These biological activities often come from their ability through reaction with cellular nucleophiles
by means of a Michael-type addition since the corresponding dihydroderiva-tives usually exhibit little or no activity (Fujita et al.
PPA and TsOH/benzene were chosen as acid catalysts, since these are strong acids and also poor nucleophiles
. The reactions proceeded under the conditions shown in Table 5 and generally gave two kinds of indolic products, namely the normally cyclized product, 7-substituted indole (74), and the rearranged product, 5-substituted indole (75).
It is important to emphasize that, as observed in previous studies , the A ring of norlichexanthone (2) has a higher electron density than the B ring, and the atoms of the fist (carbon and oxygen) are stronger nucleophiles
than the corresponding ones in B ring.
However, this type of derivatization is not applicable to all the groups to be inserted as nucleophiles
. Alcoholic solvents remain the base to dissolve properly the chlorine-nucleosides but the presence of a cosolvent has to be evaluated, accordingly .
The electrophilic character of the central carbon in isothiocyanate is usually explored as a reactive group for biologic nucleophiles
. This group is reactive toward nucleophilic attack of amines, resulting in the formation of a thiourea linkage (Figure 6).
In a number of synthetic processes, acidic hydrogen from weak organic acids namely alpha hydrogen adjacent carbonyl group quite often removed by base to generate, carbanions or nitride ion which can then be reacted further as a nucleophile
to synthesize many industrially important intermediates & products.
The thiol group is a far stronger nucleophile
than the amino groups.
These mutants of retaining glycosidases have had the catalytic glutamate nucleophile
replaced with alanine, glycine, or serine, preventing the usual double displacement mechanism for glycosidic bond cleavage.
The nonsubstitution or effective elimination of the carbon-3 moiety possibly occurred via the pathway of an antibonding orbital, which is "hidden" inside the heterocyclic C-ring (most likely in a chair conformation) rather than the [S.sub.N]2 reaction pathway; thus, it is not accessible to the approaching electron-rich nucleophile
 or by the nonnucleophilic strong base to abstract the OH proton by E2-type elimination.
In the case of P-3 and P-4, which were obtained by using [BF.sub.3][middot][EtNH.sub.2], the monoethylamine could have also acted as a nucleophile
in the termination step, thus producing a secondary amine as a linal group.
The serine amino acid has an OH group and serves as a nucleophile
on the active site of lipase, thus forming a strong hydrogen bond between the "N" atoms in the imidazole ring of the histidine.