A divergent strategy for the synthesis of secologanin derived natural products

Article abstract of DOI:10.1021/jo101775n

The syntheses of d,l-geissoschizol, d,l-corynantheidol, d,l-dihydrocorynantheol, d,l-protoemetinol, and d,l-3-epi-protoemetinol have been accomplished from a single synthetic intermediate.

Full text of DOI:10.1021/jo101775n

pubs.acs.org/joc
A Divergent Strategy for the Synthesis of Secologanin Derived
Natural Products
Brandon J. English† and Robert M. Williams*,†,‡
†Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States , and
‡University of Colorado Cancer Center, Aurora, Colorado 80045, United States
rmw@lamar.colostate.edu
Received September 20, 2010
The syntheses of D,L-geissoschizol, D,L-corynantheidol, D,L-dihydrocorynantheol, D,L-protoemetinol,
and D,L-3-epi-protoemetinol have been accomplished from a single synthetic intermediate.
Introduction
antiallergenic,5 antibacterial,6 and antiviral7 activities. Stric-
tosidine synthase catalyzes the coupling of secologanin
glucoside with tryptamine (2) to produce strictosidine (3),8
which is the common precursor of more than 250 structurally
diverse naturally occurring alkaloids including important
therapeutic agents such as quinine, vinblastine, and reserpine.
Additional classes of natural products arise from the coupling of
secologanin glucoside with dopamine (4) producing the tetra-
hydroisoquinoline deacetylipecoside (5).9 Modification of 5
gives rise to alkaloids such as the antitumor agent tubulosine10
and emetine, which displays antiprotozoic11 activity and may be
useful in the treatment of lymphatic leukemia (Scheme 1).12
Synthetic access to the secoiridoids and the secologanin
alkaloid derivatives has traditionally been pursued on a case-
by-case basis by developing novel approaches for a small
number of structurally similar synthetic targets. The Cook13
The densely functionalized monoterpene secologanin glu-
coside (1) has been shown to be the common biosynthetic
precursor of several structurally diverse classes of natural
products.1 The simple secoiridoids arise from minor mod-
ifications to the secologanin core and have been reported
to display analgesic,2 anti-inflammatory,3 antiarthritic,4
*To whom correspondence should be addressed. Phone: þ1 970 491 6747.
Fax: þ1 970 491 3944.
€
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DOI: 10.1021/jo101775n
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Published on Web 10/22/2010
J. Org. Chem. 2010, 75, 7869–7876 7869
2010 American Chemical Society

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JOCArticle
SCHEME 1. Biosynthesis of Secologanin-Derived Natural Products
SCHEME 2. Previous Conversion of 6 to Oleocanthal (7)
and Martin14 laboratories have pursued more general synthetic
strategies allowing access to a broad selection of secologanin
tryptamine alkaloids via a single synthetic strategy. Although
highly efficient and productive, these strategies confine their
synthetic scope to the tryptamine-derived alkaloids. We have
envisioned a more general strategy involving the large-scale
synthesis of a functionalized intermediate, which can be rapidly
modified to allow access to the simple secoiridoids as well as the
tryptamine- and dopamine-derived alkaloids. We recently
demonstrated that lactone 6 was a viable intermediate for the
synthesis of the simple secoiridoid oleocanthal (7) (Scheme2).15
Herein we wish to report the use of lactone 6 as a key
intermediate in the synthesis of the secologanin tryptamine
alkaloids D,L-geissoschizol (8), D,L-corynantheidol (9), and
D,L-dihydrocorynantheol (10) as well as the secologanin dopa-
mine alkaloids D,L-3-epi-protoemetinol (11) and D,L-protoeme-
tinol (12) (Figure 1).
FIGURE 1. Target natural products.
FIGURE 2. Retrosynthetic analysis.
with a similar system.16 We envisioned the requisite lactam ring
of 14 arising from the cyclization of the methyl ester moiety and
the secondary amine of a compound of type 15. This secondary
amine would be produced through the reductive amination of
an appropriately functionalized aldehyde (16) and tryptamine
or a dopamine-derived primary amine. The desired oxidation
state and relative stereochemistry of the ethyl side chain could be
established via selective reduction of the olefin of lactone 6
followed by the reduction of the resulting lactone.
Figure 2 illustrates our retrosynthetic strategy. We
planned to form the arylpiperidine rings of the target alkaloids
(13) via late-stage Bischler-Napieralski cyclization of a hydrox-
yl-protected lactam (14). We predicted that this cyclization
would proceed with complete cis-diastereoselectivity based on
observations made by Martin and co-workers while working
(14) (a) Martin, S. F.; Benage, B.; Hunter, J. E. J. Am. Chem. Soc. 1988,
110, 5925–5927. (b) Martin, S. F.; Benage, B.; Geraci, L. S.; Hunter, J. E.;
Mortimore, M. J. Am. Chem. Soc. 1991, 113, 6161–6171. (c) Martin, S. F.;
Chen, K. X.; Eary, C. T. Org. Lett. 1999, 1, 79–81. (d) Deiters, A.; Chen, K.;
Eary, T.; Martin, S. F. J. Am. Chem. Soc. 2003, 125, 4541–4550.
(15) English, B. J.; Williams, R. M. Tetrahedron Lett. 2009, 50 (23), 2713–
2715.
Results and Discussion
Scheme 3 illustrates our synthesis of D,L-geissoschizol (8).
Reduction of the lactone carbonyl of 6 followed by reductive
amination of the crude aldehyde/lactol mixture both success-
fully merged the tryptamine and secoiridoid portions of the
(16) Deiters, A.; Pettersson, M.; Martin, S. F. J. Org. Chem. 2006, 71,
6547–6561.
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SCHEME 3. Synthesis of Geissoschizol (8)
SCHEME 5. Synthesis of Corynantheidol (9)
SCHEME 6. Synthesis of Dihydrocorynantheol (9)
SCHEME 4. Selective Reduction of Lactone 6
molecule as well as forming the lactam ring of intermediate18,
presumably via amine 17, which was not isolated. Protection
ofthehydroxylfunctionalityof18asthe acetateesterfollowed
byBischler-Napieralski cyclization gave acetate19 asa single
isomer, which was deprotected to yield D,L-geissoschizol (8) in
good yield.
Next, we turned our attention to the alkaloids bearing
reduced ethyl side chains, corynantheidol (9) and dihydro-
corynantheol (10). As part of their 1992 synthesis of these
and other related alkaloids, Lounasmaa and co-workers
reported the successful hydrogenation of several geissoschi-
zol isomers.17 Unfortunately, poor diastereoselectivity was
observed for these reductions regardless of the geissoschizol
isomer or reduction method employed. These observations
lead us to pursue a strategy, which introduces the C3 stereo-
genic centers earlier in the synthesis. We speculated that
either of the desired ethyl side chain stereochemistries could
be accessed via selective reduction of lactone 6. As illustrated
in Scheme 4, catalytic hydrogenation of the olefin of lactone
6 produces exclusively the cis-isomer of the reduced lactone
(21). This selectivity is presumably due to the delivery of
hydrogen to the least sterically encumbered face of the olefin.
Conjugate reduction, conversely, produces exclusively the
thermodynamically favored trans-isomer of the reduced
lactone (20). With the ability to selectively produce lactones
20 and 21 we turned our attention to the completion of some
representative natural alkaloid syntheses.
gave tetracycle 24 in good yield and as a single isomer. Removal
of the alcohol protecting group with PPTS/MeOH gave
D,L-corynantheidol (9) in excellent yield.
Our attempts to access dihydrocorynantheol (10) began
via employment of an identical method to that used for its
epimer corynantheidol (Scheme 6). Reduction of lactone 6
gave a mixture of lactol isomers (25), which was immediately
subjected to reductive amination conditions producing the
desired lactam (26) and its C3 epimer (23) as a minor
product. Protection of the alcohol and Bischler-Napieralski
cyclization of this mixture gave a mixture of two tetracyclic
epimers (24 and 27) that were readily separated by silica gel
flash chromatography. Separate deprotection of 27 and 24
gave D,L-dihydrocorynantheol (10) and D,L-corynantheidol
(9) respectfully.
With control of the secoiridoid portion of the secologanin-
derived alkaloids established, we began our investigation of
the secoiridoids derived from dopamine. Fortunately very
little modification to the procedures developed was necessary
for this purpose. Thus, substituting 2-(3,4-dimethoxyphenyl)-
ethanamine (29) for tryptamine and adjusting Bischler-
Napieralski reaction temperature to compensate for a slightly
less reactive aromatic nucleophile, the synthesis of D,L-3-epi-
protoemetinol (11) was accomplished by the procedure devel-
oped for the synthesis of D,L-corynantheidol (9) (Scheme 7)
and D,L-protoemetinol (12) was synthesized by the procedure
developed for the synthesis of D,L-dihydrocorynantheol (10)
(Scheme 8).
Following a similar protocol to that deployed for our synthesis
of geissoschizol, our synthesis of corynantheidol (9) (Scheme 5)
began with the DIBAL-H reduction of lactone 20 followed by a
one-step reductive amination/lactam cyclization to give alcohol
23. Alcohol protection and Bischler-Napieralski cyclization
(17) Louansmaa, M.; Jokela, R.; Tirkkonen, B.; Miettinen, J.; Halonen,
M. Heterocycles 1992, 34 (2), 321–339.
J. Org. Chem. Vol. 75, No. 22, 2010 7871

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SCHEME 7. Synthesis of 3-epi-Protoemetinol (11)
round-bottomed flask containing 248 mg (1.25 mmol, 1 equiv) of
lactone 6 was added 20 mL of dry THF and this solution was
cooled to -78 °C. To this solution was dropwise added 1.50 mL
(1.2 equiv) of a 1.0 M solution of DIBAl-H in THF and the
reaction was allowed to stir for 60 min at this temperature before
the dropwise addition of 2 mL of dry MeOH and warming to
ambient temperature. To this reaction mixture was added 25 mL
of a saturated aqueous solution of Rochelle’s salt and the mixture
was stirred for 30 min at ambient temperature. This mixture was
extracted three times with EtOAc, dried over Na2SO4, and con-
centrated to yield a colorless oil, which was immediately used
without purification.
To the above produced residue was added 360 mg (2.25 mmol,
1.5 equiv) of tryptamine, 15 mL of dry THF, and then 954 mg (4.50
mmol, 3 equiv) of NaBH(OAc)3. After being stirred at ambient
temperature for 48 h the reaction was added to NaHCO3(sat.) and
extracted three times with CH2Cl2. Combined organic layers were
dried over Na2SO4 and concentrated to afford a brown oil that was
purified by silica gel chromatography eluting with 5% to 20%
MeOH in EtOAc to yield 220 mg (56%, 2 steps) of compound 18 as
a white foam.
1H NMR (300 MHz, CDCl3) δ 9.00 (br s, 1H), 7.62 (d, J = 7.8
Hz, 1H), 7.33 (d, J = 8.1 Hz, 1H) 7.13 (m, 2H), 6.97 (s, 1H), 5.28
(q, J = 5.7 Hz, 1H), 3.44-3.82 (m, 6H), 3.04 (q, J = 7.5 Hz, 2H),
2.97 (m, 1H), 2.26-2.56 (m, 2H), 1.57 (dd, J = 6.6, 1.5 Hz, 3H),
1.40-1.56 (m, 2H); 13C NMR (75 MHz, CDCl3) δ 169.7, 136.4,
132.6, 127.4, 122.5, 121.7, 120.8, 119.1, 118.6, 112.3, 111.4, 60.3, 52.5,
SCHEME 8. Synthesis of Protoemetinol (12)
47.5, 37.9, 35.2, 29.9, 22.9, 12.8; IR (NaCl, film) 3289, 1620 cm-1
;
HRMS (þTOF) [M þ H]þ 313.1911 calcd for C19H25N2O2, found
313.1916; Rf 0.18 (5% MeOH in EtOAc).
Synthesis of (E)-2-(1-(2-(1H-Indol-3-yl)ethyl)-5-ethylidene-2-
oxopiperidin-4-yl)ethyl Acetate. To a 10-mL round-bottomed
flask containing 65.0 mg (0.208 mmol, 1 equiv) of alcohol 18
dissolved in 1 mL of dry CH2Cl2 and 1 mL of dry pyridine was
added 30.0 μL (0.416 mmol, 2 equiv) of AcCl followed by a
single crystal of DMAP. The reaction was stirred at ambient
temperature for 1 h before being added to NaHCO3(sat.), and
extracted three times with CH2Cl2, dried over Na2SO4, and
concentrated. The crude residue was purified by silica gel
chromatography eluting with 1% to 10% MeOH in CH2Cl2 to
yield 70 mg (95%) of the desired product as a pale yellow oil.
1H NMR (300 MHz, CDCl3) δ 9.01 (br s, 1H), 7.63 (d, J = 7.8
Hz, 1H), 7.32 (d, J = 8.1 Hz, 1H), 7.15 (t, J = 6.6 Hz, 1H), 7.08
(t, J = 7.2 Hz, 1H), 6.97 (s, 1H), 5.33 (q, J = 6.9 Hz, 1H),
4.01-3.46 (m, 6H), 3.04 (t, J = 7.5 Hz, 2H), 2.97 (m, 1H), 2.50
(dd, J = 17.1, 5.7 Hz, 1H), 2.39 (dd, J = 16.8, 2.1 Hz, 1H), 2.01
(s, 3H), 1.63 (m, 2H), 1.56 (dd, J = 6.9, 1.5 Hz, 3H); 13C NMR
(75 MHz, CDCl3) δ 171.3, 169.2, 136.6, 132.0, 127.6, 122.6,
122.0, 121.8, 119.3, 118.8, 112.6, 111.6, 62.5, 52.6, 47.9, 38.3,
31.3, 30.3, 23.2, 21.2, 12.9; IR (NaCl, film) 1736, 1628 cm-1
;
HRMS (þTOF) [M þ H]þ 355.2016 calcd for C21H27N2O3,
found 355.2014; Rf 0.27 (5% MeOH in CH2Cl2).
Synthesis of 2-((2R,12bS,E)-3-Ethylidene-1,2,3,4,6,7,12,12b-
octahydroindolo[2,3-a]quinolizin-2-yl)ethyl Acetate (19). To 192
mg (0.542 mmol, 1 equiv) of the above produced acetate
dissolved in 5 mL of dry benzene was added 100 μL (1.08 mmol,
2 equiv) of freshly distilled POCl3. The reaction was heated to
reflux for 1.5 h before being concentrated under reduced pres-
sure. The resulting residue was taken up in 5 mL of dry MeOH
and cooled to 0 °C before 50 mg of NaBH4 was added and the
reaction was removed from the ice bath and stirred for 15 min.
The reaction was then added to 0.5 M NaOH, extracted three
times with CH2Cl2, dried over Na2SO4, and concentrated. The
resulting residue was purified by silica gel chromatography
eluting with 5% MeOH in EtOAc to provide 119 mg (65%) of
the desired product as a pale yellow foam.
The syntheses described herein of the tryptamine-derived
secologanin alkaloids geissoschizol, corynantheidol, and dihy-
drocorynantheol and the dopamine-derived secologanin alka-
loids protoemetinol and 3-epi-protoemetinol taken in context
with our previously reported synthesis of oleocanthal demon-
strate the potentially broad utility of lactone 6. The careful
manipulation of lactone 6 delivered rapid access to members
of, to date, three very distinct classes of secologanin-derived
natural products. Current efforts to produce lactone 6 in an
enantioselective manner are under investigation.
Experimental Section
Synthesis of (E)-1-(2-(1H-Indol-3-yl)ethyl)-5-ethylidene-4-(2-
hydroxyethyl)piperidin-2-one (18). To a flame-dried 50-mL
1H NMR (300 MHz, CDCl3) δ 8.40 (br s, 1H), 7.45 (d, J = 7.2
Hz, 1H), 7.34 (d, J= 7.2 Hz, 1H), 7.09 (m, 2H), 5.51 (q, J=6.9Hz,
7872 J. Org. Chem. Vol. 75, No. 22, 2010

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1H), 4.24-3.86 (m, 2H), 3.73 (d, J = 10.5 Hz, 1H), 3.04 (m, 3H),
2.64 (m, 3H), 2.08 (s, 3H), 2.05-1.70 (m, 5H), 1.58 (d, J = 6.9 Hz,
3H); 13C NMR (75 MHz, CDCl3) δ 171.5, 136.3, 135.5, 135.0,
127.5, 122.1, 121.4, 119.4, 118.2, 111.1, 108.5, 62.9, 60.1, 55.4, 53.0,
35.3, 31.0, 30.7, 21.9, 21.3, 12.9; IR (NaCl, film) 1735 cm-1; HRMS
(þTOF) [M þ H]þ 339.2073 calcd for C21H27N2O2, found
339.2072; Rf 0.45 (5% MeOH in EtOAc).
over Na2SO4, and concentrated. The resulting residue contain-
ing crude 23 was carried forward without purification.
To the crude alcohol (23) prepared above in a 50-mL round-
bottomed flask was added 22.7 mL of dry CH2Cl2 followed by
411 mg (2.72 mmol, 1.2 equiv) of TBSCl and then 309 mg (4.54
mmol, 2 equiv) of imidazole. The reaction was allowed to stir at
ambient temperature for 12 h before being added to brine,
extracted into CH2Cl2, dried over Na2SO4, and concentrated.
The resultant residue was purified by silica gel flash chroma-
tography eluting with 1:1 hex./EtOAc to yield 720 mg (67%, 3
steps) of the desired O-TBS ether derivative of 23 as a tan foam.
1H NMR (300 MHz, CDCl3) δ 8.94 (br s, 1H), 7.66 (d, J = 7.5
Hz, 1H), 7.34 (d, J = 7.5 Hz, 1H), 7.17 (t, J = 7.2 Hz, 1H), 7.10
(t, J = 7.2 Hz, 1H), 6.98 (s, 1H), 3.65 (m, 4H), 3.13 (dd, J = 12.3,
5.1 Hz, 1H), 3.03 (m, 3H), 2.37 (qd, J = 17.7, 6.0 Hz, 2H), 2.07
(m, 1H), 1.68 (m, 1H), 1.54 (m, 1H), 1.14-1.34 (m, 3H), 0.90 (s,
9H), 0.83 (t, J = 7.2 Hz, 3H), 0.05 (s, 6H); 13C NMR (75 MHz,
CDCl3) δ 169.5, 136.4, 127.4, 122.3, 121.7, 119.0, 118.6, 112.5,
111.4, 60.6, 50.6, 48.3, 38.3, 36.4, 31.5, 31.1, 26.0, 23.0, 20.5,
18.3, 11.8, -5.3; IR (NaCl, film) 3260, 1622 cm-1; HRMS
(þTOF) [M þ H]þ 429.2932 calcd for C25H41N2O2Si, found
429.2937; Rf 0.36 (1:1 hex./EtOAc)
Synthesis of Tetracycle 24. To 160 mg (0.373 mmol, 1 equiv) of
the above-produced O-TBS ether derivative of 23 dissolved in
4 mL of dry MeCN was added 1.21 mL (14.9 mmol, 40 equiv) of
dry pyridine followed by 278 μL (2.98 mmol, 8 equiv) of freshly
distilled POCl3. The reaction was stirred for 16 h at 40 °C and
concentrated under reduced pressure. To the resultant residue
was added 4 mL of dry MeOH. This solution was cooled to 0 °C,
142 mg (3.73 mmol, 10 equiv) of NaBH4 was added, and the
reaction was stirred at 0 °C for 15 min before being added to
NaHCO3(sat.), extracted into CH2Cl2, dried over Na2SO4, and con-
centrated. Purification by silica gel flash chromatography eluting
with 2:1 hex./EtOAc yielded 107 mg (69%) of the desired product
(24) as a pale yellow oil.
Synthesis of (()-Geissoshizol (8). To a 10-mL round-
bottomed flask containing 50.0 mg (0.148 mmol, 1 equiv) of
acetate 19 dissolved in 1 mL of MeOH and 0.5 mL of H2O was
added 245 mg (1.77 mmol, 12 equiv) of K2CO3. The reaction was
stirred at ambient temperature for 2 h, added to brine, extracted
three times with CH2Cl2, dried over Na2SO4, and concentrated.
The resulting oil was purified by silica gel chromatography
eluting with 10% MeOH in CH2Cl2 to yield 39.0 mg (89%) of
the desired product as a white solid.
1H NMR (400 MHz, CDCl3) δ 7.91 (br s, 1H), 7.44 (d, J = 7.6
Hz, 1H), 7.32 (d, J = 7.6 Hz, 1H), 7.15-7.06 (m, 2H), 5.51 (q,
J = 6.8 Hz, 1H), 3.67 (m, 3H), 3.25-2.97 (m, 5H), 2.72 (m, 1H),
2.62 (m, 1H), 2.08-1.46 (m, 5H), 1.62 (d, J = 6.8 Hz, 3H); 13
C
NMR (75 MHz, CDCl3) δ 136.4, 136.2, 134.8, 127.5, 121.7,
121.4, 119.5, 118.2, 110.9, 108.6, 61.3, 60.1, 55.3, 52.8, 35.4, 35.2,
31.0, 21.8, 12.9; IR (NaCl, film) 3233, 2851, 2790, 2745 cm-1
;
HRMS (þTOF) [M þ H]þ 297.1961 calcd for C19H25N2O, found
297.1961; Rf 0.14 (5% MeOH in EtOAc). Spectroscopic properties
agree in all respects with those previously reported.13e,18
Synthesis of Lactone 20. To a 250-mL round-bottomed flask
contining 1.10 g (5.55 mmol, 1 equiv) of lactone 6 dissolved in
56 mL of dry THF at -78 °C was added 6.10 mL (1.1 equiv) of a
1.0 M solution of L-Selectride in THF. The reaction was allowed
to stir at -78 °C for 60 min and was then added to NH4Cl(sat.)
,
extracted three times with EtOAc, dried over Na2SO4, concen-
trated, and purified by silica gel chromatography eluting with
2:1 hex./EtOAc to yield 918 mg (83%) of the title compound as a
colorless oil.
1H NMR (300 MHz, CDCl3) δ 4.26 (m, 2H), 3.66 (s, 3H), 2.50
(m, 2H), 2.26 (m, 2H), 2.04 (m, 1H), 1.87 (m, 1H), 1.65 (m, 2H),
0.94 (t, J = 7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3) major
conformer: δ 173.1, 172.2, 67.1, 51.9, 46.6, 38.7, 31.8, 28.4, 22.7,
10.9; IR (NaCl, film) 1732 cm-1; HRMS (þTOF) [M þ H]þ
201.1121 calcd for C10H17O4, found 201.1123; Rf 0.52 (1:1 hex./
EtOAc)
1H NMR (300 MHz, CDCl3) δ 7.82 (br s, 1H), 7.47 (d, J = 7.2
Hz, 1H), 7.30 (d, J = 7.2 Hz, 1H), 7.11 (m, 2H), 3.72 (t, J = 6.6
Hz, 2H), 3.24 (m, 1H), 3.01 (m, 3H), 2.68 (m, 1H), 2.38 (d, J =
11.4 Hz, 1H), 1.88 (m, 2H), 1.53 (m, 4H), 1.27 (m, 3H), 0.95 (s,
9H), 0.91 (m, 3H), 0.11 (s, 6H); 13C NMR (75 MHz, CDCl3) δ
136.0, 135.4, 127.6, 121.3, 119.4, 118.2, 110.9, 108.1, 66.7, 61.5,
60.6, 53.5, 39.7, 36.5, 36.4, 32.2, 29.8, 26.2, 18.5, 17.9, 12.8, -5.1;
IR (NaCl, film) 2796, 2747 cm-1; HRMS (þTOF) [M þ H]þ
413.2951 calcd for C25H41N2OSi, found 413.2948; Rf 0.46 (4:1
hex./EtOAc).
Synthesis of (()-Corynantheidol (9). To a 25-mL round-
bottomed flask containing 40 mg (0.097 mmol, 1 equiv) of
tetracycle 24 was added 1 mL of MeOH followed by 49 mg
(0.19 mmol, 2 equiv) of PPTS. The reaction was stirred 12 h at
ambient temperature, added to 1 N NaOH, extracted twice with
CH2Cl2, dried over Na2SO4, and concentrated. The resulting
residue was purified by silica gel flash chromatography eluting
with 5% to 20% MeOH in CH2Cl2 to yield 26 mg (90%) of
corynantheidol (9) as a white foam.
Synthesis of 1-(2-(1H-Indol-3-yl)ethyl)-4-(2-(tert-butyldimethyl-
silyloxy)ethyl)-5-ethylpiperidin-2-one. A 100-mL round-bottomed
flask containing 505 mg (2.52 mmol, 1 equiv) of lactone 20
dissolved in 25 mL of dry THF was cooled to -78 °C. To this
cooled mixture was slowly added 2.77 mL (2.77 mmol, 1.1 equiv) of
a 1.0 M solution of DIBAL-H in THF. The reaction was stirred for
30 min at -78 °C followed by the dropwise addition of 2 mL of
MeOH. The reaction was allowed to warm to ambient temperature
and was then added to a saturated aqueous solution of NaK
tartrate and extracted three tmes with EtOAc. Combined organic
layers were dried over Na2SO4, concentrated, and purified by silica
gel flash chromatography eluting with 1:1 hex./EtOAc to yield 459
mg of an inseparable mixture of the desired lactol mixture (22) as a
colorless oil.
To a 50-mL round-bottomed flask containing the above
prepared lactol mixture dissolved in 23 mL of dry THF was
added 546 mg (3.41 mmol, 1.5 equiv) of tryptamine and then
1.44 g (6.81 mmol, 3 equiv) of NaBH(OAc)3. The reaction was
allowed to stir at ambient temperature for 48 h before being
added to NaHCO3(sat.), extracted three times with EtOAc, dried
1H NMR (300 MHz, CDCl3) δ 8.14 (br s, 1H), 7.45 (d, J = 6.6
Hz, 1H), 7.31 (d, J = 7.2 Hz, 1H), 7.10 (m, 2H), 3.73 (m, 2H),
3.13 (dd, J = 10.8, 0.6 Hz, 1H), 2.99 (m, 3H), 2.66 (m, 1H), 2.54
(m, 1H), 2.32 (d, J = 9.9 Hz, 1H), 1.85 (m, 2H), 1.58-1.48 (m,
5H), 1.26 (m, 2H), 0.91 (t, J = 7.2 Hz, 3H); 13C NMR (75 MHz,
CDCl3) δ 136.1, 135.4, 127.5, 121.2, 119.3, 118.1, 110.9, 107.9,
60.8, 60.5, 57.9, 53.6, 39.7, 36.4, 36.0, 31.9, 21.8, 17.8, 12.8; IR
(NaCl, film) 3412, 3277, 2800, 2749 cm-1; HRMS (þTOF) [M þ
H]þ 299.2118 calcd for C19H27N2O, found 299.2120; Rf 0.25
(5% MeOH in CH2Cl2). Spectroscopic properties agreed in all
respects with those previously reported.13e,17,18
(18) (a) Lounasmaa, M.; Jokela, R. Heterocycles 1990, 31 (7), 1351–1358.
(b) Kametani, T.; Kanaya, N.; Honda, T. Heterocycles 1981, 16 (11), 1937–
1946. (c) Beard, R. L.; Meyers, A. I. J. Org. Chem. 1991, 56, 2091–2096. (d)
Itoh, T.; Yokoya, M.; Miyauchi, K.; Nagata, K.; Ohsawa, A. Org. Lett. 2006,
8, 1533. (e) Wenkert, E.; Guo, M.; Pestchanker, M. J.; Shi, Y.-J.; Vankar,
Y. D. J. Org. Chem. 1989, 54, 1166–1174.
Synthesis of Lactone 21. To a 100-mL round-bottomed flask
containing 1.07 g (5.40 mmol, 1 equiv) of lactone 6 was added
25 mL of dry MeOH then 574 mg of 10% Pd/C. Hydrogen gas
J. Org. Chem. Vol. 75, No. 22, 2010 7873

English and Williams
JOCArticle
was bubbled through this mixture for 5 min and then the
reaction was vigorously stirred for 4 h at ambient temperature
under balloon pressure of H2. The reaction was then filtered
through a thin pad of Celite, concentrated, and purified by silica
gel flash chromatography eluting with 1:1 hex./EtOAc to yield
1.07 g (>99%) of the title compound as a colorless oil. (Note:
The trans isomer (20) was not detected by 1H NMR, 13C NMR,
or TLC.)
purified by silica gel flash chromatography eluting with 1:1 to
0:1 hex./EtOAc to yield 542 mg (47%, 3 steps) of the desired
mixture of isomers as a tan foam.
1H NMR (300 MHz, CDCl3) δ 8.78 (br s, 1H), 7.66 (d, J = 7.5
Hz, 1H), 7.35 (d, J = 7.8 Hz, 1H), 7.13 (m, 2H), 6.98 (s, 1H), 3.64
(m, 4H), 3.22-2.86 (m, 4H), 2.28-2.59 (m, 2H), 2.07 (m, 1H),
1.70-1.10 (m, 6H), 0.90 (s, 9H), 0.80 (m, 4H), 0.05 (s, 6H); 13
C
NMR (75 MHz, CDCl3) δ 170.0, 169.7, 136.6, 127.7, 122.5,
122.0, 119.3, 118.9, 113.0, 112.9, 111.6, 60.9, 60.6, 51.6, 50.8,
48.5, 48.4, 39.4, 38.5, 36.6, 36.4, 33.1, 31.8, 31.3, 26.2, 23.9, 23.4,
1H NMR (300 MHz, CDCl3) (mixture of rotamers) δ 4.26 (m,
2H), 3.63 (s, 1.5H), 3.62 (s, 1.5H), 2.74-2.36 (m, 2H), 2.29-1.99
(m, 3H), 1.89-1.52 (m, 3H), 1.35 (m, 1H), 1.20 (m, 1H), 0.94 (t,
J = 7.5 Hz, 1.5H), 0.92 (t, J = 7.2 Hz, 1.5H); 13C NMR (75
MHz, CDCl3) (mixture of rotamers) δ 174.2, 173.4, 172.7, 172.4,
68.6, 67.3, 66.1, 65.6, 52.7, 52.1, 51.9, 46.7, 44.5, 38.9, 38.7, 36.1,
34.3, 33.7, 32.0, 30.3, 29.2, 28.5, 28.0, 27.0, 22.9, 22.8, 20.8, 20.2,
14.0, 12.3, 12.0, 11.1; IR (NaCl, film) 1732 cm-1; HRMS (þTOF)
[M þ H]þ 201.1121 calcd for C10H17O4, found 201.1126; Rf 0.39
(1:1 hex./EtOAc).
Synthesis of Lactams 26 and 23. To a 100-mL flame-dried
round-bottomed flask was added 1.08 g (5.40 mmol, 1 equiv) of
lactone 21 dissolved in 25 mL of dry THF. This mixture was
cooled to -78 °C before the slow addition of 5.94 mL (5.94
mmol, 1.1 equiv) of a 1.0 M solution of DIBAl-H in THF. The
reaction was stirred at -78 °C for 30 min befor the addition of
2 mL of dry MeOH. The reaction was allowed to warm to
ambient temperature before being added to a saturated solution
of Rochelle’s salt, extracted twice into CH2Cl2, dried over
Na2SO4, and concentrated to yield the desired mixture of lactols
21 as a colorless oil, which was immediately used without further
purification. (Note: The undesired trans isomer (20) was not
detected in this crude mixture; however, subjecting this crude
mixture to silica gel chromatography did result in the isolation
of 20.)
23.2, 20.7, 18.5, 12.1, 11.1; IR (NaCl, film) 3254, 1626 cm-1
;
HRMS (þTOF) [M þ H]þ 429.2932 calcd for C25H41N2O2Si,
found 429.2933; Rf 0.30 (1:1 hex./EtOAc)
To a 25-mL round-bottomed flask containing 373 mg (0.870
mmol, 1 equiv) of the above produced mixture of lactams
dissolved in 9 mL of dry MeCN was added 1.40 mL (17.4 mmol,
20 equiv) of dry pyridine followed by 406 μL (4.35 mmol,
5 equiv) of POCl3. The reaction was heated to 40 °C for 3 h
before being concentrated, taken up in 9 mL of dry MeOH, and
cooled to 0 °C. To this solution was added 33 mg of NaBH4 and
the reaction was allowed to warm to ambient temperature over
15 min before being added to NaHCO3(sat.), extracted into
EtOAc, dried over Na2SO4, and concentrated. The crude resi-
due was purified by silica gel flash chromatography eluting with
4:1 to 0:1 hex./EtOAc to yield 142 mg (40%) of the desired trans
product 27 as a tan foam along with 113 mg (31%) of the cis
product 24 as a tan foam.
1H NMR (300 MHz, CDCl3) δ 7.76 (br s, 1H), 7.47 (d, J = 7.2
Hz, 1H), 7.3 (d, J = 7.2 Hz, 1H), 7.11 (m, 2H), 3.73 (m, 2H), 3.13
(m, 3H), 3.00 (m, 1H), 2.72 (m, 1H), 2.59 (td, J = 11.1, 5.4 Hz,
1H), 2.19 (d, J = 11.7 Hz, 1H), 2.10 (t, J = 10.8 Hz, 1H), 1.94
(m, 1H), 1.68 (m, 1H), 1.26-1.51 (m, 4H), 1.16 (m, 1H), 0.932 (s,
12H), 0.10 (s, 6H); 13C NMR (75 MHz, CDCl3) δ 136.1, 135.1,
127.6, 121.4, 119.5, 118.3, 110.8, 108.3, 61.1, 60.7, 60.0, 53.4,
41.9, 37.3, 35.9, 35.8, 26.1, 23.6, 21.8, 18.5, 11.2, -5.0.; HRMS
(þTOF) [M þ H]þ 413.2983 calcd for C25H41N2O2Si, found
413.2985; Rf 0.17 (4:1 hex./EtOAc).
Synthesis of (()-Dihydrocorynantheol (10). To a 25-mL
round-bottomed flask containing 71 mg (0.17 mmol, 1 equiv)
of compound 27 was added 1 mL of dry MeOH followed by a
spatula tip of PPTS. The reaction was heated at reflux for 3 h,
added to 1 N NaOH, extracted twice with CH2Cl2, dried over
Na2SO4, and concentrated. The resultant residue was purified
by silica gel flash chromatography eluting with 10% to 20%
MeOH in CH2Cl2 to yield 46 mg (90%) of the title compound as
a white foam.
To a 100-mL round-bottomed flask containing the above
produced lactol mixture 21 dissolved in 27 mL of dry THF was
added 649 mg (4.05 mmol, 1.5 equiv) of tryptamine followed by
1.72 g (8.10 mmol, 3 equiv) of NaBH(OAc)3. The reaction was
allowed to stir at ambient temperature for 24 h before being
added to NaHCO3(sat.), extracted twice into CH2Cl2, dried over
Na2SO4, and concentrated to yield a mixture of alcohols as a tan
oil, which was used without further purification.
To a 100-mL round-bottomed flask containing the above
produced crude mixture of alcohols dissolved in 27 mL of dry
CH2Cl2 was added 488 mg (3.24 mmol, 1.2 equiv) of TBSCl
followed by 368 mg (5.40 mmol, 2 equiv) of imidazole. The
reaction was stirred at ambient temperature for 16 h before
being added to brine, extracted twice into CH2Cl2, dried over
Na2SO4, and concentrated. The resulting residue was purified
by silica gel flash chromatography eluting with 1:1 to 0:1 hex./
EtOAc to yield 542 mg (47%, 3 steps) of the desired mixture of
isomers as a tan foam.
1H NMR (300 MHz, CDCl3) δ 9.04 (br s, 1H), 7.41 (d, J = 7.5
Hz, 1H), 7.29 (d, J = 7.8 Hz, 1H), 7.08 (m, 2H), 3.61 (t, J = 6.0
Hz, 2H), 3.58 (br s, 1H), 3.00 (m, 4H), 2.69 (m, 1H), 2.47 (m,
1H), 2.16 (m, 1H), 1.88 (t, J = 11.4 Hz, 1H), 1.79 (m, 1H), 1.53
(m, 1H), 1.39 (m, 1H), 0.97-1.29 (m, 5H), 0.84 (t, J = 7.2 Hz,
3H); 13C NMR (75 MHz, CDCl3) δ 136.4, 134.5, 127.1, 121.3,
119.3, 118.1, 111.3, 107.3, 60.1, 59.9, 53.1, 50.5, 41.3, 37.0, 35.1,
1H NMR (300 MHz, CDCl3) δ 8.78 (br s, 1H), 7.66 (d, J = 7.5
Hz, 1H), 7.35 (d, J = 7.8 Hz, 1H), 7.13 (m, 2H), 6.98 (s, 1H), 3.64
(m, 4H), 3.22-2.86 (m, 4H), 2.28-2.59 (m, 2H), 2.07 (m, 1H),
34.9, 23.4, 21.4, 11.0; IR (NaCl, film) 3256, 2813, 2757 cm-1
;
1.70-1.10 (m, 6H), 0.90 (s, 9H), 0.80 (m, 4H), 0.05 (s, 6H); 13
C
HRMS (þTOF) [M þ H]þ 299.2118 calcd for C19H27N2O,
found 299.2116; Rf 0.16 (10% MeOH in CH2Cl2). Spectroscopic
properties agree in all respects with those previously reported.17,18
Synthesis of 1-(3,4-Dimethoxyphenethyl)-5-ethyl-4-(2-hydroxy-
ethyl)piperidin-2-one (30). To a 25-mL round-bottomed flask
containing 85 mg (0.42 mmol, 1 equiv) of 20 was added 4.2 mL
of dry THF, 114 mg (0.631 mmol, 1.5 equiv) of 2-(3,4-dimethoxy-
phenyl)ethanamine (29), and then 178 mg (0.840 mmol, 2 equiv) of
NaBH(OAc)3. The reaction was allowed to stir at ambient tem-
perature for 48 h before being added to NaHCO3(sat.), extracted
three times with EtOAc, dried over Na2SO4, and concentrated.
Purification by silica gel flash chromatography eluting with 5% to
20% MeOH in CH2Cl2 produced 95 mg (67%, 92% BRSM) of the
desired product as a colorless oil.
NMR (75 MHz, CDCl3) δ 170.0, 169.7, 136.6, 127.7, 122.5,
122.0, 119.3, 118.9, 113.0, 112.9, 111.6, 60.9, 60.6, 51.6, 50.8,
48.5, 48.4, 39.4, 38.5, 36.6, 36.4, 33.1, 31.8, 31.3, 26.2, 23.9, 23.4,
23.2, 20.7, 18.5, 12.1, 11.1; IR (NaCl, film) 3254, 1626 cm-1
;
HRMS (þTOF) [M þ H]þ 429.2932 calcd for C25H41N2O2Si,
found 429.2933; Rf 0.30 (1:1 hex./EtOAc)
Synthesis of Tetracycle 27. To a 100-mL round-bottomed
flask containing the above produced crude mixture of alcohols
dissolved in 27 mL of dry CH2Cl2 was added 488 mg (3.24 mmol,
1.2 equiv) of TBSCl followed by 368 mg (5.40 mmol, 2 equiv) of
imidazole. The reaction was stirred at ambient temperature for
16 h before being added to brine, extracted twice into CH2Cl2,
dried over Na2SO4, and concentrated. The resulting residue was
7874 J. Org. Chem. Vol. 75, No. 22, 2010

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JOCArticle
1H NMR (300 MHz, CDCl3) δ 6.69-6.76 (m, 3H), 3.82 (s,
3H), 3.80 (s, 3H) 3.58 (m, 3H), 3.44 (m, 1H), 3.06 (dd, J = 12.0,
4.8 Hz, 1H), 2.93 (dd, J = 12.3, 7.5 Hz, 1H), 2.81 (m, 1H), 2.75
(t, J = 7.5 Hz, 2H), 2.27 (m, 2H), 2.02 (m, 1H), 1.66 (m, 1H),
2.82 (dd, J = 10.8, 6.0 Hz, 1H), 2.56 (dd, J = 15.9, 3.0 Hz, 1H),
2.41 (td, J = 11.7, 3.9 Hz, 1H), 2.25 (dd, J = 11.4, 2.4 Hz, 1H),
1.99 (m, 1H), 1.87 (m, 2H), 1.59 (m, 3H), 1.45 (m, 1H), 1.26 (m,
2H), 0.90 (t, J = 7.5 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ
147.5, 147.2, 130.9, 127.3, 111.7, 108.2, 63.6, 61.0, 59.3, 56.3,
56.0, 53.3, 39.2, 36.9, 36.6, 34.0, 29.6, 17.7, 12.9; IR (NaCl, film)
3387, 2803, 2749 cm-1(Bohlmann bands); HRMS (þTOF)
[M þ H]þ 320.2220 calcd for C19H30NO3, found 320.2221; Rf
0.54 (10% MeOH in CH2Cl2). All spectral data were in agree-
ment with previous reports.19
1.53 (m, 1H), 1.11-1.32 (m, 3H), 0.82 (t, J = 7.2 Hz, 3H). 13
C
NMR (75 MHz, CDCl3) δ 169.4, 148.8, 147.5, 131.5, 120.7,
112.0, 111.2, 60.2, 55.9, 50.6, 49.1, 38.4, 36.3, 33.0, 31.5, 31.4,
20.5, 12.0; IR (NaCl, film) 3406, 1621 cm-1; HRMS (þTOF)
[M þ H]þ 336.2169 calcd for C19H30NO4, found 336.2174; Rf
0.23 (5% MeOH in CH2Cl2).
Synthesis of 2-(1-(3,4-Dimethoxyphenethyl)-5-ethyl-2-oxopi-
peridin-4-yl)ethyl Acetate. To a 10-mL round-bottomed flask
containing 45 mg (0.13 mmol, 1 equiv) of alcohol 30 dissolved in
1 mL of dry CH2Cl2 and 1 mL of pyridine was added 19 μL
(0.027 mmol, 2 equiv) of AcCl followed by a single crystal of
DMAP. The reaction was stirred for 30 min at ambient tem-
perature before being added to NaHCO3(sat.), extracted into
CH2Cl2, dried over Na2SO4, and concentrated. Purification by
silica gel flash chromatography eluting with 10% MeOH in
CH2Cl2 yielded 50 mg (99%) of the desired product as a pale
yellow oil.
Synthesis of a Mixture of Alcohols 30 and 33. To a 25-mL
flame-dried round-bottomed flask containing 410 mg (2.05
mmol, 1 equiv) of lactone 21 dissolved in 10 mL of dry THF
at -78 °C was dropwise added 2.15 mL (2.15 mmol, 1.05 equiv)
of a 1.0 M solution of DIBAL-H in THF. The reaction was
stirred at -78 °C for 30 min before being added to a saturated
solution of Rochelle’s salt, extracted three times with CH2Cl2,
dried over Na2SO4, and concentrated to yield 351 mg (84%) of a
mixture of lactol isomers, which was used without further
purification. To a 50-mL round-bottomed flash containing
290 mg (1.43 mmol, 1 equiv) of the above produced mixture of
lactol isomers dissolved in 7 mL of dry THF was added 390 mg
(2.15 mmol, 1.5 equiv) of 2-(3,4-dimethoxyphenyl)ethanamine
(29) dissolved in 7 mL of dry THF. To this mixture was added
909 mg (4.29 mmol, 3 equiv) of NaBH(OAc)3 and the reaction
was stirred at ambient temperature for 24 h before being added
to NaHCO3(sat.), extracted into CH2Cl2, dried over Na2SO4, and
concentrated. The resulting residue was purified by silica gel
chromatography eluting with 5% to 20% MeOH in CH2Cl2 to
yield 195 mg (41%) of the desired mixture of products.
1H NMR (300 MHz, CDCl3) δ 6.74 (m, 3H), 4.06 (m, 2H),
3.84 (s, 3H), 3.82 (s, 3H), 3.52 (m, 2H), 3.08 (dd, J = 12.3, 4.8
Hz, 1H), 2.96 (dd, J = 12.3, 7.2 Hz, 1H), 2.79 (t, J = 7.8 Hz,
2H), 2.31 (qd, J = 17.4, 6.3 Hz, 2H), 2.02 (s, 3H), 2.00 (m, 1H),
1.67 (m, 2H), 1.26 (m, 3H), 0.84 (t, J = 7.5 Hz, 3H); 13C NMR
(75 MHz, CDCl3) δ 171.2, 168.9, 149.0, 147.7, 131.7, 120.9,
112.2, 111.4, 62.6, 56.1, 50.7, 49.3, 38.4, 36.4, 33.2, 32.3, 28.1,
21.2, 20.4, 12.1; IR (NaCl, film): 1737, 1640 cm-1; HRMS
(þTOF) [M þ H]þ 378.2275 calcd for C21H32NO5, found
378.2280.
Synthesis of 2-((2R,3S,11bS)-3-Ethyl-9,10-dimethoxy-2,3,4,6,7,
11b-hexahydro-1H-pyrido[2,1-a]isoquinolin-2-yl)ethyl Acetate (31).
To a 25-mL round-bottomed flask containing 190 mg (0.503 mmol,
1 equiv) of 2-(1-(3,4-dimethoxyphenethyl)-5-ethyl-2-oxopiperidin-
4-yl)ethyl acetate dissolved in 5 mL of dry benzene was added
94.0 μL (1.06 mmol, 2 equiv) of freshly distilled POCl3. The reaction
was heated to reflux for 2 h before being concentrated, taken up in
5 mL of dry MeOH, and cooled to 0 °C. To this stirred solution was
carefully added 19 mg (0.50 mmol, 1 equiv) of NaBH4 and the
reaction was allowed to warm to ambient temperature for 15 min.
The reaction was added to NaHCO3(sat.), extracted three times with
CH2Cl2, dried over Na2SO4, and concentrated. The resulting residue
was purified by flash chromatography (10%w/w NEt3 on silica gel)
eluting with 4:1 to 0:1 hex./EtOAc to yield 142 mg (78%) of the
desired product as a colorless oil.
1H NMR (300 MHz, CDCl3) (mixture of epimers) δ 7 (m,
3H), 3.79 (m, 8H), 3.58 (m, 3H), 3.48 (m, 1H), 3.00 (m, 2H), 2.73
(t, J = 7.5 Hz, 2H, 2.52-2.18 (m, 2H), 1.97 (m, 1H), 1.72-1.51
(m, 2H), 1.19 (m, 3H), 0.77 (m, 3H); 13C NMR (75 MHz, CDCl3)
(mixture of epimers) δ 169.9, 169.6 149.0, 147.7, 131.7, 120.9,
120.8, 112.2, 112.0, 111.4, 60.3, 59.8, 56.1, 51.7, 50.8, 49.4, 41.0,
39.6, 38.6, 36.4, 36.0, 35.3, 33.3, 33.2, 31.7, 31.6, 23.8, 20.7,
12.2, 11.2; IR (NaCl, film) 3385, 1621 cm-1; HRMS (þTOF)
[M þ H]þ 336.2169 calcd for C19H30NO4, found 336.2174; Rf
0.33 (5% MeOH in EtOAc).
Synthesis of 2-((2R,3R,11bS)-3-Ethyl-9,10-dimethoxy-2,3,4,6,7,
11b-hexahydro-1H-pyrido[2,1-a]isoquinolin-2-yl)ethyl Acetate (34).
To a 25-mL round-bottomed flask containing 195 mg (0.549 mmol,
1 equiv) of a mixture of alcohols 30 and 33 dissolved in 5 mL of dry
CH2Cl2 and 1 mL of dry pyridine was added 76 mL (1.10 mmol,
2 equiv) of AcCl followed by a single crystal of DMAP. The reac-
tion was stirred at ambient temperature for 30 min before being
added to NaHCO3(sat.), extracted into CH2Cl2, dried over Na2SO4,
and concentrated. The resulting residue was purified by silica gel
flash chromatography eluting with 1% to 10% MeOH in CH2Cl2
to yield 204 mg (>99%) of the desired mixture of products.
To a 25-mL round-bottomed flask containing 164 mg (0.434
mmol, 1 equiv) of the above produced acetate mixture dissolved
in 5 mL of dry benzene was added 81.0 μL (0.869 mmol, 2 equiv)
of freshly distilled POCl3. The reaction was heated to reflux for
2 h before being concentrated, taken up in 5 mL of dry MeOH,
and cooled to 0 °C. To this stirred solution was carefully added
17 mg (0.43 mmol, 1 equiv) of NaBH4 and the reaction was
allowed to warm to ambient temperature for 15 min. The reac-
tion was added to NaHCO3(sat.), extracted three times with
CH2Cl2, dried over Na2SO4, and concentrated. The resulting
residue was purified by flash chromatography (10%w/w NEt3 on
silica gel) eluting with 4:1 to 0:1 hex./EtOAc to yield 64 mg
(41%) of the desired product (34) as a colorless oil as well as
55 mg (35%) of the C3 epimer (31).
1H NMR (300 MHz, CDCl3) δ 6.66 (s, 1H), 6.55 (s, 1H), 4.16
(t, J = 6.3 Hz, 2H), 3.83 (s, 3H), 3.82 (s, 3H), 3.11-2.94 (m, 3H),
2.82 (dd, J = 10.8, 6 Hz, 1H), 2.55 (dd, J = 15.6, 3.0 Hz, 1H),
2.41 (td, J = 11.7, 3.9 Hz, 1H), 2.25 (dd, J = 11.4, 2.4 Hz, 1H),
2.06 (s, 3H), 1.99 (m, 1H), 1.82 (m, 1H), 1.67 (m, 3H), 1.45 (m,
1H), 1.27 (m, 2H), 0.90 (t, J = 7.2 Hz, 3H); 13C NMR (75 MHz,
CDCl3) δ 171.5, 147.4, 147.2, 130.7, 127.2, 111.6, 108.0, 63.5,
63.1, 59.1, 56.3, 56.0, 53.2, 39.0, 37.5, 34.0, 32.3, 29.6, 21.3, 17.6,
12.8; IR (NaCl, film) 2802, 2748 (Bohlmann bands), 1737 cm-1
;
HRMS (þTOF) [M þ H]þ 362.2326 calcd for C21H32NO4,
found 362.2331.
Synthesis of (()-3-epi-Protoemetinol (11). To a 10-mL round-
bottomed flask containing 95.0 mg (0.263 mmol, 1 equiv) of
tetracycle 31 was added 2 mL of MeOH, 1 mL of H2O, and finally
436 mg (3.15 mmol, 12 equiv) of anhydrous K2CO3. The reaction
was stirred for 2 h at ambient temperature before being added to
brine, extracted three times into CH2Cl2, dried over Na2SO4, and
concentrated. The resulting residue was purified by silica gel flash
chromatography eluting with 2% to 10% MeOH in CH2Cl2 to
yield 84 mg (>99%) of the desired product as a white foam.
1H NMR (300 MHz, CDCl3) δ 6.67 (s, 1H), 6.56 (s, 1H), 3.83
(s, 3H), 3.82 (s, 3H), 3.71 (t, J = 6.8 Hz, 2H), 3.12-2.94 (m, 3H),
(19) Nuhant, P.; Raikar, S. B.; Wypych, J.-C.; Delpech, B.; Marazano, C.
J. Org. Chem. 2009, 74, 9413–9421.
J. Org. Chem. Vol. 75, No. 22, 2010 7875

English and Williams
JOCArticle
1H NMR (300 MHz, CDCl3) δ 6.66 (s, 1H), 6.56 (s, 1H), 4.17 (t,
J = 6.3 Hz, 2H), 3.85 (s, 3H), 3.83 (s, 3H), 3.03 (m, 4H), 2.61 (dd,
J = 15.6, 3.0 Hz, 1H), 2.45 (td, J = 11.4, 3.9 Hz, 1H), 2.31 (m, 1H),
2.05 (s, 3H), 2.00 (m, 2H), 1.66 (m, 1H), 1.42 (m, 3H), 1.27-1.07(m,
2H), 0.90 (t, J = 7.5 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 171.4,
147.6, 147.3, 130.1, 126.9, 111.6, 108.2, 62.8, 61.6, 56.3, 56.0, 52.7,
41.4, 38.2, 37.4, 31.9, 29.4, 23.7, 21.3, 11.3; IR (NaCl, film) 2802,
2748 (Bohlmann bands), 1737 cm-1; HRMS (þTOF) [M þ Na]þ
384.2145 calcd for C21H31NNaO4, found 384.2147.
J = 16.4 Hz, 1H), 2.46 (td, J = 11.6, 4.0 Hz, 1H), 2.32 (d, J =
13.2 Hz, 1H), 2.01 (m, 1H), 1.90 (m, 1H), 1.63 (m, 1H), 1.41 (m,
3H), 1.23 (m, 2H), 1.09 (m, 1H), 0.89 (t, J = 7.6 Hz, 3H); 13
C
NMR (75 MHz, CDCl3) δ 147.5, 147.2, 129.9, 126.7, 111.5,
108.4, 62.8, 61.5, 60.4, 56.2, 55.9, 52.5, 41.2, 37.7, 37.2, 35.9,
29.1, 23.5, 11.2; IR (NaCl, film) 3373, 2801, 2751 cm-1
(Bohlmann bands); HRMS (þTOF) [M þ H]þ 320.2220 calcd
for C19H30NO3, found 320.2224; Rf 0.46 (10% MeOH in
CH2Cl2). All spectral data were in agreement with previous
reports.19,20
Synthesis of (()-Protoemetinol (12). To a 10-mL round-
bottomed flask containing 61.0 mg (0.169 mmol, 1 equiv) of
acetate 34 was added 1 mL of MeOH, 0.5 mL of H2O, and finally
280 mg (2.03 mmol, 12 equiv) of anhydrous K2CO3. The
reaction was stirred for 2 h at ambient temperature before being
added to brine, extracted three times into CH2Cl2, dried over
Na2SO4, and concentrated. The resulting residue was purified
by silica gel flash chromatography eluting with 2% to 10%
MeOH in CH2Cl2 to yield 54 mg (>99%) of the desired product
as a colorless oil.
Acknowledgment. We gratefully acknowledge financial
support from the National Institutes of Health (Grant
GM068011). Mass spectra were obtained on instruments
supported by the National Institutes of Health Shared
Instrumentation Grant No. GM49631. We also gratefully
acknowledge an Eli Lilly Graduate Fellowship to B.J.E.
from Eli Lilly. This paper is dedicated to Professor Raymond
L. Funk of Pennsylvania State University on the occasion of
his 60th birthday.
1H NMR (400 MHz, CDCl3) δ 6.66 (s, 1H), 6.55 (s, 1H), 3.82
(s, 3H), 3.81 (s, 3H), 3.72 (m, 2H), 3.14-2.94 (m, 4H), 2.60 (d,
(20) (a) Battersby, A. B.; Kapil, B. S.; Bhakuni, D. S.; Popli, S. P.;
Merchant, J. R.; Salgar, S. S. Tetrahedron Lett. 1966, 4965–4971. (b) Chang,
J.-K.; Chang, B.-R.; Chuang, Y.-H.; Chang, N.-C. Tetrahedron 2008, 64,
9685–9688.
Supporting Information Available: Additional experimental
details and spectra of all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
7876 J. Org. Chem. Vol. 75, No. 22, 2010