Study of ring-opening reaction of spiro[52]octenes with aqueous hydrohalic acid: Substituent effect on the regioselectivity

Article abstract of DOI:10.1055/s-0032-1317745

We describe here the regioselective ring-opening reaction of spiro[5.2]octenes with hydrohalic acids. It was observed that the electronic nature of a substituent on the cyclopropane ring would control the regioselectivity. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0032-1317745

120▌
LETTER
lSettteur dy of Ring-Opening Reaction of Spiro[5.2]octenes with Aqueous
Hydrohalic Acid: Substituent Effect on the Regioselectivity
Ring-Opening Reaction of Spiro[5.2]octenes
Yuuki Nagamoto, Yoshiji Takemoto,* Kiyosei Takasu*
Graduate School of Pharmaceutical Sciences, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
Fax +81(75)7534604; E-mail: kay-t@pharm.kyoto-u.ac.jp
Received: 16.10.2012; Accepted after revision: 12.11.2012
MeO
MeO
OMe
Abstract: We describe here the regioselective ring-opening reac-
tion of spiro[5.2]octenes with hydrohalic acids. It was observed that
the electronic nature of a substituent on the cyclopropane ring
would control the regioselectivity.
HO
NH
N
R
CO2Me
Key words: alkenylcyclopropanes, electrophilic addition, regio-
selectivity, ring opening, spiro compound
OH
O
O
N
H
R = OH illudin S (antitumor)
R = H illudin M (antitumor)
O
duocarmycin SA (antitumor)
Cyclopropane framework is an attractive organic motif in
terms of its biological property as well as its unique chem-
ical reactivity. The relief of the high ring-strain energy,
which provides a potent enthalpic driving force, facilitates
a variety of unique chemical transformations.1 Cyclopro-
pane compounds are found as naturally occurring
substances2 such as illudins,2b,3 duocarmycins,2b,4 and
their related compounds, which display potent antitumor
activity (Scheme 1). The bioorganic studies of these natu-
ral products proved that the ring-opening reaction of the
cyclopropane ring induced by a nucleophilic attack of nu-
cleobases or proteins is a key event.5,6 The mechanism of
biological action of illudins is illustrated in Scheme 1.5a
Michael addition by a thiol nucleophile of a protein, such
as glutathione reductase (GSH), under acidic conditions
(pH ~6) gives active intermediate A. The electrophilic in-
termediate A smoothly reacts with an appropriate nucleo-
base. As the result, aromatic bioconjugate B is produced
by concurrent ring opening of the cyclopropane moiety by
the SN2′-type reaction.
illudine S
RSH
H+
(pH ~6)
NuH (DNA etc.)
NuH
HO
RS
HO
RS
Nu
HO
HO
OH
H
OH
OH
A
B
Scheme 1 Natural cancerostatic products containing a spirocyclo-
propane framework and the proposed biological action mechanism of
illudins
Recently, we reported the synthesis of spirocyclic alken-
ylcyclopropanes 1, bearing a substituent on the cyclopro-
pane ring, from fused cyclobutanols.10 The spirocyclic
core of 1 is structurally related to illudins. Our intention is
to design a new antitumor DNA-alkylating agent by using
our synthetic reaction. For this purpose, the ring-opening
reaction of alkenylcyclopropane 1, whose cyclopropane is
not conjugated with an electron-withdrawing group, must
be examined under aqueous conditions. Additionally, in-
vestigation of the regioselectivity would be an important
issue because the cationic intermediate C has three differ-
ent reactive sites with a nucleophile X– (Scheme 2). When
nucleophilic (X–) attack occurs at the substituted or non-
substituted cyclopropane carbon atoms, 1,5-adducts 2 or
3, respectively, will be obtained via a conjugate addition
(modes a and b). On the other hand, direct addition of X–
at the tertiary cationic carbon of C will give 1,2-adduct 4
(mode c). Although a number of transition-metal-cata-
lyzed ring-opening reactions of alkenylcyclopropanes
were reported,1e,11 to the best of our knowledge, only lim-
ited investigations for acid-mediated ring-opening reac-
tions of inactivated alkenylcyclopropanes can be found in
the literature.10,12 Moreover, the regioselective issues are
unexplored. Herein, we describe the regioselectivity in the
ring-cleavage reaction of spiro[5.2]octenes 1 with various
aqueous hydrohalic acids (HX).
Based on the mechanism, extensive efforts have been de-
voted to designing the illudin and duocarmycin analogues
and to understanding their biological properties.6–8 Bar-
one and co-workers examined the ring-opening reaction
of duocarmycin SA derivatives, whose spirocyclopropane
ring is electrophilically activated by the conjugation with
a carbonyl group.9 They made clear that a nucleophile at-
tacks preferably at the least substituted carbon atom of the
cyclopropane ring. Moreover, the computational studies
revealed that both the electronic and the steric effects of
the substituent can influence the reactivity of the ring-
opening reaction.9b Thus, they concluded that the substit-
uent effect would be helpful in exploring the chemical
transformations of duocarmycin SA as well as in design-
ing new analogues.
SYNLETT 2013, 24, 0120–0124
Advanced online publication: 10.12.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1317745; Art ID: ST-2012-U0893-L
© Georg Thieme Verlag Stuttgart · New York

LETTER
Ring-Opening Reaction of Spiro[5.2]octenes
121
R1
Table 1 Reaction of 1a Having an Alkyl Side Chain with Various
HXa
X
R2
2
mode
a
Ph
X–
R1
H
Ph
R1
Ph
R1
a
b
HX
X
mode
b
HX
X
X
solvent, r.t.
X–
c
+
R2
1a
2a
R2
3a
R2
3
R1
H
X–
1
H
Entry
HX
Solvent
Product ratio
Yield (%)
intermediate
mode
c
C
1
HI
MeCN
MeCN
CH2Cl2
MeCN
2aa/3aa = 100:0
2ab/3ab = 100:0
2ab/3ab = 100:0
2ac/3ac = 100:0
68
67
49
86
2
HBr
HBr
HCl
X
R2
3b
4
H
4
Scheme 2 Possible regioisomeric adducts 2–4 in the reaction of 1
with HX
a Conditions: 1a (0.1 mmol), HX (1.5 or 2.0 equiv), MeCN (0.2 M),
r.t., 1–3 h.
At the outset of our study, ring-opening reaction of 1a9
bearing an alkyl substituent under aqueous acidic condi-
tions was examined (Table 1). When 1a was treated with
aqueous hydroiodic acid (HI), which was prepared in situ
from TMSCl, NaI, and H2O (2 equiv each)13 in acetoni-
trile at ambient temperature, ring-opening reaction pro-
ceeded smoothly giving 2aa in 68% (Table 1, entry 1).
Namely, the product 2aa was formed through the nucleo-
philic substitution at the more substituted carbon of 1a
(mode a in Scheme 2). No isomeric adduct such as 3aa,
which will be produced by the C–X bond formation at the
less substituted carbon (mode b in Scheme 2), was ob-
served. As is the case in HI, secondary alkylbromide 2ab
was obtained as a single isomeric product in the reaction
of 1a with aqueous hydrobromic acid (HBr, Table 1, entry
2). The same regioselectivity as in acetonitrile was also
observed in dichloromethane, although the chemical yield
was decreased (Table 1, entry 3). With hydrochloric acid
(HCl), only electrophilic addition of Cl– to the substituted
carbon occurred to afford 2ac in 86% yield (Table 1, entry
4). It was found that, in the ring-opening reaction of 1a
with HX, a nucleophile attacks regioselectively at the
more substituted carbon regardless of the nature of the nu-
cleophiles.
b Compound 1a was recovered in 12% yield.
R1
X–
δ+
2
R2
δ+
TS D
intermediate
C
R1
δ+
3
X–
R2
δ+
TS E
Scheme 3 Plausible mechanism of regioselectivity
HI, formation of primary alkyl iodide 3ba was observed
as a minor product along with secondary alkyl iodide 2ba
(Table 2, entry 1). In the reaction of 1b with HBr and HCl,
the mixture of 2b and 3b were also obtained, respectively
(Table 2, entries 2 and 3). We guessed that an inductive ef-
fect would rationalize the formation of 3b.15 Namely, the
partially delocalized positive charge at the substituted car-
bon in TS D from 2b would be somewhat destabilized by
the ether moiety (negative inductive effect). As the result,
the regioisomeric ring-opening reaction via TS E giving
3b also proceeds as a minor path.
The regioselectivity of the ring-opening reaction can be
explained as shown in Scheme 3. After protonation of the
alkene moiety of 1, the corresponding tertiary cation inter-
mediate C is generated. Major product 2 should be ob-
tained via nucleophilic attack of X– at the more substituted
carbon [transition-state structure (TS) D], and minor prod-
uct 3 via nucleophilic attack at the less substituted carbon
(TS E). Partial positive charge (δ+) is delocalized at the
tertiary or secondary carbon in D or E, respectively.14 In
the reactions of alkyl-substituted substrate 1a, TS D
would be more stable than TS E owing to the cationic sta-
bility (primary vs. secondary carbocation), resulting in
product 2a with high regioselectivity.
In order to make clear the negative inductive effect by the
side chain on the cyclopropane ring, we investigated the
reaction of 1c having an electron-withdrawing ester sub-
stituent (Table 3). Treatment of 1c with HI in acetonitrile
afforded no 2ca but primary alkyl iodide 3ca in 75% yield
(Table 3, entry 1). The regioselectivity decreased when
the reaction of 1c was carried out in the less polar solvents
(Table 3, entries 2 and 3). The solvent effect supports that
the regioselectivity of the ring-opening reaction is depen-
dent on the electronic factor of the intermediate or transi-
tion state. When the reaction of 1c was carried out with
HBr, primary alkyl bromide 3cb was obtained along with
Next the reactions of 1b bearing an ether substituent with
HX were carried out (Table 2). When 1b was treated with
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 120–124

122
Y. Nagamoto et al.
LETTER
Table 2 Reaction of 1b Having a Benzyloxymethyl Substituent with
Table 3 Reaction of 1c Having a Benzoyloxymethyl Substituent
Various HXa
with Various HXa
OBn
OBz
OBn
OBz
HX
solvent, r.t.
OBz
HX
OBn
X
X
X
X
MeCN, r.t.
1c
2c
3c
1b
2b
3b
Entry
HX
Product ratio
Yield (%)
OH
OBz
OBz
2cd'
OH
2cd
1
2
3
HI
2ba/3ba = 71:29
2bb/3bb = 59:41
2bc/3bc = 72:28
65
88
78
HBr
HCl
OBz
a Conditions: 1b (0.1 mmol), HX (1.5 or 2.0 equiv), MeCN (0.2 M),
r.t., 0.5–3 h.
OH
4cd
regioisomeric adduct 2cb (Table 3, entry 4). The observa-
tion indicates that the regioselectivity would be also de-
pendent on the nature of the nucleophile (X–).The reaction
with HCl gave considerably complicated results (Table 3,
entries 5 and 6). At ambient temperature, ring-opening ad-
ducts 2cc and 3cc were obtained in 25% yield with 33:67
diastereomeric ratio. As a major product, cyclopropylcar-
binol 4cd was obtained in 32% yield as a 1:1 diastereo-
meric mixture. Formation of 4cd resulted in 1,2-addition
(mode c in Scheme 1) of water. As minor products, alco-
hols 2cd (R = CH2OBz, X = OH) and 2cd′ (R = OH, X =
CH2OBz) were also isolated in 8% and 2% yield, respec-
Entry HX
Solvent
Product ratio
Yield (%)
1
2
3
4
5
6c
HI
MeCN
CH2Cl2
toluene
MeCN
MeCN
MeCN
2ca/3ca/4ca = 0:100:0
2ca/3ca/4ca = 9:91:0
2ca/3ca/4ca = 38:62:0
2cb/3cb/4cb = 23:77:0
2c/3c/4c = 27:24:49b
2c/3c/4c = 72:28:0d
75
64
69
73
67
82
HI
HI
HBr
HCl
HCl
a Conditions: 1c (0.1 mmol), HX (1.5 or 2.0 equiv), solvent (0.2 M),
tively. When the reaction was carried out at 80 °C, no 1,2- r.t., 1–3 h.
b Compounds 2cc, 2cd, 2cd′, 3cc, and 4cd were obtained in 8%, 8%,
adduct 4 was detected but ring-opening products 2cc, 3cc,
2cd, and 2cd′ were produced. It indicates that 4cd is a ki-
netic product, and 4cd is transformed into conjugated ad-
ducts at higher temperature.
2%, 17%, and 32%, respectively.
c The reaction was carried out at 80 °C.
d Compounds 2cd and 2cd′ were also obtained in 29% and 14% yield,
respectively.
The regioselective formation of 3ca from ester 1c strongly
supports that the negative inductive effect of the substitu-
ent would control the position of the ring cleavage. Name-
ly, the delocalized cation in the TS D (Scheme 3) would
be delocalized by the negative inductive effect of the alk-
oxycarbonyl substituent. As the result, the ring-opening
reaction preferably proceeds via TS E. Production of alco-
hol 2cd′ (Table 3, entries 5 and 6) can be explained by the
formation of oxonium intermediate F,16 which would be
generated by an intramolecular cyclopropane ring-open-
ing reaction as shown in Scheme 4. As chloride anion (Cl–)
is less reactive than bromide and iodide anions,17 the nu-
cleophilic attack to intermediate C by the intramolecular
carbonyl group or external water molecule occurred faster
than chlorination.
O
Ph
X–
O
O
+
O
+
Ph
H2O
F
C
Scheme 4 Plausible mechanism for the formation of alcohols 2cd and
2cd′
OBz
OBz
TMSCl (2 equiv)
NaI (2 equiv)
I
H2O, MeCN
0 °C, 1 h
The ring-opening reaction of 6-oxo analogue 5 under
acidic conditions was also examined (Scheme 5). Treat-
ment of 5 with TMSCl and NaI in aqueous acetonitrile
smoothly afforded enone 6 in 83% yield even at 0 °C.
Product 6 was obtained as a 1:1 diastereomeric mixture
but no formation of its regioisomeric product was ob-
served. The nucleophile (I–) exclusively attacks at the less
O
O
6
5
83% (dr = 1:1)
Scheme 5 Regioselective ring-opening reaction of 6-oxo-spi-
ro[2.5]oct-4-ene 5 under acidic conditions
Synlett 2013, 24, 120–124
© Georg Thieme Verlag Stuttgart · New York

LETTER
Ring-Opening Reaction of Spiro[5.2]octenes
123
substituted cyclopropane carbon atom of 5 as is the case
of 1c.
Acknowledgment
This work was supported by Grants-in Aid for Young Scientists
(KT) and JSPS Fellowship for Young Scientists (YN) from the Mi-
nistry of Education, Culture, Sports and Technology (MEXT), Ja-
pan, and the Astellas Foundation for Research on Metabolic
Disorders.
In conclusion, we have examined the ring-opening reac-
tion of spirocyclic alkenylcyclopropanes under aqueous
acidic conditions. It was elucidated that the regioselectiv-
ity in the cyclopropane cleavage was significantly depen-
dent on the electronic nature of the substituent on the
cyclopropane ring. The nature of the nucleophile also
slightly influences the regioselectivity of the ring-opening
reaction. We are currently engaging in the synthesis of
new DNA-alkylating spirocyclic cyclopropane com-
pounds. The new findings on the regioselectivity would
help us in molecular designing.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
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Typical Procedure for the Ring-Opening Reaction
To a mixture of 1 (0.10 mmol), NaI (0.20 mmol), and H2O (0.20
mmol) in MeCN (0.2 M) was added TMSCl (0.20 mmol). Other-
wise, to a mixture of 1 (0.10 mmol) in MeCN was added aq concd
HCl or HBr (0.20 mmol). After being stirred for 1 h at an appropri-
ate temperature, the reaction mixture was quenched with sat.
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combined organic layers were washed with brine, dried over
Na2SO4, and concentrated in vacuo. The residue was purified by sil-
ica gel chromatography (hexane–EtOAc) to afford desired 2 and 3.
1-(2-Chloro-4-phenylbutyl)-2-methylcyclohexene (2ac)
Colorless oil. 1H NMR (400 MHz, CDCl3): δ = 7.31–7.26 (m, 2 H),
7.21–7.18 (m, 3 H), 4.01–3.94 (m, 1 H), 2.93 (ddd, J = 13.7, 9.1, 4.7
Hz, 1 H), 2.73 (ddd, J = 16.6, 9.3, 7.5 Hz, 1 H), 2.54 (dd, J = 13.9,
7.0 Hz, 1 H), 2.44 (dd, J = 13.9, 7.3 Hz, 1 H), 2.11–2.03 (m, 1 H),
1.96–1.81 (m, 5 H), 1.60 (s, 3 H), 1.55–1.53 (m, 4 H) ppm; 13C
NMR (126 MHz, CDCl3): δ = 141.2, 129.7, 128.5, 128.4, 126.3,
126.0, 61.6, 42.7, 39.3, 32.8, 31.9, 29.8, 23.2, 23.1, 19.6 ppm. IR
(neat): 2924, 2831 cm–1. LRMS (FAB+): m/z = 262 [M+]. HRMS
(FAB+): m/z calcd for C17H24Cl [M+ + H]: 263.1561; found:
263.1562.
1-[2-Benzoyloxy-1-(iodomethyl)ethyl]-2-methylcyclohexene
(3ca)
Colorless oil. 1H NMR (500 MHz, CDCl3): δ = 8.01 (d, J = 8.3 Hz,
2 H), 7.57 (t, J = 7.3 Hz, 1 H), 7.46 (dd, J = 8.3, 7.3 Hz, 2 H), 4.34
(dd, J = 11.0, 7.1 Hz, 2 H), 3.49–3.39 (m, 1 H), 3.39 (dd, J = 9.5,
6.3 Hz, 1 H), 3.19 (dd, J = 9.5, 9.0 Hz, 1H), 2.05–1.86 (m, 4 H), 1.67
(s, 3 H), 1.63–1.56 (m, 4 H) ppm. 13C NMR (126 MHz, CDCl3): δ
= 166.3, 133.0, 131.9, 130.1, 129.5, 128.4, 127.0, 65.8, 43.2, 32.5,
23.6, 23.0, 22.9, 19.4, 5.40 ppm. IR (neat): 2923, 1721 cm–1. LRMS
(FAB+): m/z = 385 [M+ + H]. Anal. Calcd for C17H21IO2: C, 53.14;
H, 5.51. Found: C, 53.34; H, 5.59.
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 120–124

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Y. Nagamoto et al.
LETTER
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