摘要：

Systematic investigations have been performed to determine the mechanism and origin of selectivity of the allylmetal-aldehyde reaction. Initial studies were performed to determine the solution structure of Lewis acid-aldehyde complexes. The intramolecular allylmetal-aldehyde reaction was next studied using three different model systems. The first two model systems were investigated to determine the relative disposition of double bonds in the transition structure of the allylmetal-aldehyde condensation. The next model system incorporated a deuterium label to allow an unbiased assessment of the syn versus anti S$\\sb{m E}\\sp\\prime$ pathways. The cyclizations of the model systems were performed with several Lewis acids. The size of the Lewis acid-aldehyde complex significantly affected the relative orientation of the double bonds. Cyclization of the deuterium-labeled model system proceeded with high selectivity via an anti S$\\sb{m E}\\sp\\prime$ pathway regardless of the proximal/distal ratio of products for all Lewis acids studied.A systematic investigation of the mechanism and origin of stereoselection in the reaction of dioxane acetals with allyltrimethylsilane was undertaken. Experimental tests for two limiting mechanisms, synchronous (S$\\sb{m N}$2-like) and dissociative (S$\\sb{m N}$1-like) substitution processes, were investigated. The meso 2,4,6-trisubstituted 1,3-dioxane acetals provided an interesting opportunity to test the timing of bond breaking and making in the substitution reactions. The modest and C(2)-substituent-dependent selectivity excluded the possibility of direct S$\\sb{m N}$2-type attack on a complexed acetal. Furthermore, when the allylation of a putative oxocarbenium ion was investigated only weak and inverted selectivity was observed, ruling out the intermediary of the extended separated ion in reactions of the cyclic acetals under similar conditions. A unified mechanistic scheme involving three distinct ion pairs is proposed to explain the dependence of allylation selectivity on structural and experimental variables. The three species: (1) an intimate ion pair, (2) an external ion pair, and (3) a separated ion each are proposed to react with a different stereochemical profile. The influence of C(2) substituent, acetal configuration, Lewis acid type and stoichiometry, nucleophile type and stoichiometry, concentration, solvent, and temperature were investigated and integrated in the proposed mechanistic scheme.

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