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亚硫酰氯处理的硅胶对多种有机化合物具有较高的化学选择性。这些硅胶的用途被扩展到合成具有生理活性的化合物。氧化铝上的硫化钠可用于在酮存在的情况下将醛选择性还原为醇，也可用于将硝基醛选择性还原为硝基醇。在硅胶存在的情况下，发现乙基< β > -三氟乙基乙烯醚在烯烃碳原子上与各种醇发生醚交换反应，定量产率地得到多种< β > -三氟乙基乙烯醚。当选择烯丙醇作为醇基时，烯丙氧基可以方便地引入体系，交换产物在加热一段时间后进行Claisen重排，得到含三氟甲基和烯丙基的< β > -二酮。这些取代的< β - > -二酮与各种双功能试剂如肼、羟胺、尿素、胍等反应更有效，分别生成吡唑、异唑、嘧啶和喹啉，含有三氟甲基和各种取代的烯基。这些含氟杂环化合物有望产生具有生理活性的化合物。结果表明，经亚硫酰氯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。即使对酸非常敏感的糠醛和烟碱醛也可以在该催化剂的作用下安全地进行缩醛化和二硫代缩醛化。将二硫代缩醛化方法推广到分子内区分甲酰基和酮羰基。在每种情况下，这种区分都进行得很完美，酮羰基完全保持不变。2-芳氧基-4-烷基-2h-1,4-噻嗪-3- 1与亲核试剂的反应采用了硅胶。例如，这种2-芳氧基化合物与噻吩和二甲苯胺在硅胶的存在下反应，分别得到了产率较高的硫取代产物和核取代产物。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