The Quinones of Nepenthes rafflesiana. The Crystal Structure of 2,5-Dihydroxy-3,8-dimethoxy-7-methylnaphtho-1,4-quinone (Nepenthone-E) and a Synthesis of 2,5-Dihydroxy-3-Methoxy-7-methylnaphtho-1,4-quinone (Nepenthone-C)

Abstract

Plumbagin (1), droserone (2), hydroxydroserone (4) and four new quinones, nepenthone-A, nepenth-one-B, nepenthone-C (17) and nepenthone-E (9), have been isolated from the roots of Nepenthes rafflesiana Jack. A fifth quinone-nepenthone-D-has been detected in the extract and has been assigned the tentative structure (29) from its spectroscopic properties.    

The crystal structure of nepenthone-E (9) was determined by X-ray diffraction; diffractometer data at 295 K were refined by full-matrix least squares to a residual of 0.061 (1040 'observed' reflections). Crystals of nepenthone-E (9) are triclinic, Pī a 7.579 (4), b 7.764 (5), c 10.806 (7) Ǻ, α 70.86 (4), β 101.40 (5), γ 91.70 (5)°, Z 2.    

The structure of nepenthone-C (17) has been confirmed by an unambiguous synthesis. A synthesis of 2,7,8-trimethoxy-3-methyl-5,6-methylenedioxynaphtho-1,4-quinone (49) has revealed that this substance (49) is not identical with the O,O-dimethyl ether of nepenthone-A, and it is suggested that nepenthone-A is either the 5- or 8-O-methyl ether of 2,5,8-trihydroxy-3-methyl-6,7-methylenedioxy-naphtho-1,4-quinone [(36) or (37), respectively].