Narcissus and Daffodil

Alkaloids of Narcissus 163
β-pro-S hydrogen (Wightman et al., 1972). Thus, it may be expected that L-phe
would be incorporated into Amaryllidaceae alkaloids with retention of the β-pro-R
hydrogen. However, feeding experiments in Narcissus pseudonarcissus cv. King Alfred
showed that tritium originally present at C-β of L-phe, whatever the configuration,
was lost in the formation of several haemanthamine and homolycorine type alkaloids,
which led to the conclusion that fragmentation of the cinnamic acids involves
oxidation of C-β to ketone or acid level, the final product being protocatechuic
aldehyde or its derivatives (Figure 6.3). On the other hand, L-tyr is degraded no
further than tyramine before incorporation into the Amaryllidaceae alkaloids.
Thus, tyramine and protocatechuic aldehyde or its derivatives are logical components
for the biosynthesis of the precursor norbelladine (83). This reaction
occupies a pivotal position since it represents the entry of primary metabolites into
a secondary metabolic pathway. The junction of the amine and the aldehyde
results in a Schiff’s base, two of which have been isolated up to now: craugsodine
(Ghosal et al., 1986) and isocraugsodine (Ghosal et al., 1988a). The existence of
Schiff’s bases in nature, as well as their easy conversion into the different ringsystems
of the Amaryllidaceae alkaloids, allows the presumption that the initial
postulate about this biosynthetic pathway was correct.
Barton and Cohen (1957) proposed that norbelladine (83) or related compounds
could undergo oxidative coupling of phenols in Amaryllidaceae plants, once ring
A had been suitably protected by methylation, resulting in the different skeletons
of the Amaryllidaceae alkaloids (Figure 6.4).
Lycorine and homolycorine types
The alkaloids of this group are derivatives of the pyrrolo[de]phenanthridine alkaloids
(lycorine type) and the 2-benzopirano-[3,4-g]indole alkaloids (homolycorine
type), and both types originate from an ortho-para′ phenol-oxidative coupling
(Figure 6.5).
The biological conversion of cinnamic acid via hydroxylated cinnamic acids into
the C 6 –C 1 unit of norpluviine (12) has been used in a study of hydroxylation
mechanisms in higher plants (Bowman et al., 1969). When [3- 3 H, β- 14 C]cinnamic
acid was fed to Narcissus pseudonarcissus cv. Texas, a tritium retention in norpluviine
(12) of 28% was observed. This is very near a predicted value of 25%, resulting from
para-hydroxylation with hydrogen migration and retention, where half the tritium
would be lost in the first hydroxylation and half the remainder in the second.
In the conversion of O-methylnorbelladine (80) into lycorine (1), the labeling
position [3- 3 H] on the aromatic ring of L-tyr afterwards appears at C-2 of norpluviine
(12), which is formed as an intermediate, the configuration of the tritium
apparently being β (Kirby and Tiwari, 1966). This tritium is retained in subsequently
formed lycorine (1), which means that hydroxylation at C-2 proceeds with
an inversion of configuration (Bruce and Kirby, 1968) by a mechanism involving
an epoxide, with ring opening followed by allylic rearrangement of the resulting
alcohol (Figure 6.6). Supporting evidence comes from the incorporation of [2β-
3 H]caranine (10) into lycorine (1) in Zephyranthes candida (Wildman and Heimer,
1967). However, a hydroxylation of caranine (10) in Clivia miniata occurring
with retention of configuration was also observed (Fuganti and Mazza, 1972b).
Further, [2α- 3 H; 11- 14 C]caranine (10) was incorporated into lycorine (1) with high