Application of 2D EXSY and qNMR Spectroscopy for Diastereomeric Excess Determination Following Chiral Resolution of β‐Lactams

Abstract

Trans-β-lactam isomers have garnered much attention as anticancer microtubule targeting agents. Currently available synthetic methods are available for the preparation of enantiopure β-lactams and favour isomeric cis/trans β-lactam mixtures. Indirect chiral resolution offers the opportunity for isolation of exclusively enantiopure trans-β-lactams. In this study, liquid chromatography chiral resolution of β-lactams derivatized as diastereomer mixtures with a panel of N-protected amino acids is explored, where N-(Boc)-L-proline served as the optimal chiral derivatising reagent. High-performance liquid chromatography failed to adequately determine diastereomeric excess (de) of resolved diastereomers. Variable temperature, 1 H NMR and 2D EXSY spectroscopic analyses of proline-derivatised diastereomers were successfully employed to characterise equilibrating rotamers of resolved diastereomers and determine their de.Integration of resolved resonances corresponding to H 3 and H 4 of the β-lactam ring served as a quantitative qNMR tool for the calculation of de following resolution.[a] E.

Analytical and behavioral characterization of N‐ethyl‐N‐isopropyllysergamide (EIPLA), an isomer of N6–ethylnorlysergic acid N,N‐diethylamide (ETH‐LAD)

“…Presumably, this reflected the steric differences found between the N ‐ethyl and N ‐isopropyl groups. A two‐dimensional 1 H/ 1 H NOESY experiment (Supporting Information) was also employed to probe spatial relationships, which indicated some in‐phase cross‐matches for the rotamer species, including δ 4.52/4.25 ppm (H‐19), 3.89/3.77 ppm (H‐8α), 3.37/3.22 ppm (H‐19), 1.24/1.12 ppm (H‐22), and 1.20/1.07 ppm (H‐20) 36 . Overall, spectral data recorded for EIPLA were distinct from those reported for ETH‐LAD, 23 which should allow for unambiguous differentiation.…”

Antiproliferative and Tubulin-Destabilising Effects of 3-(Prop-1-en-2-yl)azetidin-2-Ones and Related Compounds in MCF-7 and MDA-MB-231 Breast Cancer Cells

“…The Fmoc protecting group was removed from products 12a - e under mild basic conditions with aqueous sodium hydroxide in methanol ( Scheme 3 ) to afford the β-lactam conjugates ( 13a - e ) in 38–59% yield following purification by flash chromatography. The 1 H NMR spectra of products 12a - e and 13a - e are complex, possibly due to the presence of rotamers arising from the amino acid moieties [ 73 ]. The 1 H NMR spectrum of the diastereomeric prodrug 13e confirmed the trans stereochemistry with H-4 as a doublet, δ 5.32 ( J 3,4 = 1.48 Hz), while an additional amide carbonyl signal in the region 1680–1690 cm −1 is observed in the IR spectra of these prodrugs (see Supplementary Materials Figures S1–S20 ).…”

30th Annual GP2A Medicinal Chemistry Conference

“…A panel of 3-hydroxyl B ring meta hydroxyl substituted β-lactam racemates of combretastatin A-4 (CA-4), known as ‘the combretazets’ have demonstrated microtubule depolymerising activity in both breast cancer and CA-4 resistant colorectal cancer cells ( Figure 44 A) [ 201 , 202 ]. Chiral diastereomeric resolution has successfully resulted in enantioseparation of + and–3-hydroxyl β-lactams as the N-(Boc)- l -proline derivatised 3S,4S and 3R,4R diastereomers using liquid chromatography (LC) [ 203 ]. While diastereomeric resolution resulted in enantioseparation of the 3S,4S enantiomer in >90% enantiomeric excess ( ee ), co-elution of diastereomers resulted in lower ee of 50-80% for the 3R,4R diastereomer.…”