Stereoelectronic effects, and

Stereoelectronic effects and nonbonded interactions are non-cooperative in the reactions of (E)-allylboronates and x-heteroatom-substituted aldehydes. Thus, while transition state 8 experiences the fewest nonbonded interactions (gauche pentane type, to the extent that X has a lower steric requirement than R3), transition state 9 is expected to benefit from favorable stereoelectronic activation (Felkin-type)58f. This perhaps explains why the reaction of 2,3-[iso-propylidenebis(oxy)]propanal and ( >2-butenylboronate proceeds with a modest preference (55%) by way ol transition state 9. This result is probably a special case, how ever, since C-3 of 2.3-[isopropylidenebis(oxy)]propanal is not very stcrically demanding in 9 owing to the acetonide unit that ties back the oxygen substituent, thereby minimizing interactions with the... 

J.-F. Chapat, A. Finiels, J. Joffre, and C. Moreau, Synthesis of butyl-a and / -d-glucopyranosides in the presence of dealuminated H-Y faujasites Kinetic study, mechanism, stereoelectronic effects, and microreversibility principle, /. Catal., 185 (1999) 445—453. 

This primer by Ian Fleming on Pericyclic Reactions complements the Primer (No. 36) by Tony Kirby on Stereoelectronic Effects and will be of interest to apprentice and master chemist alike. 

This book will be particularly valuable to all investigators working with complex organic molecules, whether they be synthetic, medicinal or bioorganic chemists, since it will provide a timely view of stereoelectronic effects and how they may be applied, both to rationalise and to predict organic chemical reactivity. 

Acetals and Related Functions 5 B, and respectively. The remaining conformers , , and F possess a plane of symmetry. Consequently, the acetal function can in principle exist in the six different conformations A-F. Experimental evidence will be described which shows that the relative stabilities of these various conformers depend on stereoelectronic effects and the standard steric interactions. 

Alabugin, I. V. Zeidan, T. A. Stereoelectronic effects and general trends in hyperconjugative acceptor ability of a bonds. J. Am. Chem. Soc. 

The formation of bi- and tricyclic compounds by transannular photochemical cyclization of monocyclic reactants mostly follows special rules that are caused by the molecular rigidity, equilibria of ring conformers and distinct stereoelectronic effects and, therefore, will be discussed in a separate section. Naturally, it is no longer possible to strictly structure the reactions according to the size and the type of the ring and, furthermore, vastly different examples are known. Therefore, only some representative examples will be discussed in this section and the selection does not claim to be complete. 

Closs, G. L., Calcaterra, L. T., Green, N. J., Miller, J. R., and Penfield, K. W., 1986, Distance, Stereoelectronic Effects, and the Marcus Inverted Region in Intramolecular Electron-Transfer in Organic Radical-Anions J. Phys. Chem. 90 3673n3683. 

Gloss GL, Calcaterra LT, Green NJ, Penfield KW, Miller JR. (1986) Distance, stereoelectronic effects, and the Marcus inverted region in intramolecular electron transfer in organic radical anions. J Phys Chem 90 3673-3683. 

The optimized geometry of the biphenol MOM ether-SnCU was obtained from a partial PM3 calculation of the MOM units on the basis of a B3LYP/LANL2DZ-opti-mized geometry for a biphenol-SnCU complex (Fig. 5). It is noteworthy that the C5-04 bond is almost perpendicular to the C1-C3 axis, presumably because of the steric repulsive interactions with apical and equatorial chlorines. Also of interest is the observation that the C5-06 bond is shorter than the 04-C5 bond. This indicates that the C5-06 bond has a partial double-bond character because of the stereoelectronic effect and its most stable conformer would be the reactive oxonium intermediate. 

Rotation around hindered or retrained single bonds usually implies that the molecule reaches the energy barrier that restricts interconversion from one con-former to another. In general terms, steric hindrance is the main limitation and heating is then sufficient to cross the barrier, although additional interactions such as H-bonds, stereoelectronic effects and ionic interactions may either hamper or facilitate the rotation. 

Let us consider the reverse of acetal formation, i.e., acid hydrolysis of an acetal within the ambit of stereoelectronic effects and explore the underlying features. We begin by understanding the conformational profile and the associated conformational effects by representing the acetal in such a way that it appears to be part of a cyclohexane chair. In doing so, we understand the geometrical relationship of various bonds on this ring system much better. 

Lactone is reacted with trimethyloxonium tetrafluoroborate (Me30+BF4 ). The produce is next refluxed with sodium iodide in acetone until complete disappearance of the 5-lactone. By taking entropy effects, stereoelectronic effects and the relative ease of SN2 reaction on primary versus secondary v.v tertiary carbons into consideration, comment on the possible product(s) profile. 

Two of the factors that determine the reactivity of tethered ir-nucleophiles in Mannich-type cycliza-tions have been emphasized stereoelectronic effects and reaction medium effects. The stereoelectronics of orbital overlaps between the ir-nucleophile and the iminium electrophile are best evaluated by considerations such as antiperiplanar addition trajectories and Baldwin s rules for ring formation. The critical importance of the reaction medium has received serious attention only recently. However, it already appears clear that Tr-nucleophiles that would lead, upon cyclization, to relatively unstable carbocations can have their reactivity markedly increased by carrying out the cyclization in the presence of a nucleophilic solvent or additive which, by nucleophilic participation, can obviate the formation of high energy cyclic carbenium ion intermediates. 

The chemistry of spiroketals, especially l,7-dioxaspiro[5,5]undecanes, is well-studied and reviewed in the literature (15,16). Generally, the ratio of isomers of spiroketals may be controlled by several stabilizing factors such as stereoelectronic effects and 1,3-diaxial interaction of the substituents. Utilizing well-designed spiroketals, one can selectively prepare the most stable isomer by thermodynamic equilibration. Based on this idea, we planned to prepare a spiroketal represented by II which possesses several substituents with established (Cy and C5O and unestablished (Ca) configurations. The spiroketal center and its a-position in II can isomerize to the more stable forms by steric effects caused by the established stereocenters on the ring. The prepared most stable isomer... 

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