| RESEARCH |
Cycloaddition reactions carried out in tandem underlie the strategy that Org. s Author, has developed to meet what he describes as "the challenge of constructing many rings with many stereocenters adorned with multiple functionalities." For a decade, Author's research group has been synthesizing heterocycles by using tandem cycloaddition reactions of nitroalkenes. In the presence of Lewis acids, nitroalkenes react as dienes with electron-rich double bonds in a [4 + 2] cycloaddition--that is, the four atoms of the diene add to the two atoms of the alkene to give a six-membered ring. The product nitronates are relatively unfamiliar compounds that can undergo dipolar [3 + 2] cycloadditions with electron-deficient alkenes to make the even more unusual nitroso acetals. Simple hydrogenolysis of the nitroso acetals yields the final products. By carrying out [4 + 2]/[3 + 2] tandem cycloaddition sequences--in various combinations of inter- and intramolecular reactions--Author and coworkers can create structures that contain four new bonds, up to four new rings, and up to six new stereogenic centers. For example, they used an intermolecular [4 + 2] cycloaddition followed by an intramolecular [3 + 2] process to construct--with complete relative and absolute stereocontrol--four of the five stereocenters of the alkaloid (+)-1-epiaustraline in just two steps. The researchers' newest permutation is a double intramolecular tandem process in which the starting nitroalkene and two other double bonds are tethered together in a linear polyalkene. "By moving the points of attachment," Author said, "we can access a number of quite different structures and natural product cores." |
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