Ketal Reactions

Scheme 3b). It is instructive at this point to reiterate that the furan nucleus can be used in synthesis as a progenitor for a 1,4-dicarbonyl. Whereas the action of aqueous acid on a furan is known to provide direct access to a 1,4-dicarbonyl compound, exposure of a furan to an alcohol and an acid catalyst should result in the formation of a 1,4-diketal. Indeed, when a solution of intermediate 15 in benzene is treated with excess ethylene glycol, a catalytic amount of / ara-toluenesulfonic acid, and a trace of hydroquinone at reflux, bisethylene ketal 14 is formed in a yield of 71 %. The azeotropic removal of water provides a driving force for the ketalization reaction, and the presence of a trace of hydroquinone suppresses the formation of polymeric material. Through a Finkelstein reaction,14 the action of sodium iodide on primary bromide 14 results in the formation of primary iodide 23, a substance which is then treated, in crude form, with triphenylphosphine to give crystalline phosphonium iodide 24 in a yield of 93 % from 14.

This ketalization reaction can also be used to convert racemic 1,3-diols to (R)-1,3-diols with high optical purity. 

The product mixture consisted of three components in a 4 1 1 ratio, respectively, (IV), (V), and, (VI). Allyl, (V), was removed by selective solubility in ethyl acetate containing pyrrolidone a 4 1 ratio of (IV) and (VI) was used directly in the trans-ketalization reaction. 

Silylketene ketals. Reaction of esters with triethylsilyl perchlorate and diisopro-pylethylamine or 2,2,6,6-tetramethylpiperidine provides (Z)-silylketene ketals selectively... 

TABLE 13. Cyclopropanone ketals by the diazoalkane-ketene ketal reaction ... 

Formation of intermediate chloro derivative 112 was confirmed by its hydrolysis to benzophenone and alcoholysis to corresponding dimethyl ketal. Reaction of CBT with diazomethane resulting in formation of 1- and 2-chloromethylbenzotriazoles also confirms the proposed reaction scheme. 

Structures of mulberrofurans F (131), G (27), and K (179) have been established by correlation with chalcomoracin (103) and with mul-berrofuran C (130) using a ketalization reaction 91, 112). Therefore, the configuration of the three chiral centers in the methylcyclohexene ring of the ketal compounds is the same as those of 103 and 130. 

Reactions of carbohydrates are closely related to the normal reactions of alcohols, aldehydes, and ketones seen in previous chapters. The presence of mcd-tiple functional groups is problematic, however, because one group may interact with another. The formation of furanose and pyranose rings in the previous sections via a hemiacetal or a hemiketal reaction and of disaccharides via an acetal or a ketal reaction is one example where the functional groups interact with each other. In many other reactions, the hydroxyl units behave independently or the aldehyde or ketone unit behaves independently. This section will explore several fundamental carbohydrate reactions. 

Y., Seraji, M.S.I., and Suib, S.L. (2014) Tungsten promoted mesoporous group 4 (Ti, Zr, Hf) transition metal oxides for room temperature, solvent free acetalization and ketalization reactions. Chem. Mater., 26 (9), 2803 2813. 

Mesoporous MCM-type materials incorporating aluminum triflate were also considered for the synthesis of solketal, which is an acid-catalyzed reaction of acetone with glycerol, also known as ketalization reaction (Equation (8.50)) [119]. The selectivity to the desired product (solketal) was total, not being affected on varying the various reaction variables. On these catalysts the reaction does not produce other products such as hemiketal or six-membered cyclic ketal. 

SCHEME 2.5 Phosphoric acid-catalyzed Mannich-ketalization reaction. 

Similar to the mechanism described in Scheme 2.16, a Knoevenagel reac-tion/ketalization cascade of hydroxyacetone with 1,3-dicarbonyl compounds is assumed. In Scheme 2.16, a Knoevenagel condensation/ketalization reaction is depicted. This sequence allows a subsequent oxa-Michael addition, which yields the corresponding C-glycosides. In contrast, a Knoevenagel addition/ketalization occurs under the reaction condition described in Scheme 2.28, which is followed by an intramolecular retro-Claisen step. As a result of that, the corresponding esters were obtained (Scheme 2.29). 

---