Ketones hemithioketals

Dimethyl ketals and enol ethers are stable to the conditions of oxime formation (hydroxylamine acetate or hydroxylamine hydrochloride-pyridine). Thioketals and hemithioketals are cleaved to the parent ketones by cadmium carbonate and mercuric chloride. Desulfurization of thioketals with Raney nickel leads to the corresponding methylene compounds, while thioenol ethers give the corresponding olefin. In contrast, desulfurization of hemithioketals regenerates the parent ketone. ... 

Ketals bearing the double bond at the 4,5-position are prepared by reaction with glycol in the presence of weak acids such as adipic acid. In this case ketal formation proceeds via the intermediate 2,4-dien-3-ol ether. Hemithioketals (71) of A -3-ketones are prepared by reaction with mer-... 

Unsubstituted 20-ketones undergo exchange dioxolanation nearly with the same ease as saturated 3-ketones although preferential ketalization at C-3 can be achieved under these conditions. " 20,20-Cycloethylenedioxy derivatives are readily prepared by acid-catalyzed reaction with ethylene glycol. The presence of a 12-ketone inhibits formation of 20-ketals. Selective removal of 20-ketals in the presence of a 3-ketal is effected with boron trifluoride at room temperature. Hemithioketals and thioketals " are obtained by conventional procedures. However, the 20-thioketal does not form under mild conditions (dilution technique). ... 

Whereas aldehydes such as benzaldehyde or ketones are converted by trimethyl-silylated alkyl- or aryl-mercaptans such as 575, in the presence of catalytic amounts of cyanide, to give high yields to O-trimethylsilyl-hemithioketals such as... 

Figure 1.16 Left Schematic presentation of the 1,3-diaminopropanone core moiety as cysteine protease-directed and active site-spanning inhibitor principle (top). Upon reaction with the enzyme nucleophile, the ketone is reversibly converted to a hemithioketal (bottom). Right Peptidomimetic cysteine protease inhibitors of subsequent generations are depicted together with their inhibitory activity and primary targets.

Thioketalization. Contrary to the earlier report cited in 6, 399, the reaction of aldehydes and ketones with this reagent is initiated by traces of Lewis acids. The most satisfactory conditions use diethyl ether as solvent and zinc iodide as catalyst. The initial reaction results in O-trimethylsilyl hemithioketals, which in the presence of a second equivalent of the reagent are converted into dimethyl thioketals ... 

Alkyl- and aryl-thiosilanes react with aldehydes or ketones to form either thio-ketals or O-silylated hemithioketals. With a/3-unsaturated ketones exclusive 1,4-addition can be achieved (Scheme 80). The procedures constitute an exceptionally mild method for carbonyl group protection. ... 

An acetal is named as a dialkoxy derivative, and a thioacetal is named as a dialkylthio derivative. The name of 65 is l,l-di(ethylthio)butane. Just as ketones react with alcohols to form ketals, they also react with thiols to form the sulfur analog. The mechanism is identical, and the reaction is illustrated by the reaction of cyclohexanone (23) with propanethiol to give 66. This is a dithioketal named l,l-di(propylthio)cyclohexane. The key intermediate in this latter reaction is a hemithioketal. 

These are formed by acid-catalyzed reaction between 2-mercaptoethanol and ketones. Few if any conversions of aldehydes have been reported. Hemithioketals have also been prepared from substituted mercaptoethanols, and from mercaptopropanol (to give the corresponding 1,3-oxathians), mainly for mechanistic studies [69, 70, 71]. 

Yet another possibility is hinted at by the observation (Loewenthal, unpubl.) that the hemithioketal sulphone (26) on heating above its melting point regenerates the original ketone (25) (and, presumably, sulphur dioxide and ethylene) (Fig. 9.3). 

Both the dioxolan and the hemithioketal derived from an aromatic ketone group were found to be cleaved under the conditions of an attempted Pschorr cyclization instead of the hoped-for product a quinoline derived from the firee ketone was obtained [224] (Fig. 9.12). 

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