Electrophilic reactions stereoselective

According to the stepwise electrophilic reaction mechanism, the differences in the stereochemistries of the products from the reactions of alkenes with cyclic 49 and acyclic 51 disulfonium dications can be explained by the larger rates of the intramolecular reactions. In the case of a cyclic dication, the carbocationic center in intermediate 94, which is formed as the result of initial attack by a S-S dication on a double C=C bond reacts with nucleophile intramolecularly, thus conserving the configuration of the substituents at the double bond. On the other hand, an acyclic dication undergoes transformation to two separate particles (95 and dimethylsulfide) with a consequent loss of stereoselectivity. Additional experiments with deuteretad alkenes confirm that reaction is not stereoselective, lending further support to the stepwise mechanism (Scheme 36).106... 

The aziridines are the nitrogen analogs of the epoxides and undergo similar electrophilic reactions. No biological data were obtained for these compounds nor were they used as precursors to any CA-4, 7, analogs. They have been included since the synthesis is noteworthy, and they could be interesting intermediates. Xu et al. stereoselectively aziridinated chalcones using the nitrene precursor (PhINTS) and a copper catalyst to form compound 141 (Scheme 36) [82],... 

Addition of nucleophiles to electrophilic glycine templates has served as an excellent means of synthesis of a-amino acid derivatives [2c, 4—6]. In particular, imines derived from a-ethyl glyoxylate are excellent electrophiles for stereoselective construction of optically active molecules [32], This research and retrosyn-thetic analysis led us to believe that amine-catalyzed asymmetric Mannich-type additions of unmodified ketones to glyoxylate derived imines would be an attractive route for synthesis of y-keto-ce-amino acid derivatives [33], Initially, L-proline-catalyzed direct asymmetric Mannich reaction with acetone and N-PMP-protected a-ethyl glyoxylate was examined in different solvents. The Mannich-type reaction was effective in all solvents tested and the corresponding amino acid derivative was isolated in excellent yield and enantioselectivity (ee >95 %). Direct asymmetric Mannich-type additions with other ketones afford Mannich adducts in good yield and excellent regio-, diastereo- and enantioselectivity (Eq. 8). 

VI. AMIDOESTERS. STEREOSELECTIVE, REGIOSELECTIVE AND ELECTROPHILIC REACTIONS AT THE a-CARBON... 

It should be pointed out that in nucleophilic reactions, where the second step is protonation, as well as in the capture of the carbocation formed in the course of electrophilic reactions with bicyclobutane, the reactive intermediates are preferentially trapped from an equatorial direction. A priori, there is no obvious stereoelectronic driving force for this exceptional behavior of the reaction of PhSH with 60. However, in this specific case, it could stem from a steric repulsion between the methylene group at position 4 of the tricyclic system and the phenyl group of the PhS either entering or already present. Supporting this assumption are reactions of thiyl radical with bicyclobutane (equation 85) " . In the absence of the steric effects of the tricyclic system, the cis and trans isomers were obtained in equal amounts. Hence, at this stage, it seems that unlike the reactions of cyclobutyl carbanion or carbocation, stereoselectivity in the trapping radical... 

A related method to interpret the diastereofecial selectivities of the reactions of double bonds has been proposed by Dannenberg and coworkers [8, 13, 14,]. Tins method also relies on the 7t frontier orbitals of non symmetrical molecules, and proposes breaking the symmetry of the n or it orbitals due to polarization induced by the substituents. Application of frontier molecular orbital theory, taking into account only the substrate MOs, gives a qualitative trend of stereoselection in a number of nucleophilic (reductions of carbonyl compounds) and electrophilic reactions. 

Peracids can react with alkanes to give hydroxylated products, as shown in equation (66). This may be an electrophilic reaction because the rate increases with increasing acidity of the peracid. Radical side reactions were thought to be inhibited by added l2. CF3CO3H is also an effective oxidant. The reaction of franj-l,2-dimethylcyclohexane with PhC03H is reported to be 97% stereoselective (retention) and 97% regioselective for tertiary hydroxylation. ... 

Reactions.— The stereoselectivity of the abstraction of diastereotopic protons a to trico-ordinated sulphur and the stereochemistry of electrophilic reactions at the carbanion centres of cFlithio-thian 1-oxides still excite considerable interest. The rate factors for base-catalysed H-D exchange of a-protons in several conforma-tionally biassed or rigid thianium cations have been measured with increasing rigidity, the reactivity of decreases, while that of remains about constant. It... 

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