Transacetalization

The activation energies were computed to 3.0 (toward 183), 0.3 (toward 182), and 21.8 kcal/mol (toward 184) at the B3-LYP/6-31G level, and thus the mechanism leading to 182 is the preferred one. The transition states of all three reactions belong to concerted but asynchronous reaction paths. The transacetalization of 177 with acylium cations results in the formation of the thermodynamically stabilized 187 (Scheme 121) [97JCS(P2)2105]. 186 is less stable than 187, and 185 is destabilized by 32.5 kcal/mol. Moreover, transacetalization of 177 with sulfinyl cations is not a general reaction. Further computational studies on dioxanes cover electrophilic additions to methylenedioxanes [98JCS(P2)1129] and the influence... 

Acetals of A. A -dimethyl hydrazones 4, derived from glyoxal (1) and various enantiomerically pure diols (c.g., 3a-f)24, are readily prepared by transacetalization of 2. Addition of methyl-or butyllithium to 4 provides the corresponding hydrazines in good yield (70-88%) and moderate to excellent diastereoselectivity (see Table 3)4,5. 

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

For another example of the strong dnality in the chemical behavior of distonic cation-radicals, see Moraes and Eberlin (1998). In the gaseons phase, m- and /7-dehydrobenzoyl cation-radicals ( CgH4C =0) react selectively either as free radicals or as acylium ions, depending on the choice of the reacting partner. Transacetalization with 2-methyl-l,3-dioxolane, ketalization with 2-methoxy-ethanol and epoxide ring expansion with epichlorohydrin demonstrate their acylium reactivity. 

The reaction of aldehydes with alcohols to form acetals is rapid and reversible, and both the rate and the position of acetal-aldehyde equilibria can be affected by the pH of the reactant solution [30-32], Thus far, however, relatively few studies have made use of transacetalization as... 

Dynamic transacetalization experiments targeting cyclophane formation have also been described by Mandolini and coworkers [34]. Production of a cyclic polyether DCL by direct reaction of triethylene glycol and 4-nitrobenzaldehyde has been reported by Berkovich-Berger and Lemcoff amplification of small macrocyclic members of the library by ammonium ion was observed [35]. With these few examples demonstrating feasibility, we can anticipate increased use of transacetalization in future DCC efforts. 

The attempted transacetalization of the dimethyl acetal 135 a was partially successful only when it was boiled in excess ethanol in the presence of boron trifluoride etherate. The formation of the corresponding diethyl acetal 137 indicates that dialkyl acetals of dispiro [2.0.2. l]heptan-7-one can, in principle, enter reactions without ring opening and thus be applied as a synthetic equivalent of [3]triangulanone (Scheme 30) [125]. 

Both end groups can be determined quantitatively. A second side reaction is the transacetalization. Here a poly(oxymethylene) cation attacks an oxygen of a poly(oxymethylene) chain with formation of an oxonium ion that decomposes. Through continued cleavage and recombination of poly(oxymethylene) chains one obtains polymers which are chemically and molecularly largely homogeneous. For the case of a trioxane/ethylene oxide copolymer the following reaction scheme can be formulated ... 

The transacetalization also proceeds intramolecularly. It then leads to the formation of cyclic acetals which participate as monomers in the propagation reaction. 

The synthesis of 1,3-dioxanes and congeners by transformation of a ring of the same size is not highly developed. Only a few examples of such reactions, typically transacetalizations, have been reported. An important issue of 1,3-dithiane formation, namely the stench of the 1,3-propanedithiol, has been addressed by Liu and co-workers. They found that... 

Another interesting example of a transacetalization is the tetramerization of 1,3-dioxane 228 to the macrocycle 229 (Equation 92). This cyclooligomerization was promoted by dry HCl in Et20 and the tetrameric product was formed in 59% yield <20000L4125>. 

New reaction conditions for the odorless transacetalization providing 1,3-dithianes from aldehydes or ketones, as depicted in Scheme 108 (Section 8.11.10.1), have been published <2006S3801, 2007SC993>. 

Resin-bound aldehydes and ketones have been used as linkers for 1,2- and 1,3-diols (Entries 4-6, Table 3.33). Cleavage of acetal-based linkers is usually effected by acid-catalyzed transacetalization or by hydrolysis. 

Support-bound alcohols and thiols can be used to immobilize aldehydes and ketones as acetals. Mixed acetals of carbonyl compounds with support-bound alcohols can be prepared by transacetalization of a symmetric acetal under acidic conditions [719]. The formation of mixed acetals on solid phase is, however, not always easy to perform and control, and so prior preparation of a mixed acetal in solution followed by loading onto a support is often the preferred protocol [626,637]. Carbohydrates can be linked to resin-bound alcohols or thiols as glycosides (Table 3.40). 

Acetals are usually easy to cleave by acid-catalyzed transacetalization or hydrolysis (Table 3.40). Dithioacetals, on the other hand, tend to be more resistant to hydrolysis, but cleavage can be achieved by treatment with mercury(II) salts or by oxidation with either [bis(trifluoroacetoxy)iodo]benzene [723] or periodic acid [724]. Use of the latter reagent can, however, also lead to the conversion of methyl ketones into iodo-methyl ketones [721],... 

Preparation.l Dialkyl acetals of DMF are generally prepared by transacetal-ization of the dimethyl or diethyl acetal, but this reaction is not useful in the case of the di-r-butyl acetal. Replacement of one methoxy group of DMF dimethyl acetal occurs on refluxing in f-butyl alcohol conversion to the dw-butyl acetal is effected in the presence of 2,4,6-tri-f-butylphenol (equation I). 

Conducting the first two steps as a one-pit reaction10 was first suggested by Sharp less. Transacetalization of trimethyl orthoacetate with 10 under weakly acidic catalysis leads to the 2-methoxy-l,3-dioxolane 34. Addition of TMSC1 or acetyl chloride generates the 1,3-dioxolanylium cation 35, which is opened nucleophilically and regioselectively at the sterically least hindered position to chloro-acetate 36... 

An unsuccessful attempt was next made to simplify the problem of purifying the product by using dioxane as the extracting solvent with only enough benzaldehyde for solvolysis. Finally, on the assumption that an acetal should be as effective in transacetalization as an aldehyde or ketone, the benzaldehyde was replaced by 2,2-dimethoxypropane. In several experiments the hydrogen chloride was replaced by -toluenesulfonic acid also, dimethyl sulfoxide was tried instead of dioxane. All these experiments are summarized in Table I, and they lead to the following conclusions ... 

Possibilities include (a) the hydrolysis of the lignin-carbohydrate bond by small amounts of water present in the wood despite all precautions to make the system anhydrous, (b) the dehydration of carbohydrates under these conditions to produce water and hence hydrolytic conditions, (c) transacetalization of the extracting acetal with carbohydrates to liberate alcohol and create conditions of alcoholysis, and (d) transacetalization of the extracting acetal with free or bound keto groups in the wood to liberate acetone and create conditions of acetonolysis. 

Siloxyallyl)silane 24, acting as a synthetic equivalent of acetone a,a -dianion, readily undergoes double alkylation (equation 15)60. An acid-catalyzed transacetalization-ring closure reaction occurs in the reaction of 6-hydroxy substituted allylsilanes 25 with acetals to afford the corresponding trisubstituted tetrahydropyrans in moderate to good yield with high diastereoselectivity (equation 16)61. 

Only a few reports have dealt with this type of synthesis of the 1,3-dioxepin and 1,3-dithiepin moiety. Dithiepane 148 could be obtained by an intramolecular transacetalization of mercaptane 147 (Scheme 40) <2000T10101>. 

Higher yields of acetonides have been obtained by transacetalization of a 1,4-diol unit with dimethoxypropane, as demonstrated with the protection of 193, an intermediate in the synthesis of a cytotoxic bisabolane sesquiterpene (+)-curcuphenol (Scheme 54) <1998TA2215>. 

Optically active dithiepins 58 bearing substituents in the 2-position of the dithiepin moiety have been prepared by BF3 Et20 catalyzed transacetalization of 197 (Scheme 55) <1991JOC4467, 2002FI(57)1487>, and even sterically more demanding biaryl-derived dithiepins, such as 60, 62, and 201, have been successfully prepared by using this method (Scheme 56) ... 

Dialdehyde 207 was prepared via acid-catalyzed transacetalization of 1,1-dimethoxycyclohexane and 205 (Scheme 58) <2003SM(135)209>. 

Dithioacetals have also been used as precursors for the transacetalization. Thus, both diastereomers 213 and 214 have been prepared from glucose-derived dithioacetal 212 and rac-230 and used for the efficient resolution of rac-Y)l (Scheme 60) <1995TA295>. 

Synthesis of acetonides can also be performed using 2-methoxypropene instead of 2,2-dimethoxypropane <20050L5011> however, 4,5-dihydro-l,3-dioxepins, such as 216 (Scheme 61), can only be obtained by direct acetalization or transacetalization with special substrates. 

However, it has turned out that there are a few synthetic methods having broad scope, that is, (a) for the synthesis of dioxepins and dithiepins (i) transacetalization of the corresponding diol or dithiol, respectively, with an open chain acetal (ii) treatment of the corresponding diol or dithiol with sodium borohydride and reaction of the resulting dianion with a gem-dihalogen derivative. The latter method is particularly useful for the synthesis of dioxepins and dithiepins unsubstituted in the 2-position (b) for the synthesis of 4,5-dihydro-l,3-dioxepins (i) metal- or base-catalyzed double-bond isomerization and (ii) Heck vinylation or arylation, respectively, for the synthesis of 6-substituted 4,5-dihydro-l,3-dioxepins. 

Using a binary sensitizing system (phenanthrene P/DCNB /7-dicyanobenzene) in acetonitrile solution, O-aryl glycosides are transacetalized with alcohols after generation of aromatic radical cations [23], According to kinetic anomeric effects, the a-side attack of nucleophiles to cyclic oxocarbenium ions follows scheme 9. 

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