Phototryptophan is a new photoactivatable amino acid designed to be incorporated into tryptophan‐containing peptides for photoaffinity labeling. The synthesis of phototryptophan starts from 6‐bromoindole with the installation of an 1‐azi‐2,2,2‐trifluoroethyl moiety, followed by microwave‐assisted condensation with serine under mild conditions. Separation of the N‐acetylated enantiomers was possible by employing Aspergillus aminoacylase. Differential scanning calorimetry showed that phototryptophan is thermally stable up to 100 °C. Boc‐L‐Phototryptophan was used to assemble a diazirine analog of the marine natural product hemiasterlin.
The synthesis, thermal stability, and photoreactivity of trifluoromethyl diazirines installed at the heterocyclic section of N‐methylindole, N‐methylbenzimidazole, and at N‐methylimidazole were investigated. N‐Tosyl‐3‐diazirinylindole and N‐methyl‐2‐diazirinylbenzimidazole proved to be thermally stable, whereas the corresponding 2‐diazirinylindole was not. The least stable was 2‐diazirinylimidazole, which underwent rapid decomposition. Quenching with ethanol indicated that the corresponding carbene was formed. Decomposition is rationalized by exothermic coarctate ring‐opening of the carbene. Quantum mechanical calculations [B3LYP/6‐311G(2d,2p)] predict singlet ground states of all carbenes. Accordingly, Friedel–Crafts alkylation products were formed on irradiation (350 nm, Rayonet) of the N‐methylbenzimidazole‐based diazirine in the presence of phenol.
Amines Q 0120Access to Chiral Tertiary Amines via the Iridium-Catalyzed Asymmetric Hydrogenation of Enamines. -The iridium complex shown is found to be the most efficient catalyst to induce enantioselectivity up to 87%. Hydrogenation of the trisubstituted olefin (VII) is also possible; however, an achiral product is obtained. -(CHERUKU, P.; CHURCH, T. L.; TRIFONOVA, A.; WARTMANN, T.; ANDERSSON*, P. G.; Tetrahedron Lett. 49 (2008) 51, 7290-7293; Dep. Biochem. Org. Chem., Uppsala Univ., S-751 23 Uppsala, Swed.; Eng.) -Mais 13-064
Significance: Prior to this work several asymmetric transition-metal-catalyzed hydrogenation reactions using Ti, Rh, and Ir as metal sources have been investigated, providing chiral tertiary amines. In the present study, the authors utilized an iridium complex derived form N,P ligand L1. This system provides modest to good enantioselectivities for the hydrogenation of various acetophenone derived enamines.Comment: While one of the earliest examples of asymmetric enamine hydrogenation utilizing a titanocene catalyst was reported in 1994 by Buchwald and co-workers (J. Am. Chem. Soc. 1994, 116, 5985), synthesis of chiral tertiary amines still remains a challenging endeavor. The authors' iridium system gives relatively good selectivities for various non-cyclic amines; however, use of cyclic amines lead to a significant diminution of the ee value. A clearer understanding and resolution of this problem could lead to a more tolerant catalyst system. N PPh 2 O N L 1 R 1 N R 3 R 2 + H 2 R 1 N R 3 R 2 [L1Ir(cod)] + [BAr F ] -(0.5 mol%) CH 2 Cl 2 , r.t., 6 h (50 bar) R 1 = Ar R 2 ,R 3 = aliphatic, cyclic, Ar * 66-99% yield 20-87% ee 9 examples S e l e c t e d e x a m p l e s : NEt 2 N N N 99% yield 84% ee 99% yield 79% ee 66% yield 33% ee (R) 99% yield 20% ee SYNFACTS Contributors:
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.