Germacranolides of Erlangea cordifolia. Isolation and structures of cordifolia-54, -55, -P2, and -31 by spectral and X-ray methods

Abstract

In addition to cordifene (1) and cordifene 4β, 15-oxide (5), four new companion compounds cordifolia-54, -55, -P2, and -31 have been isolated from the insect-antifeedant plant Erlangea cordifolia. Cordifolia-54(2) is related to cordifene as the 2,3-dihydrodeoxy derivative, and cordifolia-55(3) is the acetate of a C-2,O-2-seco-cordifene resulting from a formal epoxide reduction. Cordifolia-31 is the methacrylate ester of the same sesquiterpene core as is present in cordifene 4β,15-oxide.

Cordifolia-P2 has ¹H and ¹³C n.m.r. data consistent with structure (4) but certain chemical shifts, multiplicities, and coupling constants suggest a conformation markedly different from the five other Erlangea structures. This has been confirmed by a single crystal X-ray analysis. Cordifolia-P2, in contrast to cordifene and its 4β, 15-oxide, has a negative Cotton effect near the n→π* maximum in the c.d. spectrum. This is explained by McPhail's generalisation if all three compounds have the same absolute configuration, since the C(5)–C(6)–C(7)–C(8) torsion angle is significantly <120°(99°) for -P2 but significantly >120°(157°, 159°) for cordifene and its oxide. On the other hand the idealised McPhail relationship between this torsion angle and the CC–CO torsion angle of the lactone is incompletely obeyed since the latter is measured as +0.7° in cordifolia-P2.