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

硅胶、氧化铝负载的硫化钠和经氯化亚砜处理的硅胶对多种有机化合物都有很高的化学选择性。这些硅胶的用途被扩展到合成生理活性化合物。研究发现，在酮存在下，氧化铝负载的硫化钠可用于选择性还原醛为醇，也可用于选择性还原硝基醛为硝基醇.在硅胶存在下，乙基<beta>-三氟乙酰基乙烯基醚与各种醇在烯碳原子上发生醚交换反应，定量地生成多种<beta>-三氟乙酰基乙烯基醚。选择烯丙醇作为醇时，可以方便地将烯丙氧基引入体系中，交换产物在升温时发生Claisen重排，生成<beta>带有三氟甲基和烯丙基的-二酮。这些取代的-二酮与烯丙醇<beta>发生醚交换反应，生成的-三氟乙酰基乙烯基醚具有较好的稳定性。 ...更多信息 与各种双官能试剂如肼、羟胺、脲、胍等反应，分别得到带有三氟甲基和各种取代的烯丙基的吡唑、异恶唑、嘧啶和喹啉。这些含氟杂环化合物有望得到具有生理活性的化合物.研究发现，经氯化亚砜处理的硅胶可作为一种温和的选择性催化剂用于缩醛化和二硫代缩醛化。在此催化剂的作用下，即使是对强酸非常敏感的糠醛和烟醛也能安全地进行缩醛化和二硫代缩醛化，并能定量地进行二硫代缩醛化反应。在每种情况下，区分都进行得很好，酮羰基保持完全完整。硅胶可用于2-芳酰氧基-4-烷基-2H-1，4-噻嗪-3-酮与亲核试剂的反应。例如，该2-芳酰氧基化合物与苯硫酚和二甲基苯胺在硅胶存在下反应，分别以良好的产率得到硫取代和核取代的产物。少

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