Of cyclic acetals

Catechols can be protected as diethers or diesters by methods that have been described to protect phenols. However, formation of cyclic acetals and ketals (e.g., methylenedioxy, acetonide, cyclohexylidenedioxy, diphenylmethylenedioxy derivatives) or cyclic esters (e.g., borates or carbonates) selectively protects the two adjacent hydroxyl groups in the presence of isolated phenol groups. 

Reactions of cyclic acetals with organosilicon compounds 97MI42. 

Transfomations of cyclic acetals and their heteroanalogs under the action of organosilicon reagents 97MI1. 

In practice, it s convenient to use 1 equivalent of a diol such as ethylene glycol as the alcohol and to form a cyclic acetal. The mechanism of cyclic acetal formation using ethylene glycol is exactly the same as that using 2 equivalents of methanol or other monoalcohol. The only difference is that both —OH groups are in the same molecule. 

In connection with studies on the ring-opening polymerization of cyclic acetals, we have undertaken investigations on the polymerization of bicyclic acetals, bicyclic oxalactone, and bicyclic oxalactam, which yield polysaccharide analogs, macrocyclic oligoesters, and a hydrophilic polyamide, respectively, some of which can be expected to be useful as novel speciality polymers. The monomers employed in the studies were prepared via synthetic routes presented in Scheme 1, starting from 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer) I. 

Cationic polymerization of cyclic acetals generally involves equilibrium between monomer and polymer. The equilibrium nature of the cationic polymerization of 2 was ascertained by depolymerization experiments Methylene chloride solutions of the polymer ([P]0 = 1.76 and 1.71 base-mol/1) containing a catalytic amount of boron trifluoride etherate were allowed to stand for several days at 0 °C to give 2 which was in equilibrium with its polymer. The equilibrium concentrations ([M]e = 0.47 and 0.46 mol/1) were in excellent agreement with that found in the polymerization experiments under the same conditions. The thermodynamic parameters for the polymerization of 1 were evaluated from the temperature dependence of the equilibrium monomer concentrations between -20 and 30 °C. 

Formation of cyclic acetals reaction between diketones and acids... 

The formation of cyclic acetals has also been reported by Hostettler for a series of 3-substituted 2,2,4,4-tetramethylcyclobutanones,<28)... 

Compound 9, which Is not detectably rehydrated by water or attacked by simple alcohols, Is also converted In high yield to cyclic acetals H at 70 C, pointing again to the significant thermodynamic driving force of cyclic acetal formation. This strongly favored acetal reaction Is one explanation for the excellent adhesion performance of this system on cellulosics and glass. 

This electrochemical oxidation mediated by NHPI was applicable to benzylic carbons, allylic carbons, deprotection of acetals, oxidative cleavage of cyclic acetals and amide to afford benzoylated compounds, enones ", carbonyl compounds, -hydroxyethyl esters and imides, respectively (equations 31-35). 

This ketal protective group is resistant to basic and nucleophilic reagents but is readily removed by aqueous acid. Formaldehyde, acetaldehyde, and benzaldehyde can also used as the carbonyl component in the formation of cyclic acetals. They function in the same manner as acetone. A disadvantage in the case of acetaldehyde and benzaldehyde is the possibility of forming a mixture of diastereomers, because of the new stereogenic center at the acetal carbon. 

We developed a method for the synthesis of a variety of cyclic acetals that utilizes bismuth triflate as a catalyst and does not require the use of a Dean-Stark trap for removal of water [102]. In this method, an aldehyde or ketone is treated with 1,2-bis (trimethylsiloxy)ethane in the presence of bismuth triflate. A comparison study using o-chlorobenzaldehyde showed that with ethylene glycol a low conversion to the dioxolane was observed after 2 h whereas the use of the 1,2-bis(trimethylsiloxy) ethane afforded the corresponding dioxolane in good yields. (Scheme 9). 

Krabbe SW, Spafford MJ, Mohan RS (2010) Bismuth(in) triflate catalyzed allylation of cyclic acetals and dithianes followed by in situ derivatization to generate highly functionalized esters. Org Prep Proced Int 42 363-371... 

Cyclic acetals contain at least one 1,1-dialkoxy grouping [i.e., (RO)2CH2 or (RO)2CHR] as part of a cyclic structure. A variety of cyclic acetals undergo facile cationic polymerization [Kubisa, 1996 Penczek and Kubisa, 1977, 1989b Schulz et al., 1984]. This includes various... 

Penczek, S. and P. Kubisa, Progress in Polymerization of Cyclic Acetals, Chap. 5 in Ring-Opening Polymerization, T. Saegusa and E. Goethals, eds., American Chemical Society, Washington, DC, 1977. 

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

The groups of Burczyk, Takeda, and others have made thorough studies of cyclic acetals, such as 1,3-dioxolane (five-membered ring) and 1,3-dioxane (six-membered ring) compounds, illustrated in Fig. 13. They are typically synthesized from a long-chain aldehyde by reaction with a diol or a higher polyol. Reaction with a vicinal diol gives the dioxolane [40-42] and 1,3-diols yield dioxanes [43,44]. 

Trace amounts of cyclic acetals are produced from the [2+2] dimer of cyclooctatetraene on ozonolysis in a 2 1 mixture of dichloromethane and methanol followed by stirring with Amberlyst 15 resin (Equation 39) <2003TL9313>. 

Table 15. AS Tor Three Diastereomeric Pairs of Cyclic Acetals of 3-Methylcyclohexanone (13CNMR, 100 MHz, CDC13)66...

The 6,7-dihydro-5/f -1,4-dioxepin (266) has been prepared (54CR(38)982). and more recently it has been shown that the 2,3-dihydro-5jF/-l,4-dioxepins (263) and (265) can be produced from 1,4-dioxine-halocarbene adducts (264), either by heating under reflux in xylene or by treatment with bases. The allylic chlorine atom in (263) is readily substituted by alkoxide or cyanide ions (77ZC331, 76UKZ968). Saturated rings of type (267) have been prepared by the treatment of cyclic acetals of ethane-1,2-diol with vinyl ethers in the presence of boron trifluoride, and l,4-dioxepan-5-one (268) has been prepared by the reaction of bromoform and silver nitrate with aqueous dioxane (60AG415). 

Cyclic Acetals. One of the most significant developments in the chemistry of sucrose was the synthesis of cyclic acetals which, despite many attempts, were not synthesized until 1974. The first synthesis of 4,6-O-benzylidenesucrose was achieved from the reaction of sucrose with Ot, Ot-dibromotoluene in pyridine (29). Since then, many new acetalating reagents have been used to give a variety of sucrose acetals, generally by transacetalation reactions. 

In this Section, hydrolysis, acetolysis, and isomerization of acetals are considered. Selective deprotection of acetals may also be achieved through halogenation, hydrogenolysis, ozonolysis, and photolysis, but these topics, are covered in an accompanying article in this Series, on the reactivity of cyclic acetals of aldoses and aldosides,3 and will not be discussed here. 

Of all the selective, deprotection procedures that are available to carbohydrate chemists, the partial hydrolysis of polyacetals is probably the most familiar. Articles by de Beider4,5 and Brady6 contained examples of this type of reaction for aldose and ketose derivatives, respectively, and an article by Barker and Bourne7 gave useful information from the early literature on the graded, acid hydrolysis of acetal derivatives of polyols. A discussion of the stereochemistry of cyclic acetals of carbohydrates was included in an article by Mills 70 and in one by Ferrier and Overend,76 and a survey of the formation and migration of carbohydrate cyclic acetals was made by Clode.7c... 

With due consideration of the explanations just presented for the observed, relative stabilities of cyclic acetals derived from polyols, in terms of their constitution and conformation, nearly all of the following observations on the selective hydrolysis of cyclic acetals of alditols and dialkyl dithioacetals may be readily understood. 

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