The Diels-Alder cycloadditions of heteroatom-functionalized 1,3-butadienes have provided a powerful means to synthesize a great variety of functionalized skeletons.1) Alkoxy-, trimethylsilyloxy, and sulfenyl 1,3-butadienes have been commonly used as an electron-rich diene.2,3) On the other hand, 2,4-pentadienoates and their analogs bearing alkoxy, alkylthio or arylthio groups have been developed in novel Diels-Alder reactions, which with dienophiles highly regioand stereoselectively give the [4ϩ2] cycloadducts as novel precursors for natural products or bioactive compounds.
4-7)In particular, much attention has been focused on the reactions of 5-alkoxypenta-2,4-dienoates with dienophiles bearing an electron-withdrawing group in view of the opposite regioselectivity that they show. 8,9) However, the reactions of the push-pull dienes need strict conditions such as high pressure and high temperature. 10) To find reactive push-pull dienes is highly important for synthetic organic chemistry. Recently, we have reported novel C-C bond formations of the 3-selenenyl-2-alkoxy-, 3-selenenyl-2-sulfenyl-, 3-sulfenyl-2-alkoxyallylic cations with phenyltrimethylsilyloxyethylene, which provided the 4-pentenophenones bearing 4-and 5-sulfenyl or selenenyl 4-alkoxy groups, in high yields.11) We have found that their reactions with acids or bases gave a variety of 1-phenyl-2,4-pentadien-1-ones bearing the 3-heteroatom, which are the highly reactive Diels-Alder dienes and their reactions with dienophiles were found to provide various cycloadducts. Although a small part of the results have already described in our previous report, we further investigated the Diels-Alder reactions of various dienes with dienophiles and their high reactivities were elucidated by the computational analysis. We also found the unique 1,3-hydride shift of 3a,6,7,7a-tetrahydro-1H-isoindole-1,3(2H)-diones obtained from the reactions with maleimides. Here we report the details of the Diels-Alder reactions of the 1-phenyl-2,4-pentadien-1-ones bearing 4-alkoxy or 4-sulfenyl group with dienophiles.The 1-phenyl-2,4-pentadien-1-ones 2, 3 and 4 were obtained by the treatment of 1-phenyl-4-penten-1-one 1 with NaOEt at 0°C as shown in Chart 1. The diene 2 and 4 was a mixture of 4Z-and 4E-isomers; however, the dienes 3 was exclusively obtained as a single isomer, (2E,4Z). The stereochemistries of 2 and 4 were determined by the nuclear Overhauser effects (NOE) experiments as shown in Fig. 1. Irradiation of 4E-5-H increased the intensity of the methylene protons of the ethoxy group (1%). On the other hand, the NOE enhancement of the corresponding protons of 4E-isomer was not observed. The same results were obtained from the NOE experiments of 4. 2-and 3-Olefinic protons were observed as the trans coupling constant by the 1 H-NMR spectrum (J 15 Hz). These spectral data shows the main isomer of 2 or 4 is (2E,4Z).Next, we examined the reaction of the diene 2 (4Z : 4Eϭ86 : 14) with N-phenylmaleimide to give 6-benzoyl-5-ethoxy-2-phenyl-3-(phenylselenenyl)-3a,6,7,7a-tetrahydr...