| Popis: |
Many biologically active and widely distributed natural products contain a dihydro- or a tetrahydropyran moiety. One of the best methods for the synthesis of these compounds is the hetero Diels-Aider reaction of 1-0Xia-1,3-butadienes which leads to substituted and annulated 3,4-dihydro-2H-pyrans usually in excellent yields and high selectivity. Further transformations of these substances e.g. by hydrogenation yield the tetrahydropyrans with the basic skeleton of the carbohydrates. In this article at first theoretical aspects of the cycloaddition are discussed which include ab initio calculations of the energies and the coefficients of 1-oxa-1,3-butadienes and different dienophiles. In addition, calculations of the transition structure of the cycloaddition of acrolein and ethene are presented. In the second chapter general considerations on the regia- and stereoselectivity of the cycloadditions are presented. In the following parts different hetero Diels-Aider reactions with activated and non-activated 1-oxa-1,3-butadiens and their use in diastereoselective transformations and the synthesis of natural products as carbohydrates, alkaloids, terpenes, steroids, cannabinoids, mycotoxins, ionophores and antibiotics are described. A point of emphasis is the domino Knoevenagel hetero Diels-Aider reaction in which the oxabutadienes are formed in situ and which allows the highly efficient synthesis of complex dihydropyrans starting from simple substrates. In the final chapters enantioselective cycloadditions, reactions on solid support and cycloadditions under high pressure are discussed. |
