Organic Synthesis with the Use of Silica Gel As an Effective Catalyst

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Silica gel, sodium sulfide supported on alumina and silica gel treated with thionyl chloride were found to have high chemoselectivity toword various organic compounds. Use of these silica gels were extended to synthesize physiologically active compounds.1. Sodium sulfide on alumina was found to be used for selective reduction of aldehydes to alcohols in the presence of ketones and also for selective reduction of nitroaldehydes to nitroalcohols.2. In the precense of silica gel ethyl <beta> -trifluoroacetylvinyl ether was found to undergo etherexchange reaction with various alcohols at the olefinic carbon atom and give many kinds of <beta> -trifluoroacetylvinyl ethers in quantitative yields.When allyl alcohol is chosen as an alcohol allyloxy group can be introduced conveniently into the system and the exchanged product, on warming for a while, underwent Claisen rearrangement to give <beta> -diketones bearing trifluoromethyl and allyl groups.These substituted <beta> -diketones reacted eff … More iciently with various bifunctional reagents such as hydrazines, hydroxylamine, urea, guanidine and so on to afford pyrazoles, isooxazoles, pyrimidines and quinolines, respectively, bearing trifluoromethyl and various substituted allyl groups. These fluorine-containing heterocycles are expected to be led to physiologically active compounds.3. It was found that silica gel treated with thionyl chloride can be served as a mild and therefore selective catalyst for acetalization and dithioacetalization. Even furfural and nicotinic aldehyde which are very sensitive toword acids are also acetalized and dithioacetalized safely with the use of this catalyst in quantitative yields.This dithioacetalization method was extended to discriminate formyl group and keto carbonyl group in an intramolecular manner. The discrimination proceeded in each case perfectly, with keto carbonyl groups remaining completely intact.4. Silica gel was found to be used for the reaction of 2-aroyloxy-4-alkyl-2h-1,4-thiazin-3-one with nucleophiles. For example, the reaction of this 2-aroyloxy compound with thiophenol and dimethylaniline in the presence of some silica gel gave sulfur-substituted and nuclei-substituted products in good yields, respectively. Less