Face selectivity of the Diels‐Alder additions and cheletropic additions of sulfur dioxide to 2‐ vinyl‐7‐oxabicyclo[2.2.1]hept‐2‐ene derivatives

Abstract

(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].

Site Selectivity of the Diels−Alder Reactions of 3-[1-(tert-Butyldimethylsilyloxy)vin-1-yl]furan and 3-(Propen-2-yl)furan. Synthesis of 4-Substituted Benzofurans

“…NOE experiments established that H-2, H-3, and H-4 are cis to one another in 13 and 14 . This type of stereochemistry has been observed by Vogel et al in similar systems.

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