A crucial step in the biosynthesis of the spermine alkaloid aphelandrine and its diastereoisomer orantine is an intramolecular cyclization of the intermediate (S)-dihydroxyverbacine. In order to elucidate this step of the biosynthetic pathway, microsomes from the roots of Aphelandra squarrosa Nees were incubated with unlabeled and (D8)-labeled (S)-dihydroxyverbacine. It was shown that the microsomal fraction catalyzes the intramolecular coupling of (S)-dihydroxyverbacine to aphelandrine. This was proven by microsomal transformation of (D8)-labeled (S)-dihydroxyverbacine to (D8)-labeled aphelandrine. The reaction absolutely requires NAPDH and O2. The underlying reaction mechanism is probably an oxidative phenol coupling catalyzed by an aphelandrine synthase. This enzyme is proposed to be a cytochrome P-450 oxidase. The intramolecular cyclization of (S)-dihydroxyverbacine represents an important point in the biogenesis of the aphelandrine-type alkaloids.
The asymmetric synthesis of the unlabeled and [D8]‐labeled terminal precursors, 4 ((−)‐(S)‐dihydroxyverbacine) and 19, respectively, in the biogenesis of the spermine alkaloids aphelandrine (5) and orantine (6), respectively, is described. A partial synthesis of the alkaloid (−)‐(S)‐[(E)‐4‐methoxycinnamoyl]buchnerine (10) is also presented.
Dedicated to Professor Elmar Vilsmaier on the occasion of his 60th birthday By means of the high sensitive on-line-coupled high-performance liquid chromatography and atmosphericpressure chemical-ionization mass spectrometry (HPLC/APCI-MS and HPLC/APCI-MS/MS) techniques, the new macrocyclic spermine alkaloid prelandrine (5) was detected in the roots of Aphelandra squarrosa (Acanthaceae), and its structure was elucidated as 4'-hydroxyprotoverbine ( 8-(4-hydroxyphenyl)-1,5,9,13-tetraazacycloheptadecan-6-one). It was further demonstrated that protoverbine (6) is enzymatically hydroxylated to prelandrine (5) in a reaction catalyzed by microsomes from the roots of A. squarrosa. The chemical synthesis of (À)-(S)-prelandrine is also described. The possible key role of prelandrine (5) as an intermediate in the biosynthesis of aphelandrine (1) is discussed.Introduction. ± The macrocyclic spermine alkaloid aphelandrine (1; Scheme) is a member of a rather rare class of natural products. Aphelandrine (1) has been found as the major alkaloid in the roots of several Aphelandra species [1 ± 3]. Although the structure of aphelandrine (1) displays p-coumaric acid and spermine (or spermidine and putrescine) as building blocks, little is known about the biosynthesis of this type of alkaloids. The feeding experiments with 3 H-and 14 C-labelled putrescine, spermidine, and coumaric acid confirmed the biogenetic structural units of aphelandrine; however, the question of whether mono-or dicoumaroylspermine is the precursor remained unanswered [4 ± 6]. For the further steps, there are two hypotheses: the first involves cyclization of N(1),N(5)-di(p-coumaroyl)spermine (3) leading to the species 2, which subsequently undergoes phenol coupling to give aphelandrine (1; Scheme) [7]. The second possibility is transformation of N(1)-cinnamoylspermine (4a) or N(1)(pcoumaroyl)spermine (4b) to the macrocycle 6 or 5, respectively, followed by acylation to give 2, which again undergoes phenol coupling to give aphelandrine (1; Scheme). When N(1)-cinnamoylspermine (4a) is cyclized by Michael addition, a hydroxylation step leading from 6 to 5 would be expected.So far, there exists no evidence to support one of these possibilities, presumably because of the concentration limits of the conventional analytical methods. Application of high-sensitive on-line-coupled HPLC/APCI-MS (atmospheric-pressure chemicalionization mass spectrometry) or ESI-MS (electrospray-ionization mass spectrometry) techniques allows the detection of substances, in some cases, at picomolar concentrations. Moreover, HPLC co-eluted substances can be easily differentiated by their molecular weights. The MS/MS spectra of selected quasi-molecular ions ([M H] ) very often provide substantial structural information without isolation.
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