one
(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]. 1 L C H 2 [I] 2 2 = CH=CH (21 3 2 = C<y' [3] 4 2 = C=CM% [4] C' 9X=CH2 CH [S] l o x = CI, OMe [lo] 11 [Ill 12 [Ill [I21 13 [lo] Ar = 2-N02-C6H4 4 0 18 X = CH2,O [IS] 19 [I91 20 [201 21 1211 22 [211 0 0 0 L O 2 .,< 2 0 23 24 26 26
We report here on the synthesis of new semicyclic dienes 2S26 grafted onto 7-oxabicyclo[2.2. llheptane skeletons. We shall show that their Diels-Alder additions strongly prefer the exo-face of the bicyclic systems. Under conditions of kinetic and thermodynamic control, the cheletropic additions of SO, to these 1,3-dienes also lead to adducts resulting from the addition onto their ex0 -face.
Results and
Discussion. -The racemic dienes 23 and 24 were prepared from bromoenone 29 [22] derived from (f)-7-oxabicyclo[2.2.l]hept-5-en-2-one (28) [23] (Scheme 1 ). Stille's coupling [24] of 29 with CH,=CHSnBu, and a catalytic amount of [Pd(Ph,P),] in Scheme I 27 28 1 2. H202