Cyclic acetal, hydrolysis

Acid-catalysed hydrolysis is the most common method for the deprotection of acetals and a mechanism for the reaction was given in section 1.2.2. Extensive kinetic and thermodynamic studies on cyclic acetal hydrolysis (see reviews in section 2.9.4) allow several conclusions to be drawn12-15 ... 

Figure 6.38 Examination of the slower rates of hydrolysis of cyclic acetals compared with their acyclic counterparts (a) anti-ALPH nature of dioxolane hydrolysis p ° (b) two systems with identical structure-reactivity para-meters ° (c) recombination in a dioxolane hydrolysis (d) direct measurement of the relative rates of hydrolysis and recyclisation of the acyclic oxocarbenium ion intermediate in cyclic acetal hydrolysis.

The order of reactivity toward hydrolysis of the cyclic acetals shown below is A -c B [Pg.504]

The hydrolysis of the cyclic acetal, which was used as the connecting group between the polymer chain and the lipid, was confirmed both by the IR and the proton NMR spectra of the lipid recovered from the vesicular system after standing for 3 weeks at room temperature. The lactone absorption at 1805 cm-1 disappeared from the IR spectrum (Figure 6) as the result of hydrolysis. Furthermore, a new aldehyde absorption band at 1705 cm 1 was observed in the spectrum, which is related to the substituted benzaldehyde group of the hydrolyzed product. The proton NMR spectrum (Figure 10) also clearly showed the formation of the benzaldehyde, as indicated by the peak at 810.20 ppm. 

The reactivity of the dibromo compounds 72 and 74b was also explored, and it was shown that reaction with ethylene glycol gave the corresponding cyclic acetals 76, while reaction of compound 72 with hydroxylamine furnished the fervenulin-3-carbaldoxime 77, which was found to be resistant to hydrolysis. 

Laureacetal-B (70) is a tricyclic oxetane recently isolated from a marine alga. It has the interesting features of bromine substitution and a cyclic acetal structure. In view of the extreme ease of hydrolysis of 2-methoxyoxetane, it seems surprising that a strained ring cyclic acetal would be isolated from a marine source. Its structure assignment is based on spectral studies and chemical transformations (79CL301). 

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. 

It is intended that the present Chapter shall be complementary to Haines s article in this Volume8 that deals with the selective deprotection of protected sugars. The reader interested in all aspects of the hydrolysis, the alcoholysis, the acetolysis, and the isomerization of cyclic acetals pf sugars should refer to Haines s article. A review has also been published concerning the formation and migration of cyclic acetals of carbohydrates.80... 

The formation and the hydrolysis of acyclic and cyclic acetals have been studied in rather great detail [91]. Several reviews on this topic are available [92] and some comments have been made [13] concerning the carbohydrate series. We have shown in Schemes 1,2, and 3 that a common feature of this reaction seems to be the intermediacy of an oxocarbenium ion. However, the cyclization of such an intermediate has been questioned more recently [93] in the light of the Baldwin s rules for ring closure [94]. At least for the five-membered ring, an SN2-type displacement mechanism far the protonated form (B) of die hemiacetal (A) (favorable 5-exo-tet cyclization) has been proposed rather than the unfavorable 5-endo-trig cyclization of the oxocarbenium ion (C) (Scheme 5). Except when the formation of the enol ether (D) is structurally impossible, the intermediacy of such a compound remains feasible. 

Cyclic acetals Remain unchanged ethylidene acetal45 Hydrolysis followed by oxidation... 

Compound A is a cyclic acetal and undergoes hydrolysis in aqueous acid to produce acetaldehyde, along with a dihydroxy carboxylic acid. 

An example of a simple catalyzed chemical reaction is the hydrolysis of cyclic acetal (4.1) to lactol (4.3) (Scheme 4.1).This reaction is reversible. To a rough approximation, AG... 

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