Synthesis and Expansion of Bicyclic Enol Ether: A Probable Precursor for the Synthesis of Macrolide (±)-Pyrenophorin

Article abstract of DOI:10.21577/0103-5053.20170114

A convenient procedure for the synthesis and expansion of bicyclic rings has been developed for the production of probable precursors of non-racemic pyrenophorin, an antibiotic dilactone. The major highlight for this new synthetic methodology came from the use of a readily available reagent of easy manipulation, 9-oxabicyclo[3.3.1]nonane-2,6-diol, for the preparation of the bicyclic intermediate, which sequentially was subjected to oxidative cleavage with butyl nitrite resulted in an isomeric mixture, a dioximedilactone and diisoxazoledilactone.

Full text of DOI:10.21577/0103-5053.20170114

http://dx.doi.org/10.21577/0103-5053.20170114
J. Braz. Chem. Soc., Vol. 29, No. 1, 74-78, 2018.
Printed in Brazil - ©2018 Sociedade Brasileira de Química
Article
Synthesis and Expansion of Bicyclic Enol Ether: A Probable Precursor for the
Synthesis of Macrolide ( )-Pyrenophorin
Maísa B. Costa,*,a,# Marcos P. Martins,a Hugo C. de Araújob and Inês S. Resckb
aLaboratório de Síntese, Isolamento e Modificações de Compostos Orgânicos,
Universidade Estadual de Goiás, Campus Henrique Santillo, BR 153, No. 3105,
Fazenda Barreiro do Meio, Setor Arco Verde, CP 459, 75132-400 Anápolis-GO, Brazil
bInstituto de Química, Universidade de Brasília, Campus Darcy Ribeiro,
CP 4478, 70910-970 Brasília-DF, Brazil
A convenient procedure for the synthesis and expansion of bicyclic rings has been developed
for the production of probable precursors of non-racemic pyrenophorin, an antibiotic dilactone.
The major highlight for this new synthetic methodology came from the use of a readily available
reagent of easy manipulation, 9-oxabicyclo[3.3.1]nonane-2,6-diol, for the preparation of the
bicyclic intermediate, which sequentially was subjected to oxidative cleavage with butyl nitrite
resulted in an isomeric mixture, a dioximedilactone and diisoxazoledilactone.
Keywords: enol ether, ring expansion, macrolides, pyrenophorin
Introduction
In as much of the wide spectrum of biological activities,
mainly as antibiotics, several synthetic methodologies
for the synthesis of this 16-member macrolide, racemic
or chiral, have been reported in the literature. Many are
based on the lactonization and the dimeric cyclization of
seco-ω-hydroxy-acid derivatives andγ-oxo-α, β-unsaturated
derivatives, as well as the formation of intramolecular C−C
bonds, in addition to cyclodimerization under Mitsunobu
conditions.3-7
Therefore, in this article, a new procedure is
demonstrated to obtain potential synthetic precursors that
may lead to the synthesis of ( )-pyrenophorin (1) through
the use of oxidative ring expansion methodology.8
The structural diversity of the macrolides represents
an interesting synthetic challenge for the obtainment of
molecules with medium or macrocyclic sizes. Following the
research conducted in our laboratory to obtain macrocyclic
lactones, it was developed the synthesis of a precursor that
will be later applied to the racemic synthesis of the natural
16-membered macrolide pyrenephorin1 (1), a macrolactone
dimer with high biological potential (Figure 1).
Experimental
General methods
Figure 1. Natural macrolide 1.
All reagents used were of analytical grade. The
products of all reactions were monitored by capillary gas
chromatography on VARIAN STAR 3400 CX equipment
using DB-5 and DB-1 capillary columns of length equal to
30 m × 25 mm, with a temperature range of 120-190 °C,
injector temperature of 250 °C and the detector of 300 °C.
The DEX-CB capillary chiral column with a length equal
to 25 m × 25 mm, with a temperature range of 160-200 °C,
injector temperature of 250 °C and detector of 300 °C, was
The bioactive metabolite 1, pyrephorin, was
isolated from the pathogenic fungi Pyrenophora avenae
and Stemphylium radicinum,1 and the cultures of
Drechslera avenae acting as a phytotoxin on leaves
segments of Avena sterilis.2
*e-mail: maisabc@gmail.com, maisa.costa@ueg.br
#Synthetic part of this research was initially developed at the Instituto de
Química, Universidade de Brasília, and is related to the doctoral thesis.

Vol. 29, No. 1, 2018
Costa et al.
75
used in specific cases. The products were purified on flash
chromatographic column using silica gel (230-400 mesh).
The melting points were determined in the Köfler block
9-Oxabicyclo[3.3.1]nonane-2,6-diol (3)
White solid, m.p. 124-125 °C; IR (KBr) ν / cm-1
3422, 3282, 2949, 2909, 1483, 1079, 1062, 974; 1H NMR
(300 MHz, DMSO-d6) d 4.78 (d, J 6 Hz, OH), 3.68 (2H),
3.51 (dd, J 5.0, 5.0 Hz, 2H), 2.01-1.88 (m, 2H), 1.74-1.53
(m, 6H); 13C NMR (75 MHz, DMSO-d6) d 69.4, 67.5, 28.8,
22.0; anal. calcd. for C8H14O3 (158.0946 g mol-1): C, 54.53;
H, 9.15%. Found: C, 54.52; H, 9.11%.
1
and were recorded without correction. The H and 13C
(single and two-dimensional) nuclear magnetic resonance
(NMR) spectra were obtained on the spectrometer. The
probes used were ATB and SW, with 5 mm internal
diameter, at room temperature with 45° pulse for hydrogen
and carbon. Chemical displacements (d) in 1H NMR, with
deuterated solvents CDCl3 and DMSO-d6, were referenced
with TMS and the DMSO residue (d 2.49), respectively. In
13C NMR, they were referenced with d 77.0 (CDCl3) and
d 39.7 (DMSO-d6). The multiplicities were defined in the
usual way: s (singlet), d (doublet), dd (double doublet), t
(triplet), q (quartet), m (multiplet). Low resolution mass
spectra were obtained from the Shimadzu GC-17A (Civil
Police, Distrito Federal) equipment. High resolution
mass spectra were obtained from VG-auto SPEC
(IQ, Unicamp) equipment. Elemental analyzes were
obtained from the Analytische Laboratorien Prof. Dr. H.
Malissa und G. Reuter GmbH.
Synthesis of 2,6-dioxo-9-oxabicyclo[3.3.1]nonane (4)
The 2-iodoxybenzoic acid12 (8.40 g; 30.0 mmol) was
added to the solution of diol 3 (1.58 g, 10.0 mmol) in
ethyl acetate (60 mL) and stirred magnetically at 80 °C.
After 6 h of reaction, the suspension was filtered through
a column of florisil (60-100 mesh) washed with ethyl
acetate (2 × 30 mL) and evaporated. The product obtained,
recrystallized from isopropanol, with degree of high purity
demonstrated by gas chromatographic was characterized
as a white solid (9.80 mmol; 99% m.p. 49-51 °C)
(Scheme 2).
Synthesis of 9-oxabicyclo[3.3.1]nonane-2,6-diol (3)
The isomeric mixture of 9-oxabicyclo[3.3.1]nonane-
2,6-diol (3) and 9-oxabicyclo[4.2.1]nonane-2,5-diol (3a)
was prepared by the oxidation of cycloocta-5-diene (2)
(1.18 mol) under treatment with formic acid9-11 (1.06 mol)
and under vigorous magnetic stirring and, in an ice bath,
30% H2O2 (300 mL, 9.20 mol) was added dropwise.
After the addition, the formation of a biphasic mixture
was observed, which was maintained at 50 °C (internal
temperature). The mixture became homogeneous after
two days under magnetic stirring at 50 °C. Subsequently,
10% Pd/C (0.10 g) was added to decompose the excess of
H2O2. This step was monitored with KI/starch indicator
paper. The solution was filtered under celite and subjected
to liquid-liquid extraction with ethyl acetate for 72 h. The
organic phase was concentrated on a rotary evaporator
in which provided a yellow solid of melting point equal
to 55-58 °C in 85% yield (1.50 mol). The diol 3 was
isolated from the isomeric mixture, 3 and 3a, by successive
recrystallizations from ethyl acetate. Chromatographic
analysis and NMR confirmed the structure and purity of
compound 3 (Scheme 1).
Scheme 2. Reaction conditions for synthesis of diketone 4.
2,6-Dioxo-9-oxabicyclo[3.3.1]nonane (4)
White solid, m.p. 49-51 °C; IR (KBr) ν / cm-1 3409,
2950, 2927, 1715, 1455, 1299, 1242, 1189, 1127; 1H NMR
(300 MHz, CDCl3) d 4.38 (m, 2H), 2.80-2.69 (m, 2H),
2.57-2.38 (m, 4H), 2.17-2.04 (m, 2H); 13C NMR (75 MHz,
CDCl3) d 209.6, 75.7, 33.5, 25.0; anal. calcd. for C8H10O3
(154.0629 g mol-1): C, 62.33; H, 6.54%. Found: C, 62.20;
H, 6.51%.
Synthesis of 2,8-dimethyl-3,4,6,8,9,10,11,12-octahydro-
2H,5H-pyrano[2’,3’,5,6]cycloocta[1,2-b]pyran-6,12-diol (7)
Diketone 4 (1.54 g, 10.0 mmol), anhydrous cyclohexane
(50 mL), morpholine (4.40 mL, 30.0 mmol) and nafion 417®
(2.00 mm) were refluxed under stirring on a Dean- Stark for
48 h to convert 4 into enamine 5. The solvent (cyclohexane)
and the residual reactant (morpholine) were removed under
reduced pressure (1.00 torr) resulting in a yellowish solid
(2.67 g; 9.10 mmol; 90%) which was used in the next step
without prior purification.
The solid crude enamine 5 (9.10 mmol) was subjected
to alkylation (Michael addition) in anhydrous dioxane
(50 mL) and methyl vinyl ketone (2.50 mL, 30.0 mmol)
was dripped, under controlled conditions (N2 atmosphere
Scheme 1. Reaction conditions for synthesis of diol 3.

76
Synthesis and Expansion of Bicyclic Enol Ether
J. Braz. Chem. Soc.
and CaCl2 tube).After 24 h reflux, the residual solvents and
reagents were removed under reduced pressure to provide a
viscous yellowish liquid. NaBH4 (0.76 g, 20.0 mmol) was
added to the solution of the alkylated enamine 6 in ethanol
(25 mL), cooled in an ice bath. The reagent suspension was
stirred for 0.5 h at that temperature, then for 2 h at room
temperature.
After this, the residual NaBH4 was decomposed with
20% aqueous HCl solution (ca. 15 mL). Then a further
30 mL of 20% aqueous HCl solution and fourteen hour
reflux was added to occur the hydrolysis and cyclization
of the reduced product 7 (Scheme 3). The reaction was
cooled and then the aqueous phase was extracted with
chloroform (3 × 30 mL), and the combined organic phases
were washed with saturated NaHCO3 solution (3 × 30 mL)
and brine (3 × 20 mL). After manipulation, a yellowish
viscous liquid (1.72 g, 6.10 mmol, 81%) was obtained.
Purification by dry flash chromatography column (10%
hexane/ethyl acetate) yielded yellowish crystals (1.42 g,
5.00 mmol, 50%) which recrystallized from ethyl acetate or
dichloromethane provided white crystals (m.p. 102-103 °C;
41%) with Rt (retention time) 14.18 min (50.95%) and
14.38 min (49.04%).
Synthesis for the expansion of 2,8-dimethyl-
3,4,6,8,9,10,11,12-octahydro-2H,5H-pyrano[2’,3’,5,6]
cycloocta[1,2-b]pyran-6,12-diol (7)
Butyl nitrite (0.60 mL, 5.00 mmol) was added to a
solution of enol ether 7 (0.28 g, 1.00 mmol) solubilized
in ethanol, kept under ice-bath. The reaction flask was
carefully sealed with a rubber septum to prevent the loss of
nitrous acid formed with the addition of 10% HCl (5 mL).
After 1 h of stirring at 0 °C, the reaction was allowed to
stand at −18 °C for 24 hours. The aqueous phase was
extracted with dichloromethane (5 × 20 mL), and the
organic extract was washed with brine (1 × 15 mL) and
stirred with solid NaHCO3. After the usual procedure, a
yellowish semi-solid was obtained (0.26 g, 0.72 mmol,
70%) and purified by flash column chromatography
(hexane/ethyl acetate 90:10 and 50:50). Column fractions
were characterized by spectroscopic analyzes as oxime 8
(Rt 22.8 min), and isoxazol 9 (Rt 13.4 min).
Results and Discussion
The reaction sequence for the preparation of precursors
7, 8 and 9 was initiated with the classical oxidation of
cycloocta-1,5-diene (2),9-11 to obtain diol 3, which was
obtained with its 3a isomer in 85% yield. The separation
of the diol 3 from the isomeric mixture 3 and 3a was done
by successive fractional recrystallizations of ethyl acetate.
High resolution mass spectrometry (MS) confirmed
the exact molar mass (158.0946 g mol-1) and provided the
characteristic fragments of the molecule. The elemental
analysis data (9.11% H, 54.52% C) presented a variation
when compared with the theoretical data (8.92% H,
60.74% C).9 This difference is related to the presence of
water of crystallization in the structure of diol 3, which
2,8-Dimethyl-3,4,6,8,9,10,11,12-octahydro-2H,5H-
pyrano[2’,3’,5,6]cycloocta[1,2-b]pyran-6,12-diol (7)
White solid, m.p. 102-103 °C; IR (KBr) ν / cm-1
3393, 2973, 2928, 2875, 1705, 1449, 1365, 1322, 1219,
1
1186, 1089, 890, 801, 765; H NMR (300 MHz, CDCl3)
d 4.15-4.05 (m, 3H), 3.97-3.89 (m, 3H), 3.82-3.75 (m, 1H),
2.42-1.54 (m, 22H), 1.27 (d, J 6.2 Hz, 3H), 1.22 (d, J 6.3 Hz,
3H); 13C NMR (75 MHz, CDCl3) d 145.5, 102.5, 102.0,
72.0, 71.2, 70.8, 68.9, 67.0, 66.8, 30.0, 28.9, 27.9, 25.5,
22.2, 21.2, 19.7; anal. calcd. for C16H24O4 (281 g mol-1):
C, 68.54; H, 8.63%. Found: C, 68.42; H, 8.88%.
Scheme 3. Reaction conditions for synthesis of enol ether 7.
Scheme 4. Reaction conditions for expansion of enol ether 7.

Vol. 29, No. 1, 2018
Costa et al.
77
assertion is confirmed when the data obtained are compared
with the theoretical data containing water of crystallization
in the calculations (9.15% H, 54.53% C).9
d 102.0, 102.5) tetrasubstituted not shown in the gHMQC
(heteronuclear multiple quantum correlation) spectrum.
Posteriorly, it was observed in CGC (capillary column
gas chromatography) (DB-1) that compound 7, submitted
to successive recrystallizations, was enriched by one of the
isomers. The analysis of 1H NMR (300 MHz, CDCl3) found
different intensities of the two methyl groups.
The expansion of bicyclic ethers can be promoted
by oxidative cleavage methods, such as ozonolysis,14,15
oxidation with m-chloroperbenzoic acid14 and hydrolytic
nitrosation.16-20 In this work, to expand ring 7, ozonolysis
reactions and oximation/deoximation reactions were used.
The ozonolysis reaction was studied with substrate 7
under reaction conditions described in the literature.14,15
However, the products obtained with this reaction provided
mixtures of compounds that could not be elucidated by
means of spectroscopic analyzes.
The expansion of bicyclic ether enol by hydrolytic
nitrosation with butyl nitrite was developed in our laboratory
for the production of macrolide pyrenophorin (1),8 which
resulted in an isomeric mixture (70%) formed mainly by
dioximedilactone 8 and diisoxazoledilactone 9 (Scheme 4),
in which traces of diketodilactone 9a could be observed in
the NMR and IR spectra.
The CGC analysis (DB-1) showed a mixture of
compounds 8 and 9 which were obtained (60-70%) with
the butyl nitrite oxidant.
The conversion of 9-oxabicyclo[3.3.1]nonane-
2,6-diol (3) to 2,6-dioxo-9-oxabicyclo[3.3.1]nonane (4)
was successfully achieved in 99% yield from the use of the
oxidant 2-iodoxybenzoic acid oxidant.12 High resolution
mass spectrometry (MS) confirmed the exact molar mass
(m/z 154.0629) and provided the characteristic fragments of
the molecule. Data from the elemental analysis (6.51% H,
62.20% C) were consistent with the theoretical data
(6.54% H, 62.33% C).9
Gross enamine 5, obtained from diketone 4, was
alkylated, reduced and hydrolyzed in a same pot and
provided precursor 7, which had its structure elucidated
from spectrometric techniques including 1D and 2D NMR.
The infrared spectrum analysis of enol ether 7 showed
the OH bands at 3393 cm-1, C=C at 1704 cm-1, and C−O
at the region of 1015 to 1200 cm-1, present in the cyclized
product 7. The hydroxyls present proved that the C−O
related bonds of the bridging heads of the bicyclic system 4
were broken. The absorption at 1704 cm-1 relative to the
C=C bond, although relatively high for the tetrasubstituted
unsaturated groups, has been reported in the literature for
enol ethers.13
To verify the chemical purity of product 7, a low
resolution mass spectrometry (MS) analysis was performed,
which showed molar mass 281 g mol-1 and the characteristic
fragmentations. The elemental analysis data (68.42% C,
8.88% H) are consistent with the theoretical data
(68.54% C, 8.63% H).
The 1H NMR spectrum (300 MHz, CDCl3) of product 7
showed two doublets at d 1.22 and 1.27 for the methyls of
probable isomers. The absorptions in d 1.54 to 2.42 relative
to the CH2, as well as the 7 hydrogens relative to the CH
recorded in the region of d 3.75 to 4.15, were related to the
probable stereoisomers of the product 7.
The IR spectra presented the vibrational frequencies
1
of lactone (1726 cm-1) and oxime (1604 cm-1). H NMR
(300 MHz, DMSO-d6) and 13C NMR (75.46 MHz,
DMSO-d6) spectra showed characteristic absorptions
of the oxime 8 and ketolactone 9a (see Supplementary
Information).
Subsequently, the product mixture was purified by flash
chromatographic column (hexane/ethyl acetate 90:10 and
50:50) to provide enriched fractions of Rt 15.9; 13.4 and
22.8 min, previously observed by CGC.
The 13C NMR spectrum (75.46 MHz, CDCl3) showed
the quaternary carbons of the ether enol 7 at d 101.5, 102.3,
143.8 and 145.3, absent in the DEPT spectrum.
The solid fractions of Rt 13.4 and 22.8 min observed
in the CGC (DB-1) and H NMR (300 MHz, CDCl3),
remained as main components even undergoing successive
recrystallizations.
Afterwards this fraction was maintained in CDCl3
solution and cooling for 2 days, there was enrichment of the
Rt compound of 13.4 min (DB-1). 1H NMR spectroscopic
analyzes (300 MHz, CDCl3) and 13C NMR (75.49 MHz,
CDCl3) showed a structure of the compound of Rt 13.4 min,
being probably the isoxazole 9.
In the isoxazole compound 9, the prominent singlet at
d 5.80, related to CH sp2, was characterized by the DEPT,
gHMQC, gHMBC techniques.
1
The absorptions of CH2 (d 21.0 to 35.0) and of 7 CHs
(d 66.0 to 72.0) confirmed the stereoisomers of the enol
ether 7.
By the 2D NMR techniques, some correlations of 1H,1H
1
and H,13C were observed, which aided in the structural
elucidation of the isomeric mixture of 7. The gCOSY
(correlation spectroscopy) spectrum showed the methyl
groups (d 1.22 and 1.27) coupled with CH (d 4.05-4.15
and d 3.75-3.82), respectively.
The gHMBC (heteronuclear multiple bond correlation)
experiment confirmed the sp2 carbons (d 144.3, 145.5 and
The chemical displacement of the oxime 8 in d 10.15

78
Synthesis and Expansion of Bicyclic Enol Ether
J. Braz. Chem. Soc.
was not observed maybe because of the low concentration
of this isomer in the compound mixture. In the H NMR
2. Aliferis, K.A.; Chrysayi-Tokousbalides, M.; Fasseas, C.; Plant
Physiol. Biochem. 2006, 44, 851.
1
spectra (300 MHz, DMSO-d6) doublets, with chemical shift
d 5.09, 5.11, 5.84 and 5.89 ppm, were obtained resulting
from labile hydrogen interactions of the compounds of the
mixture 9 and 9a with this deuterated solvent.
3. Kalita, D.; Klan, A. T.; Saikia, A. K.; Bez, G.; Barua, N. C.;
Synthesis 1998, 7, 975.
4. Fürstner, A.; Thiel, O. R.; Ackermann, L.; Org. Lett. 2001, 3,
449.
5. Lee, C. W.; Grubbs, R. H.; J. Org. Chem. 2001, 66, 7155.
6. Rao, K. S.; Reddy, D. S.; Mukkanti, K.; Pal, M.; Iqbal, J.;
Tetrahedron Lett. 2006, 47, 6623.
Conclusions
The key intermediate 7 was prepared from the cheap
starting material 3 in three steps and transformed into 8
and 9 in excellent yields. The conversion of the 8 and 9
compounds into dilactone 1 is under investigation and will
be reported in due course.
7. Ramakrishna, K.; Sreenivasulu, R.; Vidavalur, S.; Reddy, B. J.
M.; Lett. Org. Chem. 2016, 13, 693.
8. Mahajan, J. R.; Araújo, H. C.; Synthesis 1981, 49.
9. Duthaler, R. O.; Wicker, K.; Ackermann, P.; Ganter, C.; Helv.
Chim. Acta 1972, 55, 1809.
10. Eaton, P. E.; Millikan, R.; Synthesis 1990, 483.
11. Hegemann, K.; Fröhlich, R.; Haufe, G.; Eur. J. Org. Chem.
2004, 2181.
Supplementary Information
Supplementary information associated with this work
(NMR spectra (1H and 13C), infrared and elementary
analysis) is available free of charge at http://jbcs.sbq.org.br
as PDF file.
12. More, J. D.; Finney, N. S.; Org. Lett. 2002, 4, 3001.
13. Mahajan, J. R.; Araújo, H. C.; Synthesis 1976, 111.
14. Mahajan, J. R.; Ferreira, G. A. L.; Araújo, H. C.; Nunes, B. J.;
Synthesis 1976, 112.
15. Mahajan, J. R.; Araújo, H. C.; Synthesis 1975, 53.
16. Mahajan, J. R.; Ferreira, G. A. L.;Araújo, H. C.; J. Chem. Soc.,
Chem. Commun. 1972, 1078.
Acknowledgments
The authors are grateful to the Brazilian agencies CNPq
and FINEP (grant CT-INFRA 970/01) for financial support,
FAPEG/CAPES and PROBIP-2016/UEG.
17. Mahajan, J. R.; Araújo, H. C.; Synthesis 1980, 64.
18. Mahajan, J. R.; Resck, I. S.; Synthesis 1980, 998.
19. Mahajan, J. R.; Monteiro, M. B.; J. Chem. Res. 1980, 264.
20. Mahajan, J. R.; Araújo, H. C.; Synthesis 1981, 49.
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