A synthesis of the amaryllidaceae alkaloid pratosine

Article abstract of DOI:10.1021/np9601869

A synthesis of pratosine by thermolysis of the Michael adduct obtained from 5-hydroxy-8,9-dimethoxy-6(5H)-phenanthridone and methyl propiolate is described.

Full text of DOI:10.1021/np9601869

744
J . Nat. Prod. 1996, 59, 744-747
A Syn th esis of th e Am a r yllid a cea e Alk a loid P r a tosin e
M. Manuela A. Pereira, Sundaresan Prabhakar,* and Ana M. Lobo*
Secc¸a˜o de Qu´ımica Orgaˆnica Aplicada, Departamento de Qu´ımica and SINTOR-UNINOVA, campus FCT-UNL,
Quinta da Torre, 2825 Monte da Caparica, Portugal
Received J anuary 17, 1996X
A synthesis of pratosine by thermolysis of the Michael adduct obtained from 5-hydroxy-8,9-
dimethoxy-6(5H)-phenanthridone and methyl propiolate is described.
Pyrrolophenanthridine alkaloids, isolated from the
bulbs of several Crinum species (Amaryllidaceae), com-
prise a group of tetracyclic lactams such as hippadine
(1),1-3 pratosine (2),3 pratorinine (3),2-6 and pratorimine
(4).2-6 The ability of hippadine to reversibly inhibit
fertility in male rats4 and the antitumor7,8 activity of
some members of this family of alkaloids have prompted
the development of various synthetic methods9 for these
substances.
Experimental Section), the following substances were
isolated and identified as: pratosine (2, 17%), 8,9-
dimethoxy-6(5H)-phenanthridone13 (9, 25%), N-(formyl-
methyl)-8,9-dimethoxy-6(5H)-phenanthridone (10, 15%),
and 4-carbomethoxypratosine (11, 33%) (cf. Schemes 2
and 3).
The structure of aldehyde 10 follows from its exact
molecular weight (M+ 297.1010; C17H15O4N requires
297.1001) and spectral characteristics. The IR spectrum
contained bands at 1710, 2830, and 1640 cm-1 consis-
tent with the presence of an aldehyde and a δ-lactam
group. The 1H-NMR signals at δ 9.74 (1H, s, CHO), 5.24
(2H, s, CH2CHO), 7.07, 7.33, 7.47, 7.62, 7.91, and 8.19
ppm (1H intensity each) and the lack of any exchange-
able hydrogen with D2O showed unambigously that the
amide was tertiary.
Location of the CO2Me group at C-4 and not at C-5 of
the pyrrolophenanthridine nucleus in 11 was made on
the basis of two strongly deshielded protons at δ 8.41
(1H, dd, J 1 ) 8 and J 2 ) 3 Hz, H-3) and 8.57 (1H, s,
H-5) ppm in the 1H-NMR spectrum. The corresponding
hydrogens of pratosine resonated at δ 7.76 and 8.07
ppm, respectively.
We report here10 the application of our previously
described method11 to the synthesis of pratosine (2)
(Scheme 1), the characterization of various products
formed in the key-transformation (8 f 2) and attempts
to circumvent some of these undesired side-reactions.
Mechanistically the formation of the aldehyde 10
(Scheme 2) involves formally a 1,3-rearrangement of the
vinyl ether 8, followed by hydrolysis and decarboxyla-
tion of the resulting â-keto carboxylic acid. Heating 10
in wet DMSO under conditions of the formation of
pratosine did not give the latter, indicating that 10 is
not the precursor of 2.
Resu lts a n d Discu ssion
The benzohydroxamic acid, derived from N-phenyl-
hydroxylamine and 2-bromo-4,5-dimethoxybenzoyl chlo-
ride, and BF3‚Et2O afforded the boron compound 512 as
a white solid, mp 158-159 °C, in more than 90% yield.
Photolysis of a benzene solution of 5 using a Pyrex filter
gave the cyclic borate 612 from which the requisite
hydroxamic acid 7 was liberated by aqueous hydrolysis.
A base-catalyzed Michael addition of 7 to methyl pro-
piolate provided the enol-ether 8. Heating a solution
of 8 in wet DMSO (0.03% H2O v/v) under reflux (35 min)
followed by workup and purification by preparative TLC
led to the isolation of pratosine (2) in low yield (9%).
Slow addition of the vinyl ether 8 to boiling DMSO
containing H2O and maintaining the reflux temperature
(30 min) did not significantly alter the yield of the
alkaloid, nor did systematic variation of the H2O content
(0.03% to 1%) in DMSO or of the temperature (130-
180 °C). Optimal results were obtained by performing
the thermolysis on a small scale for a shorter length of
time (10 min) (50 mg; 3 mL wet DMSO, 0.03% in H2O),
combining five such experiments, and then working up
the reaction mixture for products. By this process (see
Although the generation of the phenanthridone 9
most probably occurs via homolysis of the weak NsO
bond and subsequent quenching of the resulting amidyl
radical,14,15 the isolation of ester 11 (cf. Scheme 3) and
pratosine (2) sheds some light on the nature of the
various competitive processes that occur on thermolysis
of 8. Formation of 2 can be envisaged to occur by a
sequence of reactions involving a 3,3-sigmatropic rear-
rangement to yield the aldehyde ester 12, a nucleophilic
attack of the nitrogen on the more electrophilic carbonyl
carbon of the aldehyde group to form 13, and hydrolysis
of the ester 13 to produce the free acid 14, which suffers
decarboxylation with simultaneous loss of the hydroxyl
group. Alternatively, the aldehyde ester 12 can suffer
hydrolysis and decarboxylation, prior to cyclization to
form aldehyde 15, which on ring closure and elimination
of H2O generates alkaloid 2. The same aldehyde ester
(12) on cyclization and dehydration leads to pratosine
4-carboxylic ester (11). It thus became apparent that,
in order to achieve a respectable yield of 2, the hydroly-
* To whom correspondence should be addressed. Phone: +351-1-
2954464. FAX: +351-1-2948550. E-mail: aml@mail.fct.unl.pt.
X Abstract published in Advance ACS Abstracts, J uly 15, 1996.
S0163-3864(96)00186-3 CCC: $12.00
© 1996 American Chemical Society and American Society of Pharmacognosy

Pratosine Synthesis
J ournal of Natural Products, 1996, Vol. 59, No. 8 745
Sch em e 1. Synthetic Scheme for Pratosine (2)
Sch em e 2. Formation of 9 and 10 from 8
sis of the ester group in 8 should occur prior to the 3,3-
rearrangement, before the intermediate 13 is formed,
or before dehydration of 13 occurs. Because the latter
two conditions are difficult to realize experimentally,
attempts were made to satisfy the first by the use of
trimethylsilyl propiolate (since selective hydrolysis of
16 to the acid 17 can be brought about under neutral
conditions, eg., by methanolysis). Accordingly, the
reaction of trimethylsilyl propiolate16 with two simple
hydroxamic acids, N-phenylacetohydroxamic and N-
phenylbenzohydroxamic acids, was performed to define
appropriate experimental conditions for the necessary
Michael addition to occur. However, no such addition
took place under a variety of conditions, and the starting
material either remained virtually unaltered at room
temperature or yielded an intractable mixture under
more drastic conditions (reflux, excess propiolate). Fail-
ure to obtain the Michael adduct with the silyl ester,
which contrasts starkly with the behavior of the corre-
sponding methyl ester, can be ascribed to one of two
reasons. Either the nucleophile attacks the silicon
atom, resulting in a simple transfer of the trimethylsilyl
group to form the O-silyl ether of the hydroxamic acid,
which regenerates the free hydroxamic acid with ex-
ceptional ease (viz. on chromatoplates used for monitor-
ing the reaction), or the presence of the silicon atom in
the â position, with respect to the carbonyl group in the
electrophile, stabilizes the â carbon to such an extent
that addition becomes unviable.
Attempts to force nucleophilic addition to the be-
taine,17 obtained from triethylamine and propiolic acid,
which had been shown to react with nucleophiles such
as alkoxide18 to furnish a â-alkoxyacrylic acid, with the
hydroxamate anion (derived from 7) proved to be
unfruitful.
Exp er im en ta l Section
Gen er al Exper im en tal P r ocedu r es. Melting points
were determined with a hot-stage microscope Reichert
Thermovar and are uncorrected. IR were recorded with
a Perkin-Elmer 157 Gand 683 grating infrared spectro-
photometer, and frequencies are reported in cm-1 1H-
.
NMR spectra were obtained at 300 MHz with a Varian
unit 300 spectrometer. Chemical shifts are reported in
ppm downfield from TMS. HREIMS were measured in
a Kratos MS-25 RF instrument using electron impact
at 70 eV. Elemental analyses were carried out at the
micro-analytical division of DTIQ-INETI, Queluz, Por-
tugal.
N-P h en yl-2-b r om o-4,5-d im et h oxyb en zoh yd r ox-
a m ic a cid : prepared, in 73% yield, from 2-bromo-4,5-
dimethoxybenzoyl chloride and N-phenylhydroxylamine
in ether, in the presence of K2CO3, mp 143-145 °C (from
C6H6/n-hexane); found C, 51.48; H, 4.38; N, 3.88; C15H14
-
BrNO4 requires C, 51.56; H, 4.01; N, 3.98%.
2,2-Diflu or o-4-p h en yl-5-(2′-b r om o-4′,5′-d im et h -
oxyp h e n yl)-1,3-d ioxo-4-a zo-2-b or ocyclop e n t -4-
en e (5). A mixture of the above acid (5.74 g) in dry

746 J ournal of Natural Products, 1996, Vol. 59, No. 8
Pereira et al.
Sch em e 3. Suggested Mechanism for the Formation of 2 and 11 from 8
ether (400 mL) and freshly distilled BF3-etherate (4 mL)
was refluxed (30 min) and then allowed to stand at room
temperature (24 h). The white crystalline solid that
separated was filtered and crystallized (C6H6/n-hexane)
to give the boron complex 5 (5.8 g, 90% yield), mp 158-
159 °C; found C, 45.17; H, 3.32; N, 3.48; C15H13BBrF2-
NO4 requires C, 45.04; H, 3.28; N, 3.50%.
5-H y d r o xy-8,9-d im e t h oxy -6(5H )-p h e n a n t h r i-
d on e (7). The boron complex 5 (1 g) dissolved in dry
C6H6 (130 mL) was irradiated (30 h) with a mercury
lamp (Philips HPR 125W) using a Pyrex filter at 37 °C.
The white crystalline precipitate of 6 that formed was
decanted, washed with dry Et2O, and dried to afford the
cyclic boron complex 6 (mp 318-320 °C; 90% yield). A
suspension of this compound (1 g) in H2O (25 mL) and
EtOH (50 mL) was heated to reflux (24 h). The mixture
was cooled to 0 °C, and the solid 7, collected by filtration,
afforded the cyclic hydroxamic acid 7 (803 mg), mp 243-
245 °C (from EtOH); found C, 66.24; H, 4.85; N, 5.27;
C15H13NO4 requires C, 66.41; H, 4.83; N, 5.17%.
experiments were combined and poured into ice-water
(100 mL). Rapid extraction with EtOAc and evapora-
tion of solvent, after drying (Na2SO4) yielded a brown
residue that was subjected to column chromatography
(silica) using EtOAc/petroleum ether (1:1) as the eluent.
The least polar substance, pratosine (2), was obtained
as a white solid (17%), mp 238-240 °C (from CH2Cl2),
lit.3 mp 238-240 °C. Its chemical shifts (δ values) for
various protons and νmax data (IR) were identical to
those recorded3 for the alkaloid. Continued elution
afforded a mixture of the aldehyde 10 and the amide 9,
which was rechromatographed on silica. Elution with
EtOAc yielded the aldehyde 10, crystallizing from ether/
CH3OH as a white solid (15%), mp 195-205 °C; νmax
(KBr) 2830 (CHO), 1710 (CHdO), 1640 (NsCdO)‚cm-1
;
1H NMR (CDCl3) δ 4.04 (3H, s), 4.11 (3H, s), 5.24 (2H,
s), 7.07 (1H, d, J ) 8 Hz), 7.33 (1H, t, J ) 8 Hz), 7.47
(1H, t, J ) 8 Hz), 7.62 (1H, s), 7.91 (1H, s), 8.19 (1H, d,
J ) 8 Hz), 9.74 (1H, s); HREIMS M+ 297.1010; C17H15
NO4 requires 297.1001.
-
tr a n s-Meth yl â-[6(5H)-Oxo-8,9-dim eth oxyph en an -
th r id in yl]-5-oxy]a cr yla te (8). A mixture of the above
compound 7 (500 mg), dry methyl propiolate (0.27 mL),
and triethylamine (0.21 mL) in dry acetonitrile (200 mL)
was stirred at room temperature (48 h). Evaporation
of the solvent under reduced pressure and crystalliza-
tion of the resulting brown solid (MeCN/n-hexane)
afforded the thermally unstable acrylate derivative 8
in a reasonably pure state, mp 134-137 °C (dec); 82%
The later fractions were evaporated to yield the
phenanthridone 9 (25%), mp 296-298 °C, [lit.12 mp
300-302 °C], νmax (KBr) 3140-2840 (NH), 1650, 1600
1
cm-1; H NMR (DMSO-d6) δ 3.91 (3H, s), 4.02 (3H, s),
7.23 (1H, t, J ) 7.5 Hz), 7.33 (1H, d, J ) 7.5 Hz), 7.43
(1H, t, J ) 7.5 Hz), 7.71 (1H, s), 7.98 (1H, s), 8.39 (1H,
d, J ) 8 Hz), 11.50 (1H, s, exchangeable with D2O).
Evaporation of the aqueous solution under reduced
pressure and high-vacuum sublimation of the residue
afforded 4-carbomethoxypratosine (11) (33%) mp 320-
yield; νmax (KBr) 1715, 1660, 1610 cm-1 1H NMR
;
(CDCl3) δ 3.65 (3H, s), 4.04 (3H, s), 4.11 (3H, s), 5.42
(1H, d, J ) 12 Hz), 7.88 (1H, d, J ) 12 Hz). Attempts
to purify the material by further crystallization served
only to cause more decomposition.
Th er m olysis of 8. The above acrylate (8) (50 mg)
in DMSO (3 mL) containing H2O (0.03%; v/v) was heated
to reflux (10 min). The reaction mixtures from five such
1
321 °C, νmax 1710, 1670 cm-1, H NMR (DMSO-d6) δ
3.94 (3H, s), 3.96 (3H, s), 4.08 (3H, s), 7.63 (1H, t, J )
7.5 Hz), 7.87 (1H, s), 8.02 (1H, s), 8.13 (1H, dd, J 1 ) 8
and J 2 ) 3Hz), 8.41 (1H, dd, J 1 ) 8 and J 2 ) 3Hz), 8.57
(1H, s); HREIMS M+ 337.0945; C19H15O5N requires
337.0950.

Pratosine Synthesis
J ournal of Natural Products, 1996, Vol. 59, No. 8 747
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Ack n ow led gm en t. We wish to thank J unta Nacio-
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Lisbon) for partial financial support, and DTIQ-INETI
(Queluz) for the microanalyses.
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(10) This work was presented in part at the 12th Annual Meeting of
the Portuguese Chemical Society, Lisbon, J anuary 1992. For a
previous synthesis of pratosine, see Ghosal et al.3
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(12) Prabhakar, S.; Lobo, A. M.; Tavares, R. J . Chem. Soc., Chem.
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407-409.
(2) Ghosal, S.; Rao, P. H.; J aiswal, D. K.; Kumar, Y.; Frahm, A. W.
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(4) Chattopadhyay, S. C.; Chattopadhyay, U.; Mathur, P.; Saini, K.
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(15) Novak, M.; Brodeur, B. A. J . Org. Chem. 1984, 49, 1142-1144.
(16) Hergott, H. H.; Simchen, G. Synthesis 1980, 626-627.
(17) McCulloh, M. W.; McInnes, A. G. Can. J . Chem. 1974, 52, 3569-
3576.
(5) For revised structures of pratorinine (3) and pratorimine (4),
see: Maddry, J . A.; Balawant, S. J .; Ali, A. A.; Newton, M. G.;
Pelletier, S. W. Tetrahedron Lett. 1985, 26, 4301-4302.
(6) For a synthesis of pratorimine, see: Balawant, S.; Haridutt, K.
D.; Pelletier, S. W. J . Nat. Prod. 1986, 49, 445-448.
(18) Buchi, G.; Vogel, D. E. J . Org. Chem. 1983, 48, 5406-5408.
NP9601869