Enantioselective synthesis of tatanans A-C and reinvestigation of their glucokinase-activating properties

Article abstract of DOI:10.1038/nchem.1597

The tatanans are members of a novel class of complex sesquilignan natural products recently isolated from the rhizomes of Acorus tatarinowii Schott plants. Tatanans A, B and C have previously been reported to have potent glucokinase-activating properties that exceed the in vitro activity of known synthetic antidiabetic agents. Here, using a series of sequential [3,3]-sigmatropic rearrangements, we report the total synthesis of tatanan A in 13 steps and 13% overall yield. We also complete a concise enantioselective total synthesis of more complex, atropisomeric tatanans B and C via a distinct convergent strategy based on a palladium-catalysed diastereotopic aromatic group differentiation (12 steps, 4% and 8% overall yield, respectively). A plausible biosynthetic relationship between acyclic tatanan A and spirocyclic tatanans B and C is proposed and probed experimentally. With sufficient quantities of the natural products in hand, we undertake a detailed functional characterization of the biological activities of tatanans A-C. Contrary to previous reports, our assays utilizing pure recombinant human enzyme demonstrate that tatanans do not function as allosteric activators of glucokinase.

Full text of DOI:10.1038/nchem.1597

ARTICLES
PUBLISHED ONLINE: 24 MARCH 2013 | DOI: 10.1038/NCHEM.1597
Enantioselective synthesis of tatanans A–C
and reinvestigation of their glucokinase-
activating properties
Qing Xiao1, Jeffrey J. Jackson1, Ashok Basak1, Joseph M. Bowler2, Brian G. Miller2
*
and Armen Zakarian1
*
The tatanans are members of a novel class of complex sesquilignan natural products recently isolated from the rhizomes
of Acorus tatarinowii Schott plants. Tatanans A, B and C have previously been reported to have potent glucokinase-
activating properties that exceed the in vitro activity of known synthetic antidiabetic agents. Here, using a series of
sequential [3,3]-sigmatropic rearrangements, we report the total synthesis of tatanan A in 13 steps and 13% overall yield.
We also complete a concise enantioselective total synthesis of more complex, atropisomeric tatanans B and C via a
distinct convergent strategy based on a palladium-catalysed diastereotopic aromatic group differentiation (12 steps, 4%
and 8% overall yield, respectively). A plausible biosynthetic relationship between acyclic tatanan A and spirocyclic
tatanans B and C is proposed and probed experimentally. With sufficient quantities of the natural products in hand,
we undertake a detailed functional characterization of the biological activities of tatanans A–C. Contrary to previous
reports, our assays utilizing pure recombinant human enzyme demonstrate that tatanans do not function as allosteric
activators of glucokinase.
a
b
mall-molecule allosteric activators of human glucokinase have
generated considerable interest as potential therapeutic agents
for the treatment of type 2 diabetes1. These molecules are effec-
OMe
4'
OMe
OMe
9''
OMe
S
H+
tive at stimulating insulin secretion from pancreatic b-cells and
thereby rapidly reducing plasma glucose levels in animal models2.
At least one glucokinase activator, piragliatin, has advanced to
phase II clinical trials in humans3. The first glucokinase activator
was described in 20034, and since that time pharmaceutical chemists
have identified a variety of unique chemical scaffolds capable of sti-
mulating glucokinase activity in vivo5. Although all glucokinase acti-
vators investigated to date are synthetic, the recent discovery of the
tatanans, a novel class of naturally occurring lignan derivatives that
reportedly possess potent glucokinase-activating capabilities,
suggests that natural products might represent a rich resource for
structurally and mechanistically new antidiabetic agents6,7. As a
first step in exploring tatanans and the derivatives thereof as poten-
tial diabetes therapeutics, we undertook the total chemical synthesis
and detailed mechanistic characterization of the glucokinase-acti-
vating properties of tatanans A, B and C.
MeO
OMe
MeO
OMe
OMe
8
8'
9'
1''
3''
7'
MeO
MeO
MeO
7
OMe
OMe
OMe
OMe
4
MeO
OMe
Tatanan A (1)
MeO
OMe
7'
OMe
1''
7
C
OMe
1
O
OMe
A
OMe
MeO
OMe
Tatanan B (2)
Lignans are a large class of polyphenolic natural products related
to oligostilbenes8,9 but distinct in their basic monomeric unit10.
Tatanans are structurally unique sesquilignans recently isolated
from the ethyl acetate extracts of rhizomes of Acorus tatarinowii
Schott plants6. Tatanan A incorporates three consecutive tertiary
stereocentres and three aryl substituents appended to an acyclic
framework. Tatanans B and C, on the other hand, are cyclic atropo-
isomers characterized by complex spirocyclic architectures that
MeO
MeO
OMe
OMe
H2O
OMe
OMe
+O
MeO
Me OMe
O
MeO
OMe
OMe
OMe
OMe
Tatanan C (3)
OMe
OMe
OMe
combine a cyclohexane ring substituted at each position with a Figure 1 | Tatanans A–C are structurally unusual novel sesquilignans
2,5-cyclohexadienone subunit. The existence of atropoisomerism isolated from the rhizomes of Acorus tatarinowii plants. These natural
in tatanans B and C is evidently a consequence of the restricted products have been reported to potently activate glucokinase. a, Structure of
rotation around the C1–C7 bond that links ring A, proximal to tatanans. b, Proposed biosynthetic relationship between acyclic tatanan A
the quaternary stereogenic centre at the spirocyclic ring junction, and spirocyclic tatanans B and C.
1Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, USA, 2Department of Chemistry and Biochemistry,
Florida State University, Tallahassee, Florida 32306-4390, USA. e-mail: zakarian@chem.ucsb.edu; miller@chem.fsu.edu
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597
ARTICLES
a
OMe
OMe
OMe
OMe
OMe
OMe
[3,3]-sigmatropic
MeO
[3,3]-sigmatropic
transposition
MeO
OMe
OMe
MeO
transposition
OMe
OSiMe3
MeO
MeO
MeO
MeO
OMe
O
OMe
O
OMe
MeO
OMe
MeO
OMe
(Z)-4
Ph
Ph
H
b
OMe
OMe
OMe
O
OMe
OMe
N
B
OMe
OMe
OMe
1. TiCl4, Et3N
2. MsCl, Et3N;
DBU
CH3CH2COCl
Py, 0 oC
MeO
BH3 Me2S
MeO
MeO
MeO
THF, –10 o
C
O
+
6
O
O
70%
98%
99%
MeO
MeO
MeO
MeO
MeO
MeO
MeO
e.e. 93%
O
O
OH
OMe
OMe
OMe
MeO
OMe
7
8
5
9
OMe
OMe
1. H2, Pd-CaCO3-Pb, MeOH
2. PPh3, I2, ImH, CH2Cl2
3. 3,4-dimethoxyphenol,
1. LiAlH4, THF
OMe
OMe
2. (COCl)2, Me2SO; Et3N
3. CBr4, PPh3, CH2Cl2
4. LDA, (CH2O)n, THF
LDA, THF, –78 oC;
Me3SiCl
MeO
MeO
O
K2CO3, acetone, 50 o
C
OH
–78 to 25 o
C
OH
68%
70%
84%
MeO
MeO
MeO
d.r. 7:1
OMe
11
MeO
OMe
10
OMe
MeO
OMe
OMe
OMe
1. NaH, MeI, THF
2. Pd-CaCO3-Pb
H2, EtOAc
OH
OMe
Me3Al, PhMe
100 oC, 2 h
+
O
–
[3,3]-shift
MeO
AlMe3
MeO
Tatanan A
OMe
83%
Me
H
H
42%
MeO
MeO
Ar
(60%
OMe
MeO
MeO
O
OMe
b.r.s.m.)
OMe
13
OMe
Me
d.r. 3:1
OMe
Ar
12
TS-12
Minimization of A1,3-strain
Figure 2 | Description of the total synthesis of tatanan A using consecutive sigmatropic rearrangements as a key element of the synthesis design, setting
the three adjacent stereogenic centres of the natural product. a, Outline of the synthesis plan for tatanan A, illustrating the application of the [3,3]-
sigmatropic transforms. b, Full reaction sequence for the enantioselective synthesis of tatanan A, 15 steps from aldehyde 6 and ketone 5. Ms,
methanesulfonyl; DBU, 1,8-diazabicycloundec-7-ene; THF, tetrahydrofuran; Py, pyridine; LDA, lithium diisopropylamide; ImH, imidazole.
to the cyclohexane subunit. This type of molecular structure is ketone (5) with 2,4,5-trimethoxybenzaldehyde promoted by
unprecedented among known lignans11,12, enhancing our interest titanium tetrachloride provided ketone
7
in 70% yield15.
in developing a concise chemical synthesis of these natural products. Enantioselective reduction of this to allylic alcohol 8 was
We postulated that acyclic tatanan A is biosynthetically interrelated accomplished using (R)-2-methyl-CBS-oxazaborolidine (CBS ¼ Corey-
to its spirocyclic congeners through a stereospecific electrophilic Bakshi-Shibata; 98% yield, 93% e.e.)16. The formation of the
ring-closing dearomatization initiated by a proton donor, as illus- propionyl ester of 8 in nearly quantitative yield was followed by
trated in Fig. 1b13. Thus, an additional goal of our synthesis the Ireland–Claisen rearrangement17, which proceeds through an
efforts was to investigate the facility of such a transformation.
initial Z-selective enolization with lithium diisopropylamide,
trapping of the lithium enolate with chlorotrimethylsilane to give
the ketene acetal (Z)-4, and its relatively rapid rearrangement via
Results and discussion
Total synthesis of tatanans A–C. Tatanan A, with its three a chair-like transition structure at 25 8C over the course of 1 h.
consecutive all-carbon tertiary stereogenic centres, became the This key constructive transformation18,19 establishes the requisite
initial goal of our synthesis efforts. Iterative application of [3,3]- configuration at the C7′ and C8′ stereocentres with good
sigmatropic rearrangements formed the basis of the synthesis stereocontrol (d.r. 7:1, 70% yield). Propargyllic alcohol 11 was
design (Fig. 2a)14, and its concise implementation is presented in accessed in four steps that included a reduction–oxidation
Fig. 1b. Aldol condensation of ethyl 2,4,5-trimethoxyphenyl sequence to reduce the carboxy group in 10 to the aldehyde,
2
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597
ARTICLES
OMe
with 2.5 mM CF3SO3H/CH2Cl2 at 0–25 8C resulted initially in no
reaction, and then decomposition to an intractable mixture of pro-
ducts at higher temperature. No reaction was observed under milder
conditions with pyridinium tosylate in CH2Cl2 or aqueous CH3OH.
Next, an approach was explored based on reversed polarity in the
cyclization, which again met with no success. Electrophilic or
radical activation of ring C in 15 by oxidation with hypervalent
iodine reagents (PhI(OAc)2, PhI(O2CCF3)2) in the presence of a
hydride donor (PhSiH3) only resulted in the formation of benzoqui-
none products, and no spirocyclization to tatanans B or C was
observed. These observations suggest that the postulated biosyn-
thetic assembly of tatanans B and C by cyclization of tatanan A is
unlikely to occur without the assistance of an enzyme.
OMe
MeO
Acidic or oxidative
electrophilic or radical
conditions
OMe
MeO
OMe
OMe
OMe
O
MeO
OMe
OMe
OMe
OMe
MeO
MeO
OR
OMe
3
14: R = Si(iPr)3
15: R = H
(PhSiH3)
PhI(OAc)2
CH2Cl2
OMe
OMe
Accomplishing the synthesis of the structurally complex tatanans
B and C required a reformulated approach. The key recourse in
developing a new synthesis plan was to reposition the ring-
opening bond scission from C7–C1′′ to C1′′–C7′′ (Fig. 4a). To
enable this strategy, a phenol dearomatization through intramolecu-
lar allylation by a p-allyl palladium complex was envisioned, which
was expected to occur with diastereotopic group differentiation and
set three consecutive stereogenic centres of the fully substituted
cyclohexane ring in one step. A precursor of acyclic intermediate
17 was to be assembled in a convergent manner by asymmetric con-
jugate addition of an (E)-crotyllithium reagent 18, where SDG is a
stereodirecting group, to cinnamic ester 19. Alkylation of the
ester enolate arising from the conjugate addition with methyl
iodide was expected to build the stereotriad found in 17 in an
expedient manner.
OMe
OMe
AcO–
MeO
OMe
MeO
OMe
+
MeO
MeO
O
MeO
O
MeO OAc
OMe
OMe
MeO
OMe
Figure 3 | Tatanans B and C can conceivably arise from cyclization of
tatanan A during their biosynthesis. The total synthesis of tatanan A
enabled an exploration of biomimetic cyclization of tatanan A derivatives to
tatanans B and C, which proved unsuccessful under a variety of cationic or
radical reaction conditions.
Implementation of the amended synthesis plan was initiated via
conversion of the aldehyde to the 1,1-dibromoalkene upon the development of a suitable conjugate addition method (Fig. 4b).
treatment with carbon tetrabromide and triphenylphosphine, and Addition of the lithiated crotyl phosphoramide to ester 19 with sub-
formation of the lithium acetylenide followed by quenching with sequent alkylation of the intermediate enolate with iodomethane,
paraformaldehyde, affording 11 in 68% overall yield. Selective following the protocol developed by Hanessian and co-workers23,
hydrogenation with Lindlar’s catalyst and two-step aryl ether delivered the expected product in high yield and with outstanding
formation provided 12 in 84% overall yield.
diastereocontrol. However, various attempts to remove the vinylic
Lewis-acid mediated [3,3]-sigmatropic rearrangement20 of (Z)- phosphoramide substituent proved to be prohibitively inefficient,
allylic aryl ether 12 secured the stereochemistry of the remaining prompting us to seek a more practical alternative. Addition of the
chiral centre while completing the carbon framework of lithiated (R)-tert-butyl (E)-2-butenyl sulfoxide24 21 occurred with
tatanan A. Phenol 13, isolated in 42% isolated yield (60% based high diastereoselectivity, delivering ester 22 as a single diastereomer
on recovered starting material, b.r.s.m.), was formed in a diastereo- in 50% yield after in situ methylation of the transient ester enolate25.
selective process that favoured the desired stereoisomer with a 3:1 Diastereoisomeric products resulting from a-alkylation of the sulf-
selectivity. The Z configuration of the double bond was necessary oxide accounted for the remainder of the mass balance. Oxidation of
to enhance diastereocontrol. The major stereoisomer presumably the sulfoxide to sulfone followed by a palladium-catalysed desulfo-
arises through a reaction pathway involving transition structure nation (Pd(acac)2, i-PrMgBr) afforded terminal alkene 24 in good
TS-12, which is favoured due to minimization of allylic strain. yield26. The diastereotopic aryl groups were introduced at this
Rearrangement of the corresponding (E)-allylic ether provided an stage by addition of 2 equiv. of the aryllithium reagent derived
approximately 1:1 mixture of stereoisomers. Starting from from 25 through lithium–bromine exchange, followed by electro-
rearrangement product 13, the completion of enantioselective syn- philic reduction of 26 with Et3SiH in the presence of BF3-etherate27.
thesis of tatanan A required only two additional steps. Methylation The cross-metathesis reaction28 between 27 and methyl acrylate
of the phenolic hydroxyl (NaH, CH3I, THF) followed by chemo- using the Hoveyda–Grubbs II catalyst effectively provided a,b-
selective hydrogenation of the terminal alkene in the presence of unsaturated ester 28, which was converted to allyl methyl carbonate
Lindlar’s catalyst delivered 26 mg of tatanan A (83% yield over in three steps and 86% overall yield.
two steps). Synthetic tatanan A showed identical spectroscopic
For the critical assembly of the spirocyclic ring system, a tran-
data (1H and 13C NMR) to those published for the natural sition metal-catalysed cyclization with concomitant dearomatiza-
product; the optical rotation for the synthetic material was of the tion and diastereotopic aryl group differentiation was selected as
same sign but of a notably higher value ([a]2D0 þ 55 (c 0.1, the preferred tactical solution (Fig. 4a)29. Although intramolecular
CH3OH); literature6 [a]D20 þ 10 (c 0.1, CH3OH)).
metal-catalysed allylic dearomatization is a powerful approach to
The synthesis of tatanan A on the aforementioned scale enabled the synthesis of spirocyclic cyclohexanediones, to date there are
a study exploring its conversion to spirocyclic tatanans B and no applications of this methodology in the total synthesis of
C, emulating the proposed biosynthetic pathway depicted in complex natural products30. Very recently, two reports describing
Fig. 1b21,22. The C4′′ triisopropylsilyl ether 15 was prepared via methodological studies on direct intramolecular C-allylation of
straightforward modification of reactions used in the total synthesis phenols have appeared in the literature, demonstrating the feasibility
of tatanan A (Fig. 3). Exposing the C4′′ phenolic hydroxyl was of this process. In 2010, Hamada and co-workers31 described a
expected to activate ring C as a nucleophlic counterpart in the cycli- palladium-catalysed spirocyclization forming a spiro[4.5]decane
zation. Under a variety of acidic reaction conditions, 15 failed to ring system. In that study, a single example was reported that
undergo cyclization to tatanans B or C. For example, treatment afforded a spirocyclic ring system containing two six-membered
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597
ARTICLES
OH
a
MeO
DGD
O
MeO
7''
MeO
O
7
PdLn
SDG
Ar
7'
O
7'
OtBu
OMe
7
1"
+
7''
8'
O
8'
1"
8
Li
7
O
MeO
OMe
OMe
OMe
OMe
MeO
Me
9
I
9
OMe
O
OH
18
19
3
SDG = stereodirecting group
17
Key features of design:
Diastereotopic group differentiation (DGD)
Conjugate addition to set the acyclic stereotriad
Dearomatization with π-allyl palladium complex
OTIPS
MeO
b
25
OMe
OMe
MeO
OMe
OMe
MeO
Br
MeO
LDA, LiBr, THF, –78oC;
then 19, –78 oC; CH3I
Pd(acac)2 (10 mol%)
iPrMgBr, THF
mCPBA, CH2Cl2
0 oC, 40 min
nBuLi, THF,
25 oC, 3 h
O
S
O
S
OMe
CO2tBu
OMe
OMe
CO2tBu
tBuSO2
CO2tBu
21
50%
86%
71%
89%
O
P
22
23
24
N
N
20
OTIPS
OH
OTIPS
OTIPS
BF3 Et2O, Et3SiH
CH2Cl2
1. (iBu)2AlH, CH2Cl2
2. ClCO2Me, Py
3. nBu4NF, THF
CH2=CHCO2Me
HGII (5 mol%), CH2Cl2
40 oC, 72 h
MeO
MeO
MeO
MeO
–78 oC, 30 min
OCO2Me
Ar
OMe
Ar
OMe
Ar
OMe
Ar
OMe
OH
OMe
OMe
OMe
MeO2C
OMe
7
93%
93%
88%
MeO
OH
MeO
OTIPS
MeO
OTIPS
MeO
OTIPS
29
28
26
27
MeO
1. NaH, CH3I, THF
2. H2, Lindlar cat.
EtOAc
Pd(dba)2 (20 mol%)
P(OPh)3 (48 mol%)
CH2Cl2, 25 oC, 12 h
MeO
O
MeO
MeO
MeO
MeO
Tatanan B
(24%)
1"
OMe
R
OMe
7
1"
7
7
1"
OMe
+
+
OMe
MeO 7"
78%
OMe
84% (92% b.r.s.m.)
d.r. 6:3:2
O
O
MeO
OMe
OMe
OMe
OH
Tatanan C
(48%)
A
OMe
A
OMe
OMe
MeO
MeO
OH
see Table 1
OR'
34: R = CH=CH2, R'=H
32
33
35: R = CH2CH3, R'=CH3
c
MeO
MeO
MeO
MeO
PdLn
PdLn
PdLn
PdLn
O
O
O
O
O
7"
OMe
OMe
OMe
OMe
OMe
OMe
OMe
7
7
1"
Ar
Ar
Ar
1
OMe
–O
–O
–O
–O
MeO
OMe
OH
G
E
C
A
O
O
O
O
O
vs
vs
vs
vs
7
LnPd
MeO
MeO
MeO
PdLn
PdLn
MeO
Ar
PdLn
O
7"
OMe
OMe
OMe
O
OMe
OMe
7
OMe
OMe
OMe
1"
Ar
1
OMe
–O
Ar
–O
–O
–O
O
O
O
O
MeO
OH
B
H
D
F
Figure 4 | Total synthesis of tatanans B and C by a palladium-catalysed cyclodearomatization. a, Reformulated synthesis plan centred on a convergent
strategy enabled by a stereochemically complex palladium-catalysed cyclization featuring diastereotopic aromatic group differentiation with concomitant
construction of the central quaternary centre. b, Successful implementation of the convergent synthesis plan leading to a separable mixture of atropisomeric
tatanans B and C in 12 steps from sulfoxide 21. c, Four pairs of transition structures rationalizing all four stereogenic events in the complex palladium-
catalysed cyclodearomatization of substrate 29. Structures A and B explain the observed selectivity for the formation of the C1′′ quaternary centre. Structures
C and D provide the basis for the observed high selectivity during the diastereotopic group differentiation establishing stereochemistry at C7. Structures E
and F explain the preference for the observed selectivity at C7′′, favouring the vinyl substituent at the equatorial position. Structures G and H rationalize the
observed atropselectivity, with a moderate preference for G over H, which places the ortho-MeO group in ring A in a sterically crowded environment of the
forming cyclohexane ring. LDA, lithium diisopropylamide; MCPBA, 3-chloroperbenzoic acid; acac, acetylacetonate; TIPS, triisopropylsilyl; HGII, Hoveyda–
Grubbs II catalyst; dba, dibenzylideneacetone.
4
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597
ARTICLES
rings. In 2011, You and colleagues reported a related iridium-cata-
The synthesis of tatanans B and C was readily accomplished in
lysed dearomatization producing a 3-azaspiro[5.5]undecadienone, two direct steps from 33 and 32, respectively: (i) phenol methylation
together with numerous examples of intramolecular allylation and (ii) chemoselective hydrogenation of the vinyl group (H2,
giving spiro[4.5]decane ring systems32.
Lindlar’s catalyst, EtOAc; 78% yield over two steps). The atropiso-
In the context of this total synthesis endeavour, intramolecular meric tatanans B and C were separated by preparative reverse-
allylic dearomatization of complex substrate 29 was investigated phase HPLC. Completion of the total chemical synthesis delivered
concentrating on palladium(0) and iridium(I) catalysis (Table 1). 30 mg of (–)-tatanan C and 15 mg of (–)-tatanan B. The synthetic
We found that iridium-catalysed reactions were ineffective (entries material was spectroscopically identical to the natural products
1 and 2)32. In contrast to Hamada’s findings, the rate of allylic de- (1H and 13C NMR in two different solvents: CD3OD and
aromatization of 29 under palladium catalysis is substantially (CD3)2CO); however, the data for the absolute optical rotation
enhanced with decreasing s-donor capacity of the phosphine were substantially different (tatanan B: [a]2D0–30 (c 0.10, CH3OH);
ligand (entries 3–7). Although little to no conversion has been literature6 [a]D20 0 (c 0.1, CH3OH); tatanan C: [a]D20–47 (c 0.11,
observed with PPh3 or P(o-tol)3, the reaction took place with com- CH3OH); literature6 [a]D20 0 (c 0.1, CH3OH)).
plete conversion in the presence of P(2-furyl)3, P(OPh)3 or P(OEt)3.
Using palladium(II) acetate and triphenylphosphine as the source of Biological studies. Following the initial discovery and structural
the catalyst resulted in the formation of only a trace amount of the characterization of tatanans by Yu and co-workers, bioactivity
expected product (entry 9). Interestingly, a somewhat slower rate studies suggested that these novel natural products display
was observed when Pd2(dba)3 was used in place of Pd(dba)2 as antidiabetic activity through an ability to allosterically activate
the source of the active catalyst (entry 10).
human pancreatic glucokinase6. This finding was intriguing, as
Under optimized conditions, the desired cyclization could be tatanans appear to represent the first example of a biogenic
achieved cleanly upon treatment with Pd(dba)2 and P(OPh)3, glucokinase activator. Moreover, the reported bioactivities of
affording an inseparable mixture of atropisomeric products 32 tatanans A–C exceed that of a known, potent synthetic activator,
and 33 together with C1′′,C7′′-diastereomer 34 in 84% combined derivatives of which are currently under investigation as
yield (92% based on recovered starting material, d.r. 6:3:2, respect- therapeutic agents3. However, initial characterization of the
ively). During this operation, three of the six stereocentres of the glucokinase-activating properties of tatanans was limited,
target molecule, including the quaternary centre at the core of the presumably due to the small quantities of tatanan A, B and C
spirocyclic ring system, are set with a high degree of stereocontrol. isolated from natural sources. Our successful total synthesis efforts
Only three of 16 possible stereoisomers have been observed.
provided substantial quantities of all three compounds, thus
The stereochemical course of the spirocyclization reaction can be enabling a detailed mechanistic investigation of the glucokinase-
understood by considering chair-like transition structures A–H activating characteristics of the tatanans. Previous work indicated
(Fig. 4c), where the non-reacting aryl group resides in the equatorial that tatanan C was the most potent congener, with an EC1.5 value
position, and minimization of steric interactions between the ortho- (concentration of compound required to increase enzyme activity
methoxy substituents delivers the observed products. Each of the by 50%) of 0.16 mM toward human glucokinase6. Based on this
four pairs of transition structures demonstrates preferences within finding, our initial biochemical characterization efforts focused
the four corresponding stereochemical categories operative in the on tatanan C.
palladium-catalysed cyclization, namely (i) selectivity in the for-
Recombinant human pancreatic b-cell glucokinase was pro-
mation of the quaternary centre at C1′′ (A versus B); (ii) diastereo- duced and purified to greater than 95% purity using established pro-
selectivity at C7 (C versus D); (iii) selectivity at the vinyl-substituted cedures33, and the enzyme was used in assays to evaluate the impact
centre C7′′ (E versus F); and (iv) atropselectivity around the C1–C7 of tatanan C upon the kinetic parameters of glucokinase. As a
bond (G versus H). Although stereocontrol for the formation of the control, we also characterized the activation properties of RO-28-
quaternary centre is high, control over the axial selectivity 1675, a previously described synthetic glucokinase activator that
around the C1–C7 bond only reached a moderate level of ꢀ3:1. has served as a lead compound for therapeutic development4.
In minor isomer 33, the 2-MeO group of ring A is situated in the Consistent with previous reports, RO-28-1675 is a potent gluco-
sterically hindered position, eclipsing the cyclohexane subunit. kinase activator that elicits a tenfold decrease in the glucose K0.5
Nevertheless, the formation of atropisomers 32 and 33 enabled value and a 30% increase in the kcat value (Fig. 5). Similar to
eventual access to both natural products: tatanan B and tatanan C. other synthetic activators, RO-28-1675 also produces a reduction
A minor amount of 34 is also formed, probably by a competing in the kinetic cooperativity of human glucokinase, as reflected by
pathway proceeding through a boat-like transition structure.
a lower Hill number. In contrast to RO-28-1675, the presence of
Table 1 | Optimization studies for the key intramolecular allylic spiro-dearomatization.
Entry
Catalyst
Ligand
T (8C)
Time (h)
Yield of 32133 (%)
32:33
1
2*
3
4
5
[Ir(COD)Cl]2
[Ir(COD)Cl]2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(OAc)2
Pd2(dba)3
P(OPh)3
P(OPh)3
PPh3
P(o-tol)3
P(2-furyl)3
P(OEt)3
P(OPh)3
P(OPh)3
PPh3
25
25
25
25
25
25
25
25
50
25
36
36
36
36
12
12
24
12
0
10
30
0
69
75
76
1:1
3.5:1
2.5:1
2.5:1
1.9:1
2.0:1
6
7
8†
9‡
10
84 (92% b.r.s.m.)
,1
51 (76% b.r.s.m.)
24
24
P(OPh)3
1.4:1
Typical reaction conditions: substrate (15 mg) was stirred with catalyst (20 mol%) and ligand (48 mol%) in CH2Cl2 under argon.
*Cs2CO3 (2 equiv.) was used as an additive.
†Performed on 100 mg scale.
‡Ti(OPr-i)4 (2 equiv.) was used as an additive.
COD, 1,5-cyclooctadiene; dba, dibenzylideneacetone; b.r.s.m., based on recovered starting material.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597
ARTICLES
the densely substituted cyclohexane unit present in these molecules.
The atropisomeric products are formed in ꢀ2:1 ratio and converted
to atropisomeric tatanans B and C, completing the total synthesis in
13 steps from 2,4,5-trimethoxybenzaldehyde. Tatanan A was pre-
pared concisely by a strategy based on consecutive [3,3]-sigmatropic
rearrangements, allowing us to experimentally probe its putative
biosynthetic relation to spirocyclic tatanans B and C.
a
50
40
30
20
The observation that tatanans exhibit no glucokinase activation
under all conditions investigated herein strongly supports the con-
clusion that tatanans are not antidiabetic, allosteric activators of glu-
cokinase. The reason for the discrepancy between our bioactivity
assays and previous reports6 is unclear, as both investigations uti-
lized the same enzymatic assay for glucokinase activity. Notably,
Ni and co-workers measured glucokinase activity at only a single
glucose concentration6, whereas we determined the full kinetic
profile of glucokinase in the presence and absence of potential acti-
vators. In addition, the assay conditions used by Ni and co-workers
involved a non-optimal ratio of reagents; however, we also con-
ducted assays under these non-ideal conditions and failed to
observe glucokinase activation. The possibility that another
natural product of unknown structure co-purified with tatanans
during the initial isolation of these compounds, and is responsible
for the previously observed activation of glucokinase, remains to
be investigated. Our successful development of a concise and
high-yielding synthetic strategy for tatanans, and derivatives
thereof, will enable a broad investigation of the biological activities
of these structurally unique natural products.
k
cat (s–1
)
K
0.5 (mM)
12.0
1.2 0.1
11.0
Hill
Control
38
48
33
1
1
1
1
1.7 0.1
1.4 0.1
1.8 0.1
10
0
RO-28-1675
Tatanan C
1
0
20
40
60
80
100
[Glucose] (mM)
Relative change in glucokinase activity
b
Compound
RO-28-1675
Tatanan A
Tatanan B
Tatanan C
12.6 3.0
0.85 0.15
0.75 0.10
0.89 0.04
Figure 5 | In vitro enzymatic assays demonstrate that tatanans A–C do not
activate human glucokinase. a, Glucokinase activation assays conducted in
the absence (control) and presence (20
mM) of tatanan C and a previously
described synthetic activator, RO-28-1675 (20
mM). RO-28-1675 activates
glucokinase by increasing the kcat value, decreasing the glucose K0.5 value,
and reducing the Hill coefficient. Tatanan C has no effect on these catalytic
constants. Experimental rate data for each glucose concentration represent
the average of two or more independent assays, with the standard deviation
of the mean represented by the vertical bars. Kinetic constants were obtained
from the fit of the experimental data to the Hill equation and represent the
average of two or more complete kinetic profiles. G-6-P, glucose-6-phosphate.
b, RO-28-1675 produces a 12.6-fold increase in relative glucokinase activity,
while tatanans A, B and C do not increase enzyme activity. Activity changes
were calculated from the ratio of enzymatic second-order rate constants
([kcat/K0.5 activated]/[kcat/K0.5 control]) determined in the presence and
Received 22 August 2012; accepted 7 February 2013;
published online 24 March 2013; corrected online 4 April 2013
References
1. Matschinsky, F. M. Assessing the potential of glucokinase activators in diabetes
therapy. Nature Rev. Drug Discov. 8, 399–416 (2009).
2. Meininger, et al. Effects of MK–0941, a novel glucokinase activator, on glycemic
control in insulin-treated patients with type 2 diabetes. Diabetes Care 34,
2560–2566 (2011).
3. Bonadonna, R. C. et al. Piragliatin (RO4389620), a novel glucokinase activator,
lowers plasma glucose both in the postabsorptive state and after a glucose
challenge in patients with type 2 diabetes mellitus: a mechanistic study. J. Clin.
Endocrinol. Metab. 95, 5028–5036 (2010).
absence of 20 mM activator.
4. Grimsby, J. et al. Allosteric activators of glucokinase: potential role in diabetes
therapy, Science 301, 370–373 (2003).
5. Grimsby, J., Berthel S. J. & Sarabu, R. Glucokinase activators for the potential
treatment of type 2 diabetes. Curr. Top. Med. Chem. 8, 1524–1532 (2008).
6. Ni, G. et al. Glucokinase-activating sesquilignans from the rhizomes of Acorus
tatarinowii Schott. J. Org. Chem. 76, 2056–2061 (2011).
7. Hung, H.-Y., Qian, K., Morris-Natschke, S. L., Hsu, C.-S. & Lee, K.-H. Recent
discovery of plant-derived anti-diabetic natural products. Nat. Prod. Rep. 29,
580–606 (2012).
8. Snyder, S. A., Gollner, A. & Chiriac, M. I. Regioselective reactions for
programmable resveratrol oligomer synthesis. Nature 474, 461–466 (2011).
9. Snyder, S. A., Breazzano, S. P., Ross, A. G., Lin, Y. & Zografos, A. L. Total
synthesis of diverse carbogenic complexity within the resveratrol class from a
common building block. J. Am. Chem. Soc. 131, 1753–1765 (2009).
10. Snyder, S. A., ElSohly, A. M. & Kontes, F. Synthetic approaches to oligomeric
natural products. Nat. Prod. Rep. 28, 897–924 (2011).
11. Whiting, D. A. Lignans and neolignans. Nat. Prod. Rep. 2, 191–211 (1985).
12. Pan, J. Y., Chen, S. L., Yang, M. H., Wu, J., Sinkkonen, J. & Zou, K. An update on
lignans: natural products and synthesis. Nat. Prod. Rep. 26, 1251–1292 (2009).
13. Snyder, S. A. & Kontes, F. Explorations into neolignan biosynthesis: total
synthesis of helictenin B, helisorin, and helisterculin A from a common
intermediate. J. Am. Chem. Soc. 131, 1745–1752 (2009).
14. Ilardi, E. A., Stivala, C. E. & Zakarian, A. [3,3]-Sigmatropic rearrangements:
recent applications in the total synthesis of natural products. Chem. Soc. Rev. 38,
3133–3148 (2009).
15. Harrison, C. R. Transient titanium enolate aldol condensations. Tetrahedron
Lett. 28, 4135–4138 (1987).
16. Corey, E. J. & Helal, C. J. Reduction of carbonyl compounds with chiral
oxazaborolidine catalysts: a new paradigm for enantioselective catalysis
and a powerful new synthetic method. Angew. Chem. Int. Ed. 37,
1986–2012 (1998).
tatanan C at concentrations up to 20 mM failed to produce measur-
able changes in the kinetic profile and properties of human glucoki-
nase (Fig. 5). Higher concentrations of tatanan C were not explored
due to the limited solubility of this compound under aqueous con-
ditions; however, the concentrations used in our study are more
than an order of magnitude higher than the EC1.5 values reported
in the initial description of the tatanans.
We next investigated the bioactivities of the two remaining ses-
quilignans, tatanan A and B. Although the reported activities for
tatanan A and B towards glucokinase are somewhat lower than
that of tatanan C, both compounds displayed EC1.5 values (1.85
and 0.52 mM, respectively6) that are readily accessible with
enzyme activity assays. Similar to the tatanan C results, the inclusion
of tatanan A and B at concentrations up to 20 mM did not
produce an increase in glucokinase activity over control reactions
(Fig. 5b). In contrast, the known activator RO-28-1675 produced
a 12.6-fold increase in the second-order rate constant for enzyme
catalysis, kcat/K0.5
.
Conclusions
The concise, step-economical34,35 synthesis of novel sesquilignans
tatanans B and C was accomplished by an intramolecular palla-
dium-catalysed allylic phenol dearomatization of a complex sub-
strate. This transformation includes a diastereotopic aryl group
differentiation and effectively sets three of the six stereocentres of
6
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© 2013 Macmillan Publishers Limited. All rights reserved.

NATURE CHEMISTRY DOI: 10.1038/NCHEM.1597
ARTICLES
17. Ireland, R. E., Mueller, R. H. & Willard, A. K. The ester enolate Claisen
rearrangement. Stereochemical control through stereoselective enolate
formation. J. Am. Chem. Soc. 98, 2868–2877 (1976).
18. Newhouse, T., Baran, P. S. & Hoffmann, R. W. The economies of synthesis.
Chem. Soc. Rev. 38, 3010–3021 (2009).
19. Young, I. S. & Baran, P. S. Protecting-group-free synthesis as an opportunity for
invention. Nature Chem. 1, 193–205 (2009).
20. Castro, A. M. M. Claisen rearrangement over the past nine decades. Chem. Rev.
104, 2939–3002 (2004)
21. Stadler, D. & Bach, T. Concise stereoselective synthesis of (–)-podophyllotoxin
by an intramolecular iron(III)-catalysed Friedel–Crafts alkylation. Angew. Chem.
Int. Ed. 47, 7557–7559 (2008).
22. Stadler, D. & Bach, T. Highly diastereoselective Friedel–Crafts alkylation
reactions via chiral alpha-functionalized benzyllic carbocations. Chem. Asian J.
3, 272–284 (2008).
23. Hanessian, S. & Gomtsyan, A. Highly stereocontrolled sequential asymmetric
Michael addition reactions with cinnamate esters—generation of three and four
contiguous stereogenic centers on seven-carbon acyclic motifs. Tetrahedron Lett.
35, 7509–7512 (1994).
30. Qi, J., Beeler, A. B., Zhang, Q. & Porco, J. A. Jr. Catalytic enantioselective
alkylative dearomatization–annulation: total synthesis and absolute
configuration assignment of hyperibone K. J. Am. Chem. Soc. 132,
13642–13644 (2010).
31. Nemoto, T., Ishige, Y., Yoshida, M., Kohno, Y., Kanematsu, M. & Hamada, Y.
Novel method for synthesizing spiro[4.5]cyclohexadienones through a
Pd-catalyzed intramolecular ipso-Friedel–Crafts allylic alkylation of phenols.
Org. Lett. 12, 5020–5023 (2010).
32. Wu, Q. F., Liu, W. B., Zhuo, C. X., Rong, Z. Q., Ye, K. Y. & You, S. L. Iridium-
catalyzed intramolecular asymmetric allylic dearomatization of phenols. Angew.
Chem. Int. Ed. 50, 4455–4458 (2011).
33. Larion, M. & Miller, B. G. 23-residue C-terminal alpha-helix governs
kinetic cooperativity in monomeric human glucokinase. Biochemistry 48,
6157–6165 (2009).
34. Wender, P. A., Verma, V. A., Paxton, T. J. & Pillow, T. H. Function-oriented
synthesis, step economy, and drug design. Acc. Chem. Res. 41, 40–49 (2008).
35. Wender, P. A. & Miller, B. L. Synthesis at the molecular frontier. Nature 460,
197–201 (2009).
Acknowledgements
The authors thank Hongjun Zhou for continued assistance with NMR spectroscopy. This
work was supported by the US National Institutes of Health (NIGMS GM077379 to A.Z.,
NIDDK DK081358 to B.G.M.) and additional kind donations from Eli Lilly and Amgen.
24. Davis, F. A., Thimma Reddy, R., Han, W. & Carroll, P. J. Chemistry of
oxaziridines. 17. N-(Phenylsulfonyl)(3,3-dichlorocamphoryl)oxaziridine: a
highly efficient reagent for the asymmetric oxidation of sulfides to sulfoxides.
J. Am. Chem. Soc. 114, 1428–1437 (1992).
25. Binns, M. R., Haynes, R. K., Katsifis, A. G., Schober, P. A. & Vonwiller, S. C.
Aprotic conjugate addition of allyllithium reagents bearing polar groups to cyclic
enones. 1. 3-Alkylallyl systems. J. Am. Chem. Soc. 110, 5411–5423 (1988).
26. Fabre, J. L. & Julia, M. Organic synthesis with sulfones XXIX. Stereospecific
hydrogenolysis of vinylic sulfones with Grignards and transition metal catalysts.
Tetrahedron Lett. 24, 4311–4314 (1983).
Author contributions
Q.X., A.B., J.J.J. and J.M.B. planned, conducted and analysed the experiments. A.Z. and
B.G.M. designed and directed the project. A.Z. and B.G.M. wrote the manuscript. All
authors contributed to discussions.
27. Rye, C. E. & Barker, D. Asymmetric synthesis of (þ)-Galbelgin, (–)-Kadangustin Additional information
Supplementary information and chemical compound information are available in the
J, (–)-Cyclogalgravin and (–)-Pycnanthulignenes A and B, three structurally
distinct lignan classes, using a common chiral precursor. J. Org. Chem. 76,
6636–6648 (2011).
online version of the paper. Reprints and permissions information is available online at
www.nature.com/reprints. Correspondence and requests for materials should be addressed to
B.G.M. (bioassays) and A.Z. (chemical synthesis).
28. Connon, S. J. & Blechert, S. Recent developments in olefin cross metathesis.
Angew. Chem. Int. Ed. 42, 1900–1923 (2003).
29. Trost, B. M. & Crawley, M. L. Asymmetric transition-metal-catalyzed allylic
alkylations: applications in total synthesis. Chem. Rev. 103, 2921–2944 (2003).
Competing financial interests
The authors declare no competing financial interests.
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CORRIGENDUM
Enantioselective synthesis of tatanans A–C and reinvestigation of their glucokinase-
activating properties
Qing Xiao, Jeffrey J. Jackson, Ashok Basak, Joseph M. Bowler, Brian G. Miller and Armen Zakarian
Nature Chemistry http://dx.doi.org/10.1038/nchem.1597 (2013); published online 24 March 2013; corrected online 4 April 2013.
In the original version of this Article published online, the term Acorus tatarinowii was spelled incorrectly. is has now been corrected
in all versions of the Article.
© 2013 Macmillan Publishers Limited. All rights reserved.