Total Synthesis of (-)-Lasonolide A

Article abstract of DOI:10.1021/jacs.6b05127

The lasonolides are novel polyketides that have displayed remarkable biological activity in vitro against a variety of cancer cell lines. Herein we describe our first-generation approach to the formal synthesis of lasonolide A. The key findings from these studies ultimately allowed us to go on and complete a total synthesis of lasonolide A. The convergent approach unites two highly complex fragments utilizing a Ru-catalyzed alkene-alkyne coupling. This type of coupling typically generates branched products; however, through a detailed investigation, we are now able to demonstrate that subtle structural changes to the substrates can alter the selectivity to favor the formation of the linear product. The synthesis of the fragments features a number of atom-economical transformations which are highlighted by the discovery of an engineered enzyme to perform a dynamic kinetic reduction of a β-ketoester to establish the absolute stereochemistry of the southern tetrahydropyran ring with high levels of enantioselectivity.

Full text of DOI:10.1021/jacs.6b05127

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Article
Total Synthesis of (–)-Lasonolide A
Barry M. Trost, Craig E. Stivala, Daniel R Fandrick, Kami L.
Hull, Audris Huang, Caroline Poock, and Rainer Kalkofen
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 22 Aug 2016
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Journal of the American Chemical Society
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Total Synthesis of (–)-Lasonolide A
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Barry M. Trost,* Craig E. Stivala, Daniel R. Fandrick, Kami L. Hull, Audris Huang, Caroline Poock,
Rainer Kalkofen
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Department of Chemistry, Stanford University, Stanford, California 94305-5080
ABSTRACT: The lasonolides are novel polyketides that have displayed remarkable biological activity in vitro against a variety of
cancer cell lines. Herein we describe our first generation approach to the formal synthesis of lasonolide A. The key findings from
these studies ultimately allowed us to go on and complete a total synthesis of lasonolide A. The convergent approach unites two
highly complex fragments utilizing a Ru-catalyzed alkene-alkyne coupling. This type of coupling typically generates branched
products, however through a detailed investigation we are now able to demonstrate that subtle structural changes to the substrates
can alter the selectivity to favor the formation of the linear product. The synthesis of the fragments features a number of atom eco-
nomical transformations which are highlighted by the discovery of an engineered enzyme to perform a dynamic kinetic reduction of
a b-ketoester to establish the absolute stereochemistry of the southern tetrahydropyran ring with high levels of enantioselectivity.
INTRODUCTION
Our pursuit of the target molecule was not only inspired by its
unique biological activity, but also its novel molecular archi-
tecture that presented us with the opportunity to execute and
expand on many of the organic transformations that had previ-
ously been developed in our laboratory. We aimed at develop-
ing a more concise and efficient synthesis compared to those
previously known. 2,6,7 A preliminary report of our efforts has
been communicated.7a
Lasonolide A was discovered in 1994 by McConnell and
coworkers in an effort to identify new and diverse antitumor
agents from marine organisms.1 The connectivity of the la-
sonolides and relative stereochemistry of each tetrahydropyran
were determined by NMR correlation spectroscopy from iso-
lated materials. In 2002, the correct fully elucidated structure
of lasonolide A was disclosed in Lee’s total synthesis.2 This
seminal work established the unknown relative stereochemis-
try of the C28 stereocenter and corrected the C17-C18 and
C25-C26 olefin geometries, which had been incorrectly as-
signed (Figure 1). Importantly, the synthesis also established
the absolute stereochemistry of the natural product and re-
vealed that the levorotatory or (–)-lasonolide A was the bio-
logically active enantiomer, contrary to what had been report-
ed in the isolation paper. Despite the remarkable activity la-
sonolide A displayed in the NCI’s 60-cell line screen,3 very
few analogs have been prepared.4,5
We recognized that the main point of diversification between
lasonolides A-F occurred at the C28 and C30 positions (Figure
1. Subtle structural differences had dramatic effects on the
biological profiles between each of the six natural analogs.
While a comprehensive understanding of the structure activity
relationships remains unclear, we felt that it was important to
devise a synthesis plan (Scheme 1) that could allow for the
installation of a range of diverse structural elements at these
positions. A late stage Wittig olefination could install the re-
quired Z-olefin geometry present in all lasonolides and enable
a flexible strategy for analog synthesis. The macrocycle can
then be disconnected into two equally complex
RESULTS AND DISCUSSION
OH
R1
OH
26
18
O
O
17
R1
28
28
18
Structural
Reassignment
O
O
26
17
R2O
O
O
R2O
Lee (Ref 2)
O
O
OH
Revised Lasonopyran Skeleton
Lasonopyran Skeleton
OH
lasonolide C (3)
HO
lasonolide B (2)
O
lasonolide D (4)
R1 = H
lasonolide E (5)
lasonolide F (6)
lasonolide G (7)
HO
lasonolide A (1)
O
O
O
O
HO
O
R1
=
R1
=
R1
=
R1
=
R1
=
R1 =
O
O
O
O
O
HO
R2 = H
R2 = H
R2 = H
R2 = H
R2 = H
R2 = CH3(CH2)10CO
R2 = H
Figure 1. Reassigned Lasonopyran Skeleton – Lasonolides A – G
1
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tetrahydropyrans, which could be joined together by an esteri-
of a tetrasubstituted propargylic center inhibits cycloisomeri-
zation of complex D to the point that the reaction is now fa-
vored via complex A.
fication/macrolactonization and a Ru-catalyzed alkene-alkyne
coupling. These disconnections of the lasonopyran skeleton
allowed us to identify three sub-targets 9, 10, and 11 for syn-
thesis.
In all our prior total syntheses involving terminal alkyne part-
ners, only branched products have been formed – alternaric
acid,9 amphidinolides A10 and P,11 and laulimalide.12 The par-
ticular efficiency of the Ru catalyzed macrocyclization in the
laulimalide synthesis stimulates exploration of the Ru cata-
lyzed process making linear rather than branched products. To
be applied to the total synthesis of lasonolide A, there are two
possible approaches that could afford the desired product
(Scheme 2). The first would feature an intramolecular reaction
to generate the macrocycle (14) of lasonolide A. The second
would be an intermolecular approach to form 19. Theoretical-
ly, in each scenario the linear isomer can be formed using ei-
ther functional group orientation (i.e. 12 vs. 13 and 15/16 vs.
17/18).
Scheme 1. Synthesis Plan for (–)-Lasonolide A
OH
Wittig Olefination
Alkene-Alkyne Coupling
R
O
O
O
O
26
9
25
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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41
42
43
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45
46
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60
O
O
O
O
HO
HO
O
OH
OH
Lasonopyran Skeleton
Macrolactonization
8
OR
O
O
O
O
O
12
23
19
7
O
10
PPh3+Br-
OMe
OH
HO
11
OH
9
10
We, therefore, initiated studies on representative model sys-
tems. Tetrahydropyrans 2013 and 21 were subjected to several
reaction conditions to test the feasibility of the intermolecular
coupling and to gain insight concerning the levels of line-
ar:branched selectivity. We screened several solvents that have
been previously utilized in alkene-alkyne couplings. No reac-
tion was observed in non-coordinating solvents such as
CH2Cl2 and DCE (Table 1, entries 1 and 2), likely due to cata-
lyst decomposition. Next we screened coordinating solvents
(acetone and DMF) (Table 1, entries 3-5) in an effort to stabi-
lize the coordinatively unsaturated Ru-catalyst. To our delight,
reactions run in DMF and acetone delivered the desired cou-
pled products. The linear:branched ratios were much higher in
acetone (3.5:1) than in DMF (1:1), suggesting that DMF pro-
motes a reversible cycloisomerization and therefore Curtin-
Hammett conditions. Increasing the amount of alkene 21 (5
equiv.) delivered the desired 1,4-diene (22) as a 4:1 mixture of
linear:branched isomers in 56% yield.14 Importantly, 74% of
the excess alkene was recovered at the end of the reaction.
The hallmark of our synthetic plan, the Ru-catalyzed alkene-
alkyne coupling, was unique in the fact that we aimed to ob-
tain a linear 1,4-diene product. Previously we demonstrated
that branched selectivity is generally observed, as shown in
eq.1.8 However, in a few cases, the linear product dominated
(eq. 2). We have put forth a mechanistic rationale to under-
stand this behavior. The proposed mechanism for the trans-
formation, via ruthenacyclopentene formation, is depicted in
Figure 2. The initial oxidative cycloisomerization with the
Ru-catalyst is believed to be reversible and can form one of
two possible regioisomers (A or B). A subsequent β-hydride
and reductive elimination, from the metallacycles, can afford
the linear or branched isomers. Steric factors suggest that in-
termediate B is more stable than intermediate A which ac-
counts for the preferential formation of the branched product.
However, the tautomerization of the initial alkene-alkyne
complex C is thought to be faster than that of complex D then
forms the new C-C bond between the sterically less-hindered
terminus of each unsaturated partner. If the rate of β-hydride
elimination of complex A can outcompete its cycloreversion to
complex C, a linear product would result. Indeed, introduction
O
[CpRu(MeCN)3]PF6
(CH2)7CO2CH3
+
(CH2)7CO2CH3
(eq. 1)
(CH2)7CO2CH3
+
86%
O
5:1 branched:linear
OH
O
[CpRu(MeCN)3]PF6
91%
(CH2)7CO2CH3
+
(CH2)7CO2CH3
+
(CH2)7CO2CH3
(eq. 2)
OH
OH
1:32 branched:linear
Proposed Catalytic Cycle
R1
R2
R1
R2
+
+
R1
R1
Ru
H
Ru
H
+
R2
+
R2
+
H
H
linear
product
branched
product
Ru
Ru
S
S
R1
Ru
S
S
S
R1
+
+
A
R2
R2
B
H
H
Ru
S
Ru
S
R1
R1
H
+
R2
R2
D
C
R2
R1
2
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Figure 2. Known Ru-Catalyzed Alkene-Alkyne Couplings Favoring Linear and Branched Products – Proposed Catalytic Cycle
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2
3
4
Scheme 2. Proposed Ru-catalyzed Alkene-Alkyne Couplings
Intramolecular Approach - Macrocyclization
5
6
7
O
O
O
O
O
O
O
O
O
cat.
cat.
[CpRu(MeCN)3]PF6
[CpRu(MeCN)3]PF6
O
O
O
HO
HO
HO
8
9
O
O
O
OH
OH
OH
12
Intermolecular Approach - Fragment Coupling
14
13
10
11
12
13
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15
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17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
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O
O
O
O
O
O
cat.
cat.
[CpRu(MeCN)3]PF6
[CpRu(MeCN)3]PF6
O
15
17
OH
HO
HO
OH
OR
OH
OR
HO
O
O
OH
O
O
O
OH
OH
OR
16
19
18
The alternative substitution pattern was also explored on a
similar model system using the optimal conditions discovered
in Table 1 - entry 5 (Scheme 3). In this case, the branched
isomer (27) was exclusively formed in 33% yield.
Scheme 3. Additional Model Studies for Alkene-Alkyne
Coupling
O
O
O
not observed
26
[CpRu(MeCN)3]PF6
acetone, rt
24
Linear
Table 1. Model Studies for Alkene-Alkyne Coupling
+
+
O
O
PMBO
O
33%
isolated yield
O
PMBO
O
O
27
25
22
Me3Si
[CpRu(MeCN)3]PF6
20
Branched
Linear
+
Me3Si
conditions
(see table)
+
O
PMBO
The synthesis of alkyne 28 commenced with an asymmetric
aldol reaction between ynone 32 and aldehyde 31.15 Under our
standard set of conditions significant elimination was observed
when reactions were run at room temperature (Table S2, entry
1), and at 4°C (Table S2, entry 2).16 The elimination could be
suppressed by running the reaction at -20°C (Table S2, entry
3) for 72 h. This affords the desired β-hydroxy ketone (30) in
a moderate 54% yield but with high levels of enantioselectivi-
ty (99% ee). Elimination of the β-hydroxy group could be
entirely suppressed by modifying the work-up procedure.
Switching from an aqueous work-up to a direct filtration
through celite improved the isolated yield of 30 to 78% (Table
S2, entry 4).
O
O
21
23 Me3Si
Branched
Entrya
Solvent (Conc.)
CH2Cl2 (5 M)
DCE (5 M)
21 (equiv)b
Convc
0%
22:23c
1
2
3
4
5
1.2
1.2
1.2
1.2
5.0
- : -
- : -
0%
DMF (5 M)
33%
37%
56%d
1 : 1
Acetone (5 M)
Acetone (1 M)
3.5 : 1
4.0 : 1
aAll reactions were run using 0.1 mmol (20), and 10 mol%
b
Scheme 4. Synthesis Plan for Alkyne 28
[CpRu(MeCN)3]PF6 at rt for 14 h. 21 was added as a solution
drop-wise over 1h using a syringe pump. cDetermined by 1H
NMR. dIsolated yield.
Oxy-Michael
Addition
HO
O
O
O
HWE
22
Transacetalization
The synthesis plan for alkyne 28 is presented in Scheme 4. A
dinuclear Zn-catalyzed aldol reaction between ynone 32 and
aldehyde 31 is envisioned to establish the absolute stereo-
chemistry of fragment 30. It is important that high enantiose-
lectivity is attained, since the C21 hydroxyl group will dictate
the formation of each subsequent stereocenter. The C22 qua-
ternary stereocenter will be created from the thermodynamic
formation of benzylidene acetal 29 via transacetilization. An
analogous approach was previously reported by the Kang
group in their total synthesis of (+)-lasonolide A.7d Finally,
Horner-Wadsworth-Emmons (HWE) olefination with subse-
quent oxy-Michael addition should favor formation of the
desired equatorial C23 stereocenter to complete the synthesis
of alkyne 28.
O
O
O
O
OH
28
O
Ph 29
O
SiEt3
O
O
+
21
21
O
SiEt3
32
OH
O
30
31
Zn-Prophenol
With an optimized protocol in hand, we then investigated a
1,3-syn reduction of β-hydroxy ketone 30 (Scheme 5). Under
standard conditions (Et2BOMe and NaBH4)17 the reduction of
30 only afforded a ~4:1 mixture of diastereomers favoring the
syn-diol. Switching to Kiyooka’s conditions (DIBAL-H, -
78°C) increased the diastereoselectivity to 17:1 and the desired
diol (33) was isolated in 98% yield.18 Selective protection of
3
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Page 4 of 13
the less sterically hindered secondary propargyl alcohol with
TBDPSCl gave 34.
with acetic acid20 suppressed formation of the carboxylic acid
and improved the isolated yield to 94%.
1
2
3
4
5
Scheme 5. Preparation of 34
Horner-Wadsworth-Emmons olefination and concomitant
intramolecular oxy-Michael addition generated tetrahydropy-
HO
OH
Ph
1
N
OH N
Ph
Ph
ran 39 as a single diastereomer detectable by H NMR in 94%
Ph
isolated yield.21 The stereochemistry of the tetrahydropyran
was assigned by ROESY correlation.
(5 mol%)
6
CH3
OH
O
O
O
7
8
9
Et2Zn, i-PrOH
O
O
Scheme 6. Synthesis of Alkyne 28
Ph
21
+
O
SiEt3
SiEt3
78%, 99% ee
Ph
O
O
O
30
31
32
O
O
OTBDPS
O
O
OTBDPS
TBAF, AcOH
94%
TPAP, NMO
76%
OH OTBDPS
OH OH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
TBDPSCl, Pyr.
92%
DIBAL-H
HO
O
SiEt3
19
SiEt3
O
19
R
SiEt3
SiEt3
98%
R = H (36)
R = OH (37)
35a
O
34
33
>17:1 dr
O
O
We then aimed to establish the C22 quaternary stereocenter
via transacetalization of 34 to form a thermodynamically fa-
vored benzylidene acetal.7d Utilization of Kang’s conditions
(PhCHO, TFA, PhMe -20°C to rt, 3.5 h), led primarily to the
formation of isomers 35c and 35d (Figure 3). Only trace
EtO
OEt
O
HO
O
O
O
O
O
P
EtO
LiBF4,
aq. MeCN
O
OEt
NaH
O
O
O
96%
OH
28
94%
Ph
39
Ph
38
Ph
1
amounts of 35a and 35b were detected in the crude H NMR
O
H
H
spectrum. Although the cyclization favored the incorrect iso-
mer, we were encouraged that these products were isolated in
near quantitative yield with little evidence of decomposition
that may have arisen from the acidic conditions. Assuming
that the observed products were not the thermodynamically
preferred ones, we decided to conduct the acetal formation in a
more polar solvent (CHCl3) for an extended period of time.
Under these new conditions the desired acetal (35a) was
strongly favored in good yield, 5:1 chemoselectivity and 10:1
diastereoselectivity. Importantly, the undesired isomers (35b,
35c and 35d) could be separated from the predominant dias-
teromer (35a) by silica gel chromatography and re-subjected
to the reaction conditions. After 2 rounds of recycling the un-
desired diastereomers, acetal 35a, containing the newly
formed C22 quaternary stereocenter, was isolated in 93%
yield.
O
H
H
H
H
CO2Et
ROESY
H
H
O
CH3
The completion of the alkyne 28 involved hydrolysis of the
benzylidene acetal and cyclization of the resulting primary
alcohol. Toward this end, a variety of acids were screened to
effect the desired hydrolysis/cyclization sequence. The use of
p-toluenesulfonic acid (MeOH)22 or acetic acid (THF, 50 °C)23
gave only recovered starting material. Reactions involving
BCl3 (DCM, -78 °C to rt),24 2M HCl (MeOH, 65 °C),25 or
Amberlyst A-15 (4 Å MS, MeCN)26 led to complicated mix-
tures of partially hydrolyzed products and alkyne 28 in <40%
isolated yield. This partial conversion to the lactone was not
unexpected since the equilibrium to remove the benzylidene
requires the presence of both acid and water, whereas the equi-
librium to form the lactone requires the presence of acid, but
the exclusion of water/alcohol. Along these lines, we discov-
ered that the desired lactol could be obtained in 53-82% isolat-
ed yield by utilizing a two-step procedure, which involved
hydrolysis of the acetal with HCl in THF at 80 °C and then
ring closure using catalytic p-TsOH in refluxing toluene.27 We
later discovered that formation of alkyne 28 could be accom-
plished in a single step using LiBF4 in aq. MeCN in 96%
yield.28
With 35a in hand we pushed forward towards the completion
of the alkyne 28 (Scheme 6). The sequence began with explor-
ing an oxidation of the primary neopentylic alcohol. Surpris-
ingly, 35a was resistant to oxidation using Dess-Martin perio-
dinane, PDC, TEMPO, and under Moffatt-Swern conditions.
Fortunately, Ley’s catalytic TPAP/NMO oxidation reliably
delivered aldehyde 36 in 76% yield.19 TBAF mediated depro-
tection of the TBDPS and TES protecting groups followed by
concomitant cyclization produced lactol 38 in 59% yield. In
addition to lactol 38, carboxylic acid 37 was also isolated in
20% yield. We postulated the undesired side product was aris-
ing from a Cannizzaro reaction promoted by the hydroxide
typically present in TBAF solutions. Buffering the reaction
Transacetalization Results:
Ph
Ph
OH OTBDPS
OH OTBDPS
OH OTBDPS
O
O
OTBDPS
O
O
OTBDPS
PhCHO, TFA
O
O
+
+
+
O
SiEt3
SiEt3
SiEt3
Ph
O
Ph
O
O
HO
HO
SiEt3
SiEt3
34
35a
35b
35c
35d
Mole Ratios
Conditions
Toluene, -20°C - rt, 3.5 h
96%
trace
:
:
trace
1
:
:
:
:
4
1
CHCl3, rt, 18 h
10
<1
<1
(2 recycles, 93% of 35a)
Figure 3. Selective Formation of Benzylidene Acetal 35a
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The synthesis plan for alkene 11 is presented in Scheme 7. A tion decreased to 4:1. The relative stereochemistry was further
1
2
3
4
5
6
7
8
confirmed by nOe and J-coupling analysis on an advanced
intermediate.36These results were in direct contrast to the
trends observed from the literature, therefore we decided to
screen a number of isolated enzymes,37 generously donated to
us by Codexis Inc., to determine their performance in the dy-
namic kinetic asymmetric reduction of β-ketoester 41. Two
types of enzymes were examined in the reaction; the first was
derived from Bakers’ yeast and utilized GDH (glucose dehy-
drogenase) and NADP with glucose as the stoichiometric re-
ductant.16 The pH of these reactions must be carefully con-
trolled due to the accumulation of gluconic acid. The second
type of enzyme utilizes NADPH as the cofactor and 2-
propanol as the reductant. Table S5 of the SI provides a com-
prehensive screening of enzymatic methods.
dynamic kinetic asymmetric reduction of β-ketoester 41 was
envisioned to establish the absolute stereochemistry. The C7
stereocenter would be formed through a Michael addition,
which could occur after the installation of an appropriate ac-
ceptor. Several possible routes were considered for the for-
mation of the dienoate fragment. Our first tier of experiments
would focus on incorporating this side chain via Horner-
Wadsworth-Emmons olefination. Alternatively, if complica-
tions arose from the proposed HWE reaction, we also devel-
oped a convenient new strategy to form 2,4-dienoates that
evolved from the reaction of terminal alkynes with ethyl di-
azoacetate29 followed by subsequent phosphine-catalyzed
isomerization.30 A series of model studies, as illustrated in the
conversion of alkyne 43 to dienoate 46 via intermediate 45,
validated this latter approach (see Scheme 7).16
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
NADP dependent ketoreductases, in general, showed en-
hanced selectivity for the syn product. However, under the
standard conditions (Conditions A) using triethanolamine as a
buffer, significant olefin isomerization of β-ketoester 41 was
observed. After an exhaustive investigation of various buffers
(pH 4.5 – 10 screened), we found that a pH 4.5 phosphate
buffer completely suppressed the isomerization. CDX-024
demonstrated the highest diastereoselectivity favoring the syn-
diastereomer (48) with high levels of enantioselectivity (see
SI, Table S5, entries 7 and 8). Using this enzyme we were
able to obtain the desired syn product (48) in 75% yield, with a
4 : 1 diastereoselectitivy and greater than 95% ee (Table S5,
entry 8).38
Scheme 7. Synthesis Plans for Alkene 11
Coupling
BnMe2Si
O
O
HWE
1,4-Addition
12
O
O
O
O
OEt
7
OMe
DYKAT
OH
11
OR 40
41
Cross Metathesis
Diazoacetate - alkyne cross coupling and
Phosphine catalyzed isomerization
Alternative
Strategy
O
CuI (10 mol%),
MeCN
O
+
OEt
9
9
53%
44 19 : 1 (alkyne : allene)
N2
OEt
O
43
45
20% PCy3, AcOH,
Et3N, benzene,
60°C, 2d
O
OPg
42
OEt
Scheme 9. Synthesis of Diol 60
CDX-O24,
9
94%
46
O
O
OH
O
RO
O
NADP,
i-PrOH
R3SiOTf,
2,6-lutidine
Initially, we were intrigued by the prospect of utilizing a tran-
sition metal catalyzed hydrogenation reaction to establish the
absolute stereochemistry of fragment 11.31 Although these
highly atom economical32 methods proceed with high enantio-
and diastereoselectivities for a range of substrates, the chiral
catalysts developed so far exhibit poor diastereoselectivities
for methyl substituted β-ketoesters such as 41.33 As a result,
we believed it would be worthwhile to investigate a microbial
transformation that could establish the necessary absolute and
relative stereochemistry via dynamic reduction of a racemic β-
ketoester. After surveying the literature, we discovered that
Baker’s yeast reductions of methyl-substituted β-ketoesters
could provide the requisite products.16,34
10
9
OEt
OEt
OEt
75%,
>95% ee
R = TBS (50), 96%
R = TIPS (51), 99%
41
48 - syn
1. i-PrMgCl,
HN(OMe)Me • HCl
(S)-CBS,
catecholborane
RO
O
RO
OH
MgBr
2.
R = TBS (52), 83% (2 steps)
R = TIPS (53), 94% (2 steps)
R = TBS (54), 56%
R = TIPS (55), 71%
[Cp*Ru(MeCN)3]PF6
BnMe2SiH
HGII,
ethyl acrylate
RO
OH SiMe2Bn
13
R = TBS (56), 70%
R = TIPS (57), 86%
R = TBS (58), 72%
R = TIPS (59), 85%
12
CuI, TBAF, THF
EtO
O
HO
OH
EtO
O
RO
OH SiMe2Bn
Br
Scheme 8. Relative Stereochemical Assignment of 49
CN
86%
Bakers' Yeast,
MVK, H2O, 2d
60
OH
O
O
O
O
Zn
Br
*
OEt
OEt
OEt
*
67%
10 : 1 dr
The synthesis of fragment 11 was advanced using the synthet-
ic sequence depicted in Scheme 9. It began with the silyl pro-
tection of the C9 alcohol, which was subsequently followed by
a 2-step Weinreb amide synthesis and Grignard addition to
obtain ynone 52/53 in excellent yield.39 (S)-CBS reduction
under standard conditions ((S)-CBS, BH3•DMS, THF) deliv-
ered the desired propargyl alcohols (54/55) in good yields (50-
70%) as a 6.7 : 1 mixture of diastereomers. Yu has reported
that the use of nitroethane as solvent has a profound effect on
both reaction rate and stereoselectivity.40 Pleasingly, we found
that when we made this switch and used freshly distilled cate-
chol borane the diastereoselectivity improved to >20:1.
47
41
48
8%
12%
Ph
3%
H
H
nOe and
Coupling
Constants
steps
O
O
H
O
Ph
CH3
O
H
49
J = 10 Hz
Encouraged by this precedent, we embarked on our synthesis
of fragment 11 first by preparing the necessary starting mate-
rial (Scheme 8). β-ketoester 41 was generated through a Blaise
reaction with commercially available α-bromoester 47 and
allyl cyanide.35 Interestingly, when we investigated the reduc-
tion of β-ketoester 41 with Bakers’ yeast (Sigma-Aldrich,
YSC2, batch 035K0169) the undesired anti-product was
formed with 10:1 diastereoselectivity (30% ee). Without heat
treatment or additives, the diastereoselectivity for the reduc-
Propargyl alcohol 54/55 was then subjected to a Ru-catalyzed
hydrosilylation to generate trisubstituted (Z)-vinylsilane
56/57.41 In each case (R=TBS, or R=TIPS) high selectivities
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Page 6 of 13
for a single geometric isomer (>15:1) were obtained. In prep- cyclization was observed upon oxidizing allylic alcohol 67
1
2
3
4
5
6
7
8
aration for tetrahydropyran formation via oxy-Michael addi-
tion, the enoate acceptor was installed through cross metathe-
sis with ethyl acrylate.42 Finally, the C13 allyl moiety was
introduced by a copper mediated cross coupling to form diol
60.43
with MnO2, and the desired tetrahydropyran (68) was isolated
as a single diastereomer in an un-optimized 47% yield.
Scheme 11. Revised Synthetic Approach to Access THP 68
Current Route
O
O
steps
HO
TIPSO
67
OH
13
NaH, THF
0°C to rt
O
O
O
O
low
yielding
sequence
EtO
O
HO
60
OH
+
68
OTIPS
Hiyama
Cross
Coupling
89%
1 : 1 dr
EtO
EtO
OH
OH
61a
61b
Alternate Sequence
9
BnMe2Si
O
HO
TIPSO
69
OH SiMe2Bn [O]
13
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
EtO
O
TIPSO
OH
O
O
O
O
NaH, THF
12
70
+
9
2.4 : 1 dr
EtO
EtO
OTIPS
OTIPS
OTIPS
62
63a
63b
While encouraged by these results, access to allylic alcohol 67
was problematic due to several low yielding steps that were
not amenable to scale-up. To circumvent these issues we de-
cided to pursue an alternative reaction sequence that would
feature a THP cyclization, bearing the necessary TIPS protec-
tion on the C9 hydroxyl, before installation of the C13 allyl
segment (Scheme 11). However, in this alternate sequence, the
alcohol required for the copper-mediated cross-coupling reac-
tion would no longer be available. Therefore we decided to
investigate a Hiyama cross coupling45 for the late stage instal-
lation of the allyl moiety. At the time this work was executed,
the use of allyl acetate in the Hiyama cross coupling had not
been described.
Figure 4. THP Cyclizations of 60 and 62
Deprotonation of diol 60 with sodium hydride generated tetra-
hydropyrans 61a/61b as a 1:1 mixture of diastereomers. Un-
fortunately, no improvement in diastereoselectvity was ob-
served after screening various bases (t-BuOK and EtONa),
solvents (DMF, THF, and DMSO) and reaction temperatures
(-78°C, 0°C, rt and 60°C). Furthermore, equilibration of the
diastereomeric mixture under basic conditions did not improve
diastereoselectivity. We were aware that a similar approach to
the southern tetrahydropyran ring was utilized in Kang’s total
synthesis of (+)-lasonolide A;7d during their studies, an en-
hancement in diastereoselectivity was observed upon switch-
ing the protecting group on the C9 hydroxyl from TBS to
TIPS. Likewise, we observed a modest increase in diastereose-
lectivity (1:1 to 2.4:1) for the cyclization when a TIPS protect-
ing group was incorporated (62). This experiment supported
the notion that this structural feature was important for im-
proving selectivity in the cyclization.
Scheme 12. Synthesis of Vinyl Silane 71
DIBAL-H
HO
TIPSO
69
OH SiMe2Bn
EtO
O
TIPSO
OH SiMe2Bn
CH2Cl2
87%
59
O
O
P
BnMe2Si
BnMe2Si
O
EtO
OEt
OEt
O
O
O
MnO2
99%
4Å MS, LiOH
Scheme 10. Further Studies Investigation THP Cycliza-
tions
OEt
90%
71
OTIPS
70
OTIPS
DIBAL-H
MnO2
47%
EtO
O
HO
OH
HO
HO
OH
The revised synthesis of fragment 11 began with the DIBAl-H
reduction of ester 59 (Scheme 12). Allyic oxidation of alcohol
69 with MnO2 afforded tetrahydropyran 70 directly in nearly
quantitative yield as a single diastereomer.46 Attempts to per-
form the Hiyama cross coupling with aldehyde 70 and allyl
acetate were unsuccessful, presumably due to the insufficient
stability of the starting aldehyde. To eliminate this problematic
functionality, we first installed the dienoate moiety, before re-
exploring the Hiyama coupling. Elaboration of the aldehyde
into E,E-dienoate 71 was accomplished via Horner-
Wadsworth-Emmons olefination using 4 Å MS and LiOH.
70%
60
64
O
O
DBU
R = H (66) or
R = TIPS (68)
HO
OH
> 90%
O
4 – 5:1 dr
OR
65
single
diastereomer
MnO2
TIPSO
67
47%
DIBAL-H
70%
EtO
O
TIPSO
62
OH
HO
OH
Our investigation of the Hiyama coupling16 between allyl ace-
tate47 and vinyl silane 71 began with utilizing 2 mol%
Pd2dba3•CHCl3 and 4.2 equivalents of TBAF in a solution of
THF (see Scheme 13 and SI Table S6). After 19 hours, a mix-
ture of the desired product (72) along with an equimolar
amount of silanol 73 (entry 1) was obtained in 75% combined
yield. Extending the reaction time from 19 hours to 3 days also
generated a 1:1 mixture of 72 : 73 but diminished the yield to
44% (entry 2). Increasing both the catalyst loading (10 mol%)
and amount of TBAF (6.4 equiv.) was found to decrease the
proportion of the remaining intermediate silanol 73, and al-
kene 72 could be isolated in 78% yield (entry 3). It is im-
portant to note that, when we repeated the experiment, we
noticed that the highest yields for this reaction (85%) were
Several reports in the literature have indicated that cyclizations
of secondary alcohols onto a proximal α,β-unsaturated alde-
hyde could enhance diastereoselectivity.44 It is conceivable
that the lower pKa of the proton adjacent to the aldehyde
would render the reaction more reversible, and thereby favor
the formation of the thermodynamically preferred 2,6-cis tet-
rahydropyran ring. To test this hypothesis, α,β-unsaturated
aldehyde 65 was prepared (Scheme 10) in two steps from ester
60 and subjected to cyclization using DBU. We found that the
diastereoselectivity was improved to ~4–5 : 1. Recalling our
previous experience, we suspected that the incorporation of a
protecting group at C9 could further enhance distereoselectivi-
ty. Upon preparing the TIPS protected substrate, spontaneous
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Journal of the American Chemical Society
obtained when fresh TBAF solutions were used (entry 4). This the concentration from 0.02 to 0.04 M (entry 4) also had no
1
2
3
4
5
6
7
8
modest increase in yield may be related to the water concen-
tration present in TBAF solutions.48 Alkene 75 was completed
in two steps from 72; saponification using lithium hydroxide,
followed by TBS protection and in situ hydrolysis of the re-
sulting silyl ester (Scheme 13).
effect on conversion.
Scheme 14. Attempted Macrocyclization via Ru-Catalyzed
Alkene-Alkyne Coupling
O
O
Cl
O
Cl
Cl
O
O
O
O
Scheme 13. Synthesis of Alkene 75
Pd2(dba)3 • CHCl3
Cl
OH
28
O
Et3N, DMAP, benzene
+
O
OAc
TBAF
R
BnMe2Si
O
60%
O
LiOH
90%
O
O
OTBS
O
9
76
85%
O
OEt
OH
OEt
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
OH
O
O
71
OTIPS
OTBS
75
R = CH2CHCH2 (72)
R = SiMe2OH (73)
[CpRu(MeCN)3]PF6,
Acetone
TBSOTf
2,6-lutidine
O
O
O
O
O
or
O
O
O
O
DCE
O
O
OH
OH
91%
X
O
O
74
75
OH
OTBS
decomposition
O
O
OTBS
OTBS
76
77
Having achieved a synthesis of both fragments, we were
poised to examine our key Ru-catalyzed alkene-alkyne cou-
pling. Initial efforts were dedicated to exploring an intramo-
lecular reaction to forge the macrocycle, since to date only
three examples of intramolecular Ru-catalyzed alkene-alkyne
couplings have been reported. These have appeared in the total
syntheses of (+)-amphidinolide A, (+)-pinnatoxin A,49 and
laulimalide. However, in each of these examples only
branched products were generated. Figuring we possessed the
necessary coordinating groups to favor the generation of a
linear isomer, we were excited by the prospect of investigating
a linear selective macrocyclization. Two substrates were ex-
amined in the macrocyclization reaction (76 and 78), each of
which could easily be accessed using a Yamaguchi esterifica-
tion.50 Unfortunately, after extensive investigation we were
never able to form the desired macrocycle as rapid decomposi-
tion was observed in all experiments that were attempted
(Scheme 14).
O
OiPr
O
O
OiPr
O
[CpRu(MeCN)3]PF6
acetone
X
decomposition
O
O
O
O
TBSO
O
O
TBSO
OTBS
OTBS
78
79
Content with our reaction conditions (entry 3), we increased
the scale of the reaction from 5 µmol to 60 µmol in an attempt
to obtain an isolated yield for the coupling. To our surprise,
this seemingly minor modification was not tolerated and the
conversion diminished significantly to ~20%. In this case,
increasing the concentration to 0.12 M (entry 5) rescued the
conversion slightly (35%) but not to the levels that were seen
previously. Increasing both the catalyst loading and concentra-
tion had a deleterious effect on the overall conversion (entry
6).
We then decided to turn our attention to the exploration of an
intermolecular alkene-alkyne coupling (Figure 5, eq 3).16 We
were concerned that the π systems of electron deficient ester,
present in alkene 11, could be coordinating to the ruthenium
catalyst and inhibiting the coupling. As a result, we chose to
use alkyne 28 in excess in an attempt to diminish this potential
interaction (Table S7, entries 1-6). We observed 50% conver-
sion of the alkene to the desired coupled products (80a and
80b) as an inseparable 2:1 mixture of linear : branched iso-
mers (entry 1). Unfortunately, the excess alkyne that was used
could not be recovered as it seems to decompose over the
course of the reaction, likely to the hydrated dimer.9 Heating
the reaction to 50 °C had little effect on the overall conversion
(entry 2). However, when the alkyne was added to the reac-
tion drop-wise over 15 minutes and then heated at 50°C we
noticed an increase in conversion to 70% (entry 3). Increasing
Based on the experimental evidence presented in Table S7,
entries 1-5 (see SI), we realized that our initial concerns relat-
ed to catalyst deactivation via coordination to alkene 11 were
largely attenuated due to the fact that in each of these entries,
which successfully generated product, a 5-fold excess of al-
kene 11 to catalyst was present. Therefore we decided to
screen reactions that used the alkene in excess. This change
resulted in the complete consumption of alkyne 28 on a 68
µmol scale (Table S7, entry 7). Furthermore, after brief opti-
mization, we determined that only 3 equiv. of alkene 11 were
needed (using 10 mol% catalyst, in a 0.094 M solution of ace-
tone) to achieve a 66% yield (entry 8).
O
O
O
O
OH
O
O
O
[CpRu(MeCN)3]PF6
acetone
O
O
O
O
O
+
+
O
OH
OH
O
OH
OH
OMe
O
O
OH
conditions
(see table S7)
O
O
O
OH
OH
OH
O
proposed hydrated dimer
28
11
OMe
80a
80b
eq. 3
O
OMe
Figure 5. Intermolecular Ru-Catalyzed Alkene-Alkyne Coupling
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Page 8 of 13
1
2
Scheme 15. Attempted Macrolactonization and Revised Synthetic Strategy
O
O
O
O
O
O
O
3
4
5
6
7
8
9
21
O
O
macrolactonization
(see text)
1. LiOH, THF/H2O
2. p-TsOH, PhMe/MeCN
O
O
O
OH
OH
O
X
60% (2 steps)
OH
OH
O
O
O
OH
OMe
80a
OH
81
82
H
OH
H
H
H
O
OH
O
RO2C
RO
O
vs.
R
R
O
21
21
H
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Me
Me
Rigid
Flexible
OPg
O
PgO
O
O
O
O
1. Reduction
2. Wittig
Target Substrates
OH
PgO
O
for Macrolactonization
3. Saponification
OH
PgO
OPg
O
OPg
OH
84
O
83
OH
With a reliable synthetic route, we advanced our synthesis
adhering to the current synthesis plan (Scheme 15). Saponifi-
cation of the ester and lactone, followed by re-lactonization
delivered seco acid 81. Macrolactonization was attempted with
a variety of reagents including 2,4,6-trichlorobenzoyl chloride,
2-methyl-6-nitrobenzoic anhydride,51 and dibutyltin oxide.52
Unfortunately, in all cases the macrocycle was never observed
and the substrate decomposed under each of the reaction con-
ditions. We speculated that the restricted conformational mo-
bility of the C21 alcohol could be unable to access its optimal
conformation for macrolactonization. Therefore, opening the
lactone could enable further flexibility of the C21 alcohol and
could allow for the desired cyclization to occur. As a result we
decided to incorporate this idea into two modified substrates,
83 and 84.
Table 2. Probing at Structure and Solvent Effects in Ru-
Catalyzed Alkene-Alkyne Coupling
HO
O
R1O
O
O
OR3
R3O
25
OTBS
R2O
OR2
93
O
95a
OMe
[CpRu(MeCN)3]PF6
+
+
conditions
(see table)
HO
O
O
O
O
OR3
95b
R3O
OMe
OTBS
OTBS
94
O
OMe
Scheme 16. Preparation of Wittig Salt 90 and Attempted
Olefination with Lactol 91
Entrya
1
R1
R2
R3
Solvent
acetone
Yieldb 95a:95bc
O
O
H
H
H
H
32%
39%
3.2 : 1
3.8 : 1
Br
MgBr
2
cyclo-
pentanone
88
86
O
OH
CuI
p-TsOH • H2O
OH
85%
78%
3
4
H
H
acetone
41%
62%
3.7 : 1
3 : 1
85
87
1. TBSOTf,
2,6-lutidine
2. PPh3
OH
OTBS
cyclo-
pentanone
O
O
O
O
90
86% 2 steps
Br
PPh3+Br-
89
5
TBS
cyclo-
pentanone
36%
3.7 : 1
OTBS
aAll reactions were run using 3.0 equiv. 94, 15 mol%
[CpRu(MeCN)3]PF6, in 0.047 M solvent, at 55°C for 2 h 10 min. b
Mixture of 95a:95b . c Determined by 1H NMR.
O
HO
O
90, KHMDS, THF
O
O
O
X
OH
91
92
HO
OH
At this point, we decided to take a step back and, in turn, we
began investigating a synthesis of seco acid 84. Ideally, an
alkene-alkyne coupling between triol 93 and alkene 94 could
provide rapid access to seco acid 84. Accordingly, we began
screening conditions similar to those found to be optimal pre-
viously (Table S7). When acetone was used as solvent, we
were able to obtain the desired products as their acetonides in
32% yield, as a 3.2:1 mixture of linear:branched isomers (Ta-
ble 2, entry 1). Upon switching the solvent to cyclopentanone,
the product ratio was enhanced to 3.8:1 (entry 2) – this was the
highest ratio we had ever observed in our studies. We specu-
We had hoped to access the elaborated seco acid (83) from
lactone 80a via reduction to the lactol, subsequent Wittig ole-
fination, and saponification. Disappointingly, after completing
the synthesis of Wittig salt 90, the proposed reaction sequence
was unsuccessful in our model system (Scheme 16). Warming
the reaction above -15°C decomposed the unstabilized Wittig
reagent rapidly.
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Journal of the American Chemical Society
late that the enhanced selectivity could be related to several ratio is highly dependent on the identity of the substrate and is
1
2
3
4
5
6
7
8
factors. Because the ruthenium catalyst has one open coordi-
nation site, the size of the solvent could have an effect on the
product ratios. Alternatively, triol 93 might have been protect-
ed as its acetonide or cyclopentanone ketal before coupling
occurs and the linear to branched ratios may be substrate spe-
cific. Finally, it is known that coordinating substituents in the
substrates have dramatic effects on product distributions,53 and
the presence of the C25 primary alcohol may be contributing
to selectivity.
less influenced by the solvent (i.e. cyclopentanone vs. ace-
tone).
From diene 95a, protection of the primary alcohol as its
TBDPS ether and hydrolysis of the cyclopentanone ketal pro-
vided diol 96, an intermediate that was used in Shishido’s total
synthesis of (+)-lasonolide A (Scheme 17).7e The physical data
for diol 96 was in complete agreement with the reported data.
The optical rotation of 96 was +11.4 (c 0.75, CHCl3), opposite
to that reported in the Shishido synthesis ( [ꢀ]!!" –19.3 (c 1.04,
CHCl3), and consistent with our absolute stereochemical as-
signment for (–)-lasonolide A.
9
We designed a series of experiments to probe these hypothe-
ses. To determine the extent of contribution from solvent, we
tested the cyclopentanone-protected substrate (Table 2, entry
3) in the coupling reaction using acetone as solvent. Only a
slight decrease in the linear:branched ratio from 3.8:1 to 3.7:1
was observed. When the acetonide-protected substrate (entry
4) was tested, this time using cyclopentanone as solvent, we
saw a decrease in linear:branched ratio to from 3.8:1 to 3:1.
Finally, to probe the role of the C25 alcohol, we incorporated
a TBS protecting group onto the cyclopentanone protected
substrate (entry 5), expecting the linear:branched ratios to
decrease if coordination was no longer possible. An identical
3.7:1 ratio of linear:branched isomers was obtained. The fact
that the TBS protecting group was cleaved during the reaction
invalidates any conclusions about the role of the hydroxyl
group since the desilylation could have occurred prior to or
after the alkene-alkyne coupling.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 18. Synthesis of Alkyne 92
O
O
P
1.
OEt
t-BuO
O
HO
O
O
O
OEt
NaH, THF
2. DIBAl-H, CH2Cl2
76% 2 steps
O
O
O
Ph
Ph
38
97
OH
O
1. 90, KHMDS, THF
O
O
2. LiBF4, aq. MeCN
69% 2 steps
OH
HO
92
At this point, we decided to further streamline our synthesis of
lasonolide A to help fulfill the need for material that will help
advance biological investigations. We turned our attention
back to the Ru-catalyzed coupling reaction and questioned if a
more elaborated coupling partner could be tolerated under the
reaction conditions. Alkyne 92, which contains the fully elabo-
rated side chain could be obtained in 4-steps from lactol 38
(Scheme 18). Although alkyne 92 contains two alkenes, disub-
stituted olefins are typically unreactive in the Ru-catalyzed
coupling.53
Scheme 17. Formal Synthesis of (–)-Lasonolide A
OH
OTBDPS
1. TBDPSCl, ImH
CH2Cl2
2. CSA, MeOH
O
O
OH
HO
O
O
O
OH
O
HO
61% (2 steps)
OTBS
OTBS
O
O
The alkene-alkyne coupling between 92 and 75 (Scheme 19),
run under our standard set of conditions, took place in 43%
yield with a linear:branched ratio of 3:1. The coupling be-
tween 92 and 74 also successfully occurred albeit with dimin-
ished linear:branched ratios (2:1) but in 69% yield. Reactions
run in cyclopentanone led to poor conversion and slightly di-
minished linear:branched ratios in each case. Additionally, for
the coupling between 92 and 74, decreasing the amount of
catalyst (from 15 mol% to 5 mol%) while also increasing the
concentration (from 0.047 M to 0.14M) decreased the yield to
28% (51% of 92 as its acetonide was recovered) without af-
fecting the linear:branched ratio. It is also important to note,
that in each of the examples described above, >80% of the
alkene 74/75 was recovered.
OMe
OMe
96
95a
FORMAL SYNTHESIS
(Shishido's Intermediate)
Several proposed ruthenacyclopentene intermediates for both
the linear and branched products are depicted in Figure 5.
After oxidative coupling, ruthenium has one open coordination
site and can be occupied by solvent (S) or by a coordinating
atom present in the substrate. For the linear isomer, the oxy-
gen in the tetrahydropyran ring or the primary alcohol could
occupy this site. For the branched isomer only coordination of
the primary alcohol may be possible but, as depicted in Figure
6, seems to be rather unfavorable. The facile desilylation that
was observed in the reaction (Table 2, entry 5) suggests that
the alcohol may be coordinated to the Lewis acidic ruthenium
rendering it more susceptible towards desilylation. Taken to-
gether, these results begin to provide evidence that the product
+
+
+
+
+
H
H
H
H
H
H
O
HO
HO
Ru
Ru
Ru
Ru
Ru
O
R
S
R
R
R
R
S
H
O
O
O
O
O
O
vs.
HO
O
O
O
O
O
O
O
O
O
Linear Product
Branched Product
Figure 6. Possible Ruthenacyclopentene Intermediates for Linear and Branched Products
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Page 10 of 13
1
2
3
4
5
6
7
8
Scheme 19. Final Reaction Sequence Completing the Synthesis of (–)-Lasonolide A and Lasonolide Analogs
OH
OH
OH
O
O
O
O
O
O
O
O
O
O
21
O
30
[CpRu(MeCN)3]PF6
O
O
+
OR
+
O
OH
O
HO
O
92
R=H; 69%
R=TBS; 43%
OR
R=H (98a)
R=TBS (99a)
R=H (98b)
R=TBS (99b)
O
O
R=H (74)
or
OH
OH
9
9
O
OH
R=TBS (75)
OR
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
OH
OTBS
O
O
Cl
O
28
Cl
Cl
O
O
O
O
Cl
TBSCl, ImH;
1M HCl
Et3N, DMAP
40-62%
CSA
O
O
OH
HO
OH
TBSO
R=H; 86%
R=TBS; 68%
R=H; 72%
R=TBS; 84%
OR
OTBS
R=H (100)
O
102
O
R=TBS (101)
OH
OH
OH
HO
HO
O
OR
O
O
O
O
OH
O
O
O
O
O
O
O
O
HO
OH
O
O
HO
28
O
RO
O
OH
O
O
O
O
O
104
O
OR
106
O
R=TBS (103)
HF • Pyr.
75%
105
(–)-lasonolide A (1)
Lasonolide Analogs
R=H
OH
Completion of the synthesis was accomplished from both in-
termediates 98a and 99a.54 Removal of the acetonide with
CSA followed by protection of the most accessible alcohols in
both 100 and 101 provided seco acid 102, a common interme-
diate in both routes. Macrolactonization using the Yamaguchi
reagent occurred without incident to provide the TBS protect-
ed lasonolide (103) in yields ranging from 40-62%. At this
point, the undesired branched isomer that was generated dur-
ing the coupling could be separated from the linear isomer. A
final desilylation, using HF•Pyr, provided the target molecule,
(–)-lasonolide A in 75% yield.
intermolecular coupling between fully unprotected polyhy-
droxy dieneyne 92 and tetraene 74/75 demonstrates a remark-
able chemoselectivity.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Experimental procedures, analytical data (1H NMR, 13C NMR,
MS, IR, and [a]D) for all new compounds as well as additional
reaction optimization tables.
In addition to synthesizing the natural product we have also
generated three analogs (Scheme 19). Compound 104 came
from macrolactonization of the fully deprotected precursor
hydroxyl acid. Compounds 105 and 106 were straightforward-
ly derived from the branched by-products of the alkene-alkyne
couplings. The synthetic lasonolide A and the 3 analogs were
submitted to in vitro testing in an attempt to explore their ac-
tivity against various cell lines.16 Each analog tested was es-
sentially inactive compared to the synthetic (–)-lasonolide A in
all assays except for the HCT116.
AUTHOR INFORMATION
Corresponding Author
*bmtrost@stanford.edu
Funding Sources
We thank the NIH (GM33049) for generous support to our pro-
grams. C.E.S., K.L.H., and A.H. are the recipients of NIH Ruth
L. Kirschstein National Research Service Awards (GM101747,
GM086093, and GM077840, respectively). C.P. is the recipient
of a DAAD ISAP fellowship.
SUMMARY
In conclusion, a synthesis of (–)-lasonolide A has been de-
scribed. The synthesis has been accomplished in 16 linear
steps and 34 total steps from commercially available starting
materials. A formal intermediate was also prepared and
matches the physical data that was reported in Shishido’s syn-
thesis of (+)-lasonolide A. Biological studies, utilizing the
synthetic material generated from this work, are currently un-
derway in an effort to further understand the mechanism of
action. Importantly also, these studies verify that the Ru cata-
lyzed alkene-alkyne coupling is amenable for making linear as
well as branched 1,4-diene motifs en route to natural products,
notably biologically active macrocycles. This success of the
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank Johnson-Matthey for generous gifts of palladium and
ruthenium salts and Codexis, Inc. for the generous donation of
enzymes. Finally we would like to thank Genentech (Gail Phil-
lips, Vishal Verma and John Flygare) for screening the synthetic
(–)-lasonolide A and lasonolide analogs.
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(12) (a) Trost, B. M.; Amans, D.; Seganish W. M.; Chung,
1
C. K. J. Am. Chem. Soc., 2009, 131, 17087–17089. (b) Trost,
B.M.; Amans, D.; Seganish, W.M.; Chung, C.K. Chem. Eur. J.
2012, 18, 2961–2971. (c) Trost, B. M.; Seganish, W. M.;
Chung, C. K.; Amans, D. Chem. Eur. J. 2012, 18, 2948–2960.
(13) Trost, B. M.; Yang, H.; Brindle, C. S.; Dong, G.
Chem– Eur. J. 2011, 17, 9777–9788.
(14) It is possible that alkyne 20 can dimerize under the re-
action conditions. For examples of Ru-catalyzed alkyne di-
merizations see: (a) Trost, B. M.; Rudd, M. T. J. Am. Chem.
Soc. 2001, 123, 8862–8863. (b) Trost, B. M.; Rudd, M. T. J.
Am. Chem. Soc. 2002, 124, 4178 – 4179.
(15) Trost, B. M.; Fettes, A.; Shireman, B. T. J. Am. Chem.
Soc. 2004, 126, 2660–2661.
(16) See Supporting Information for details.
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OH
O
1
2
3
4
5
O
O
O
O
HO
6
7
O
OH
8
(–)-lasonolide A
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
13
ACS Paragon Plus Environment