A new and unnatural type of phospholipids with the head group attached to the 2-position of the glycerol backbone has been synthesized and shown to be a good substrate for secretory phospholipase A2 (sPLA2). To investigate the unexpected sPLA2 activity, we have compared three different phospholipids by using fluorescence techniques and HPLC, namely: (R)-1,2-dipalmitoyl-glycero-3-phosphocholine (hereafter referred to as 1R), (R)-1-O-hexadecyl-2-palmitoyl-glycero-3-phoshocholine (2R), and (S)-1-O-hexadecyl-3-palmitoyl-glycero-2-phosphocholine (3S). Furthermore, to understand the underlying mechanisms for the observed differences, we have performed molecular dynamics simulations to clarify on a structural level the substrate specificity of sPLA2 toward phospholipid analogues with their head groups in the 2-position of the glycerol backbone. We have studied the lipids above 1R, 2R, and 3S as well as their enantiomers 1S, 2S, and 3R. In the simulations of sPLA2-1S and sPLA2-3R, structural distortion in the binding cleft induced by the phospholipids showed that these are not substrates for sPLA2. In the case of the phospholipids 1R, 2R, and 3S, our simulations revealed that the difference observed experimentally in sPLA2 activity might be caused by reduced access of water molecules to the active site. We have monitored the number of water molecules that enter the active site region for the different sPLA2-phospholipid complexes and found that the probability of a water molecule reaching the correct position such that hydrolysis can occur is reduced for the unnatural lipids. The relative water count follows 1R > 2R > 3S. This is in good agreement with experimental data that indicate the same trend for sPLA2 activity: 1R > 2R > 3S.

A robust and scaleable route to chiral 1-isopropylamino-2-(diphenylphosphino)ethanes is described via the ring-opening of chiral, cyclic sulfamidates with potassium diphenylphosphide (KPPh(2)). The novel protocol offers a robust access to gram quantities of chiral amino phosphinoethanes in high yields. The Li-amides of the chiral aminophosphines were evaluated as chiral ligands in the asymmetric addition of n-butyllithium (BuLi) to benzaldehyde, yielding 1-phenylpentanol up to 98% ee.

Scheme 1. Lithium ligand libraries from amino acids.

In the presence of a chiral lithium N,P amide, alkylation of benzaldehyde results in an enantioselective formation of 1-phenyl-pentanol. This stereoselective addition reaction has herein been studied using dispersion-corrected density functional theory. For five different chiral ligands originating from amino acids the resulting enantioselectivity has been computationally determined and compared with experimentally available enantiomeric ratios (e.r.). In all cases the experimentally preferred enantiomer could be reproduced by the computational model. The selectivity trend among the ligands was found strongly sensitive to the amount of dispersion correction included. The origin of selectivity in the alkylation reaction is found to be composed of many combined interactions. For the most selective ligand 2A the most important factors found, which are favouring the (R)-TS, are a CH-π interaction between benzaldehyde-dimethyl ether (DME), stronger Li-solvation, and Li-π interactions with the phenyl ring in the backbone of the chiral lithium N,P amide. In addition, solvation by the bulk solvent and the size of the substituent on the nitrogen are also found important factors for the enantioselectivity.

Chiral lithium amides with a chelating diphenylphosphine group have previously been found to mediate excellent enantioselectivity in the asymmetric addition of alkyllithiums to benzaldehyde. NMR studies reveal for the first time that chiral lithium amides can form chelating dimeric complexes with P-Li interactions in both diethyl ether and THF. The two lithium atoms in the dimer are found to be non-equivalent, one of them is coordinated to two phosphines, detected by 6 Li, 31 P-couplings, while the other lithium is only solvent bound. Slow ethereal ligand exchange on the NMR time scale below -50 °C has been detected by 13 C NMR. Excess of n-butyllithium generates mixed dimeric complexes in both diethyl ether and THF.