Conclusions:I. The ACD/Structure Elucidator program quickly generated eight

candidate structures with the top ranked candidate being the final structure.II. This greatly simplified the analysis and gave confidence in the final structure.

The preferred conformation would appear to be 1A from Figure 5 based on the deshielding of H-1. The ROESY and DFT calculations confirm this interpretation. The DFT calculations indicate a roughly 30:1 preference for 1A over 1B.

The compound, referred to here as AD-10, was isolated as an optically active orange gum. IR and HR-ESI-MS indicated the molecular formula of C17H14O4 which requires 11 units of unsaturation. Regular 1D proton and carbon as well as COSY, HSQC, HMBC and ROESY experiments were performed on a Varian VNMRS600 with a OneNMR Probe. The spectra quickly demonstrated that a rather common catechol group (here a 3,4-dihydroxyphenyl moiety), is attached to the methine carbon of the above CH-CH3 group to give the fragment in Figure 1.

After this the process became especially di�cult. The remainder of the structure was proton de�cient and there were several carbons with a large number of HMBC connectivities. There was also an exceptionally deshielded (9.13 ppm) proton attached to a carbon at 152.09 which was hard to explain. An aldehyde was indicated in the undetermined part of the structure, but there were too many combinations to rule out easily.

We thank Yiu-Fai Lam, the UMCP NMR director for his generous help. Also the SMSB branch at CFSAN for their support. We also thank Osvaldo Gutierrez for performing the DFT calculations.

The exhaustive search of the possible structures lends strong support to the �nal structure being correct. It is consistent with all of the NMR data as well.

[1] G. P. P. Kamatou, I. Vermaak, A. M. Viljoen, S. Afr. J. Bot. 2011, 77, 908-919.[2] F. J. Chadare, A. R. Linnemann, J. D. Hounhouigan, M. J. R. Nout, M. A. J. S.

Van Boekel, Crit. Rev. Food Sci. Nutr. 2009, 49, 254-274.[3] E. De Caluwe, K. Halamova, P. Van Damme, Afr. Focus 2010, 23, 11-51.[4] C. Buchmann, S. Prehsler, A. Hartl, C. R. Vogl, Ecol. Food Nutr. 2010, 49,

145-172.[5] ACD/Structure Elucidator, version 14.02, Advanced Chemistry Development,

Inc., Toronto, Ont., Canada www.acdlabs.com, 2015.[6] C. B. Naman, J. Li, A. Moser, J. M. Hendrycks, P. A. Benetrehina, H. Chai, C. Yuan,

W. J. Keller, A. D. Kinghorn, Org. Lett. 2015, 17, 2988-2991.[7] C. Lorenc, J. Sauri, A. Moser, A. V. Buevich, A. J. Williams, R. T. Williamson,

G. E. Martin, W. M. Peczuh, ChemistryOpen 2015, 4, 577-580.[8] R. B. Williams, M. O’Neill-Johnson, A. J. Williams, P. Wheeler, R. Pol, A. Moser,

Org. Biomol. Chem. 2015, 13, 9957-9962.

H-1

H-1

H-8 H-6

H-6

H-81B 1A

IntroductionAdansonia digitata, commonly known as “Baobab”, is found mainly in Africa.1 The tree occurs naturally in dry areas, mainly in the Sahelian, Soudano-Sahelian, and Soudanian zones. The baobab is a multi-purpose tree with products having numerous food uses and medical properties. The fruits, seeds and leaves are all utilized and consumed daily by rural popula-tions in Africa.2,3 Baobab fruit pulp is been approved by statutory bodies for use in certain nutritional products. In 2008 the European Commission recognized the dried fruit pulp of baobab as a novel food.4

A new terpenoid aldehyde, with an unusual fused 5/6-ring skeleton has been isolated. The complete proton and carbon assignments of this com-pound were determined by a series of 1D and 2D NMR experiments includ-ing 1H-1H COSY, 1H-13C HSQC, 1H-13C HMBC and 1H-1H ROESY experiments. The structural elucidation proved more difficult than expected. After arriving at various combinations of fused 5/6-membered ring systems, all of which gave structures that were more or less consistent with the HMBC data and required a great deal of analysis time, we decided to make use of a Computer-Assisted Structural Elucidation (CASE) program. Specifically, the ACD/Structure Elucidator Suite program (v.14.04 Jul 30, 2015 running on a Windows 7 64 bit, 8 GB RAM, Dual Core 2.67 GHz computer) from Advanced Chemistry Development Inc.5-8

Acknowledgements

References

Conclusions

ACD/Structure Elucidator OutputAs we were stumped on the final portion of this structure, we thought this would be an excellent test for the CASE program from ACD/Labs. A set of experimental NMR data (1H, 13C, HSQC, and HMBC) was entered into ACD/Structure Elucidator Suite, and the program generated 152,400 candidate structures in under eight minutes. This list was filtered to 61 by comparison of predicted carbon chemical shifts to experimental data. These candidates were ranked by using another comparison to the data with neural net statistics [dN(13C)]. The best eight structures are shown in Figure 3.

The Difficult Natural Product

Further examination (see below) showed that the top ranked structure (candidate #1 in Figure 3) was the correct one. More refinement of the conformation was also made based on the NMR and DFT calculations.

Eliminating CandidatesAn examination of the other seven structures shows that these com-pounds have many common deficiencies: (i) the unusually deshielded proton (9.13 ppm) is not part of an observed 3-spin system [Structures 2, 3, 6-8], (ii) the centrally located proton (7.94 ppm) does not exhibit nearly enough HMBC connectivities [Structures 2, 4, 5, 7, 8], (iii) certain HMBC connectivities would be expected to be seen between an alkenic proton and an alkoxyl-bearing carbon [Structures 2-7], where none are actually observed, and (iv) there is no 3-spin proton system [Structures 4 and 5, the latter of which has two 2-spin systems], where one such system is found. In addition, Structure #7 contains a bis-exocyclic-butene system, which is an unusual ring system.

Determining Conformation

Figure 3: The top eight candidates generated by ACD/Structure Elucidator Suite. Structure ranking is based on overall differences between experimental and predicted 13C chemical shifts. The lower the dN(13C) value, the better the ranking.

Figure 1: Regular 1 and 2D NMR indicated the fragment to the left was present. The remainder of the molecule proved much more dificult to determine.

Figure 5: The two possible conformations of the final structure for AD-10.

Figure 4: HMBC connections indicated on structure #1 from figure 3. Only structure #1 fits the expected connectivities without any that are unexplained. In addition, the oxygen on C-8 is close enough to explain the exceptionally deshielded H-1.

Figure 2: A close inspection of the HMBC revealed an exceptionally large long-range carbon-proton coupling which indicated the presence of an aldehyde

Figure 6: A section of the ROESY spectrum of AD-10 showing the correlations for protons H-1, H-6 and H-8 as indicated in red. The red peaks are negative and the blue are positive. A horizontal trace of H-8 is overlaid in black showing the much stronger correlation for conformation 1A over 1B.

H

HO

OH

H

H

H

H3C ~

9.659.709.759.80

122

123

124

125

126

127

1H δ(ppm)

13C

δ(p

pm)

23 Hz

O

H

OH

OHH

HH

H

HH

6'

3

4

3'

1

7

6

7a

5

2' 4'

1'7'8'

4a

5'

H

H3C

8

O

H

.....

dN(13C): 2.044

Formula: C17H14O4[M+H]+: 283.096485

1

CH3

OH

OH

O

O

dN(13C): 3.002

Formula: C17H14O4[M+H]+: 283.096485

2

CH3OH

OH

O

O

dN(13C): 3.367

Formula: C17H14O4[M+H]+: 283.096485

3

CH3

OH

OH

O

O

dN(13C): 3.777

Formula: C17H14O4[M+H]+: 283.096485

4

CH3

OH

OH

O

O

dN(13C): 3.798

Formula: C17H14O4[M+H]+: 283.096485

5

CH3

OHOH

O

O

dN(13C): 3.984

Formula: C17H14O4[M+H]+: 283.096485

6

CH3

OHOH

O

O

dN(13C): 4.792

Formula: C17H14O4[M+H]+: 283.096485

7

CH3

OH

OH

O

O

dN(13C): 5.014

Formula: C17H14O4[M+H]+: 283.096485

8

OO

CH3

OH

OH

7.88.08.28.48.68.89.09.29.49.69.810.0

8.0

8.4

8.8

9.2

9.6

10.0

1H δ(ppm) f2

1 H δ

(ppm

) f1

O

H

OH

OHH

HH

H

HH

1A

6'

3

4

3'

1

7

6

7a

5

2' 4'

1'7'8'

4a

5'

H

H3C

O

OH

OH

OHH

HH

H

HH

6'

3

4

3'

1

7

8

6

7a

5

2' 4'

1'7'8'

4a

5'

H

H

H3C

1B

8

O

H

Application of a Computer-Assisted Structure Elucidation Program for the Structural Determination of a Difficult Natural ProductClark D. Ridge1*, Xing-Nuo Li2,3, Eugene P. Mazzola4, Jianghao Sun3, Pei Chen3, Arvin Moser5, Joseph C. DiMartino5, and Scott A. MacDonald5

1U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, College Park, MD 207402College of Pharmaceutical Science, Zhejiang University of Technology, Hangzhou 310014, P. R. China

3Food Composition and Methods Development Laboratory, Beltsville Human Nutrition Research Center, ARS, USDA, Beltsville, MD 20705, USA4Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742

5Advanced Chemistry Development, Inc. (ACD/Labs) Toronto Department, 8 King Street East, Suite 107, Toronto, Ontario M5C 1B5, Canada