Cychc acetals

We can either protect the two hydroxyl groups in A as a cychc acetal or use four mols of PhMgBr and waste two of them. 

Acetals are readily formed with alcohols and cycHc acetals with 1,2 and 1,3-diols (19). Furfural reacts with poly(vinyl alcohol) under acid catalysis to effect acetalization of the hydroxyl groups (20,21). Reaction with acetic anhydride under appropriate conditions gives the acylal, furfuryUdene diacetate... 

Furan can be catalyticaHy oxidized in the vapor phase with oxygen-containing gases to maleic anhydride (93). Oxidation with bromine or in an electrochemical process using bromide ion gives 2,5-dimethoxy-2,5-dihydrofuran [332-77-4] (19) which is a cycHc acetal of maleic dialdehyde (94—96). 

Reactions with Alcohols, Mercaptans, and Phenols. Alcohols add readily to acetaldehyde in the presence of trace quantities of mineral acid to form acetals eg, ethanol and acetaldehyde form diethyl acetal [105-57-7] (65). Similarly, cycHc acetals are formed by reactions with glycols and other polyhydroxy compounds eg, ethylene glycol [107-21-1] and acetaldehyde give 2-methyl-1,3-dioxolane [497-26-7] (66) ... 

Heating butanediol with acetylene in the presence of an acidic mercuric salt gives the cycHc acetal expected from butanediol and acetaldehyde (128). A commercially important reaction is with diisocyanates to form polyurethanes (129) (see Urethane POLYMERS). 

The addition of certain glycols and polyols to acroleki leads to the production of cycHc acetal derivatives. 

Cyclic Acetals. One of the most significant developments in the chemistry of sucrose was the synthesis of cycHc acetals which, despite many attempts, were not synthesized until 1974. The first synthesis of 4,6-0-benzyhdenesucrose was achieved from the reaction of sucrose with a, a-dibromotoluene in pyridine (29). Since then, many new acetalating reagents have been used to give a variety of sucrose acetals, generally by transacetalation reactions. 

Acetates. Because of the significant interest in selective acetylation reactions of sucrose, the need for a convenient and unambiguous method of identification has been recognized (34,35). The position of an acetyl group in a partially acetylated sucrose derivative can be ascertained by comparison of its H-nmr acetyl methyl proton resonances after per-deuterioacetylation with those of the assigned octaacetate spectmm. The synthesis of partially acetylated sucroses has generally been achieved either by way of selectively protected derivatives such as trityl ethers and cychc acetals or by direct selective acetylation and deacetylation reactions. 

Conversion to acetates, trifluoroacetates (178), butyl boronates (179) trimethylsilyl derivatives, or cycHc acetals offers a means both for identifying individual compounds and for separating mixtures of polyols, chiefly by gas—Hquid chromatography (glc). Thus, sorbitol in bakery products is converted to the hexaacetate, separated, and determined by glc using a flame ionisation detector (180) aqueous solutions of sorbitol and mannitol are similarly separated and determined (181). Sorbitol may be identified by formation of its monobensylidene derivative (182) and mannitol by conversion to its hexaacetate (183). 

Reaction of acetic acid and a catalytic amount of sulfuric acid at reflux temperatures for 6—8 hours with dihydromyrcene can cause rearrangement of the dihydromyrcenyl acetate to give a mixture of the cycHc acetates analogous to the cycHc formate esters (108). The stereochemistry has also been explained for this rearrangement, depending on whether (+)- or (—)-dihydromyrcene is used (109). The cycHc acetates are also commercially avaUable products known as Rosamusk and CyclocitroneUene Acetate. 

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. 

Ester, ether, and cycHc acetal groups are used as blocking groups to allow regiospecific reactions to take place, ie, reaction at specific unblocked hydroxyl groups. 

Maltol. Otsuka Chemical Co. in Japan has operated several electroorganic processes on a small commercial scale. It has used plate and frame and aimular cells at currents in the range of 4500—6000 A (133). The process for the synthesis of maltol [118-71 -8], a food additive and flavor enhancer, starts from furfural [98-01-1] (see Food additives Flavors and spices). The electrochemical step is the oxidation of a-methylfurfural to give a cycHc acetal. The remaining reaction sequence is acid-catalyzed ring expansion, epoxidation with hydrogen peroxide, and then acid-catalyzed rearrangement to yield maltol, ie ... 

It could be established that the hydrolysis of 1,3-dithianes derived from pc/ra-substi luted acetophenones (4) is promoted by silver ions. Kinetic study using a 10% (v/v) diox-ane/water solvent shows that, when [Ag+] <0.2 mol L 1, the p-NC)2, —Cl, —H and —Me derivatives hydrolyze via rapidly formed 1 Ag+ 1 dithiane complexes, but that the / -McO derivative forms an unreachve 1 1 complex and hydrolyses via a 2 Ag+ 1 dithiane complex. Comparison of the kinetic parameters with those available for analogous open-chain 5,5-acetals reveals that cychzahon leads to a substantial (>104-fold) overall loss of reactivity and that this loss arises both from a lowering of acetal basicity toward Ag+ and from a slower rate of hydrolysis of the 1 1 complex. The imphcations for hydrogen ion-catalyzed hydrolysis of cychc acetals have been discussed, as have the reasons for the lower reactivity of the cychc acetals343. 

Other cychc acetals that have been studied are 1,3,5-trioxane, 1,3,5-trioxepane, 1,3,6,9-tetraoxacycloundecane, and 1,3,5,7-tetroxocane (also referred to as 1,3,5,7-tetroxane) [Kawakami and Yamashita, 1979 Munoz-Escalona, 1978 Schulz et al., 1984 Szwarc and Perrin, 1979]. Polymerization of bicychc acetals has been of interest for synthesizing polysaccharides [Good and Schuerch, 1985 Hirasawa et al., 1988 Okada, 1991 Okada et al., 1989 Sumitomo and Okada, 1984]. 

Using NMR to deduce stereochemistry and seeing how stereoelectronics decide the conformation of a cychc acetal. 

The composites based on the PFO resols cured ester type (UP-632, XXIV) and cychc acetal type (UP-612, XXVI) CECs exhibit higher mechanical strength and heat resistance in comparison with similar materials cured with dicarboxylic acid anhydrides [86]. 

The Ti +.mont also acted as a reusable heterogeneous acid catalyst for esterification of carboxylic acids vdth alcohols [101], deprotection of acetals [102], and the condensation of glycerol with ketones or aldehydes into cycHc acetals under mild reaction conditions [103]. 

The side chain unit was prepared from L-mahc acid in 8 steps as shown in Scheme 13. The cycHc acetal 56 was condensed with the alcohol 57 [45], to afford 58, which was converted to the phosphonium salt 59. 

Benzylidene acetals of diols undergo radical-chain cleavage to give benzoate esters via the formation and subsequent selective fragmentation of a di(a -alkoxyl)henzyl radical. Thus, cychc acetal 24 was quantitatively converted to henzoate 25 under the catalysis of TBST (eq 10). Apphcation to the carhohydrate derived cyclic acetal 26 led to the formation of the 6-deoxy glycoside 27 in high yield (eq 11). ... 

Monohydric alcohols react with transient aldehydes to give open-chain dialkoxy or diaryloxy acetals. For steric reasons, usually linear alcohols give higher yields of acetals than branched alcohols [6]. In the reaction with diols, cychc acetals are formed. The preference for the formation of these compounds may be so high that even traces of water (technical feed ) do not inhibit the acetahzation reaction. 

When a diol such as ethylene glycol is used in place of two equivalents of ROH, a cyclic acetal is formed. Both oxygen atoms in the cychc acetal come from the diol. 

Step [1] The ketone carbonyl is protected as a cychc acetal by reaction of the starting material with HOCH2CH2OH and TsOH. 

Furanose (Section 22.2C) A sugar in which the cychc acetal or hemiacetal ring is five membered. 

Finally, we shall look at cycHc acetals as rigidifying templates in conformation directed reactions and for the generation of stable configurations and... 

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