Stretched poly(methyl methacrylate) gel aligns small organic molecules in chloroform.: Stereochemical analysis and diastereotopic proton NMR assignment in ludartin using residual dipolar couplings and 3J coupling constant analysis

[GRAPHICS] Poly(methyl methacrylate) (PMMA) gels prepared by copolymerizing methyl methacrylate (MMA) and various amounts of ethylene glycol dimethacrylate (EGDMA) in the presence of the radical initiator V-70 (2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile)) can orient small organic molecules when swollen in NMR tubes with CDCl3. The aligning properties of the stretched PMMA gels were evaluated by monitoring the quadrupolar splitting of the H-2 NMR signal of CDCl3, and the aligning degree is proportional to the cross-linking density. Natural abundance one-bond H-1-C-13 residual dipolar couplings (RDCs) for menthol measured in the gels depended on the cross-link density. The stereochemistry and assignment of the diastereotopic protons of the gastroprotective and nonsteroidal aromatase inhibitor sesquiterpene lactone ludartin, isolated from Stevia yaconensis var. subeglandulosa, were unambiguously determined using a combination of natural abundance one-bond H-1-C-13 RDCs measured in a PMMA gel and a (3)J coupling constant analysis.