| Abstract
| - Treatment of α-aryl-β-bromo(or chloro)-α-nitrosoethylene, prepared in situ from α-monobromo(orchloro)ketoximes and sodium carbonate in ether at rt, with allytrimethylsilane afforded exclusivelytrans-(4S,6S)- and trans-(4R,6R)-3-aryl-4-halo-6-[(trimethylsilyl)methyl]-5,6-dihydro-4H-1,2-oxazines10 albeit in low yields. Similar treatment of β-halo-α-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 predominantformation of trans-11a by a radical reaction. On the other hand, similar reactions with tert-butylvinyl ether at 30 °C gave diastereomeric mixtures of cis-(4S,6S)-, cis-(4R,6R)-, trans-(4S,6R)-, andtrans-(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] cycloadditionreaction 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 1H NMR couplingconstants, which were unambiguously determined by the decoupling experiments. All reactionsleading to compounds 10−12 proceed with very high regioselectivity. Diastereoselectivity and highregioselectivity 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 ofHClO4 (72%) in CHCl3 at rt.
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