3 research outputs found
Relative Tendency of Carbonyl Compounds To Form Enamines
Equilibria between carbonyl compounds and their enamines (from <i>O</i>-TBDPS-derived prolinol) have been examined by NMR spectroscopy in DMSO-<i>d</i><sub>6</sub>. By comparing the exchange reactions between pairs (enamine A + carbonyl B → carbonyl A + enamine B), a quite general scale of the tendency of carbonyl groups to form enamines has been established. Aldehydes quickly give enamines that are relatively more stable than those of ketones, but there are exceptions to this expected rule; for example, 1,3-dihydroxyacetone acetals or 3,5-dioxacyclohexanones (2-phenyl-1,3-dioxan-5-one and 2,2-dimethyl-1,3-dioxan-5-one) show a greater tendency to afford enamines than many α-substituted aldehydes
Nucleophile-Catalyzed Additions to Activated Triple Bonds. Protection of Lactams, Imides, and Nucleosides with MocVinyl and Related Groups
Additions
of lactams, imides, (<i>S</i>)-4-benzyl-1,3-oxazolidin-2-one,
2-pyridone, pyrimidine-2,4-diones (AZT derivatives), or inosines to
the electron-deficient triple bonds of methyl propynoate, <i>tert</i>-butyl propynoate, 3-butyn-2-one, <i>N</i>-propynoylmorpholine, or <i>N</i>-methoxy-<i>N</i>-methylpropynamide in the presence of many potential catalysts were
examined. DABCO and, second, DMAP appeared to be the best (highest
reaction rates and <i>E/Z</i> ratios), while RuCl<sub>3</sub>, RuClCp*(PPh<sub>3</sub>)<sub>2</sub>, AuCl, AuCl(PPh<sub>3</sub>), CuI, and Cu<sub>2</sub>(OTf)<sub>2</sub> were incapable of catalyzing
such additions. The groups incorporated (for example, the 2-(methoxycarbonyl)ethenyl
group that we name MocVinyl) serve as protecting groups for the above-mentioned
heterocyclic CONH or CONHCO moieties. Deprotections were accomplished
via exchange with good nucleophiles: the 1-dodecanethiolate anion
turned out to be the most general and efficient reagent, but in some
particular cases other nucleophiles also worked (e.g., MocVinyl-inosines
can be cleaved with succinimide anion). Some structural and mechanistic
details have been accounted for with the help of DFT and MP2 calculations
Cyclostreptin Derivatives Specifically Target Cellular Tubulin and Further Map the Paclitaxel Site
Cyclostreptin is the first microtubule-stabilizing agent
whose mechanism of action was discovered to involve formation of a
covalent bond with tubulin. Treatment of cells with cyclostreptin
irreversibly stabilizes their microtubules because cyclostreptin forms
a covalent bond to β-tubulin at either the T220 or the N228
residue, located at the microtubule pore or luminal taxoid binding
site, respectively. Because of its unique mechanism of action, cyclostreptin
overcomes P-glycoprotein-mediated multidrug resistance in tumor cells.
We used a series of reactive cyclostreptin analogues, 6-chloroacetyl-cyclostreptin,
8-chloroacetyl-cyclostreptin, and [<sup>14</sup>C-<i>acetyl</i>]-8-acetyl-cyclostreptin, to characterize the cellular target of
the compound and to map the binding site. The three analogues were
cytotoxic and stabilized microtubules in both sensitive and multidrug
resistant tumor cells. In both types of cells, we identified β-tubulin
as the only or the predominantly labeled cellular protein, indicating
that covalent binding to microtubules is sufficient to prevent drug
efflux mediated by P-glycoprotein. 6-Chloroacetyl-cyclostreptin, 8-chloroacetyl-cyclostreptin,
and 8-acetyl-cyclostreptin labeled both microtubules and unassembled
tubulin at a single residue of the same tryptic peptide of β-tubulin
as was labeled by cyclostreptin (219-LTTPTYGDLNHLVSATMSGVTTCLR-243),
but labeling with the analogues occurred at different positions of
the peptide. 8-Acetyl-cyclostreptin reacted with either T220 or N228,
as did the natural product, while 8-chloroacetyl-cyclostreptin formed
a cross-link to C241. Finally, 6-chloroacetyl-cyclostreptin reacted
with any of the three residues, thus labeling the pathway for cyclostreptin-like
compounds, leading from the pore where these compounds enter the microtubule
to the luminal binding pocket