Study On Some Tandem Reactions Involving 2,3-Allenoates And Allene Intermediates In Organic Synthesis

Sequential reaction represents an elegant and efficient way to access novel and complex molecules from simple, readily available starting materials. These reactions often involve a series of inter- or intra- molecular processes wherein the product of one reaction is programmed to be the substrate for the next. Thus the consecutive transformations of the involved intermediates finally incorporate the powerful sequences into a designed scheme, which are now routinely employed to construct core skeletons of many important natural products. More importantly, without isolating the intermediates, changing the reaction conditions or adding reagents, these reactions often allow an ecologically and economically favorable chemical process which is well appreciated in modern synthetic chemistry. Thus tremendous attention has been paid to the development and application of sequential reaction nowadays.Allenes, featured by the presence of two cumulated C=C double bonds, have shown a variety of unique chemical reactivity. The past decades have witnessed an explosive progress as a result of the rapid development and application of a large number of novel reactions based on allenes to the fields of the synthesis of natural products, Pharmaceuticals, dyes, polymers, etc.Intrigued by the concept of sequential reaction and the remarkable advance on allene chemistry, in this dissertation we initially focus our attention on studying the reactivity and synthetic application of 2,3-allenoates, a class of readily available allene derivatives, and mainly, two types of sequential reactions involving 2,3-allenoates were developed. Then we designed and realized a class of novel and interesting sequential reactions via allene intermediates for the rapid synthesis of structurally diverse polycyclic molecules. The whole work can be divided into four parts.1. We found that hydroazidation of 2,3-allenoates can give vinyl azides in excellent yields with high regio- and stereoselectivities. We also developed a novel synthesis of polysubstituted pyrroles from the reaction of 1-allylic 2,3-allenoates and sodium azide. The latter reaction proceeds through a tandem nucleophilic addition, cycloaddition, denitrogenation, and aromatization process.2. We have developed a facile and moderate-yielding method for the preparation of polysubstituted 5-functionalized 2-pyrone derivatives via a K2CO3-catalyzed tandem nucleophilic addition/lactonization reaction starting from 2,3-allenoates and a-substituted ketones with electron-withdrawing groups. This method would be potentially useful due to a general lack of simple and efficient procedures for synthesis of these compounds in the literature.3. We first disclosed an unique electrophilic addition reaction of 2,3-allenoates with N,N-dibromo-p-toluenesulfonamide (TsNBr2), leading to an unexpected efficient and highly stereoselective synthesis of (lE,2E)-3-bromo-4-oxo-N-tosyl-2-alkenoxylimidic acid ethyl esters. A plausible mechanism involving a cyclic cation intermediate was proposed to rationalize the reaction.4. We reported a class of well-devised palladium-catalyzed sequential reactions utilizing an allene-intermediate-involved intramolecular [4+2] cycloaddition as the key step, resulting in a very facile and efficient method for the preparation of a variety of structurally diverse polycyclic compounds from electron-deficient vinyl iodides and propargyl ethers. This work is still being carried out in our group.