And regiospecificity

Fluorinated a., -unsaturated carbonyl compounds also are reactive dipo-larophiles Because ol the highly activating carbonyl substituent, these 1,3-dipolar cycloadditions are rapid and regiospecific Good examples are the additions of... 

Note that the terms regiose-lective and regiospecific, however, are defined in terms of each other. A re-giospecific reaction is one that is 100% regioselective. 

Tabic 1.2 Relative Rate Constants and Regiospecificities for Addition of Radicals... 

The rate of radical addition is most dramatically affected by substituents either at the site of attack or at the radical center. Remote substituents generally have only a small influence on the stereochemistry and regiospecificity of addition unless these groups are very bulky or the geometry of the molecules is constrained (e.g. intramolecular addition - Section 1,2.4). 

The fact that pentacarbonyl carbene complexes react with enynes in a chemo-selective and regiospecific way at the alkyne functionality was successfully applied in the total synthesis of vitamins of the Kj and K2 series [58]. Oxidation of the intermediate tricarbonyl(dihydrovitamin K) chromium complexes with silver oxide afforded the desired naphthoquinone-based vitamin K compounds 65. Compared to customary strategies, the benzannulation reaction proved to be superior as it avoids conditions favouring (E)/(Z)-isomerisation within the allylic side chain. The basic representative vitamin K3 (menadione) 66 was synthesised in a straightforward manner from pentacarbonyl carbene complex 1 and propyne (Scheme 38). 

Completely stereospecific and regiospecific ring-opening reactions were observed with the just mentioned nucleophiles when the 3-aryl substituent is a phenyl or a p-nitrophenyl group (Scheme 20) [30]. 

Addition of sulfonyl iodide to alkenes ensues stereo- and regiospecifically (equation 41). 

To summarize, when the kinetic data predict that only bromonium ions or only bromocarbocations are formed, the bromination products are obtained stereospecifically and regiospecifically, respectively, whatever the solvent. Olefin brominations involving open intermediates lead to more solvent-incorporated products in methanol or acetic acid than those involving bridged ions. This chemoselectivity can be interpreted in terms of the hard and soft acid and base theory (Dubois and Chretien, 1978). Methanol assistance to intermediate formation also plays a role in determining product-selectivity (Ruasse et al, 1991). 

A remaining class of dimerizations is represented by the reactions shown in Eqs. 36—39. These are singlet state reactions, and they are all characterized by stereospecificity and regiospecificity. Production of the trans isomer from the steroidal dienone in Eq. 39 is anomalous and poses a... 

Cyclic imidate esters, 2-ethoxypyrrolin-5-one and 2-ethoxy-1II -indol-3-one, undergo 1,3-dipolar cycloaddition reactions with nitrile oxides, the reaction site being at the pyrroline C=N bond (317). Rigid and sterically congested pyrroline spiro compounds 148 demonstrate complete diastereofacial selection in site and regiospecific cycloaddition reactions with nitrile oxides to give products 149 (318). 

Reactions of 2,3-dihydro-17/-1,4-diazepines with mesitonitrile oxide proceed with site- and regiospecific 1,3-dipolar cycloaddition leading to bis[ 1,2,4] oxadiazolo[l,4]diazepine derivatives 160 (326). Of the three compounds 160 only the one with R = R = Ph is formed with trails arranged substituents. The two other products (R = R = Me and R = Me, R = Ph) are mixtures of diastereoiso-mers. The heterotricyclic 6,1 Oa, 11,11 a-tetrahydro-5//-bis[ 1,2,4]oxadiazolo[4,5-d 5 -g][, A diiazeipm.e structure 160 of the obtained bis-adducts indicates that the hetero double bonds are much more reactive than the olefinic ones. No evidence for the formation of monoadducts was obtained. 

Grigg discovered that a 5-exo-dig Pd-catalyzed cyclization of IV-acetylenic-o-haloanilines 291 to give 3-exo-alkylidene indolines 292 occurs in the presence of a hydride source, such as formic acid [346]. The reaction is stereoselective and regiospecific. 

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