Allylic strain element

It should also be noted that there is a strong conformational bias for only one of the product chelate conformers. For example, erythro chelate D should be strongly disfavored by both 1,3-diaxial Rj L and CH3 Xq steric control elements. Consequently, it is assumed that the transition states leading to either adduct will reflect this conformational bias. Further support for these projections stems from the observations that the chiral acetate enolates derived from 149a exhibit only poor diastereoface selection. In these cases the developing Rj CH3 interaction leading to diastereomer A is absent. Similar transition state allylic strain considerations also appear to be important with the zirconium enolates, which are discussed below. 

High levels of enantioselectivity are achieved when the stereochemistry-inducing group is fixed in the transition state, thereby preventing one side of the molecule from attacking the carboxenium ion. Because of steric and electronic effects, the crotyl silane confor-m ations are stabilized when the dihedral angle between the C-Si bond and the ally lie double bond is about 120°. This structural element is still relevant in the transition state of the attack of 12 to a carboxenium ion such as 4 or 8. Therefore, attack cannot come fi om conformation III, but only from conformations I or II. Conformation II, however, is disfavored over conformation I because of 1,3-allylic strain. The bulky silyl group in conformation I then demands that the attack to the electrophile comes from the opposite si face. 

Gephyrotoxin An Example of Allylic Strain as a Stereocontrol Element ... 

Among the earliest examples of symmetrical bifunctional radical cations, the distonic trimethylene species (103) invoked by Williams and coworkers [293, 296, 297] are stabilized solely by hyperconjugation. The main rationale for their formation would be the relief of ring strain. On the other hand, the non-vertical radical cations 137 derived from cyclopentadiene dimers [386-389] are favored by two elements of allylic stabilization. This radical cation has three eonformat-... 

The Patemo-Buchi reaction is the photocycloaddition of an alkene with an aldehyde or ketone to form oxetanes. This transformation has been shown to proceed through a biradical intermediate, and up to three new stereocenters can be formed as a result of this reaction. A general mechanism for the reaction between an aldehyde and a chiral enol silyl ether is shown in Eq. (13.7) [18]. Allylic 1,3-strain is cited as the control element in reactions of this type, and diastereomeric ratios of >95 5 are reported for products 30 containing four contiguous stereocenters. Examples of photocyclizations of amino acid derivatives proceeding through biradical intermediates have been repotted [19]. 

In general, if the enolate contains a stereocenter in the a-position, excellent stereocontrol may be realized. The effect of stereocenters on the p-position is negligible. " " An important factor that should be considered is the geometry of the enolate. For ( )-enolate, the dominant control element is the allylic 1,3-strain, whereas in the case of a (Z)-enolate, the dominant control element is the avoidance of 1,3-diaxial interactions. 

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