Molecular library synthesis using complex substrates: Expanding the framework of triterpenoids

Article abstract of DOI:10.1021/jo302211f

The remodeling of a natural product core framework by means of diversity-oriented synthesis (DOS) is a valuable approach to access diverse/biologically relevant chemical space and to overcome the limitations of combinatorial-type compounds. Here we provide proof of principle and a thorough conformational analysis for a general strategy whereby the inherent complexity of a starting material is used to define the regio- and stereochemical outcomes of reactions in chemical library construction. This is in contrast to the traditional DOS logic employing reaction development and catalysis to drive library diversity.

Full text of DOI:10.1021/jo302211f

Article
pubs.acs.org/joc
Molecular Library Synthesis Using Complex Substrates: Expanding
the Framework of Triterpenoids
Vasily A. Ignatenko, Yong Han, and Gregory P. Tochtrop*
Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
S
* Supporting Information
ABSTRACT: The remodeling of a natural product core framework by means of
diversity-oriented synthesis (DOS) is a valuable approach to access diverse/
biologically relevant chemical space and to overcome the limitations of
combinatorial-type compounds. Here we provide proof of principle and a thorough
conformational analysis for a general strategy whereby the inherent complexity of a
starting material is used to define the regio- and stereochemical outcomes of reactions
in chemical library construction. This is in contrast to the traditional DOS logic
employing reaction development and catalysis to drive library diversity.
product will have dramatic effects on the resultant products.
Consequently, once a diversification strategy is devised, it can
be applied to many members of a natural product family to
afford significantly different resultant products. Coupled with a
reliance on readily available natural products, we argue that this
general concept will prove to be a powerful and transferable
method for the development of diverse molecular libraries.
In this paper, we describe a synthetic strategy for remodeling
a triterpenoid skeleton based on the reactivity patterns of
lanosterol (Figure 1) and application of the devised strategy to
a pentacyclic triterpenoid, bryonolic acid. Lanosterol was
initially chosen because of the unsaturated B/C ring fusion,
which was shown by the groups of Snatzke,28 Fox,29
Sicinski,30,31 and Marsaioli32,33 to undergo iterative allylic
oxidation/oxidative cleavage to produce transannular poly-
ketones, followed by the aldol addition reactions to form
distinct molecular skeletons. A literature search revealed that
bryonolic acid is the major documented pentacyclic triterpe-
noid with the unsaturated B/C ring fusion.34 We hypothesized
that based on this structural similarity, bryonolic acid will react
in a complementary fashion producing novel pentacyclic
triterpenoids, each distinctively different from the others and
the prototype structure. In addition, bryonolic acid can be
isolated in gram-quantities from the sprouts of Cucurbita pepo
L. (common zucchini)35 and is known to have anti-
inflammatory properties.36
INTRODUCTION
■
The construction of combinatorial libraries coupled with high-
throughput screening has been a primary engine of the drug
discovery process over the past three decades.1,2 A central tenet
of this scientific model is that such libraries confer a massive
number of diverse chemical entities and drug leads.3 Because of
this reliance on library-derived molecules, natural products,
despite their long history of success in pharmaceutical
research,4,5 have seen a decline in drug discovery efforts.6,7
The subsequent realization that combinatorial-type molecules
are substantially less diverse in their chemical structures as
compared to molecules from natural sources8,9 has led to the
syntheses around “privileged scaffolds”,10−13 chemical alteration
of crude natural product extracts,14−17 and diversity-oriented
synthesis (DOS), which was introduced to construct complex
and diverse molecular skeletons from simple and similar
starting materials.18−21
The synthetic approaches for the aforementioned library
types have generally followed the traditional logic of organic
synthesis: generating libraries of complex molecules from
simple substrates using new reaction development and/or
catalysis. The approach we illustrate herein deviates from this
traditional model in that we set out to identify chemical
functionality that can be exploited to rearrange the carbocyclic
skeleton of an abundant natural product. The central
hypothesis behind our approach was that instead of relying
on reaction development and catalysis to impart stereochemical
and regiochemical selectivity, we postulated that the inherent
complexity of the natural product-derived substrates can drive
stereoselective and regioselective reactions. Herein we provide
proof of principle for this concept. While a number of studies
have produced novel scaffolds using the rearrangement of
functionalities embedded within a natural product core
structure,22−27 this is the first report to carefully test and
rationalize through conformational analysis the hypothesis that
subtle perturbations to the structure of the parent natural
RESULTS AND DISCUSSION
■
Substrate 3 was prepared via methylation of C-29 carboxylic
acid of bryonolic acid 1 with diazomethane to initially form
ester 2, followed by the protection of the hydroxyl group at C3
to generate acetate 3 (Scheme 1).
Received: October 15, 2012
© XXXX American Chemical Society
A
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regioselective allylic oxidation at C7 to give α,β-unsaturated
ketone 5 (Scheme 1). This transformation was fast and robust
providing the complete conversion of starting material in 20
min as detected by TLC. However, with an extended reaction
time (24 h), intermediate 5 was further oxidized in situ by
ruthenium tetroxide to give the desired triketone 6 and the
product of competing CH activation 7.
Transannular aldol addition of diketone 4 was next
investigated38−41 (Table 1). Examination of the structure of
Table 1. Aldol Addition of Diketone 4 via Pathways a and b
Figure 1. Diversity-oriented synthesis strategy based on the reactivity
patterns of lanosterol.
Scheme 1. Oxidation of the Double Bond at the B/C Ring
a
Fusion of the Substrate 3
isolated yield (%)
a
entry
reagent i
8
9
10
11
1
2
3
4
5
6
7
8
9
TFA
45
56
80
71
65
48
79
81
50
0
0
0
0
0
0
0
7
0
40
6
0
0
0
0
0
0
0
0
0
NaH
pyrrolidine
LDA
8
22
25
43
5
LiHMDS
LiTMP
Ph3CLi
Al2O3
0
TiCl4
32
a
See the Experimental Section for detailed experimental procedures.
diketone 4 reveals two possible enolization sites, potentially
leading to four different regioisomeric aldol adducts (pathways
a and b). Since aldol addition is one of the most well-known
reactions in organic synthesis,42,43 our initial efforts were
focused on the standard aldol protocols. The use of catalytic
TFA at room temperature led to the formation of the mixture
of syn product of pathway a (8) and the syn product of pathway
b (10) in good overall yield (85%, Table 1, entry 1). The use of
sodium hydride as well as pyrrolidine as an organocatalyst for
enamine-mediated aldol reaction gave a similar distribution of
products 8 and 10, although reaction with sodium hydride led
to some degradation (Table 1, entries 2 and 3). Predominant
formation of aldol adduct 8 suggested that it was the more
thermodynamically stable product of the reaction. It is
noteworthy that no products of aldol condensation were
observed.
Our next step was to force the transannular aldol reaction of
diketone 4 into pathway b under the kinetic conditions. Upon
reaction of diketone 4 with LDA (1.1 equiv, THF, −78 °C to
rt), aldol adducts 8 and 10 were formed in 71% and 22% yield,
respectively (Table 1, entry 4). The increase in the steric bulk
of the amide base led to the increased formation of aldol adduct
10. Thus, the use of LiHMDS produced 10 in 25% yield and
a
Key: (a) CH2N2, THF, rt for 5 min (2, 88% yield); (b) (CH3CO)2O,
pyridine, 50 °C for 24 h (3, 84% yield).
In order to gain access to ketones 4 and 6, protected
bryonolic acid 3 was subjected to catalytic oxidation with
ruthenium tetroxide under Sharpless conditions.37 Under these
conditions, substrate 3 underwent competing oxidative cleavage
of Δ8,9 double bond to form the desired diketone 4 and
B
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LiTMP proved to be the reagent of choice, giving the highest
yield of aldol adduct 10 (43%, Table 1, entry 6).
Scheme 2. Aldol Addition/Condensation of Diketone 4 via
Pathways c and d
a
However, upon treatment of diketone 4 with Al2O3 (basic)
in DCM, aldol adduct 8 was isolated in 81% yield but 10 was
not observed. Instead, the anti product of pathway a (9) was
formed in 7% yield (Table 1, entry 8). By the virtue of substrate
bias, the lowest energy conformation of the cyclodecane ring of
diketone 4 is “boat (with bow at C-5 and stern between C-8
and C-11) -chair−chair” which determines the α-face position
of the carbonyl group at C-8 and β-face position of the carbonyl
at C-9 (Figure 2). In both pathways a and b, the pseudoaxial α-
a
Key: (a) BF3·Et2O (1.1 equiv), DCM, −78 °C to rt, 4 days; (b)
BF3·Et2O (10 equiv), DCM, −78 °C to rt, 18 h.
Figure 2. Working model demonstrates the role of the enolate
geometry and the conformation of cyclodecane ring of diketone 4
toward stereoselectivity.
Scheme 3. Mechanistic Considerations for the Formation of
Ketones 12, 13, and 14 from the Corresponding Aldol
Adducts 8, 10, and 11
hydrogen is antiperiplanar with the carbonyl and the
pseudoequatorial α-hydrogen is orthogonal to the CO
bonds, thus producing the trans (Z)-enolate of diketone 4 as
the key intermediate after deprotonation by the loss of the
pseudoequatorial hydrogen. The intermediate enolate is
geometrically predisposed for the formation of transannular
C−C bond with syn configuration at the ring junction. To
rationalize the formation of anti product 9, we hypothesized
that when treated with a Lewis acid such as Al2O3, the key
intermediate engages in the Zimmerman−Traxler transition
state44 (Figure 2) which determines the β-face position of H-11
and α-face position of the keto group at C-9. When purified
aldol adduct 8 was resubjected to the same reaction conditions
(100 equiv Al2O3, DCM, rt, 16 h), no interconversion of these
products was observed. The absence of product interconversion
under the same set of conditions led to a conclusion that the
relative stereochemistry of anti product 9 is the result of Lewis
acid-controlled transition state of the aldol addition reaction of
the diketone 4.
It is worth noting that the lithium cation, which is generally
known to follow the Zimmerman−Traxler model, did not give
any anti products in reactions with amide bases (Table 1,
entries 4−6). The reaction of diketone 4 with TiCl4 in the
presence of tertiary amine (1.1 equiv, −78 °C to rt) led to the
formation of aldol adducts 8 and 10 in 1.6:1 ratio and no anti
products were observed (Table 1, entry 9). In due course,
treatment of diketone 4 with BF3·Et2O (1.1 equiv, −78 °C to
rt) unexpectedly gave aliphatic ketones 12 and 13 and α,β-
unsaturated ketone 14 (Scheme 2, pathway c).
The absolute stereochemistry of H-11 suggested that ketone
12 was the result of aldol addition via pathway a to form aldol
adduct 8 followed by a cascade of methide shifts and an
elimination of H-18 to form internal double bond at Δ13,18
(Scheme 3). The control experiment (Scheme 2, pathway d)
confirmed our prediction when isolated aldol adduct 8 was
treated with BF3·Et2O to form aliphatic ketone 12 in 92% yield.
Thermodynamic stability of 8 and trans arrangement of the C-8
hydroxyl group and the methyl at C-26 add to the robustness of
this transformation. A similar rationale was then proposed for
the formation of ketones 13 and 14 via pathway c in 1:4 ratio.
β-Facial position of H-7 in ketone 13 predisposed for the
formation of aldol adduct 10 via pathway b to be followed by
either a C-25 methide shift or an elimination of H-7 through a
common carbocationic intermediate and E1-type transition
state to yield ketones 13 or 14, respectively.
Both of these possibilities seem equally plausible due to the
fact that both the C-25 methyl group and the H-7 are in cis
C
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arrangement with the hydroxyl group at C-9. The control
experiment (Scheme 2, pathway d) with isolated aldol adduct
10 yielded compounds 13 and 14 in 2.4:1 ratio. The reversal of
selectivity observed for the production of ketone 13 over 14 via
pathway d led to the conclusion that the reaction through
pathway c gives intermediately aldol adduct 10 and an anti
product of pathway b (11), which has the antiperiplanar
arrangement of H-7 and the hydroxyl group at C-9 (Scheme 3),
thus raising the possibility of the increased production of
ketone 14, if the overall elimination of water is to occur
through E2-type transition state.
together with the product of oxidative destruction 17 in 18%
and 21% yield, respectively.
The structure of the carboxylic acid 17 was tentatively
suggested after a thorough NMR study but appeared to be
difficult to establish due to slow conformational equilibrium
and identical chemical shifts of C-8 and C-29 carbonyls at 179.5
ppm. The structure was established unambiguously after
carboxylic acid 17 was treated with diazomethane to form the
ester 18, which showed three singlets centered around 3.6 ppm
1
in H spectrum as well as four ester carbonyls and two keto
groups in the 13C spectrum.
In the case of triketone 6 (Scheme 4), we also located the
lowest energy conformation, the cyclodecane part of which
In order to obtain the final aldol adduct in the proposed
molecular library, tetraketone 16 was treated with LDA (1.1
equiv, THF, −78 °C to rt) to give the aldol adduct 20 and the
product of transannular hemiketalization 19 in 9% and 64%
yield, respectively (Scheme 6). As expected, tetraketone 16 was
not deprotonated at C-12 since it is considerably more sterically
hindered as compared to C-6.
Scheme 4. Aldol Addition Reaction of Triketone 6
Scheme 6. Aldol Addition Reaction of Tetraketone 16
In the ground-state conformation46 of tetraketone 16,
pseudoaxial H-6 is coplanar with the carbonyl at C-7, giving
rise to the cis (E)-enolate as the key intermediate upon the
abstraction of pseudoequatorial H-6 by the base (Figure 4).
Figure 3. Working model demonstrates the role of the enolate
geometry and the conformation of cyclodecane ring of triketone 6
toward stereoselectivity.
closely resembles that of diketone 4 (Figure 3). Abstraction of
pseudoequatorial α-hydrogen at C-11 upon treatment of
triketone 6 with Al2O3 in DCM at room temperature produces
trans (Z)-enolate, which is structurally biased to adopt the
Zimmerman−Traxler transition state with C-7 carbonyl, thus
forming anti product 15.
In an attempt to obtain the last transannular polyketone in
the desired series, we performed the oxidative cleavage of the
enone 7 (Scheme 5). As in the case of protected bryonolic acid
3, ruthenium tetroxide under Sharpless conditions proved to be
the reagent of choice for this otherwise hardly achievable
transformation. However, larger amounts of ruthenium catalyst
and reoxidant and a longer reaction time were required to
afford, after careful column chromatography, tetraketone 16
Figure 4. Working model demonstrates the role of the enolate
geometry and the conformation of cyclodecane ring of tetraketone 16
toward regio- and stereoselectivity.
The higher yield of 19 over the aldol adduct 20 can be
explained in view of the fact that the transition state leading to
the aldol adduct 20 seems to be markedly higher in energy in
contrast with the transition state leading to the product of
transannular hemiketalization 19. The formation of 19 suggests
a mechanism which presumably proceeds through an
intermediate with the charge localized on the oxygen of the
pyranyl ring,45 thereby bringing a novel triterpenoid structure
into the library by virtue of an unanticipated reaction. This
proposed mechanism was confirmed when the reaction was
quenched with D2O, resulting in a partial deuterium
incorporation at C-6.
Scheme 5. Oxidative Cleavage of 7 with Ruthenium
Tetroxide under Sharpless Conditions
D
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37.3, 37.1, 34.8, 34.5, 33.0, 31.3, 31.0, 30.9, 30.4, 30.0, 28.1, 27.5, 25.1,
CONCLUSION
■
24.3, 22.2, 21.5, 20.8, 20.1, 19.2, 17.1, 16.8. HRMS (EI): m/z calcd for
+
C33H52O4 [M]+ 512.38656, found 512.38532.
Here we have illustrated what we consider to be a new principle
in the construction of molecular libraries. We have shown a
strategy for the remodeling of the triterpenoid skeleton devised
on the basis of the reactivity of the steroidal triterpenoid
lanosterol. We hypothesized that the subtle differences between
the steroid and triterpeoid core structures would impart
differential reactivity. This hypothesis was confirmed by
illustrating that despite the common unsaturation at the B/C-
ring fusion, the differences between the parent molecules led to
dramatic differences in reactivity and, concomitantly, the
composition of the resultant chemical library. Specifically, the
aldol addition of diketone 4 was shown to follow three different
pathways. Moreover, the aldol reactivity of the tetraketone 16
appeared to be completely different from that derived from
lanosterol, giving rise to the novel bridged structure 19 as a
major reaction product. We would argue that the strategy
outlined here is a general approach that could be applied to a
wide range of natural product families, allowing access to broad
categories of novel and potentially biologically relevant
molecules that would be otherwise very difficult to attain.
Preparation of 4, 6, and 7. In a 200 mL single-neck round-
bottom flask, RuCl3 (81 mg, 0.39 mmol) was added in one portion to
the solution of NaIO4 (2.09 g, 9.75 mmol) in 59 mL of H2O, and the
resulting suspension was stirred open to atmosphere for 15 min,
followed by the addition of 39 mL of acetonitrile. The solution of 3 (1
g, 1.95 mmol) in 39 mL of CCl4 was then added dropwise to the
reaction mixture by a syringe-pump. The flask was sealed with a glass
stopper, and the resulting biphasic mixture was vigorously stirred for
24 h, at which time 10 mL of ethanol was added to the solution, layers
were separated, and the aqueous layer was extracted with DCM. The
organic layer was dried with Na2SO4 and concentrated under vacuum
and the products further separated by chromatography. Silica column
chromatography yielded an inseparable mixture (351 mg) of 4 and 7,
Rf = 0.45 (EA/hex =25/75), and pure 6 as a yellow solid (416 mg,
38%). Mp: 168−171 °C. Rf = 0.38 (EA/hex =25/75). Compounds 4
and 7 were further separated by semiprep HPLC (Agilent C18 column
21.2 × 250 mm, isocratic elution CH3CN/H2O = 9/1, flow rate 5 mL/
min) to yield 7 as a yellow oil (96 mg, 9%), tR = 52 min, and 4 as white
foam (231 mg, 22%), tR = 66 min.
(2R,4aS,6aS,9aS,11S,13aS,16aS,16bR)-Methyl 11-Acetoxy-
2,4a,6a,10,10,13a,16a-heptamethyl-7,14-dioxodocosahydro-
benzo[6,7]cyclodeca[1,2-a]naphthalene-2-carboxylate (4). 1H
NMR (400 MHz, CDCl3, δ): 4.61 (m, 1H), 3.67 (s, 3H), 2.65−2.57
(2H), 2.39 (d, J = 16 Hz, 1H), 2.22 (d, J = 16 Hz, 1H), 2.08 (s, 3H),
1.174 (s, 3H), 1.170 (s, 3H), 1.14 (s, 3H), 1.12 (s, 3H), 1.04 (s, 3H),
0.94 (s, 3H), 0.87 (s, 3H). 13C NMR (100 MHz, CDCl3, δ): 218.1,
216.6, 179.0, 171.1, 80.2, 54.1, 52.2, 52.0, 45.52, 45.51, 44.2, 40.8, 38.7,
36.4, 36.3, 35.2, 34.7, 32.9, 31.7, 31.6, 31.5, 29.83, 29.79, 29.2, 28.5,
28.4, 28.1, 23.5, 21.4, 19.3, 17.4, 16.4, 16.0. HRMS (EI): m/z calcd for
EXPERIMENTAL SECTION
■
(2R,4aS,6aS,8aR,10S,12aS,14aS,14bR)-10-Hydroxy-2,4a,-
6a,9,9,12a,14a-heptamethyl-1,2,3,4,4a,5,6,6a,7,8,8a,9,-
10,11,12,12a,13,14,14a,14b-icosahydropicene-2-carboxylic
Acid (1). Bryonolic acid (1) was isolated in gram quantities from the
sprouts of “Spineless beauty” zucchini squash by previously published
method:35 mp 274−278 °C (lit.35 mp 274−278 °C).
+
C33H52O6 [M]+ 544.37639, found 544.37685.
(2R,4aS,6aS,8aR,10S,12aS,14aS,14bR)-Methyl 10-Hydroxy-
2,4a,6a,9,9,12a,14a-heptamethyl-1,2,3,4,4a,5,6,6a,7,8,8a,9,10,-
11,12,12a,13,14,14a,14b-icosahydropicene-2-carboxylate (2).
In a round-bottom flask open to atmosphere, bryonolic acid (1) (3
g, 6.57 mmol) was dissolved in 65 mL of freshly distilled THF. Freshly
prepared diazomethane in diethyl ether (15 mmol) was added
dropwise to the resulting solution by a pipet, and the reaction mixture
was stirred until full conversion detected by TLC (approximately 5
min). The reaction was quenched by dropwise addition of glacial acetic
acid until the yellow color of the solution disappeared. The reaction
mixture was concentrated under vacuum and the crude product was
purified by column chromatography on silica to give 2 as a white solid
(2R,4aS,6aS,9aS,11S,13aS,16aS,16bR)-Methyl 11-Acetoxy-
2,4a,6a,10,10,13a,16a-heptamethyl-7,8,14-trioxodocosa-
hydrobenzo[6,7]cyclodeca[1,2-a]naphthalene-2-carboxylate
(6). 1H NMR (400 MHz, CDCl3, δ): 4.67 (dd, J1 = 12 Hz, J2 = 4 Hz,
1H), 3.67 (s, 3H), 2.78 (dd, J1 = 20 Hz, J2 = 8 Hz, 1H), 2.71 (dd, J1 =
16 Hz, J2 = 4 Hz, 1H), 2.32 (d, J = 16 Hz, 1H), 2.27 (dd, J1 = 8 Hz, J2
= 4 Hz, 1H), 2.23 (d, J = 12 Hz, 1H), 2.06 (s, 3H), 1.79 (dd, J1 = 16
Hz, J2 = 4 Hz, 1H), 1.73 (dd, J1 = 16 Hz, J2 = 8 Hz, 1H), 1.54 (d, J = 8
Hz, 1H), 1.25 (s, 3H), 1.18 (s, 3H), 1.07 (s, 3H), 1.03 (s, 3H), 1.01 (s,
3H), 0.98 (s, 3H), 0.91 (s, 3H). 13C NMR (100 MHz, CDCl3, δ):
214.1, 212.6, 203.1, 179.2, 171.0, 79.6, 53.1, 52.0, 50.7, 46.3, 44.4, 43.5,
40.8, 39.3, 39.2, 35.4, 33.6, 33.0, 32.3, 31.9, 31.8, 31.7, 31.2, 30.9, 29.6,
28.5, 27.5, 23.4, 21.4, 17.6, 16.9, 16.6, 16.1. HRMS (ESI): m/z calcd
for C33H50O7Na+ [M + Na]+ 581.3454, found 581.3465.
(2R,4aS,6aS,8aR,10S,12aS,14aS,14bR)-Methyl 10-Acetoxy-
2,4a,6a,9,9,12a,14a-heptamethyl-7,13-dioxo-1,2,3,4,4a,5,6,-
6a,7,8,8a,9,10,11,12,12a,13,14,14a,14b-icosahydropicene-2-
carboxylate (7). 1H NMR (400 MHz, CDCl3, δ): 4.56 (m, 1H), 3.66
(s, 3H), 2.58 (ddd, J1 = 12 Hz, J2 = J3 = 4 Hz, 1H), 2.51−2.47 (3H),
2.07 (s, 3H), 1.93 (ddd, J1 = J2 = 12 Hz, J3 = 4 Hz, 1H), 1.28 (s, 3H),
1.24 (s, 3H), 1.20 (s, 3H), 1.06 (s, 3H), 0.989 (s, 3H), 0.987 (s, 3H),
0.96 (s, 3H), 0.89 (s, 3H). 13C NMR (100 MHz, CDCl3, δ): 200.0,
199.8, 179.1, 171.1, 154.9, 152.7, 79.6, 52.1, 52.0, 48.1, 44.2, 42.1, 40.5,
39.1, 38.1, 37.6, 36.8, 36.3, 34.1, 33.6, 32.5, 31.0, 30.9, 30.6, 29.8, 29.6,
27.5, 25.3, 23.9, 21.4, 21.0, 20.6, 17.9, 16.1. HRMS (ESI): m/z calcd
for C33H48O6Na+ [M + Na]+ 563.3348, found 563.3352.
1
(2.72 g, 88%). Mp: 142−144 °C. Rf = 0.5 (EA/hex, 2/3). H NMR
(400 MHz, CDCl3, δ): 3.61 (s, 3H), 3.23 (dd, J1 = 12 Hz, J2 = 4 Hz,
1H), 2.37 (d, J = 12 Hz, 1H), 2.19 (m, 1H), 2.1 (s, 3H), 1.02 (s, 3H),
0.99 (s, 3H), 0.95 (s, 3H), 0.94 (s, 3H), 0.79 (s, 3H), 0.74 (s, 3H). 13C
NMR (150 MHz, CDCl3, δ): 179.4, 134.1, 133.9, 79.0, 51.6, 50.5, 44.8,
41.9, 40.5, 38.9, 37.6, 37.2, 37.1, 35.1, 34.5, 33.0, 31.3, 31.0, 30.9, 30.4,
30.0, 28.1, 27.9, 27.7, 25.1, 22.1, 20.8, 20.0, 19.3, 17.2, 15.7. HRMS
+
(EI): m/z calcd for C31H50O3 [M]+ 470.37600, found 470.37553.
(2R,4aS,6aS,8aR,10S,12aS,14aS,14bR)-Methyl 10-Acetoxy-
2,4a,6a,9,9,12a,14a-heptamethyl-1,2,3,4,4a,5,6,6a,7,8,8a,9,10,-
11,12,12a,13,14,14a,14b-icosahydropicene-2-carboxylate (3).
In a round-bottom flask, bryonolic acid methyl ester 2 (2.35 g, 5
mmol) was dissolved in 10 mL of dry pyridine. Acetic anhydride (1.02
g, 10 mmol) was added to the resulting solution by a quick syringe
transfer. The reaction mixture was stirred at 50 °C for 24 h, after which
time pyridine was removed at reduced pressure and the crude mixture
was taken into ethyl acetate. The organic layer was washed successively
with dilute HCl, water and saturated solution of sodium bicarbonate. It
was subsequently dried with Na2SO4 and evaporated in vacuo. The
crude product was purified by column chromatography to give 3 as a
white solid (2.16 g, 84%). Mp: 164−167 °C. Rf = 0.62 (EA/hex =3/7).
1H NMR (400 MHz, CDCl3, δ): 4.47 (dd, J1 = 12 Hz, J2 = 4 Hz, 1H),
Preparation of 8−10. Table 1 (Entry1). In a round-bottom flask
open to atmosphere, trifluoroacetic acid (1 μL, 35 mol %) was added
to the solution of 4 (20 mg, 0.0367 mmol) in 1 mL of DCM. The flask
was sealed with a glass stopper, and the mixture was stirred vigorously
for 72 h, at which time the solvent was removed in vacuo, and the
mixture of products was separated by column chromatography on
silica without additional workup to yield 8 (9 mg, 45%) and 10 (8 mg,
40%).
3.62 (s, 3H), 2.37 (d, J = 16 Hz, 1H), 2.19 (d, J = 12 Hz, 1H), 2.05 (s,
3H), 1.17 (s, 3H), 1.02 (s, 3H), 0.96 (s, 3H), 0.94 (s, 3H), 0.87 (s,
3H), 0.86 (s, 3H), 0.73 (s, 3H). 13C NMR (100 MHz, CDCl3, δ):
179.3, 171.1, 134.1, 134.0, 81.0, 51.7, 50.7, 44.9, 41.9, 40.6, 37.8, 37.5,
Table 1 (Entry 2). NaH (95%, 2 mg, 0.0792 mmol) was placed in a
flame-dried (under vacuum) round-bottom flask, followed by the
addition of 0.5 mL of dry THF. The resulting suspension was cooled
to 0 °C, and the solution of 4 (36 mg, 0.066 mmol) in 0.5 mL of dry
E
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Article
THF was added dropwise at this temperature. The reaction mixture
was stirred at 0 °C for 30 min and then warmed to rt and stirred at rt
for 19 h, at which time the solvent was removed in vacuo, the crude
mixture was treated with 5% acetic acid in H2O, and the aqueous layer
was extracted with DCM. The organic layer was dried with Na2SO4
and concentrated under vacuum, and the crude mixture of products
was separated by column chromatography on silica to give 8 (20 mg,
56%) and 10 (2 mg, 6%).
mL) was added to the solution, layers were separated, and the aqueous
layer was extracted with DCM. The organic layer was dried with
Na2SO4 and concentrated under vacuum and the crude mixture of
products was separated by column chromatography on silica to give 8
(49.5 mg, 50%) and 10 (31.5 mg, 32%).
(2R,4aS,6aS,6bR,8aS,10S,12aS,13aS,14aS,14bR)-Methyl 10-
Acetoxy-6b-hydroxy-2,4a,6a,9,9,12a,14a-heptamethyl-13-
oxodocosahydrobenzo[f ]naphtho[2,1-a]azulene-2-carboxy-
late (8). White solid. Mp: 235−238 °C. Rf = 0.53 (EA/hex =3/7). Rf =
0.4 (EA/hex =25/75). 1H NMR (600 MHz, CDCl3, δ): 4.45 (m, 1H),
3.62 (s, 3H), 3.57 (dd, J1 = J2 = 12 Hz, 1H), 2.43 (dd, J1 =12.6 Hz, J2 =
4.2 Hz, 1H), 2.17 (2H), 2.04 (s, 3H), 1.18 (s, 3H), 1.16 (s, 3H), 1.08
(s, 3H), 1.05 (s, 3H), 0.96 (s, 3H), 0.95 (s, 3H), 0.59 (s, 3H). 13C
NMR (150 MHz, CDCl3, δ): 215.4 (CO), 179.3 (CO), 171.0 (CO),
84.5 (COH), 80.2 (CH), 56.8 (CH), 52.8 (C), 51.8 (CH3), 49.6 (C),
46.8 (C), 46.0 (CH), 44.1 (CH), 40.7 (C), 38.5 (CH2), 37.8 (C), 37.4
(CH2), 35.9 (CH2), 34.9 (CH2), 33.0 (CH2), 32.7 (CH3), 31.4 (C),
31.1 (CH3), 30.6 (CH2), 30.3 (CH2), 27.8 (CH3), 24.0 (CH2), 23.7
(CH2), 21.4 (CH3), 21.2 (CH3), 19.7 (CH2), 17.8 (CH3), 16.6 (CH3),
16.1 (CH3). HRMS (ESI): m/z calcd for C33H52O6Na+ [M + Na]+
567.3661, found 567.3660.
(2R,4aS,6aS,6bR,8aS,10S,12aS,13aR,14aS,14bR)-Methyl 10-
Acetoxy-6b-hydroxy-2,4a,6a,9,9,12a,14a-heptamethyl-13-
oxodocosahydrobenzo[f ]naphtho[2,1-a]azulene-2-carboxy-
late (9). White foam. Rf = 0.4 (EA/hex =25/75). 1H NMR (600 MHz,
CDCl3, δ): 4.48 (dd, J1 =17.4 Hz, J2 = 6 Hz, 1H), 3.70 (s, 3H), 3.60
(dd, J1 =16.2 Hz, J2 = 4.2 Hz, 1H), 2.35 (m, 1H), 2.04 (s, 3H), 1.18 (s,
3H), 1.10 (s, 3H), 1.07 (s, 3H), 1.04 (s, 3H), 0.91 (s, 3H), 0.90 (s,
3H), 0.89 (s, 3H). 13C NMR (150 MHz, CDCl3, δ): 212.4 (CO),
179.1 (CO), 170.8 (CO), 87.7 (COH), 80.4 (C), 53.3 (CH), 53.0
(C), 52.1 (CH3), 49.5 (C), 48.5 (C), 47.1 (CH), 44.2 (CH), 40.7 (C),
40.0 (C), 37.6 (CH2), 37.4 (CH2), 35.8 (CH2), 35.7 (CH2), 32.8
(CH2), 32.6 (CH3), 32.3 (CH2), 31.5 (C), 31.3 (CH3), 30.2 (CH2),
28.0 (CH3), 24.2 (CH2), 23.7 (CH2), 23.5 (CH2), 21.7 (CH3), 21.5
(CH3), 21.2 (CH3), 18.1 (CH3), 16.4 (CH3). HRMS (ESI): m/z calcd
for C33H52O6Na+ [M + Na]+ 567.3661, found 567.3654.
Table 1 (Entry 3). To a solution of 4 (25 mg, 0.0459 mmol) in 0.5
mL of DCM in a flame-dried (under vacuum) round-bottom flask was
added pyrrolidine (1.9 μL, 1.63 mg, 0.023 mmol) dropwise at rt. The
resulting mixture was stirred at that temperature for 24 h, at which
time the solvent was removed under vacuum and the crude mixture of
products was separated by column chromatography on silica without
additional workup to give 8 (20 mg, 80%) and 10 (2 mg, 8%).
General Procedure for Reaction of 4 with Amide Bases. The
solution of a secondary amine (1.1 equiv) in dry THF (0.027 M in
amine) in a flame-dried (under vacuum) round-bottom flask was
cooled to −78 °C, followed by a dropwise addition of n-butyllithium
(2.5 M in hexanes, 1.1 equiv). The reaction mixture was warmed to 0
°C, stirred at this temperature for 5 min, and subsequently cooled to
−78 °C. A solution of 4 (1 equiv) in dry THF (0.07 M in 4) was then
added dropwise to the solution of lithium amide by syringe at −78 °C.
The reaction mixture was allowed to warm to rt overnight and stirred
at rt for a total of 24 h. After the solvent was removed in vacuo, the
residue was taken up in DCM and washed with water and brine, and
the organic layer was dried over Na2SO4. Evaporation of the solvent in
vacuo gave crude product mixtures that were further separated by
chromatography.
Table 1 (Entry 4). Following the general procedure, the use of
diisopropylamine (20 mg, 29 μL, 0.202 mmol), n-butyllithium (0.202
mmol, 81 μL), 4 (100 mg, 0.1836 mmol), and THF (10 mL) gave,
after column chromatography on silica, 8 (71 mg, 71%) and 10 (22
mg, 22%).
Table 1 (Entry 5). Following the general procedure, the use of
hexamethyldisilazane (6.5 mg, 8.5 μL, 0.0404 mmol), n-butyllithium
(0.0404 mmol, 16.2 μL), 4 (20 mg, 0.0367 mmol), and THF (2 mL)
gave, after column chromatography on silica, 8 (13 mg, 65%) and 10
(5 mg, 25%).
Table 1(Entry 6). Following the general procedure, the use of
2,2,6,6-tetramethylpiperidine (5.7 mg, 0.0404 mmol), n-butyllithium
(0.0404 mmol, 16.2 μL), 4 (20 mg, 0.0367 mmol), and THF (2 mL)
gave, after column chromatography on silica, 8 (9.6 mg, 48%) and 10
(8.6 mg, 43%).
Table 1 (Entry 7). Following the general procedure for amide bases,
the use of Ph3CH (9.9 mg, 6.9 μL, 0.0404 mmol), n-butyllithium
(0.0404 mmol, 16.2 μL), 4 (20 mg, 0.0367 mmol), and THF (2 mL)
gave, after column chromatography on silica, 8 (16 mg, 79%) and 10
(1 mg, 5%).
Table 1 (Entry 8). A round-bottom flask open to atmosphere was
charged with basic alumina (1.9 g, 18.4 mmol), followed by the
addition of DCM (1 mL). A solution of 4 (100 mg, 0.184 mmol) in
DCM (1 mL) was then added to the resulting suspension. The flask
was sealed with a glass stopper, and the reaction mixture was
vigorously stirred at rt for 16 h, at which time the mixture was filtered
over a fine sinter funnel and washed successively with ethyl acetate.
After removal of the solvent in vacuo, the column chromatography on
silica yielded an inseparable mixture (90 mg) of 8 and 9: Rf = 0.4 (EA/
hex =25/75). Compounds 8 and 9 were further separated by semiprep
HPLC (Agilent C18 column 21.2 × 250 mm, isocratic elution
CH3CN/H2O = 95/5, flow rate 5 mL/min) to yield 8 (81 mg, 81%),
tR = 53 min, and 9 (7 mg, 7%), tR = 68 min.
Table 1 (Entry 9). Dry DCM (1 mL) was added to a flame-dried
(under vacuum) round-bottomed flask. The flask was cooled to −78
°C, and TiCl4 (41.8 mg, 0.22 mmol, 24.2 μL) was added at that
temperature by a quick syringe transfer, followed by a dropwise
addition of diisopropylethylamine (33.2 mg, 0.257 mmol, 44.8 μL). A
solution of 4 (100 mg, 0.1836 mmol) in dry DCM (1 mL) was added
to the reaction mixture at −78 °C, and the reaction mixture was
allowed to warm to rt and stirred at rt for 24 h, at which time water (2
(2R,4aS,6aS,7aR,8aS,10S,12aS,12bS,14aS,14bR)-Methyl 10-
Acetoxy-12b-hydroxy-2,4a,6a,9,9,12a,14a-heptamethyl-7-
oxodocosahydrobenzo[a]naphtho[2,1-f ]azulene-2-carboxy-
late (10). White solid. Mp: 187−190 °C. Rf = 0.46 (EA/hex =3/7).
1H NMR (600 MHz, CDCl3, δ): 4.45 (dd, J1 = 12 Hz, J2 = 4.8 Hz,
1H), 3.64 (s, 3H), 3.14 (d, J = 9.6 Hz, 1H), 2.42−2.38 (2H), 2.22 (d, J
= 14.4 Hz, 1H), 2.16 (ddd, J1 = J2 = 14.4 Hz, J3 = 4.8 Hz, 1H), 2.06
(ddd, J1 = J2 = 14.4 Hz, J3 = 4.8 Hz, 1H), 2.05 (s, 3H), 1.84 (dd, J1 = J2
= 13.2 Hz, 1H), 1.75−1.47 (12H), 1.45 (s, 3H), 1.42−1.27 (4H), 1.18
(s, 3H), 1.13 (dd, J1 = 13.8 Hz, J2 = 6 Hz, 1H), 1.08 (s, 3H), 1.04 (s,
3H), 0.96 (s, 3H), 0.92 (s, 3H), 0.64 (s, 3H). 13C NMR (150 MHz,
CDCl3, δ): 214.6 (CO), 179.2 (CO), 171.2 (CO), 82.1 (C), 81.0
(CH), 58.5 (CH), 56.3 (C), 52.0 (CH3), 50.5 (CH), 49.7 (C), 45.7
(CH), 40.8 (C), 39.1 (C), 37.5 (C), 36.5 (CH2), 34.0 (CH2), 33.0
(CH3), 32.8 (CH2), 31.8 (C), 31.40 (CH3), 31.35 (CH2), 31.0 (CH2),
30.8 (CH2), 29.8 (CH2), 29.0 (CH3), 27.6 (CH2), 25.0 (CH2), 22.8
(CH2), 21.4 (CH3), 18.0 (CH3), 17.8 (CH3), 17.0 (CH3), 16.1 (CH3).
HRMS (ESI): m/z calcd for C33H52O6Na+ [M + Na]+ 567.3661,
found 567.3647.
Preparation of 12−14. Solution of 4 (30 mg, 0.055 mmol) in dry
DCM (1 mL) in a flame-dried (under vacuum) round-bottom flask
was cooled to −78 °C, followed by a dropwise addition of BF3·Et2O
(purified, redistilled) (8.6 mg, 0.0605 mmol, 7.6 μL) at this
temperature. The reaction mixture was allowed to warm to rt
overnight and stirred at this temperature for a total of 4 days, at which
time the solvent was removed under vacuum, water (2 mL) was added
to the solution, layers were separated, and the aqueous layer was
extracted with DCM. The organic layer was dried with Na2SO4 and
concentrated under vacuum, and the crude mixture of products was
separated by column chromatography on silica to give 13 as
transparent oil (2.5 mg, 9%). Rf = 0.3 (EA/hex =15/85); 12 as
white solid (13 mg, 45%). Mp: 214 °C. Rf = 0.27 (EA/hex =15/85).
Compound 14 was obtained as a white solid (10 mg, 35%). Mp: 198−
202 °C. Rf = 0.22 (EA/hex =15/85).
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(2R,4aS,6aS,6bR,8aS,10S,12aS,13aS)-Methyl 10-Acetoxy-
2,4a,6a,6b,9,9,12a-heptamethyl-13-oxo-1,2,3,4,4a,5,6,6a,6b,7,-
8,8a,9,10,11,12,12a,13,13a,14-icosahydrobenzo[f ]naphtha
round-bottom flask open to atmosphere was charged with basic
alumina (11 g, 107.4 mmol), followed by the addition of DCM (5.7
mL). A solution of 6 (600 mg, 1.074 mmol) in DCM (5 mL) was then
added to the resulting suspension. The flask was sealed with a glass
stopper, and the reaction mixture was vigorously stirred at rt for 24 h,
at which time the mixture was filtered on a fine sinter funnel and
washed successively with ethyl acetate. After the removal of the solvent
in vacuo, column chromatography on silica yielded 15 as white solid
(480 mg, 80%). Mp: 292 °C. Rf = 0.37 (EA/hex =3/7). 1H NMR (600
MHz, CDCl3, δ): 4.52 (m, 1H), 3.59 (s, 3H), 2.90 (m, 1H), 2.61(dd,
J1 = 16 Hz, J2 = 8 Hz, 1H), 2.43 (m, 1H), 2.15 (m, 1H), 2.05 (s, 3H),
1.96 (ddd, J1 = J2 = 16 Hz, J3 = 4 Hz, 1H), 1.31 (s, 3H), 1.20 (s, 3H),
1.15 (s, 3H), 1.06 (s, 3H), 0.97 (s, 3H), 0.91 (s, 3H), 0.62 (s, 3H). 13C
NMR (150 MHz, CDCl3, δ): 216.6 (CO), 213.8 (CO), 179.1 (CO),
171.0 (CO), 80.2 (CH), 75.5 (COH), 52.7 (C), 51.9 (CH3), 47.3
(CH), 46.3 (C), 45.9 (CH), 43.8 (CH), 40.7 (C), 39.7 (C), 38.2 (C),
35.6 (CH2), 35.4 (CH2), 33.9 (CH2), 32.7 (CH3), 31.8 (CH3), 30.9
(CH2), 30.8 (C), 30.4 (CH2), 29.9 (CH2), 28.1 (CH2), 27.7 (CH3),
24.7 (CH2), 23.8 (CH2), 21.3 (CH3), 21.0 (CH3), 20.3 (CH3), 19.8
(CH3), 16.7 (CH3). HRMS (EI): m/z calcd for C33H50O7 [M]+
558.35565, found 558.35386.
Preparation of 16 and 17. A round-bottom flask open to
atmosphere was charged with RuCl3 (161 mg, 0.777 mmol) and
NaIO4 (1.66 g, 7.77 mmol), followed by the addition of H2O (11.7
mL) and CH3CN (7.8 mL). To the resulting dark suspension was
added a solution of 7 (420 mg, 0.777 mmol) in CCl4 (7.8 mL) by a
quick syringe transfer. The flask was sealed with a glass stopper, and
the resulting biphasic mixture was vigorously stirred for 7 days, at
which time 5 mL of ethanol was added to the solution. Subsequently,
layers were separated and aqueous layer was extracted with DCM. The
organic layer was dried with Na2SO4 and concentrated under vacuum,
and the crude mixture of products was separated by flash column
chromatography with gradient elution to give 16 as a yellow solid (75
mg, 18%). Mp: 186−188 °C. Rf = 0.76 (EA/hex =1/1); unreacted 7
(18 mg), Rf = 0.76 (EA/hex =1/1). Compound 19 was obtained as a
transparent oil (26 mg, 6%): Rf = 0.56 (EA/hex =1/1). Compound 17
was obtained as a yellow solid (92 mg, 21%). Mp: 93−96 °C. Rf = 0.1
(EA/hex = 1/1).
1
[2,1-a] azulene-2-carboxylate (12). H NMR (600 MHz, CDCl3,
δ): 4.52 (dd, J1 = 10.2 Hz, J2 = 6 Hz, 1H), 3.65 (dd, J1 = J2 = 9 Hz,
1H), 3.57 (s, 3H), 2.92 (dd, J1 = 18 Hz, J2 = 8.4 Hz, 1H), 2.74 (dd, J1
= 13.8 Hz, J2 = 2.4 Hz, 1H), 2.14 (dd, J1 = 17.4 Hz, J2 = 9.6 Hz, 1H),
2.05 (s, 3H), 1.25 (s, 3H), 1.17 (s, 3H), 0.99 (s, 6H), 0.96 (s, 3H),
0.89 (s, 3H), 0.48 (s, 3H). 13C NMR (150 MHz, CDCl3, δ): 217.4
(CO), 177.4 (CO), 171.0 (CO), 138.3 (C), 130.2 (C), 80.1 (CH),
51.5 (C), 51.33 (CH3), 51.26 (CH), 50.7 (CH), 49.8 (C), 49.7 (C),
45.9 (C), 39.7 (CH2), 38.8 (C), 36.8 (CH2), 36.6 (CH2), 36.1 (CH2),
35.7 (CH2), 34.1 (C), 32.2 (CH2), 28.2 (CH3), 28.1 (CH3), 26.8
(CH2), 25.1 (CH2), 24.8 (CH3), 23.7 (CH2), 22.7 (CH3), 21.8 (CH2),
21.4 (CH3), 19.0 (CH3), 17.5 (CH3), 16.9 (CH3). HRMS (ESI): m/z
calcd for C33H50O5Na+ [M + Na]+ 549.3556, found 549.3551.
(2R,4aS,6aS,7aR,10S,12bR,14aS,14bR)-Methyl 10-Acetoxy-
2,4a,6a,9,9,12b,14a-heptamethyl-7-oxo-1,2,3,4,4a,5,6,6a,7,-
7a,8,9,10,11,12,12b,13,14,14a,14b-icosahydrobenzo[a]-
1
naphtho[2,1-f ]azulene-2-carboxylate (13). H NMR (600 MHz,
CDCl3, δ): 4.75 (ddd, J1 = 10.2 Hz, J2 = 3 Hz, J3 = 1.8 Hz, 1H), 3.58
(s, 3H), 3.32 (ddd, J1 = 9 Hz, J2 = 3 Hz, J3 = 1.2 Hz, 1H), 2.96 (m,
1H), 2.42 (d, J = 15.6 Hz, 1H), 2.05 (s, 3H), 1.32 (s, 3H), 1.25 (s,
3H), 1.17 (s, 3H), 1.07 (s, 6H), 1.00 (s, 3H), 0.67 (s, 3H). 13C NMR
(150 MHz, CDCl3, δ): 214.8 (CO), 179.1 (CO), 171.2 (CO), 137.5
(C), 137.3 (C), 78.2 (CH), 55.5 (C), 54.5 (CH), 51.8 (CH3), 50.9
(C), 45.3 (CH), 40.7 (C), 39.4 (C), 36.7 (CH2), 36.2 (C), 34.9
(CH2), 32.9 (CH3), 32.2 (CH2), 31.8 (CH2), 31.7 (CH3), 31.5 (C),
31.4 (CH2), 30.4 (CH2), 29.9 (CH2), 27.0 (CH2), 26.2 (CH3), 25.6
(CH3), 24.5 (CH2), 21.6 (CH3), 21.4 (CH3), 19.7 (CH2), 19.3 (CH3),
17.5 (CH3). HRMS (ESI): m/z calcd for C33H50O5Na+ [M + Na]+
549.3556, found 549.3544.
(2R,4aS,6aS,8aR,10S,12aS,14aS,14bR)-Methyl 10-Acetoxy-
2,4a,6a,9,9,12a,14a-heptamethyl-7-oxo-1,2,3,4,4a,5,6,6a,-
7,8,8a,9,10,11,12,12a,13,14,14a,14b-icosahydrobenzo[a]-
1
naphtho[2,1-f ]azulene-2-carboxylate (14). H NMR (600 MHz,
CDCl3, δ): 4.52 (dd, J1 = 11.4 Hz, J2 = 4.8 Hz, 1H), 3.59 (s, 3H), 2.68
(dd, J1 =15 Hz, J2 = 6.6 Hz, 1H), 2.35 (d, J = 16.2 Hz, 1H), 2.05 (s,
3H), 1.19 (s, 3H), 1.08 (s, 3H), 1.01 (s, 3H), 0.98 (s, 3H), 0.97 (s,
3H), 0.89 (s, 3H), 0.77 (s, 3H). 13C NMR (150 MHz, CDCl3, δ):
206.9 (CO), 179.4 (CO), 171.1 (CO), 158.9 (C), 135.5 (C), 80.9
(CH), 56.2 (CH), 55.6 (C), 51.8 (CH3), 50.8 (C), 46.1 (CH), 40.8
(C), 39.3 (C), 37.2 (C), 36.4 (CH2), 34.0 (CH2), 33.0 (CH3), 32.7
(C), 32.6 (CH2), 32.5 (CH2), 32.0 (CH2), 31.3 (CH3), 29.6 (CH2),
29.3 (CH2), 28.5 (CH3), 26.1 (CH2), 24.9 (CH2), 24.0 (CH2), 21.9
(CH3), 21.4 (CH3), 17.2 (CH3), 17.1 (CH3), 16.7 (CH3). HRMS
(ESI): m/z calcd for C33H51O5+ [M + H]+ 527.3737, found 527.3736.
General Procedure for Reaction via Pathway d. A flame-dried
(under vacuum) round-bottom flask was cooled to −78 °C and
charged with BF3·Et2O (purified, redistilled) (10 equiv) followed by
the addition of DCM (0.14 M in starting material) The solution of
aldol adduct (1 equiv) in DCM (0.14 M in starting material) was then
added dropwise to the reaction mixture at −78 °C, allowed to warm to
rt overnight, and stirred at this temperature for a total of 18 h, at which
time the solvent was removed under vacuum, water (2 mL) was added
to the solution, the layers were separated, and the aqueous layer was
extracted with DCM. The organic layer was dried with Na2SO4 and
concentrated under vacuum, and the crude mixture of products was
separated by column chromatography on silica.
(2R,4aS,6aS,9aS,11S,13aS,16aS,16bR)-Methyl 11-Acetoxy-
2,4a,6a,10,10,13a,16a-heptamethyl-7,8,14,15-tetraoxodocosa-
hydrobenzo[6,7]cyclodeca[1,2-a]naphthalene-2-carboxylate
1
(16). H NMR (600 MHz, CDCl3, δ): 4.74 (dd, J1 =12 Hz, J2 = 3.6
Hz, 1H), 3.74 (s, 3H), 3.51 (d, J = 13.2 Hz, 1H), 3.28 (dd, J1 =14.4
Hz, J2 = 4.2 Hz, 1H), 2.76 (ddd, J1 = J2 = 13.8 Hz, J3 = 3.6 Hz, 1H),
2.59 (dd, J1 = 10.2 Hz, J2 = 3.6 Hz, 1H), 2.34 (d, J = 16.2 Hz, 1H),
2.28 (d, J = 14.4 Hz, 1H), 2.07 (s, 3H), 1.48 (s, 3H), 1.26 (s, 3H), 1.22
(s, 3H), 1.05 (s, 3H), 1.01 (s, 3H), 1.00 (s, 3H), 0.96 (s, 3H). 13C
NMR (150 MHz, CDCl3, δ): 207.4 (CO), 205.9 (CO), 203.6 (CO),
201.1 (CO), 178.7 (CO), 170.6 (CO), 79.1 (CH), 53.3 (C), 52.1
(CH3), 51.1 (C), 45.8 (CH), 44.0 (C), 42.5 (CH2), 41.6 (CH), 40.6
(C), 38.7 (C), 38.5 (CH2), 35.1 (CH2), 34.7 (CH2), 33.7 (CH2), 32.7
(CH3), 32.0 (C), 31.6 (CH2), 31.4 (CH3), 29.69 (CH2), 29.67 (CH2),
27.7 (CH3), 23.4 (CH2), 21.4 (CH3), 18.0 (CH3), 17.4 (CH3), 17.2
(CH3), 16.5 (CH3). HRMS (ESI): m/z calcd for C33H48O8Na+ [M +
Na]+ 595.3247, found 595.3256.
(1S,2S,4aR,7R,8aR)-1-(3-((1S,2S,4S)-4-Acetoxy-2-(carboxy-
methyl)-1,3,3-trimethylcyclohexyl)-2,3-dioxopropyl)-7-(me-
thoxycarbonyl)-1,2,4a,7-tetramethyldecahydronaphthalene-
2-carboxylic Acid (17). 1H NMR (600 MHz, CDCl3, δ): 9.65 (br s),
4.73 (dd, J1 =11.4 Hz, J2 = 4.2 Hz, 1H), 3.62 (s, 3H), 3.52 (d, J = 19.8
Hz, 1H), 2.80 (dd, J1 = J2 = 4.8 Hz, 1H), 2.64 (d, J = 19.8 Hz, 1H),
2.47 (dd, J1 =17.4 Hz, J2 = 5.4 Hz, 1H), 2.18 (dd, J1 =17.4 Hz, J2 = 5.4
Hz, 1H), 2.07 (s, 3H), 1.32 (s, 3H), 1.23 (s, 3H), 1.20 (s, 3H), 1.13 (s,
3H), 0.97 (s, 3H), 0.94 (s, 3H), 0.94 (s, 3H). 13C NMR (150 MHz,
CDCl3, δ): 204.6 (CO), 199.7 (CO), 183.9 (CO), 179.5 (CO), 179.5
(CO), 170.8 (CO), 79.l (CH), 51.8 (CH3), 50.4 (C), 47.2 (C, br),
45.8 (CH2, br), 43.6 (CH), 42.9 (C, br), 42.6 (C, br), 40.4 (CH, br),
38.9 (C), 37.9 (CH2, br), 33.7 (C), 32.9 (CH2, br), 32.4 (CH2), 32.2
(CH2, br), 30.5 (CH3), 29.8 (CH2, br), 29.4 (CH2), 28.6 (CH2, br),
27.7 (CH3), 24.5 (CH3, br), 23.1 (CH2), 22.7 (CH3, br), 21.4 (CH3),
Reaction of 8 via Pathway d. Following the general procedure for
reaction via pathway d, the use of 8 (75 mg, 0.1377 mmol), BF3·Et2O
(195.4 mg, 1.377 mmol, 173 μL), and DCM (2 mL) gave, after
column chromatography, 12 (67 mg, 92%): Rf = 0.58 (EA/hex = 3/7).
Reaction of 10 via Pathway d. Following the general procedure for
reaction via pathway d, the use of 10 (37 mg, 0.068 mmol), BF3·Et2O
(96.5 mg, 0.68 mmol, 85 μL), and DCM (1.4 mL) gave, after column
chromatography, 13 (19 mg, 53%), Rf = 0.5 (EA/hex =25/75), and 14
(8 mg, 22%), Rf = 0.43 (EA/hex =25/75).
(2R,4aS,6aS,7aS,8aS,10S,12aS,13aS,14aS,14bR)-Methyl 10-
Acetoxy-7a-hydroxy-2,4a,6a,9,9,12a,14a-heptamethyl-7,13-
dioxodocosahydrobenzo[a]tetracene-2-carboxylate (15). A
G
dx.doi.org/10.1021/jo302211f | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
20.5 (CH3, br), 17.6 (CH3, br), 17.3 (CH3, br). HRMS (ESI): m/z
calcd for C33H50O10Na+ [M + Na]+ 629.3301, found 629.3299.
(1S,2S,4aR,7R,8aR)-Dimethyl 1-(3-((1S,2S,4S)-4-Acetoxy-2-(2-
methoxy-2-oxoethyl)-1,3,3-trimethylcyclohexyl)-2,3-dioxo-
propyl)-1,2,4a,7-tetramethyldecahydronaphthalene-2,7-dicar-
boxylate (18). Solution of 17 (30 mg, 0.05 mmol) in diethyl ether (1
mL) in a round-bottomed flask was cooled to 0 °C, followed by a
dropwise addition of solution of diazomethane in ether (about 2 mL
total), and the reaction mixture was stirred at 0 °C until full conversion
of the starting material was detected by TLC (approximately 15 min).
The excess of diazomethane was quenched by a drop of glacial acetic
acid, the solvent was removed in vacuo, and the crude product was
further purified by column chromatography on silica to give trimethyl
ester 18 as a white solid (29.5 mg, 93%). Mp: 114−117 °C. Rf = 0.7
2.06 (s, 3H), 1.33 (s, 3H), 1.24 (s, 3H), 1.22 (s, 3H), 1.09 (s, 3H),
1.05 (s, 3H), 1.02 (s, 3H), 0.51 (s, 3H). 13C NMR (150 MHz, CDCl3,
δ): 216.1 (CO), 209.9 (CO), 209.7 (CO), 178.9 (CO), 170.8 (CO),
80.0 (CH), 78.9 (COH), 57.6 (CH), 54.2 (C), 52.1 (CH3), 50.0
(CH), 47.3 (C), 44.5 (CH), 43.6 (CH2), 40.6 (C), 40.0 (C), 39.9 (C),
35.7 (CH2), 33.9 (CH2), 32.5 (CH3), 32.1 (C), 31.52 (CH2), 31.45
(CH3), 30.9 (CH2), 29.8 (CH2), 27.3 (CH3), 23.8 (CH2), 23.7 (CH2),
21.4 (CH3), 21.2 (CH3), 19.1 (CH3), 18.0 (CH3), 17.8 (CH3). HRMS
+
(EI): m/z calcd for C33H48O8 [M]+ 572.33492, found 572.33395.
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental methods, mechanistic considerations for
the formation of 10 and 11, conformational analysis of 4, 6, 16;
complete ref 46; 1H and 13C NMR spectra of all new
compounds, as well as HMQC, HMBC, COSY, and NOESY
NMR spectra and the key COSY, HMBC, and NOESY
correlations of compounds 6, 8−10, 12−15, 17, 19, and 20.
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
(EA/hex =1/1). H NMR (400 MHz, CDCl3, δ): 4.69 (dd, J1 = 11.6
Hz, J2 = 4 Hz, 1H), 3.66 (s, 3H), 3.63 (s, 3H), 3.62 (s, 3H), 3.51 (d, J
= 20 Hz, 1H), 2.77 (dd, J1 = 6.4 Hz, J2 = 5.2 Hz, 1H), 2.72 (d, J = 20
Hz, 1H), 2.44−2.31 (3H), 2.06 (s, 3H), 1.33 (s, 3H), 1.26 (s, 3H),
1.20 (s, 3H), 1.08 (s, 3H), 0.97 (s, 3H), 0.95 (s, 3H), 0.92 (s, 3H). 13C
NMR (150 MHz, CDCl3, δ): 205.5, 200.9, 179.5, 177.9, 174.0, 170.7,
79.1, 52.0, 46.9, 46.1, 44.2, 43.0, 42.6, 40.1, 38.8, 38.2, 33.7, 33.6 (br),
32.4, 31.9, 30.7, 30.0, 29.5, 28.4 (br), 27.7, 24.4, 23.1, 23.0, 21.4, 19.9
+
(br), 17.31 (br), 17.28 (br). HRMS (EI): m/z calcd for C35H54O10
[M]+ 634.37170, found 634.36992.
AUTHOR INFORMATION
■
Preparation of 19 and 20. A solution of diisopropylamine (10.7
mg, 0.1056 mmol, 15 μL) in dry THF (2 mL) in a flame-dried (under
vacuum) round-bottom flask was cooled to −78 °C, followed by a
dropwise addition of n-butyllithium (2.5 M in hexanes, 0.1056 mmol,
42 μL). The reaction mixture was warmed to 0 °C, stirred at this
temperature for 5 min, and then cooled to −78 °C. A solution of 16
(55 mg, 0.096 mmol) in dry THF (3 mL) was then added dropwise to
the solution of LDA by syringe at −78 °C. The reaction mixture was
stirred at this temperature for 30 min, at which time the reaction was
quenched by H2O (D2O) (2 mL). Following the addition of DCM (2
mL), the layers were separated, and the aqueous layer was extracted
with DCM. The organic fractions were combined, washed with water
and brine, and dried over Na2SO4. Evaporation of the solvent in vacuo
gave crude product mixtures that were further separated by
chromatography. Column chromatography on silica yielded an
inseparable mixture of compounds (7 mg) containing 20, Rf = 0.43
(EA/hex = 4/6), and pure 19 as a transparent oil (36 mg, 64%), Rf =
0.31 (EA/hex = 4/6). A mixture of unidentifiable compounds
containing 20 was further purified by semiprep HPLC (Agilent C18
column 21.2 × 250 mm, isocratic elution CH3CN/H2O = 8/2, flow
rate 5 mL/min) to yield 20 as a white foam (5 mg, 9%). tR = 30 min.
(2R,4aS,6aS,8S,9aS,11S,13aS,14R,16aS,16bR)-Methyl 11-Ace-
toxy-8,14-dihydroxy-2,4a,6a,10,10,13a,16a-heptamethyl-7,15-
dioxodocosahydro-8,14-epoxybenzo[6,7]cyclodeca[1,2-a]-
naphthalene-2-carboxylate (19, 6-H). 1H NMR (600 MHz,
CDCl3, δ): 4.65 (dd, J1 =11.4 Hz, J2 = 3.6 Hz, 1H), 3.74 (s, 3H),
3.14 (d, J = 12 Hz, 1H), 3.08 (br s, 1H), 2.91 (br S, 1H), 2.75 (2H),
2.37 (d, J = 15.6 Hz, 1H), 2.31−2.23 (2H), 2.18 (d, J = 12 Hz, 1H),
2.05 (s, 3H), 1.82 (s, 3H), 1.19 (s, 3H), 1.18 (s, 3H), 1.08 (s, 3H),
0.92 (s, 3H), 0.91 (s, 3H), 0.71 (s, 3H). 13C NMR (150 MHz, CDCl3,
δ): 207.6 (CO), 205.1 (CO), 178.6 (CO), 170.9 (CO), 100.9 (COH),
99.0 (COH), 79.9 (CH), 55.0 (C), 52.1 (CH3), 44.8 (CH), 44.6 (C),
43.6 (CH2), 40.6 (C), 40.5 (CH), 40.1 (C), 37.3 (C), 35.8 (CH2),
34.7 (CH2), 32.7 (CH3), 31.6 (CH2), 31.54 (CH3), 31.47 (C), 29.8
(CH2), 29.4 (CH2), 29.0 (CH2), 28.7 (CH2), 27.8 (CH3), 23.6 (CH2),
21.4 (CH3), 16.92 (CH3), 16.88 (CH3), 16.5 (CH3), 16.0 (CH3).
HRMS (ESI): m/z calcd for C33H50O9Na+ [M + Na]+ 613.3353,
found 613.3352.
Corresponding Author
*E-mail: tochtrop@case.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was begun in the laboratory of Stuart L. Schreiber
during the postdoctoral tenure of G.P.T. and was supported by
a grant from the National Institute of General Medical Sciences
(GM38627). The completion of the work occurred in the
laboratory of G.P.T. and was supported by grants from the
National Cancer Institute (CA157735 and CA132168), NSF
(Grant No. MCB-084480), Reuter Foundation, and Landon
Foundation INNOVATOR award from AACR to G.P.T.
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I
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