Diels-Alder reactions of [(S)R]-(1E,3E
)-l-p-tolylsulfinyl-l,3-pentadiene with 1-pyrrolidinyl-l-cyclohexene and methyl acrylate evidenced: i) low reactivity of the diene regardless of the electronic character of the dienophile (high pressures were always required); ii) a complete regio-and endo-selectivity (both controlled by the sulfoxide), but a low rr-facial diastereoselectivity in the cycloaddition with enamine; iii) diastereoselectivity (facial and endo/exo) for the reaction with methyl acrylate is dependent on the regiochemistry (mainly controlled by the methyl substituent). @ 1997 Elsevier Science Ltd
The increasing interest of asymmetric Diels-Alder reactions with sulfinyl dienophiles is due to the achievement of efficient control of the diastereoselectivity. 1 By contrast, reactions with enantiomerically pure sulfinyldienes have received much less attention 2 in spite of their synthetic potential. In preceding papers, we have studied the Diels-Alder reactions of 1-p-tolylsulfinyldienes with dienophiles such as N-methylmaleimide 3 and maleic anhydride 2a showing a high "if-facial diastereoselectivity for such cycloadditions. An additional interesting feature of these reactions was the spontaneous [2,3]-sigmatropic rearrangement of the allylic sulfoxide in the adducts which opens an easy access to highly functionalized cyclohexenols, 3,4 not easily available by other ways. Applications of these tandem reactions were limited by the low reactivity of these dienes with electron poor dienophiles which required high pressures and/or long reaction times. Taking into account the expected electron withdrawing character of the sulfinyl group, this low reactivity was not surprising. According to previous work of Evans 4 on racemic l-phenylsulfinylbutadiene, inverse electron demand Diels-Alder reactions with dienophiles such as vinylethers and enamines should run better. The high stereoselectivity observed for the reactions with maleimide 3 or maleic anhydride, 2a had been explained on the base of strong electrostatic repulsions between the oxygen atoms of diene and dienophile in the transition state leading to the resulting adducts. In order to support this explanation, the study of dienophiles unable to exhibit such interaction in the TS was necessary. In this paper we report the results obtained in reactions of en antiomerically pure [(S)R]-( 1E,3E)-1 -p-tolylsulfinyl-1,3-pentadiene 1 with ethyl vinyl ether, 1-pyrrolidinyl-1-cyclohexene and methyl acrylate which allowed us to clarify several of the mentioned questions as well as to establish the regiochemistry of the cycloadditions with 1-sulfinyl substituted dienes.
All attempts to achieve the cycloaddition between enantiomerically pure diene 13'5 and ethyl vinylether (different solvents, temperatures, Lewis acids and high pressures) were unsuccessful. 6 Compound 1 reacted with 1-pyrrolidinyl-1-cyclohexene 2 in refluxing acetonitrile 7 giving a complex mixture where the cycloadducts could only be detected in trace amounts. A clean reaction was achieved when a mixture of diene 1 and enamine 2 (6 eq) was submitted to high pressure (13 Kbar) in ether solution. The results are shown in Scheme 1.