Hetero-Diels-Alder reactions of alpha-aryl-beta-monohalo-(alpha-nitrosoethylenes: Diastereoselective synthesis of 6-substituted 3-Aryl-4-halo-5,6-dihydro-4H-1,2-oxazines
- Authors
- Yoon, SC; Kim, K; Park, YJ
- Issue Date
- Nov-2001
- Publisher
- AMER CHEMICAL SOC
- Citation
- JOURNAL OF ORGANIC CHEMISTRY, v.66, no.22, pp 7334 - 7341
- Pages
- 8
- Journal Title
- JOURNAL OF ORGANIC CHEMISTRY
- Volume
- 66
- Number
- 22
- Start Page
- 7334
- End Page
- 7341
- URI
- https://scholarworks.sookmyung.ac.kr/handle/2020.sw.sookmyung/16637
- DOI
- 10.1021/jo010476g
- ISSN
- 0022-3263
1520-6904
- Abstract
- Treatment of alpha -aryl-beta -bromo(or chloro)-alpha -nitrosoethylene, prepared in situ from alpha -monobromo(or chloro)ketoximes and sodium carbonate in ether at rt, with allytrimethylsilane afforded exclusively trans-(4S,6S)- and trans-(4R,6R)-3-aryl-4-halo-6-[(trimethylsilyl)methyl]-5,6-dihydro-4H-1,2-oxazines 10 albeit in low yields. Similar treatment of beta -halo-alpha -nitrosoethylenes with ethyl vinyl ether, however, gave single stereoisomers, i.e., cis-(4S,6S)- and cis-(4R,6R)-6-ethoxy-4-halo-5,6-dihydro-4H-1,2-oxazines 11, in moderate to good yields. The result is in contrast to the reported predominant formation of trans-11a by a radical reaction. On the other hand, similar reactions with tert-butyl vinyl ether at 30 degreesC gave diastereomeric mixtures of cis-(4S,6S)-, cis-(4R,6R)-, trans-(4S,6R)-, and trans-(4R,6S)-6-(tert-butoxy)-4-halo-5,6-dihydro-4H-1,2-oxazines 12. In contrast to compounds 11, the major isomers have (4S,6R) and (4R,6S) configurations. The tendency of a [4 + 2] cycloaddition reaction is consistent with that observed in the Diels-Alder reaction with inverse-electron demand. The stereochemistries of compounds 10-12 were assigned on the basis of the H-1 NMR coupling constants, which were unambiguously determined by the decoupling experiments. All reactions leading to compounds 10-12 proceed with very high regioselectivity. Diastereoselectivity and high regioselectivity are understood in terms of the frontier orbital method. It has been found that cis-12g is isomerized to a mixture of stereoisomers in favor of the trans-isomer in the presence of HClO4 (72%) in CHCl3 at rt.
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