Facial stereoselectivity

On the other hand, addition of methylmagnesium bromide, as well as (triisopropoxy)methyl-titanium, to the O-benzyl-protected analog 4 proceeds with the facial stereoselection predicted by Cram s open-chain model, although the selectivities are very low72. 

Aqueous cycloaddition using glyco-organic substrates. Facial stereoselectivity in Diels-Alder reactions of a chiral diene derived from D-glyceraldehyde [102]... 

Summary of Facial Stereoselectivity in Aldol and Mukaiyama Reactions. The examples provided in this section show that there are several approaches to controlling the facial selectivity of aldol additions and related reactions. The E- or Z-configuration of the enolate and the open, cyclic, or chelated nature of the TS are the departure points for prediction and analysis of stereoselectivity. The Lewis acid catalyst and the donor strength of potentially chelating ligands affect the structure of the TS. Whereas dialkyl boron enolates and BF3 complexes are tetracoordinate, titanium and tin can be... 

The use of chiral Br0nsted acids is illustrated in Eq. 93 as a method for catalyst-controlled double diastereoselective additions of pinacol allylic boronates. Aside from circumventing the need for a chiral boronate, these additions can lead to very good amplification of facial stereoselectivity. For example, compared to both non-catalyzed (room temperature, Eq. 90) and SnCU-catalyzed variants, the use of the matched diol-SnCU enantiomer at a low temperature leads to a significant improvement in the proportion of the desired anti-syn diastereomer in the crotylation of aldehyde 117 with pinacolate reagent (Z)-7 (Eq. 93). Moreover, unlike reagent (Z)-ll (Eq. 91) none of the other diastereomers arising from Z- to E-isomerization is observed. 

The diastereoselectivities in the nucleophilic addition reactions of l,3-dioxane-5-ones 37 and l,3-dithiane-5-ones 38 were studied by employing two newly available theoretical tools, the exterior frontier orbital electron (EFOE) density of the 7tc=o -orbitals and the 7t-plane-divided accessible space (PDAS) as quantitative measures of the 7t-facial steric effects <1999CRV1243, 1999CC621, 1999CL1161, 2000H(52)1435, 2001HAC358>. The two parameters predict correctly the experimentally observed stereochemical reversal of 37 and 38 (R = Ph see Table 1) in particular, the PDAS values for both substrates clearly show the opposite steric environment about the carbonyl carbon atom of these heterocyclic ketones and prove sizeable ground-state conformational differences to be responsible for the observed reversed facial stereoselection. 

Electronic and conformational effects on jt-facial stereoselectivity in nucleophilic additions to carbonyl compounds have been studied by the use of RHF/3-21G and RHF/6-31G methods ". Figure 10 shows a comparison of predicted and experimental selectivities for methyl Grignard additions. Satisfactory agreement of the ratios of anti and equatorial attacks of MeMgX on the carbonyl carbon atoms was reported. 

Asymmetric epoxidation, dihydroxylation, aminohydroxylation, and aziridination reactions have been reviewed.62 The use of the Sharpless asymmetric epoxidation method for the desymmetrization of mesa compounds has been reviewed.63 The conformational flexibility of nine-membered ring allylic alcohols results in transepoxide stereochemistry from syn epoxidation using VO(acac)2-hydroperoxide systems in which the hydroxyl group still controls the facial stereoselectivity.64 The stereoselectivity of side-chain epoxidation of a series of 22-hydroxy-A23-sterols with C(19) side-chains incorporating allylic alcohols has been investigated, using m-CPBA or /-BuOOH in the presence of VO(acac)2 or Mo(CO)6-65 The erythro-threo distributions of the products were determined and the effect of substituents on the three positions of the double bond (gem to the OH or cis or trans at the remote carbon) partially rationalized by molecular modelling. 

Both experimental and theoretical studies of the electrophilic additions to vinylic sulphoxides have demonstrated that the jr-facial stereoselection can be rationalized by the transition state 939. 

Similar geometric optimization has been reported for bicyclo[3.2.2]nona-6,8-diene (BND). The double bond situated in the opposite direction to the methylene group was found to be more exo-pyramidalized than the other double bond and the electron density (qi, HOMO) of the former double bond in HOMO of the molecule higher than that of the latter double bond. The exo and endo faces of exo-pyramidalized double bonds proved not to be equal and the electron density was found to be higher on the endo faces. The endo molecular complexes with bromine have been found by the HF/321G method to be more stable than their exo congeners this was attributed to electronic and steric factors. As a result, endo-facial stereoselectivity of bromination ( ) predominates.21 A related theoretical study of facial selectivity and regioselectivity of the electrophilic addition of chlorine to exo-tricyclo[4.2.1.02,5]nona-3,7-diene (exo-TND) has also been reported.22... 

The 1,3-dipolar cycloaddition reactions of racemic as well as enantiopure azetidin-2-one-tethered nitrones 364 with alkenes and alkynes 365 (Equation 136) yielded isoxazolidinyl- or isoxazolinylazetidin-2-ones 366, exhibiting good regio- and facial stereoselectivity (Equation 136) <2002JOC7004>. 

Wong, S. S. Paddon-Row, M. N. Theoretical evidence in support of the Anh-Eisenstein electronic model in controlling ji-facial stereoselectivity in... 

Wu, Y. D. Houk, K. N. Electronic and conformational effects on Jt-facial stereoselectivity in nucleophilic additions to carbonyl compounds, J. Am. Chem Soc. 1987, 908-910. 

Williams, L. Paddon-Row, M. N. Electrostatic and steric control of Jt-facial stereoselectivity in nucleophilic additions of LiH and McLi to endo-5,6-disubstituted norbornen-7-ones an ab initio MO study, J. Chem. Soc. Chem. Commun. 1994, 353-355. 

In addition to the 1,3-ally lie strain concept, Houk has employed a model for 7t-facial stereoselection of electrophilic additions to chiral alkenes, such as hydroboration, epoxidation, and dihydroxylation, with similar predictive success. ... 

Washington, I., Houk, K. N. CH...O hydrogen bonding influences -facial stereoselective epoxidations. Arrgew. Chem., Int. Ed. Engl. 2001, 40, 4485-4488. 

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