Stereo electronic requirement

A stereospecific total synthesis of polyoxin C and related nucleosides is reported, in which thereacdon of l-Cphenylthioi-l-nitroalkenes v/ith nucleophiles and siibseqiientozono lysis are key reacdons Addidonof potassium trimethylsilanoate to l-Cphenylthioi-nitroalkenes derived from D-ribose followed by ozonolysis gives the cr-hydroxy thioester, which is formed v/ith excellent diastereoselecdvity fScheme 4 5 This conformadon meets the stereo-electronic requirements for andperiplanar addition of the nucleophile with the result of high 5-fS stereochemical bias in the reacdons... 

A stereoselective reaction on the other hand is one in which the stereo-electronic requirement of the reaction mechanism is such that two equally valid alternative pathways are available for the same mechanistic interaction between reactant and reagent. However, either the free energies of activation of the alternative reactions or the thermodynamic stabilities of the products differ, so that one isomer is formed in preference to the other selection has occurred. An example is provided by the reduction of cholestan-3-one (32). Equatorial attack (i) or axial attack (ii) of the hydride ion is mechanistically equally feasible and stereoelectronically defined. However, steric interactions between the hydride ion source and the conformationally fixed steroid molecule, together with considerations as to whether the reaction was under kinetic or thermodynamic control, would determine that the reaction is proceeding in a stereoselective manner. 

The variety of structural modifications carried out on the ionone ring and the observation that pigment analogs could be made from all these derivatives strongly support the idea that the binding site of opsin is indeed very lenient as far as stereo-electronic requirements of the ring binding site are concerned. 

The mechanism of the transformation of 2-cycloalkenols to chlorooxiranes with hydroxy group participation has been studied by taking into consideration stereo-electronic requirements (Eq. 64). ... 

Whilst these biosynthetically-patterned biotransformations of terpenoids have been described in the context of studies with G. fujikuroi, studies with other fungi have shown that a similar pattern of results can be obtained using other biosyntheses. Although the yields of the metabolites are often poor, the transformations do have some value in the preparation of rare or labelled compounds. Furthermore, the results reveal some constraints on the biosynthetic pathways and provide information on the stereo-electronic requirements of enzymatic processes that could not be obtained from conventional studies. 

Bromination of androstan-i6-ones is complicated by rearrangements and by dibromination, but appears to favour 17a (pseudo-axial) attack initially. Enolisation of the 16-0x0 group is slow and inefficient compared with a 17-ketone, at least when isopropenyl acetate is used [x i], although the same (A ) double bond is produced. The enol acetate bromi-nates selectively at the lya-position [152]. The alternative enol acetate (A ) is formed in minor amount, and gives the 15 -bromoketone. Both 17a- and i5) -attack correspond to pseudo-axial approach of the reagent as required by stereo-electronic factors. 

Thermochemical data, and in particular the enthalpies of formation of oxygen-and sulfur-containing six-membered heterocycles, provide essential information on the factors responsible for the contrasting behaviour (structural, conformational and reactivity) between these types of compounds. Understanding of the experimental thermochemical observations has required theoretical modeling to confirm the relative importance of the steric, electronic, electrostatic and stereo-electronic interactions that are responsible for the enthalpies of formation for 1,3- and 1,4- dioxanes, 1,3,5-trioxane and their... 

Having shown earlier that in 4-vinylcyclohexene, the main product from the dimerization of butadiene, rearrangement of the vinyl group into the ring on thermal automerization is faster than the loss of optical activity, Doering and Brenner now report rate constants for the three processes involved, after resolution of products into optical antipodes and analysis of isotope transpositions in each. Stereo-electronic effects guide the cycloaddition of dichloroketen to 3,3-dimethylcyclohexene and l-methyl-5,5-dimethylcyclohexa-l,3-diene ° whereas steric effects predominate in the additions to 3,3-dimethylpentene and the spiro-diene (56). Evidence was obtained for a non-parallel transition state for addition as required by the [2s + 2a] or [2s + 2s + 2s] mechanisms this was also found in additions of dichloroketen to methylenecyclohexenes. 

Reduction of all of the reported geminal diaryl substrates was completely under the control of the third stereo defining R-group. Particularly difficult were the sterically demanding 29 and the electron poor olefin 30, both of which required heating to produce poor to moderate yields. 

In subsequent studies, methyl vinyl ketone (2.0 mmole) was chosen as the dienophile so as to determine the combined effect of the ionic liquid (2 mL) and the Lewis acids (0.2 and 0.5 wt%) upon the yield and selectivity. Without the Lewis acid catalyst, this system demonstrated a 52% conversion of the cyclopentadiene (2.2 mmol) in 1 h with the endojexo selectivity being 85/15. The cerium triflate-catalyzed reaction was quantitative in 5 min and the endo. exo selectivity was very good for this experiment as well (94 6, endo. exo). Also with the scandium or yttrium salts tested, reactions came to completion in a short time with high stereo-selection. Cerium, scandium and yttrium triflates are strong Lewis acids known to be quite effective catalysts in the cycloadditions of cyclopentadiene with acyclic aldehydes, ketones, quinones and cycloalkenones. These compounds are expected to act as strong Lewis acids because of their hard character and the electron-withdrawing triflate group. On the other hand, reaction times of 1 hour were required for... 

As you should recall from general chemistry, a favorable equilibrium constant is not sufficient to ensure that a reaction will occur. In addition, the rate of the reaction must be fast enough that the reaction occurs in a reasonable period of time. The reaction rate depends on a number of factors. First, the reactants, in this case the acid and the base, must collide. In this collision the molecules must be oriented properly so that the orbitals that will form the new bond can begin to overlap. The orientation required for the orbitals of the reactants is called the stereoelectronic requirement of the reaction. (,Stereo means dealing with the three dimensions of space.) In the acid-base reaction, the collision must occur so that the atomic orbital of the base that is occupied by the unshared pair of electrons can begin to overlap with the is orbital of the acidic hydrogen. 

Without the need for complicated magnetic lenses and electron beams, the STM is far less complex than the electron microscope. The tiiiy tunneling current can be simply amplified through electronic circuitry similar to that which is used in other electronic equipment, such as a stereo. In addition, the sample preparation is usually less tedious. Many samples can be im ed in air with essentially no preparation. For more sensitive samples which react with air, imaging is done in vacuum. A requirement for the STM is that the samples be electrically conductir, such as a metal. 

From the above discussion, it is easy to identify those acetals that will react with ozone and also those acetals that either do not react or react, but with great difficulty. Note the three different conformers of 74. The conformers 74a and 74b meet the requirement for reaction with ozone for having two electron pair orbitals antiperiplanar to the crc H bond. The conformer 74c does not meet this requirement, as it has only one such electron pair orbital. To test whether 74c indeed does not react or reacts with ozone but slowly in comparison to the conformers 74a and 74b, one needs to freeze the conformer 74c as in 75. The species 75 was indeed discovered to be inert to ozone. Likewise, the reactive conformer 74b could be frozen as in 76 and 77. Note that the key structural features of the three conformers of 74 resemble that of a p-glycoside. Also note further the stereo-functional similarity between 75 and 78 the [Pg.16]

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