Yohimban system

Thus the critical synthetic 1,6-dihydropyridine precursor for the unique isoquinuclidine system of the iboga alkaloids, was generated by reduction of a pyridinium salt with sodium borohydride in base (137-140). Lithium aluminum hydride reduction of phenylisoquinolinium and indole-3-ethylisoquinolinium salts gave enamines, which could be cyclized to the skeletons found in norcoralydine (141) and the yohimbane-type alkaloids (142,143). 

The quaternary alkaloid anhydroalstonatine (70), which has a completely aromatic yohimbane ring system, has been isolated from Alstonia venenata R.Br. (64). 

The previously known reserpine-type alkaloids (3) are shown below. They are reserpine (109), deserpidine (110), rescinnamine (111), veneserpine (112), pseudoreserpine (113), raunescine (114), rescidine (115), raugustine (116), and isoraunescine (117). It should be mentioned that all reserpine-type alkaloids are trisubstituted at ring E and that the substituents at C-16, C-17, and C-18 are all in equatorial positions on the epiallo yohimbane ring system existing predominantly... 

Compounds containing condensed yohimbane and pyrazole/or pyrimidine ring systems (616, 617) are potent central nervous system depressants (294, 297). 

On going from [363] to the pentacyclic system exemplified by yohimbane [367] similar A(5 values are encountered for the C(14) and... 

Several groups of workers have reported briefly on new or modified syntheses of the yohimbane ring system.59 The last of these communications59e describes syntheses of 14-isogambirtannine and related compounds. 

The numbering system for the alkaloids in this chapter is based on biogenetic relationships, i.e., an atom is assigned the same number as its supposed equivalent in yohimban,... 

The chiroptical properties and conformations inferred therefrom were correlated " with n.m.r. and dehydrogenation (at C-3—N-4) measurements, the traditional methods for the analysis of configuration and conformation in these systems. The C-3 equilibration of all the yohimbane and yohimban-17-ones was studied. One interesting point which emerged was the differences observed between equilibrium positions determined by the usual organic acid method and... 

The numbering system employed in the chapter assigns to the ring atoms the same numbers as their presumed equivalents in yohimbane ). 

One application of this reaction was in the synthesis of yohimban 11.17 (Scheme 11.6). The cycloaddition precursor 11.14 could be easily built up from tryptamine 11.13 and the cyclization employed a nickel/phosphite catalyst system. Selective reduction of the less-hindered alkene, protio-desilylation and a Bischler-Napieralski reaction gave the desired pentacyclic system 11.16. Reduction of the remaining alkene gave yohimban 11.17... 

---