Stereoselective reactions at the

If R and R are different, the two faces of the double bond become nonequivalent, permitting stereoselective reactions at the double bond. These effects have been explored, for example, using 4-silyl-2-pentenes. Reactions such as epoxidation and hydroboration proceed by preferential addition fiom the face opposite the bulky silyl substituents. 

V. AMIDES, ELECTROPHILIC REGIO AND STEREOSELECTIVE REACTIONS AT THE a-CARBON.1514... 

To achieve a stereoselective reaction at the new stereogenic centre (shown in black) the green stereogenic centre would somehow have to influence the direction of attack of the nucleophile on the C=0 group. Separated from it by three bonds, in a molecule with a high degree of flexibility, makes this a very tall order. A more or less 50 50 mixture of the two diastereoisomers would be expected. 

Conjugated dienes, upon complexation with metal carbonyl complexes, are activated for Friedel-Crafts acylation reaction at the akyhc position. Such reactions are increasingly being used in the stereoselective synthesis of acylated dienes. Friedel-Crafts acetylation of... 

The N,0- and N,S-heterocyclic fused ring products 47 were also synthesized under radical chain conditions (Reaction 53). Ketene acetals 46 readily underwent stereocontrolled aryl radical cyclizations on treatment with (TMSlsSiH under standard conditions to afford the central six-membered rings.The tertiary N,0- and N,S-radicals formed on aryl radical reaction at the ketene-N,X(X = O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from (TMSlsSiH was investigated and found to provide higher selectivities than BusSnH. 

The optical stability of organotin compounds is then discussed and the stereoselectivity (or nonstereoselectivity) of a series of substitution reactions at the tin atom is described and used to distinguish between possible mechanisms. 

The optical purity of almost all the organotin compounds described in this chapter is not yet known. In order to determine the stereoselectivity of substitution reactions at the tin atom of these organotin compounds, it is almost always necessary to know the optical purity of the starting compound and of the final product. The method described in this section can be used not only for the resolution of racemic organotin compounds but also for the determination of their optical purity 50). It will be a valuable tool for the determination of the stereoselectivity of the reactions described in Chapter 5, and of other reactions which will be studied. 

The prochirality concept is not necessarily an expression of a precursor-product relationship because there exist stereoselective reactions at pro-chiral elements that do not generate elements of chirality. An illustration of this is the reversible enzymatic dehydration of citric to cu-aconitic acid. In this process two prochiral centers of citric acid disappear and we obtain an achiral line of stereoisomerism that physically coincides with a prochiral plane of prostereoisomerism. 

Lewis acids can be efficiently used to increase the reactivity of reactants in cycloaddition reactions. At the same time improved regio- and stereoselectivities can be achieved. 

Acylation of limonene at the disubstituted double bond is favoured by a factor of 2.3 over reaction at the trisubstituted double bond using acetyl hexachloroan-timonate. Mixed alkylcuprate alkylation of tricarbonylcyclohexadienyliron salts has been used to synthesize the a-phellandrene tricarbonyliron complex. Dichlorocarbene addition to limonene in the presence of 1,4-diazabicy-clo[2,2,2]octane is almost 100% stereoselective at the trisubstituted double bond (no yield given) (cf. Vol. 6, p. 31) in contrast to dibromocarbene addition to carvone (Vol. 7, p. 34), dichlorocarbene addition to the carveols is not regio-specific. ... 

Treatment of the potentially electrophilic Z-xfi-unsaturated iron-acyl complexes, such as 1, with alkyllithium species or lithium amides generates extended enolate species such as 2 products arising from 1,2- or 1,4-addition to the enone functionality are rarely observed. Subsequent reaction of 2 with electrophiles results in regiocontrolled stereoselective alkylation at the a-position to provide j8,y-unsaturated products 3. The origin of this selective y-deproto-nation is suggested to be precoordination of the base to the acyl carbonyl oxygen (see structures A), followed by proton abstraction while the enone moiety exists in the s-cis conformation23536. 

Remarkable changes in the product profiles were recognized when the reaction was carried out at low (5%) conversion or at low temperature (-78°C). In the former case, a good ee value (77% ee) was obtained with exclusive product formation (>95% as observed by H-NMR of the crude mixture). Under these conditions, the molecules were supposed to be still under topochemical control leading to the better stereoselective reaction in the crystals. Additionally, the reaction at -78°C gave similar results (50% yield, 65% ee) for the same reason, where the molecules were strongly frozen in the crystal lattices. Obviously, the topochemical control was much more effective at lower temperature since 65 %ee was obtained even at 50 % conversion, which value corresponded to about 39 %ee for 0°C on estimation of the curve. Apart... 

Every reaction at the molecular level is stereospecific (axiom 1). For a collection of molecules the elementary reaction is stereoselective... 

Olefins react with benzenetellurinyl trifluoroacetate in acetonitrile in the presense of boron trifluoride etherate to give 2-acetaminoalkyl phenyl tellurium oxides that are converted, upon treatment with triethylamine in tetrahydrofuran at 30° for 4 h, to 4,5-dihydro-l,3-oxazoles4. These highly regio- and stereoselective reactions allow the one-pot, high-yield conversion of olefins to 1,3-oxazoles. Ethyl cyanide and phenyl cyanide can be used instead of acetonitrile. 

Disubstituted 1,3-oxathianes 316, prepared from benzaldehyde and the corresponding 3-mercaptoalkanol, have been stereoselectively deprotonated at the equatorial position to yield, after reaction with electrophiles, 2,2-disubstituted products. t.S j-Benzoin was obtained in 75% ee in the case of using compound 316 (R = OTBS), after reaction with benzaldehyde and final deprotection of the major product 317 with NCS and silver nitrate (Scheme 83)490. 

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

Although the subsequent discussion describes the stereoselection at the steady state through the example of radical reactions, the analysis and principles are general for any reaction profile that fits into the scheme of complex stereoselective reactions. In the process proposed and analyzed by Curran et al., the activation of compounds of type 1 is done, for example, by radical formation. The group selectivity in this first step has again no effect on the stereomeric nature of the product. To obtain a stereoconvergent process it is crucial, however, that the reaction is operating at the steady state. This means that the concentrations of the radial intermediates (compounds in brackets in Scheme 2) is low and stationary, while their absolute concentrations are determined by the different rates of reaction. 

Matsuo et al. 2004). Several kinds of double reactions at the left and right sides were efficiently used throughout the synthesis. Thus, the total synthesis of BTX-B (1) was efficiently achieved with high stereoselectivity in 59 steps as the longest linear sequence and in total 90 steps with an average of 93% yield for each step. 

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