| Abstract
| - Highly useful C−C bond formation using 2-ethoxy-3-(phenylseleno)prop-2-enal acetal 2 wasexamined with various Lewis acids. The reaction of 2 with the silyl enol ether in the presence ofBF3·Et2O, ZnBr2, or SnBr4 regioselectively provided (Z)-3,4-diethoxy-5-(phenylseleno)pent-4-enophenone 5a in high yields. On the other hand, the reaction with other Lewis acids such as EtAlCl2 orSnCl4 gave 5-(phenylseleno)- 6 or non-selenopentane-1,4-dione 7, respectively. Novel prop-2-enalacetals 2−4 and 13−15 reacted with various nucleophiles to give pent-4-enophenones 5a,b, 10a,12, and 16−18, S-ethyl pent-4-enoate 5b, alkylated vinylic sulfide 10b, 3-pentenenitrile 5d, and10c. A versatile pent-4-enophenone 5a could be converted to tetrahydrofuran 20 and penta-2,4-dienophenone 19, the Diels−Alder reactions of which with dienophiles gave the adducts 24 and25.
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