Cleavage of acetals

AI2O3, microwaves, 12 min, 93% yield. " Cleavage of acetates occurs similarly. 

The acid-catalyzed cleavage of acetals and ketals is greatly influenced by the substitution on the acetal or ketal carbon atom. The following values for k illustrate the magnitude of the effect ... 

Cleavage of acetals or ortho esters with Grignard reagents... 

Because of the polyfunctional nature of carbohydrates, protective-group strategy plays an important role in synthetic methodology involving this class of compounds. In the present Chapter, results are described from a study of the utility of N-trimethylsilyl- and N-tert-butyldimethylsilyl-phthalimide for the selective silylation of primary hydroxyl groups in carbohydrates. Also described, is a new, facile method for cleavage of acetals and dithioacetals in carbohydrate derivatives the method involves treatment of the derivatives with a dilute solution of iodine in methanol. 

Cleavage of Acetals and Dithioacetals in Carbohydrate Derivatives Using Iodine in Methanol... 

Developments in protective-group strategy, including the formation and cleavage of acetals, continue to be of interest in synthetic carbohydrate chemistry (95,96). The attractiveness of the reagent system described here stems from its simplicity, convenience, versatility, and the high yields of the cleavage products. 

Coppola GM (1984) Amberlyst-15, a superior acid catalyst for the cleavage of acetal. Synthesis 1021-1023... 

Sabitha G, Babu RS, Reddy EV, Yadav JS (2000) A novel, efficient, and selective cleavage of acetals using bismuth(III) chloride. Chem Lett 9 1074—1075... 

Kim KS, Song YH, Lee BH, Hahn CS (1986) Efficient and selective cleavage of acetals and ketals using ferric chloride adsorbed on silica gel. J Org Chem 51 404 -06... 

Reduction of acetals or ortho esters 0-80 Reduction of epoxides 0-92 Cleavage of acetals or ortho esters with Grignard reagents... 

Cleavage of acetals. Ethylene acetals of /J,y-unsaturatcd carbonyl compounds arc cleaved to dienol ethers by NaH in DMSO. 

Resin-bound aldehydes and ketones have been used as linkers for 1,2- and 1,3-diols (Entries 4-6, Table 3.33). Cleavage of acetal-based linkers is usually effected by acid-catalyzed transacetalization or by hydrolysis. 

Olah, G.A., Yamato, T., Iyer, P.S. and Prakash, G.K.S., Catalysis by solid superacids. 20. Nafion-H catalyzed reductive cleavage of acetals and ketals to ethers with triethylsilane, /. Org. Chem., 1986, 51, 2826. 

Figure 8.9 Possible mechanisms for cleavage of acetals and ketals. I A-l, Mechanism I II A-2, Mechanism II III SB2, Mechanism III.

Scheme 10.7. Oxidative cleavage of acetals to yield monoacylated diols [26].

The cleavage of acetals with catalytic iodine in methanol is a reaction first reported by Walter Szarek and co-workers6 in the 1980s. Although a mechanism has never been proposed in the literature, it is conceivable that the C(l)-exo-acetal oxygen of 10 could attack iodine to form 25 and iodide ion (Scheme 12.8). Loss of the C(l)-0-substituent could then occur to create 26, which could capture iodide ion and methanol to form 27 and acetone. Such a mechanism would regenerate the catalytic quantity of iodine needed to propagate the catalytic cycle. 

This feature of PLNM immediately explains why least motion effects are not observed in cleavage of acetals, but are observed in cleavage of orthoesters and other substrates at the acyl level of oxidation the transition state for acetal cleavage is late, whereas that for orthoester hydrolysis is more central (Sinnott, 1984 Cordes and Bull, 1974). 

There is a competition between alkoxymethylation and acylation in this reaction. Alkoxymethylation by cleavage of acetals is also applicable to alcoholates obtained by condensing Reformatsky reagents with aldehydes and ketones (98, 174a). 

Cleavage of acetals. In the presence of a Pd(II) catalyst, /-butyl hydroperoxide oxidatively cleaves a five- or six-membered acetal to an ester of a diol. Pd(OAc), and PdCl2 can catalyze this reaction, but CFjCOjPdtJOCfCHj), (10, 299) is most effective. The cleavage of acetals derived from unsymmeti ical diols is not regioselective. Example ... 

Cleavage of acetals. The acetals derived from (2R,4R)-2,4-pentanediol (this volume) are cleaved by organotitanium reagents of the type RTiCI, or R,TiCf. with high chemo- and stereoselectivity. Removal of the chiral auxiliary gives chiral secondary alcohols in high purity. Acetals complexed with TiCb arc also cleaved by treatment with an alkyllithium. 

Cleavage of acetals by RCu or R2CuLi,n Acetals are cleaved by organocopper and cuprate reagents complexed with BF3 with substitution of one alkoxy group. The reaction in ether is rapid, even at — 30°. 

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