A-attack

The pyrazole molecule resembles both pyridine (the N(2)—C(3) part) and pyrrole (the N(l)—C(5)—C(4) part) and its reactivity reflects also this duality of behaviour. The pyridinic N-2 atom is susceptible to electrophilic attack (Section 4.04.2.1.3) and the pyrrolic N-1 atom is unreactive, but the N-1 proton can be removed by nucleophiles. However, N-2 is less nucleophilic than the pyridine nitrogen atom and N(1)H more acidic than the corresponding pyrrolic NH group. Electrophilic attack on C-4 is generally preferred, contrary to pyrrole which reacts often on C-2 (a attack). When position 3 is unsubstituted, powerful nucleophiles can abstract the proton with a concomitant ring opening of the anion. 

The 5a-isomer is obtained from hydrogenation of A -7-keto steroids over platinum or palladium. In the latter case the addition of pyridine to the reaction mixture greatly increases the amount of 5a- product formedd a Attack also takes place on hydrogenation (deuteration or tritiation )... 

Hydrogenation of 19-nor-A -3-keto steroids also gives 5a- and 5 -product mixtures under the usual conditions but with ruthenium oxide at high pressures only the 5j8-isomer is formed.The presence of a 4-methyl group on a A -3-keto steroid increases the amount of a attack as compared to the parent enone. ... 

In light of the previous discussions, it would be instructive to compare the behavior of enantiomerically pure allylic alcohol 12 in epoxidation reactions without and with the asymmetric titanium-tartrate catalyst (see Scheme 2). When 12 is exposed to the combined action of titanium tetraisopropoxide and tert-butyl hydroperoxide in the absence of the enantiomerically pure tartrate ligand, a 2.3 1 mixture of a- and /(-epoxy alcohol diastereoisomers is produced in favor of a-13. This ratio reflects the inherent diasteieo-facial preference of 12 (substrate-control) for a-attack. In a different experiment, it was found that SAE of achiral allylic alcohol 15 with the (+)-diethyl tartrate [(+)-DET] ligand produces a 99 1 mixture of /(- and a-epoxy alcohol enantiomers in favor of / -16 (98% ee). 

Bromination of 4//-furo[3,2-b]indole (50) occurred at the 2-position and was assisted by prior N-benzylation with the 4-benzoyl compound a 61% yield of the 2-bromo derivative was obtained. Thus, a-attack in the furan moiety predominates (78JHC123). When the pyrazolofuran (51) was treated with one molar equivalent of bromine, a mixture of the 2-bromo... 

The reaction of the anion of an aryl allyl sulfoxide with benzaldehyde can take place via an a or y attack. The a attack leads to a product with three stercogcnic centers (four possible diastereomers) whereas the y attack results in a product which has only two stereogenie centers and geometric isomerism is possible. 

Addition of the anions of allyl aryl sulphoxides 407 to benzaldehyde proceeds readily and affords a mixture of products resulting from both a- and y-attack of the allyl anion483 (equation 240). In the case of the a-attack a mixture of all four possible diastereoisomers is observed, while in the case of the y-attack, the diastereoisomer ratio exceeds 2 1. 

A. Attack by NH2, NHR, or NR2 at an Alkyl Carbon 10-44 Alkylation Of Amines Amino-de-halogenation (alkyl)... 

Potential energy sections were calculated for R = CH3 and R = Ph, and X = F, Cl, Br, I, in the geometrical scheme shown in Fig. 16 a. The relative ease of approach of the thiolate anion in the xy plane was discussed in terms of the relative energies and positions of potential energy minima, possibly corresponding to reaction intermediates. Fig. 16b and 16c show, for a-attack, the positions and relative energies of these minima. No such minima were found for /3-attack, while hints for a possible linear RCC—X. .. SH intermediate were found for bromo- and iodophenylacetylene only. The conclusions drawn were a) a-attack is favoured over /3-attack, and b) the order of reactivity with respect to the... 

Such secondary bonds are formed by donation from the lone pair of a nucleophile into the a orbital of a covalent bond ( n —> a attack ). Weak covalent bonds (implying low-energy a levels) are easier attacked by n - a overlap, leading to unsymmetric or symmetric 3c le bonds, than strong bonds this is why supramolecular arrays, due to secondary interactions or 3c-4e bonds, play a particular role in the chemistry of the heavier main group elements. 

Radical reduction of 1-nitro-C-glycosyl compounds. In 1983, Vasella and co-workers125 reported a stereoselective method for the synthesis of a-C-mannopyranosyl compounds by reduction of 1-nitro-C-mannopyranosyl derivatives with Bu3SnH in the presence of a,a -azoisobutyronitrile (AIBN) radical initiator. These reactions involve the formation of anomeric centered radicals. Thus, in the case of d-manno configuration as in 140, the 1,2-cts-C-pyranosyl compound 145 was obtained in 84% yield. The intermediate pyranosyl radicals 143 prefer a-attack by the tin hydride. Thus for D-glucopyranosyl derivatives, the corresponding l,2-tra x-C-pyranosyl compound 144 is obtained preferentially (Scheme 47). 

The lowest TS pc>2l5b lies at almost the same energy as the TS F2b of front-side attack of 2b because of the higher stability of the preceding intermediate 15b while the activation barrier is much higher (27.5 kcal/mol) than that of 2b. The relative stability of 2b and 15b may be sensitive to environmental effects that were not taken into account in the models. Note also that TS al5a of a attack lies only 4.4 kcal/mol higher than TS F3b. 

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