Boron- and silicon-substituted [3]-1-heterodendralenes as versatile building blocks for the rapid construction of polycyclic architectures

Article abstract of DOI:10.1002/chem.201101624

B and Si, additional assets: Skeletally diverse polycyclic compounds with up to six stereocenters were assembled with excellent stereoselectivity from simple 2-vinyl α,β-unsaturated aldehydes possessing a trimethylsilyl or a boronate group at position 3 (

Full text of DOI:10.1002/chem.201101624

DOI: 10.1002/chem.201101624
Boron- and Silicon-Substituted [3]-1-Heterodendralenes as Versatile Building
Blocks for the Rapid Construction of Polycyclic Architectures
Fabien Tripoteau,[a] Tristan Verdelet,[b] Alain Hercouet,[a] FranÅois Carreaux,[a] and
Bertrand Carboni*[a]
In recent years, dendralenes 1 have received increasing at-
tention as building blocks for multiple Diels–Alder cycload-
ditions and rapid generation of molecular complexity.[1]
These acyclic cross-conjugated polyenes are particularly
well-suited to diversity-oriented synthesis (DOS), a promis-
ing strategy for lead generation in chemical genetics and
drug discovery.[2] They have found numerous applications
Figure 1. [3]-1-Heterotrienes 4, 5, 6 and 7.
for rapid access to polycyclic frameworks,[3] and have also
been engaged in targeted syntheses of natural products, such
as vinigrol[4] or triptolide.[5] Since the early report of Blom-
quist and Verdol,[6] several routes have been developed to
synthesize [n]dendralenes. Surprisingly, only few examples
related to their heteroatom analogues are hitherto reported.
Tsuge et al., Motoki et al. and Saito et al. independently de-
scribed diene-transmissive hetero-Diels–Alder reactions of
compounds 2, in which the heteroatom occupies the central
position of the triene skeleton.[7–9] More recently, cross-con-
jugated dioxatrienes 3 with two heteroatoms were synthe-
sized and were engaged in sequential reactions to provide
pyran-fused aza- and thia-heterocycles.[10,11]
In parallel, Pi and Bodwell described normal and inverse
electron demand Diels–Alder reactions of dienes 4,[12] while,
more recently, Spino and Perreault have intensively investi-
gated a very elegant intramolecular strategy to construct the
quassinoid framework through tandem cycloadditions in-
volving cyclic compounds 5 (Figure 1).[13]
On the basis of these precedents, we initiated a research
program with the aim of exploring the synthesis and reactiv-
ity of acyclic 2-vinyl a,b-unsaturated aldehydes, 6 and 7
(R1 =SiMe3 or Bpin), which we named [3]-1-heterodendra-
lenes by analogy with the corresponding carbotrienes. These
compounds were designed as potential useful starting build-
ing blocks for rapid access to a rich structural diversity with
control of multiple stereocenters. Indeed, sequential Diels–
Alder/hetero-Diels–Alder reactions (or vice versa) can be
carried out chemoselectively, and the structures of the final
polycyclic products are determined by the intrinsic reactivity
of each 1,3-dienyl system or by the order of introduction of
the reagents. Furthermore, the first cycloaddition step or the
combination of two Diels–Alder reactions generates an al-
lylsilane or an allylboronic ester that constitutes an addition-
al asset for creating structural diversity.[14] Finally, most of
the final products share a common motif characterized by a
cyclic lactol ether system, a structural element that, as such
or as precursor of the corresponding lactones or tetrahydro-
pyrans, has been found in many bioactive compounds and
drugs.[15] Herein, we report the first results to illustrate the
multiple synthesis possibilities offered by the polyfunctional
building blocks 6 and 7 in diene-transmissive Diels–Alder
reactions (Scheme 1).
[a] F. Tripoteau, Dr. A. Hercouet, Dr. F. Carreaux, Dr. B. Carboni
Sciences Chimiques de Rennes, UMR 6626 CNRS
Universitꢀ de Rennes 1, Campus de Beaulieu
35042 Rennes CEDEX (France)
Fax : (+33)0299236747
E-mail: bertrand.carboni@univ-rennes1.fr
[b] T. Verdelet
Present address: Laboratoire de Chimie Bioorganique
IRCOF, UMR 6014, Rue Tesniꢁres
76130 Mont Saint Aignan (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101624.
Scheme 1. DOS strategy based on boron- and silicon-substituted
[3]-1-heterodendralenes 6 and 7.
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The starting dendralenes were prepared according to the
following sequences (Scheme 2). Direct addition of vinyl-
magnesium chloride to 3-trimethylsilylpropargyl alcohol af-
Scheme 3. a) ethyl vinyl ether, YbACHTNUTRGNENG(U fod)3 (5 mol%), room temperature,
overnight; 10, 81%; 11, 63%; b) N-phenylmaleimide, toluene, room tem-
perature, 12 h; 12a, 90%; 13a, 88%.
various other partners, and the results are presented in
Table 1.
Interestingly, adducts derived from ethyl vinyl ether and
a,b-unsaturated alkenes, activated azo compounds and
naphtoquinone participated well in this multicomponent re-
action (entries 1–4). The yield was lower with methyl acety-
lenedicarboxylate (entry 5), which was also the case when
the silicon was replaced by boron, probably for reasons of
stability rather than reactivity (entries 6–7). Other enol
ether, such as 2,3-dihydrofuran, can also be engaged in this
HDA/DA sequence as electron-rich dienophiles (entry 8). It
is worth noting that 6 can also play the dual role of hetero-
diene and dienophile to give the cycloadduct 12 f when the
reaction was conducted in a default of enol ether (entry 9).
Taking advantage of the presence of an allylboronate or
an allylsilane moiety, we also engaged the N-phenylmalei-
mide derivatives 12a and 13a, chosen as examples, in vari-
ous transformations (Scheme 4). The fluorination of 12a
was performed by using Selectfluor in acetonitrile to afford
a single diastereomer 14 in 81% yield, while bromination
with NBS afforded the corresponding allylic bromide 15.[21]
Concerning 13a, oxidation with hydrogen peroxide gave the
allylic alcohol 16. More interesting, in terms of additional di-
versity, is the addition to aldehydes that provided access to
the corresponding homoallylic alcohols 17a–c in 77–86%
yield (Scheme 4).[22]
Scheme 2. Synthesis of boron- and silicon-substituted [3]-1-heterodendra-
lene 6 and 7. a) MnO2, CH2Cl2, reflux, 4 h, 95%; b) TBDMSCl, imida-
zole, CH2Cl2, room temperature, 18 h, 90%; c) NIS, MeCN, 08C, 4 h,
98%; d) B2pin2, KOAc, PdCl2ACTHUNTRGNEU(GN dppf) (3 mol%), DMSO, 508C, 4 h, 88%
(three steps, 77%); e) AcOH, THF/H2O (1:1), 508C, 5 h, 89%; f) Dess–
Martin periodinane, CH2Cl2, room temperature, 3 h, 93% (two steps,
83%).
forded the diene 8 in 91% yield according to the procedure
of Fallis et al.[16] Oxidation with manganese dioxide in re-
fluxing CH2Cl2 produced the aldehyde 6. In parallel, after
protection of the alcohol as a TBDMS ether, 8 was treated
with N-iodosuccinimide; this resulted in the replacement of
the trimethylsilyl group by iodine with retention of configu-
ration. Borylation was carried out with bis(pinacolato)dibor-
on and a catalytic amount of PdCl2ACTHUNTGRNEUNG(dppf) in the presence of
potassium acetate.[17] Deprotection of 9 with acetic acid in a
mixture THF/water was followed by the oxidation of the al-
cohol with the Dess–Martin reagent (83%, two steps).
Unlike the parent compound, 2-formyl-1,3-butadiene (R1 =
H), which shows a high propensity toward dimerization,[18]
the boron- and silicon-substituted derivatives can be isolated
and kept for several days, at room temperature for 6 and at
À208C for 7.
With the starting building blocks in hand, we first ex-
plored their reactivity as heterodienes. 3-Boronoacrolein
esters are viable substrates in metal-catalyzed inverse elec-
tron demand hetero-[4+2] cycloaddition with enol
ethers.[15,19] With 6 or 7 and ethylvinyl ether in the presence
Alternatively, as the cycloadduct 11 bears a boronate
group in allylic position, the allylation and the cycloaddition
steps can be inverted to generate a supplementary skeletal
diversity from the same starting heterodendralene. Accord-
ingly, as an illustration of this sequence, 11 and 4-nitrobenz-
of Yb
G
ACHTUNGTRENaNGNU ldehyde were allowed to react at room temperature to
2H-pyrans 10 or 11 were isolated in 81 and 63% yields, re-
spectively. In the presence of N-phenylmaleimide, they un-
derwent normal Diels–Alder reactions to afford the corre-
sponding cycloadducts as single diastereomers (Scheme 3).
The stereochemical outcome of this sequence (two consecu-
tive endo cycloadditions) was assigned on the basis of X-ray
crystallographic analysis of the final product 12a.[20] These
reactions can be also advantageously conducted in a one-pot
process without formation of any product resulting from a
first cycloaddition of the 1,3-butadienyl moiety to the elec-
tron-poor alkene. These results encouraged us to examine
the reactivity of heterodendralenes 6 and 7 with respect to
afford 18, which underwent a further cycloaddition with N-
phenylmaleimide to furnish the single tricyclic compound 19
(Scheme 5). Its stereochemistry, which was established by X-
ray crystallographic analysis,[23] results first from a cyclic
chair-like transition state A for the allylation reaction in-
volving an equatorial position for the 4-nitrophenyl substitu-
ent, and then from an endo approach (transition state B) of
the dienophile from the less hindered bottom face of the
new created diene.
To continue the exploration of the reactivity of boron-
and silicon-substituted [3]-1-heterodendralenes in diene
transmissive Diels–Alder reactions, we engaged 6 and 7
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B. Carboni et al.
Table 1. One-pot HDA/DA sequence from heterodendralenes, 6 and 7.
Entry Heterodendralene Enol ether
Electron-poor
dienophile
Product
Yield
[%][a]
1
71
64
63
2
3
4
68
5
6
41
54
7
8
9
45
43
63[b]
[a] Yields refer to pure products (one pot, two steps); [b] mixture of diastereomers (94:6).
through their 1,3-butadienyl moiety in normal electron-
demand cycloadditions. Indeed, the cycloadducts 20a/b and
21a/b were obtained as single diastereomer in good yields
after heating in toluene for N-phenylmaleimide,[24] while 4-
phenyl-1,2,4,-triazoline-3,5-dione reacted at room tempera-
ture (Scheme 6).
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[3]-1-Heterodendralenes for Constructing Polycyclic Compounds
COMMUNICATION
amine the reactivity of similar cyclic enals 22 (R1 =H) and
23 (R1 =Ph) to determine if this unexpected low reactivity
can be attributed to the presence of the TMS or Bpin sub-
stituents in the allylic position (Scheme 7). When the cyclo-
Scheme 7. Eu
vinyl ether. Reagents and conditions: a) 22 (R1 =H), ethyl vinyl ether
(9 equiv), CH2Cl2, 48 h, Eu(fod)3 (5%), 408C, 74%; b) ethyl vinyl ether
ACHTUNGRTEN(NNUG fod)3-catalysed [4+2] cycloadditions of enals 20–22 to ethyl
AHCTUNGTRENNUNG
Scheme 4. Synthesis of tricyclic imides derivatives 14–17. a) Selectfluor,
MeCN, room temperature, 5 h, 81%; b) NBS, MeCN, 3 h, À208C to 08C,
84%; c) H2O2, NaOAc, THF, 08C, 6 h, 87%; d) RCHO, toluene, 808C,
(9 equiv), CH2Cl2, 16 kbar, EuACHTNUGRTNEUNG
(fod)3 5%, 508C, 4 days, 20a (R1 =TMS,
30%) or 23 (R1 =Ph, 41%).
À
16 h, (R=Ph, 77%, 4-NO2 C6H4, 86%, EtO2C, 79%).
addition took place at 408C (CH2Cl2, EuACHTNUTRGNE(NUG fod)3, 5%) in a
74% yield in the case of 22, application of 16 kbar pressure
at 508C for 4 days was necessary for 23 to afford the corre-
sponding cycloadduct 25 as a mixture of two diastereomers
in a moderate 41% yield.[25] Under these conditions, a simi-
lar result was obtained with the trimethylsilyl derivative
20a; this suggests that the presence of a substituent at posi-
tion 6 of the 1-cyclohexene-1-carboxaldehyde fragment
greatly decreases the reactivity of the heterodienes. With
R1 =Bpin we were unable to isolate any defined product,
probably due to a lower stability of the starting boronated
species.
Finally, we decided to examine the behavior of 21a as al-
lylating reagent. When a test reaction with benzaldehyde ef-
fectively afforded the expected homoallyllic alcohols, yields
were very low (ꢀ10%). Gratefully, this drawback can be
circumvented by using a modification of the order of the re-
actions and by converting the initial boronic ester group in a
trifluoroborate substituent that greatly increases the reactiv-
ity of the dienyl moiety.[26] This transformation was easily
achieved by treatment of 9 with KHF2 in MeOH/H2O with a
concomitant deprotection of the alcohol. The resulting
diene 27, which could be isolated if necessary,[27] was directly
engaged in a Diels–Alder cycloaddition with N-phenylmalei-
mide at 508C in acetone for 12 h in the presence of various
aldehydes.[28] The diols 28a–c were obtained as major diaste-
reomers (ꢁ95%) in good overall yields (one pot, four steps;
Scheme 8).[29]
Scheme 5. Synthesis of 19 via a hetero-Diels–Alder (HDA)/allylboration/
Diels–Alder (DA) sequence.
First attempts to perform the hetero-Diels–Alder cycload-
dition of 20a or 21b with ethyl vinyl ether under standard
conditions (EuACHTUNGTRENNUNG(fod)3 or YbHCATUNGTERN(NUGN fod)3, 1,2-dichloroethane, at
room temperature or 608C, or by using EtOCH=CH2 as sol-
vent) proved to be unsuccessful, and led mainly to the start-
ing materials or degradation. We, therefore, decided to ex-
In conclusion, we have developed efficient procedures for
the rapid construction of substituted polycyclic structures
from a common starting acyclic cross-conjugated diene with
control of the multicreated stereogenic centers. The range of
accessible scaffolds can be greatly extended by varying the
nature of the different building blocks, such as the substitu-
ents of the starting heterodendralenes. Furthermore, most of
the resulting products have also functional groups for easy,
further diversification by classical reactions. The develop-
Scheme 6. Synthesis of 20–21. a) N-phenylmaleimide, toluene, 908C, 15 h;
20a, 70%; 21a, 64%; b) 4-phenyl-1,2,4,-triazoline-3,5-dione, toluene,
room temperature, 15 h; 20b, 65%; 21b, 63%.
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Acknowledgements
We thank Dr. I. Chataignier (IRCOF, Rouen) for help and advice in high
pressure experiments, and T. Roisnel and L. Toupet (University of
Rennes 1) for crystallographic analysis. This work was supported by a
grant from MESR (F.T.), the CNRS and the University of Rennes 1.
Keywords: boronic esters · cycloadditions · Diels–Alder
reaction · diversity oriented synthesis · silicon
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[3]-1-Heterodendralenes for Constructing Polycyclic Compounds
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b) M. Gravel, K. A. Thompson, M. Zak, C. Berube, D. G. Hall, J.
Org. Chem. 2002, 67, 3–15, and references therein.
[27] A single isomer 27 was detected, the structure of which is in agree-
ment with an endo transition state. The relative configuration of the
Received: May 27, 2011
Published online: November 8, 2011
Chem. Eur. J. 2011, 17, 13670 – 13675
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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13675