvinyl Chloride Poly(chloroethene) PVC, Self Assessment Activity - Hemiacetal Formation, Self Assessment Activity - Hemiacetal formation, Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License. and you may need to create a new Wiley Online Library account. It is unstable with respect to intramolecular reaction, otherwise it is stable. Acetal additions to β‐substituted vinyl ethers having a variety of substituents (alkenyl ethers) were stereochemically investigated as model reactions for their cationic polymerization. Example 2: Hemiacetal Reversibility Formation of acetals. Based on the characteristics of 2-O-(2-hydroxypropyl)-d-glucose and derivatives thereof in aqueous acid, these bicyclic products are shown to comprise a mixture of two furanose and two pyranose species that differ widely in relative stability, depending on the chirality of C-8 of the 1,4-dioxane ring of the compounds. The Grignard reagent is formed by inserting magnesium into an alkyl halide, an R-X group. J. This product is an ‘acetal’. Careful! Back in the day, acetals were diethers of geminal diols (ie. The controlling influence of an acetal of this type is similarly overridden by what appears to be a chelation effect to an oxygen substituent in a 1,3-relationship to the acetal oxygens.90, Allenylsilanes react with acetals, as they do with aldehydes, by addition, but a simple elimination step completes the substitution reaction (Scheme 48).91 Propargylsilanes likewise react with acetals in the presence of Lewis acids (Scheme 49).92 The reaction has been used intramolecularly (Scheme 50),93 where the first step is likely to be acetal or hemiacetal formation followed by ring closure, and in reactions at the anomeric position of sugars with high levels of axial attack giving allenes.94, Vinylsilanes react with acetals in the presence of Lewis acids to give allylic ethers, which go on to react further.95 Intramolecular reactions are less susceptible to this difficulty78 and provide an opportunity for controlling the configuration of exocyclic double bonds (Scheme 51). We also use third-party cookies that help us analyze and understand how you use this website. Fragmentation involving formation of enamine. The reactions catalyzed by BF3O(C2H5)2 in CH2Cl2 at O°C gave 1:1 adducts, the steric structure of which was determined by means of 13C‐NMR spectroscopy. 3) Deprotonation to form a hemiacetal. Learn more. It is mandatory to procure user consent prior to running these cookies on your website. These cookies will be stored in your browser only with your consent. Richard M. Kellogg. If you do not receive an email within 10 minutes, your email address may not be registered, 4) Protonation of the alcohol. Remember that these reactions are reversible so even if the intermolecular attack proceeded, the product would be unstable with respect to intramolecular attack by the free OH that is within the molecule. Working off-campus?