Narcissus and Daffodil

Alkaloids of Narcissus 177
Lycorine type
This group has been subject to several 1 H-NMR studies, and lycorine (1) as well as
its main derivatives have been completely assigned. The general characteristics of
the 1 H-NMR spectra are:
1 Two singlets for the para-oriented aromatic protons in the range δ 6.5–7.2
ppm.
2 A unique olefinic proton around 5.5 ppm.
3 Two doublets as an AB system corresponding to the benzylic protons of C-6.
4 The deshielding observed in the β protons of positions 6 and 12 in relation to
their α-homologs is due to the effect of the cis-lone pair of the nitrogen atom.
5 Like almost all other lycorine type examples, the alkaloids isolated from
narcissus show a trans B/C ring junction, the coupling constant being
J 4a,10b ~11Hz.
In the plant, the alkaloid lycorine (1) is particularly vulnerable to the oxidation
processes, giving several ring-C aromatised products.
Homolycorine type
This group includes lactone, hemiacetal or cyclic ether alkaloids. The general
traits for these types of compounds could be summarised as follows:
1 Two singlets for the para-oriented aromatic protons. In lactone alkaloids,
differentiation of the H-7 and H-10 signals is readily made by virtue of the
deshielding of H-7 by effect of the peri-carbonyl group.
2 The hemiacetal alkaloids always show the substituent at C-6 in α-disposition, and
the benzylic proton H-6β appears as a singlet between 5–6 ppm, depending on
the substituent at C-6.
3 The majority of compounds belong to a single enantiomeric series containing
a cis B/C ring junction, which is made clear by the small size of the coupling
constant J 1,10b . In the Narcissus genus no exception to this rule has been
observed.
4 The large coupling constant between H-4a and H-10b (J 4a,10b ~10Hz), is only
consistent with a trans-diaxial relationship.
5 In general, the C ring presents a vinylic proton around 5.5 ppm.
6 The singlet corresponding to the N-methyl group is in the range δ 2.0–2.2
ppm, its non-existence being very unusual.
7 If position 2 is substituted by an OH, OMe or OAc group, it always displays an
α-disposition.
8 The H-12α is more deshielded than H-12β as a consequence of the cis-lone
pair of the nitrogen atom.
An interesting study of homolycorine type alkaloids with a saturated ring C
has been made by Jeffs et al. (1988). They describe empirical correlations of
N-methyl chemical shifts with stereochemical assignments of the B/C and C/D
ring junction.