Erythrina alkaloids lactonic

On route to the Erythrina alkaloid 3-dimethoxyerythratidinone, Wang and Padwa encountered the interesting acid catalyzed rearrangement of lactam 151 to the tetracyclic hydroxyindole 153 via the lactone 152 <060L601>. 

Further experiments have established the aromatic Erythrina alkaloids as precursors for the lactonic bases (106). [17- H]Erysodine [as (104)], [14,17- H2]erysopine [as (105)], and ( )-[l,17- H2]erysodienone [as (102)] were incorporated satisfactorily into a- and /S-erythroidine (106) degradation of the material obtained after feeding [17- H]erysodine established that the label was confined to C-17. This is the expected labelling site and the absence of scrambling is established." ... 

These results led to the postulation of the spirocyclic erythrinane skeleton and this was confirmed by synthesis of the parent nucleus by Belleau (1953). The structures of the two lactonic alkaloids (3) were also elucidated by Boekelheide (1960) and coworkers, who recognised their close structural identity to the other aromatic erythrina alkaloids (1) the lactonic alkaloids also underwent an apo -type rearrangement under drastic acidic conditions. Subsequently the structures of both the aromatic and lactonic alkaloids were confirmed by X-ray crystallography of erythra-line hydrobromide (Nowacki and Bonsma 1958) and of the erythroidines (Hanson 1963). The spiro centre was shown to have the same configuration in all the erythrina alkaloids by use of optical rotatory measurements (Weiss and Ziffer 1963, Beecham 1971). 

In our own studies in Cardiff we have also isolated 8-oxo-a- and j3-erythroidine (4a) and (4b) (Chawla et al. 1982) from E.herteroana as well as 8-oxo-erysodine from E.tahitensis (Chunchatprasert 1983) it thus seems likely that 8-oxo- analogues of many of the other dienoid alkaloids are also likely to be found accompanying the parent alkaloid, albeit in small amounts. However, the relatively high polarity makes them less amenable to GC-MS and only 8-oxo-i3-erythroidine (4b) has been detected by GC, presumably because the derivatization of the lactone residue by the silylat-ing reagent makes it much more volatile. (The TMS derivative of 3-erythroidine itself has the shortest retention time of all the Erythrina alkaloids we have studied so far). 

The biosynthesis of these novel pyridine analogues of the dienoid Erythrina alkaloids is of considerable interest and it seems likely that they arise by a variant of the pathway leading to the lactonic alkaloids a- and jS-erythroidine (cf. Fig. 5). Thus oxidative ring openings of the C16-C17 bond of erysovine (lb) followed by amination could afford erymelanthine directly. 

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