Chemotaxonomy of Pseudowintera: Sesquiterpene dialdehyde variants are species markers

Abstract

Two sesquiterpene dialdehydes, the 1β-E-coumaroyl-5α-hydroxypolygodial plus the known 1β-E-cinnamoyl-5α-hydroxypolygodial, were isolated from the recently described species Pseudowintera insperata. This discovery is a further example of the rare sesquiterpene dialdehyde coumarate/cinnamate combination being found exclusively in the family Winteraceae. Another sesquiterpene dialdehyde, isopaxidal, with the rare rearranged drimane skeleton, was isolated from Pseudowintera axillaris. The sesquiterpene dialdehyde contents of leaves of 25 individual plants of the four Pseudowintera species, all endemic to New Zealand, were measured by HPLC. P. insperata individuals all had high levels (3.0–6.9% of leaf dry wt.) of the coumarate, P. axillaris had high levels (2.2–6.9%) of paxidal, and Pseudowintera colorata from different areas of New Zealand contained varying levels of polygodial (1.4–2.9%) and 9-deoxymuzigadial (0–2.9%). Therefore the sesquiterpene dialdehydes are good species markers.

Introduction

Sesquiterpene dialdehydes occur in a variety of organisms, including fungi, marine sponges, liverworts and flowering plants (Jansen and de Groot, 2004). This is probably because the various biological activities of these compounds (Jansen and de Groot, 2004) protect these organisms. The most widespread sesquiterpene dialdehyde is polygodial (1) with a drimane skeleton. Dialdehydes with the rearranged drimane skeleton of 9-deoxymuzigadial (2) are much rarer, having only been reported from the plants Canella winterana (Al-Said et al., 1990, Ying et al., 1995), Warburgia salutaris (Clarkson et al., 2007), Warburgia ugandensis (Kioy et al., 1990, Kubo and Ganjian, 1981), Warburgia stuhlmannii (Kioy et al., 1990, Kubo and Ganjian, 1981, Wube et al., 2005), Pseudowintera colorata (Gerard et al., 1993) and Pseudowintera axillaris (Brennan et al., 2006). Canella and Warburgia are in the Canellaceae and Pseudowintera is in the Winteraceae, the two families in the order Cannelales (Takhatajan, 2009). Another variant on the sesquiterpene dialdehyde structure is substitution with cinnamate or coumarate at C1, as in paxidal (3), 1β-E-p-coumaroyl-6α-hydroxypolygodial (8) and 1β-E-cinnamoyl-5α-hydroxypolygodial (9). Such compounds have only been found in the Winteraceae species: Drimys brasiliensis (Vichnewski et al., 1986), Drimys granadensis (Ferreto et al., 1988), Drimys winteri (Cechinel Filho et al., 1998, Malheiros et al., 2001), P. axillaris (Brennan et al., 2006), P. colorata (Larsen et al., 2007) and most recently in Zygogynum spp. (Allouche et al., 2009, Fotsop et al., 2008). In the light of these chemotaxonomic relationships, the discovery of a new species of Pseudowintera, Pseudowintera insperata (Heenan and de Lange, 2006), led us to expect that this plant might contain new sesquiterpene dialdehydes.

Pseudowintera (horopito) is a New Zealand endemic genus of shrubs and small trees comprising four species: P. axillaris (J.R. Forst and G. Forst) Dandy, P. colorata (Raoul) Dandy, Pseudowintera traversii (Buchan.) Dandy (Allan, 1961) and the recently described P. insperata Heenan & de Lange (Heenan and de Lange, 2006). P. axillaris and P. colorata are wide ranging on both North and South Islands of New Zealand, though P. axillaris is mainly northerly. P. traversii is much less common, found only in the north-west of the South Island. P. insperata is confined to the volcanic plugs above the Whangarei Harbour, north of Auckland on the Northland Peninsula (Fig. 1), and is a threatened species known only from 46 mature individuals (de Lange et al., 2009). In Northland P. axillaris and P. colorata are scarce, and there are plants with foliage, fruit and flower characteristics suggestive of hybridism between these species (Vink, 1970). Sampson (1980) argued against such hybrids, based on morphology and experimental attempts to generate them. While the naming of P. insperata resolved some of the uncertainty about Northland Pseudowintera, further morphological intermediates remain. In particular, Heenan and de Lange (2006) could not resolve the status of Northland plants from Logue’s Bush (unknown, Fig. 1), which were more similar to P. insperata than to any other Pseudowintera collection.

Leaves of P. colorata contain polygodial (1) and 9-deoxymuzigadial (2) with anti-Candida, insect antifeedant and anthelmintic activities (Gerard et al., 1993, Lorimer et al., 1996, McCallion et al., 1982). Polygodial (1) is also intensely pungent to humans (Szallasi et al., 1996), so its presence at high levels in P. colorata leaves is the basis of the common name pepper tree (Riley, 1994). Individual P. colorata plants varied in their levels of 1 and 2, with plants from four populations on the South Island of New Zealand showing two chemotypes: a mixed chemotype with similar levels of polygodial and 9-deoxymuzigadial, and a polygodial chemotype with very low levels of 9-deoxymuzigadial (Fig. 1) (Perry et al., 1996b). The leaves of P. axillaris contain sesquiterpene dialdehyde cinnamates, paxidal (3) and the 6-hydroxy derivatives 4 and 5, with activity against fungal plant pathogens (Brennan et al., 2006). P. axillaris leaves are much less pungent than those of P. colorata (Riley, 1994), despite paxidal (3) possessing the unsaturated 1,4-dialdehyde moiety associated with pungency (Szallasi et al., 1996). The only reported work on the rare P. traversii showed an unidentified dialdehyde (Larsen et al., 2007). Leaves of P. colorata also yielded the sesquiterpene cyclocolorenone (6) (Corbett and Speden, 1958), common flavonols and flavones (Williams and Harvey, 1982), and antioxidant anthocyanins and other flavonoid glycosides, which were at higher levels in the red portions of leaves around wounds (Gould et al., 2002). Fruits of P. colorata contained very high levels of 1 and 2, plus sesquiterpene dialdehyde cinnamate 7 and coumarate 8 (Larsen et al., 2007).

We now report that leaves of P. insperata contain high levels of a new sesquiterpene dialdehyde, coumarate (10), plus the rare cinnamate (9). Another new sesquiterpene dialdehyde, isopaxidal (12), with the rare rearranged drimane skeleton, was isolated from P. axillaris. We also report HPLC analyses of the sesquiterpene dialdehyde contents of leaves of individual plants of the four Pseudowintera species, and discuss the chemistry of potential hybrids.