Spectroscopic studies on the TiCl4-promoted reaction of allylsilanes with aldehydes and α,β-enones

Stereochemistry of the allylation and crotylation reactions of alpha-methyl-beta-hydroxy aldehydes with allyl- and crotyltrifluorosilanes. Synthesis of anti,anti-dipropionate stereotriads and stereodivergent pathways for the reactions with 2,3-anti- and 2,3-syn-alpha-methyl-beta-hydroxy aldehydes

A new method for the stereoselective synthesis of the anti,anti-dipropionate stereotriad via the reaction of alpha-methyl-beta-hydroxy aldehydes with (Z)-crotyltrifluorosilane (24) is described. These reactions were designed to occur through bicyclic transition states (e.g., 31) in which the silane reagent is covalently bound to the beta-hydroxyl group of the aldehyde and the crotyl group is transferred intramolecularly. This methodology was used to synthesize the C(7)-C(16) segment (58) of zincophorin, which contains a synthetically challenging all-anti stereopentad unit. Surprisingly, 2,3-anti- and 2,3-syn-alpha-methyl-beta-hydroxy aldehydes react in a stereodivergent manner with 24: 2,3-anti-beta-hydroxy aldehydes give the targeted anti,anti-dipropionate adducts with high selectivity, but the reactions of 2,3-syn-beta-hydroxy aldehydes are poorly selective. The stereodivergent behavior of 2,3-syn- vs 2,3-anti-alpha-methyl-beta-hydroxy aldehydes is also exhibited in their reactions with the allyl- (68) and (E)-crotyltrifluorosilanes (27). Competition experiments performed with beta-hydroxy aldehydes 37a (anti) and the corresponding p-methoxybenzyl (PMB) ether 48, and between aldehyde 39 (syn) and the PMB ether 90, established that the 2,3-anti-beta-hydroxy aldehydes react predominantly through bicyclic transition states while the 2,3-syn aldehydes react predominantly through conventional Zimmerman-Traxler transition states. NMR studies established that both the 2,3-syn and the 2,3-anti aldehydes form stable, pentavalent silicate intermediates (98 and 100) with PhSiF(3), but chelated structures 99 and 101 could not be detected. The activation energies for the competing bicyclic and conventional Zimmerman-Traxler transition states were calculated by using semiemperical methods (MNDO/d). These calculations indicate that the stereodivergent behavior of the 2,3-syn-beta-hydroxy aldehydes and the 2,3-anti-beta-hydroxy aldehydes is due to differences in nonbonded interactions in the bicyclic transition states. Specifically, nonbonded interactions in the bicyclic transition states for the allylation/crotylation reactions of the 2,3-syn-beta-hydroxy aldehydes permits the traditional Zimmerman-Traxler transition states to be preferentially utilized.

Lewis acid promoted annulation of p-quinoneimines by allylsilanes: a facile entry into benzofused heterocycles

[reaction: see text] Lewis acid promoted addition of allyltriisopropylsilane to p-quinoneimines afforded different benzofused heterocycles in moderate to good yields depending on the conditions employed.

Zirconium tetrachloride-formaldehyde sigma-complexes: A computational and spectroscopic investigation

We have carried out a combined theoretical-experimental study of the structures and energies of ZrCl(4)-aldehyde complexes using (13)C NMR spectroscopy and a DFT (B3LYP) computational approach. The computational investigation has demonstrated the existence of different types of complexes: a 1:1 complex (H(2)CO-ZrCl(4)), various 2:1 complexes ((H(2)CO)(2)-ZrCl(4)), and several dimeric species. The analysis of the energies involved in the formation of the various complexes has indicated that the dimeric species should correspond to the only adduct observed in the (13)C NMR spectra (carbonyl resonance at 226.96 ppm) when a 1:1 ZrCl(4)/aldehyde molar ratio is used, while the 2:1 complex should be responsible for the signal at 224.30 ppm that is recorded when this molar ratio is 1:2.