Intramolecular (4+2) -Cycloadditions of Three Heterodienes: Nitrosoalkenes, Nitroalkenes, and Z-Alpha,beta-Unsaturated Aldehydes
Sternberg, Jeffrey Arthur
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https://hdl.handle.net/2142/70380
Description
Title
Intramolecular (4+2) -Cycloadditions of Three Heterodienes: Nitrosoalkenes, Nitroalkenes, and Z-Alpha,beta-Unsaturated Aldehydes
Author(s)
Sternberg, Jeffrey Arthur
Issue Date
1987
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Abstract
Intramolecular (4+2) -cycloadditions employing nitrosoalkenes, nitroalkenes, and Z -$\alpha,\beta$-unsaturated aldehydes as the 4$\pi$-components are discussed. Nitrosoalkenes, generated in situ from the corresponding $\alpha$-chloroketone silyloximes, engaged in intramolecular cycloadditions with enol ethers and vinyl sulfides at ambient temperature. The yield and stereoselectivity of the tricyclic octahydronaphthalene (6-6)- and hexahydroindene (6-5)-oxazine cycloadducts varied. In the enol ether cases, the success of the cycloaddition depended on the olefin geometry. Only the dienophile bearing the proper geometry for secondary orbital overlap cyclized in high yield and with good stereoselectivity. The vinyl sulfides demonstrated no such geometry dependence and afforded high yields of cycloadducts in both 6-6 and 6-5 ring systems. Despite extensive effort, a general method for cleaving the cyloadducts could not be found.
Intramolecular cycloadditions of nitroalkenes were also investigated in the 6-6 system. At $-78\sp\circ$C in the presence of tin tetrachloride, a cyclic nitroalkene was observed to undergo cycloaddition with an isolated double bond. The tricyclic nitronate formed in 70% yield was a single stereoisomer that maintained the E -stereochemistry of the starting olefin. Cleavage of the N-O bond afforded bicyclic ketooximes and hydroxylamines with preservation of the ring fusion stereochemistry. Intermolecular cycloadditions between 1-nitrocyclohexene and various cyclic alkenes were also examined. An X-ray crystal structure of the major nitronate isomer obtained from the cyclization with cyclohexene provided the stereochemical assignments in the products.
Intramolecular Z -$\alpha,\beta$-unsaturated aldehyde cycloadditions were investigated as a general method of constructing the cyclopenta(c) pyran ring system of the iridoid monoterpenes. Using a vinyl sulfide as the dienophile, the Z -enal cyclized at $-78\sp\circ$C using BF$\sb{3}\cdot$OEt$\sb{2}$ catalyst to give exclusively cis-fused products in 91% yield. Contrastingly, the analogous E -enal cyclization afforded a mixture of cis- and trans-fused products under conditions of kinetic control. The cis-fused isomers could be exclusively obtained under more forcing conditions but the cyclization yields were lower than those in the Z -enal reactions. This methodology was utilized in a short, efficient, and stereoselective synthesis of (+)-nepetalactone. The asymmetry was introduced using known chiral hydrazone alkylation technology. The Z -enal cyclization provided the desired cis ring fusion and also established the proper relative configuration of the methyl group at C(7).
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