Biomimetic diversity-oriented synthesis of benzannulated medium rings via ring expansion

Article abstract of DOI:10.1038/nchembio.1130

Nature has exploited medium-sized 8- to 11-membered rings in a variety of natural products to address diverse and challenging biological targets. However, owing to the limitations of conventional cyclization-based approaches to medium-ring synthesis, these structures remain severely underrepresented in current probe and drug discovery efforts. To address this problem, we have established an alternative, biomimetic ring expansion approach to the diversity-oriented synthesis of medium-ring libraries. Oxidative dearomatization of bicyclic phenols affords polycyclic cyclohexadienones that undergo efficient ring expansion to form benzannulated medium-ring scaffolds found in natural products. The ring expansion reaction can be induced using three complementary reagents that avoid competing dienone-phenol rearrangements and is driven by rearomatization of a phenol ring adjacent to the scissile bond. Cheminformatic analysis of the resulting first-generation library confirms that these molecules occupy chemical space overlapping with medium-ring natural products and distinct from that of synthetic drugs and drug-like libraries.

Full text of DOI:10.1038/nchembio.1130

article
puBLIsHed OnLIne: 18 nOveMBer 2012 | dOI: 10.1038/nCHeMBIO.1130
Biomimetic diversity-oriented synthesis of
benzannulated medium rings via ring expansion
renato a Bauer1, todd a Wenderski2 & derek s tan1–3*
Nature has exploited medium-sized 8- to 11-membered rings in a variety of natural products to address diverse and challenging
biological targets. However, owing to the limitations of conventional cyclization-based approaches to medium-ring synthesis,
these structures remain severely underrepresented in current probe and drug discovery efforts. To address this problem,
we have established an alternative, biomimetic ring expansion approach to the diversity-oriented synthesis of medium-ring
libraries. Oxidative dearomatization of bicyclic phenols affords polycyclic cyclohexadienones that undergo efficient ring expan-
sion to form benzannulated medium-ring scaffolds found in natural products. The ring expansion reaction can be induced using
three complementary reagents that avoid competing dienone-phenol rearrangements and is driven by rearomatization of a
phenol ring adjacent to the scissile bond. Cheminformatic analysis of the resulting first-generation library confirms that these
molecules occupy chemical space overlapping with medium-ring natural products and distinct from that of synthetic drugs and
drug-like libraries.
edium-ring structures (8−11 atoms) are found in a variety
Indeed, the synthesis of medium rings is a long-standing chal-
of natural products with diverse biological activities. Of lenge in organic synthesis17. Classical cyclization-based approaches
particular interest are benzannulated medium rings, which are subject to both entropic effects (frequency of encounters of reac-
M
include aryl ethers such as the heliannuols1 (allelopathic) and bra- tive groups at chain ends and additional losses in torsional degrees
zilone2 (anticoagulant); diaryl ethers such as aspercyclide A3 (IgE of freedom) and enthalpic effects (ring strain due to unfavorable
receptor inhibitor); aryl esters such as puerol A4 (Chinese tradi- bond angles and torsion). Moreover, medium rings suffer from
tional medicine component) and kurzichalcolactone5 (anticancer); unfavorable transannular steric interactions that are not present
and biaryls such as pterocaryanin C6 (anticancer, antiviral), stegana- in smaller and larger rings. As a result, the efficiency of medium-
cin7 (antileukemic) and rhazinilam8 (anticancer) (Supplementary ring cyclizations can be highly variable and subject to unpredictable
Results, Supplementary Fig. 1a).
substrate effects. For example, although ring-closing metathesis was
These cyclic frameworks are useful for organizing the overall pre- used in the total synthesis of aspercyclide C, it proved ineffective
sentation of functional groups to biological targets9. Furthermore, for accessing the closely related aspercyclides A and B19,20. Variable,
the conformational constraint provided by cyclic scaffolds can afford substrate-dependent yields (30–94%) have been reported in ring-
enhanced binding affinity compared to corresponding linear struc- closing metathesis approaches to heliannuol A and related helianane
tures10 (although not necessarily due to reduced entropic costs11,12). congeners21–26. In the sphere of library synthesis, elegant cyclization-
Conformational restriction has also been correlated with improved based approaches to medium rings have been reported27–31 but are
bioavailability13 and, in some cases, enhanced cell permeability14,15
.
generally limited in their ability to accommodate diverse back-
However, despite their occurrence in many important natural bone substituents, ring sizes and other structural factors that affect
products, medium rings are absent among the current top 200 cyclization efficiency.
brand-name and top 200 generic drugs (http://cbc.arizona.edu/
An alternative, potentially more flexible approach to the syn-
njardarson/group/top-pharmaceuticals-poster)16 (Supplementary thesis of medium rings involves ring expansion of polycyclic
Fig. 1b). In contrast, aliphatic five- and six-membered rings occur substrates32,33. Indeed, such ring expansion reactions have been
widely in these drugs, along with lower incidences of other small proposed in biosynthetic pathways to various medium-ring natural
rings (three, four or seven atoms) and macrocyclic rings (twelve or products. For example, ring-expanding rearomatization of a polycy-
more atoms). Although numerous compounding factors influence clic cyclohexadienone intermediate, formed via an earlier oxidative
the progression of molecules that ultimately become commercially dearomatization reaction, was proposed in a biosynthetic route to
successful approved drugs, synthetic accessibility has an important the alkaloid protostephanine in 1964 (ref. 34), and the chemical fea-
role at multiple stages of drug discovery and development. Thus, sibility of this route was demonstrated shortly thereafter (Fig. 1a)35.
the absence of medium rings in current drugs may be attributed Related oxidative dearomatization–ring-expanding rearomatization
to challenges associated with their synthesis17 that lead to their (ODRE) sequences have also been invoked in biosynthetic propos-
scarcity in commercial and proprietary small-molecule screen- als and biomimetic syntheses of erythrina and homoerythrina alka-
ing collections. Analogous academic efforts to discover biological loids (Fig. 1b)36–40. Notably, in all of these cases, the ring expansion
probes are likewise dependent on the composition of screening reaction is aided by a strategically placed nitrogen atom that drives
collections18, and a recent PubChem substructure search of the electron flow toward the developing aromatic ring.
>360,000-membered US National Institutes of Health Molecular
Thus, we envisioned that a biomimetic ring expansion approach
Libraries Small Molecule Repository identified only ten ben- might provide efficient and flexible access to a variety of benzan-
zannulated medium-ring molecules of the types targeted herein nulated medium-ring scaffolds found in natural products (Fig. 1c).
(Supplementary Figs. 2 and 3).
This ring expansion approach would circumvent the challenges
1tri-institutional training program in chemical biology, Memorial Sloan-Kettering cancer center, new York, new York, uSA. 2Molecular pharmacology &
chemistry program, Memorial Sloan-Kettering cancer center, new York, new York, uSA. 3tri-institutional Research program, Memorial Sloan-Kettering
cancer center, new York, new York, uSA. *e-mail: tand@mskcc.org
nature CHeMICaL BIOLOGY | AdvAnce online publicAtion | www.nature.com/naturechemicalbiology
1

NaTure CHemiCal biOlOgy dOI: 10.1038/nCHeMBIO.1130
article
a
OMe
NMe
O
MeO
OMe
HO
OMe
Reduction;
methylation
Ring-expanding
rearomatization
NMe
NMe
MgI2
LiAlH4;
diazomethane
MeO
MeO
MeO
OR
OMe
OR
Biosynthetic proposal: R = H
Laboratory synthesis: R = Me
Protostephanine
OH
b
HO
HO
MeO
Alternative
HO
dienone-phenol
rearrangement
(1,2-alkyl shift)
MeO
MeO
Ring-expanding
rearomatization
MeO
O
O
n
H
N
O
n
MeO
MeO
B
NaOH, MeOH
(n = 1, 2)
MeO
BF3·OEt2
(n = 2)
N
CF3
N
CF3
N
CF3
A
MeO
OH
O
HO
O
Pathway A
Pathway B
BF3
(Homo)erythrina skeletons
(Homo)proerythrinadienones
Homoaporphine skeleton
O
R2
i
i
c
R1
O
X
Br
RO
R1MgX
or
Sakurai-type
alkylation
HO
R1
1
H/OH
H/OH
1
or
OsO4, NMO
Br
OSiR´3
OH
ii
( )1–2
HO
HO
TBSO
TBSO
TBSO
TBSO
TMS
n
n
n
HO
HO
5
5
RO
O
O
TMS
ii
R2
R2
HO
X
R2
HO
R1
Oxidative
dearomatization
R1
R3
Ring-expanding
rearomatization
X
X
1–3 tailoring steps
R3
OMe
OH
R1
Y
R3
MeO
HO
O
HO
n
n
R4
R4
R4
Figure 1 | Overall strategy and precedents for the synthesis of benzannulated medium rings. (a) proposed biosynthetic route34 and biomimetic
synthesis35 of protostephanine. (b) biomimetic syntheses of (homo)erythrina alkaloids38,39 (pathway A) and alternative dienone-phenol rearrangement
(1,2-alkyl shift) pathway leading to a homoaporphine scaffold43 (pathway b). (c) biomimetic odRe approach to benzannulated medium-ring synthesis.
nMo, N-methylmorpholine N-oxide; tbS, tert-butyldimethylsilyl.
associated with cyclization-based approaches, particularly in the polycyclic cyclohexadienone substrates. The products are amenable
context of diversity-oriented synthesis41, where diverse linear sub- to further downstream modifications, and cheminformatic analysis
strates would otherwise need to cyclize efficiently to access these confirms that they occupy regions of chemical space that overlap
scaffolds31. Key linkages found in medium-ring natural products with medium-ring natural products and are distinct from current,
(for example, aryl ethers, diaryl ethers, lactones and biaryl linkages) high-profile synthetic drugs and related libraries.
would be installed via initial oxidative dearomatization of simple
bicyclic phenols, and a subsequent aromatization-driven ring reSulTS
expansion of the intermediate polycyclic cyclohexadienones would Development of biomimetic ring expansion reactions
provide a variety of medium-ring scaffolds. Modular synthetic To test the feasibility of ring expansion of non–nitrogen-containing
access to the bicyclic phenol precursors would be readily achieved polycyclic cyclohexadienones, we synthesized tricycle 5 in five steps
in three to five steps from simple starting materials.
on a gram scale from 6-hydroxytetralone via a modular route involv-
At the outset, we noted that other acid-catalyzed reactions that ing oxidative dearomatization of bicyclic phenol 4 (Supplementary
convert cyclohexadienones to phenols, such as dienone-phenol Fig. 4). The C1-methyl substituent blocked benzylic oxidation dur-
(1,2-alkyl shift) and Cope rearrangements42, are well known and ing the oxidative dearomatization step (Supplementary Fig. 5)
that our synthetic approach would need to avoid these competing and was also envisioned to facilitate the desired ring expansion by
pathways leading to other undesired products. Indeed, the alter- formation of a stable tertiary carbocation intermediate (described
native dienone-phenol rearrangement was observed during early below). Cyclohexadienone 5 was then exposed to a variety of
studies of related biomimetic ring expansions (Fig. 1b)43. In the reaction conditions aimed at inducing ring expansion (Table 1).
context of a diversity-oriented synthesis, our overall goals were Treatment with mild Lewis acids, including MgI2, which was used
to establish ring expansion conditions that (i) provide access to a in the biomimetic synthesis of protostephanine35, led to no reaction
wide variety of medium-ring sizes and linkages, (ii) do not require even at elevated temperatures (entries 1,2 in Table 1), highlighting
a backbone nitrogen for ring expansion, (iii) are tolerant of diverse the importance of the backbone nitrogen in the protostephanine
functional groups and (iv) avoid competing pathways such as system. Meanwhile, stronger Lewis acids caused decomposition
dienone-phenol rearrangements.
of the starting material (entries 3,4). In contrast, treatment with
We describe herein the development of this biomimetic ODRE Me3O·BF4 (where Me represents methyl), MeAlCl2 or TiCl4 led to
strategy, resulting in the synthesis of a first-generation library the formation of alternate tricycles 9 or 10 (entries 5–7), presum-
of diverse benzannulated medium rings. Three complementary ably via the undesired dienone-phenol rearrangement followed by
reagent classes have been discovered that induce this relatively a subsequent, second 1,2-alkyl shift (described below)42. Exposure
neglected mode of cyclohexadienone reactivity while avoiding to methanolic hydroxide, which was used in previous biomimetic
competing dienone-phenol rearrangements for a wide variety of syntheses of homoerythrina and erythrina alkaloids38–40, led to no
2
nature CHeMICaL BIOLOGY | AdvAnce online publicAtion | www.nature.com/naturechemicalbiology

NaTure CHemiCal biOlOgy dOI: 10.1038/nCHeMBIO.1130
Table 1 | reactivity of a tricyclic cyclohexadienone substrate.
article
The TsOH and Cu(BF4)2-induced ring expansion reactions both led
to a 3:1 mixture of olefin 6 and methanol adduct 8 (entries 14,15,19).
Notably, these processes yielded anisoles rather than the corres-
ponding phenols, providing a key mechanistic insight (described
below). A third ring expansion using trifluoromethanesulfonic
anhydride (Tf2O) was also discovered and provided triflate 7 as a
mixture of three olefin regioisomers in excellent overall yield (entry
20). Although ring expansion of this particular substrate led to a
mixture of olefin regioisomers, we envisioned that increased regio-
control might be possible using other functionalized substrates, as
described below.
O
O
O
OMe
RO
MeO
O
5
6: R = Me (single isomer)
7: R = tf (three regioisomers)
8
HO
O
RO
HO
OMe
modular synthesis of cyclohexadienone substrates
With three complementary reaction conditions for ring expansion
in hand, we sought to evaluate the scope of the ODRE sequence
across a range of additional substrates. Thus, a variety of polycyclic
cyclohexadienones were synthesized via the same modular synthetic
approach used for 5, starting from simple precursors (Fig. 1c; full
details are in Supplementary Note 1).
9: R = Me
10: R = H
11
entry
Conditionsa
temperature (°C) product Yield (%)
1
Zncl2, 1,2-dce
84
80
no rxn
no rxn
decomp
decomp
9
–
–
2
3
4
5
Mgi2, c6H6
bF3·oet2, cH2cl2
inbr3, cH2cl2
0 → 25
0 → 25
40
–
Ketone alkylation or arylation of silyl-protected bicyclic
ketophenols with commercially available Grignard reagents or
dihydroxylation of 6-hydroxy-1-methyl-3,4-dihydronaphthalene
afforded intermediate benzylic alcohols. Convenient introduction
of various side chains was then achieved via addition of allyl trim-
ethylsilanes or trialkylsilyl ketene acetals under Sakurai conditions
(TiCl4 or ZnCl2)44. The resulting ester or olefin side chains were
tailored using a variety of straightforward transformations to
provide tethered acid, aryl, phenol, primary alcohol and tertiary
alcohol nucleophiles. Optional bromination at C5 provided a
handle for subsequent introduction of other functional groups in
the western half of the scaffold.
The key oxidative dearomatization of phenols 12–30 with iodo-
sobenzene diacetate (PhI(OAc)2) then provided efficient access
to polycyclic cyclohexadienone substrates that could be further
modified as desired to provide 31–50 (Table 2). Overall, the sub-
strate syntheses are straightforward, scalable and operationally
simple, and several steps can be carried out without purification of
the intermediates.
–
Me3o·bF4,
60
proton-sponge, cH2cl2
6
ticl4, cH2cl2
MeAlcl2, cH2cl2
naoH, MeoH
25
25
80b
10
10
90
65
20c
79c
–
7
8
11
9
naoMe, MeoH
tsoH, cH2cl2
80b
11
10
11
25
no rxn
no rxn
no rxn
no rxn
6 + 8
6 + 8
6 + 8
tsoH, cH3cn
80
–
12
13
14
15h
16
tsoH, dMF
80
–
tsoH, Meno2
70
–
tsoH, MeoH
25 (32 h)
25 (12 h)
25 (24 h)
81d
86d
30e
tsoH, 1:1 MeoH/Meno2
tsoH, 10 equiv. MeoH,
Meno2
17
tsoH, 1:1 H2o/tHF
tsoH, 1:1 H2o/MeoH
cu(bF4)2, tMoF, MeoHf
tf2o, dtbMp, cH2cl2
100
60
50
0
no rxn
no rxn
6 + 8
7
–
18
19h
20h
–
88d
90g
Scope of ring-expanding rearomatization reaction
We were pleased to find that polycyclic cyclohexadienones 31–50
all underwent effective ring expansion under one or more of our
three reaction conditions, generating 8- to 12-membered rings
52–81 (Table 2; full details are in Supplementary Note 1). The
larger [6,6,6]-tricycle 31 reacted efficiently under all three condi-
tions to give ten-membered ring products 52 or 53 after extended
treatment with TsOH to converge olefin isomers. Incorporation of
an olefin in substrate 32 led to MeOH adduct 54, presumably by SN1′
addition of MeOH during the ring expansion reaction. The [6,5,5]-
a2 equiv. of each reagent unless otherwise noted, 0.1 M substrate concentration. bSealed reaction
vessel. cSingle diastereomer. d3:1 ratio of olefin 6 and methanol adduct 8. e10:1 ratio of olefin 6
and methanol adduct 8. f20 mol% cu(bF4)2, 3 equiv. trimethylorthoformate (tMoF). gMixture
of three olefin regioisomers (major isomer 7 shown). hthese conditions were used to investigate
substrate scope in Table 2. Rxn, reaction; decomp, decomposition of starting material; 1,2-dce,
1,2-dichloroethene; bF3·oet2, boron trifluoride diethyl etherate; tHF, tetrahydrofuran; dtbMp,
2,6-di-tert-butyl-4-methylpyridine.
reaction at 25 °C, again emphasizing the importance of a backbone tricyclic cyclohexadienone 33 led to eight-membered ring product
nitrogen in those systems, which was released by hydrolysis of a 55, comprising the core benzooxocane scaffold of heliannuol A. The
corresponding amide. At 80 °C, a distinct bicycle product 11 was Tf2O ring expansion was most effective for this acid-labile tertiary
formed (entries 8 and 9), presumably by γ-enolization and solvolytic ether system, with the TsOH and Cu(BF4)2 ring expansions afford-
SN2′ displacement of the alkoxyethyl side chain.
ing low yields (<25%). Methyl, bromo and aryl substituents on the
However, several conditions were found to effect the desired ring cyclohexadienone ring (34–36) were well tolerated under all three
expansion of 5. Although p-toluenesulfonic acid (TsOH) did not ring expansion conditions to provide aryl-substituted medium rings
induce ring expansion in aprotic solvents (entries 10–13), the reac- 56–61, although increased reaction temperatures were required to
tion did proceed in alcoholic solvents such as MeOH (entry 14) and induce ring expansion of the bromide 34.
was further accelerated by the inclusion of MeNO2 (entry 15). When
In contrast to reactions of the C1-methyl cyclohexadienone 5,
the amount of MeOH was decreased to 10 equiv., the reaction pro- TsOH- and Cu(BF4)2-induced ring expansions of the correspond-
ceeded more slowly and afforded a lower yield (entry 16). The ring ing C1-phenyl cyclohexadienone 37 afforded not only the expected
expansion also proceeded in other alcohols such as ethanol, isopro- anisole products but also the free phenol 62 and could also be
panol and ethylene glycol, albeit with decreased efficiency (10–40% carried out under ‘MeOH-free’ conditions to produce the phenol
yields). Moreover, the reaction did not proceed in aqueous medium 62 exclusively, providing important mechanistic insights, as
(entries 17,18). A complementary Lewis acid–promoted ring described below. Moreover, the [6,7,5]-tricyclic cyclohexadienone
expansion was also identified using Cu(BF4)2 in MeOH (entry 19). 38 also yielded the corresponding free phenol 63 under the standard
nature CHeMICaL BIOLOGY | AdvAnce online publicAtion | www.nature.com/naturechemicalbiology
3

NaTure CHemiCal biOlOgy dOI: 10.1038/nCHeMBIO.1130
article
expansion
Table 2 | Scope of the oxidative dearomatization–ring
expansion sequence.
phenol
Cyclohexadienonea
Medium ring
yield (%)b
HO
expansion
phenol
HO
Cyclohexadienonea
Medium ring
yield (%)b
O
O
RO
HO
O
O
O
20
41 (26)
69 (R = H)
70 (R = tf)
90n (tsoH′)
HO
O
RO
59p (tf2o)
12
13
31 (45)
52 (R = Me)
53 (R = tf)
82c (tsoH)
86c (cu(bF4)2)
91c (tf2o)
O
O
O
HO2C
O
OMe
HO
HO
O
TfO
64i (tf2o)
76 (tf2oj)
O
21
42 (53)
O
71
O
O
HO
HO2C
O
MeO
O
O
32 (55d)
54
55
90 (tsoH)
49 (cu(bF4)2)
HO
O
TfO
22
43 (84)
72
HO
O
O
O
O
HO2C
O
O
HO
O
TfO
75e (tf2o)
HO
O
RO
14
33 (94)
23
44 (95)
73 (R = H, Dγ,δ
)
)
56n (tsoH′)
HO
74 (R = tf, Dβ,γ
56i (tf2o)
O
O
O
O
HO2C
O
O
RO
HO
R′
R′
O
Br
RO
15
R′ = br
34 (74)
56 (R = Me)
57 (R = tf)
50 (tsoHf)
HO
O
h
85g (cu(bF4)2 )
24
45 (42)
75 (R = H)
76 (R = tf)
51p (tsoH′)
96i (tf2oj)
85g (tsoH)
84g (cu(bF4)2)
92i (tf2o)
68i (tf2o)
R′ = Me
35 (89k)
58 (R = Me)
59 (R = tf)
O
O
OH
O
R′ = pMp
36 (95l)
60 (R = Me)
61 (R = tf)
89g (tsoH)
85g (cu(bF4)2)
83i (tf2o)
O
HO
O
MeO
25
46 (50q)
77
(Gram scale)
89 (tsoH)
HO
Ph
Ph
O
Ph
O
74 (cu(bF4)2)
OMe
OMe
HO
O
HO
OMe
MeO
MeO
OMe
16
37 (86)
62
57m (tsoH)
54n (tsoH′)
67 (in cH2cl2)b
77° (cu(bF4)2)
88 (in cH2cl2)b
TfO
HO
O
(Gram scale)
26
27
47 (72)
78
84 (tf2o)
OH
HO
OH
O
O
HO
O
O
O
HO
O
RO
17
38 (96)
63 (R = H)
64 (R = tf)
95g (tsoH)
98 (tsoH′)
57c (tf2o)
HO
O
MeO
48 (55)
79
71 (tsoH)
74 (cu(bF4)2)
HO
OH
HO
O
O
HO
HO
O
O
O
RO
HO
18
19
39 (39)
65 (R = H)
66 (R = tf)
63i (tsoH′)
HO
O
MeO
52i (tf2o)
28 (1.2:1 dr)
49 (71, 1.2:1 dr)
80
73e (tsoH)
44e (cu(bF4)2)
HO
HO
TIPS
O
O
O
O
OH
O
RO
O
HO
O
HO
O
MeO
40 (67)
67 (R = H)
68 (R = tf)
82i (tsoH′)
29
50 (81r)
81
0 (tsoH)
80 (cu(bF4)2)
87i (tf2o)
4
nature CHeMICaL BIOLOGY | AdvAnce online publicAtion | www.nature.com/naturechemicalbiology

NaTure CHemiCal biOlOgy dOI: 10.1038/nCHeMBIO.1130
article
bicyclic phenol 30. This transformation can be envisioned to occur
via an intermediate Adler-Becker oxidation45 product 51, followed
by an unexpected, but precedented, spontaneous C-C bond cleavage
of the epoxide moiety46.
Table 2 | (continued).
expansion
phenol
Cyclohexadienonea
Medium ring
yield (%)b
HO
HO
mechanistic analysis of ring expansion reactions
OH
O
O
O
We next set out to determine whether the TsOH- and Cu(BF4)2-
mediated ring expansion reactions produce kinetic or thermo-
dynamic mixtures of medium-ring products in the parent system
(Table 1). Separation and re-exposure of olefin 6 and methanol
adduct 8 to the reaction conditions confirmed that they are kinetic
products of the ring expansion reaction (Supplementary Fig. 6).
However, methanol adduct 8 could be converted to olefin 6 by treat-
ment with a stronger Lewis acid, MeAlCl2. In contrast, treatment of
6 or 8 with TiCl4 led unexpectedly to the alternate tricycle 9. This
ring contraction reaction is envisioned to involve initial forma-
tion of a tertiary carbocation, which then undergoes transannular
Friedel-Crafts alkylation and subsequent 1,2-alkyl shift to form 9
(Supplementary Fig. 7), intercepting the same undesired dienone-
phenol rearrangement pathway discussed earlier.
O
HO
HO
30s
51
82
odRe cascade
42 (from 30a)
aphi(oAc)2 (1–2 equiv.), K2co3 (2–3 equiv.), cF3cH2oH, 0 → 25 °c; yields shown in parentheses.
btsoH = 2 equiv. tsoH, 1:1 MeoH/Meno2, 25 °c; tsoH′ = 2 equiv. tsoH, Meno2, 25 °c;
cu(bF4)2 = 20 mol% cu(bF4)2, 3 equiv. trimethylorthoformate, MeoH, 50 °c; ‘in cH2cl2’ indicates
the reaction in the preceding entry was performed again using methylene chloride as the solvent;
tf2o = 1.1 equiv. tf2o, 2 equiv. 2,6-di-tert-butyl-4-methylpyridine, cH2cl2, 0 °c. cYield after
subsequent treatment with tsoH to converge olefin regioisomers. doverall two-step yield after
treatment of initially formed exocyclic olefin with tsoH, cH2cl2, 40 °c. eMixture of endocyclic and
exocyclic olefin regioisomers (major isomer shown). f50 °c. g3:1 ratio of olefin to MeoH adduct
(see 8). h80 °c. iMixture of three olefin regioisomers (major isomer shown). j25 °c. kYield after
treatment of 34 with tetramethylstannane, tetrakis(triphenylphosphine)palladium(0) (pd(pph3)4),
cubr, N,N-dimethylformamide, 80 °c. lYield after treatment of 34 with p-methoxyphenylboronic
acid, pd(pph3)4, na2co3, 1:1 toluene/ethanol, 50 °c. m1:3 ratio of phenol 62 to corresponding
anisole (see 6). nMixture of two endocyclic olefin regioisomers (major isomer shown). o1.0:2.3:1.4
ratio of phenol olefin 62 to corresponding anisole olefin (see 6) to anisole MeoH adduct (see 8).
p>10:1 ratio of olefin regioisomers (major isomer shown). qoverall three-step yield after
treatment of initially formed exocyclic olefin with oso4, N-methylmorpholine N-oxide, 1,4-
diazabicyclo[2.2.2]octane (dAbco), 3:1 tetrahydrofuran (tHF)/H2o, 45 °c, then lead(iv)
tetraacetate, 1:1 ethyl acetate/cH2cl2, 0 °c. roverall two-step yield after treatment of initially
formed triisopropylsilyl-protected tricyclic cyclohexadienone with tetrabutylammonium fluoride
(tbAF), tHF, 0 °c. sprepared by treatment of a silyl-protected precursor with tbAF, tHF, 0 °c and
used immediately without further purification. complete details are in Supplementary Note 1.
dr, diastereomeric ratio.
TiCl4 was the only reagent observed to drive ring contraction of
6 and 8. MeAlCl2 induced MeOH elimination in 8 to form 6 with-
out any observed ring contraction, and 6 and 8 were completely
unreactive to TsOH, Me3O·BF4, ZnCl2 and MgI2 at room tempera-
ture, suggesting that the TiCl4-mediated pathway may not involve
simple Lewis or Brønsted acid catalysis (Supplementary Fig. 7).
Indeed, treatment of 6 and 8 with anhydrous HCl (CH2Cl2, 0 °C)
did not induce this ring contraction to 9, instead resulting in
eliminative cleavage of the cyclic aryl ether bond followed by reclo-
sure to a distinct vinyl benzooxepane rearrangement (1,3-phenoxide
TsOH-induced ring expansion, and all of the cyclohexadienone shift) product.
substrates having larger 7- and 8-membered rings (38–41) were
Notably, in the aryl ether series above, we observed that substrates
competent to undergo MeOH-free TsOH-induced ring expansions in which the cyclohexadienone is fused to a six-membered ring (5, 31)
to 10- to 12-membered rings 63, 65, 67 and 69.
require MeOH for both the TsOH and Cu(BF4)2-induced ring expan-
Carboxylate, phenol and C-aryl side chain nucleophiles in 21–26 sion reactions and incorporate MeOH to afford anisole products
affordedpolycycliccyclohexadienones42–47, whichalsounderwent (6, 52). In contrast, analogous substrates with a fused seven- or eight-
efficient ring expansion reactions to afford aryl lactone (71–76), membered ring (38–41) do not require MeOH for ring expansion and
diaryl ether (77) and biaryl (78) scaffolds related to those found in proceed directly to the corresponding phenols (63, 65, 67, 69). Similar
natural products. In the lactone series (71–76), Tf2O proved most trends were observed in the aryl lactone series, where seven- and
generally effective for ring expansion. The position of the double eight-membered ring substrates (44, 45) underwent effective MeOH-
bond in the ring expansion product could be modulated predict- free TsOH-induced ring expansions to phenol products (73, 75).
ably by incorporation of an additional olefin in the substrate (71
To investigate the underlying mechanistic basis for these ring-
versus 72). Notably, the same trend in ring size effects observed in size effects, we carried out molecular modeling of cyclohexadi-
the aryl ether series above (5 and 31 versus 38–41) was also observed enone ethers 5, 31 and 38–41 and cyclohexadienone lactones 42,
in the lactone series, with the seven- and eight-membered ring sub- 44 and 45 (Supplementary Fig. 8). Within each substrate series,
strates 44 and 45 able to undergo MeOH-free TsOH-induced ring calculated strain energies increased as the ring fused to the cyclo-
expansion directly to phenols 73 and 75. Phenol and C-aryl side hexadienone ring increased in size from having six to seven to eight
chain nucleophiles in 25 and 26 afforded polycyclic cyclohexadi- members. Moreover, the scissile bond aligned more favorably with
enones 46 and 47, which underwent ring expansion reactions to the π-system of the cyclohexadienone ring as ring size increased.
afford diaryl ether (77) and biaryl (78) scaffolds, and both of these This stereoelectronic effect has been used to rationalize ring size
processes were readily performed on a gram scale. As expected, effects upon cyclohexadienone reactivity in duocarmycin analogs47.
the Tf2O ring expansion was incompatible with the enolizable ketone These results suggest that the increased reactivity of seven- and
in 46, resulting in a complex mixture.
eight-membered ring substrates compared to the corresponding
As we hoped, installation of an additional alcohol side chain in six-membered rings arises from a combination of inherent ring
cyclohexadienone 48 allowed ring expansion with concurrent cycl- strain and stereoelectronic alignment.
ization of the alcohol to afford spiroether 79. In contrast, placing
Having investigated the various competing reaction manifolds,
an additional alcohol on the side chain in 49 led to olefin 80 rather substrate scope and kinetic nature of the desired ring expansion,
than the corresponding bridged ether product, presumably because we propose a mechanism for ring expansion that takes into account
of unfavorable ring strain in the latter. Meanwhile, incorporation of all of the current data (Fig. 2). Initial activation of the cyclohexadi-
an alcohol functionality vicinal to the scissile bond in cyclohexadi- enone carbonyl in 5 is followed by an aromatization-driven cationic
enone 50 afforded ketone 81, presumably via 1,2-hydride shift after ring expansion to form medium rings 6–8. An obligate O-methyl
ring expansion. These ring expansion reactions were carried out oxocarbenium intermediate 83a is proposed for the TsOH- and
with TsOH or Cu(BF4)2, avoiding undesired side reactions of the Cu(BF4)2-induced ring expansion reactions of 5, consistent with the
alcohol functionalities with Tf2O. Finally, consistent with this general inability to induce ring expansion of 5 in aprotic solvents or aque-
reactivity of polycyclic cyclohexadienones, we observed direct for- ous mixtures (Table 1, entries 10–13,17,18) and with the formation
mation of spiroketal 82 during the oxidative dearomatization of of anisole (O-methylated) products in all cases in which MeOH
nature CHeMICaL BIOLOGY | AdvAnce online publicAtion | www.nature.com/naturechemicalbiology
5

NaTure CHemiCal biOlOgy dOI: 10.1038/nCHeMBIO.1130
article
TsOH, MeOH
or Cu(BF4)2, MeOH
Ring
expansion
O
O
O
O
MeOH
1
MeOH
or
Tf2O
or
Y
or Tf2O
–H+
RO
RO
O
RO
H
X
5
83a (R = Me)
83b (R = Tf)
84a (R = Me)
84b (R = Tf)
6,8 (R = Me)
7 (R = Tf)
X = H, Y = OMe
or X = Y
83c (R = TiCl4)
83d (R = AlMeCl2)
Dienone-phenol
rearrangement
Transannular
Friedel-Crafts
O
O
O
Alternative undesired
rearrangement pathways
–H+
1,2-shift
RO
RO
RO
H
85a (R = Me)
86a (R = Me)
9
(R = Me)
85b (R = Tf)
85c (R = TiCl4)
85d (R = AlMeCl2)
86c (R = TiCl4)
86d (R = AlMeCl2)
87c (R = TiCl4)
87d (R = AlMeCl2)
Workup
10 (R = H)
Figure 2 | Proposed mechanisms for aromatization-driven ring expansion of 5 to benzannulated medium rings 6–8 and for formation of alternate
tricycles 9 and 10. in the desired pathway, polarization of the cyclohexadienone carbonyl in oxocarbenium intermediates 83 induces bond scission to form
tertiary carbocations 84, leading to ring expansion products 6–8. in alternative undesired pathways, dienone-phenol rearrangement of 83 or transannular
Friedel-crafts alkylation of 84 affords 85, which then undergoes 1,2-alkyl shift to 86, leading to tricycles 9 and 10.
is required for these ring expansions (Table 2). The enhanced leading to the same undesired rearrangement products 9 or 10
reactivity of O-alkyl oxocarbenium intermediates compared to (Fig. 2). Thus, one reason for the success of TsOH, Cu(BF4)2 and
their O-proteo congeners has been highlighted previously48,49. The Tf2O in inducing the desired ring expansion pathway is likely the
analogous O-triflyl oxocarbenium intermediate 83b is proposed for ability of these reagents to generate kinetic ring expansion products
Tf2O-induced ring expansion reactions.
that do not react further (reversibly) under these reaction condi-
Notably, the carbonyl group can be activated by two distinct tions, in contrast to reactions with TiCl4.
mechanisms, in which it acts as either an electrophile (TsOH- or
In addition, polar solvent effects in the ring expansion reactions
Cu(BF4)2-catalyzed MeOH addition) or a nucleophile (Tf2O sul- using TsOH in 1:1 MeOH/MeNO2 and Cu(BF4)2 in MeOH seem
fonylation). This characteristic provides useful flexibility when to have a role in directing the key oxocarbenium intermediate 83a
working with electron-rich or electron-deficient cyclohexadienone toward the desired ring-expanded tertiary carbocation 84a rather
substrates. The O-methyl and O-triflyl oxocarbenium intermedi- than the undesired cation 85a. In the Me3O·BF4, MeAlCl2 and TiCl4
ates 83a and 83b then undergo the desired ring expansions to the reactionscarriedoutinCH2Cl2, thelattercationismostlikelyfavored
tertiary carbocations 84a and 84b, which are quenched by either because its charge is internally stabilized by delocalization, leading
addition of a MeOH nucleophile or elimination to form the kinetic to alternative tricycles 9 and 10. Ring expansion of the C1-phenyl–
ring expansion products 6−8.
substituted cyclohexadienone 37 is feasible in CH2Cl2 because of
The proposed cationic manifold is supported by the observation the additional stabilization of the tertiary carbocation provided
that the TsOH-induced ring expansion reaction is accelerated by the by the phenyl substituent. The Tf2O-induced ring expansion can
addition of the charge-stabilizing solvent MeNO2 (ε = 36) (Table 1, also be carried out in CH2Cl2, and we postulate that oxocarbenium
entries 14,15). Additional support for a cationic pathway is pro- intermediate 83b favors ring expansion to tertiary carbocation 84b
vided by the ability of C1-phenyl–substituted cyclohexadienone 37 rather than the alternative cation 85b because of the instability
to undergo ring expansion using TsOH or Cu(BF4)2 in the absence of the latter caused by the electron-withdrawing triflyl group.
of MeOH (Table 2). Presumably, stabilization of the corresponding Overall, it is apparent that reagent selection (Tf2O in CH2Cl2 versus
tertiary cation intermediate by the C1-phenyl substituent (see 84) Me3O·BF4 in CH2Cl2), solvent polarity (MeNO2 versus CH2Cl2) and
allows ring expansion to occur directly from the O-proteo and substrate structure (C1-methyl in 5 versus C1-phenyl in 37) all have
O-cupric oxocarbenium intermediates, without requiring formation complementary roles in promoting ring expansion and in avoiding
of a more reactive O-methyl oxocarbenium intermediate (see 83a). undesired alternative rearrangement reactions.
Similarly, other larger-ring substrates that accommodate MeOH-free
TsOH-induced ring expansions (38–41, 44, 45) directly to phenol Downstream transformations of medium-ring scaffolds
products (63, 65, 67, 69, 73, 75) are thought to proceed via O-proteo To establish the ability of the ring expansion products to undergo
oxocarbenium intermediates that are sufficiently reactive to undergo further functionalizations that might be useful in library synthesis,
ring expansion owing to increased ring strain and improved stereo- we synthesized benzannulated medium-ring scaffolds 77 and 78 on
electronic overlap of the scissile bond with the nascent aromatic ring a gram scale and investigated a variety of downstream transforma-
in these systems (Supplementary Fig. 8)47. The undesired dienone- tions. The investigated diaryl ether 77 underwent effective cyclo-
phenol rearrangement of cyclohexadienone 5 to form tricycles propanation (88), epoxidation (89) and dihydroxylation (90) of the
9 and 10 using Me3O·BF4, MeAlCl2 or TiCl4 in CH2Cl2 (Table 1, olefin functionality as well as Luche reduction of the ketone (91)
entries 5–7) indicates that alternative rearrangement pathways can (Fig. 3a). The methyl ether–capping group could also be removed
indeed compete with the desired ring expansion reactivity under to afford free phenol 92, which then allowed Mitsunobu alkylation
some conditions. Moreover, the TiCl4-catalyzed ring contraction of to lactate derivative 93. The biaryl scaffold 78 also underwent selec-
medium-ring products 6 and 8 to tricycle 9 (Supplementary Fig. 6) tive dihydroxylation on the trisubstituted olefin to afford diol 94
suggests that tertiary carbocations 84 are competent to undergo an (Fig. 3b). The triflate capping group could then be removed
alternative transannular Friedel-Crafts reaction to form cations 85, under mild conditions to afford free phenol 95, poised for further
6
nature CHeMICaL BIOLOGY | AdvAnce online publicAtion | www.nature.com/naturechemicalbiology

NaTure CHemiCal biOlOgy dOI: 10.1038/nCHeMBIO.1130
article
a
O
(i)
I
O
O
Me
S
Me
O
Me
(v) BBr3
O
O
O
NaH
HO
MeO
MeO
77
92
88
(ii) m-CPBA
(vi) DIAD,
MeO
(iii) OsO4,
NMO
(iv) NaBH4,
CeCl3
PPh3
OH
O
O
O
OH
O
OH
OH
O
O
O
O
O
MeO
MeO
MeO
MeO
O
89
90
91
93
O
OMe
OMe
OMe
b
OMe
OMe
OMe
(iii) OsO4, NMO
(vii) LiOH
OH
OH
OH
OH
TfO
TfO
HO
78
94
95
HO
O
OH
c
O
Ph
OH
OH
O
O
O
OH
OH
OH
OH
MeO
MeO
HO
R
TfO
96 (from 6; (iii))
97 (from 62; (iii))
98 (R = Br; from 56; (iii))
99 (R = Me; from 58; (iii))
100 (R = PMP; from 60; (iii))
101 (from 72; (iii))
Figure 3 | Downstream modifications of ODre-derived benzannulated medium-ring scaffolds. (a) diversification reactions of diaryl ether 77.
(b) dihydroxylation and detriflation of biaryl 78. (c) Additional dihydroxylated products synthesized from odRe-derived benzannulated medium-
ring scaffolds. (i) trimethylsulfoxonium iodide, naH, dMSo, 25 °c, 20 h, 80%. (ii) 3-chloroperoxybenzoic acid (m-cpbA), cH2cl2, 0 → 25 °c,
14 h, 61%. (iii) oso4, N-methylmorpholine N-oxide (nMo), 3:1 acetone/H2o, 0 °c, 15 min; 90: 82%; 94: 97%; 96: 50%; 97: 62%; 98: 80%; 99:
83%; 100: 77%; 101: 66%. (iv) nabH4, cecl3, MeoH, 0 → 15 °c, 3 h, 77%. (v) bbr3, cH2cl2, 0 → 25 °c, 2 h, 85%. (vi) methyl (2R)-lactate, diisopropyl
azodicarboxylate (diAd), triphenylphosphine (pph3), tetrahydrofuran, 0 → 25 °c, 12 h, 84%. (vii) lioH, MeoH, 0 → 25 °c, 30 min, 100%. pMp,
p-methoxyphenyl.
modifications. Several other benzannulated medium-ring products by the pharmaceutical industry, owing to increased ring system
also underwent efficient dihydroxylation reactions to afford diols size and complexity (Supplementary Fig. 15).
96–101 (Fig. 3c).
DiSCuSSiON
Cheminformatic analysis of ODre-derived library
Although nature has successfully exploited medium-ring natural
To evaluate the effectiveness of the biomimetic ODRE method- products to address diverse biological targets, such structures
ology in providing libraries that access regions of chemical space remain underrepresented in current probe and drug discovery
targeted by natural products, we used a cheminformatic approach efforts. Synthetic challenges in accessing these molecules, relat-
involving principal component analysis (PCA; full details are in ing to slow cyclization kinetics and transannular ring strain, have
Supplementary Note 2) to compare the structural and physico- presented obstacles to making natural product–inspired medium
chemical properties of the 47 benzannulated medium-ring scaf- rings available for biological evaluation. The ring expansion
folds and 25 polycyclic cyclohexadienones synthesized herein approach to medium-ring synthesis developed herein, inspired by
(Supplementary Fig. 9) with a collection of benzannulated biosynthetic proposals for alkaloid natural products, is designed
medium-ring natural products (Supplementary Fig. 10) and our to avoid the drawbacks associated with conventional cyclization-
established reference sets of brand-name drugs, natural products based approaches.
and commercial drug-like library members18,41,50 (Supplementary
The identification of three distinct reagent classes (Brønsted
Figs. 11–13). The benzannulated medium-ring scaffolds syn- acid, Lewis acid and sulfonyl anhydride) that induce this ring
thesized herein overlapped with the benzannulated medium- expansion allows selection among three complementary reac-
ring natural products in this analysis (Supplementary Fig. 14 tion conditions based on the functional groups present in a given
and Supplementary Data Set 1), whereas the corresponding substrate. Notably, these reactions access a relatively neglected mode
polycyclic cyclohexadienones were even more distinct from the of cyclohexadienone reactivity while avoiding undesired compet-
narrow range of drug and drug-like structures currently targeted ing dienone-phenol rearrangement pathways. Moreover, they do
nature CHeMICaL BIOLOGY | AdvAnce online publicAtion | www.nature.com/naturechemicalbiology
7

NaTure CHemiCal biOlOgy dOI: 10.1038/nCHeMBIO.1130
article
not require embedded backbone nitrogen functionalities that seem 14. Rezai, T., Yu, B., Millhauser, G.L., Jacobson, M.P. & Lokey, R.S. Testing the
conformational hypothesis of passive membrane permeability using synthetic
to be essential for previously reported biomimetic variants of this
cyclic peptide diastereomers. J. Am. Chem. Soc. 128, 2510–2511 (2006).
transformation. Taken together, the reactions showed excellent
15. Kwon, Y.-U. & Kodadek, T. Quantitative comparison of the relative cell
functional group tolerance and were compatible with various cyclo-
permeability of cyclic and linear peptides. Chem. Biol. 14, 671–677 (2007).
hexadienone substituents (34–36) in the substrates and with alcohol
(80), ketone (77, 81), Michael acceptor (72, 77), ester (71–74) and
acid-sensitive skipped diene (78) and tertiary ether (55) functional-
ities in the products.
16. McGrath, N.A., Brichacek, M. & Njardarson, J.T. A graphical journey of
innovative organic architectures that have improved our lives. J. Chem. Educ.
87, 1348–1349 (2010).
17. Illuminati, G. & Mandolini, L. Ring closure reactions of bifunctional chain
molecules. Acc. Chem. Res. 14, 95–102 (1981).
Cheminformatic analysis using PCA demonstrated that mole-
cules derived from medium-ring scaffolds 6, 7 and 52–82 have
structural and physicochemical properties that recapitulate those
found in naturally occurring benzannulated medium rings. Indeed,
the scaffolds obtained by ODRE show key structural features
found in natural products, such as the benzooxocane framework
of heliannuol A1 (55); the aryl ether linkage in brazilone2 (6–8);
the diaryl ether linkage in the aspercyclides3 (77); the aryl lactones
in kurzichalcolactone5 and puerol A4 (71–76); and the biaryl link-
ages in steganacin7, rhazinilam8 and various ellagitannins6 (78).
Downstream modifications of ODRE products such as olefin
dihydroxylation provide additional structures that present the polar
and stereochemically rich motifs found in benzannulated medium-
ring natural products. Thus, these scaffolds probe attractive areas
of chemical space that are not currently targeted by high-profile
synthetic drugs and related libraries. Biological evaluation of the
resulting libraries is ongoing and will be reported in due course.
18. Bauer, R.A., Wurst, J.M. & Tan, D.S. Expanding the range of ′druggable′
targets with natural product–based libraries: an academic perspective. Curr.
Opin. Chem. Biol. 14, 308–314 (2010).
19. Fürstner, A. & Mueller, C. Total synthesis of aspercyclide C. Chem. Commun.
(Camb.) 5583–5585 (2005).
20. Pospíšil, J., Mueller, C. & Fürstner, A. Total synthesis of the aspercyclides.
Chemistry 15, 5956–5968 (2009).
21. Stefinovic, M. & Snieckus, V. Connecting directed ortho metalation and olefin
metathesis strategies. Benzene-fused multiring-sized oxygen heterocycles.
First syntheses of radulanin A and helianane. J. Org. Chem. 63, 2808–2809
(1998).
22. Kishuku, H., Shindo, M. & Shishido, K. Enantioselective total synthesis of
(–)-heliannuol A. Chem. Commun. (Camb.) 350–351 (2003).
23. Biswas, B., Sen, P.K. & Venkateswaran, R.V. Bargellini condensation of
coumarins. Expeditious route to [o-(carboxyvinyl)phenoxy]isobutyric acids
and application to the synthesis of sesquiterpenes helianane, heliannuol A
and heliannuol C. Tetrahedron 63, 12026–12036 (2007).
24. Macías, F.A., Chinchilla, D., Molinillo, J.M.G., Fronczek, F.R. & Shishido, K.
A stereoselective route towards heliannuol A. Tetrahedron 64, 5502–5508 (2008).
25. Sabui, S., Ghosh, S., Sarkar, D. & Venkateswaran, R.V. Expeditious synthesis
of helianane and C-10 halogenated heliananes employing ring-closing
metathesis. Tetrahedr. Lett. 50, 4683–4684 (2009).
received 28 July 2011; accepted 22 October 2012;
published online 18 November 2012
26. Deb, I., John, S. & Namboothiri, I.N.N. Synthesis of benzo-fused medium
ring cyclic ethers via a Michael addition–ring closing metathesis strategy
involving nitroaliphatic compounds. Tetrahedron 63, 11991–11997 (2007).
27. Souers, A.J., Virgilio, A.A., Rosenquist, A., Fenuik, W. & Ellman, J.A.
Identification of a potent heterocyclic ligand to somatostatin receptor subtype
5 by the synthesis and screening of β-turn mimetic libraries. J. Am. Chem.
Soc. 121, 1817–1825 (1999).
28. Spring, D.R., Krishnan, S., Blackwell, H.E. & Schreiber, S.L. Diversity-oriented
synthesis of biaryl-containing medium rings using a one bead/one stock
solution platform. J. Am. Chem. Soc. 124, 1354–1363 (2002).
29. Khadem, S. et al. A solution- and solid-phase approach to
tetrahydroquinoline-derived polycyclics having a 10-membered ring. J. Comb.
Chem. 6, 724–734 (2004).
30. Brown, N. et al. Design and synthesis of medium-ring lactam libraries
inspired by octalactin A. A convergent-divergent approach. J. Comb. Chem.
10, 628–631 (2008).
meTHODS
Methods and any associated references are available in the online
version of the paper.
references
1. Macías, F.A., Varela, R.M., Torres, A., Molinillo, J.M.G. & Fronczek, F.R.
Allelopathic studies on cultivar species. 2. Novel sesquiterpene from
bioactive fractions of cultivar sunflowers. Tetrahedr. Lett. 34, 1999–2002
(1993).
2. Shimokawa, T., Kinjo, J., Yamahara, J., Yamasaki, M. & Nohara, T. Two novel
aromatic compounds from Caesalpinia sappan. Chem. Pharm. Bull. (Tokyo)
33, 3545–3547 (1985).
31. Marcaurelle, L.A. et al. An aldol-based build/couple/pair strategy for the
synthesis of medium- and large-sized rings: discovery of macrocyclic histone
deacetylase inhibitors. J. Am. Chem. Soc. 132, 16962–16976 (2010).
32. Roxburgh, C.J. Syntheses of medium sized rings by ring expansion reactions.
Tetrahedron 49, 10749–10784 (1993).
3. Singh, S.B. et al. Aspercyclide A–C, three novel fungal metabolites from
Aspergillus sp. as inhibitors of high-affinity IgE receptor. Tetrahedr. Lett. 45,
7605–7608 (2004).
4. Shirataki, Y., Tagaya, Y., Yokoe, I. & Komatsu, M. Studies on the constituents
of Sophora species. Part 21. Sophoraside A, a new aromatic glycoside from
the roots of Sophora japonica. Chem. Pharm. Bull. (Tokyo) 35, 1637–1640
(1987).
33. Hesse, M. Ring Enlargement in Organic Chemistry (VCH, Weinheim, 1991).
34. Barton, D.H.R. Some studies in the biogenesis of plant products. Pure Appl.
Chem. 9, 35–48 (1964).
5. Fu, X. et al. Flavanone and chalcone derivatives from Cryptocarya kurzii.
J. Nat. Prod. 56, 1153–1163 (1993).
35. Battersby, A.R., Bhatnagar, A.K., Hackett, P., ornber, C.W. & Staunton, J.
Synthesis of protostephanine by a route related to the biosynthetic pathway.
Chem. Commun. 1214–1215 (1968).
6. Quideau, S. & Feldman, K.S. Ellagitannin chemistry. Chem. Rev. 96, 475–504
(1996).
36. Bhakuni, D.S. & Jain, S. e biosynthesis of laurifinine. Tetrahedron 37,
3171–3174 (1981).
7. Kupchan, S.M. et al. Tumor inhibitors. LXXX. Steganacin and steganangin,
novel antileukemic lignan lactones from Steganotaenia araliacea. J. Am.
Chem. Soc. 95, 1335–1336 (1973).
37. euns, H.G. et al. Neodihydrothebaine and bractazonine, two dibenz[d,f]
azonine alkaloids of Papaver bracteatum. Phytochemistry 23, 1157–1166
(1984).
8. Abraham, D.J., Rosenstein, R.D., Lyon, R.L. & Fong, H.H.S. e structure
elucidation of rhazinilam, a new class of alkaloids from the Apocynaceae, by
X-ray analysis. Tetrahedr. Lett. 13, 909–912 (1972).
38. Marino, J.P. & Samanen, J.M. Biogenetic-type approach to homoerythrina
alkaloids. J. Org. Chem. 41, 179–180 (1976).
9. Majhi, T.P., Achari, B. & Chattopadhyay, P. Advances in the synthesis and
biological perspectives of benzannulated medium ring heterocycles.
Heterocycles 71, 1011–1052 (2007).
39. Kupchan, S.M., Kim, C.-K. & Lynn, J.T. Biomimetic synthesis of a key
Erythrina alkaloid precursor. J. Chem. Soc. Chem. Commun. 86 (1976).
40. Kupchan, S.M., Dhingra, O.P. & Kim, C.-K. New biogenetic-type approach
to Cephalotaxus alkaloids. J. Chem. Soc. Chem. Commun. 847–848 (1977).
41. Kopp, F., Stratton, C.F., Akella, L.B. & Tan, D.S. A diversity-oriented synthesis
approach to macrocycles via oxidative ring expansion. Nat. Chem. Biol. 8,
358–365 (2012).
10. Khan, A.R. et al. Lowering the entropic barrier for binding conformationally
flexible inhibitors to enzymes. Biochemistry 37, 16839–16845 (1998).
11. Benfield, A.P. et al. Ligand preorganization may be accompanied by entropic
penalties in protein-ligand interactions. Angew. Chem. Int. Ed. Engl. 45,
6830–6835 (2006).
42. Miller, B. Too many rearrangements of cyclohexadienones. Acc. Chem. Res. 8,
245–256 (1975).
12. Udugamasooriya, D.G. & Spaller, M.R. Conformational constraint in protein
ligand design and the inconsistency of binding entropy. Biopolymers 89,
653–667 (2008).
43. Marino, J.P. & Samanen, J.M. Chemistry of prohomoerythrinadienone I.
Tetrahedr. Lett. 14, 4553–4556 (1973).
44. Hosomi, A. Characteristics in the reactions of allylsilanes and their applications
to versatile synthetic equivalents. Acc. Chem. Res. 21, 200–206 (1988).
13. Veber, D.F. et al. Molecular properties that influence the oral bioavailability of
drug candidates. J. Med. Chem. 45, 2615–2623 (2002).
8
nature CHeMICaL BIOLOGY | AdvAnce online publicAtion | www.nature.com/naturechemicalbiology

NaTure CHemiCal biOlOgy dOI: 10.1038/nCHeMBIO.1130
article
was generously provided by ChemAxon. Financial support from the US National
45. Becker, H.D., Bremholt, T. & Adler, E. Oxidative formation and
photochemical isomerization of spiro-epoxy-2,4-cyclohexadienones.
Tetrahedr. Lett. 13, 4205–4208 (1972).
Institutes of Health (P41 GM076267 to D.S.T., P41 GM076267-03S1 to R.A.B. and T32
CA062948-Gudas to T.A.W.), W.H. Goodwin and A. Goodwin and the Commonwealth
Foundation for Cancer Research, the Memorial Sloan-Kettering Cancer Center
Experimental Therapeutics Center and the Tri-Institutional Stem Cell Initiative is
gratefully acknowledged.
46. Morton, J.G.M., Kwon, L.D., Freeman, J.D. & Njardarson, J.T. An Adler-
Becker oxidation approach to vinigrol. Tetrahedr. Lett. 50, 1684–1686
(2009).
47. MacMillan, K.S. & Boger, D.L. Fundamental relationships between structure,
reactivity, and biological activity for the duocarmycins and CC-1065. J. Med.
Chem. 52, 5771–5780 (2009).
author contributions
R.A.B., T.A.W. and D.S.T. designed the experiments, analyzed the data and wrote the
manuscript. R.A.B. and T.A.W. performed the synthesis and characterization. T.A.W.
performed the molecular modeling studies. R.A.B. performed the PCA analysis.
48. Roush, W.R., Gillis, H.R. & Essenfeld, A.P. Hydrofluoric acid catalyzed
intramolecular Diels-Alder reactions. J. Org. Chem. 49, 4674–4682 (1984).
49. Harmata, M. & Rashatasakhon, P. Cycloaddition reactions of vinyl
oxocarbenium ions. Tetrahedron 59, 2371–2395 (2003).
50. Moura-Letts, G., DiBlasi, C.M., Bauer, R.A. & Tan, D.S. Solid-phase synthesis
and chemical space analysis of a 190-membered alkaloid/terpenoid-like library.
Proc. Natl. Acad. Sci. USA 108, 6745–6750 (2011).
Competing financial interests
The authors declare no competing financial interests.
additional information
acknowledgments
Supplementary information and chemical compound information is available in the
online version of the paper. Reprints and permissions information is available online at
http://www.nature.com/reprints/index.html. Correspondence and requests for materials
should be addressed to D.S.T.
This work is dedicated to the memory of our colleague and mentor, David Y. Gin
(1967–2011). We thank D. Boger for helpful discussions; C. Stratton for assistance
with PCA calculations; J. Njarðarson for providing drug structures; and G. Sukenick,
H. Liu, H. Fang and S. Rusli for expert mass spectral analyses. Instant JChem
nature CHeMICaL BIOLOGY | AdvAnce online publicAtion | www.nature.com/naturechemicalbiology
9

orthoformate (3 equiv.) were added. The solution was warmed to 50 °C
and stirred for 5–16 h. The reaction could be accelerated by adding
more Cu(BF4)2 with no deterioration in yield (for example, 50 mol% or
100 mol% catalyst). Once complete, the reaction was quenched with satd
aq NH4Cl and extracted with EtOAc (2×). The combined organic extracts
were washed with brine, dried (Na2SO4), filtered and concentrated by
rotary evaporation. The product (or products) was purified by silica gel
chromatography (hexanes/EtOAc).
ONliNe meTHODS
The following general procedures were used for the ring expansion of polycyclic
cyclohexadienones as described in Tables 1 and 2. All ring expansions were run
on a scale of 30−60 mg, with the exception of 46→77 (TsOH and CuBF4) and
47→78 (Tf2O), which were run on a 1-g scale. Full details on synthetic methods
are in Supplementary Note 1. Full details on principal component analysis are in
Supplementary Note 2. NMR spectra are shown in Supplementary Note 3.
TsOH-mediated ring expansion. The cyclohexadienone was dissolved in
1:1 MeOH/MeNO2 or MeNO2 for MeOH-free ring expansions (0.1 M), and
TsOH·H2O (2.0 equiv.) was added. The solution was stirred at 25 °C for 5–20 h.
Once complete, the reaction was quenched with satd aq NaHCO3 and extracted
with EtOAc (2×). The combined organic extracts were washed with brine,
dried (Na2SO4), filtered and concentrated by rotary evaporation. The product
(or products) was purified by silica gel chromatography (hexanes/EtOAc).
Tf2O-mediated ring expansion. The cyclohexadienone and 2,6-di-tert-butyl-
4-methylpyridine (1.5 equiv.) were dissolved in CH2Cl2 (0.1 M) at 0 °C, and
triflic anhydride (1 M in CH2Cl2, 1.3 equiv.) was added. The reaction was
allowed to stir for 0.25–6 h. An extra 0.5 equiv. of triflic anhydride could be
added to accelerate sluggish ring expansions. The reaction was quenched
at 0 °C with satd aq NaHCO3, warmed to room temperature and extracted
with EtOAc (2×). The combined organic extracts were washed with brine,
dried (Na2SO4), filtered and concentrated by rotary evaporation. The product
(or products) was purified by silica gel chromatography (hexanes/EtOAc).
Cu(BF4)2-mediated ring expansion. The cyclohexadienone was dis-
solved in MeOH (0.1 M), and Cu(BF4)2·xH2O (20 mol%) and trimethyl
nature CHeMICaL BIOLOGY
doi:10.1038/nchembio.1130