“…The key step in the palladium-catalyzed preparation of bis-allylic compounds is the nucleophilic attack on a β-substituted (η 3 -allyl)palladium complex (Scheme ). For electron-withdrawing β-substituents, the nucleophilic attack proceeds with remarkably high 1,4-regioselectivity. − In some recent studies we have discussed the effects of electronegative β-substituents on the structure and properties of (η 3 -allyl)palladium complexes. − It was concluded that polar β-substituents are involved in conjugative electronic interactions with the allyl−palladium system, inducing asymmetric electron distribution on the allylic fragment, which makes the less substituted allylic terminus more reactive to nucleophilic attack. 1 …”
Three new β-silyl-substituted (η3-allyl)palladium complexes were prepared (3−5) in order
to study the substituent effects of the silyl functionality on the allyl−metal system. Complexes
3−5 show a downfield 13C NMR shift for the more substituted allylic terminus (C3). The
deshielding effect of the silyl group on C3 is strongest when PPh3 ligands are coordinated to
palladium. Both internal and external nucleophiles attack the less substituted allylic
terminus (C1) with high selectivity. A theoretical analysis of the structure and stability of
slightly simplified model compounds 7−10 was performed by applying density functional
theory at the B3PW91 level. The theoretical results indicate hyperconjugative interactions
between the π-allyl−metal system and the C−Si σ-bond. The intensity of these interactions
depends on the conformation of the silyl substituent and on the σ-donor/π-acceptor character
of the ancillary ligand on palladium. The β-substituent effects increase the thermodynamic
stability of the complexes, weaken the Pd−C3 bond, and facilitate the heterolytic fission of
the C−Si bond. It was also concluded that regioselection of the nucleophilic attack is enhanced
in the presence of strong β-silyl effects.
“…The key step in the palladium-catalyzed preparation of bis-allylic compounds is the nucleophilic attack on a β-substituted (η 3 -allyl)palladium complex (Scheme ). For electron-withdrawing β-substituents, the nucleophilic attack proceeds with remarkably high 1,4-regioselectivity. − In some recent studies we have discussed the effects of electronegative β-substituents on the structure and properties of (η 3 -allyl)palladium complexes. − It was concluded that polar β-substituents are involved in conjugative electronic interactions with the allyl−palladium system, inducing asymmetric electron distribution on the allylic fragment, which makes the less substituted allylic terminus more reactive to nucleophilic attack. 1 …”
Three new β-silyl-substituted (η3-allyl)palladium complexes were prepared (3−5) in order
to study the substituent effects of the silyl functionality on the allyl−metal system. Complexes
3−5 show a downfield 13C NMR shift for the more substituted allylic terminus (C3). The
deshielding effect of the silyl group on C3 is strongest when PPh3 ligands are coordinated to
palladium. Both internal and external nucleophiles attack the less substituted allylic
terminus (C1) with high selectivity. A theoretical analysis of the structure and stability of
slightly simplified model compounds 7−10 was performed by applying density functional
theory at the B3PW91 level. The theoretical results indicate hyperconjugative interactions
between the π-allyl−metal system and the C−Si σ-bond. The intensity of these interactions
depends on the conformation of the silyl substituent and on the σ-donor/π-acceptor character
of the ancillary ligand on palladium. The β-substituent effects increase the thermodynamic
stability of the complexes, weaken the Pd−C3 bond, and facilitate the heterolytic fission of
the C−Si bond. It was also concluded that regioselection of the nucleophilic attack is enhanced
in the presence of strong β-silyl effects.
“…The reaction proceeded with an overall retention of configuration as expected and the 1 H NMR spectra of the crude reaction mixture only showed the desired diastereomer 17 . Several known catalytic systems were tested [ 29 ]: Pd(PPh 3 ) 4 /PPh 3 in THF/DMF [ 33 ], Pd 2 (dba) 3 /diphos in THF/DMF [ 34 ], Pd 2 (dba) 3 /dppf in THF [ 35 ]. The first set of conditions proved to be the most successful, although addition of DMF was required to improve the solubility of the reactants.…”
SummaryA convergent and stereoselective synthesis of chiral cyclopentyl- and cyclohexylamine derivatives of nucleoside Q precursor (PreQ0) has been accomplished. This synthetic route allows for an efficient preparation of 4-substituted analogues with interesting three-dimensional character, including chiral cyclopentane-1,2-diol and -1,2,3-triol derivatives. This unusual substitution pattern provides a useful starting point for the discovery of novel bioactive molecules.
“…The mixture was stirred at room temperature under an argon atmosphere for 30 min. Tetrakis(triphenylphosphine)palladium (1.0 g, 0.87 mmol), Ph 3 P (0.50 g, 1.91 mmol), and a solution of 13 7 (2.0 g, 14.08 mmol) in dry THF (30 mL) was added . The mixture was stirred at 55 °C for 2 days.…”
A recent observation that (+)-7-deaza-5'-noraristeromycin (1), as an L-like analogue of aristeromycin, possessed meaningful anti-trypanosomal properties has prompted a search of other 7-deazapurines with similar or improved anti-trypanosomal responses. In that direction a series of pyrazolo[3,4-d]pyrimidines (that is, 8-aza-7-deaza-5'-noraristeromycin derivatives, 2-11) related to 1 have been prepared. These derivatives were evaluated against bloodstream forms of Trypanosoma brucei brucei and Trypanosoma brucei rhodesiense grown in vitro. Of these compounds, the parent L-like derivative 2 was less potent (IC50 40-70 microM) than 1 (IC50 0.165-5.3 microM) whereas the D-like analogue 3 was inactive, which is the same trend observed previously with 7-deaza-5'-noraristeromycin. Interestingly, some moderate activity (IC50 12.2-16.8 microM) was seen in the D-like 4'-methyl derivative 7 while its L-like partner was inactive.
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