1. Proline-catalyzed asymmetric addition reaction of 9-tosyl-3,4-dihydro-beta-carboline with ketones
Takashi Itoh, Masashi Yokoya, Keiko Miyauchi, Kazuhiro Nagata, Akio Ohsawa Org Lett. 2003 Nov 13;5(23):4301-4. doi: 10.1021/ol030103x.
[reaction: see text] 9-Tosyl-3,4-dihydro-beta-carboline (1) reacted with a ketone in the presence of (S)-proline as a catalyst to give the corresponding addition product in good yield and high enantioselectivity. In the process, a small amount of water was found to affect the stereoselectivity of the products. The system was applied to reaction of compound 1 and 3-buten-2-one to give 3,4,6,7,12,12b-hexahydro-1H-indolo[2,3-a]quinolizin-2-one, which is a versatile precursor for the synthesis of some indole alkaloids.
2. N-Tosyl-L-proline benzene hemisolvate: a rare example of a hydrogen-bonded carboxylic acid dimer with symmetrically disordered H atoms
Joanna Wojnarska, Katarzyna Ostrowska, Marlena Gryl, Katarzyna Marta Stadnicka Acta Crystallogr C Struct Chem. 2019 Sep 1;75(Pt 9):1228-1233. doi: 10.1107/S2053229619010829. Epub 2019 Aug 7.
The carboxylic acid group is an example of a functional group which possess a good hydrogen-bond donor (-OH) and acceptor (C=O). For this reason, carboxylic acids have a tendency to self-assembly by the formation of hydrogen bonds between the donor and acceptor sites. We present here the crystal structure of N-tosyl-L-proline (TPOH) benzene hemisolvate {systematic name: (2S)-1-[(4-methylbenzene)sulfonyl]pyrrolidine-2-carboxylic acid benzene hemisolvate}, C12H15NO4S·0.5C6H6, (I), in which a cyclic R22(8) hydrogen-bonded carboxylic acid dimer with a strong O-(1/2H)...(1/2H)-O hydrogen bond is observed. The compound was characterized by single-crystal X-ray diffraction and NMR spectroscopy, and crystallizes in the space group I2 with half a benzene molecule and one TPOH molecule in the asymmetric unit. The H atom of the carboxyl OH group is disordered over a twofold axis. An analysis of the intermolecular interactions using the noncovalent interaction (NCI) index showed that the TPOH molecules form dimers due to the strong O-(1/2H)...(1/2H)-O hydrogen bond, while the packing of the benzene solvent molecules is governed by weak dispersive interactions. A search of the Cambridge Structural Database revealed that the disordered dimeric motif observed in (I) was found previously only in six crystal structures.
3. Convenient synthesis of L-proline benzyl ester
A Córdova, N N Reed, J A Ashley, K D Janda Bioorg Med Chem Lett. 1999 Nov 1;9(21):3119-22. doi: 10.1016/s0960-894x(99)00546-6.
Mesylates or tosylates of delta-hydroxy-L-norvaline esters spontaneously afford L-proline esters upon exposure to aqueous buffer in near quantitative yield. This novel reaction has led to the development of a simple route to optically active proline esters.
