Regioselectivity factors affecting

Chiral cyclopentanones.2 The regioselective cyclization of a-diazo-p-keto esters to cyclopentanones (11, 459) is also enantioselective with substrates derived from chiral alcohols. Preliminary studies show that steric factors affect the diastereoselectivity the highest diastereoselectivity is obtained with esters of the alcohol 1, which is available from camphor,3 and in which both the bomane and the naphthalene rings can exert steric effects on the diastereoselectivity. 

Song, C. and A. Schmitz. Shape-Selective Isopropylation of Naphthalene over Dealuminated Mordenite. Key Factors Affecting Regioselectivity. Extended Abstracts, Program. 11th Internat. Congress on Catalysis, Baltimore, Maryland, June 30-July 5, 1996, Po-164. 

The factors affecting HheC-mediated regioselectivity are summarized in Scheme 11. First, the hydrogenbonding pattern in the active site affects the regioselectivity by restricting the relative movement of the... 

There has been a review of factors affecting positional selectivity in substitutions on heteroaromatics, and a theoretical study of reactivity and regioselectivity in... 

This chapter will discuss the factors affecting the reaction regioselectivity of several alkenyl substrates and the related mechanistic aspects arising fi-om in situ IR spectroscopic studies and deuterioformylation experiments. 

Acetate pyrolysis is normally carried out using a flow apparatus at temperatures in die region of 3(X)-600 °C. The syn elimination mechanism (equation 48) was established by pyrolysis of the stereospecifi-cally deuterium-labeled acetates (116) and (117), which gave labeled and unlabeled products as shown in equations (49) and (50). 1,2-Disubstituted acyclic alkenes are normally obtained with the ( )-stereo-chemistry, as for the other eliminations discussed in this chapter. However, the regioselectivity of acetate pyrolysis can be rather modest. This is because the regioselectivity is affected by several factors, in particular the stability of the product alkene and the number of available 3-hydrogens. If these two factors oppose each other only low regioselectivity is observed, e.g. a 50 50 mixture of alkenes (119) and (120) was obtained from the pyrolysis of acetate (118 equation 51). 

A series of a-donor-cyclopenten-l-ones (74 X = H, Cl, Br, OMe, SPh, etc.) have been reacted with a variety of alkyllithiums to explore the factors affecting 1,2-versus 1,4-addition. Calculations using density functional theory show that, in general, the regioselectivity of addition (in THF, at —78 °C) does not accord with the thermodynamic stability of the products. 

The results of the alkylation of 6 377a 426-427 429 are summarized in Table XI. In addition to the above factors, the regioselectivity of electrophilic attack on 518 was strongly affected by the structure of the alkylating agents, e.g., methyl iodide, ethyl bromide, cyclohexyl bromide, and rm-butyl bromide. 

Under Payne rearrangement conditions, sodium /-butylthiolate provides 1 -/-butylthio-2,3-diols with very high regioselectivity. The selectivity is affected, however, by many factors including reaction temperature, base concentration, and the rate of addition of the thiol. These sulfides can then be converted to the l,2-epoxy-3-alcohols, which in turn react with a wide variety of nucleophiles specifically at the 1-position (Scheme 9.6). This methodology circumvents the problems associated with the instability of many nucleophiles under Payne conditions.85... 

The regioselectivity of the iodolactonization of l,6-hcptadicnc-4-carboxylic acid derivatives24 is strongly affected by electronic factors. In fact, the electronic control, as a consequence of the differential alkene substitution, was proven in the iodolactonization of 2-methyl-l,6-heptadi-ene-4-carboxylic acid and 2-methyl-l,6-octadiene-4-earboxylie acid. The conditions employed play an important role in the selectivity low temperature, tetrahydrofuran as solvent and butyllithium as base strongly increase the trans/cis selectivity in favor of the methallyl moiety. 

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