Acetals symmetrical cyclic

Acid Anhydrides. Symmetrical anhydrides of monocarboxylic acids, when unsubstituted, are named by replacing the word acid by anhydride. Anhydrides of substituted monocarboxylic acids, if symmetrically substituted, are named by prefixing bis- to the name of the acid and replacing the word acid by anhydride. Mixed anhydrides are named by giving in alphabetical order the first part of the names of the two acids followed by the word anhydride, e.g., acetic propionic anhydride or acetic propanoic anhydride. Cyclic anhydrides of polycarboxylic acids, although possessing a... 

While a variety of techniques are available for the monoprotection of symmetrical diols, there are few methods that allow for the chemoselective functionalization of the more hindered hydroxyl in an unsymmetrical 1,3-diol.5 The acid-catalyzed reaction of an unsymmetrically substituted cyclic formal with acetyl chloride described here invariably proceeds via preferential rupture of the less congested C(2)-0 bond to give a product having an acetate at the less congested site... 

Chinese researchers reported a synthetic route to photochrome 16 (Scheme 7), in which the aldehyde functions are then transformed into cyclic acetals and thioacetals, methylol and dicyanoethylene groups (07T5437, 08T2576). Scheme 7 also gives (at the bottom) symmetrical photochrome 17 (where R are ferrocenyl substituents), which does not exhibit fluorescence in the initial state, but shows fluorescence in the cyclic form and which is also synthesized starting from dialdehyde 16 (08AFM302). 

Aldehydes, symmetric and asymmetric ketones, such as formaldehyde, acetaldehyde, substituted benzaldehydes and cyclic ketones, were introduced into the reaction along with acetone. The reaction is reversible azirenoimidazoles undergo reverse transformation forming tnms-aziridinyl ketones in acetic acid. 

Chiral diols have also been prepared starting from meso-compounds [68-71]. Since meso-compounds are, in essence, symmetric molecules, the same applies as for the other symmetric starting materials. Indeed, this is exactly what was found Even though the stereocenters of the protected heptane tetrol are far away from the ester groups that are to be hydrolysed stereoselectively, this is what happens [69, 70]. The high selectivity is partly due to the fact that the secondary alcohol groups are protected as a cyclic acetal, giving additional structural information to the enzyme (Scheme 6.20 A). A cyclic acetal also provides additional structural information in the enantioselective hydrolysis of a pentane tetrol derivative (Scheme 6.20 B) [71]. In both cases Pseudomonas fluorescens lipase (PFL) proved to be the enzyme of choice. 

Trithia-[3]-peristylane 706, a novel Civ symmetric thiabowl, can be synthesized by the reaction of cyclic acetal 705 with LR. The acetal is derived by ozonolysis and subsequent acetalization of bullubalene (Scheme 107) <20040L1617>. 

The total synthesis of the proteasome inhibitor cyclic peptide TMC-95A was accomplished by. S.J. Danishefsky and co-workers. The biaryl linkage in the natural product was constructed by a Suzuki cross-coupling between an aryl iodide and an arylboronic ester derived from L-tyrosine. The required arylboronic pinacolate substrate was prepared using the Miyaura boration. The aryl iodide was exposed to b/s(pinacolato)diboron in the presence of a palladium catalyst and potassium acetate in DMSO. The coupling proceeded in high yield and no symmetrical biaryl by-product was observed. 

Values for F-H and Cl-Fl complexes are from refs. (9) and (2). Only in the case of F-H- -OHj can Qg and be unambiguously distinguished. Values for acetic acid are from ref. (3). The symmetry of the cyclic dimers drawn in Figure 4.4 leads to modes that are symmetric (A) or antisymmetric (B) with respect to the twofold rotation Cj around axis perpendicular to the plane of symmetry and symmetric (g) or antisymmetric (u) with respect to inversion (see ref. (3) for visualisation of these modes). IR bands are of type u, Raman bands of type g. 

Type III Reactions and Other Reactions of Acetals. Treatment of symmetric acetals with alkenols or al-kynols in the presence of TiCU results in transacetalization to give mixed acetals such as (285) which react further via cation (286) to give cyclic products such as (287) in 50-99% yield. In this one-pot procedure the alkenic acetal need not be isolated. Acetoxyalkoxyacetic esters, such as (288), prepared in two steps from methyl glyoxylate and unsaturated alcohols, cyclize under the influence of SnCU in CH2CI2 to give oxacyclic carboxylic esters such as (290). These reactions proceed through the intermediacy of methoxycarbonylcarbonium ion (289). 

A process for the asymmetric cyclopropanation of the enol ethers of cyclic and acyclic ketones has been developed by Tai [109-111]. In this process, a 2-symmetric acetal is isomerized to a hydroxy enol ether which serves as substrate or the Simmons-Smith cyclopropanation, as shown in Scheme 6.29. The stereoselectivity is nearly perfect, but a mechanistic hypothesis has not been proposed. The auxiliary may be removed either by hydrolysis, to give the methyl ketone, or by oxidation of the alcohol and p-elimination [111]. 

When warmed, vinyl ethers react with higher alcohols in the presence of hydrochloric acid but with rearrangement to give symmetrical acetals only.176 Cyclic acetals are formed from ethylene glycol and vinyl ethers at 140-1800.177... 

Acetal Fission in Desymmetrization of 1,2-Diols. The asymmetric desymmetrization of cyclic i eso-l,2-diols was accomplished via diastereoselective acetal fission. After acetalization of ffiei >-l,2-diols with the C2-symmetric bis-sutfoxide ketone, the resulting acetal was subjected to base-promoted acetal fission, followed by acetylation or benzylation to give the desymmetrized diol derivatives. Interestingly, the counter-cation of the base had a remarkable effect on the diastereoselectivity of the reaction. While LHMDS produce the desired compound with low diastereoselectivity, 90% and >96% diastereomeric excesses were obtained... 

This reaction was first reported by Bouveault and Blanc in 1903, and was further extended by Bouveault and Locquin. It is the synthesis of symmetrical a-hydroxy ketones via the reductive condensation of esters in an inert solvent in the presence of sodium. Since symmetrical a-hydroxy ketones, the aliphatic analogs of benzoins, are generally known as acyloins, the formation of a-hydroxy ketones from esters is simply referred to as acyloin condensation. In a few cases, it is also referred to as acyloin reaction." For the individual acyloin, the name is derived by adding the suffix oin to the stem name of corresponding acid, e.g., acetoin prepared from acetate. The most common method used to make acyloin is the reductive condensation of aliphatic esters with sodium in inert solvents, such as ether, xylene or even in liquid NH3 The yield of this reaction can be greatly improved when trimethylchlorosilane presents." " Intromolecular acyloin condensation from aliphatic diesters affords cyclic ketones of different ring sizes. 

As noted in Section 20.6.1.1, the reactions of racemic cyclic allylic esters that generate symmetrical allylic intermediates and reactions of cyclic allylic diacetates in which the two acetate groups are enantiotopic have been developed. Examples of these reactions and catalysts that are most selective for these reactions are described in the next two sections. 

Similarly, but in the cyclic series, the thio-Claisen rearrangement of substrates derived from chiral thiolactams was reported to be facile but poorly stereoselective [129]. The low stereocontrol observed in both cases may be explained by the lack of facial selectivity resulting from the free rotation around the C-N bond of the N,S-ketene acetals. This critical issue has been solved either by constructing a rigid bicyclic framework or by using C2-symmetric amines as chiral inductors. The former strategy, developed by Meyers et al. [45], involved bicycUc thiolactams, which were transformed into N,S-ketene acetals by deprotonation with LDA, followed by S-allylation with various allyl halides (Scheme 9.27). 

Introduction. (l/ ,5i -2//-l,5-Benzodithiepin-3(4/ -one 1,5-dioxide (C2-symmetric his-sulfoxide 1) has been used as a chiral auxiliary for asymmetric desymmetrization of cyclic meso-1,2-diols via diastereoselective acetal cleavage reaction. The procedure consists of three steps (eq 1), that is, acetalization (step 1), acetal cleavage reaction followed by benzylation (step 2), and hydrolysis of the vinyl ether (step 3). Due to the Ca-symmetry of 1, the chiral auxiliary gives only one product in step 1. In addition, no regio- or geometric isomers of the enol ether are formed in step 2. This reagent can be recovered by acid-promoted hydrolysis and reused. 

Lewis acid-mediated addition of silyl ketene acetals to a chiral sulfimine gives precursors of /1-amino acids in fair to excellent deP Mixed diesters of both symmetrical and unsymmetrical diols have been prepared by reaction of carboxylic acids with cyclic ketene acetals of the diols, with the less substituted carbon of the cyclic dioxonium ion intermediate being attacked in most cases. Hydration of trifluoroacetylketene is discussed later under Enolization. 

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