Diols form acetals

Reaction is acid catalyzed Equilibrium constant normally favorable for aide hydes unfavorable for ketones Cyclic acetals from vicinal diols form readily... 

Nucleophilic addition of an alcohol to the carbonyl group initially yields a hydroxy ether called a hemiacetal, analogous to the gem diol formed by addition of water. HcmiacetaJs are formed reversibly, with the equilibrium normally favoring the carbonyl compound. In the presence of acid, however, a further reaction occurs. Protonation of the -OH group, followed by an El-like loss of water, leads to an oxonium ion, R2C=OR+, which undergoes a second nucleophilic addition of alcohol to yield the acetal. The mechanism is shown in Figure 19.12. 

The diol (43) obtained from dihydroxylation of acrolein benzene-1,2-dimethanol acetal (entry 11) is a masked glyceraldebyde and has the potential to be a very useful synthon. Although the enantiomeric purity of the crude diol formed in this reaction is 84% ee, one recrystallization from ethyl acetate improves it to 97% ee in 55% recovery yield. The masked glyceraldehyde 43 is converted via the tosylate 44 to the masked glycidaldehyde 45 in an overall yield of 85%. Both these masked aldehydes are superior to the free aldehydes in terms of handling ease, stability, and safety. The aldehydes can be released from the acetal under the mild conditions of catalytic hydrogenolysis [45]. 

Acetals can serve as protecting groups for 1,2-diols, as well as for aldehydes and ketones. When the acetal is formed from acetone plus the diol, the acetal is called an acetonide. Show the acetonides formed from these diols with acetone under acid catalysis. 

An efficient pinacol rearrangement mediated by trialkyl orthoformate has been developed [83]. The reactions of various types of 1,2-diol with a catalytic amount of SnCU in the presence of trimethyl orthoformate afford the rearranged product in good yields via a cyclic ortho ester intermediate (Eq. 50). This combined system is applicable not only to cyclic and acyclic tri- and tetrasubstituted diols but also to diols forming acid-sensitive acetals. 

Acetals, however, are easily regioselectively introduced (Schemes 3.11 and 3.12) [9]. Benzylidene acetals are preferentially formed as six-membered dioxane-type acetals (i.e., hexopyranosides form 4,6-0-benzylidene derivatives), whereas isopropylidene acetals are more stable as five-membered dioxolane acetals formed on vicinal cw-diols. However, the selectivity is not complete and methyl mannoside, for example, produces mixtures of the mono- and di-acetal compounds. The use of methyl propenyl ether yields mainly the kinetic product (the 4,6-0-isopropylidene acetal). Complementary to these traditional acetals is a rather recently developed acetal — the BDA-acetal [21]. In contrast to other acetals, this exclusively forms acetals between... 

The given conversion from the diacetate form to the diol form is shown below. It leads to sodium acetate. 

Acetalization of 1,2- and 1,3-diols plays an important role in manipulating the reactivity of cyclic and acyclic polyhydroxy compounds. Acetals derived from 1,2- and 1,3-diols are readily accessible via their reactions with ketones or aldehydes in the presence of an acid catalyst. Once they are formed, acetals are very stable to basic conditions but are labile toward acids. 

Diols Only vicinal cw-OH groups of cyclic 1,2-diols readily form acetals. " Acetal exchange is the most common method for preparing isopropylidene acetals (1,3-dioxolanes). 

Diols Both cis- and tran5-l,3-diols form cyclic acetals with aldehydes in the presence of an acid catalyst to furnish the corresponding benzylidene and ethylidene derivatives, respectively. 

The Prins-pinacol rearrangement was utilized during the first enantioselective total synthesis of briarellin diterpenes by L.E. Overman and co-workers. The cyclohexadienyl diol substrate was condensed with a (Z)-a,p-unsaturated aldehyde at low temperature in the presence of catalytic amounts of acid and MgS04 as dehydrating agent. The initially formed acetal was then exposed to 10 mol% of SnCU to afford the desired tetrahydroisobenzofuran as a single stereoisomer that was later converted to briarellin F. 

Niobium. Szymoniak has developed a niobium-based method for the pinacol coupling of aliphatic aldehydes, aromatic aldehydes, and aromatic ketones (Eq. 3.18) [37]. In the presence of NbCl3, intermolecular couplings proceed with consistently high diastereoselectivity. In many cases, the diol forms an acetal with the remaining aldehyde, and this is isolated at the end of the reaction. The stereoselection of... 

Several diols form inclusion compounds.67 The bicyclic one shown (7.10) contains tubular canals that guests can fit into.68 The crystals form inclusion compounds with the vapor of 1,2-dimethoxyethane, 1,2-dichloroethane, 1,4-diox-ane, ethyl acetate, toluene, chlorobenzene, and others. The tetrols (7.11 and 7.12) also form inclusion compounds with various guests. 

Templates containing diols can be reacted with monomers carrying a carbonyl functionality to form acetals or ketals. In the same way, carbonyl containing templates can be derivatized with polymerizable groups by reaction with diol containing monomers. This approach has, for example, been applied to the imprinting of mono- and di-ketones [60-63] and alcohols [64],... 

Diols may also be reacted under similar conditions to form acetals and ketals [54]. Deloxan ASP catalyst at 200 bar and 150 C in the presence of scCOa solvent can catalyze the general reaction shown in Scheme 12.4. 

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