Diterpenoids from the Aerial Parts of Plectranthus ornatus

Abstract

Phytochemical investigation of a hexane extract of the aerial parts of Plectranthus ornatus yielded three new neoclerodane diterpenoids (1-3), two labdane diterpenes (4 and 5) obtained for the first time as natural products, and several previously known substances. The structures and relative stereochemistry of 1-5 were established mainly on the basis of NMR spectroscopic studies and by comparison with related compounds.

Miscellaneous Diterpenes from the Aerial Parts of Plectranthus ornatus Codd

“…16 Compound Tables 1 and 2) of 1 were similar to those reported for the diterpenes 11-acetoxyneoclerodane, which were previously isolated of the P. ornatus. 19 The only difference between these two compounds was the replacement of the carboxyl function at C-15 (d C 171.7) for a methyl ester (d C 166.7, 51.0 / d H 3.66).…”

“…It is worthy of notice that both structures of 2 and 4 were also supported by comparison of the 1 H and 13 C NMR data of these with those reported to the rearranged (4→2)-abeo-clerodane diterpenes. 22 The relative stereochemistry of 4, as depicted in Figure 3, was defined by NOESY and literature data, 16,18 which was similar to compound 2. Thus, the structure of compound 4, a new (4→2)-abeo-clerodane diterpene, was established and named as ornatin D.…”

“…The chromatographic fractionation of the n-hexane extract from the aerial parts of P. ornatus was performed, leading to the isolation of five new diterpenes which were designated as ornatins A-E (1-5), as well as the known diterpenes: coleon R (6), 14 ent-3a-hydroxycleroda-4(18), 13E-dien-15-oic acid (7), 15 plectrornatin A (8), 16 labd-13-en-8β-hydroxy-15-oic acid (9), 17 11-acetoxy-2-oxo-entcleroda-3,13E-dien-15-oic acid (10) 18 and ent-11R-acetoxy-2-oxocleroda-3,13E-dien-15-oic acid (11). 16 Compound Tables 1 and 2) of 1 were similar to those reported for the diterpenes 11-acetoxyneoclerodane, which were previously isolated of the P. ornatus.…”

“…The double bond C-13/C-14 was defined as E based on chemical shifts (see Table 1) in agreement with previous diterpenes isolated of P. ornatus. 16 As can be seen in the Experimental section, no MeOH has been used during any extraction or chromatographic procedures, thus, eliminating the possibility of 1 being an artifact. The final structure of 1 was confirmed by the heteronuclear multiple bond correlation (HMBC) spectrum through the selected long-range correlations depicted by arrows in Figure 2, while its relative stereochemistry was established by the nuclear Overhauser spectrum (NOESY) (see Supplementary Information).…”

“…The observed NOE's for Me-20 with Me-17 and Me-11' (methyl of the acetoxyl group) confirmed the a-position inferred for both AcO-11' and Me-17 ( Figure 3), also in agreement with the stereochemistry reported previously for 11R* diterpenes from P. ornatus. 16,18 , H-11'). The 13 C NMR spectrum exhibited 22 carbon signals, which in accordance with the DEPT 135 and HSQC spectra were defined as six methyls, four methylenes, five monohydrogenated and seven non-hydrogenated carbons, including an olefinic carbon d C 160.6 (C-13), two rid carboxyls d C 180.3 (C-3) and d C 171.6 (C-15), one of which a,β-unsaturated, and an acetoxyl moiety d C 170.9 (C-11').…”