Abstract
Chiral tertiary and quaternary amine solvating agents for NMR spectroscopy were synthesized from the wood resin derivative (+)-dehydroabietylamine (2). The resolution of enantiomers of model compounds [Mosher’s acid (3) and its n-Bu4N salt (4)] (guests) by (+)-dehydroabietyl-N,N-dimethylmethanamine (5) and its ten different ammonium salts (hosts) was studied. The best results with 3 were obtained using 5 while with 4 the best enantiomeric resolution was obtained using (+)-dehydroabietyl-N,N-dimethylmethanaminium bis(trifluoromethane-sulfonimide) (6). The compounds 5 and 6 showed a 1:1 complexation behaviour between the host and guest. The capability of 5 and 6 to recognize the enantiomers of various α-substituted carboxylic acids and their n-Bu4N salts in enantiomeric excess (ee) determinations was demonstrated. A modification of the RES-TOCSY NMR pulse sequence is described, allowing the enhancement of enantiomeric discrimination when the resolution of multiplets is insufficient.
Highlights
Wood resin is a natural product that after losing its volatile components hardens and is called rosin
Rosin can be derivatized to an amine mixture containing 60% of (+)-dehydroabietylamine (2, Figure 1) [3,4,5], able to resolve chiral carboxylic acids by crystallization [6,7]
We have previously reported [8] that neutral and ammonium forms of secondary amines derived from dehydroabietylamine may be used in the chiral recognition of enantiomers of certain carboxylic acids
Summary
Wood resin is a natural product that after losing its volatile components hardens and is called rosin. To evaluate the corresponding tertiary amines for chiral recognitions we have examined in the present study the behaviour of dehydroabietylamine (2)-derived tertiary amines and their ammonium salts as chiral solvating agents (CSAs) that can be used in the discrimination of enantiomers of both aromatic and aliphatic carboxylic acids by NMR. The quaternization of the tertiary amine seems to make the recognition of enantiomers unfeasible This may be due to a crowding in the active bonding site, i.e., ammonium terminus, caused by the side chain (methyl, benzyl, 2-hydroxyethyl) so that the guest cannot get close enough. The best enantiomeric resolutions are obtained with racemic carboxylic acids bearing an electronegative atom (e.g., O, N, Br) at the α site. Et2O and the orgEatn2Oic apnhdastehewoarsgwanaischpehdawseitwh awsawtearsuhnedtilwnietuhtwraal.teTrhuenotrilgnaneuictrpahl.aTsehewoarsgdarniiecdpohvaesre was dried over
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