Transacetalization reaction

Usually, the copolymerization is carried out in bulk and the reaction is very rapid, completed in few minutes. Although the comonomer, e.g., ethylene oxide, is preferentially consumed at the beginning, the copolymer obtained has a statistical distribution of removal units, which is due to the occurring transacetalization reaction. 

Comonomer is exhausted at relatively low conversion (20), but a random copolymer is nevertheless obtained. This is because a very facile transacetalization reaction allows for essentially random redistribution of the comonomer units (18) and also results in a polydispersity index near 2.0 (21). 

Some peculiarities of this system have already been described in connection with transacetalization reaction. As indicated by Jaacks 53), TXN-DXL copolymerization cannot be analysed by using the conventional Mayo plot because all propagation reactions are reversible and propagation is accompanied by chain transfer to polymer (transacetalization). Jaacks developed a method which, although based on some simplifying assumptions, gives the reactivity ratio r4. The final equation... 

The chemical structure of the acetal influences the thermal properties of the resin such as glass transition temperature, decomposition temperature and thermal flow stability (32-35). This paper describes the improved thermal properties by increasing the molecular weight via a transacetalization reaction. The polymers containing such crosslinking units were evaluated in two-component positive pWoresists. 

The concept of acid labile crosslinked units in polymers for positive DUV resist was evaluated with poly-(4-hydroxystyrene/4-vinylcyclohexanol) and implemented on a poly-4-hydroxystyrene backbone. The principle to increase the molecular weight by a transacetalization reaction was used to improve acetal protected phenolic resins for advanced chemically amplified resists. The polymer chains were linked via diols with acetal bridges. This type of linkage can be applied to a wide range of phenolic polymer binders. Resolution and thermal stability of the photoresist patterns were improved. Gel permeation chromatography and dissolution rate measurements prove that the crosslinked units remain essentially intact in the unexposed areas, thus enhancing... 

Direct reaction (an initial dehydration of the glucose monohydrate is required at an elevated temperature and high vacuum before the direct reaction), transacetahzation reaction Direct reaction, transacetalization reaction... 

Direct reaction (possible on a continuous basis with lower-molecular-weight alcohols such as mixtures of primary octyl, nonyl, or decyl alcohols), transacetalization reaction... 

Acetalization. Cyclic acetals of pyruvic acid are not easily prepared by using commonly available acetalization methods. Cyclic pyruvic acid acetals of carbohydrates were prepared by DMTST-promoted transacetalization reaction between methyl pyruvate diphenyl dithioacetal and carbohydrate... 

An early example of an ion/molecule reaction carried out in an atmospheric-pressure capillary inlet reactor based on an ESI interface to a quadrupole mass spectrometer involved the study of proton transfer between amines and multiply protonated proteins by Smith (333, 334]. Smith also observed protonated adenosine 5 -monophosphate (AMP) as the product of charge inversion in the reaction of [AMP-H] with multiplyatmospheric pressure 335j. In addition, the formation of TNT Meisenheimer complex (336) and the interaction between piperidine and multiply-charged lysozyme ions generated by electrosonic spray ionization (ESSI) [337] under atmospheric pressure has been reported. Eberlin reactions (i.e., the gas-phase transacetalization reaction) of acylium ions have also been observed to occur at atmospheric pressure under in-source ion/molecule reaction conditions [338]. 

We have developed a catalytic enantioselective synthesis of 0,0-acetals via a transacetalization reaction [24], Transacetalization reactions usually require relatively strong Brpnsted acid catalysts and proceed via oxocarbenium ion intermediates (Scheme 11a). An obvious issue related to the transacetalization reaction is the functional similarity of the starting material and the product. 

Scheme 11 (a) Intermolecular transacetalization reaction (b) proposed bifunctional activation of... 

The chiral phosphoric acid TRIP (3) was found to be an efficient and highly enantioselective catalyst of the intramolecular transacetalization reaction of hydroxyacetals enabling the asymmetric synthesis of acetals with the acetal carbon... 

Although initially developed as a rather exotic reaction, the intramolecular transace-talization reaction can also be a very useful tool in asymmetric synthesis of important chiral compounds and natural products. Both, starting materials and products of the intramolecular transacetalization reaction are protected y-hydroxycarbonyl compounds, orhomoaldols (Scheme 13). Homoaldols are versatile motifs in organic synthesis that can be easily transformed into a vast array of important chiral compounds. However, due to the problematic homoaldol disconnection, these compounds are not readily available in a catalytic asymmetric fashion. 

A highly enantioselective kinetic resolution of protected homoaldols via a catalytic asymmetric transacetalization reaction could be achieved with a novel phosphoric acid STRIP (6) (Table 1) [25]. A catalyst loading of 1 mol% could be routinely used at 20°C, and even 0.1 mol% of the catalyst can give very similar enantiomeric ratios. The method is applicable to the resolution of a wide range of secondary and, most remarkably, of tertiary homoaldols. In most cases, both acyclic homoaldols 7 and cyclic homoaldols 8 could be obtained in enantiomeric ratios exceeding 95 5. Although chemical kinetic resolutions of secondary alcohols by other methods are well developed [26], these are not readily applicable to kinetic resolutions of tertiary alcohols [27-34]. 

The reaction of a linear aliphatic-substituted homoaldol illustrates an interesting feature of this kinetic resolution (Scheme 14). As an additional acetal stereocenter is created in the transacetalization reaction, the high catalyst control of its formation results in ahighly diastereoselective reaction [26, 35, 36]. Thus, even in cases where the enantiodifferentiation of the starting material is not very pronounced, the less reactive enantiomer is converted into the minor frans-diastereomer (Scheme 14). 

Such kinetic resolutions via intramolecular transacetalization reaction represent an atom economic method that does not require any stoichiometric reagents and forms ethanol as the only by-product. This reaction also is the first example of a kinetic resolution of alcohols via acetal formation and the first example of a chiral Brpnsted acid-catalyzed kinetic resolution of alcohols. The utility of the reaction... 

Although 0,0-acetals are among the most common stereocenters in organic molecules, their catalytic asymmetric syntheses have only been achieved recently. Due to problems associated with asymmetric additions to oxocarbenium ions using chiral Brpnsted acid catalysts, intermediates in the synthesis of acetals, design of a suitable reaction was necessary. We have presented the invention of a catalytic asymmetric intramolecular transacetalization reaction for the synthesis of acetals... 

Chen C, Lin Y, Liu C. Catal3dic carbon-sulfur bond formation by amphoteric vanadyl trifiate exploring with thia-Michael addition, thioacetalization, and transacetalization reactions. Tetrahedron 2009 65 10470-10476. 

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