The Chemistry of Mycotoxins: 97 (Progress in the Chemistry of Organic Natural Products)

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  1. Invited Participation in Symposia and Conferences
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Invited Participation in Symposia and Conferences

Rearrangement of Dopachrome. Polymerization of DHI. Melanin Formation Under Udenfriend Conditions. Melanocytotoxicity: Antimelanocyte-Antibodies Formation. The Immune Hypothesis. The Neural Hypothesis. The Self-Destruction Hypothesis. The Composite Hypothesis. Chemotherapy of Vitiligo.

Psoralen Action and UV Light. Psoralen Action on Melanogenesis.


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Abnormal Biochemical Parameters in Vitiligo. Status of Tryptophan in the Melanogenic System. A Composite Hypothesis on Vitiligo. Author Index. Grove, Dr. The history of erythrina research begins at the end of the 19th century. During the last two decades of that time extracts from species of Erythrina have been found to exhibit curare-like neuromuscular blocking activities which are caused by alkaloids occurring therein It was Altamirano 2 , who obtained a silky shining, crystallinic acetate as well as Greshoff 3 who already isolated several basic unspecied compounds.

Because of their remarkable biological activity he suggested a systematic phytochemical examination of the genus Erythrina. But it still has taken at least half a century before this has been realized for the rst time by Folkers. He has shown that more than fty Erythrina species as an example, E. Moreover, his group succeeded in isolating the rst crystallized erythrinane alkaloid named erythroidine 14; Fig. Soon after nu- merous alkaloids have been isolated, e.

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Another decade later the fundamental investigations of Prelog References, pp. In the late s ring C-homologues of erythrinane alkaloids have been anticipated from the biosynthetic pathway of certain alkaloids, which are known to be generated from 1-phenethyl-isoquinoline derivatives as precursors 15, Only a short time later such compounds named homoerythrinanes, homoerythrina alkaloids, or schelhammeranes indeed have been found in the plant kingdom 17 parent compound 2, Fig.

Due to the increasing attraction and rapid extension in this eld the Erythrina alkaloids have been regularly reviewed concerning occur- rence, structure, analytic and spectral properties, biosynthesis, total syn- thesis, and biological activities covering the literature up to The most important reviews are cited in Refs. The erythrina-type alkaloids are characterized by their unique tet- racyclic spiroamine framework.

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They are generally classied into two main groups: Alkaloids predominantly possessing a membered indoloisoquinoline core are called erythrinanes and those exhibiting a membered indolobenzazepine skeleton are generally called schel- hammeranes or homoerythrinane alkaloids see Fig. Depending on the nature of the D ring both groups in turn may be subdivided into aromatic and non-aromatic alkaloids, the latter of which References, pp.

General classication of Erythrina alkaloids: Dienoid and alkenoid type alkaloids and D ring modications. In addition, in both series there have been isolated alkaloids containing a pyridyl instead of a phenyl unit, which are known as erymelanthine 11 and holidine 12 azaerythrinane and aza- homoerythrinane derivatives belonging to two further different subtypes of these alkaloids 25, 26 see below and Fig. Several D-seco-derivatives in the homoerythrinane group should also be mentioned see e.

Finally, the typical position and the number of olenic bonds in the A and B ring have led to a further subdivision into dienoids and alkenoids in both alkaloid series. The former are characterized by a conjugated diene unit covering C atoms 1, 2, 6, and 7, while the latter possess only one double bond in the 1,6-position see Fig. Table 1. New Erythrinane Alkaloids 1 Nr. References, pp. C23H29NO8 dark 3. Table 2. New Homoerythrinane Alkaloids 1 Nr. C20H27NO5 3. C19H25NO4 3.

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C18H23NO3 3. Furthermore, there is a small group of Cdeoxygenated erythrinane alkaloids. The alkaloids are generally dextrorotatory and their absolute conguration at the spiro- atom C5 is S with respect to the basic framework 17, 22, 24, 30, Due to an anomalous substitution pattern there are few exceptions, for instance wilsonine , with R -conguration of C5.

Atom C3 of ery- thrinanes is always R -congured, that of homoerythrinanes, however, is also found exhibiting S -conguration. For an overview concerning structures, substitution patterns, and properties see e. Nevertheless, the trans-erythrinane skeleton has been synthesized by Mondon 31 and Desmaele In this chapter all the new compounds reported in literature from to are listed including the natural source, structure, analytical and spectroscopical informations. All in all, there are 12 erythrinane and 6 homoerythrinane type compounds compiled in Tables 1 and 2.

The rst generally accepted pathway for the biosynthesis of erythrinane alkaloids was established by Bartons group. This route starts with the benzylisoquinoline S -norprotosinomenine 34 as the main precursor, which is cyclized by para-para phenol coupling to the neoproaporphine derivative This in turn undergoes rearrangement yielding the sym- metrical dibenzazonine Its hydrogenation product 37 is oxidized to the corresponding diphenoquinone Finally, intramolecular Michael type addition proceeds to afford erysodienone 39 possessing the characteristic erythrinane skeleton Both precursors, 34 and 37, are naturally occurring compounds 49, 50 Scheme 1.

Scheme 1. Biosynthesis of erythrinane alkaloids according to Barton et al. However, the relative small incorporation rate 0. Thus, in detailed investigations it has been unequivocally shown that S -norreticuline 40 in fact is also the most important biosynthetic precursor for the erythrinane alkaloids, and its incorporation rate exceeds by far that of the isomeric S -norprotosinomenine 34 previously found 7.

Furthermore, the latter was not converted into any of the alkaloids in the reinvestigation concerned. These results required a new route and mechanism for the biosynthesis of the ery- thrinane alkaloids depicted in Scheme 2. According to model reactions in this eld previously carried out 51 , the initial para-para coupling of S -norreticuline 40 should led differently from the previous route to the morphinandienone derivative norisosalutaridine 41 rather than to the neoproaporphine derivative The latter, after generation of the benzo[1,3]dioxole function giving noramurine 42 , can rearrange via the neospirinic ion 43 to the unsymmetrical dibenzazonine A symmetrical intermediate of the diphenoquinone type 38 postulated previously could be excluded based on feeding experiments with 13C-labelled precursors.

Thus, e. However, the enrichment has been observed exclusively at C10 The subsequent steps leading to erythraline 3 are similar to those already established The biosynthesis of homoerythrinane alkaloids has been proposed to proceed by the same pathway as that of the erythrinanes according to References, pp.

Acknowledgements

Phenethylisoquinolines 47 and 48 isolated from plants, which produce homoerythrinane and dibenzazecine alkaloids 49, 50, and Scheme 1 starting from the homologous S phenethyltetrahydro- isoquinoline precursor 34, n 2 via the corresponding dibenzazecine 37, n 2. Supports for these assumptions are based on the isolation of several naturally occurring phenethylisoquinolines, e.

But until now there is no experimental evidence for this route concerned, which possibly with regard to that of the erythrinane alkaloids will demand a revision. Because of their unique structures and their biological activities there has been much interest in the total synthesis of Erythrina alkaloids. Contrarily, only few routes have been reported for the synthesis of the ring C-homologous schelhammeranes.

A practical classication of all syntheses especially concerning the erythrinane ring system is based on the idea which of the three alicyclic rings A, B, or C is completed in the nal step. Thus in principle two alternatives are conceivable: The completion of one of the rings mentioned in the following called route A, B, or C or the sequential or simultaneous formation of more than one cycle e.

Schemes 3 and 4. Depending on the bond s to be formed several subtypes result from both cases as outlined in Schemes 3 and 4: e. Scheme 3. Strategies for the synthesis of aromatic erythrinane alkaloids 5. Scheme 4.

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In this classication C-homoerythrinane syntheses remain out of con- sideration, since the construction of their framework is far more difcult than that of the erythrinanes. Nevertheless, several useful approaches to this alkaloid group are included in this section see below. Concerning the non-aromatic erythrinane alkaloids, only one single total synthesis has been achieved until now. Furthermore, several synthesis approaches to ring D oxaerythrinane frameworks are known Erythrinanes 5.

Final Formation of One Ring 5. Ring C Route C 5. Scheme 5.