2.1. Structural Elucidation of Compounds 1–3
Compound 1, = +14.3 (c = 0.2, MeOH), was isolated as colorless crystals. Its molecular formula was determined to be C25H40O9 according to its high-resolution (HR) ESI-MS (found: m/z 507.2567 [M + Na]+, calcd.: 507.2570) and 1H- and 13C-NMR spectra. IR absorptions at 3550, 3484, 1752 and 1729 cm−1 were indicative of hydroxyl and ester carbonyl functional groups. In the 1H-NMR spectrum (Table 1), four tertiary methyls at δH 0.91, 1.11, 1.27 and 1.41 (each 3H of singlet), one acetyl methyl at δH 2.10 (s, 3H), and one primary methyl at δH 1.11 (3H, t, J = 7.6 Hz) were clearly observed. Additionally, four singlets (δH 3.06, 3.39, 4.93 and 5.59), two triplets (δH 3.60 and 3.62, J = 5.0 Hz), and one doublet (δH 4.20, d, J = 5.4 Hz) were ascribable to either oxygenated methine or free hydroxyl groups. Other signals which were mostly overlapped centered between 1.58 and 2.85 ppm, resonating from either methine or methylene signals. The 13C-NMR spectrum revealed 25 carbon resonances, which were further classified by DEPT-90 and DEPT-135 spectra as six methyls, five methylenes, seven methines including four oxygenated ones (δC 72.4, 82.1, 82.5, and 85.0), seven quaternary carbons including three oxygenated ones (δC 78.4, 79.6, and 84.1), and two carbonyl carbons (δC 170.3 and 173.4). By analysis of the HSQC spectral data, all proton signals, except for the two singlets at δH 3.06 and 3.39 and the doublet at δH 4.20, could be assigned unambiguously to their respective carbons, suggesting that the signals at δH 3.06, 3.39 and 4.20 were assignable to free hydroxyl groups. Moreover, the existence of a propionyloxy-group was determined from analysis of the 1H-1H COSY and HMBC spectra. The above spectroscopic evidence suggested a highly oxygenated grayanane diterpenoid for 1.
Detailed analysis of the 1D (1H and 13C) and 2D (1H-1H COSY, HSQC, and HMBC) NMR spectra (Figures S1–S6 in supplementary data) of 1 revealed that its structure closely resembled that of asebotoxin VIII [19,26], a known grayanane diterpenoid previously isolated from both P. japonica and P. formosa. The obvious difference between the two compounds was that the acetoxy group located at C-6 in asebotoxin VIII migrated to C-15 in 1, as indicated by the HMBC correlations from H-15 (δH 4.93, s) to the acetoxy carbonyl group at δC 170.3 and from H2-6 (δH 2.05, m) to C-5 (δC 84.1) and C-7 (δC 72.4) (Figure 3a). In the ROESY spectrum, the correlations of H-15 with Me-18 as well as Me-20 suggested that H-15, Me-18, and Me-20 were all in the same β-orientation (Figure 3d). In addition, the correlations of 3-OH with Me-18, and of 15-H with 7-H and Me-17 suggested that 3-OH, H-7 and Me-17 were also in β-orientation (Figure 3d). Further analysis of the ROESY spectrum indicated the configurations of the remaining functional groups in 1 were the same as those in asebotoxin VIII, namely 3β, 5β, 7α, 10α, 16α-pentahydroxy, and 14β-propionyloxy. Accordingly, the structure of 1 was deduced as shown in Figure 2, and was named pierisoid C.
Compound 2 was obtained as colorless oil with a molecular formula of C23H36O8, as determined by a combination of HR-EI-MS and NMR spectra (including 1H, 13C, and DEPT) (Figures S7–S2 in supporting information). The resemblance of the NMR spectra of 2 (Table 1) with those of 1 disclosed that 2 was another grayanane diterpenoid structurally similar to 1. The major difference was the replacement of a methylene carbon in 1 by an oxygen-occurring methine in 2 (δC 78.2), suggesting that either C-6, or C-11, or C-12 of 2 was oxygenated. In the HMBC spectrum of 2, the HMBC correlations from 5-OH to the methine carbon at δC 78.2 indicated that this methine was ascribable to C-6 (Figure 3b). Carefully comparison of 13C-NMR spectral data of 2 with those of 1 (Table 1) obviously found that the upfield-shift of C-15 (δC 68.6) and C-16 (δC 61.3) in 2, indicated an oxygen bridge, was formed between C-15 and C-16; this was supported by the HR-EI-MS spectrum. In the ROESY spectrum of 2, the correlations of Me-17 with H-15; of 3-OH and 5-OH with Me-18; and of 5-OH with 6-OH and H-7 indicated that 3-OH, 5-OH, 6-OH, H-7, H-15, and Me-17 were in the same β-orientation (Figure 3e). Consequently, the structure of 2 was determined as shown in Figure 2 and was named pierisoid D.
Compound 3, colorless crystals, has a molecular formula of C31H42O14, as determined by a combination of HR-EI-MS and NMR spectra (including 1H-, 13C-, and DEPT) (Figures S13–S18 in supplementary data). Its spectroscopic data were very similar to those of secorhodomollolide B, a 3,4-secograyanane diterpenoid also isolated from P. formosa . The only difference between them was that the terminal double bond between C-4 (δC 146.0) and C-18 (δC 116.7) in secorhodomollolide B was replaced by a 4,18-oxirane group (δC 62.5 and 52.8) in 3, which was confirmed by the HMBC correlations from Me-19 (δH 1.34, s) to C-4, C-5 (δC 87.5) and C-18 (Figure 3c). Such an oxirane moiety has also been found in pierisoid A, another 3,4-secograyanane diterpenoid we reported from the flowers of P. formosa . In the ROESY spectrum of 3, the correlations of Me-19 with H-1, and of Me-20 with H-7 indicated that Me-19 and H-1 were in α-orientation and Me-20 coupled with H-7 were in β-orientation (Figure 3f). Therefore, compound 3 was identified as shown in Figure 2 and was named pierisoid E.
Ten known diterpenoids (Figure 2), namely, pierisformotoxin C (4) , secorhodomollolides C (5), D (6), and F (7) , asebotoxins I (8), II (12), IV (10), and VIII (11) [26,29], pieristoxin I (9) , and pierisformosin (13)  were also isolated from P. formosa and were identified by comparison of their spectroscopic data with those reported in the literature.
Classification essays rank the groups of objects according to a common standard. For example, popular inventions may be classified according to their significance to the humankind.
Classification is a convenient method of arranging data and simplifying complex notions.
When you select a topic, do not forget about the length of your paper. Choose the topic you will be able to cover in your essay, do not write about something global or general.
Consider these examples:
- Evaluate the best to worst methods of upbringing.
- Rate the films according to their influence on people.
- Classify careers according to the opportunities they offer.
You should point out the common classifying principle for the group you are writing about. It will become the thesis of your essay.
It is important for you to use clear method of classification in your essay, especially when you are dealing with subjective categories such as "quality" or "benefit". Make sure you explain what you mean by this term.
To organize a classification essay, the writer should:
- categorize each group.
- describe or define each category. List down the general characteristics and discuss them.
- provide enough illustrative examples. An example should be a typical representative of the group.
- point out similarities or differences of each category, using comparison-contrast techniques.