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Showing posts with label structure elucidation. Show all posts
Showing posts with label structure elucidation. Show all posts

Sunday 21 July 2013

Computed NMR spectra predicts the structure of Nobilisitine A


Nobilisitine A was isolated by Evidente and coworkers, who proposed the structure 1.1 Banwell and co-workers then synthesized the enantiomer of 1, but its NMR did not correspond to that of reported for Nobilisitine A.; the largest differences are 4.7 ppm for the 13C NMR and 0.79 ppm for the 1H NMR.2

1
Lodewyk and Tantillo3 examined seven diastereomers of 1, all of which have a cis fusion between the saturated 5 and six-member rings (rings C and D). Low energy conformations were computed for each of these diasteromers at B3LYP/6-31+G(d,p). NMR shielding constants were then computed in solvent (using a continuum approach) at mPW1PW91/6-311+G(2d,p). A Boltzmann weighting of the shielding contants was then computed, and these shifts were then scaled as described by Jain, Bally and Rablen4 (discussed in this post). The computed NMR shifts for 1 were compared with the experimental values, and the mean deviations for the 13C and 1H svalues is 1.2 and 0.13 ppm, respectively. (The largest outlier is 3.4 ppm for 13C and 0.31 for 1H shifts.) Comparison was then made between the computed shifts of the seven diasteomers and the reported spectrum of Nobilisitine A, and the lowest mean deviations (1.4 ppm for 13C and 0.21 ppm for 1H) is for structure 2. However, the agreement is not substantially better than for a couple of the other diasteomers.

2
They next employed the DP4 analysis developed by Smith and Goodman5 for just such a situation – where you have an experimental spectrum and a number of potential diastereomeric structures. (See this post for a discussion of the DP4 method.)The DP4 analysis suggests that 2 is the correct structure with a probability of 99.8%.
Banwell has now synthesized the compound with structure 2 and its NMR matches that of the original natural product.6 Thus Nobilisitine A has the structure 2.

References

(1) Evidente, A.; Abou-Donia, A. H.; Darwish, F. A.; Amer, M. E.; Kassem, F. F.; Hammoda, H. A. m.; Motta, A., "Nobilisitine A and B, two masanane-type alkaloids from Clivia nobilis,"Phytochemistry, 1999, 51, 1151-1155, DOI: 10.1016/S0031-9422(98)00714-6.
(2) Schwartz, B. D.; Jones, M. T.; Banwell, M. G.; Cade, I. A., "Synthesis of the Enantiomer of the Structure Assigned to the Natural Product Nobilisitine A," Org. Lett., 2010, 12, 5210-5213, DOI:10.1021/ol102249q
(3) Lodewyk, M. W.; Tantillo, D. J., "Prediction of the Structure of Nobilisitine A Using Computed NMR Chemical Shifts," J. Nat. Prod., 2011, 74, 1339-1343, DOI: 10.1021/np2000446
(4) Jain, R.; Bally, T.; Rablen, P. R., "Calculating Accurate Proton Chemical Shifts of Organic Molecules with Density Functional Methods and Modest Basis Sets," J. Org. Chem., 2009, DOI:10.1021/jo900482q.
(5) Smith, S. G.; Goodman, J. M., "Assigning Stereochemistry to Single Diastereoisomers by GIAO NMR Calculation: The DP4 Probability," J. Am. Chem. Soc., 2010, 132, 12946-12959, DOI:10.1021/ja105035r
(6) Schwartz, B. D.; White, L. V.; Banwell, M. G.; Willis, A. C., "Structure of the Lycorinine Alkaloid Nobilisitine A," J. Org. Chem., 2011, ASAP, DOI: 10.1021/jo2016899

Welwitindolinones structure

A quick note here on the use of computed NMR to determine stereochemical structure. The Garg group synthesized two “oxidized welwitindolines”, compounds 1 and 2.1 The relative stereochemistry at the C3 position (the carbon with the hydroxy group) was unknown.

1

2
Low energy gas-phase conformers of both epimers of 1 and 2 were optimized at B3LYP/6-31+G(d,p). (These computations were done by the Tantillo group.) See Figure 1 for the optimized lowest energy conformers. Using these geometries the NMR chemical shifts were computed at mPW1PW91/6-311+G(d,p) with implicit solvent (chloroform). The chemical shifts were Boltzmann-weighted and scaled according to the prescription (see this post) of Jain, Bally and Rablen.2 The computed chemical shifts were then compared against the experimental NMR spectra. For both 1and 2, the 13C NMR shifts could not readily distinguish the two epimers. However, the computed 1H chemical shifts for the S epimer of each compound was significantly in better agreement with the experimental values; the mean average deviation for the S epimer of 2 is 0.08 ppm but 0.36ppm for the R epimer. As a check of these results, DP4 analysis3 (see this post) of 2 indicated a 100% probability for the S epimer using only the proton chemical shifts or with the combination of proton and carbon data.

1

2
Figure 1. B3LYP/6-31+G(d,p) optimized geometries of the
lowest energy conformations of 1 and 2.

References

(1) Quasdorf, K. W.; Huters, A. D.; Lodewyk, M. W.; Tantillo, D. J.; Garg, N. K., "Total Synthesis of Oxidized Welwitindolinones and (-)-N-Methylwelwitindolinone C Isonitrile," J. Am. Chem. Soc. 2011,134, 1396-1399, DOI: 10.1021/ja210837b
(2) Jain, R.; Bally, T.; Rablen, P. R., "Calculating Accurate Proton Chemical Shifts of Organic Molecules with Density Functional Methods and Modest Basis Sets," J. Org. Chem. 2009, 74, 4017-4023, DOI: 10.1021/jo900482q.
(3) Smith, S. G.; Goodman, J. M., "Assigning Stereochemistry to Single Diastereoisomers by GIAO NMR Calculation: The DP4 Probability," J. Am. Chem. Soc. 2010, 132, 12946-12959, DOI:10.1021/ja105035r

Thursday 18 July 2013

Malleobactin Structure Elucidated

A team of scientists based in Germany have elucidated the structure and absolute configuration of malleobactin, which is the siderophore of the pathogenic Burkholderia mallei. Siderophores are strong iron-binding agents, and act as virulence factors for pathogenic bacteria.



The virulence factor malleobactin has been shown to contain an unprecedented nitro-bearing amino acid
Read more



http://www.chemistryviews.org/details/ezine/4997511/Malleobactin_Structure_Elucidated.html