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|Title:||Intramolecular (4+2); and tandem intermolecular (4+2);/intramolecular (3+2); cycloaddition of nitroalkenes with achiral and chiral dienophiles|
|Author(s):||Senanayake, Chandrawansha Bandara Weerasinghe|
|Doctoral Committee Chair(s):||Denmark, Scott E.|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Nitroalkenes tethered with activated and unactivated olefins were prepared. Intramolecular cycloadditions with both acyclic and cyclic dienophiles in the presence of SnCl$\sb4$ were investigated. Nitroalkenes tethered with three methylene units to a Z-dienophile afforded trans fused nitronates. The cycloaddition towards larger rings were unsuccessful. Cyclic dienophiles cyclized to form fused ring nitronates with extremely high selectivity. The ring fusion between the carbocycles was dependent on the ring size of the dienophile. However, cycloaddition to afford spiro ring nitronates were unsuccessful.
The tandem intermolecular (4+2) intramolecular (3+2) cycloaddition with 2,3-dimethyl-2-butene afforded nitroso acetals in good yields. Using enol ethers as dienophiles, nitroso acetals were obtained as a mixture of anomers. These anomers were transformed to either $\alpha$-hydroxy lactams or 1,3-amino alcohols upon hydrogenolysis. The rate of the (3+2) cycloaddition is dependent on the tether length, geometry and the substitution on the dipolarophile and nitronate. For two methylene tethered nitroalkenes, the (3+2) cycloaddition undergoes via a syn endo pathway while three methylene tethered nitroalkenes undergo a syn exo pathway.
The use of camphor derived chiral enol ethers allowed the demonstration asymmetric tandem cycloadditions. These nitroso acetals were transformed to optically active $\alpha$-hydroxy lactams with high recovery of the chiral auxiliary. The optical purities $>$98% ee were achieved with the vinyl and the (E)-1-propenyl ethers, while only 50% ee was obtained with the (Z)-1-propenyl ether. The high diastereoselectivity arises from a strong endo preference for the enol ethers during the (4+2) cycloaddition step. The mechanism and transition structures for cycloadditions are described.
|Rights Information:||Copyright 1991 Senanayake, Chandrawansha Bandara Weerasinghe|
|Date Available in IDEALS:||2011-05-07|
|Identifier in Online Catalog:||AAI9136726|