Although it was expected that this elimination should provide a single isomer from the pure diastereomer 12, a partial E, elimination through a benzyl anion intermediate resulted in the formation of the mixture of two dienes. The conformational analysis of 6A based on MM2* calculations (see Scheme 2) predicts that the dominant conformation (>95%) has a twisted diene moiety and an axial orienration of the alkyne side chain at C(2). Attack of the dienophile from the less hindered convex p-face of the diene can explain the slightly preferential formation of 5 . These calculations also suggest "remote control" of the Diels-Alder reaction of 6A, in which intermediate binding of the dienophile (acetylenedicarboxylate) to the alkynyl group attached to the diene moiety in 6a, would help produce compound 5 as the sole product. Crystal structure data of 14: monoclinic, space group P2,/a, N = 11.888(4), h = 27.454(8). r = 10.509(3) A, p =103.68(2)', V = 3333(2) A3. Z = 4: pca,cd = 1.16 gcm-3; 4705 measured reflections, 2525 classed as observed with I2 3.0u(!). Solution of structure by direct methods (Monte-Carlo MUL-TAN), R = 0.073. Further details of the crystal structure investigation may be

No abstract

Rate studies of the lithiation of benzene and related alkoxy-substituted aromatics by n-BuLi/TMEDA mixtures implicate similar mechanisms in which the proton transfers are rate limiting with transition structures of stoichiometry [(n-BuLi)2(TMEDA)2(Ar−H)]⧧ (Ar−H = benzene, C6H5OCH3, m-C6H4(OCH3)2, C6H5OCH2OCH3, and C6H5OCH2CH2N(CH3)2). Cooperative substituent effects and an apparent importance of inductive effects suggest a mechanism in which alkoxy−lithium interactions are minor or nonexistent in the rate-limiting transition structures. Supported by ab initio calculations, transition structures based upon triple ions of general structure [(n-Bu)2Li]-//+Li(TMEDA)2 are discussed.

No abstract

No abstract