Synthesis of 7-epineoptilocaulin, mirabilin B, and isoptilocaulin. A unified biosynthetic proposal for the ptilocaulin and batzelladine alkaloids. Synthesis and structure revision of netamines E and G

Article abstract of DOI:10.1021/jo801956w

(Chemical Equation Presented) Addition of guanidine to a 6-methylhexahydroindenone in MeOH at 85°C afforded 7-epineoptilocaulin. A similar reaction with a 6-propylhexahydroindenone afforded netamine E. MnO ₂ oxidation of 7-epineoptilocaulin and netamine E afforded mirabilin B and netamine G, respectively. The netamines have the side chains trans, not cis as was initially proposed. A unified biosynthetic scheme for the batzelladines and ptilocaulin family is proposed. Conjugate addition of guanidine to a bis enone followed by an intramolecular Michael reaction of the enolate to the other enone, aldol reaction, dehydration, and enamine formation will lead to a tricyclic intermediate at the dehydroptilocaulin oxidation state. 1,4-Hydride addition will lead to ptilocaulin or 7-epineoptilocaulin depending on which face the hydride adds to. 1,2-Hydride addition will lead to isoptilocaulin. The key tricyclic intermediate was prepared from a tetrahydroindenone and guanidine and reduced with NaBH₄ to give a mixture rich in ptilocaulin and isoptilocaulin.

Full text of DOI:10.1021/jo801956w

Synthesis of 7-Epineoptilocaulin, Mirabilin B, and Isoptilocaulin. A
Unified Biosynthetic Proposal for the Ptilocaulin and Batzelladine
Alkaloids. Synthesis and Structure Revision of Netamines E and G
Min Yu, Susan S. Pochapsky, and Barry B. Snider*
Department of Chemistry MS 015, Brandeis UniVersity, Waltham, Massachusetts 02454-9110
snider@brandeis.edu
ReceiVed September 4, 2008
Addition of guanidine to a 6-methylhexahydroindenone in MeOH at 85 °C afforded 7-epineoptilocaulin.
A similar reaction with a 6-propylhexahydroindenone afforded netamine E. MnO2 oxidation of 7-epin-
eoptilocaulin and netamine E afforded mirabilin B and netamine G, respectively. The netamines have
the side chains trans, not cis as was initially proposed. A unified biosynthetic scheme for the batzelladines
and ptilocaulin family is proposed. Conjugate addition of guanidine to a bis enone followed by an
intramolecular Michael reaction of the enolate to the other enone, aldol reaction, dehydration, and enamine
formation will lead to a tricyclic intermediate at the dehydroptilocaulin oxidation state. 1,4-Hydride addition
will lead to ptilocaulin or 7-epineoptilocaulin depending on which face the hydride adds to. 1,2-Hydride
addition will lead to isoptilocaulin. The key tricyclic intermediate was prepared from a tetrahydroindenone
and guanidine and reduced with NaBH4 to give a mixture rich in ptilocaulin and isoptilocaulin.
Introduction
(8, 9, 10, 20, 21, and 2) as their N-acetyl derivatives from
Arenochalina mirabilis in 1996.4 Patil and Freyer isolated
mirabilin B (9), 8R-hydroxymirabilin B (13), and compounds
5 and 6 from Batzella sp. in 1997.5 Compound 5, which they
called 8a,8b-dehydroptilocaulin is epimeric to ptilocaulin at C-7.
It is at the same oxidation state as ptilocaulin so the dehydro
prefix is incorrect. We have renamed this compound as
7-epineoptilcaulin, with the neo prefix used to indicate a double
bond between C-8a and C-8b because the iso prefix was already
used to indicate a double bond between C-7 and C-8. Using
Ptilocaulin (1) and isoptilocaulin (4) were isolated by Rinehart
from two different samples of Ptilocaulis aff. P. spiculifer
collected at slightly different locations and depths in 1981 (see
Chart 1).1a The structures were initially determined by NMR
experiments. An X-ray structure determination of ptilocaulin
(1) established that H-5a and H-8b are cis despite the 10 Hz
coupling constant between them. Isoptilocaulin (4) also has the
same stereochemistry at C-5a, 1b although it was also originally
depicted as the trans isomer on the basis of the large coupling
constant.1a Both ptilocaulin (1) and isoptilocaulin (4) have been
reisolated from this and related sponges along with the more
complex batzelladines, ptilomycalins, and crambescidins.2 Since
1995, nineteen additional members of this family of tricyclic
guanidine alkaloids have been isolated from a variety of other
marine sponges. Braekman isolated 8b-hydroptilocaulin (15)
from Monanchora arbuscula.3 Capon isolated mirabilins A-F
(2) (a) Patil, A. D.; Kumar, N. V.; Kokke, W. C.; Bean, M. F.; Freyer, A. J.;
De Brosse, C.; Mai, S.; Truneh, A.; Faulkner, D. J.; Carte, B.; Breen, A. L.;
Hertzberg, R. L.; Johnson, R. K.; Westley, J. W.; Potts, B. C. M. J. Org. Chem.
1995, 60, 1182–1188. (b) Gallimore, W. A.; Kelly, M.; Scheuer, P. J. J. Nat.
Prod. 2005, 68, 1420–1423. (c) Kossuga, M. H.; de Lira, S. P.; Nascimento,
A. M.; Gambardella, M. T. P.; Berlinck, R. G. S.; Torres, Y. R.; Nascimento,
G. G. F.; Pimenta, E. F.; Silva, M.; Thiemann, O. H.; Oliva, G.; Tempone, A. G.;
Melhem, M. S. C.; de Souza, A. O.; Galetti, F. C. S.; Silva, C. L.; Cavalcanti,
B.; Pessoa, C. O.; Moraes, M. O.; Hadju, E.; Peixinho, S.; Rocha, R. M. Quim.
NoVa 2007, 30, 1194–1202.
(3) Tavares, R.; Daloze, D.; Braekman, J. C.; Hajdu, E.; Van Soest, R. W. M.
J. Nat. Prod. 1995, 58, 1139–1142.
(4) Barrow, R. A.; Murray, L. M.; Lim, T. K.; Capon, R. J. Aust. J. Chem.
1996, 49, 767–773.
(1) (a) Harbour, G. C.; Tymiak, A. A.; Rinehart, K. L., Jr.; Shaw, P. D.;
Hughes, R. G., Jr.; Mizsak, S. A.; Coats, J. H.; Zurenko, G. E.; Li, L. H.;
Kuentzel, S. L. J. Am. Chem. Soc. 1981, 103, 5604–5606. (b) Harbour, G. C.
The isolation and structure determination of antimicrobial, cytotoxic, and
antiViral marine natural products. Ph. D. Dissertation, University of Illinois,
Urbana, IL, 1983;. Diss. Abstr. Int. B 1984, 44 (7), 2159.
(5) Patil, A. D.; Freyer, A. J.; Offen, P.; Bean, M. F.; Johnson, R. K. J. Nat.
Prod. 1997, 60, 704–707.
10.1021/jo801956w CCC: $40.75
Published on Web 10/18/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 9065–9074 9065

Yu et al.
CHART 1. Structures of the Ptilocaulin (1) Family
SCHEME 1. Syntheses of Ptilocaulin (1)
Several members of this family have significant biological
activity. Ptilocaulin (1) and isoptilocaulin (4) have ID50s of 0.39
and 1.4 µg/mL, respectively, against L1210 leukemia cells and
1 has a minimum inhibitory concentration of 3.9 µg/mL against
Streptococcus pyogenes.1 Ptilocaulin (1) showed a fairly broad
spectrum of in Vitro activity against colon and mammary
adenocarcinomas, melanomas, leukemias, transformed fibro-
blasts, and normal lymphoid cells with IC50s of 0.05 to >10
µg/mL.9 Mirabilin B (9) exhibited antifungal activity against
Cryptococcus neoformans with an IC50 value of 7.0 µg/mL and
antiprotozoal activity against Leishmania donoVani with an IC50
value of 17 µg/mL. A mixture of 13 and 14 is active against
the malaria parasite Plasmodium falciparum with an IC50 value
of 3.8 µg/mL.7
We reported the first synthesis of ptilocaulin (1) in 1983 by
the reaction of bicyclic enone 22 with guanidine in benzene at
reflux (see Scheme 1).10 Under equilibrating conditions, con-
jugate addition occurs from the desired R-face and enamine
formation affords the desired double bond between C-8 and
C-8a. Addition of nitric acid affords ptilocaulin nitrate identical
to the natural product.10 Roush prepared amino ketone 24 and
converted it to a guanidine which led to a mixture of tricyclic
products. Heating the mixture in benzene at reflux, as in the
reaction of 22 with guanidine, resulted in the equilibration of
this tricyclic mixture to give ptilocaulin (1).11 Several more
recent ptilocaulin syntheses prepared 22 by alternate methods
and completed the synthesis by reaction with guanidine as we
described.12-16
Although minor stereo- or double bond position isomers are
also formed from the reaction of 22 with guanidine, they were
not characterized with the exception of 23. Schmalz found that
23 cocrystallized with ptilocaulin and the structure was deter-
mined crystallographically.16 It is noteworthy that 23 is formed
by the presumably kinetic addition of guanidine to the less
hindered top convex face of 22, whereas ptilocaulin is formed
by the presumably thermodynamic addition of guanidine to the
more hindered bottom face of 22.
this system, compound 6 is named 8R-hydroxy-7-epineoptilo-
caulin. Capon isolated mirabilin G (3) from a Clathria sp. in
2001.6 Hamann isolated mirabilin B (9) and both 8R- and 8ꢀ-
hydroxymirabilins (13 and 14) from Monanchora unguifera.7
Finally, Kashman isolated netamines A-G (16-19, 7, and
11-12) from Biemna laboutei in 2006.8
Most of these compounds have 7-epi stereochemistry; only
mirabilin F (2), mirabilin G (3), and 8b-hydroxyptilocaulin (15)
have the same C-7 stereochemistry as ptilocaulin (1). Mirabilins
A (8), B (9), and C (10), and netamines F (11) and G (12) are
more highly oxidized with an aromatic 2-aminopyrimidine ring.
Netamines A-D (16-19) are more highly reduced with a
tetrahydro-2-aminopyrimidine ring. Compounds 6, 15, 20, and
21 are hydroxylated at the dihydro-2-aminopyrimidine oxidation
state, while 13 and 14 are hydroxylated at the aromatic
2-aminopyrimidine oxidation state. In the aromatic series or with
the double bond between C-8a and C-8b as in the neoptilcaulins,
two stereoisomers are possible at C-8. The side chains are trans
in 7-epineoptilocaulin (5), mirabilin B (9), and mirabilin C (10).
The side chains were assigned to be cis in mirabilin A (8) on
the basis of a 7-9% NOE enhancement between H-7 and H-8.4
The cis stereochemistry in all the netamines (16-19, 7, 11, 12)
was also assigned based on an NOE between H-7 and H-8.8
(9) Ruben, R. L.; Snider, B. B.; Hobbs, F. W., Jr.; Confalone, P. N.; Dusak,
B. A. InVest. New Drugs 1989, 7, 147–154.
(10) (a) Snider, B. B.; Faith, W. C. Tetrahedron Lett. 1983, 24, 861–864.
(b) Snider, B. B.; Faith, W. C. J. Am. Chem. Soc. 1984, 106, 1443–1445.
(11) (a) Roush, W. R.; Walts, A. E. J. Am. Chem. Soc. 1984, 106, 721–723.
(b) Walts, A. E.; Roush, W. R. Tetrahedron 1985, 41, 3463–3478.
(12) (a) Uyehara, T.; Furuta, T.; Kabasawa, Y.; Yamada, J.-i.; Kato, T. Chem.
Commun. 1986, 53, 9–540. (b) Uyehara, T.; Furuta, T.; Kabawawa, Y.; Yamada,
J.-i.; Kato, T.; Yamamoto, Y. J. Org. Chem. 1988, 53, 3669–3673.
(13) (a) Hassner, A.; Murthy, K. S. K. Tetrahedron Lett. 1986, 27, 1407–
1410. (b) Murthy, K. S. K.; Hassner, A. Isr. J. Chem. 1991, 31, 239–246.
(14) Asaoka, M.; Sakurai, M.; Takei, H. Tetrahedron Lett. 1990, 31, 4759–
4760.
(6) Capon, R. J.; Miller, M.; Rooney, F. J. Nat. Prod. 2001, 64, 643–644.
(7) Hua, H.-m.; Peng, J.; Fronczek, F. R.; Kelly, M.; Hamann, M. T. Bioorg.
Med. Chem. 2004, 12, 6461–6464.
(8) Sorek, H.; Rudi, A.; Gueta, S.; Reyes, F.; Martin, M. J.; Aknin, M.;
Gaydou, E.; Vacelet, J.; Kashman, Y. Tetrahedron 2006, 62, 8838–8843.
(15) Cossy, J.; BouzBouz, S. Tetrahedron Lett. 1996, 37, 5091–5094.
(16) (a) Schmalz, H.-G.; Schellhaas, K. Angew. Chem., Int. Ed. Engl. 1996,
35, 2146–2148. (b) Schellhaas, K.; Schmalz, H.-G.; Bats, J. W. Chem.sEur. J.
1998, 4, 57–66.
9066 J. Org. Chem. Vol. 73, No. 22, 2008

7-Epineoptilocaulin, Netamines, and Isoptilocaulin
SCHEME 2. Hassner’s Studies with Hexahydroindenone 25
CHART 2. Calculated Energies of Ptilocaulin- and
Neoptilocaulin-Like Ureas
In 1986, Hassner prepared a mixture of 22, which he
converted to ptilocaulin, and 25, which was treated with
guanidine in benzene at reflux to give a compound he character-
SCHEME 3. Synthesis of Hexahydroindenone 25
ized as 7-epiptilocaulin (26) (see Scheme 2).13 The 1H and 13
C
NMR spectral data of 26 in CDCl3 are very similar to those
reported in 1997 for 7-epineoptilocaulin (5) in CD3OD.5 This
suggests that the position of the double bond of Hassner’s
product 26 has been incorrectly assigned. However, no definitive
conclusions can be drawn because the spectra were recorded in
different solvents. Heating 25 with guanidine neat at >130 °C
resulted in disproportionation to give a mixture of an aromatic
compound that is probably mirabilin B (9) and a saturated
compound 27 as only one of 16 possible stereoisomers.13b
Unfortunately, the 1H and 13C NMR spectral data of Hassner’s
aromatic compound were recorded in CDCl3 in 1991, whereas
those of natural mirabilin B (9) were recorded in CD3OD in
1997.5
Results and Discussion
We decided to develop a practical, efficient, and stereospecific
route to optically pure 25 and to unambiguously determine
whether reaction with guanidine afforded 7-epiptilocaulin (26)
or 7-epineoptilocaulin (5). We also wanted to explore routes to
the aromatic, tetrahydroaromatic, and hydroxylated members
of this growing family of tricyclic alkaloids. Ptilocaulin (1) and
mirabilins F (2) and G (3) with a 7ꢀ-methyl group have the
double bond between C-8 and C-8a, whereas 7-epineoptilocaulin
(5) and netamine E (7) with a 7R-alkyl group have the double
bond between C-8a and C-8b. We therefore started by consider-
ing the effect of the stereochemistry of the 7-alkyl group on
the stability of the three possible tricyclic enamines. MMX
calculations were performed on ureas, which are better para-
materized than guanidinium cations, with side chains truncated
to methyl groups to minimize rotational freedom.17
The ptilocaulin-like urea 28 is calculated to be 0.11 kcal/
mol less stable than the trans-neoptilocaulin-like urea 29 (see
Chart 2). However the 7-epiptilocaulin-like urea 31 is calculated
to be 2.45 kcal/mol less stable than the trans-7-epineoptilocau-
lin-like urea 32. Both cis-neoptilocaulin isomers 30 and 33 are
less stable than the corresponding trans-neoptilocaulin isomers
29 and 32. The 0.11 kcal/mol calculated energy difference
between 28 and 29 is so small that it is not surprising that 28
is experimentally observed to be more stable. However changing
the C-7 methyl group stereochemistry makes the trans-7-
epineoptilocaulin-like urea 32 much more stable than the
7-epiptilocaulin-like urea 31 suggesting that the change in double
bond position in the 7-epi series results from thermodynamic
rather than enzymatic control.
We developed a six-step route to indenone 25 starting from
2E,6Z-nonadienal (34), which contains a double bond that serves
as a latent aldehyde and is readily available because of its
valuable fragrance properties (see Scheme 3).18 Michael addi-
tion19 of tert-butyl 3-oxooctanoate (35)20 to dienal 34 followed
by aldol reaction with 0.25 equiv of KO-t-Bu in t-BuOH at 0
°C to reflux afforded a keto ester that was hydrolyzed and
decarboxylated with TsOH in toluene at 80 °C to provide
cyclohexenone 36 in 77% yield. Addition of excess MeLi and
CeCl321 afforded the tertiary allylic alcohol, which was treated
with PCC in CH2Cl2 containing 0.8 equiv of NaOAc to give
79% of cyclohexenone 37.22 House reported that Birch reduction
of 3,5-dimethyl-2-cyclohexenone afforded cis-3,5-dimethylcy-
clohexanone containing 6-12% of the trans isomer.23 We
(18) Kula, J.; Sadowska, H. Perfum. FlaVor. 1993, 18, 23–25; Chem. Abstr.
1994, 120, 6947.
(19) Chong, B.-D.; Ji, Y.-I.; Oh, S.-S.; Yang, J.-D.; Baik, W.; Koo, S. J.
Org. Chem. 1997, 62, 9323–9325.
(20) (a) Oikawa, Y.; Sugano, K.; Yonemitsu, O. J. Org. Chem. 1978, 43,
2807–2808. (b) Murakami, M.; Kobayashi, K.; Hirai, K. Chem. Pharm. Bull.
2000, 48, 1567–1569.
(17) PCModel (version 8.0); Serena Software: Bloomington, IN.
J. Org. Chem. Vol. 73, No. 22, 2008 9067

Yu et al.
SCHEME 4. Synthesis of 7-Epineoptilocaulin and
Mirabilin B
FIGURE 1. MMX-calculated structure of 27 with NOEs shown that
establish the stereochemistry.
B (9) in 80% yield. Heating 25 with guanidine neat for 4 h at
130-140 °C followed by workup with 1% HNO3 afforded
mirabilin B (9) in 39% yield and 27 in 31% yield as described
by Hassner.13b The NMR spectral data of synthetic 9 in CDCl3
are identical to those reported by Hassner13b and the data in
CD3OD are identical to those reported for natural mirabilin B5
establishing that synthetic 9 is mirabilin B.
therefore expected that Birch reduction of 37 would control the
stereochemistry at C-3. Reduction of 37 with Li in NH3
containing 9 equiv of t-BuOH at -33 °C afforded 38 in 73%
yield as an irrelevant 10:1 mixture of stereoisomers at the butyl
group. Minor amounts of the two isomers with a ꢀ-methyl group
at C-3 are probably also formed, but analysis is complicated
by the mixture of isomers at C-2. Ozonolysis of the side chain
double bond followed by reduction of the ozonide with Ph3P
provided aldehyde 39 in 93% yield. Intramolecular aldol reaction
was most effectively carried out in a 30:1 mixture of DME/6
M aqueous HCl at 55 °C under microwave irradiation for 10
min to give 25 in 76% yield as a 4:1 mixture of isomers.10
Poorer results were obtained in THF, which has been widely
used in intramolecular aldol reactions that form 22.15,24
Remarkably, tricycle 27 is formed as one of sixteen possible
stereoisomers. The spectral data for 27 are quite different from
those of saturated netamines A-D (16-19) indicating that the
stereochemistry is different. H-3a absorbs at δ 3.79 (br dd, J )
5.6, 5.6 Hz) and H-8a absorbs at δ 3.77 (br dd, J ) 2.7, 2.7
Hz). H-3a, H-8a, H-8b are on the same face of the ring because
a large coupling constant would be observed if either H-3a or
H-8a was trans to and therefore in a diaxial orientation relative
to H-8b. This restricts the 16 possible isomers to only four.
These three hydrogens can be all up or all down and the butyl
group can be either cis or trans to the methyl group. Protons
and carbons in CDCl3 were assigned on the basis of COSY,
HMQC, and HMBC experiments at 800 MHz (see Table S1 in
the Supporting Information). A ROESY experiment showed
NOEs between H-8b and both H-3a and H-8a confirming that
they are all on the same face of the molecule as expected from
the analysis of coupling constants. NOEs between H-8 and both
H-6R and H-8b establish that all these hydrogens are on the
R-face. NOEs were also observed between NH-1 at δ 7.63 and
the side chain hydrogens H-1′ at δ 1.52 and 1.70 and between
NH-3 at δ 8.2 and H-4ꢀ at δ 1.72 confirming the stereochemical
assignment shown in Figure 1.
Compounds 27 and mirabilin B (9) are presumably formed
by disproportionation of intermediates at the oxidation state of
7-epineoptilocaulin (5) as suggested by Hassner. It is quite
surprising that the stereochemistry of 27 at C-3a is the opposite
of that of 5 and identical to that of the minor byproduct 40.
7-Epineoptilocaulin (5) is clearly formed under thermodynamic
conditions. A kinetically formed intermediate with the C-3a
stereochemistry of 40 must undergo reduction in the dispro-
portionation reaction more readily than intermediates with the
C-3a stereochemistry of 5.
Treatment of 25 with guanidine in benzene at reflux gave
mixtures containing 7-epineoptilocaulin (5). Better results were
obtained with guanidine in MeOH25 at 85 °C in a sealed tube
for 24 h followed by workup with 1% nitric acid, which led to
7-epineoptilocaulin (5) in ∼50% yield, 3a,7-bisepiptilocaulin
(40) in 10% yield, and minor isomers with absorptions for H-3a
at δ 4.01 to 3.66 that were not characterized (see Scheme 4).
1
The H and 13C NMR spectral data of synthetic 5 in CD3OD
are identical to those reported by Patil and Freyer5,26 and the
data in CDCl3 are identical to those reported by Hassner for
7-epitilocaulin (26)13 whose structure should therefore be revised
to 7-epineoptilocaulin (5). H-3a of 5 absorbs at δ 4.25,
considerably downfield from δ 3.77 of ptilocaulin (1), as
expected for an allylic hydrogen. H-3a of a minor isomer that
was not obtained in pure form absorbs at δ 3.15 (ddd, J ) 11.6,
11.6, 5.2 Hz). The structure is tentatively assigned as 40, based
on the very similar chemical shift and coupling pattern for H-3a
of 23 at δ 3.13 (ddd, J ) 11.6, 11.6, 5.3 Hz).16,27
28
Oxidation of 7-epineoptilocaulin (5) with activated MnO2
in CH2Cl2 at 55 °C in a sealed tube for 1 d afforded mirabilin
Kashman reported that hydrogenation of netamine E (7) over
Pd/C afforded a compound with the same stereochemistry as
netamines A-D (16–19).8 We therefore hydrogenated 7-epin-
eoptilocaulin (5) over Pd/C under 3 atm of H2 for 4 h to give
saturated tricyclic guanidine 41 in ∼90% yield (see equation
(21) The yield of 37 was lower and starting enone 36 (10%) was recovered
if MeLi was used without CeCl3: Imamoto, T.; Takiyama, N.; Nakamura, K.;
Hatajima, T.; Kamiya, Y. J. Am. Chem. Soc. 1989, 111, 4392–4398.
(22) Dauben, W. G.; Michno, D. M. J. Org. Chem. 1977, 42, 682–685.
(23) (a) House, H. O.; Giese, R. W.; Kronberger, K.; Kaplan, J. P.; Simeone,
J. F. J. Am. Chem. Soc. 1970, 72, 2800–2810. (b) Pradhan, S. K. Tetrahedron
1986, 42, 6351–6358.
(24) (a) Bal, S. A.; Marfat, A.; Helquist, P. J. Org. Chem. 1982, 47, 5045–
5050. (b) Johansson, M.; Sterner, O. Org. Lett. 2001, 3, 2843–2845. (c) Tsantali,
G. G.; Takakis, I. M. J. Org. Chem. 2003, 68, 6455–6458.
(27) This proton was reported to absorb as a dt, J ) 11.6, 5.3 Hz. It actually
absorbs as a dt, J ) 5.3, 11.6 Hz. Prof. Hans-Guenther Schmalz, University of
Cologne, personal communication, January 8, 2008. We thank Prof Schmalz
for a copy of the 1H NMR spectra of the mixture of 1 and 23.
(28) Vanden Eynde, J. J.; Audiart, N.; Canonne, V.; Michel, S.; Van
Haverbeke, Y.; Kappe, C. O. Heterocycles 1997, 45, 1967–1978, and references
cited therein.
(25) For the reaction of guanidines with enones in EtOH and t-BuOH see:
(a) Kim, Y. H.; Yoon, C. M.; Lee, N. J. Heterocycles 1981, 16, 49–52. (b) El-
Rayyes, N. R.; Ramadan, H. M. J. Heterocycl. Chem. 1987, 24, 1141–1147.
(26) We thank Dr. Alan J. Freyer, GlaxoSmithKline Pharmaceuticals, for a
1
copy of the H NMR spectrum of 5.
9068 J. Org. Chem. Vol. 73, No. 22, 2008

7-Epineoptilocaulin, Netamines, and Isoptilocaulin
SCHEME 5. Synthesis of Hydroxymirablins (13 and 14)
FIGURE 2. MMX-calculated structures of 41 and 67 with NOEs shown
that establish the stereochemistry.
2). As expected, the spectral data for 41 were quite different
from those of the stereoisomer 27 obtained in the dispropor-
tionation reaction. We therefore assigned all the protons and
carbons in 41 in CDCl3 as shown in Table S1 in the supporting
material on the basis of COSY, HMQC, and HMBC experiments
at 800 MHz. H-3a absorbs at δ 3.89 (br dd, J ) 6, 4.2 Hz) and
H-8a absorbs at δ 3.54 (br dd, J ) 4.9, 1.1 Hz). As in 27, H-3a,
H-8a, H-8b are on the same face of the ring because a large
coupling constant would be observed if either H-3a or H-8a
was trans to and therefore in a diaxial arrangement with H-8b.
A ROESY experiment showed NOEs between H-8b and both
H-3a and H-8a confirming that they are all on the same face of
the molecule as expected from the analysis of coupling
constants. The cyclohexane ring is calculated to be a boat (see
Figure 2) making H-6R and H-8 3.52 Å apart so that no NOE
is observed. However, the guanidinium nitrogens are observed
as three separate peaks in CDCl3. The NOE observed between
NH-1 at δ 7.64 and H-8 at δ 1.46 confirms that side chains in
41 are trans.
components of the mixture with the enamine double bond
between H-8 and H-8a as in ptilocaulin, such as 40, to give 42
and then 13 and 14. Neither 5 nor 9 can be intermediates in the
formation of 13 and 14 because oxidation of 5 affords only
mirabilin B (9). Dioxiranes have been reported to effect benzylic
hydroxylations.31 Unfortunately treatment of 9 with dimethyl-
dioxirane afforded no 13 and 14 and a complex mixture that
appeared to contain N-oxides, which are known to be readily
formed from 2-aminopyrimidines.32
Synthesis of (+)-7-Epineoptliocaulin (5) and (+)-Mirabi-
lin B (9). The syntheses described above were carried out in
the racemic series because the Michael reaction of 34 and 35
afforded cyclohexenone 36 as a racemic mixture. Jørgensen
recently reported an asymmetric organocatalytic protocol for
such Michael reactions.33 Reaction of 34 and 35 neat with 10
mol% catalyst 43 and then treatment with 20 mol% p-
toluenesulfonic acid in toluene at 80 °C afforded (R)-36 in 55%
yield and 90% ee (see Scheme 6). The absolute stereochemistry
was assigned by analogy to Jørgensen’s examples.33 Reduction
of (R)-36 with NaBH4 and CeCl3 afforded cis allylic alcohol
44 which was converted to the Mosher esters 45 and 46. This
established that the ee was 90% as observed by Jørgensen in
related examples. The upfield shifts of the protons proximate
to the phenyl group confirmed the assignment of absolute
stereochemistry.34
The spectral data of saturated tricyclic guanidine 41 are
remarkably similar to those of netamines A-D (16-19) in which
the side chains are reported to be cis rather than trans as in 41.
The spectral data of 41 are virtually identical to those of
netamine C (18), which has a hexyl rather than a butyl group
on C-8 (see Table S1 in the Supporting Information). The
stereochemistry of netamines A (16), C (18), E (7), and G (12)
was therefore reexamined and established to be trans as di-
scussed below.
We now turned our attention to the preparation of hydroxy-
lated members of this family. Oxidation of 7-epineoptilocaulin
29
(5) with K3Fe(CN)6 in aqueous KOH and benzene afforded
only mirabilin B (9). However treatment of 25 with guanidine
in MeOH in a sealed tube in an 85 °C oil bath and oxidation of
the crude product mixture with K3Fe(CN)6 as described above
afforded mirabilin B (9) in 25% yield and a 1:1 mixture of
8-hydroxymirabilins B (13 and 14) in 5% yield (see Scheme
5). The spectral data of this 1:1 mixture are identical to those
of the 1:1 mixture isolated by Hamann.7,30 We suspect that 13
and 14 are formed by hydroxylation and oxidation of minor
The same series of reactions as in the racemic series afforded
(+)-7-epineoptliocaulin (5) and (+)-mirabilin B (9) (see equa-
tion 2). The optical rotation for synthetic 5, [R]D25 + 45.9, has
the same sign, but is much larger than that of natural 5, [R]D +
13.3.5 The optical rotation for synthetic 9, [R]D + 123.6
25
(31) (a) Brown, D. S.; Marples, B. A.; Muxworthy, J. P.; Baggaley, K. H.
J. Chem. Res. 1992, 28–29. (b) Bovicelli, P.; Lupattelli, P.; Mincione, E.;
Prencipe, T.; Curci, R. J. Org. Chem. 1992, 57, 2182–2184.
(32) Buscemi, S.; Pace, A.; Piccionello, A. P.; Vivona, N.; Pani, M.
Tetrahedron 2006, 62, 1158–1164.
(33) Carlone, A.; Marigo, M.; North, C.; Landa, A.; Jørgensen, K. A. Chem.
Commun. 2006, 4928–4930.
(29) (a) Deli, J.; Lo´ra´nd, T.; Szabo´, D.; Fo¨ldesi, A.; Zschunke, A. Collect.
Czech. Chem. Commun. 1985, 50, 1602–1610. (b) Thyagarajan, B. S. Chem.
ReV. 1958, 58, 439–460.
(30) We thank Prof. Mark T. Hamann, University of Mississippi for a sample
of the mixture of 13 and 14 and a copy of the 1H NMR spectrum of the mixture
of 13 and 14.
J. Org. Chem. Vol. 73, No. 22, 2008 9069

Yu et al.
SCHEME 6. Synthesis of (-)-(R)-36
SCHEME 7. Unified Biosynthetic Proposal for Ptilocaulin,
Isoptilocaulin, 7-Epineoptilocaulin, and Batzelladine K
(MeOH), has the same sign, but is much larger than that initially
reported for 9, [R]D + 41.6 (MeOH).5 However the value for
25
synthetic 9, [R]D + 145 (CHCl3), compares favorably with
22
that of the mirabilin B (9) that was reisolated in 2004, [R]D
+ 129 (CHCl3).35 The absolute stereochemical assignment of
synthetic 5 and 9 is based on the expected product of the
Jørgensen asymmetric Michael reaction and analysis of the
NMR spectra of the Mosher esters. Therefore the absolute
stereochemistry of natural 5 and 9 is as shown.
SCHEME 8. Precedents for the Proposed Biosynthesis
54b provided 55b in 80% yield.37 Dione 49 would have to
undergo an intramolecular aldol and dehydration reaction,
epimerization of H-8b, and iminium ion formation in unspecified
order to give the key tricyclic intermediate 52. There is precedent
for these steps in the work of Roush who reported that addition
of PMe3 to bis enone 56 gives 57, which undergoes an
intramolecular aldol and dehydration reaction to give bicyclic
dienone 58 after elimination of PMe3.38
The key step in this biosynthetic scheme is the reduction of
52 which can form ptilocaulin (1), isoptilocaulin (4), and
7-epineoptilocaulin (5). 1,2-Reduction of 52 from the less
hindered ꢀ-face will give isoptilocaulin (4). 1,4-Reduction from
the R-face will give ptilocaulin (1). 1,4-Reduction from the
ꢀ-face and isomerization to the more stable enamine in the 7-epi
series will give 7-epineoptilocaulin (5).
A Proposal for a Unified Biosynthesis of the Ptilocaulin
and Batzelladine Family of Alkaloids. It is possible that
ptilocaulin (1) and 7-epineoptilocaulin (5) are biosynthesized
by addition of guanidine to enones 22 and 25, respectively, as
can be easily achieved in one pot in ∼50% yield in the
laboratory. However, it is not clear how nature could adapt this
route to the biosynthesis of isoptilocaulin (4), which has an
allylic guanidine. Furthermore, this would require the presence
of two different precursor enones in the sponge. A unified
biosynthetic proposal that would lead to ptilocaulin (1), isop-
tilocaulin (4), 7-epineoptilocaulin (5), and the batzelladines such
as batzelladine K (53) is shown in Scheme 7.
Conjugate addition of guanidine to bis enone 47 will give
guanidinium enolate 48, which could undergo an intramolecular
Michael addition to give cyclopentane dione 49. There are ample
precedents for tandem conjugate additions leading to trans,trans-
cyclopentanes analogous to 49. For example, addition of a
thiophenoxide anion to bis enone 54a gave 55a in 58% yield
(see Scheme 8).36 Addition of an amide anion to bis enoate
A proton shift will convert guanidinium enolate 48 to
guanidine ketone 50. An intramolecular conjugate addition of
the guanidine can form pyrrolidine 51. Iminium ion formation
and 1,2-reduction will give rise to batzelladine K (53). Numerous
batzelladines are known with different side chains;2a batzelladine
K (53) which would be formed from the same precursor as the
(34) (a) Hoye, T. R.; Jeffrey, C. S.; Shao, F. Nat. Prod. 2007, 2, 2451–
2458. (b) Yasuhara, F.; Yamaguchi, S.; Takeda, M. Bull. Chem. Soc. Jpn. 1991,
64, 3390–3394.
(35) Professor Mark Hamann, University of Mississippi, personal com-
munication, April 28 and 29, 2008.
(36) Brown, P. M.; Ka¨ppel, N.; Murphy, P. J.; Coles, S. J.; Hursthouse, M. B.
Tetrahedron 2007, 63, 1100–1106.
(37) Urones, J. G.; Garrido, N. M.; D´ıez, D.; El Hammoumi, M. M.;
Dominguez, S. H.; Casaseca, J. A.; Davies, S. G.; Smith, A. D. Org. Biomol.
Chem. 2004, 2, 364–372.
(38) Thalji, R. K.; Roush, W. R. J. Am. Chem. Soc. 2005, 127, 16778–16779.
9070 J. Org. Chem. Vol. 73, No. 22, 2008

7-Epineoptilocaulin, Netamines, and Isoptilocaulin
SCHEME 9. Biomimetic Synthesis of Ptilocaulin and
Isoptilocaulin
SCHEME 10. Synthesis of Netamines E (65) and G (66)
ptilocaulin natural products was isolated in 2007.39 Reaction
of guanidine with bis enones, differing from 47 only in the alkyl
side chains, followed by reduction affords batzelladines analo-
gous to 53 in excellent yields.40 It is therefore not straightfor-
ward to develop the reaction of guanidine with bis enones as a
laboratory route to the ptilocaulin family of natural products.
Fortunately it is fairly easy to develop an alternate route to
the key intermediate 52 and explore whether reduction can lead
to ptilocaulin (1), isoptilocaulin (4), and 7-epineoptilocaulin (5).
The cyclohexene double bond of 52 was easily installed simply
by using enone 37 without the Birch reduction that afforded
38. Selective ozonolysis of the more electron rich double bond
of 37 to give 59 in 66% yield was easily accomplished in the
presence of pyridine,41 which also served to reduce the ozonide
(see Scheme 9). A complex mixture of products was obtained
in the absence of pyridine. Intramolecular aldol reaction with
30:1 DME/6 M aqueous HCl gave bicyclic dienone 60 in 70%
yield. Reaction of 60 with guanidine in MeOH in a sealed tube
in an 85 °C oil bath for 24 h presumably afforded tricyclic
iminium ion 52. Cooling the reaction to room temperature and
addition of NaBH4 afforded an inseparable 1:1 mixture of
ptilocaulin (1) and isoptilocaulin (4) and various minor byprod-
ucts completing the first synthesis of isoptilocaulin (4) and
establishing the plausibility of the proposed biosynthetic route.
The presence of isoptilocaulin in this mixture was established
by comparison with data for the natural product.1b,2c We were
frustrated by our inability to separate ptilocaulin and isoptilo-
caulin, but note that isoptilocaulin has only been obtained pure
when it has been isolated from a source that does not also
produce ptilocaulin.1,2b,2c
Stereochemistry of the Netamines. The close similarity of
the 1H NMR spectra of hydrogenation product 41 and netamine
C (18), which has a hexyl rather than a butyl side chain,
suggested that the side chains in the netamines might be trans,
not cis. The stereochemistry of the netamines was assigned on
the basis of an NOE between H-8 and both H-7 and H-8a in
the saturated netamines A-D. However, the MMX-calculated
distances17 between H-8 and both H-7 and H-8a in saturated
netamines A-D (16-19) are 2.37 Å and 2.25 Å, respectively,
for the cis isomer and 2.87 Å and 2.51 Å, respectively, for the
trans isomer so these NOEs are inconclusive. Mirabilin A (8)
was also assigned to have side chains cis based on an NOE
between H-7 and H-8. In the aromatic series, the calculated
distances17 are 2.28 Å for the cis isomer mirabilin A (8) and
3.08 Å for the trans isomer mirabilin C (10), which was also
isolated, so this assignment seems secure. The 1H and 13C NMR
spectra of aromatic netamines F (11) and G (12) correspond
closely to those of mirabilin B (9) and N-acetyl mirabilin C
with trans side chains and differ considerably from N-acetyl
mirabilin A with cis side chains (see Supporting Information)
suggesting that these netamines also have the side chains trans.
The trans isomers of netamines E (65) and G (66) should be
readily accessible by a slight modification of the route used to
prepare 7-epineoptilocaulin (5) and mirabilin B (9). Addition
of PrMgCl and CeCl3 to (R)-36 followed by PCC oxidation
afforded cyclohexenone 61 in 78% yield (see Scheme 10). Birch
reduction afforded 62 in 78% yield as a 7:1 mixture of isomers
at C-2. This Birch reduction is not as selective as that of 37 for
the required 3,5-cis isomer, probably because of steric interac-
tions between the larger butyl and propyl substituents. We
unsuccessfully explored a variety of copper hydride conjugate
reductions of 61.42 Apparently the double bond of 61 is too
(39) (a) Hua, H.-M.; Peng, J.; Dunbar, D. C.; Schinazi, R. F.; de Castro
Andrews, A. G.; Cuevas, C.; Garcia-Fernandez, L. F.; Kelly, M.; Hamann, M. T.
Tetrahedron 2007, 63, 11179–11188. For reviews of these alkaloids see: (b)
Berlinck, R. G. S.; Burtoloso, A. C. B.; Kossuga, M. H. Nat. Prod. Rep. 2008,
25, 919–954. (c) Berlinck, R. G. S.; Kossuga, M. H. Nat. Prod. Rep. 2005, 22,
516–550, and previous reviews in this series.
(42) (a) Ito, H.; Ishizuka, T.; Arimoto, K.; Miura, K.; Hosomi, A. Tetrahedron
Lett. 1997, 38, 8887–8890. (b) Lipshutz, B. H.; Keith, J.; Papa, P.; Vivian, R.
Tetrahedron Lett. 1998, 39, 4627–4630. (c) Jurkauskas, V.; Sadighi, J. P.;
Buchwald, S. L. Org. Lett. 2003, 5, 2417–2420. (d) Deutsch, C.; Krause, N.;
Lipshutz, B. H. Chem. ReV. 2008, 108, 2916–2927.
(40) (a) Black, G. P.; Murphy, P. J.; Thornhill, A. J.; Walshe, N. D. A.;
Zanetti, C. Tetrahedron 1999, 55, 6547–6554. (b) Snider, B. B.; Busuyek, M. V.
J. Nat. Prod. 1999, 62, 1707–1711.
(41) Slomp, G., Jr.; Johnson, J. L. J. Am. Chem. Soc. 1958, 80, 915–921.
J. Org. Chem. Vol. 73, No. 22, 2008 9071

Yu et al.
hindered. Ozonolysis of 62 afforded keto aldehyde 63 in 91%
yield, which was treated with HCl in DME in a microwave oven
at 55 °C for 10 min to give bicyclic enone 64 in 78% yield as
an 8:4:2:1 mixture of trans-64, cis-64, and the two isomers with
a ꢀ-propyl group, respectively. The selectivity for the trans
isomer of 64 (2:1) is much lower than for the trans isomer of
25 (4:1). MMX calculations17 indicate that trans-64 is only 0.5
kcal/mol more stable than the cis isomer, whereas trans-25 is
1.2 kcal/mol more stable than the cis isomer. Bicyclic enone
64 may also contain some of the two isomers with the propyl
group on the top face resulting from the less selective Birch
reduction.
FIGURE 3. Revised structures of netamines A and C.
group at C-7 (see Figure 3). The structure of netamine A should
therefore be revised to 67.
The structures of netamines E and G have been revised to
65 and 66, respectively, on the basis of the identity of their
spectra to those of the natural products. The NOE between NH-1
and H-8 of natural netamine A establishes that the side chains
are trans and the structure should be revised from 16 to 67.
The similarity of the spectra of 41 and netamine C, which has
a hexyl rather than a butyl side chain, establishes that structure
of netamine C should be revised to 68. This suggests that
netamines B, D, and F may also have the side chains trans with
a ꢀ-alkyl group at C-8.
Heating 64 with guanidine in MeOH in a sealed tube in an
85 °C oil bath for 24 h followed by neutralization with 1%
HNO3 afforded a more complex mixture of products than the
analogous reaction of 25. Repeated careful chromatography gave
65 that was >90% pure in 38% yield. Oxidation of the crude
reaction mixture with MnO2 in CH2Cl2 in a sealed tube in a 55
°C oil bath for 48 h afforded 66 in 25% overall yield from 64.
In conclusion, we have prepared optically pure indenone 25
in six steps in 31% overall yield and converted it to 7-epin-
eoptilocaulin (5, ∼50%), mirabilin B (9, 39%), and 8-hy-
droxymirabilins (13 and 14, 5%). Optically pure indenone 64
was prepared analogously and converted to netamine E (65,
38%) and netamine G (66, 25%). These studies establish that
the structures of netamines A (67), C (68), E (65), and G (66)
should be revised so that the 8-alkyl group is trans to the alkyl
substituent at C-7. A unified proposal for ptilocaulin, isoptilo-
caulin, and 7-epineoptilocaulin family of alkaloids suggests that
all of these natural products can be obtained by 1,2- or 1,4
reduction of tricyclic iminium salt 52, which could be formed
by addition of guanidine to bis enone 47. Tricyclic iminium
salt was generated in the laboratory by addition of guanidine
to dienone 60. Reduction with NaBH4 generated a mixture rich
in ptilocaulin (1) and isoptilocaulin (4) providing experimental
support for this hypothesis.
The spectral data of 66 in CDCl3 vary significantly as a
function of the amount of acid in solution as is well-known for
2-aminopyrimidines.43 We titrated 66 with a solution of TFA
in CDCl3 to obtain a complete set of partially protonated spectra.
The spectrum of 66 in CDCl3 containing 9% TFA matched the
published spectra of netamine G very well.8,44 At this degree
of protonation, H-5 (δ 2.38) and H-8 (δ 2.30) are well resolved
and H-8 absorbs as a ddd, J ) 8.8, 4.4, 4.4 Hz, as calculated17
and observed in trans isomers mirabilin B (9) and N-acetyl
mirabilin C (10).4,5 The 8.8 Hz coupling constant between H-7
and H-8 establishes that these two hydrogens are trans. With
more TFA, H-5 and H-8 appear together as a broad multiplet
at δ 2.40. H-8 absorbs as a ddd, J ) 6, 6, 5 Hz, as calculated,17
in the cis isomer N-acetyl mirabilin A (8).4 This analysis
establishes that the side chains of natural netamine G are trans
and the structure should therefore be revised to 66. The rotation
of synthetic netamine G (66), [R]D25 + 26.3, is identical to that
21
of natural netamine G, [R]D + 27.0, establishing that the
absolute stereochemistry of the natural product is as shown
for 66.
Experimental Section
(()-(3aS,5aS,7R,8S)-8-Butyl-1,3a,4,5,5a,6,7,8-octahydro-7-me-
thylcyclopenta[de]quinazolin-2-amine (7-Epineoptilocaulin, 5).
A 0.18 M solution of guanidine in MeOH was prepared by adding
guanidinium hydrochloride (86 mg, 0.90 mmol) to a solution of
NaOMe in MeOH prepared by adding Na (21 mg, 0.90 mmol) to
anhydrous MeOH (5.0 mL) under nitrogen.
The trans stereochemistry of the side chains of 65 was
established by a 1D NOESY experiment in CD3OD with
irradiation of axial H-6R at δ 0.77 which showed an NOE to
both H-8 at δ 1.95 and the propyl CH2 group at δ 1.20 and
1.60. This analysis establishes that the side chains of natural
netamine E are trans and the structure should therefore be
Guanidine in MeOH (0.18 M, 2.5 mL, 0.45 mmol) was
transferred to a resealable tube containing indenone 25 (41 mg,
0.20 mmol) in 2.5 mL of MeOH under nitrogen. The tube was
sealed and heated in an 85 °C oil bath for 24 h. The mixture was
cooled, quenched with 1% HNO3 (3.0 mL, 0.33 mmol) and diluted
with CHCl3 (20 mL). The layers were separated and the aqueous
layer was extracted with CHCl3 (2 × 30 mL). The combined organic
layers were dried over MgSO4 and concentrated to yield 82 mg of
crude 5 as a brown oil. Repeated flash chromatography on silica
gel (20:1 CHCl3/MeOH) gave 13 mg (21%) of >95% pure 5 as a
colorless oil and 23 mg (37%) of 80% pure 5 as a light yellow oil:
1H NMR (CDCl3) 8.99 (br s, 1), 8.21 (br s, 1), 7.60 (br s, 2), 4.25
(br dd, 1, J ) 7.0, 6.4), 2.45-2.32 (m, 1), 2.19-2.05 (m, 1),
1.98-1.83 (m, 3), 1.82-1.51 (m, 4), 1.41-1.20 (m, 3), 1.20-1.05
(m, 2), 1.00 (d, 3, J ) 6.4), 0.89 (t, 3, J ) 7.2), 0.86 (ddd, 1, J )
12, 12, 12); (CD3OD) 4.27 (br dd, 1, J ) 7.0, 6.4), 2.51-2.40 (m,
1), 2.25-2.12 (m, 1), 2.04-1.92 (m, 2), 1.92-1.81 (m, 1),
1.79-1.54 (m, 4), 1.40-1.20 (m, 4), 1.20-1.10 (m, 1), 1.06 (d, 3,
J ) 6.8), 0.93 (t, 3, J ) 7.2), 0.90 (ddd, 1, J ) 12, 12, 12); 13C
NMR (CDCl3) 153.6, 127.9, 117.1, 52.3, 42.5, 38.9, 36.6, 32.9,
25
revised to 65. The rotation of synthetic netamine E (65), [R]D
+ 15.6, has the same sign as that of natural netamine E, [R]D
25
+ 35.0, establishing that the absolute stereochemistry of the
natural product is as shown for 65.
The stereochemistry of the side chains of saturated tricycle
41 was assigned by analysis of the NOEs to the NH protons,
which are separately observable in CDCl3. We carried out a
ROESY experiment at 800 MHz on a sample of natural
netamine A (16) kindly provided by Prof. Kashman.44 NOEs
between NH-1 at δ 7.76 and H-8a at δ 3.57 and H-8 at δ 1.51,
but not the hexyl group H-1′ at δ 1.27, established that H-8 is
down and the hexyl group is up and therefore trans to the propyl
(43) Riand, J.; Chenon, M. Th.; Lumbroso-Bader, N. J. Am. Chem. Soc. 1977,
99, 6838–6845.
(44) We thank Prof. Yoel Kashman, Tel-Aviv University, for helpful
discussions, copies of the proton spectra of netamine G, and samples of netamines
A and F.
9072 J. Org. Chem. Vol. 73, No. 22, 2008

7-Epineoptilocaulin, Netamines, and Isoptilocaulin
32.0, 29.7, 27.0, 25.9, 23.1, 20.0, 14.2; 13C NMR (CD3OD) 154.7,
128.7, 119.5, 53.9, 44.1, 40.1, 37.8, 33.9, 33.7, 30.6, 28.4, 27.4,
24.4, 20.4, 14.6; IR (neat) 3223, 1674, 1576, 1455, 1380; HRMS
(EI) calcd for C15H25N3 (M+) 247.2048, found 247.2050.
29.0, 27.4, 22.7, 20.1, 14.2; IR (neat) 3225, 3087, 1667, 1621, 1463,
1380; HRMS (EI) calcd for C15H27N3 (M+) 249.2205, found
249.2202.
(()-Mirabilin B (9) and (()-(5aS,7R,8R)- and (()-(5aS,7R,8S)-
2-Amino-8-butyl-4,5,5a,6,7,8-hexahydro-7-methylcyclopenta[de]-
quinazolin-8-ol ((()-8r-Hydroxymirabilin B, 13 and (()-8ꢀ-
Hydroxymirabilin B, 14). Crude 5, containing isomers (47 mg)
was prepared as described above from indenone 25 (33.5 mg, 0.16
mmol). This material was taken up in water (1.9 mL) and benzene
1
Our H NMR (CD3OD) data are 0.01 to 0.02 ppm downfield
and our 13C NMR (CD3OD) data are shifted by up to 0.1 to 0.2
ppm from those reported.5 The appearance of the 1H NMR spectrum
in CD3OD corresponds well with an authentic spectrum provided
by Dr. Alan J. Freyer.26 The spectral data in 1H NMR data in CDCl3
are identical to those reported for “7-epiptilocaulin” by Hassner.
The 13C NMR data are shifted upfield by 0 to 0.2 ppm.13
(1.9 mL). KOH (17.1 mg, 0.30 mmol) and K
3[Fe(CN)6] (105 mg,
0.32 mmol) were added and the mixture was stirred at 25 °C for
16 h. The organic layer was separated and the aqueous layer was
extracted with benzene (2 × 15 mL). The combined organic layers
were washed with water, dried over MgSO4 and concentrated to
yield 26 mg of crude 9, 13, and 14 as a yellow oil. Flash
chromatography on silica gel (70:1 ∼ 20:1 CH2Cl2/MeOH) gave
12.5 mg (25%) of 9 followed by 2.8 mg (5%) of a 1:1 mixture of
13 and 14: 1H NMR 4.96 (br s, 2), 3.05-2.83 (m, 2), 2.66 (dd, 0.5
× 1, J ) 17.2, 8), 2.60 (dd, 0.5 × 1, J ) 17.2, 8.4), 2.44-2.27 (m,
1), 2.27-2.16 (m, 0.5 × 1), 2.10 (ddd, 0.5 × 1, J ) 12.4, 12.4,
4.4), 2.08-1.98 (m, 0.5 × 1), 1.97-1.81 (m, 2), 1.76 (ddd, 0.5 ×
1, J ) 12.4, 12.4, 4.4), 1.71-1.47 (m, 1), 1.40 (ddd, 0.5 × 1, J )
11.6, 11.6, 12.4), 1.34-1.11 (m, 3.5), 1.17 (d, 0.5 × 3, J ) 6.8),
1.06 (d, 0.5 × 3, J ) 6.8), 1.01-0.72 (m, 1), 0.85 (t, 0.5 × 3, J )
7.2), 0.82 (t, 0.5 × 3, J ) 7.2); 13C NMR (500 MHz) (13) 176.3,
164.7, 125.5, 75.3, 38.3, 37.2, 36.7, 35.2, 34.1, 33.5, 27.1, 23.5,
15.0, 14.0 (the peak at 163.5 was not observed); (14) 125.5, 74.1,
42.0, 37.8, 37.4, 36.8, 33.8, 33.3, 26.9, 23.1, 15.6, 13.8 (the peaks
at 175.7, 163.7 and 163.2 were not observed); IR (neat) 3416, 3314,
3184, 1607, 1584, 1464, 1385; HRMS (EI) calcd for C15H23ON3
(M+) 261.1841, found 261.1842.
1
The H NMR spectrum of the crude mixture in CDCl3 showed
the presence of 40 [∼10%, δ 3.15 (ddd, 1, J ) 11.6, 11.6, 5.2)]
and uncharacterized isomers with peaks at δ 4.01 (br dd, 1, J )
6.4, 5.6), 3.89 (ddd, 1, J ) 6.8, 5.6, 1.1), and 3.82-3.66 (m).
25
An identical reaction with (7aS)-25 afforded (5aS)-5: [R]D
+
45.9 (c 0.135, MeOH); [lit.5 [R]D + 13.3 (c 1.2, MeOH)].
(()-(5aS,7R,8S)-8-Butyl-4,5,5a,6,7,8-hexahydro-7-methylcy-
clopenta[de]quinazolin-2-amine (Mirabilin B, 9). A mixture of
5 (7 mg, 0.023 mmol) and activated MnO2 (13.1 mg, 0.15 mmol)
in CH2Cl2 (1 mL) was stirred in a sealed tube in a 55 °C oil bath
for 24 h. The mixture was cooled and filtered through Celite. The
residue was washed with CH2Cl2 (3 × 4 mL). The combined
organic layers were concentrated to yield 7 mg of crude 9 as a
yellow oil. Flash chromatography on silica gel (70:1 CH2Cl2/MeOH)
1
gave 5.7 mg (80%) of 9 as a light yellow oil: H NMR (CDCl3)
4.90 (br s, 2), 2.97-2.83 (m, 2), 2.58 (dd, 1, J ) 16.4, 8.8), 2.35
(ddd, 1, J ) 12.4, 7.2, 7.2), 2.20 (br ddd, 1, J ) 10.2, 4.4, 4.4),
2.09-1.96 (m, 2), 1.91-1.80 (m, 1), 1.80-1.69 (m, 1), 1.59-1.45
(m, 1), 1.38-1.19 (m, 3), 1.09 (d, 3, J ) 6.8), 1.16-1.05 (m, 1),
0.91 (ddd, 1, J ) 11.6, 11.6, 12.4), 0.87 (t, 3, J ) 6.8); (CD3OD)
2.99-2.83 (m, 2), 2.55 (dd, 1, J ) 16.4, 8.4), 2.38 (ddd, 1, J )
12.4, 7.2, 7.2), 2.19 (br dt, 1, J ) 9.6, 4,4), 2.12-1.96 (m, 2),
1.95-1.70 (m, 2), 1.60-1.44 (m, 1), 1.38-1.19 (m, 3), 1.11 (d, 3,
J ) 6.8), 1.14-1.01 (m, 1), 0.91 (ddd, 1, J ) 12, 12, 12), 0.88 (t,
3, J ) 7.2); 13C NMR (CDCl3) 174.6, 166.1, 163.1, 126.1, 47.0,
39.7, 37.7, 34.1, 33.6, 33.1, 30.3, 27.7, 23.3, 21.0, 14.1; 13C NMR
(CD3OD) 176.3, 167.6, 164.8, 126.9, 48.4, 41.0, 39.1, 35.3, 34.3,
34.3, 31.2, 28.7, 24.5, 21.4, 14.5; IR (neat) 3317, 3184, 1622, 1587,
1456, 1386; HRMS (EI) calcd for C15H23N3 (M+) 245.1893, found
245.1892.
1
The H NMR spectral data match those of an authentic sample
provided by Prof. Hamann.30 The 13C NMR spectral data match
the literature data,7 except that some aromatic carbons were not
observed due to the low sample concentration.
(()-(3aS,5aS,7R,8S,8aR,8bS)-8-Butyl-1,3a,4,5,5a,6,7,8,8a,8b-
decahydro-7-methylcyclopenta[de]quinazoline-2-amine (41). 7-Epi-
neoptilocaulin (5) (4.5 mg, 0.0145 mmol) in methanol (5 mL) was
hydrogenated over 10% Pd/C (5 mg) for 4 h at 3 atm. The solution
was filtered through Celite and concentrated to afford 5 mg (111%)
of 80% pure 41: 1H NMR (800 MHz) 7.64 (br s, 1), 7.61 (br s, 1),
7.05 (br s, 2), 3.89 (dd, 1, J ) 6, 4.2), 3.54 (dd, 1, J ) 4.9, 1.1),
2.34 (ddd, 1, J ) 11.4, 5.7, 5.7), 2.14-2.06 (m, 1), 1.97 (ddd, 1,
J ) 12.9, 7.1, 7.1), 1.93 (dd, 1, J ) 13.2, 5.8), 1.71-1.58 (m, 2),
1.48-1.45 (m, 1), 1.40-1.20 (m, 8), 1.13 (ddd, 1, J ) 13, 13, 13),
1.09 (d, 3, J ) 6.7), 0.92 (t, 3, J ) 6.7); 13C NMR (800 MHz)
154.8, 53.8, 50.0, 44.7, 35.8, 35.2, 34.8, 34.6, 34.5, 33.4, 30.5, 29.8,
23.4, 22.9, 14.0; IR (neat) 3268, 1668, 1634, 1456, 1373; HRMS
(Qtof) calcd for C15H28N3 (MH+) 250.2283, found 250.2281.
Attempted purification by chromatography on silica gel gave even
less pure 41.
(()-3aS,5aS,7S,8bR)-8-Butyl-1,3a,4,5,5a,6,7,8b-octahydro-7-
methylcyclopenta[de]quinazolin-2-amine (Ptilocaulin, 1) and
(()-3aS,5aS,8aS,8bR)-8-Butyl-1,3a,4,5,5a,6,8a,8b-octahydro-7-
methylcyclopenta[de]quinazolin-2-amine (Isoptilocaulin, 4). A
0.36 M solution of guanidine in MeOH was prepared by adding
guanidinium hydrochloride (170 mg, 1.8 mmol) to a solution of
NaOMe in MeOH prepared by adding Na (42 mg, 1.8 mmol) to
anhydrous MeOH (5.0 mL) under nitrogen.
Guanidine in MeOH (0.36 M, 0.84 mL, 0.30 mmol) was
transferred to a resealable tube containing indenone 60 (35 mg,
0.17 mmol) in 2.5 mL methanol under nitrogen. The tube was sealed
and heated in an 85 °C oil bath for 24 h. The mixture was cooled,
treated with water (1 mL) and NaBH4 (13 mg, 0.34 mmol), stirred
overnight at 25 °C, quenched with 1% HNO3 (2.5 mL, 0.28 mmol)
and diluted with CHCl3 (30 mL). The layers were separated and
the aqueous layer was extracted with CHCl3 (2 × 30 mL). The
combined organic layers were dried over MgSO4 and concentrated
to yield 50 mg of crude 1 and 4 as a brown oil. Flash chromatog-
The 1H NMR (CD3OD) data and 13C NMR (CD3OD) correspond
to the literature data,5 except that our 1H NMR (CD3OD) data are
0.01 to 0.02 ppm downfield and our 13C NMR (CD3OD) data are
0.1 to 0.2 ppm downfield. The 1H NMR data in CDCl3 are identical
and the 13C NMR data are within 0.1 ppm to those reported.13b
25
An identical reaction with (5aS)-5 afforded (5aS)-9: [R]D
+
123.6 (c 0.09, MeOH), [R]D25 + 145 (c 0.06, CHCl3); [lit.5 [R]D +
41.6 (c 0.48, MeOH)], [lit.35 [R]D + 129 (c 0.1, CHCl3)].
22
(()-Mirabilin B (9) and (()-(3aR,5aS,7R,8S,8aS,8bR)-8-
Butyl-1,3a,4,5,5a,6,7,8,8a,8b-decahydro-7-methylcyclopenta-
[de]quinazoline-2-amine (27). Guanidine in MeOH (0.18 M, 1.2
mL, 0.22 mmol) was transferred to a 50 mL flask containing
indenone 25 (31 mg, 0.15 mmol). The MeOH was evaporated and
the reaction mixture was heated in a 130-140 °C oil bath under
nitrogen for 4 h. The solution was cooled, treated with benzene
(15 mL) and 1% HNO3 (2 mL), and stirred for 30 min. The organic
layer was separated and the aqueous layer was extracted with CHCl3
(2 × 30 mL). The combined organic layers were dried over MgSO4
and concentrated to yield 53 mg of crude 9 and 27 as a brown oil.
Flash chromatography on silica gel (70:1 CH2Cl2/MeOH) gave 18
mg (39%) of 9 as a yellow oil. Flushing the silica gel with MeOH
gave 14 mg (31%) of 27.
Compound 27: 1H NMR (800 MHz, CDCl3) 8.20 (br s, 1), 7.63
(br s, 1), 7.32 (br s, 2), 3.79 (br dd, 1, J ) 5.6, 5.6), 3.77 (br dd,
1, J ) 2.7, 2,7), 2.10 (m, 1), 1.98-1.85 (m, 2), 1.78-1.65 (m, 3),
1.55-1.42 (m, 4), 1.42-1.22 (m, 2), 1.22-1.10 (m, 3), 0.96 (d, 3,
J ) 7.2), 0.94 (t, 3, J ) 8), 0.86 (ddd, 1, J ) 12, 12, 12); 13C
NMR 154.2, 51.6, 47.0, 46.2, 46.1, 39.6, 36.3, 32.4, 31.7, 29.1,
J. Org. Chem. Vol. 73, No. 22, 2008 9073

Yu et al.
raphy on silica gel (18:1 CHCl3/MeOH) gave 20.6 mg (39%) of
an inseparable mixture of 1 (∼30%), 4 (∼30%) and numerous other
isomers.
14.6 (the peak at 155 was not observed); IR (neat) 3205, 1674;
HRMS (EI) calcd for C17H29N3 (M+) 275.2361, found 275.2363.
A 1D NOESY experiment with irradiation of H-6R at δ 0.77
showed NOE enhancements of H-6ꢀ at δ 2.15 (very strong), H-5R
at δ 1.30 (strong), H-8 at δ 1.95 (medium strong), H-1” at δ 1.60
and δ 1.20 (medium strong), and H-5a at δ 2.43 (weak).
1
Data for ptilocaulin (1) were determined from the mixture: H
NMR (CDCl3) 8.97 (br s, 1), 8.35 (br s, 1), 7.44 (br s, 2), 3.80-3.68
(m, 1), 2.52-1.81 (m, 6), 1.81-1.52 (m, 2), 1.52-1.18 (m, 7),
1.04 (d, 3, J ) 6.8), 0.89 (m, 3); (CD3OD) 3.84 (br d, 1, J ) 10),
2.58-2.33 (m, 3), 2.33-2.05 (m, 3), 1.89-1.56 (m, 2), 1.56-1.21
(m, 7), 1.09 (d, 3, J ) 6.8), 0.94 (t, 3, J ) 7.2); 13C NMR (CDCl3)
151.7, 126.9, 121.0, 53.2, 36.5, 33.9, 32.2, 31.5, 29.7, 29.6, 27.7,
24.6, 22.4, 19.5, 14.0; (CD3OD) 152.9, 128.1, 122.9, 51.3, 37.8,
The 1H NMR (CD3OD) and 13C NMR (CD3OD) data correspond
to the literature data,8 except that our 1H NMR (CD3OD) data are
0.01 to 0.03 ppm upfield and our 13C NMR (CD3OD) data are all
about 0.1 to 0.4 ppm upfield.
(5aS,7R,8S)-8-Butyl-4,5,5a,6,7,8-hexahydro-7-propylcyclopen-
ta[de]quinazolin-2-amine (Netamine G, 66). Crude 65, containing
isomers, (105 mg) was prepared as described above from indenone
64 (63 mg, 0.27 mmol). CH2Cl2 (6 mL) and activated MnO2 (103
mg, 1.2 mmol) was added and the mixture was stirred in a sealed
tube in a 55 °C oil bath for 48 h. The mixture was cooled, filtered
through Celite. The residue was washed with CH2Cl2 (3 × 20 mL).
The combined organic layers were washed with 10% K2CO3
solution (10 mL), dried over MgSO4 and concentrated to yield 45
mg of crude 66 as a yellow oil. Flash chromatography on silica
gel (70:1 CH2Cl2/MeOH) gave 22.5 mg (25%) of 66 as a yellow
1
35.5, 34.2, 33.3, 30.9, 29.2, 28.1, 25.6, 23.7, 20.1, 14.5. The H
NMR (CDCl3 and CD3OD) data and 13C NMR (CDCl3 and CD3OD)
correspond to the literature data,1,10 except that our 1H NMR (CDCl3
and CD3OD) data are 0.01 to 0.04 ppm downfield, our 13C NMR
(CDCl3) data are 0.1 to 0.4 ppm upfield, and our 13C NMR
(CD3OD) data are all about 0.1 to 0.3 ppm shifted.
Data for isoptilocaulin (4) were determined from the mixture:
1H NMR (CDCl3) 7.73 (br s, 1), 7.33 (br s, 1), 7.09 (br s, 2), 3.96
(d, 1, J ) 4.8), 3.90-3.80 (m, 1), 2.52-1.81 (m, 7), 1.81-1.52
(m, 1), 1.74 (s, 3), 1.52-1.18 (m, 6), 0.91 (m, 3); (CD3OD) 4.02
(br d, 1, J ) 4.8), 3.93-3.84 (m, 1), 2.58-2.33 (m, 1), 2.33-2.05
(m, 4), 2.05-1.88 (m, 2), 1.89-1.56 (m, 1), 1.78 (s, 3), 1.56-1.21
(m, 6), 0.95 (t, 3, J ) 7.2); 13C NMR (CD3OD) 156.5, 136.4, 133.1,
54.6, 50.8, 41.0, 37.6, 37.2, 35.5, 32.9, 31.9, 31.8, 23.9, 19.9, 14.4;
IR (neat); HRMS (EI, from mixture) calcd for C15H25N3 (M+)
247.2048, found 247.2049. The 1H NMR (CDCl3 and CD3OD) data
and 13C NMR (CD3OD) correspond to the literature data,1,2c except
8
25
21
oil: [R]D + 26.3 (c 0.255, CH2Cl2); [lit. [R]D + 27.0 (c 0.20,
CH2Cl2)]; 1H NMR (CDCl3 9% protonated by TFA) 5.19 (br s, 2),
3.00-2.80 (m, 2), 2.60 (br dd, 1, J ) 16.4, 8.4), 2.38 (ddd, 1, J )
12.0, 7.2, 7.2), 2.30 (ddd, 1, J ) 8.8, 4.4, 4.4, H-8), 2.17 (ddd, 1,
J ) 12.4, 3.8, 3.8), 2.09-1.91 (m, 1), 1.83-1.64 (m, 2), 1.64-1.41
(m, 3), 1.41-1.15 (m, 5), 1.15-1.02 (m, 1), 0.93 (t, 3, J ) 6.8),
0.87 (t, 3, J ) 6.8), 0.78 (ddd, 1, J ) 11.6, 11.6, 11.6); 13C NMR
(CDCl3 9% protonated by TFA) 174.5, 166.4, 163.1, 126.3, 45.2,
38.8, 37.5, 37.1, 36.2, 33.6, 33.0, 31.0, 27.7, 23.2, 20.2, 14.4, 14.0;
IR (neat) 3334, 3193, 1588; HRMS (EI) calcd for C17H27N3 (M+)
273.2205, found 273.2203.
1
that our H NMR (CD3OD) data are 0.04 to 0.05 ppm downfield
and our 13C NMR (CD3OD) data are 0.1 to 0.2 ppm downfield.
Partial data for minor isomers: 1H NMR (CDCl3) 3.76-3.64 (m,
1), 3.53 (br d, 1, J ) 4.4), 1.15 (d, 3, J ) 7.2), 0.97 (d, 3, J ) 7.2);
(CD3OD) 3.77 (d, 1, J ) 5.6), 3,59 (br d, 1, J ) 4.8), 1.19 (d, 3,
J ) 7.2), 1.06 (d, 3, J ) 6.8), 1.03 (d, 3, J ) 6.8).
1
The H NMR data and 13C NMR correspond to the literature
data,8,44 except that our 1H NMR data are 0.02 to 0.05 ppm upfield
and our 13C NMR data are all about 0.1 to 0.5 ppm shifted, possibly
because of a slightly different extent of protonation.
(3aS,5aS,7R,8S)-8-Butyl-1,3a,4,5,5a,6,7,8-octahydro-7-propyl-
cyclopenta[de]quinazolin-2-amine (Netamine E, 65). A 0.22 M
solution of guanidine in MeOH was prepared by adding guani-
dinium hydrochloride (104 mg, 1.1 mmol) to a solution of NaOMe
in MeOH prepared by adding Na (25 mg, 1.1 mmol) to anhydrous
MeOH (5.0 mL) under nitrogen.
Acknowledgment. We are grateful to the National Institutes
of Health (GM-50151) for support of this work. The 800 MHz
spectrometer in the Landsman Research Facility, Brandeis
University was purchased under NIH RR High-End Instrumen-
tation program, 1S10RR017269-01. We thank Dr. Alan J.
Freyer, GlaxoSmithKline Pharmaceuticals, for a copy of the 1H
NMR spectrum of 5. We thank Prof. Hans-Guenther Schmalz,
University of Cologne, for a copy of the NMR spectrum of the
mixture of 1 and 23. We thank Prof. Mark T. Hamann,
University of Mississippi, for copies of the 1H NMR spectra of
9 and the mixture of 13 and 14, a sample of the mixture of 13
and 14, and for determining the [R]D of mirabilin B. We thank
Prof. Yoel Kashman, Tel-Aviv University, for helpful discus-
sions, copies of the proton spectra of netamine G, and samples
of netamines A and F.
Guanidine in MeOH (0.22 M, 2.0 mL, 0.44 mmol) was
transferred to a resealable tube containing the 8:4:2:1 mixture of
indenone 64 and stereoisomers (58 mg, 0.25 mmol) in 3.0 mL of
MeOH under nitrogen. The tube was sealed and heated in an 85
°C oil bath for 24 h. The mixture was cooled, quenched with 1%
HNO3 (5.0 mL, 0.55 mmol) and diluted with CHCl3 (30 mL). The
layers were separated and the aqueous layer was extracted with
CHCl3 (2 × 30 mL). The combined organic layers were dried over
MgSO4 and concentrated to yield 82 mg of crude 65 as a yellow
oil. Repeated flash chromatography on silica gel (25:1 CHCl3/
MeOH) gave 31.5 mg (37.5%) of 90% pure 65 as a yellow oil:
[R]D + 15.6 (c 0.09, CH2Cl2); [lit.8 [R]D + 35.0 (c 0.80,
CH2Cl2)]; 1H NMR (CDCl3) 8.85 (br s, 1), 8.26 (br s, 1), 7.63 (br
s, 2), 4.26 (br t, 1, J ) 6.8), 2.51-2.21 (m, 1), 2.14 (br dd, 1, J )
13.6, 6.8), 2.10-2.00 (m, 1), 2.00-1.87 (m, 2), 1.77-1.03 (m, 13),
0.90 (t, 3, J ) 6.8), 0.89 (t, 3, J ) 6.8), 0.72 (ddd, 1, J ) 11.2,
11.2, 11.2); (CD3OD) 4.27 (br t, 1, J ) 6.8), 2.46-2.34 (m, 1),
2.19 (br dd, 1, J ) 12.8, 6.8), 2.16-2.05 (m, 1), 2.05-1.80 (m, 2),
1.78-1.10 (m, 13), 0.94 (t, 3, J ) 7.2), 0.93 (t, 3, J ) 7.2), 0.77
(ddd, 1, J ) 12, 12, 12); 13C NMR (CD3OD) 128.8, 119.8, 54.1,
42.3, 38.4, 37.7, 37.3, 36.5, 34.0, 30.4, 28.7, 27.5, 24.4, 21.4, 14.8,
25
21
Supporting Information Available: Experimental details for
the preparation of indenones 25, 60, and 64, tables comparing
the spectral data of various synthetic and natural products, and
1H and 13C NMR spectral data. This material is available free
of charge via the Internet at http://pubs.acs.org.
JO801956W
9074 J. Org. Chem. Vol. 73, No. 22, 2008