Short Report
J. Braz. Chem. Soc., Vol. 16, No. 5, 1074-1077, 2005.
Printed in Brazil - ©2005 Sociedade Brasileira de Química
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The Preparation of a 10-Membered Ring Macrolactone by Selective Ozonolysis and the Role of
the Dihydropyran-Substituent on the MCPBA-Oxidation Reaction Profile of β-Lapachone
Phenazines
a
a
a
a
Raphael S. F. Silva , Tiago T. Guimarães , Daniel V. Teixeira , Ana Paula G. Lobato ,
a
b
b
Maria do Carmo F. R. Pinto , Carlos Alberto de Simone , Janaína G. Soares ,
b
,b
,a
Alessandra G. Cioletti , Marilia O. F. Goulart* and Antonio V. Pinto*
a
Núcleo de Pesquisas de Produtos Naturais, Universidade Federal do Rio de Janeiro,
Centro de Ciências da Saúde, 21944970 Rio de Janeiro – RJ, Brazil
b
Departamento de Química/CCEN, Universidade Federal de Alagoas, Campus A. C. Simões,
Tabuleiro do Martins, 57072-970 Maceió – AL, Brazil
A benzofenazina derivada da β-lapachona foi submetida a ozonólise em condições que forneceram
seletivamente a lactona macrocíclica 7,7-dimetil-7,8,9,10-tetrahidro-5H-benzo[3,4]oxecino[5,6b]quinoxalina-5,10-diona, com rendimento de 52%. O efeito de substituintes localizados no anel
diidropirânico das fenazinas, no caso C2’–OH, C2’–Br e C2’–I, em reações de oxidação com
MCPBA, foi também investigado.
The benzophenazine from β-lapachone was submitted to ozonolysis under conditions that
selectively furnished the corresponding rigid macrocyclic lactone 7,7-dimethyl-7,8,9,10-tetrahydro5H-benzo[3,4]oxecino[5,6-b]quinoxaline-5,10-dione in an yield of 52%. The effect of substituents
located at the dihydropyrane moiety of the phenazines, namely C2’-OH, C2’-Br and C2’-I, in the
oxidation with MCPBA, was also investigated.
Keywords: β-lapachone, ozonolysis, 10-membered ring macrolide, selectivity, effect of
substituent
Introduction
In general, natural and synthetic phenazines have
attracted considerable attention because of their interesting
biological activities,1 including broad-spectrum antibiotic,2
antimalarial,3,4 trypanocidal5 along with anti hepatitis C viral
replication activities.6 Benzo[a]phenazines are also efficient
DNA intercalating ligands, exhibiting antitumor activity in
leukaemia and solid tumors.7 They have also been described
as dual inhibitors of topoisomerase I and II.8
In our search for preparing new derivatives, for synthetic
purposes and biological screening, we recently reported9 that
peroxidation of the b-lapachone-derived benzophenazine,
named lapazine (1), with m-ClC6H4CO2H/CH2Cl2 generated,
unexpectedly, a macrocyclic lactone 2 with a rigid
10-membered ring (27% yield), the corresponding N-oxide 3
(13% yield), and the dihydrobenzophenazine-5-one 4 (35%
yield) (Figure 1). Macrocyclic compounds 2 and 3 were formed
* e-mail: mofg@qui.ufal.br; ventura@nppn.ufrj.br
via oxidative cleavage of the aromatic double bond at the
site of fusion of the dihydropyran moiety with the phenazine
component. This region (rings C/E, in 1) (Figure 1) would
behave as a conjugated enol ether, which, after the expected
epoxide formation and fragmentation would lead to 2 and 3
(further oxidation). An acid-catalysed epoxide-acetal or
α-hydroxy hemiacetal would furnish 4.9
Compounds 2 and 3 belong to the class of macrolides,
an important group of biologically active compounds,
especially recognized as antibiotics. 10 Among
10-membered macrolactones, the antibiotic activity of
apicularen A and B was reported.11
In the present work, considering 2 and 3, the presence
of diaza heterocyclic groups close to the ion-binding
macrolactone moiety constitutes attractive molecular
features.
In searching for a more general application of the
oxidative cleavage by MCPBA,9 we have analysed the role
of substituent, at C-2’, in the dihydropyrane moiety of
lapazine, namely -OH (5), -Br (6) and -I (7) (Figure 1).
Vol. 16, No. 5, 2005
The Preparation of a 10-Membered Ring Macrolactone
Figure 1. Lapazine (1), its derivatives (5, 6, 7) and oxidation products (2-4).
Additionally, in an attempt to develop a more selective
and simpler method by which to bring about this oxidative
cleavage,9 we have evaluated the use of ozone, a versatile
reagent capable of oxidizing both cyclic and acyclic
olefins.12,13 Since double bonds connected to electrondonating groups react with ozone many times faster than
those connected to electron-withdrawing groups,14 this
gaseous reagent possesses significant selective reaction
potential. Ozone has been used for the selective cleavage of
double bonds in alicyclic compounds, a strategy that gave
rise to the expanded C-15 macrocyclic system in the synthesis
of muscone.15 Aromatic bonds can also be cleaved through
ozonolysis as shown, for example, in the formation of
biphenyl compounds from benzo[c]phenanthrene and
phenanthrene.16,17 A number of studies have concerned the
ozonolysis of heterocyclic molecules,18-22 but none has
focused on the reaction with phenazines.
1075
a versatile intermediate for the synthesis of new derivatives.
In the case of 6, a complex mixture of products was
obtained, from which it was possible to isolate 9, as the
corresponding N-oxide bromomacrolide, in low (10%)
yield (Figure 2). Supplementary crystallographic data is
given.23
The structure was solved by direct methods and refined
by full-matrix least squares calculations (Figure 3). The
refinement was conducted until all atomic parameters shifts
were smaller than their standard deviations. All H atoms
were located by geometric considerations. In the final
difference Fourier map there are no peaks greater than 0.34
Å-3. Bond lengths and angles are in good agreement, to
within experimental accuracy, with the values found in
the literature.24
The oxidation of compound 7 with MCPBA, due to
the liability of the C-I bonding, was unsuccessful.
From these results, it is possible to envisage that the
method would be valuable for the synthesis of substituted
macrolides, specially for substituents, which are not easily
broken in the oxidation conditions.
Concerning oxidation of phenazines, a natural
development would be to try ozonolysis. A solution of
lapazine (1) in methylene dichloride was, then, subjected
to ozonolysis, and the ozonides thus formed were reduced
with Zn/water.25 The progress of the reaction was followed
by TLC using pure samples of 2, 3 and 4 as reference
standards. The macrocyclic lactone 2 was formed as the
major product of the reaction, together with some minor
polar by-products that were not identified. Neither 3 nor 4
could be detected in the crude reaction mixture by TLC
analysis. Compound 2 was isolated from the reaction
mixture by column chromatography as pure white crystals
in an overall yield of 52% (Figure 2).
Results and Discussion
As described before,9 the oxidation with MCPBA
caused the oxidative cleavage of the aromatic double bond
at the site of fusion of the dihydropyran moiety with the
phenazine component. To know the role of the substituent
on the reaction profile, we have studied the phenazines (57) derived respectively from 3-hydroxyl, 3-bromo and 3iodo-β-lapachone.
The phenazine 5 reacted with MCPBA to produce the
macrolide 8, as the sole product, in 52% yield, along with
unchanged 5 (Figure 2). This hydroxymacrolide constitutes
Figure 2. Oxidation of phenazines. A: ozonization of lapazine 1
furnishing 2; B: oxidation of 5 furnishing 8; C: oxidation of 6
furnishing 9.
1076
Silva et al.
J. Braz. Chem. Soc.
silica gel column chromatography using hexane/ethyl
acetate (95:5) as eluent. Pure 2 (179 mg; 0.52 mmol, 52%
yield) was obtained as a colourless crystalline product
(melting point 162-163 oC) that was shown to be identical
to an authentic sample of 7,7-dimethyl-7,8,9,10tetrahydro-5H-benzo[3,4]oxecino[5,6-b]quinoxaline5,10-dione (2) by mixed melting point and TLC (cochromatography). All of the physico-chemical data of 2
were in full accordance with those previously published.9,26
Figure 3. Perspective view of 9 showing the atom-labelling scheme.
Displacement of ellipsoids plotted at the 50% probability level, are
shown for the non-H atoms.
In addition to providing an easier protocol and cleaner
conditions for the preparation and isolation of 2, the
reaction of 1 with ozone was also more selective and
efficient than the reported direct formation of 2.9
To the best of our knowledge, this is the first example
of the ozone-mediated formation of a rigid macrocyclic
lactone. Although a more general view of the synthetic
aspects of the use of ozone is not addressed in the present
communication, the formation of 2 suggests the possible
use of this reagent to cleave phenanthrene rings linked
face-to-face with a heterocyclic moiety. Experiments along
this line are in progress.
Experimental
NMR experiments were performed in deuteriochloroform
with TMS as the internal standard with a Bruker AVANCE
DRX-400 or Gemini-200 MHz instruments; chemical shifts
are given on the δ scale, and J values are given in Hz. For
elemental analysis, we used Perkin-Elmer CHN 2400 and
for IR spectra, a Perkin-Elmer 783. The ozonolysis was
performed in a T-408 Welsbach, with ozone pressure of 2
Lb in-2.
Preparation of 7,7-dimethyl-7,8,9,10-tetrahydro-5Hbenzo[3,4]oxecino[5,6-b]quinoxaline-5,10-dione (2).
Yellow solution of lapazine, was prepared as described4,9
(1; 314 mg; 1.0 mmol) in 20 mL of methylene chloride,
treated with ozone at –5.0 oC until all substrate had reacted
(1h). The colourless reaction product was reduced with
Zn/water and, after the usual workup, was submitted to
Preparation of 7,7-dimethyl-8-hydroxy-7,8,9,10tetrahydro-5H-benzo[3,4]oxecino[5,6-b]quinoxaline-5,10dione (8). The hydroxyphenazine 5 (0.330 g, 1 mmol) in a
solution of 10 mL of methylene chloride, at room
temperature, was treated with m-chloroperbenzoic acid
(0.690 g, 4 mmol) in small portions until total consumption.
The reaction was followed by TLC. After 24 hours, the
analysis by silica gel TLC revealed the presence of a sole
spot, together with remaining substrate. After the usual work
up, with sodium carbonate (10%), the reaction residue was
submitted to chromathography in a silica gel column, using
hexane/ethyl acetate, in increasing polarity, as eluents. 8
(0.188 g, 0.52 mmol, 52%) was eluted with a mixture of
hexane/ethyl acetate (7:3) and recrystallized from this
fraction, as light yellow needles, mp 195 o C, with
decomposition. δ H [200 MHz, CDCl3; J(Hz)]: 1.35 (3H, s,
C7-Me); 1.45 (3H, s, C7-Me); 2.60 (1H, dd, J 13; 2 Hz, C9H); 2.80 (1H, dd, J 2; 13 Hz, C9-H), 4.90 (1H, m, C8-H); 5.80
(1H, bs, -OH, exchangeable with D2O); 7.70 (3H, m, ArH);
8.0 (3H, m, ArH); 8.25 (1H, m, ArH); 8.50 (1H, m, ArH); 13C
NMR (50 MHz, CDCl3) δ C: 17.3 (CH3); 24.5 (CH3); 45.0
(CH2); 73.8 (CHOH); 86.4 (C-O); 119.7 (CH); 129.7 (C);
129.8 (C); 130.2 (CH); 131.5 (CH); 131.7 (CH); 133.1 (CH);
132.3 (CH); 132.4 (CH); 132.7 (CH); 137.6 (C); 138.9 (C);
142.2 (C); 153.6(C); 163.9 (COO); 196.0 (C=O). IR (KBr)
νmax/cm-1: 3407, 2926, 1723, 1597, 1574, 1368, 1350, 1257,
1038, 766, 754. Found C 69.50 H 5.13 N 7.60. Calc. for
C21H18N2O4, C 69.60 H 5.01 N 7.73.
Preparation of 8-bromo-7,7-dimethyl-5,10-dioxo7,8,9,10-tetrahydro-5H-benzo[3,4]oxecino[5,6b]quinoxaline-16-N-oxide (9). The bromophenazine 6
(0.393 g, 1 mmol) in a solution of 10 mL of methylene
chloride, at room temperature, was treated with
m-chloroperbenzoic acid (MCPBA) (0.690 g, 4 mmol) in
small portions until total consumption. The reaction was
followed by TLC. After 24 hours, the reaction was
complete. After the usual work up, with sodium carbonate
(10%), the reaction residue, a complex mixture of products,
was submitted to chromathography in a silica gel column,
using hexane/ethyl acetate, in increasing polarity, as
Vol. 16, No. 5, 2005
The Preparation of a 10-Membered Ring Macrolactone
eluents. 9 (0.044 g, 0.1 mmol) was eluted with a mixture of
hexane/ethyl acetate (95:5) and recrystallized from this
fraction, as colourless needles, mp 194-198 oC, with
decomposition. Any other product was isolated in a pure
form. δ H [400 MHz, CDCl3; J(Hz)]: 1.65 (3 H, s, C7-Me);
1.71 (3H, s, C7-Me); 3.14 (1H, dd, J 14.8; 4.0 Hz, H-9);
4.17 (1H, dd, J 14.8; 12.8 Hz, C9-H), 4.38 (1H, dd, J 12.8;
4.0 Hz, C8-H); 7.60 (2H, m, C1-H and C3-H); 7.73 (1H, td,
J 8.0; 1.6 Hz, C2-H); 7.85 (2H, td, J 7.2, 1.6 Hz, C14-H);
7.93 (1H, td, J 7.2; 1.6 Hz, C13-H); 8.15 (1H, dd, J 8.0, 1.6
Hz, C4-H); 8.22 (1H, dd, J 8.0, 1.2 Hz, C12-H); 8.60 (1H,
dd, J 8.0, 1.2 Hz, C15-H). 13C NMR (100 MHz, CDCl3) δ C:
18.2 (C7-Me); 25.4 (C7-Me); 47.1 (C-9); 52.5 (C-8); 87.2
(C-7); 119.7 (C-15); 128.9 (C-1a); 129.2 (C-4a); 129.9 (C1 or C-3); 130.4 (C-12); 131.6 (C-14); 132.0 (C-4); 132.2
(C-13); 132.7 (C-2); 132.8 (C-3); 137.7 (C-15a); 138.8 (C16a); 142.3 (C-11a); 152.9 (C-10a); 163.7 (C-5); 195.1
(C-10). IR (KBr) νmax / cm-1: 3096, 2987, 2963, 1716, 1638,
1597, 1574, 1480, 1346, 1291, 1256, 1121, 768, 762, 703,
661, 624, 587. Found C 57.10 H 4.06 N 6.17. Calc. for
C21H17BrN2O4, C 57.16 H 3.88 N 6.35.
1077
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Received: December 21, 2004
Published on the web: July 07, 2005