(25) and their ethylene acetals 24 and 26, respectively, were derived from the Diels-Alder adduct of furan to I-cyanovinyl acetate (27). The Diels-Alder additions of 26 to dimethyl acetylenedicarboxylate, to methyl propynoate, to N-phenylmaleimide, and to methyl acrylate were highly exo-face selective, as were the cycloadditions of methyl propynoate to dienones 23 and 25 and of dimethyl acetylenedicarboxylate to ethylenedioxy-diene 24. The cheletropic additions of SO2 to 2>26 gave exclusively the corresponding sulfolenes 57-60 resulting from the exo-face attack of the semicyclic dienes under conditions of kinetic and thermodynamic control.Introduction. -The face selectivity of the Diels-Alder additions of conformationally rigid s-cis-butadiene systems attached to bicyclic skeletons (e.g. 1-13) were studied extensively [ 1-1 51. The face selectivity depends on the nature of the bridges that constitute the bicyclic system, on the nature of the dienophile, and in some cases on the solvent [ 131. The results were interpreted in terms of stereoelectronic factors [ le,f,g,h], of torsional effects [4] [14], and of steric hindrance [8] [ll] [15]. In the case of semicyclic 1,3-dienes grafted onto monocyclic skeletons (e.g. 14-22), the face 'anti' to the substituent the closest to the diene moiety is generally preferred for the Diels-Alder addition, suggesting that repulsive steric factors govern the stereoselectivity of these cycloadditions [16][17][18][19][20][21].
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Homochiral Matching in the Diels-Alder Cyclodimerization of 2-Vinyl-7-oxabicyclo(2.2.1)hept-2-ene Derivatives.-Whereas ketone (I) dimerizes spontaneously at 0 • C in solution, the analogues (II)-(IV) react much more slowly. Generally, the dimerizations are highly stereoselective and give excellent yields for both the racemic and chiral series. -(MEERPOEL, L.; VRAHAMI, M.-M.; ANCEREWICZ, J.; VOGEL, P.; Tetrahedron Lett. 35 (1994) 1, 111-114; Sect. Chim., Univ. Lausanne, CH-1005 Lausanne, Switz.; EN)
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