Ketals and Acetals

The hydrogenolysis of acetals and ketals does not take place over most catalysts under normal hydrogenation conditions, even at relatively high temperatures and pressures. About the only reasonable hydrogenolytic cleavage of these compounds takes place over rhodium at 50°-80°C under 3-4 atmospheres of hydrogen in the presence of a trace of acid. O jhe formation of ethers in this 

As mentioned in Chapter 18, aryl ketones are hydrogenolyzed to the methylene over palladium catalysts under mild conditions. The addition of a little acid to the hydrogenation mixture facilitates the reaction. Since this reaction takes place under mild conditions few, if any, side reactions take place making this palladium catalyzed hydrogenolysis the preferred method for the removal of aryl carbonyl groups 

The palladium catalyzed hydrogenolysis of 5,5-dimethyl 1,3-cyclohexane-dione (59) in acidic medium gave good yields of the cyclohexanone, 60 (Eqn. 20.41). 5 In iiie absence of acid, a platinum catalyst gave a higher selectivity for the hydrogenolysis of one of the carbonyl groups of 1,3-cyclohexanedione than did palladium.  

It is to be noted that when a and / are attached to some iso-propylidene ketals of fructose they are part of a trivial name and are not used in the customary anomeric sense the quotation marks may serve for easy distinction. 

Ohle and Just112 have carried out the following sequence 1,2-isopro-pylidene-D-fructopyranose 3-p-toluenesulfonate — l,2-isopvopylidene-3,4-anhydro-D-psicose — l,2-isopropylidene-4-methyl-D-sorbose (VII), ([a]D -81.6°). Since the introduction of a methyl group does not alter greatly the specific rotation, it is to be expected that the parent 1,2-iso- 

Bell114 has suggested that the very ready hydrolysis of a -diisopro-pylidene-D-fructose might possibly indicate a furanose structure, namely, that a-diisopropylidene-D-fructose is l,2 4,6-diisopropylidene-D-fructo-furanose. Hydrolysis would have to be accompanied by change in the size of the ring since the structure of the mono-isopropylidene derivative is, without doubt, pyranose. In this connexion, however, it should be 

Another mono-isopropylidene-D-fructose, crystalline and non-reducing, was made by Zervas and Sessler86 by debenzoylation of the product obtained by condensing D-fructofuranose 1,6-dibenzoate (see page 63) with acetone. From the method of preparation this must be 2,3-iso- 

Another monobenzylidene-D-fructose was prepared as a sirup by Brigl and Widmaier128 from D-fructofuranose 1,6-dibenzoate. Possible structures suggested by these workers are 2,3- or 2,4-benzylidene-D-fructofuranose, but this work is incomplete. 

Catechols can be protected as diethers or diesters by methods that have been described to protect phenols. However, formation of cyclic acetals and ketals (e.g., methylenedioxy, acetonide, cyclohexylidenedioxy, and diphenylmethyl-enedioxy derivatives) or cyclic esters (e.g., borates or carbonates) selectively protects the two adjacent hydroxyl groups in the presence of isolated phenol groups. 

The methylenedioxy group, often present in natural products, is stable to many reagents. Efficient methods for both formation and removal of the group are available. 

CH2Br2, NaOH, H2O, Adogen, reflux, 3 h, 76-86% yield, [Adogen = R3N CH3CP, phase transfer catalyst (R = Cg-C,o straight-chain alkyl groups)]. Earlier methods required anhydrous conditions and aprotic solvents. 

73-78% yield. Aluminum bromide cleaves aryl and alkyl methyl ethers in high yield methyl esters are stable. 

CH3SCH3, CICH2CH2CI, 83°, 98% yield. Selective cleavage of an aryl methylenedioxy group or an aryl methyl ether by boron trichloride has been investigated.  

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The primary and secondary alcohol functionahties have different reactivities, as exemplified by the slower reaction rate for secondary hydroxyls in the formation of esters from acids and alcohols (8). 1,2-Propylene glycol undergoes most of the typical alcohol reactions, such as reaction with a free acid, acyl hahde, or acid anhydride to form an ester reaction with alkaU metal hydroxide to form metal salts and reaction with aldehydes or ketones to form acetals and ketals (9,10). The most important commercial appHcation of propylene glycol is in the manufacture of polyesters by reaction with a dibasic or polybasic acid. 

Cyclic carbonates and cyclic boronates have also found considerable use as protective groups. In contrast to most acetals and ketals the carbonates are cleaved with strong base and sterically unencumbered boronates are readily cleaved by water. 

CYCLIC ACETALS AND KETALS 123 7. Acetonide (Isopropylidene Ketal) (Chart 3)... 

Formylbenzenesulfonate, 170 PROTECTION FOR CATECHOLS CYCLIC ACETALS AND KETALS... 

Methyl-5-2-(methylthio)ethyl, 209 Cyclic Monothio Acetals and Ketals... 

Methods similar to those used to form and cleave dimethyl acetal and ketal derivatives can be used for other dialkyl acetals and ketals. 

Bis(2,2,2-trichloroethyl) Acetals and Ketals R2C(OCH2CCl3)2 (Chart 5) Formation ... 

Dibenzyl Acetals and Ketals R2C(OCH2Ph)2 Fonnation/Cleayage ... 

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