Total synthesis of (-)-solanapyrone A via enzymatic Diels-Alder reaction of prosolanapyrone

Article abstract of DOI:10.1021/jo980743r

The syntheses of prosolanapyrones I (6) and II (7) via the aldol reactions of pyrone and dienal segments have been achieved in five steps in 31% overall yield for 6 and seven steps in 5% overall yield for 7. An improved synthetic route starting from vinylpyrone 27 provided 7 in 11 steps in 12% overall yield. The enzymatic Diels-Alder reaction of 7 affords (-)- solanapyrone A (1) with high enantioselectivity and with good exo- selectivity, which is difficult to attain by chemical methods. In addition, a crude enzyme preparation from Alternaria solani has been used to perform a kinetic resolution of (±)-3.

Full text of DOI:10.1021/jo980743r

8748
J . Org. Chem. 1998, 63, 8748-8756
Tota l Syn th esis of (-)-Sola n a p yr on e A via En zym a tic Diels-Ald er
Rea ction of P r osola n a p yr on e
Hideaki Oikawa,* Tomonori Kobayashi, Kinya Katayama, Yuichi Suzuki, and
Akitami Ichihara*
Department of Bioscience and Chemistry, Faculty of Agriculture, Hokkaido University,
Sapporo 060, J apan
Received April 21, 1998
The syntheses of prosolanapyrones I (6) and II (7) via the aldol reactions of pyrone and dienal
segments have been achieved in five steps in 31% overall yield for 6 and seven steps in 5% overall
yield for 7. An improved synthetic route starting from vinylpyrone 27 provided 7 in 11 steps in
12% overall yield. The enzymatic Diels-Alder reaction of 7 affords (-)-solanapyrone A (1) with
high enantioselectivity and with good exo-selectivity, which is difficult to attain by chemical methods.
In addition, a crude enzyme preparation from Alternaria solani has been used to perform a kinetic
resolution of (()-3.
In tr od u ction
subsequent study, we found enzymatic activity catalyzing
the Diels-Alder reaction in cell-free extracts of A. solani,7
and we reported the partial purification and properties
of the enzyme, solanapyrone synthase,8 which is the first
example of a “Diels-Alderase”. In addition, we showed
that in the presence of molecular oxygen the crude
enzyme converted 7 to 1 and 2 with accompanying
formation of hydrogen peroxide. Current evidence sug-
gests that the single enzyme catalyzes oxidation and
Diels-Alder reactions in a two-step conversion, although
the enzyme was not purified as a single band on SDS-
PAGE.8
Solanapyrones were isolated as phytotoxic substances
from phytopathogenic fungi Alternaria solani1-3 and
Ascochyta rabiei.4 The solanapyrone family consists of
diastereomers A (1) and D (2) and their reduced forms B
(3), E (4),5 and C (5) that possess alternate side chains
at C-13. Isolation of these substances as optically active
Enzymes are now firmly established as useful tools for
syntheses of organic molecules with their capability for
performing asymmetric transformation under mild condi-
tions.9,10 In organic synthesis, however, use of enzymes
for carbon-carbon bond formation is limited to aldolases,
prenyl diphosphate synthase, and a few other ex-
amples.9,10 In this paper, we illustrate the application
of a Diels-Alderase in the context of an efficient asym-
metric synthesis of (-)-solanapyrone A (1) with good exo-
selectivity, which is difficult to achieve by chemical
methods.
Syn th esis of P r osola n a p yr on es via Ald ol Con -
d en sa tion s of P yr on es a n d Dien a l. Total synthesis
of (()-1 was completed by intramolecular Diels-Alder
reaction of a triene intermediate which was prepared by
the aldol reaction of a pyrone segment with a dienal
segment.11 Although this convergent route was straight-
forward and short, a major drawback was the capricious
yield of the aldol condensation. We essentially adopted
this biomimetic route and made an effort to improve the
forms strongly suggest that solanapyrones are biosyn-
thesized from the achiral linear triene precursor pro-
solanapyrone III (8) via an enzyme-catalyzed Diels-Alder
reaction.3,6 Incorporation of an isotopically labeled bio-
synthetic precursor, prosolanapyrone II (7) into (-)-
solanapyrones unambiguously confirmed the biosynthetic
pathway of solanapyrones as shown in Scheme 1.5 In a
(1) Ichihara, A.; Tazaki, H.; Sakamura, S. Tetrahedron Lett. 1983,
24, 5373-5376.
(7) Oikawa, H.; Katayama, K.; Suzuki, Y.; Ichihara, A. J . Chem.
Soc., Chem. Commun. 1995, 1321-1322. Ichihara, A.; Oikawa, H.
Dynamic Aspects of Natural Products Chemistry: Molecular Biological
Approaches; Ogura, K., Sankawa, U., Eds.; Koudannsha Scientific:
Tokyo, 1997; pp 119-144.
(2) Ichihara, A.; Miki, M.; Sakamura, S. Tetrahedron Lett. 1985, 26,
2453-2454.
(3) Oikawa, H.; Yokota, T.; Ichihara, A.; Sakamura, S. J . Chem. Soc.,
Chem. Commun. 1989, 1284-1285.
(8) Katayama, K.; Kobayashi, T.; Oikawa, H.; Honma, M.; Ichihara,
A. Biochim. Biophys. Acta 1998, 1384, 387-395.
(4) Alam, S. S.; Bilton, J . M.; Slawin, M. Z.; Williams, D. J .;
Sheppard, R. N.; Strange, R. M. Phytochemistry 1989, 28, 2627-2630.
(5) Oikawa, H.; Suzuki, Y.; Naya, A.; Katayama, K.; Ichihara, A. J .
Am. Chem. Soc. 1994, 116, 3605-3606.
(9) Drauz, K.; Waldmann, H. Enzyme Catalysis in Organic Synthe-
sis; VCH Publishers: New York, 1995.
(10) Santaniello, E.; Ferraboschi, P.; Grisenti, P.; Manzocchi, A.
Chem. Rev. 1992, 92, 1071-1140.
(11) Ichihara, A.; Miki, M.; Tazaki, H.; Sakamura, S. Tetrahedron
Lett. 1987, 28, 1175-1178.
(6) Oikawa, H.; Yokota, T.; Abe, T.; Ichihara, A.; Sakamura, S.;
Yoshizawa, Y.; Vederas, J . C. J . Chem. Soc., Chem. Commun. 1989,
1282-1284.
10.1021/jo980743r CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/10/1998

Total Synthesis of Solanapyrone A
J . Org. Chem., Vol. 63, No. 24, 1998 8749
Sch em e 1. Biosyn th etic P a th w a y of Sola n a p yr on esa
Sch em e 2
Sch em e 3
condensation yield. In addition, we modified the route
which is applicable to the synthesis of multiply labeled
compounds for the biosynthetic study of solanapyrones.5
First, 3-methyl-2-pyrone 12 was synthesized from the
known copper complex 912 (Scheme 2). Following litera-
ture precedent,12 alkylation of 9 with 15 equiv of iodo-
methane (MeI) under reflux provided the multiply meth-
ylated products. The result showed that MeI is highly
reactive unlike other alkyl halides tested.12 Use of MeI
(0.5 equiv) at 0 °C predominantly gave the monometh-
ylated product 10 in 88% yield based on MeI. Treatment
of the unstable diketoester 10 with DBU followed by
O-methylation gave pyrone 12.
toluenesulfonate (TsOMe) followed by oxidation with
mCPBA gave sulfoxide 14 in quantitative yield. How-
ever, Pummerer rearrangements of the sulfoxide 14
under various conditions failed.
Hydroxymethyl-substituted pyrone segments were syn-
thesized as follows (Scheme 4). Starting from pyrone 16,
methylation with TsOMe in the presence of K2CO3
followed by TiCl4 mediated formylation14 and a modified
workup procedure gave formylpyrone 1514 in improved
yield (57%). Reduction of pyrone 15 with NaBH4 and
subsequent silylation or tetrahydropyranylation afforded
the pyrone segment 19a or 19b, respectively.
Previously, the synthesis of dienal 21 employed a Li2-
CuCl4-catalyzed cross coupling between the correspond-
ing Grignard reagent and the acetate of sorbic alcohol.11
The need for multiply labeled prosolanapyrones prompted
us to synthesize 21 by an alternative route (Scheme 5)
via Wittig reaction between a C6-aldehyde and a crotyl
phosphonium reagent, which would be prepared from
Alternatively, pyrone 12 was synthesized from the
known pyrone 1313 via 11 by desulfurization and methyl-
ation (Scheme 3). Transformation of 13 to formylpyrone
15 was then attempted. Methylation with methyl p-
(12) Cervello, J .; Marquet, J .; Man˜as, M. M. Tetrahedron 1990, 46,
2035-2046.
(13) The pyrone 13 was prepared by condensation of formaldehyde,
thiophenol, and commercially available pyrone 16, see: Man˜as, M. M.;
Pleixats, R. Synthesis 1984, 430-431.
(14) Poulton, G. A.; Cyr, T. D. Can. J . Chem. 1982, 60, 2821-2829.

8750 J . Org. Chem., Vol. 63, No. 24, 1998
Oikawa et al.
Sch em e 4
Sch em e 6
Sch em e 5
protected pyrone 19b, 2H-tetrahydropyran-2-ol was formed
as a byproduct. This indicated that an elimination
competed with enolate formation as shown in eq 1. To
Ta ble 1. Ald ol Rea ction s of Va r iou s 3-Su bstitu ted
P yr on es
suppress the elimination, the reaction with the dianion
prepared from 18 and use of in situ protection15 of
aldehyde 15 with lithium N-methyl-N-(2-pyridyl)amide
or lithium N-methylpiperazide were tested. All attempts,
however, could not improve the yield.16
Transformation of the aldol adducts to prosolanapy-
rones was employed as shown in Scheme 6. Tosylation
of 23a and subsequent elimination with DBU provided
prosolanapyrone I (6) in 87% yield. Similarly, adduct 23b
was converted to the unsaturated product 24b. The
deprotection of the THP-protected pyrone 24b was prob-
lematic as deprotection under usual conditions (PPTS in
MeOH at 60 °C) failed and harsher conditions (2 M HCl,
EtOH, reflux) caused decomposition. When PPTS-
catalyzed acid hydrolysis of the tert-butyldiphenylsilyl
group protected pyrone 25 was attempted, the methyl
ether 26 was obtained in low yield. This result suggests
substrate
R
yield (%) recovered SM (%)a
product
12
19b
22
19a
18
H
66
28
17
17
10
0
0
23a
23b
23c
23d
23e
OTHP
SPh
OTES
OH
trace
26
0
a
b
SM ) starting material. Base (2.2 equiv), -10 °C.
propargyl alcohol via alkylation and reduction of the
alkyne. Starting from ꢀ-caprolactone, dienol 20 was
synthesized in an one-pot manner. In the event, treat-
ment of a toluene solution of the lactone with DIBALH
at -78 °C resulted in the corresponding hemiacetal which
was reacted with the phosphorane prepared from crotyl
triphenylphosphonium bromide and n-BuLi at 0 °C to
afford a 1:1 mixture of E,E- and E,Z-dienols 20 in 66%
yield. Isomerization with I2 to improve the E,E-:E,Z-
ratio to 5:1, and Swern oxidation furnished dienal 21.
With the pyrone and dienal segments in hand, the aldol
reactions between various 2-pyrones and E,E-dienal 21
were examined. The reaction of 3-methyl-2-pyrone 12
proceeded smoothly to yield the adduct 23a as a single
product (Table 1). On the other hand, in the case of the
pyrones with the substituent at C-1′, the yields of the
reactions did not exceed 30%. In the reaction of the THP-
F igu r e 1.

Total Synthesis of Solanapyrone A
J . Org. Chem., Vol. 63, No. 24, 1998 8751
Sch em e 7
Ta ble 2. Non en zym a tic Diels-Ald er Rea ction s of
that cleavage the removal of the C-1′-O bond occurred
under acidic conditions and was followed by either
addition of MeOH or decomposition (Figure 1). Among
the protecting groups tested (THP, SEM, and TES), the
TES group gave the best result. Thus, tosylation-
elimination of 23d followed by desilylation provided
prosolanapyrone II (7) in 83% overall yield. Oxidation
of 7 with Dess-Martin periodinane17 furnished pro-
solanapyrone III (8) in 63% yield.
Im p r oved Syn th esis of P r osola n a p yr on e II. The
observation that 3-methyl-2-pyrone (12) gave adduct 23a
in a good yield prompted us to use 3-vinyl-2-pyrone (27),11
which would allow subsequent conversion to the hy-
droxymethyl derivative. Indeed, the reaction of 27 with
aldehyde 28 prepared from hexan-1,6-diol in two steps
proceeded smoothly to give adduct 29 in 85% yield
(Scheme 7). Oxidative cleavage of the terminal olefin was
employed in a two-step manner [(1) OsO4, N-methylmor-
pholine N-oxide, (2) NaIO4] to afford aldehyde 30. After
reduction and subsequent acetylation of the resultant diol
31, elimination of the secondary acetoxy group with DBU
provided olefin 32 in good overall yield (70%, five steps).
To introduce the diene moiety into the side chain, the
terminal alkoxy group in 32 was converted to vinyl iodide
in 35 as follows. After deprotection of the silyl group,
oxidation followed by Takai reaction18 afforded a 4:1
mixture of E- and Z-vinyl iodide 35. Efforts to increase
E-selectivity by changing solvents19,20 and temperature
P r osola n a p yr on esa
reaction yield recovery
entry substrate solvent
time
(%) of SM (%) endo/exo
1
2
3
6
6
7
7
7
7
8
8
8
8
toluene
H2O
toluene
CHCl3
CH3CN
H2O
toluene
CHCl3
CH3CN
H2O
48 h
7 d
48 h
2 h
24 h
2 d
1 h
1 h
1 h
3 h
12
7
55
7
71
19
68
64
82
62
11
93
2
91
18
81
27
28
10
28
1.9
>10
2.2
3.6
5.6
4b
5b
6
20
7
8
9
10
2.7
3.4
4.4
23
a
All reactions were carried out at 110 °C except entries 1 (180
b
°C) and 2, 6, and 10 (30 °C). Prolonged reaction time caused
degradation of products.
were unsuccessful likely due to instability of the pyrone
moiety. A Stille reaction21 of 35 with 1-propenyltribu-
tylstannane prepared from (E)-1-bromopropene gave the
diene 36, which was finally converted to prosolanapyrone
II (7) by alkaline hydrolysis.
Non en zym atic Diels-Alder Reaction of P r osolan a-
p yr on es. To assess the diastereoselectivity and the
intrinsic reactivity of prosolanapyrones, Diels-Alder
reactions were examined under various conditions. The
results are shown in Table 2. Endo-/exo-selectivities with
6-8 were essentially the same in various solvents while
the endo-selectivity was increased with increasing solvent
polarity. The slight preference of endo-selectivity in less
polar solvents suggest that there is little steric conges-
tion, in both endo- and exo-transition states as reported
in the reactions of simple decatriene systems.22
(15) Comins, D. L. Synlett 1992, 615-625.
(16) Due to the low solubility of the pyrones, we could not test other
solvents.
(17) Dess, D. B.; Martin, J . C. J . Org. Chem. 1983, 48, 4155-4156.
For preparation of Dess-Martin periodinane, see: Ireland, R. E.; Liu,
L. J . Org. Chem. 1993, 58, 2899.
(18) Takai, K.; Nitta, K.; Utimoto, K. J . Am. Chem. Soc. 1986, 108,
7408-7410.
(19) Evans, D. A.; Black, W. C. J . Am. Chem. Soc. 1993, 115, 4497-
4513.
In less polar solvents, heating was required for the
effective cycloaddition of all three compounds. Rate
acceleration depended on the oxidation level of the
3-substituent in prosolanapyrones (Table 2). These are
explained by the LUMO energy of the dienophile moiety
(20) Following Evans modification,19 the model reaction using
decanal was employed in dioxane/THF (6:1) in the presence of pyrone
19a , we could improve an isomer ratio of the vinyl iodide (E/Z ) 19:1)
but none of 19a was recovered. We found that iodine liberated during
the reaction caused decomposition of the pyrone moiety in removal of
solvent. However, we could not identify other factors for the decom-
position.
(21) Goure, W. F.; Wright, M. E.; Davis, P. D.; Labadie, S. S.; Stille,
J . K. J . Am. Chem. Soc. 1984, 106, 6417-6422.
(22) Raimondi, L.; Brown, F. K.; Gonzalez, J .; Houk, K. N. J . Am.
Chem. Soc. 1992, 114, 4796-4804.

8752 J . Org. Chem., Vol. 63, No. 24, 1998
Oikawa et al.
F igu r e 2. LUMO energies of prosolanapyrone analogues.
Calculated by MOPAC 6.0 with PM3.
in the pyrones (Figure 2). Use of Lewis acid (1 equiv of
Et2AlCl, CH2Cl2, -78 °C to rt) caused simply degradation
of starting materials in the case of 6 and 8. We noted a
significant rate acceleration of prosolanapyrone III (8)
in aqueous medium. The increased reactivity could be
explained by “hydrophobic effect”23 and/or hydrogen
bonding24 between water and the dienophile carbonyl
group. To our knowledge, this enormous rate accelera-
tion in the reaction of decatrienes has not been reported
previously. Since there are numbers of natural products
with trans-fused decalin ring systems,25 it could be useful
that proper modification of the dienophile, such as
introducing a reactive electron-withdrawing group, would
result in predominant formation of trans-fused decalin
systems with rate acceleration. We chose 7 as a sub-
strate to study the enzymatic Diels-Alder reaction, to
suppress the nonenzymatic reaction which reduces the
optical purity of 1.
En zym a tic Diels-Ald er Rea ction of P r osola n a p y-
r on e II. In an analytical-scale experiment, the crude
enzyme containing the solanapyrone synthase provided
(-)-1 with excellent enantioselectivity (99% ee) and
relatively high exo-selectivity (6:1).7 During optimizing
reaction conditions, we found that the reactions in
semipreparative scale were not reproducible and provided
solanapyrones with low enantio- and diastereoselectivity.
This is mainly due to the fact that cycloaddition activity
was lost more rapidly than oxidation activity and this
caused accumulation of 8. Addition of 30% glycerol8 was
not enough to prolong the cycloaddition activity. Use of
catalase to remove coproduct hydrogen peroxide, which
might inactivate the enzyme, did not affect the reaction
rate or selectivity. Finally, we found that freezing in
liquid nitrogen of the freshly prepared enzyme containing
30% glycerol is effective for maintaining activity for
months.
F igu r e 3. Enzymatic kinetic resolution of (-)- and (()-
solanapyrone B (3).
converted to a 6:1 mixture of solanapyrones A (1) and D
(2) in 61% yield with >98 and 67% ee,26 respectively, and
10% of prosolanapyrone III (8) recovered (eq 2). For our
purpose, contamination with the hardly separable Z-
isomer of substrate 7 was not a problem since the
corresponding E,Z-isomer of 8 was not cyclized under the
reaction conditions. On the basis of the chromatographic
behavior of the enzyme,8 we proposed that the single
enzyme catalyzes the oxidation from the alcohol 7 to the
reactive aldehyde 8 which is further converted to the
adducts 1 and 2 by the Diels-Alder reaction. Erosion of
the ee of 2 may be due to the partially distorted
conformation of the bifunctional enzyme. Thus, as 8 is
released from the active site, it is then converted to (()-2
by the nonenzymatic Diels-Alder reaction.
Kin etic Resolu tion of (()-Sola n a p yr on e B. During
the enzymatic study, we found that the crude enzyme
containing solanapyrone synthase catalyzes not only the
oxidation of prosolanapyrone II (7) to III (8) but that of
solanapyrone B (3) to A (1) in the presence of oxygen.
The same enzyme preparation converted 84% of (-)-3 to
afford (-)-1 compared with the conversion of 7 as a
control. On the basis of this result, we anticipated that
the enzyme, prior to the oxidation, has a transition states
structure of 7 similar to those of Diels-Alder reaction
adduct. If so, the enzyme would oxidize racemic 3 with
different rates. Thus, we employed kinetic resolution of
(()-solanapyrone B (3).
The hydrophobic nature of 7 prompted us to examine
the effect of substrate concentration of the substrate on
enzyme activity. A substrate concentration range of
0.05-0.75 mM was tested, without any loss of reaction
rate or stereoselectivity. Since reactions are performed
under relatively dilute conditions, normal workup such
as solvent extraction or lyophilization was a major
disadvantage of the enzymatic reaction. We found that
hydrophobic resin HP-20 adsorbed both substrate and
products, and the adsorbed materials were easily eluted
with ethyl acetate, simplifying products recovery.
Using the enzyme preparation shown above, a 5:1
mixture of 7′′E,9′′E- and 7′′Z,9′′E-7 at 0.75 mM was
First, the enzymatic reaction rates of (-)-3 and (()-3
prepared in the thermal cycloaddition of 7 were examined
(Figure 3). As expected, (-)-3 was oxidized more rapidly
than (()-3 but the difference between them was relatively
small. In larger scale incubation (30 °C, 2 h), (-)-1 was
obtained in 55% yield with 27% ee and (+)-3 was
recovered in 18% yield with 91% ee (eq 3). This kinetic
resolution allowed us to obtain the unnatural form of
solanapyrone B (3).
(23) Breslow, R. Acc. Chem. Res. 1991, 24, 159-164.
(24) Ruiz-Lo´pez, M. F.; Assfeld, X.; Garc´ıa, J . I.; Mayoral, J . A.;
Salvatella, L. J . Am. Chem. Soc. 1993, 115, 8780-8787.
(25) Ichihara, A.; Oikawa, H. Curr. Org. Chem. 1998, 2, 365-394.
(26) Suzuki, E. Synthesis 1978, 144-146.

Total Synthesis of Solanapyrone A
J . Org. Chem., Vol. 63, No. 24, 1998 8753
added DBU (48 mg, 0.31 mmol). The mixture was stirred
under reflux for 1h. After being cooled to ambient tempera-
ture, 1 M HCl was added and the resulting solution was
extracted with EtOAc. The combined organic extracts were
washed with brine, dried over anhydrous Na2SO4, filtered and
concentrated in vacuo. The residue was crystallized from
CHCl3 and MeOH to afford hydroxypyrone 11 (35 mg, 81%)
as white crystals: mp 205-207 °C, (lit.13 209-211 °C); 1H
NMR (90 MHz, CDCl3 + CD3OD) δ 1.86 (s, 3H), 2.20 (s, 3H),
5.90 (s, 1H).
4-Meth oxy-3,6-d im eth yl-2H-p yr a n -2-on e (12). To a so-
lution of hydroxypyrone 11 (104 mg, 0.47 mmol) in 2-butanone
(20 mL) were added K2CO3 (0.93 g, 6 mmol) and dimethyl
sulfate (0.054 mL, 0.55 mmol). The mixture was stirred for
3.5 h under reflux. It was cooled to ambient temperature and
filtered. After concentration of the solution in vacuo, the
residue was purified by silica gel flash chromatography
(CHCl3/acetone, 95:5) to give methoxypyrone 12 (91.7 mg, 81%)
as white crystals: mp 87-89 °C (from CHCl3), lit.13 82-83 °C;
1H NMR (500 MHz, CDCl3) δ 1.90 (s, 3H), 2.26 (s, 3H), 3.88
(s, 3H), 6.03 (s, 1H).
Con clu sion . Convergent, efficient syntheses of pro-
solanapyrones via the aldol reactions of pyrone and dienal
segments are described. Through these syntheses, we
found that 3-alkoxymethyl-substituted 2-pyrones were
labile under both acidic and basic conditions. This
drawback was overcome by the proper choice of protecting
group and use of vinylpyrone 27. Enzymatic Diels-Alder
reaction of 7 on semipreparative scale afforded (-)-1
without loss of enantio- and diastereoselectivity. We also
showed an enzymatic kinetic resolution of (()-3 furnished
unnatural (+)-3. Our synthetic approaches to prosolana-
pyrones allow syntheses of various analogues which
would be useful for the study of the substrate specificity
of the Diels-Alderase. Despite current unavailability of
large amounts of solanapyrone synthase, use of the
enzyme for the enantioselective construction of the cis-
fused decalin system has greatly simplified the synthetic
route of (-)-1. This approach would be practical when
information on amino acid sequence and the gene encod-
ing the enzyme is cloned and overexpressed.
4-Meth oxy-6-m eth yl-2H-p yr a n -2-on e (17). To a solution
of hydroxypyrone 16 (7.3 g, 58 mmol) in DMF (290 mL) were
added K2CO3 (16 g, 116 mmol) and methyl p-toluenesulfonate
(22 g, 116 mmol) at 3 °C. The mixture was allowed to warm
to ambient temperature. After 12 h of stirring, the mixture
was poured into water and the aqueous layer was separated
and extracted with CH2Cl2. The combined organic extracts
were washed with water and brine, dried over anhydrous
MgSO4, filtered, and concentrated in vacuo. The residue was
crystallized from EtOAc to afford methoxypyrone 17 (5.00 g)
as white crystals. The mother liquor was concentrated in
vacuo and purified by silica gel flash chromatography (CHCl3/
acetone, 4:1) to give 17 (1.54 g, total 79%) as white crystals:
Currently, solanapyrone synthase is proposed to be an
oxidase which provides the reactive aldehyde 8 and
catalyzes cycloaddition. This implicates that some oxi-
dases or dehydrogenases could be good sources of Diels-
Alderase as a bifunctional enzyme when they can provide
reactive dienophile species and stabilize them as transi-
tion states.
mp 86 °C (from EtOAc), (lit.26 86-87.5 °C); IR (KBr) 1710 cm-1
;
1H NMR (270 MHz, CDCl3) δ 2.20 (br s, 3H), 3.79 (s, 3H), 5.40
(d, 1H, J ) 2.0 Hz), 5.77 (br d, 1H, J ) 2.0 Hz); EI-MS m/z
140 (M+); EI-HR-MS calcd for C7H8O3 m/z 140.0473 (M+), found
140.0471.
3-F or m yl-4-m eth oxy-6-m eth yl-2H-p yr a n -2-on e (15). To
a solution of 17 (5.8 g, 41.6 mmol) in CH2Cl2 (40 mL) was added
TiCl4 (46 mL, 416 mmol). Under ice-cooled conditions, dichlo-
romethyl methyl ether (37 mL, 416 mmol) was added over 30
min with stirring, and evolved HCl was removed by continuous
nitrogen flow. The mixture was allowed to warm to ambient
temperature and stirred for 6 h. The mixture was carefully
poured into crushed ice, and the aqueous layer was extracted
with CH2Cl2. The combined organic extracts were washed
with saturated aqueous NaHCO3, and brine, dried over an-
hydrous Na2SO4, filtered, and concentrated in vacuo. Purifica-
tion by silica gel flash chromatography (CHCl3/acetone, 4:1)
gave formylpyrone 15 (4.0 g, 57%) and starting material 17
(0.5 g, 9%). For 15: yellow crystals: mp 172-175 °C; IR (KBr)
1750 cm-1; 1H NMR (270 MHz, CDCl3) δ 2.30 (br s, 3H), 4.01
(s, 3H), 6.18 (s, 1H), 10.06 (s, 1H); EI-MS m/z 168 (M+); EI-
HR-MS calcd for C8H8O4 m/z 168.0422 (M+), found 168.0442.
3-(H yd r oxym et h yl)-4-m et h oxy-6-m et h yl-2H -p yr a n -2-
on e (18). To a solution of formylpyrone 15 (2.76 g, 16.4 mmol)
in MeOH (150 mL) was added NaBH4 (1.59 g, 49.2 mmol) at
3 °C. After the mixture was stirred for 1 h, the excess reagent
was quenched with acetone. The volatile materials were
removed in vacuo. Purification by silica gel flash chromatog-
raphy (CHCl3/acetone, 4:1) gave alcohol 18 (2.34 g, 84%) as
Exp er im en ta l Section
Gen er a l Meth od s. Unless otherwise noted, nonaqueous
reactions were carried out under argon atmosphere. Tetrahy-
drofuran (THF) and toluene were distilled from sodium metal/
benzophenone ketyl. Benzene, CH2Cl2, N,N-dimethylform-
amide (DMF), dimethyl sulfoxide, and triethylamine were
distilled from calcium hydride and were stored under argon
atmosphere. Methanol was distilled from magnesium meth-
oxide. Dess-Martin periodinane17 was prepared according to
literature procedures. All other commercially obtained re-
agents were used as received. Melting points are uncorrected.
The enzyme preparation containing solanapyrone synthase
was prepared as described.8 The specific activity of the enzyme
was 0.096 mU/µL (one unit will convert 1 mmol of 7 per min
at 30 °C to 1, 2, and 8).
Meth yl 2-Meth yl-3,5-d ik etoh exa n oa te (10). To a solu-
tion of copper complex 9 (12.8 g, 32 mmol) in THF (384 mL)
was added NaH (40% in mineral oil, 1.28 g, 32 mmol). After
1 h of stirring at ambient temperature, iodomethane (2.05 mL,
32 mmol) was added dropwise at 0 °C. The mixture was
stirred at ambient temperature for 20 h before the reaction
was quenched with 1 M HCl and extracted with CH2Cl2 The
combined organic extracts were washed with brine, dried over
anhydrous Na2SO4, filtered, and concentrated in vacuo. Pu-
rification by silica gel flash chromatography (hexane/acetone,
7:1) gave copper-decomplexed methyl 3,5-diketohexanoate
(5.19 g) and ester 10 (4.97 g, 88% based on iodomethane) as a
white crystals: mp 149-151 °C; IR (KBr) 3406, 1683 cm-1
;
1H NMR (270 MHz, CDCl3) δ 2.29 (s, 3H), 2.90 (t, 1H, J ) 5.9
Hz), 3.90 (s, 3H), 4.54 (d, 2H, J ) 5.9 Hz), 6.05 (s, 1H); EI-MS
170 (M+); EI-HR-MS calcd for C8H10O4 m/z 170.0579, found
170.0606 ((M+).
3-[[1-(Tr iet h ylsilyl)oxy]m et h yl]-4-m et h oxy-6-m et h yl-
2H-p yr a n -2-on e (19a ). To a solution of alcohol 18 (2.34 g,
13.8 mmol) in CH2Cl2 (200 mL) was added 2,6-lutidine (5.1
mL, 44 mmol). Under ice-cooled conditions, triethylsilyl
trifluoromethanesulfonate (4.7 mL, 21 mmol) was added
dropwise. After 1 h of stirring, the mixture was poured into
brine and aqueous layer was separated and extracted with
colorless oil: IR (KBr) 1742, 1610 cm-1 1H NMR (90 MHz,
;
CDCl3) δ 1.37 (d, 3H, J ) 8.0 Hz), 2.04, (s, 3H), 3.36 (d, 1H, J
) 8.0 Hz), 3.70 (s, 3H), 5.52, (s, 1H); FI-MS m/z 172 (M+); FI-
HR-MS calcd for C8H12O4 172.1809 (M+), found m/z 172.1807.
4-Hyd r oxy-3,6-d im eth yl-2H-p yr a n -2-on e (11). To a so-
lution of ester 10 (53 mg, 0.31 mmol) in benzene (1 mL) was

8754 J . Org. Chem., Vol. 63, No. 24, 1998
Oikawa et al.
CH2Cl2. The combined organic extracts were washed with 1
M HCl, saturated aqueous NaHCO3, and brine, successively,
dried over anhydrous Na2SO4, filtered, and concentrated in
vacuo. Purification by silica gel flash chromatography (CHCl3/
EtOAc, 9:1) gave silyl ether 19a (2.94 g, 75%) as white
3H), 2.04-2.10 (m, 2H), 2.52 (dd, 1H, J ) 9.2, 14.5 Hz), 2.69
(dd, 1H, J ) 3.3, 14.5 Hz), 3.88 (s, 3H), 4.06 (m, 1H), 5.49-
5.61 (m, 2H), 5.94-6.03 (m, 2H), 6.10 (s, 1H); EI-MS m/z 306
(M+); EI-HR-MS calcd for C18H26O4 (M+) m/z 306.1831, found
306.1840.
crystals: mp 111-112 °C (from CHCl3); IR (KBr) 1680 cm-1
;
P r osola n a p yr on e I (6). To a solution of adduct 23a (50
mg, 0.161 mmol) in CH2Cl2 (3 mL) were added DMAP (203
mg, 1.61 mmol) and p-toluenesulfonyl chloride (79 mg, 0.40
mmol). The mixture was stirred for 24 h at ambient temper-
ature. To this was added DBU (304 mg, 0.59 mmol). After
an additional 4 h of stirring, the mixture was diluted with CH2-
Cl2. The resultant mixture was washed with 1 M HCl,
saturated aqueous NaHCO3, and brine, successively, dried over
anhydrous Na2SO4, filtered, and concentrated in vacuo. Pu-
rification by silica gel column chromatography (hexane/ether,
9:1) gave 6 (41 mg, 87%) as a colorless oil: IR (KBr) 3400,
1H NMR (270 MHz, CDCl3) δ 0.57 (q, 6H, J ) 7.92 Hz), 0.88
(t, 9H, J ) 7.92 Hz), 2.18 (s, 3H), 3.83 (s, 3H), 4.45 (s, 2H),
5.98 (s, 1H); EI-MS m/z 255 (M+ - Et); EI-HR-MS calcd for
C
12H19O4Si (M+ - Et) m/z 255.1052, found 255.1045.
(6E,8E)-6,8-Deca d ien ol (20). To a solution of ꢀ-caprolac-
tone (3.5 mL, 31.5 mmol) in THF (60 mL) was added dropwise
DIBALH (1.0 M THF solution, 33.3 mL, 33.3 mmol) at -78
°C. After 45 min of stirring, it was transferred via cannula to
a
solution prepared with 2-butenyltriphenylphosphonium
bromide (15 g, 37.8 mmol) in THF (60 mL) and n-BuLi (1.6 M
THF solution, 22.5 mL, 36.0 mmol) at 3 °C. After the mixture
was allowed to warm to ambient temperature and stirred for
2 h, saturated aqueous NH4Cl was added. The mixture was
extracted with ether. The combined organic extracts were
washed with brine, dried over anhydrous Na2SO4, filtered, and
concentrated in vacuo. Purification by silica gel flash chro-
matography (hexane/ether, 5:1) gave dienol 20 (3.20 g, 66%,
(6E,8E):(6E,8Z) ) 1:1).
1
1680 cm-1; H NMR (270 MHz, CDCl3) δ 1.40-1.49 (m, 6H),
1.72 (d, 3H, J ) 6.6 Hz), 1.93 (s, 3H), 2.02-2.10 (m, 2H), 2.17-
2.22 (m, 2H), 3.87 (s, 3H), 5.50-5.61 (m, 2H), 5.92-6.03 (m,
3H), 6.11 (s, 1H), 6.07 (dt, 1H, J ) 7.3, 15.2 Hz); EI-MS m/z
288 (M+); EI-HR-MS calcd for C18H24O3 (M+) m/z 288.1726,
found 288.1727.
3-[[1-(Tr ieth ylsilyl)oxy]m eth yl]-6-[(7E,9E)-2-h yd r oxy-
7,9-u n d eca d ien yl]-4-m eth oxy-2H-p yr a n -2-on e (23d ). To
a solution of pyrone 19a (515 mg, 1.78 mmol) in THF (20 mL)
at -78 °C was added dropwise LHMDS (1.0 M THF solution,
2.55 mL, 2.55 mmol) over 10 min. After 15 min, a solution of
dienal 21 (310 mg, 2.01 mmol) in THF (6.3 mL) was added
dropwise, and the mixture was stirred for 1 h before saturated
aqueous NH4Cl was added. The mixture was extracted with
EtOAc, and the combined organic extracts were washed with
brine, dried over anhydrous Na2SO4, filtered, and concentrated
in vacuo. Purification by silica gel flash chromatography
(CHCl3/acetone, 9:1) and then preparative thin layer silica gel
chromatography (CHCl3/acetone, 6:1) gave adduct 23d (134
mg, 17%) as a colorless oil and the starting materials 19a (82
mg, 16%) and 21 (56 mg, 18%). For 23d : IR (KBr) 3400, 1680
To a solution of dienol 20 (2.56 g, 16.7 mmol) in benzene
(72 mL) was added iodine (115 mg, 91 mmol). The mixture
was stirred avoiding light for 24 h before the reaction was
quenched with saturated aqueous Na2S2O3. After 15 min of
stirring, the mixture was extracted with ether. The combined
organic extracts were washed with brine, dried over anhydrous
Na2SO4, filtered, and concentrated in vacuo. Purification of
the resultant oil ((6E,8E):(6E,8Z) ) 5:1) by 15% AgNO3/silica
gel chromatography (hexane/ether, 95:5) gave (6E,8E)-dienol
20 (0.66 g, 29%) and a (6E,8Z)- and (6E,8E)-mixture (1.40 g,
55%) which was subjected to additional isomerization and
recovery of 20. For the (6E,8E)-isomer: a colorless oil; IR
1
(KBr) 970 cm-1; H NMR (270 MHz, CDCl3) δ 1.34-1.47 (m,
1
4H), 1.57 (m, 2H), 1.72 (d, 3H, J ) 6.6 Hz), 2.06 (m, 2H), 3.63
(t, 2H, J ) 6.6 Hz), 5.48-5.63 (m, 2H), 5.94-6.07 (m, 2H);
EI-MS m/z 154 (M+); EI-HR-MS calcd for C10H18O (M+)
154.1358, found 154.1352.
cm-1; H NMR (270 MHz, CDCl3) δ 0.64 (q, 6H, J ) 7.9 Hz),
0.95 (t, 9H, J ) 7.9 Hz), 1.38-1.50 (m, 6H), 1.72 (d, 3H, J )
5.9 Hz), 2.06 (m, 2H), 2.51 (dd, 1H, J ) 8.6, 14.5 Hz), 2.66
(dd, 1H, J ) 4.0, 14.5 Hz), 3.89 (s, 3H), 4.04 (m, 1H), 4.51 (s,
2H), 5.49-5.61 (m, 2H), 5.97-6.05 (m, 2H), 6.11 (s, 1H); EI-
(6E,8E)-6,8-Deca d ien a l (21). To a solution of oxalyl
chloride (0.28 mL, 3.2 mmol) in CH2Cl2 (10 mL) was added
DMSO (0.32 mL, 4.5 mmol) in CH2Cl2 (10 mL) at -78 °C. After
the mixture was stirred for 10 min, 20 (250 mg, 1.62 mmol) in
CH2Cl2 (5 mL) was introduced, and the resulting mixture was
stirred for 20 min. Triethylamine (1.7 mL, 12.2 mmol) was
added, and the mixture was allowed to warm to ambient
temperature and stirred for 30 min. The reaction was
quenched with water, and the aqueous layer was separated
and extracted with ether. The combined organic extracts were
washed with brine, dried over anhydrous Na2SO4, filtered, and
concentrated in vacuo. Purification by silica gel flash chro-
matography (hexane/ether, 9:1) gave dienal 21 (210 mg, 85%)
MS m/z 407 (M+ - Et); EI-HR-MS calcd for C22H35O5Si (M+
Et) m/z 407.2250, found 407.2250.
-
P r osola n a p yr on e II (7). To a solution of adduct 23d (20
mg, 0.046 mmol) in CH2Cl2 (0.25 mL) were added DMAP (84
mg, 0.69 mmol) and p-toluenesulfonyl chloride (26 mg, 0.14
mmol). The mixture was stirred for 30 min at ambient
temperature. To this was added DBU (90 mg, 0.59 mmol),
and the resulting mixture was stirred for further 1 h before
dilution with EtOAc. The resultant mixture was washed with
1 M HCl, saturated aqueous NaHCO3, and brine, successively,
dried over anhydrous Na2SO4, filtered, and concentrated in
vacuo. The crude unsaturated product thus obtained was used
without further purification.
as a colorless oil: IR (KBr) 1710 cm-1 1H NMR (270 MHz,
;
CDCl3) δ 1.36-1.47 (m, 2H), 1.58-1.69 (m, 2H), 1.72 (d, 3H,
J ) 6.6 Hz), 2.07 (m, 2H), 2.42 (m, 2H), 5.46-5.64 (m, 2H),
5.94-6.05 (m, 2H), 9.75 (dd, 1H, J ) 2.0, 1.3 Hz); EI-MS m/z
152 (M+); EI-HR-MS calcd for C10H16O (M+) m/z 152.1201,
found 15.1221.
To a solution of the product above in THF (0.3 mL) was
added Bu4NF (1.0 M THF solution, 0.06 mL, 0.06 mmol). The
mixture was stirred at ambient temperature for 30 min before
the mixture was diluted with EtOAc. The resultant mixture
was washed with saturated aqueous NH4Cl and brine, suc-
cessively, dried over anhydrous MgSO4, filtered, and concen-
trated in vacuo. Purification by preparative thin-layer silica
gel chromatography (hexane/EtOAc, 1:2) gave 7 (12 mg, 83%)
6-[(7E,9E)-2-Hyd r oxy-7,9-u n d eca d ien yl]-4-m eth oxy-3-
m eth yl-2H-p yr a n -2-on e (23a ). To a solution of LDA in THF
(2.2 mL) (generated from n-BuLi (1.6 M hexane solution, 1.33
mL, 1.99 mmol) and N,N-diisopropylamine (0.32 mL, 2.29
mmol) at 0 °C) at -78 °C was added dropwise pyrone 12 (273
mg, 1.74 mmol) in THF (6 mL) over 50 min. A solution of
dienal 21 (289 mg, 1.9 mmol) in THF (6.3 mL) was added
dropwise, and the mixture was stirred for 4 h before saturated
aqueous NH4Cl was added. The mixture was extracted with
EtOAc, and the combined organic extracts were washed with
brine, dried over anhydrous Na2SO4, filtered, and concentrated
in vacuo. Purification by silica gel column chromatography
(CHCl3/acetone, 95:5) gave adduct 23a (350 mg, 66%) as a
as a colorless oil: IR (KBr) 3400, 1680 cm-1 1H NMR (270
;
MHz, CDCl3) δ 1.34-1.55 (m, 4H), 1.73 (d, 3H, J ) 6.6 Hz),
2.04-2.11 (m, 2H), 2.23-2.2.25 (m, 2H), 2.90 (dd, 1H, J ) 6.6,
7.3 Hz), 3.90 (s, 3H), 4.55 (d, 2H, J ) 6.6, d, J ) 7.3 Hz), 5.50-
5.62 (m, 2H), 5.96-6.03 (m, 3H), 6.00 (s, 1H), 6.78 (dt, 1H, J
) 7.4, 15.2 Hz); EI-MS m/z 304 (M+); EI-HR-MS calcd for
C
18H24O4 (M+) m/z 304.1674, found 304.1686.
P r osola n a p yr on e III (8). To a solution of 7 (5.7 mg, 19
mmol) in CH2Cl2 (0.2 mL) was added Dess-Martin periodi-
nane (14 mg). The mixture was stirred at ambient tempera-
ture for 2 h, when saturated aqueous Na2CO3 and saturated
aqueous Na2S2O3 was added. The resultant mixture was
1
colorless oil: IR (KBr) 3350, 1680 cm-1; H NMR (270 MHz,
CDCl3) δ 1.27-1.56 (m, 6H), 1.73 (d, 3H, J ) 5.9 Hz), 1.91 (s,

Total Synthesis of Solanapyrone A
J . Org. Chem., Vol. 63, No. 24, 1998 8755
stirred for 10 min and then extracted with CH2Cl2. The
combined organic extracts were washed with brine, dried by
passing through a short column of anhydrous MgSO4, filtered,
and concentrated in vacuo. Purification by preparative thin-
layer silica gel chromatography (hexane/EtOAc, 1:2) gave 8
(3.5 mg, 63%) and the starting material 7 (1.2 mg, 21%). For
6.6 Hz), 6.82 (dt, 1H, J ) 6.6, 15.8 Hz); EI-MS m/z 385 (M+
-
CH3); EI-HR-MS calcd for C19H33O6Si (M+ - CH3) m/z 385.2015,
found 385.2052.
3-(Acet oxym et h yl)-6-[(1E)-7-[[(1,1-d im et h ylet h yl)d i-
m eth ylsilyl]oxy]h epten yl]-4-m eth oxy-2H-pyr an -2-on e (32).
To a solution of diol 31 (8.3 mg, 0.020 mmol) in CH2Cl2 (0.3
mL) were added DMAP (10.8 mg, 0.082 mmol) and acetic
anhydride (0.01 mL, 0.091 mmol) at 0 °C. After 1 h of stirring
at ambient temperature, the reaction was quenched with ice
and water. The resultant mixture was extracted with CHCl3,
and the combined organic layers were washed with 1 M-HCl,
saturated aqueous NaHCO3, and brine, dried over anhydrous
Na2SO4, filtered, and concentrated in vacuo. The crude
diacetate thus obtained was used without further purification.
8: a colorless oil: IR (KBr) 1732 cm-1 1H NMR (270 MHz,
;
CDCl3) δ 1.42-1.54 (m, 4H), 1.73 (d, 3H, J ) 5.9 Hz), 2.04-
2.12 (m, 2H), 2.26-2.2.33 (m, 2H), 4.06 (s, 3H), 5.47-5.63 (m,
2H), 5.98-6.08 (m, 3H), 6.03 (s, 1H), 7.01 (dt, 1H, J ) 7.3,
15.2 Hz), 10.15 (s, 1H); EI-MS m/z 302 (M+); EI-HR-MS calcd
for C18H22O4 (M+) m/z 302.1518, found 302.1535.
6-[2-Hyd r oxy-7-[[(1,1-d im eth yleth yl)d im eth ylsilyl]oxy]-
h ep tyl]-4-m eth oxy-3-vin yl-2H-p yr a n -2-on e (29). To a so-
lution of pyrone 27 (1.62 g, 9.85 mmol) in THF (50 mL) was
added dropwise LHMDS (1.5 M hexane solution, 9.85 mL, 14.7
mmol) at -78 °C over 10 min. To this was added dropwise
the aldehyde 28 (4.85 g, 19.7 mmol) in THF (25 mL) over 20
min. After 70 min, additional 28 (2.42 g, 9.85 mmol) in THF
(12 mL) was added, and the mixture was stirred for 2 h before
saturated aqueous NH4Cl was added. The mixture was
extracted with EtOAc. The combined organic extracts were
washed with brine, dried over anhydrous Na2SO4, filtered, and
concentrated in vacuo. Purification by silica gel flash chro-
matography (CHCl3/EtOAc, 9:1) gave adduct 29 (3.29 g, 85%)
1
For the diacetate: a colorless oil, H NMR (270 MHz, CDCl3)
δ 0.02 (s, 6H), 0.87 (s, 9H), 1.30-1.70 (m, 4H), 2.03 (s, 6H),
3.57 (t, 2H, J ) 6.6 Hz), 3.88 (s, 3H), 4.94 (s, 2H), 6.08 (s, 1H),
5.16 (quintet, 1H, J ) 6.6 Hz), 6.82 (dt, 1H, J ) 6.6, 15.8 Hz).
To a solution of diacetate prepared above in CH2Cl2 (0.3 mL)
was added DBU (0.01 mL, 0.065 mmol) at 0 °C. After 40 min
of stirring at ambient temperature, the mixture was diluted
with 1 M HCl. The resultant mixture was extracted with
CHCl3. The combined organic layers were washed with
saturated aqueous NaHCO3 and brine, dried over anhydrous
Na2SO4, filtered, and concentrated in vacuo. Purification by
silica gel thin-layer chromatography (CHCl3/EtOAc, 15:1) gave
olefin 32 (7.6 mg, 81%) as a colorless oil: IR (KBr) 1735, 1698
as a colorless oil: IR (KBr) 3371, 1684 cm-1 1H NMR (270
;
MHz, CDCl3) δ 0.05 (s, 6H), 0.89 (s, 9H), 1.30-1.60 (m, 8H),
2.54 (dd, 1H, J ) 8.9, 14.9 Hz), 2.70 (dd, 1H, J ) 3.6, 14.5
Hz), 3.61 (t, 2H, J ) 6.3 Hz), 3.93 (s, 3H), 4.10 (m, 1H), 5.35
(dd, 1H, J ) 2.3, 11.9 Hz), 6.16 (s, 1H), 6.25 (dd, 1H, J ) 2.3,
17.8 Hz), 6.72 (dd, 1H, J ) 11.9, 17.8 Hz); EI-MS m/z 381 (M+
- CH3); EI-HR-MS calcd for C20H33O5Si (M+ - CH3) m/z
381.2097, found 381.2126.
3-F or m yl-6-[7-[[(1,1-d im eth yleth yl)d im eth ylsilyl]oxy]-
2-(h yd r oxyh ep tyl)]-4-m eth oxy-2H-p yr a n -2-on e (30). To
a solution of adduct 29 (120 mg, 0.293 mmol) in H2O/dioxane
(11 mL, 5:3) were added K2OsO4‚2H2O (1.1 mg, 0.003 mmol)
in water (0.02 mL) and N-methylmorpholine N-oxide (68 mg,
0.581 mmol). The mixture was stirred overnight at ambient
temperature before the mixture was quenched with 5% aque-
ous Na2S2O3. The resultant mixture was stirred for further
30 min. To this was added saturated aqueous NH4Cl, and the
resultant mixture was extracted with EtOAc. The combined
organic layers were washed with brine, dried over anhydrous
Na2SO4, filtered, and concentrated in vacuo. The crude diol
thus obtained was used without further purification.
1
cm-1; H NMR (270 MHz, CDCl3) δ 0.04 (s, 6H), 0.89 (s, 9H),
1.30-1.60 (m, 4H), 2.05 (s, 3H), 2.24 (ddt, 2H, J ) 1.3, 6.6,
7.9 Hz), 3.60 (t, 2H, J ) 5.9 Hz), 3.91 (s, 3H), 4.98 (s, 2H),
5.98 (s, 1H), 6.00 (dt, 1H, J ) 15.8, 1.3 Hz), 6.82 (dt, 1H, J )
15.8, 6.6 Hz); EI-MS m/z 424 (M+); EI-HR-MS calcd for
C
18H27O6Si (M+ - t-Bu) m/z 367.1577, found 367.1580.
3-(Acet oxym et h yl)-6-[(1E)-7-h yd r oxy-1-h ep t en yl]-4-
m eth oxy-2H-p yr a n -2-on e (33). PPTS (33.9 mg, 0.277 mmol)
was added to a solution of olefin 32 (454 mg, 1.07 mmol) in
MeOH (30 mL), and the mixture was stirred at ambient
temperature for 11 h. To this was added additional PPTS (33.9
mg, 0.277 mmol), and the resultant mixture was stirred for
another 24 h before the mixture was diluted with saturated
aqueous NaHCO3 and extracted with CHCl3. The combined
organic layers were washed with water and brine, dried over
anhydrous Na2SO4, filtered, and concentrated in vacuo. Pu-
rification by silica gel flash chromatography (CHCl3/MeOH,
20:1) gave alcohol 33 (302 mg, 91%) as a colorless oil: IR (KBr)
1
3378, 1735, 1684 cm-1; H NMR (270 MHz, CDCl3) δ 1.30-
To a solution of the diol prepared above in H2O/dioxane (11
mL, 5:3) was added sodium metaperiodate (94 mg, 0.439
mmol). The mixture was stirred at ambient temperature for
1.5 h and then extracted with EtOAc. The combined organic
layers were washed with brine, dried over anhydrous Na2SO4,
filtered, and concentrated in vacuo. Purification by silica gel
flash chromatography (CHCl3/MeOH, 20:1) gave formylpyrone
30 (108 mg, 2 steps, 88%) as a colorless oil: IR (KBr) 3373,
160 (m, 4H), 2.04 (s, 3H), 2.25 (ddt, 2H, J ) 1.3, 6.6, 7.3 Hz),
3.64 (t, 2H, J ) 5.9 Hz), 3.91 (s, 3H), 4.97 (s, 2H), 5.99 (s, 1H),
6.00 (dt, 1H, J ) 15.8, 1.3 Hz), 6.80 (dt, 1H, J ) 15.8, 6.6 Hz);
EI-MS m/z 310 (M+); EI-HR-MS calcd for C16H22O6 (M+) m/z
310.1417, found 310.1425.
3-(Acetoxym eth yl)-6-[(1E)-6-for m yl-1-h exen yl]-4-m eth -
oxy-2H-p yr a n -2-on e (34). To a solution of alcohol 33 (10.8
mg, 34.8 µmol) in CH2Cl2 (1 mL) was added Dess-Martin
periodinane (14.8 mg, 34.8 µmol) with stirring at ambient
temperature for 1 h. To this was added additional Dess-
Martin periodinane (2.8 mg, 6.6 µmol). The resultant mixture
was directly purified by silica gel chromatography (CHCl3/
MeOH, 20:1) to give aldehyde 34 (8.9 mg, 83%) as a colorless
1
1733, 1685 cm-1; H NMR (270 MHz, CDCl3) δ 0.05 (s, 6H),
0.88 (s, 9H), 1.30-1.70 (m, 8H), 2.59 (dd, 1H, J ) 9.2, 14.5
Hz), 2.74 (dd, 1H, J ) 2.6, 14.5 Hz), 3.61 (t, 2H, J ) 5.9 Hz),
4.06 (s, 3H), 4.10 (m, 1H), 6.25 (s, 1H), 10.1 (s, 1H); EI-MS
m/z 365 (M+ - (t-Bu + H2O)); EI-HR-MS calcd for C16H23O5Si
(M+ - (t-Bu - H2O)) m/z 323.1315, found 323.1343.
1
oil: IR (CHCl3) 1717, 1684 cm-1; H NMR (270 MHz, CDCl3)
3-(Hyd r oxym eth yl)-6-[7-[[(1,1-d im eth yleth yl)d im eth yl-
silyl]oxy]-2-(h yd r oxyh ep t yl)]-4-m et h oxy-2 H -p yr a n -2-
on e (31). To a solution of formylpyrone 30 (58 mg, 0.141
mmol) in MeOH (3 mL) was added NaBH4 (6.2 mg, 0.164
mmol) at 0 °C. The mixture was stirred at ambient temper-
ature for 30 min. To this was added acetone (0.3 mL), and
the resultant mixture was diluted with brine and extracted
with EtOAc. The combined organic layers were washed with
water, brine, dried over anhydrous Na2SO4, filtered, and
concentrated in vacuo. Purification by silica gel flash chro-
matography (CHCl3/MeOH, 20:1) gave diol 31 (53 mg, 91%)
δ 1.44-1.54 (m, 4H), 2.04 (s, 3H), 2.27 (ddt, 2H, J ) 1.3, 7.3,
5.9 Hz), 2.47 (dt, 2H, J ) 1.3, 7.3 Hz), 3.92 (s, 3H), 4.98 (s,
2H), 6.00 (s, 1H), 6.02 (dt, 1H, J ) 15.8, 1.3 Hz), 6.79 (dt, 1H,
J ) 15.8, 7.3 Hz), 9.77 (t, 1H, J ) 1.3 Hz); EI-MS m/z 308
(M+); EI-HR-MS calcd for C16H20O6 (M+) m/z 308.1250, found
308.1243.
3-(Acet oxym et h yl)-6-[(1E,7E)-8-iod o-1,7-oct a d ieyl]-4-
m eth oxy-2H-p yr a n -2-on e (35). To a suspension of CrCl2-
(II) (36.3 mg, 20.3 µmol) in THF (0.03 mL) were added
aldehyde 34 (9.1 mg, 29.5 µmol) and iodoform (36.3 mg, 92.1
µmol) in THF (0.07 mL) at 0 °C. The mixture was stirred for
4 h before the reaction was quenched with saturated NaHCO3
and extracted with CHCl3. The combined organic layers were
washed with brine, dried over anhydrous Na2SO4, filtered, and
concentrated in vacuo. Purification by preparative thin layer
1
as a colorless oil: IR (NaCl) 3389, 1716 cm-1; H NMR (270
MHz, CDCl3) δ 0.02 (s, 6H), 0.87 (s, 9H), 1.30-1.70 (m, 4H),
2.03 (s, 6H), 2.74 (d, 2H, J ) 5.9 Hz), 3.57 (t, 2H, J ) 6.6 Hz),
3.88 (s, 3H), 4.94 (s, 2H), 6.08 (s, 1H), 5.16 (quintet, 1H, J )

8756 J . Org. Chem., Vol. 63, No. 24, 1998
Oikawa et al.
silica gel chromatography (CHCl3/EtOAc, 4:1) gave vinyl iodide
determined as follows. After the concentration of the isolated
products was determined from the absorption at 320 nm (ꢀ )
7.29 × 103, 1, ꢀ ) 7.73 × 103, 2), the optical purities were
calculated by comparison between the peak areas of the
products in the CD spectra and those of enantiomerically pure
natural solanapyrones with predefined concentration as fol-
lows: (-)-1 >98% ee, CD λmax/nm (EtOH) 320 ([θ] -7474;
natural [θ] -7324); (-)-2 67% ee, CD λmax/nm (EtOH) 320 ([θ]
-1946; natural -2906).
35 (7.3 mg, 57%, E/Z ) 4:1) and the starting material 34 (4.2
mg, 19%). For 35: a colorless oil; IR (CHCl3) 1734, 1717 cm-1
;
1H NMR (270 MHz, CDCl3) (7E-isomer) δ 1.40-1.50 (m, 4H),
2.04 (s, 3H), 2.00-2.10 (m, 2H), 2.24 (m, 2H), 3.92 (s, 3H), 4.98
(s, 2H), 5.99 (s, 1H), 5.90-6.00 (m, 1H), 6.49 (dt, 1H, J ) 14.5,
7.3 Hz), 6.79 (dt, 1H, J ) 15.8, 6.6 Hz); EI-MS m/z 432 (M+);
EI-HR-MS calcd for C17H21O5I (M+) m/z 432.0434, found
432.0466.
3-(Acetoxym eth yl)-4-m eth oxy-6-[(1E,7E,9E)-1,7,9-u n d e-
ca tr ien yl]-2H-p yr a n -2-on e (36). To bis(acetonitrile)dichlo-
ropalladium(II) (0.5 mg, 1.3 µmol) were added 1-(tributylstan-
nyl)propene (16.7 mg, 50.0 µmol) in DMF (0.1 mL) and vinyl
iodide 35 (10.9 mg, 25.2 µmol) in DMF (0.1 mL). The mixture
was stirred at ambient temperature for 10 min and then
filtered through a pad of Celite. The filtrates were diluted
with water and extracted with CHCl3. The combined organic
layers were washed with brine, dried over anhydrous Na2SO4,
filtered, and concentrated in vacuo. Purification by prepara-
tive thin-layer silica gel chromatography (CHCl3/ EtOAc, 6:1)
gave triene 36 (5.0 mg, 57%) as a colorless oil: IR (NaCl) 1717
cm-1; 1H NMR (270 MHz, CDCl3) (7E-isomer) δ 1.40-1.50 (m,
4H), 1.73 (d, 3H, J ) 6.6 Hz), 2.05 (s, 3H), 2.00-2.10 (m, 2H),
2.24 (m, 2H), 3.92 (s, 3H), 4.99 (s, 2H), 5.40-5.60 (m, 2H), 5.98
(s, 1H), 5.90-6.10 (m, 3H), 6.81 (dt, 1H, J ) 15.8, 6.6 Hz);
EI-MS m/z 346 (M+); EI-HR-MS calcd for C20H26O5 (M+) m/z
346.1780, found 346.1754.
P r osola n a p yr on e II (7). To a solution of triene 36 (3.9
mg, 11 µmol) in MeOH (0.5 mL) was added K2CO3 (20.0 mg,
0.145 mmol). After 50 min of stirring at ambient temperature,
the mixture was diluted with water and extracted with EtOAc.
The combined organic layers were washed with brine, dried
over anhydrous Na2SO4, filtered, and concentrated in vacuo.
Purification by silica gel flash chromatography (CHCl3/EtOAc,
2:1) gave 7 (2.4 mg, 70%) and 36 (0.3 mg, 8%) as a colorless
oil.
HP LC An a lysis of Non en zym a tic Diels-Ald er Rea c-
tion s of P r osola n a p yr on es. Prosolanapyrone I (6) was
diluted with solvents indicated in Table 2 at a concentration
of 1 mg/mL. The glass tubes were sealed and heated at
indicated temperature over the indicated time (Table 2).
Similar experiments for prosolanapyrones II (7) and III (8)
were carried out. An aliquot of each solution was analyzed
by HPLC: condition (1) Wakosil-5Sil (φ6 × 250 mm, Wako),
UV 320 nm, hexane/ EtOAc, 9:1, flow rate 1 mL/min, tR (min)
1 9.7, 2 12.1, 8 15.2; 3 13.2, 4 14.8, 7 19.5; condition (2) Inertsil
ODS-2 (φ4.6 × 250 mm, GL science), i-PrOH/CH3CN/H2O, 3:2:
5, flow rate 0.85 mL/min, tR (min) deoxysolanapyrone (exo)
23.3, (endo) 24.8, 6 27.5. The values of peak areas were
corrected by calibration.
En zym a tic Con ver sion of P r osola n a p yr on e II (7). To
a 50 mM potassium phosphate buffer (pH 7.0, 15 mL) contain-
ing 30% glycerol, the enzyme preparation (5.1 mL) was added.
After preincubation at 30 °C for 5 min, a solution of 7 (4.8
mg, 15 µmol) in ethanol (1.5 mL) was added, and the assay
mixture was vortexed and then incubated at 30 °C for 2 h.
The reaction mixture was passed through a Diaion HP-20
column (3 mL). The column was washed with water, and then
adsorbed materials were eluted with EtOAc. The eluent was
washed with brine, dried over anhydrous Na2SO4, filtered, and
concentrated in vacuo. Purification by silica gel thin-layer
chromatography (CHCl3/EtOAc, 2:1) gave 1 (2.4 mg, 51%), 2
(0.48 mg, 10%), and recovered starting material 8 (0.50 mg,
10%). The optical purities of the reaction products were
(()-Sola n a p yr on e B (3). The sealed tube containing 7 (7.5
mg, 24.5 µmol) in benzene (25 mL) was heated at 140 °C for
24 h. After cooling to ambient temperature, the reaction
solution was concentrated in vacuo. Purification by silica gel
thin-layer chromatography (CHCl3/EtOAc, 6:1, multiple de-
velopments) gave (()- 3 (1.9 mg, 26%) and (()-4 (4.9 mg, 66%)
as colorless oils.
En zym a tic Con ver sion of (-)- a n d (()-Sola n a p yr on e
B (3). To a 50 mM potassium phosphate buffer (pH 7.0, 0.6
mL) containing 30% glycerol, the enzyme preparation (13.5
µL) was added. After preincubation for 5 min at 30 °C, a
solution of (-)-3 in ethanol (17.4 mM, 0.15 mmol, 8.6 µL) was
added to the mixture, and the assay mixture was vortexed and
then incubated at 30 °C. At the indicated time (Figure 3), the
reaction solution (150 µL) was taken and extracted with
EtOAc. To the reaction solutions, 5 µL of 1.25 mM ethyl
acetate solution of coumarin was added as an internal stan-
dard. An aliquot of the solutions was analyzed by normal
phase HPLC [Wakosil-5Sil (φ6 × 250 mm, Wako), UV 320 nm,
CHCl3/ EtOAc, 2:3, flow rate 1 mL/min, tR (min) coumarin 6.7,
1 8.5, 3 11.8]. Compound (()-3 was also incubated, and the
reaction products were analyzed in a similar way.
Kin etic Resolu tion of (()-Sola n a p yr on e B (3). After
removing glycerol with gel filtration column, the enzyme
preparation (90 µL) was added to a 50 mM potassium
phosphate buffer (pH 7.0, 5 mL). After preincubation for 5
min at 30 °C, (()-3 in ethanol (0.31 mg, 1 µmol, 0.24 mL) was
added to the mixture and the assay mixture was vortexed and
then incubated at 30 °C. After 2 h, the reaction was quenched
by addition of EtOAc and vortexed. The organic layer was
separated and dried over anhydrous Na2SO4, filtered, and
concentrated in vacuo. Purification by silica gel thin-layer
chromatography (CHCl3/EtOAc, 7:1) gave 1 (0.17 mg, 55%) and
3 (0.06 mg, 18%). After concentrations of the isolated products
were determined (ꢀ ) 2.79 × 103, 3 at 320 nm), the optical
purity of the reaction products were determined as described:
(-)-1 27% ee, CD λmax/nm (EtOH) 320 ([θ] -1993); (+)-3 91%
ee, CD λmax/nm (EtOH) 320 ([θ] +4050; natural -4450)
Ack n ow led gm en t. We are grateful to Prof. M.
Honma, in our department for his technical advice on
handling the enzyme. This work was supported by a
Grant-in-Aids from the Ministry of Education, Science
and Culture of J apan, and partly by a grant from
Suntory Institute for Bioorganic Research.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
compounds 6-8, 10-12, 18, 23a ,d , and 29-36 (17 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, and
can be ordered from the ACS; see any current masthead page
for ordering information.
J O980743R