Catalysts acetalation

Oxidative decarboxylation Cupric acetale(catalyst). Lead dioxide. Lead tetraacetate. 

Acetalation. As polyhydroxy compounds, carbohydrates react with aldehydes and ketones to form cycHc acetals (1,13). Examples are the reaction of D-glucose with acetone and a protic or Lewis acid catalyst to form l,2 5,6-di-0-isoprop5lidene-a-D-glucofuranose [582-52-5] and its reaction with benzaldehyde to form 4,6-0-benzyhdene-D-glucopyranose [25152-90-3]. The 4,6-0-(l-carboxyethyhdine) group (related to pymvic acid) occurs naturally in some polysaccharides. 

The search for new catalysts that would be effective for acetalations still stimulates interest. Better yields, and increased rates, have been claimed for the reaction of monosaccharides with acetone in the presence of ferric chloride.48 (Ferric chloride was already known to catalyze acetalation in other series.47) Not yet applied in the carbohydrate field, but potentially useful for sugars, is the separate use of two different catalysts. The first one recommended is pyridinium p-toluenesul-fonate as a mild and efficient catalyst for the tetrahydropyranylation of alcohols.48 The main interest in this catalyst lies in the excellent yields... 

Important in this quite general strategy is that, for practically all instances, die reaction is under thermodynamic control, and the control of the stoichiometry is extremely difficult It follows that only the more stable acetals are produced (see Sec. H.B) and usually multiacetals are obtained if several hydroxyl groups are available within die same molecule. This has been a major concern in acetalation reactions in neutral conditions. For instance, use of copper(II)sulfate either in acetone alone or in N, N-dunethylformamide without any additional catalyst leads to acetals with structures that differ from those resulting from reactions in the presence of an acid The reaction depends on the temperature [31] however, the strict neutrality of a medium in which copper(II)sulfate and polyols are interacting can be questioned. 

Once again, efforts have been made to find neutral conditions that can modulate the course of the reaction. For instance, use of 2,2-diincthoxypropane in solution in 1,2,-dimethoxyethane (which probably plays a role through its interaction with polyols) has been suggested as a reagent for acetalation in neutral conditions (no catalyst) of D-mannitol [42] and o-glucitol [43],... 

It has been claimed that acetalation of25 with acetone-sulfuric acid (or other dehydrating agent) in the presence of ultrasonic waves lessens the time of reaction (to <70 min), and 26 may be isolated in 76% yield.109 In addition, several reports have been published on the use of other acid catalysts, namely, zinc chloride-phosphoric acid,110... 

The copolymerisation of ethylene oxide and phenyl isocyanate has been found [266] to proceed in the presence of the triethylaluminium-water (2 1) catalyst, although phenyl isocyanate alone could not be polymerised by the same catalyst. The copolymer formed was characterised by an alternating comonomer distribution [scheme (39)] and contained acetalic units in its chains (Table 9.4) ... 

There are a few reports dealing with the coordination polymerisation of acetone in the presence of organometallic catalysts. Such a situation results from long-term instability of acetone polymers [283]. The polymerisation of acetone with metal alkyls such as triethylaluminium as catalysts has been reported [284,285] to produce polymers containing acetalic units (Table 9.3), which was confirmed by IR spectroscopy. 

Interestingly, the titanium(IV) isopropoxide catalyzed process can still be highly diastereoselec-tivc28. In certain cases with more Lewis acidic catalysts (Cl, TiO-/-Pr or EtAlCl2) even acetalic oxygen in a homoallylic position can induce stereospecificity22. 

The mechanism of the catalysis in the reaiTangements of 4-acetoxyhepta-2,5-dienes was investigated by using the O-labeled acetate 362 in the presence of Pd and Pd catalysts. It was shown by NMR spectroscopy that the reaction catalyzed by Pd affords a 1 1 mixture of the dienes 363 and 364 which results from the Pd-coordinated pentadienyl species interaiediate and O-acetale (equation 132). By using two Pd -catalysts, viz. (RCNiaPdCh (R = Me, Ph) [Pd"(l) and (Ph3P)4Pd [i.e. Pd"(2), two rearrangement products 365 and 366, respectively, were obtained. The heterocyclic 1,3-dioxanium cations 367 and 368 were assumed to be intermediates of these isomerizations (equation 133). 

Acetals are constructed using a pair of oxygens from hydroxyl groups. In the acetalation of a sugar derivative which has more than two free hydroxyl groups, several acetals are, therefore, possible. Most of the time, an acid catalyst is used to accelerate the equilibrium between possible acetals, and the most stable one is finally isolated as the major product. 

NBS is an effective catalyst for the acetalation of alcohols under mild conditions 4 4 aldehydes are converted to 1,1-diacetates by reaction of acetic anhydride with NBS as a catalyst. ... 

In this context, a one-pot regioselective protection of persilylated monosaccharides, using copper(ii) triflate as a single catalyst was able to promote acetalation-reductive etherification-acylation reactions. In the same way, the easy to handle, cheap and environment-friendly FeCls 6H2O complex was a very efficient catalyst to promote regioselective acetalation and reductive etherification on glucopyranosides. Hence, treatment of per-O-silylated a-methyl-u-glucopyranoside 50 with benzaldehyde (3 equiv) and triethylsilane (1.1 equiv) in the presence of... 

They also demonstrated that inexpensive molecular iodine can be used as a cheap, nontoxic, general, and fast catalyst for one-pot tandem acetalation-esteri-fication reactions of glycosides in good to excellent yields without the need of purification after every reaction step. Further, the addition of catalytic DMAP can be used to accelerate the esterification step and thus shorten the reaction times. The method is mild and compatible with different thioglycosides and 0-glycosides, applicable to the formation of 4,6-0-benzylidene and 4,6-0-p-methoxybenzylidene acetals in tandem reaction with either 2,3-0-di-acetate or 2,3-0-di-benzoate esters and also amenable to commonly used amino-protecting groups (e.g., phthalimides and 2,2,2-trichloroethoxycarbonyl chloride). 

Another important modification of poly(vinyl acetate) is its derivatization with aldehydes to poly(vinyl acetal)s [269]. This can be accomplished by first hydrolyzing the poly(vinyl acetate) to poly(vinyl alcohol) and then carrying out a subsequent acetalation reaction with an aldehyde and a strong mineral acid in water. Alternatively, poly(vinyl acetate) can be converted in a single one-pot reaction with acetic acid as a solvent directly to the poly(vinyl acetal) by reaction with water, an aldehyde, and a mineral acid catalyst. 

Diethyl chloroacetal refluxed with an equimolar amount of neopentyl glycol in the presence of 1 mole-% p-toluenesulfonic acid under a fractionating column with distillation of the resulting ethanol, benzene added when most of the ethanol has been removed after ca. 2 hrs., the remaining ethanol removed as benzene azeotrope, the catalyst destroyed with solid K-carbonate, and the product distilled 2-chloromethyl-5,6-dimethyl-m-dioxane. Y 95%. Numerous e., mostly m-dioxanes and other cyclic acetals, also direct acetalation of 0x0 compounds, e. g. acrolein, s. C. S. Rondestvedt, Jr., J. Org. Chem. 26, 2247 (1961) transaeetalation s. a. S. J. Angyal and R. M. Hoskinson, Soc. 1962, 2985 N. B.Lorette and W. L. Howard, Org. Synth. A2, 1 (1962). 

---