Characterizing the Epothilone Binding Site on ß-Tubulin by Photoaffinity Labeling: Identification of ß-Tubulin Peptides TARGSQQY and TSRGSQQY as Targets of an Epothilone Photoprobe for Polymerized Tubulin.

Photoaffinity labeling with an epothilone A photoprobe led to the identification of the ß-tubulin peptides TARGSQQY and TSRGSQQY as targets of the photoprobe for polymerized tubulin. These peptides represent residues 274-281 in different ß-tubulin isotypes. Placing the carbene producing 21-diazo/triazolo moiety of the photoprobe in the vicinity of the TARGSQQY peptide in a homology model of TBB3 predicted a binding pose and conformation of the photoprobe that are very similar to the ones reported for 1) the high resolution cocrystal structure of epothilone A with an α,ß-tubulin complex and for 2) a saturation transfer difference NMR and transferred NOESY NMR study of dimeric and polymerized tubulin. Our findings thus provide additional support for these models as physiologically the most relevant among several modes of binding that have been proposed for epothilone A in the taxane pocket of ß-tubulin.

Enantioselective synthesis of 3,4-dihydro-1,2-oxazepin-5(2H)-ones and 2,3-dihydropyridin-4(1H)-ones from ß-substituted ß-hydroxyaminoaldehydes.

The synthesis of 3,4-dihydro-1,2-oxazepin-5(2H)-ones and 2,3-dihydropyridin-4(1H)-ones from ß-substituted ß-hydroxyaminoaldehydes is reported. The ß-hydroxyaminoaldehydes were prepared by enantioselective organocatalytic 1,4-addition of N-tert-butyl (tert-butyldimethylsilyl)oxycarbamate to α,ß-unsaturated aldehydes (MacMillan protocol). Alkyne addition to the aldehydes followed by alcohol oxidation furnished N-Boc O-TBS-protected ß-aminoynones. Removal of the TBS protecting group initiated a 7-endo-dig cyclization to yield previously unknown 3,4-dihydro-1,2-oxazepin-5(2H)-ones. Reductive cleavage of the N-O bond of the oxazepinones and Boc-deprotection provided 2-substituted 2,3-dihydropyridin-4(1H)-ones via 6-endo-trig cyclization. 2,3-Dihydropyridin-4(1H)-ones are versatile intermediates that have been used for the synthesis of many alkaloids. The new protocol allows the synthesis of 3-dihydropyridin-4(1H)-ones carrying an array of substituents at C2 that cannot be prepared from commercial ß-amino acids or by one-carbon homologation of proteinogenic amino acids. The use of readily available ß-hydroxylaminoaldehydes expands the utility of our previously reported method to prepare 2,3-dihydropyridin-4(1H)-ones from ß-amino acids as the source of diversity and chirality. A broad substrate scope is possible because ß-aminoaldehydes can be prepared from α,ß-unsaturated aldehydes by an enantioselective organocatalytic process.