Highly functionalized spirocyclohexadienones from Morita-Baylis-Hillman adducts

Article abstract of DOI:10.1055/s-0029-1217725

Spirohexadienone is a structural moiety present in several biologically active natural products isolated both from terrestrial and marine natural sources. This structural moiety is commonly and strictly related to the biological properties exhibited by th

Full text of DOI:10.1055/s-0029-1217725

LETTER
2333
Highly Functionalized Spirocyclohexadienones from
Morita–Baylis–Hillman Adducts
Intramolecular
H
o
R
eactiondrigo V. Pirovani, Bruno R. V. Ferreira, Fernando Coelho*
Laboratory of Synthesis of Natural Products and Drugs, University of Campinas, DQO/IQ, PO Box 6154, 13084-971, Campinas, SP, Brazil
Fax +55(19)37883023; E-mail: coelho@iqm.unicamp.br
Received 14 April 2009
The authors dedicate this paper in memorium of the recently deceased Prof. Octávio Antunes for his outstanding contributions to Brazilian
palladium chemistry.
The structural complexity of their derivatives associated
Abstract: Spirohexadienone is a structural moiety present in sever-
with the relevant biological and physical properties of
spirohexadienone moiety have been used as inspiration to
develop diverse synthetic approaches towards this partic-
al biologically active natural products isolated both from terrestrial
and marine natural sources. This structural moiety is commonly and
strictly related to the biological properties exhibited by the natural
products. We show herein that the combination of intermolecular ular spiro system. Spirocyclohexadienones have been
synthesized using radical chemistry,8 electrophilic cy-
Heck reactions with a phenolic oxidation induced by hypervalent
clization of bis(4-methoxybenzylthio)acetylenes,9 addi-
iodine form these structural moieties from Morita–Baylis–Hillman
adducts in moderate to good overall yields.
tion of the enolate of the carbene complex to a carbonyl
compound,10 or by a three-component reaction involving
Key words: Morita–Baylis–Hillman, Heck reaction, spiro com-
naphthalene derivatives, isobutyraldehyde and nitrile,11 or
pounds, palladacycles, hypervalent iodine
in palladium-mediated reactions.12 However, despite the
several alternatives to synthesize spirocyclohexadienone,
Natural products, isolated both from terrestrial or marine
sources, display great structural diversity and serves as
sources of inspiration for synthetic organic chemists to de-
velop prototypes in designing new drugs.
the most employed are those based on phenolic oxidation
mediated by hypervalent iodine.13
Under normal circumstances, this reaction involves first
the formation of a complex between the hypervalent io-
dine reagent and the hydroxy group of phenol with the for-
mation of a carbocation on the aromatic ring. This cation
is then trapped by an internal nucleophile to form the spi-
rocyclohexadienone system. Amines, hydroxy groups,
imines, carbon–carbon double bonds, and oximes have
been used as nucleophiles to intercept the intermediate
carbocation (Scheme 1).13b Alternatively this reaction can
occur without the formation of a cation.
Among this huge diversity we find terrestrial and marine
natural products bearing in their structures a spirohexadi-
enone moiety.1 Owing to the high conjugation, provided
by the presence of a carbonyl group and two double
bonds, this structural moiety acts as an efficient Michael
acceptor and this chemical property is normally associat-
ed with the biological activity. Figure 1 depicts some
examples of natural products bearing spirocyclohexa-
dienone moieties.
OMe
O
Annosqualine (1), a tetrahydroisoquinoline alkaloid iso-
lated from Annona squamosa (sugar apple), presents
strong purgative activity.2 Futoenone (2) is a neolignan
isolated from Piper futokadsura,3 which inhibits selec-
tively metalloproteins4 and also PAF.5 Futoenone displays
cardiovascular protective properties and is used to control
anti-inflammatory processes.5
MeO
MeO
HO
N
annosqualine (1)
O
O
O
O
O
Interiorin A (3) is a dibenzocyclooctadiene lignan isolated
from stems of Kadsura interior A. C. Smith. An extract of
these plant stems is used to prepare the Chinese medicine
Fufang Jixueteng Gao, which treats menstrual irregulari-
ties, blood deficiencies, and other feminine disorders.6
Spirocyclohexadienes have been used as building blocks
for the design of new organic materials exhibiting non-
conventional optoelectronic properties.7a Spirohexadi-
enones also show important fungicide activities.7b
futoenone (2)
O
O
MeO
MeO
O
interiorin A (3)
SYNLETT 2009, No. 14, pp 2333–2337
x
x
.x
x
.2
0
0
9
Figure 1 Examples of biologically active natural products bearing
the spirocyclohexadienone moiety
Advanced online publication: 31.07.2009
DOI: 10.1055/s-0029-1217725; Art ID: S03909ST
© Georg Thieme Verlag Stuttgart · New York

2334
R. V. Pirovani et al.
LETTER
X
I
Ph
O
– PhI
– X
OH
R2
O
OH
R1
PHIX2
– HX
O
O
O
O
R1
R2
R1
R2
spiro compounds
β-keto ester
– PhI
– X
OH
R2
OH
R2
R1
R1
R1 = CO2Me
R2 = alkyl or aryl
Scheme 1 Plausible mechanism of the oxidative dearomatization of phenolic substrates using b-keto esters synthezised from MBH adducts
(adapted from ref. 13b)
As far as we know, b-keto esters with phenol groups have catalyst. Apparently, they form in situ a nanoparticulate
not been used as substrates for an oxidative dearomatiza- palladium under the reaction conditions, thus increasing
tion of phenols mediated by hypervalent iodine reagents.
the reaction efficiency.21
b-Keto esters are important building blocks in organic With the adducts in hand, we then evaluated the Nájera
synthesis and many methods are available for their prepa- catalysts. Initially, we worked with catalyst 16, since it is
ration.14
the most adequate for reactions performed in organic sol-
vents. The reaction between MBH adduct 10 and phenyl
iodide was used as a model for our study. Initially, the cat-
alyst was used in a concentration of 0.001 mol%.
Morita–Baylis–Hillman (MBH) adducts15,16 can be effi-
ciently transformed into b-keto esters by using an inter-
molecular Heck reaction with an adequate palladium
catalyst.
Under these conditions, no Heck coupling product was
observed after 15 hours. We then varied the amount of
catalyst (0.01, 0.025, 0.05, 0.1, 0.25, and 0.5 mol%). For-
tunately, when the reactions were performed using 0.5
mol% we are able to observe a clean reaction, which fur-
nished the corresponding a-hydroxybenzyl-b-keto esters
We and others have already described intermolecular
Heck reaction using MBH adducts, but either the yields
are moderate or the reaction times are too long.17
In a research program directed towards the preparation of
spirocyclohexadienones with pesticide activities, we de-
cided to revisite the Heck reaction with MBH adducts. In
this letter we disclose a very efficient intermolecular Heck
reaction with MBH adducts using palladacycles as cata-
lysts leading to a-hydroxybenzyl-b-keto esters and their
further transformation into spirocyclohexadienones.
Table 1 Preparation of the Morita–Baylis–Hillman Adducts
OH
O
O
CO2R2
R1
OR2
R1
H
DABCO
ultrasound
We started our investigation by preparing a set of different
MBH adducts using a method previously described by our
research group.18 Thus, aldehydes were treated with
an excess of methyl or ethyl acrylate in the presence of
DABCO. The mixtures were placed in a ultrasound bath
for some hours to form the corresponding adducts in good
to excellent yields (Table 1).
a
4–9
10–15
[bmim]PF6
Entry
Aldehyde
R2
Time (h) MBH (%)b
1
2
3
4
5
6
4 R1 = Ph
Me
Me
Me
Me
Me
Et
96
96
96
8
10 91
11 70
12 73
13 96
14 90
15 85
5 R1 = piperonyl
6 R1 = 4-MeOC6H4
7 R1 = 4-O2NC6H4
8 R1 = 2-thienyl
9 R1 = Et
Palladacycles have recently attracted great attention as ef-
ficient Heck catalysts,19 eliminating the need for phos-
phine addition. An example of palladacycles used as Heck
catalysts are the oxime-derived palladacycles developed
by Nájera (Figure 2).20
4
8
The palladacycles 16 and 17 are thermally robust and in-
sensitive to both oxygen and moisture and have been
shown to function as efficient phosphine-free Heck pre-
a Traces of ionic liquid [bmim]PF6 was used to catalyze the MBH
reactions.
b Yields refer to isolated and purified products.
Synlett 2009, No. 14, 2333–2337 © Thieme Stuttgart · New York

LETTER
Intermolecular Heck Reaction
2335
fication, the required spirocyclohexadienones. Table 3
summarizes the results of this oxidative dearomatization.
OH
Cl
Cl
Table 3 Spirocyclizations Using Hypervalent Iodine
N
Pd Cl
N
Pd Cl
HO
HO
F3COCO
OCOCF3
(PIFA)
I
O
2
2
16
O
O
17
Figure 2 Nájera’s oxime-derived palladacycles
R1
OR2
MeCN, –10 °C,
15 min
O
in good yield after three hours. Table 2 summarizes the re-
sults.
OH
R1
CO2R2
Table 2 Intramolecular Heck Reaction with MBH Adducts Cata-
lyzed by Nájera Catalyst
α-hydroxybenzyl-β-keto
esters
spirocyclohexadienones
(18–23)
(24–29)
O
O
OH
O
Nájera
catalyst (16)
Entry a-Benzyl-b-keto ester
Time (min) Yield (%)a–c
R1
OR2
R1
OR2
DMF, 110 °C
HOPhI, Et3N
1
2
3
4
5
6
18 R1 = Ph, R2 = Me
15
15
15
15
15
15
24 60
25 70
26 36
27 36
28 40
29 40
MBH adducts
(10–15)
19 R1 = piperonyl, R2 = Me
20 R1 = 4-MeOC6H4, R2 = Me
21 R1 = 4-O2NC6H4, R2 = Me
22 R1 = 2-thienyl, R2 = Me
23 R1 = Et, R2 = Et
OH
intermolecular Heck
products (18–23)
Entry MBH
Time (h) Yield (%)a–d
1
2
3
4
5
6
10 R1 = Ph, R2 = Me
3
3
3
3
3
3
18 82
19 85
20 90
21 70
22 80
23 70
11 R1 = piperonyl, R2 = Me
12 R1 = 4-MeOC6H4, R2 = Me
13 R1 = 4-O2NC6H4, R2 = Me
14 R1 = 2-thienyl, R2 = Me
15 R1 = Et, R2 = Et
a Yields refer to isolated and purified products.
b See reference section for experimental details.
c All spectral data (1H NMR, 13C NMR, IR, HRMS are compatible to
the proposed structures).
The reaction works therefore nicely and furnishes the cor-
responding spirocyclohexadienone compounds in moder-
ate yields. These yields are, however, acceptable for this
type of transformation when compared with those report-
ed in literature.13b
a Yields refer to isolated and purified products.
b For all cases 4-iodophenol was used as halide in the Heck coupling.
c See reference section for experimental details.22,
d All spectral data (1H NMR, 13C NMR, IR, HRMS are compatible
with the proposed structures).
The combination of two reactions allows one to prepare
spiro compounds starting from MBH adducts. Since these
adducts are very easy to prepare, the strategy disclosed
herein is promising as a valuable alternative to synthesize
sophisticated new carbon arrangements from simple start-
ing materials.
In all reactions, the yields and reaction times were very
good. This seems to be one of the most efficient Heck
reaction performed with MBH adducts. Basavaiah and
Muthukumaran already described a very efficient Heck
reaction using the ‘Jefferey protocol’ [Pd(OAc)2, NaHCO3,
n-Bu4NBr], however, with much longer reaction times.17b
In summary, a-hydroxybenzyl-b-keto esters can be pre-
pared from MBH adducts with efficiency via an intermo-
lecular free-phoshine Heck reaction catalyzed by Nájera
palladacycles in good to excellent yield and short reaction
times. Allylic alcohols and derivatives are very complicat-
ed as substrates for a Heck reaction. When these com-
pounds are used in this reaction the control of selectivity
becomes a serious issue and normally a mixture of prod-
ucts is obtained.24 Despite the numerous options of exper-
imental conditions available in literature to perform a
Heck reaction, most of them did not work properly with
allylic alcohols and derivatives. In this letter, we demon-
strate that Najera palladacycles could be considered as a
valuable alternative to circumvent this drawback. This
seems to be the first report on the use of this type of cata-
lyst for a Heck reaction with MBH adducts.
Phenyliodine(III) bis(trifluoroacetate) (PIFA) has attract-
ed considerable attention due to its ready availability, low
toxicity, easy handling, and reactivity similar to that of
heavy-metal reagents.22 Its efficient utilization in metal-
free transformations relies on both the extremely mild re-
action conditions required and its ability to oxidize
chemoselectively a wide range of functionalities such as
phenols, amines, sulfides, and carbonyl compounds.
Then, we decided to investigate this derivative as oxidants
for the a-benzyl keto esters obtained in the Heck reac-
tion.22
A solution of the a-hydroxybenzyl-b-keto esters in anhy-
drous acetonitrile at –10 °C was therefore treated with
PIFA23 for 15 minutes, to furnish, after workup and puri-
Synlett 2009, No. 14, 2333–2337 © Thieme Stuttgart · New York

2336
R. V. Pirovani et al.
LETTER
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reaction were easily transformed into spiro compounds in
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spiro compounds.
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Further studies focused on determining the scope and lim-
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and the results will be disclosed in due time.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
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Acknowledgment
R.V.P. and B.R.V.F. thank CNPq/Capes for fellowships. F.C.
thanks Fapesp for financial support (06/06347-9) and CNPq for a
research fellowship (CNPq 301832/2007-2).
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Synlett 2009, No. 14, 2333–2337 © Thieme Stuttgart · New York