Ketals dialkyl

Methods similar to those used to form and cleave dimethyl acetal and ketal derivatives can be used for other dialkyl acetals and ketals. 

Enol ethers of 17-ketones are formed by pyrolysis of the corresponding dialkyl ketals and enol acetates are readily prepared by the exchange procedure.The latter derivatives are widely used as reactive intermediates for the introduction of substituents at C-16. 

An alternative method for dialkyl peroxide synthesis is the nucleophilic addition of an alkyl hydroperoxide to an alkene under acid catalysis reported by Davies and coworkers (Scheme 31, path B) ". A similar reaction is the nucleophilic addition of alkylhy-droperoxides to vinyl ethers under acid catalysis, producing perketals. Perketals can be deprotected under mild conditions (THF/water/acetic acid) and this hydroperoxide protection-deprotection sequence has been used by Dussault and Porter as a means for the resolution of racemic hydroperoxides (see also Section II.A.2) . In this respect more detailed studies were carried out with the perketals 75, which were prepared via reaction of alkyl hydroperoxides with vinyl ethers (Scheme 33). Weissermel and Lederer reported that in the presence of teri-butyl hypochlorite, a-chlorodialkyl peroxides can be formed in yields between 12% and 45% (Scheme 31, path C)". a-Alkoxydialkyl peroxides and diperoxyacetals were prepared by Rieche and coworkers via acid catalyzed reaction of one or two equivalents of alkyl hydroperoxides with acetals, ketals or aldehydes (Scheme 31, path D)" or by methylation of the corresponding a-alkoxy hydroperoxides with diazomethane (yields 11%, 27%)" . The diperoxyacetals 76 were isolated in yields ranging from 39 to 77%. 

Reaction of 2,2-dichlorocyclopropanone ketals with potassium /-butoxide to form dialkyl /-butyl orthopropiolates [176]. 

An alternative method of hydroboration is to use diisopinocampheylborane (12) (Scheme 4). This reaction is particularly useful for sterically hindered alkenes. Diisopinocampheylborane (12) is prepared from borane-dimethyl sulfide and (+)-pinene.[23-24] Treatment of 4-meth-ylenecyclohexanone ethylene ketal with diisopinocampheylborane (12) gives the borane 13.[25] Further treatment with 2 equivalents of an aldehyde results in the elimination of pinene and the formation of a new dialkyl boronate, e.g. treatment of 13 with acetaldehyde gives the diethyl cyclohexylmethylboronate 14J261 The dialkyl boronates thus produced can be transesterified with pinanediol to give 15[26] or with other cyclic diols. 

Explain why general acid catalysis is found in the hydrolysis of tropone diethyl-ketal (1), despite the fact that hydrolysis of dialkyl ketals ordinarily shows only specific hydronium ion catalysis. 

In analogy with Eq. (79), HFA forms unstable hemiketals 93a which are converted into stable ketals 93b with diazomethane (163) or dialkyl sulfates (185). The ketals 93b are almost stable in 2 M HCl, only slight decomposition occurring. 

Reaction with chiral acetals. The chiral ketals derived from (2R,4R)-(-)-2,4-pentanediol (1) can be cleaved with high diastercoselectivity by aluminum hydride reagents, in particular DIBAH, CI2AIH, and Br,AlH. Oxidative removal of the chiral auxiliary affords optically active alcohols. This process provides a useful method for highly asymmetric reduction of dialkyl ketones. ... 

Ethylene ketals. The exchange reaction between dialkyl acetals or dialkyl ketals and 1,2-glycols as a route to cyclic acetals and ketals was first described by Delepine. This method in combination with Claisen s orthoester ketalization procedure (1, 1206) constitutes a convenient route to ethylene ketals (1, 376). The mixed orthoester 1 is probably the actual reagent involved in the Delepine method. It is convenient to prepare and use for ketalization of carbonyl compounds under mild conditions. A striking example is the ready conversion of the acid-sensitive 2 into 3, a reaction that proceeds by other known methods in yields of only 30%. 

There is no reason to doubt that the sequence of steps in the mechanism of hydrolysis of the ring compounds is the same as in the hydrolyses of dialkyl acetals and ketals. However, it must be expected that carbonium ion formation in the second step is reversible for the reactions of the ring compounds because the intramolecular return step (ring-closure) can successfully compete with the attack of water in the third step, i.e. 

Pyruvate ketals can be synthesized [161] by direct condensation of a pyruvate ester with a diol in the presence of a Lewis acid, but this is less preferred because of the electron-withdrawing effect of the adjacent carboxylate group [162,163]. Therefore, several indirect methods for the acetalization have been introduced including condensation with pyruvate derivatives [164,165] or generation of the carboxylate group by oxidation of a suitable precursor [166,167,168,169]. A more efficient route to pyruvic acid acetals starts from silylated diols [170] or by the reaction between diols and methyl pyruvate dialkyl dithioacetal [171,172] activated by methyl triflate, dimethyl(methylthio)sulfonium trifluoromethane sulfonate (DMTST), nitroso tetraflu-oroborate (NOBF4), S02Cl2-trifluoromethanesulfonic acid, or Al-Iodosuccinimide (NIS) and trifluoromethanesulfonic acid [173] (O Scheme 24). 

Dialkyl acetals and ketals can easily be formed from carbonyl compounds with alcohols under acidic conditions. Some representative examples for the great variety of methods available for this transformation are given in Scheme 77. As is demonstrated, both simple alcohols themselves or formic acid ortho esters can be used for acetal formation in the presence of hydrochloric acid, toluenesulfonic acid °° or activated alumina (Montmorrilonite clay K-10). ° Owing to different carbonyl reactivities, regio- and chemo-selective differentiation is often realizable, as has been shown, for example, on androstane-3,17-dione (78). Acid-catalyzed acetalization selectively delivers the 3-ketal, whereas the sterically hindered 17-carbonyl function remains unaffected. Under neutral conditions the reactions are promoted by cata-... 

The possibility that olefin-carbon monoxide copolymers can exist in the isomeric poly-spiro ketal structure (Scheme 8.5) was recognized soon after the first synthesis of the copolymers of propene. Using dialkyl diphosphines as the modifying ligands [27], copolymers were obtained with blocks with the spiroketal form [28]. 

The preparation of acetals of the unknown formylphosphonic acid was reported by treating a dialkyl hydrogenphosphonate or a dialkyl trimethylsilyl phosphite wth trialkyl orthoformate in the presence of boron trifluoride. Another method which is reported to yield acylphosphonate ketals is the addition of dialkyl hydrogenphosphonates to ketene acetals (equation 55). For dithioketals, see Section II. C. 4. b. 

Ketals 201 from cyclopentenone and dialkyl tartrates have a rigid cyclopentene ring. Submitted to 2 -i- 2 photocycloaddition with a cyc-lohexenone and a cyclopentenone carboxylic ester, diastereoselectivities of up to 84% were obtained. The selectivity was very sensitive to the steric hindrance of the chiral auxiliary and tartrate derivatives gave better results than the corresponding threitoldibenzylethers [162]. When the chiral auxiliary was introduced into an acylic enone system (204), the diastereoselec-tivity of the cycloaddition was low. This indicates that the chiral alkene in the ground state exists as a mixture of conformers leading to opposite facial diastereoselection [163]. 

Easily accessible acetals and ketals of a,p-unsaturated aldehydes and ketones derived fi-om C2-symmetric chiral 1,2-diols have been successfully used with Simmons-Smith reagents furnishing cyclopropane aldehydes with high selectivity and recovery of the auxiliary. Thus, dialkyl tartrates proved to be superior compared to 1,2-diphenyl-ethanediols as chiral auxiliaries in reactions of a,p-unsaturated aldehydes. 

Benzil dialkyl ketals are also very efficient photoinitiators. They too are believed to decompose by the Norrish Type I cleavage ... 

BFj-etherate added to a soln. of cyclohexanone ethylene ketal in methylene chloride at —78° under N2, the mixture treated dropwise with a soln. of l,2-bis(trimethyl-siloxy)cyclopentene in the same solvent, and the stirred mixture allowed to warm to room temp, overnight product. Y 89%. The method allows efficient preparation of 2,2-dialkylated 1,3-cyclohexanediones and is a useful alternative to routine double alkylation procedures. F.e. and from dialkyl ketals s. Y.-J. Wu, D.J. Burnell, Tetrahedron Letters 30, 1021 (1989). 

The world s most popular method of PVC polymerization is the suspension method. Around 80% of PVC is produced this way. The difference in this method is that it uses initiators soluble in the monomer. They are dialkyl and diacyl peroxides, ketone peroxides, peroxo-dicarbonates, peroxo-ketals, alkyl peresters or azo compoimds. Seldom is the role of emulsifiers played by alkalies or buffers in order to improve the plasticizer adsorption in PVC. In this process, in order to obtain proper porosity and particle granulation, so-called suspension stabilizers are used, which are derivatives of meth-ylhydroxypropyl cellulose, karboxymethyl cellulose and poly(vinyl alcohols). PVC obtained this way is of high purity. Its molecular mass depends on the temperature of polymerization. Other parameters depend on the interfacial tension at the water-monomer interface. 

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