The formation of carboxylic esters via reaction of carboxylic acids with O-alkylisoureas proceeds in excellent yields with very short reaction times when conducted in a monomode microwave synthesizer. Efficient processes were developed using preformed or commercially available isoureas derived from primary and secondary alcohols, with a reaction time of only 5 min or less. It was demonstrated that under these microwave conditions, ester formation proceeded in good yields with clean inversion of configuration where appropriate. The process was validated using menthol, a hindered substrate for S(N)2 reactions. In addition, starting from primary alcohols, ester formation was successfully accomplished using ail in situ isourea formation procedure. A polymer-assisted solution-phase procedure was also developed by employing preformed solid-supported isoureas and by an efficient "catch and release" ester formation procedure whereby primary alcohols were caught on resin as isoureas by reaction with immobilized carbodiimide and released as esters by subsequent treatment with a carboxylic acids.
Nucleophilic substitutions using <i>O</i>
-alkyl-<i>N</i>
,<i>N</i>
′-dialkylisoureas. Applications to ephedrines
作者:Martin A. Poelert、Richard M. Kellogg、L. A. Hulshof
DOI:10.1002/recl.19941130705
日期:——
hydroxyl group of N-methylated (1R,2S)-ephedrine and (1S,2S)-pseudoephedrine. These adducts react with nucleophiles like alkyl and aryl thiols as well as thioic acids and phthalimide to form the substitution products with overall retention of configuration. It is postulated that intramolecular participation of the amino group via an SN2 reaction leads to aziridium salts, which are subsequently opened
在Cu 1催化剂存在下,二烷基碳二亚胺与N-甲基化的(1 R,2 S)-麻黄碱和(1 S,2 S)-伪麻黄碱的羟基完全反应。这些加合物与亲核试剂如烷基和芳基硫醇以及硫代酸和邻苯二甲酰亚胺反应,形成具有整体保留构型的取代产物。据推测,氨基通过S N 2反应在分子内参与会形成叠氮鎓盐,随后该叠氮鎓盐会由亲核试剂通过第二个S N打开2反应。这种合成方法也可用于简单仲醇的转化。在先用二环己基碳二亚胺处理,再用苯甲酸进行处理,再用LiAlH 4处理,薄荷醇以高收率转化为新薄荷烷硫醇。