Palladium-catalyzed asymmetric allylic alkylation of cyclic dienol carbonates: Efficient route to enantioenriched γ-butenolides bearing an all-carbon α-quaternary stereogenic center

Article abstract of DOI:10.1002/anie.201206368

Alpha, beta, gamma: Allyl dienol carbonates (1) served as substrates for the title reaction to afford the furanones 2 in both high yields and high enantioselectivities. These furanones were eventually converted into valuable building blocks including γ-tertiary and γ-quaternary furanones (3) as well as β-quaternary butyrolactones (4). This method was used as a key step in the total synthesis of (-)-nephrosteranic acid and (-)-roccellaric acid. Copyright

Full text of DOI:10.1002/anie.201206368

Angewandte
Chemie
DOI: 10.1002/anie.201206368
Asymmetric Catalysis
Palladium-Catalyzed Asymmetric Allylic Alkylation of Cyclic Dienol
Carbonates: Efficient Route to Enantioenriched g-Butenolides Bearing
an All-Carbon a-Quaternary Stereogenic Center
Jeremy Fournier, Oscar Lozano, Candice Menozzi, Stellios Arseniyadis,* and Janine Cossy*
Since its introduction almost simultaneously by Tsuji et al.[1]
and Saegusa et al.[2] in the beginning of the 1980s, the
palladium-catalyzed decarboxylative allylic alkylation reac-
tion has been the focus of intensive efforts and has become
ꢀ
one of the most valuable methods for the construction of C C
bonds.[3] Interestingly however, despite all the developments
made during the next two decades, it was only in 2004 that
Stoltz et al.,[4] and Burger and Tunge[5] reported the first
asymmetric versions of this class of reaction by applying it to
cyclic allyl enol carbonates and 1,3-disubstituted allylic b-
ketoesters, respectively. Immediately after, Trost et al.
reported the use of a new family of ligands derived from 2-
diphenylphosphinobenzoic or 1-naphthoic acid and a chiral
scalemic diamine enabling the asymmetric synthesis of both
cyclic[6] and acyclic[7] ketones bearing either a tertiary or
a quaternary a-stereogenic center. Since then, this reaction
has been successfully applied to a wide variety of substrates
including a-sulfonyl,[8] a-nitro,[9] a-cyano,[10] a-imino,[11] or a-
heteroaromatic[12] allyl esters as well as various enol carbo-
nates[13] and silyl enol ethers,[14] thus illustrating its broad
functional-group tolerance.
Our work in this field began after observing that cyclic
dienol carbonates such as A could also serve as valuable
substrates for the palladium-catalyzed decarboxylative allylic
alkylation (Pd-AA) reaction, thereby affording, predomi-
nantly, the a-allylated product B (Scheme 1a). The resulting
1,5-diene could then be engaged in a microwave-mediated
Cope rearrangement[15] followed by a nucleophilic addition
and a dehydration reaction to afford the corresponding 2,4-
disubstituted or 2,3,4-trisubstituted furan D.[16] We thus
reasoned that by combining a source of Pd0 with an
adequately chosen chiral ligand, this reaction would offer
the possibility to access 2(3H)-furanones B bearing an a-
quaternary stereogenic center (a-quaternary buteno-
lides)[17,18] in a highly straightforward and enantioselective
fashion. We present here the results of our endeavors
(Scheme 1b).
Scheme 1. a) One-pot, four-step, sequence for the synthesis of poly-
substituted furans by Pd-AAA. b) Enantioselective synthesis of a-
quaternary butenolides and b-quaternary butyrolactones by Pd-AAA.
DIBAL-H=diisobutylaluminum hydride, PCC=pyridinium chlorochro-
mate, MW=microwave.
latter, prepared in three steps and 39% overall yield starting
from commercially available a-methylene-g-butyrolactone by
cross-metathesis with 3-butenylbenzene,[19] subsequent
RhCl3-mediated isomerization,[20] and a final O-acylation,
was first engaged in a reactivity and enantioselectivity screen
across an array of ligands by performing the reactions in THF
at 08C. The results of this survey are summarized in Table 1.
As a general trend, with the exception of (R)-binaphane
(L7; Table 1, entry 7), all the reactions proceeded efficiently,
independently of the ligand used, to afford predominantly the
a-allylated product 2a. It is worth pointing out however that
palladium catalysts derived from C2-symmetric diphosphines
such as L1, L2, and L3 displayed higher levels of selectivity
(entries 1–3) than the mixed P/N-type ligands such as the
phosphine oxazoline (PHOX) ligand L4, the axially dissym-
metric C2-chiral diphosphines (R)-binap (L5; entry 5),
biphenyl L6 (entry 6) and binaphthyl L7 (entry 7), and the
dimethylphospholane-derived ligands (L8; entry 8). In the
ideal case, the use of the (R,R)-DACH-phenyl ligand (L1,
10 mol%) developed by Trost et al. in conjunction with
[Pd2(dba)3·CHCl3] (5 mol%) led to the highest level of
selectivity, thus affording the a,a-disubstituted 2(3H)-fura-
none in 82% yield and up to 54% ee (entry 1).
To test our hypothesis, we decided to initiate our study
using the cyclic dienol carbonate 1a as a model substrate. The
[*] J. Fournier, Dr. O. Lozano, Dr. C. Menozzi, Dr. S. Arseniyadis,
Prof. Dr. J. Cossy
Encouraged by these preliminary results, we next exam-
ined the influence of the solvent. Interestingly, as the solvent
system became more polar, a distinct increase in both the
regio- and the enantioselectivity was observed. Accordingly,
when performing the reaction in n-hexnae, we were able to
isolate the a,a-disubstituted 2(3H)-furanone 2a in 72% yield
(2a/3a = 3:1) and 16% ee (Table 1, entry 9). Solvents such as
Laboratoire de Chimie Organique, ESPCI ParisTech, CNRS (UMR
7084), 10 rue Vauquelin 75231 Paris Cedex 05 (France)
E-mail: stellios.arseniyadis@espci.fr
janine.cossy@espci.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206368.
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Table 1: Selected optimization studies.[a]
Entry Solvent Ligand
2a/3a[b] 2a
Yield [%][c] ee [%][d] ee [%][d]
3a
1
2
3
4
5
6
7
8
THF
THF
THF
THF
THF
THF
THF
THF
hexane
toluene (R,R)-L1 3:1
EtOAc
CHCl3
acetone (R,R)-L1 13:1
CH3CN (R,R)-L1 16:1
DMSO
DMF
(R,R)-L1 6:1
(R,R)-L2 15:1
(R,R)-L3 12:1
82
95
90
82
78
88
15
80
72
74
85
78
82
88
90
92
82
84
54
41
29
4
7
4
92
–
40
10
28
2
(S)-L4
12:1
(R,R)-L5 4:1
(R,R)-L6 7:1
(R,R)-L7 6:1
10
1
24
–
(S)-L8
15:1
9
(R,R)-L1 3:1
16
40
51
58
70
72
72
75
77
82
74
98
95
90
88
–
–
–
90
–
10
11
12
13
14
15
16
17
18
(R,R)-L1 7:1
(R,R)-L1 4:1
(R,R)-L1 15:1
(R,R)-L1 15:1
(R,R)-L1 12:1
(R,R)-L1 15:1
NMP
NMP[e]
[a] All reactions were run on a 0.25 mmol scale. [b] Determined by
1H NMR analysis of the the crude reaction mixture. [c] Yield of isolated 2.
[d] Enantiomeric excess determined by SFC analysis using a chiral
stationary phase. [e] Reaction run at ꢀ208C. dba=dibenzylideneace-
tone, NMP=N-methyl-2-pyrrolidone.
Scheme 2. Scope of the palladium-catalyzed allylic alkylation of allyl
dienol carbonates 1a–p. All reactions were run on a 0.25 mmol scale.
Enantiomeric excess was determined by SFC analysis using a chiral
stationary phase. [a] Reaction run in CH3CN.
itive g allylation rather than a [3,3]-sigmatropic Cope rear-
rangement which could have been triggered during the Pd-
AAA process.
Having identified a useful set of reaction conditions, we
next examined the reaction scope by subjecting various alkyl-
(1a–c), aryl- (1d–l, 1o–p), heteroaryl- (1m), and vinyl-
substituted (1n) dienol carbonates to the Pd-AAA process.
The results are summarized in Scheme 2.
Overall, the expected butenolides bearing an a-quater-
nary stereogenic center (2a–n) were obtained in high yields
(74-88%, a/g > 10:1) independently of the substitution pat-
tern on the starting allyl dienol carbonate (1a–n), except
perhaps for the two substrates bearing a substitutent at the
ortho position of the aromatic ring (1k and 1l) for which
a lower regioselectivity was observed (a/g = 3:1) therefore
resulting in a slightly lower yield (50–51%). Nonetheless, the
enantioselectivities were generally high ranging from 77% to
85% ee for the alkyl derivatives (2a–c), 72% to 91% ee for
the corresponding aryl derivatives (2d–l), 80% to 81% ee for
the heteroaryl- and vinyl-substituted butenolides (2m–n), and
around 70% ee for the 1,3-disubstituted allyl dienol carbo-
nates 2o and 2p. Finally, to demonstrate the practicality and
scalability of the method, a test reaction was performed on
a gram scale using the allyl dienol carbonate 1b as a model
substrate. Unsurprisingly, the corresponding a,a-disubsti-
DMSO and DMF led almost exclusively to the a-allylated
product in over 90% yield (2a/3a = 15:1) and with up to
75% ee (entries 15 and 16). Ultimately, conducting the
experiment in N-methyl-2-pyrrolidone (NMP) under other-
wise identical reaction conditions offered the best selectivity
with an ee value of 77% (entry 17), which could be further
increased to 82% by running the reaction at ꢀ208C
(entry 18). Additionally, it is worth noting that a difference
in selectivity was always observed between the two products
2a and 3a, independent of the solvent used. In toluene, for
instance, the g-allylated product was obtained in almost
optically pure form (98% ee), whereas the a-allylated prod-
uct was isolated in only 40% ee (entry 10), thus confirming
our initial assumption[16] that the formation of the g-mono-
substituted 2(5H)furanone was solely the result of a compet-
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Scheme 3. Microwave-assisted Cope rearrangement of a-quaternary
butenolides: a straightforward access to g-tertiary and g-quaternary
2(5H)furanones. All reactions were run on a 0.2 mmol scale. Enantio-
meric excess was determined by SFC analysis using a chiral stationary
phase.
tuted 2(3H)-furanone 2b was isolated in 84% yield and
77% ee, which compared favorably with the result obtained
on a smaller scale.
With an enantioselective route to enantioenriched bute-
nolides bearing an a-quaternary stereogenic center in hand,
we next wanted to illustrate their synthetic utility in the
preparation of useful heterocyclic intermediates.[21] In this
context, we subjected several a,a-disubstituted butenolides
obtained through our catalytic asymmetric alkylation chemis-
try to a microwave-assisted Cope rearrangement.[22] Indeed,
we reasoned that the stereospecificity of this [3,3]-sigma-
tropic-type transposition would offer straightforward access
to enantioenriched 2(5H)furanones bearing either a g-tertiary
Scheme 4. Asymmetric synthesis of b-quaternary butyrolactones. All
reactions were run on a 0.25 mmol scale. Yield is that of product
isolated after two steps. Enantiomeric excess was determined by SFC
analysis using a chiral stationary phase.
provide a remarkably efficient tool for the synthesis of
enantioenriched b-quaternary butyrolactones. To our delight,
treating a solution of the a-quaternary butenolides 2a–n with
2.1 equivalents of DIBAL-H (CH2Cl2, ꢀ788C) and oxidation
of the resulting lactol afforded the corresponding b-quater-
nary butyrolactones 4a–n in high yields and, once again, with
no erosion of their optical purity (Scheme 4).
To determine the absolute configuration of the a-quater-
nary stereogenic center formed during the palladium-cata-
lyzed allylic alkylation process and also demonstrate the
synthetic utility of our method, we undertook the total
synthesis of two members of the paraconic acid family[23] of
natural products, namely (ꢀ)-nephrosteranic acid 8[24] and
(ꢀ)-roccellaric acid 9,[25] both of which exhibit interesting
antifungal, antibiotic, antitumor, and antibacterial properties
(Scheme 5).
The synthesis of the two natural products began by first
converting commercially available 3-methyl-2(5H)-furanone
5 into the corresponding ally dienol carbonate 1b by treat-
ment with NaHMDS and then allyl chloroformate (THF,
ꢀ608C). The resulting allyl dienol carbonate species was then
subjected to our optimized Pd-AAA conditions using the
(R,R)-DACH-phenyl Trost ligand in conjunction with [Pd2-
(dba)3·CHCl3] (NMP, ꢀ208C) to afford the a-quaternary
butenolide 2b in 80% yield and 77% ee. The latter was then
converted into the corresponding g-tertiary furanone 3b by
(3a–l) or
a
g-quaternary (3o–p) stereogenic center
(Scheme 3). To our delight, the microwave irradiation of
a solution of the a-quaternary butenolides 2a–p in toluene
(closed vessel, 400 W, 1808C, 1 h) cleanly afforded the desired
products 3a–p in quantitative yield and, most importantly,
with no erosion of their optical purity.
Following these results, and with the idea of developing
a straightforward and enantioselective route to butyrolac-
tones bearing a b-quaternary stereogenic center, we next
decided to subject our a,a-disubstituted butenolides to a two-
step sequence featuring a DIBAL-H reduction and a PCC-
mediated oxidation. Indeed, we reasoned that in the presence
of a hydride source, the a-quaternary butenolides could be
readily reduced to the corresponding aluminoxy acetal
intermediates G, which would be in equilibrium with the
aldehyde aluminium enolate species H (Scheme 4). Further
reduction with a second equivalent of DIBAL-H could then
afford the related aluminium alkoxide enolate intermediates I
which would in turn be converted into the corresponding
lactol E upon aqueous work-up. Finally, a PCC-mediated
oxidation should ultimately afford the desired butyrolactone
F bearing a b-quaternary stereogenic center (b-quaternary
butyrolactone). If successful, this three-step sequence starting
from readily available prochiral allyl dienol carbonates would
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Figure 1. Model for the explanation of the selectivity.
utility of our method by applying it as a key step in the total
syntheses of (ꢀ)-nephrosteranic acid 8 and (ꢀ)-roccellaric
acid 9.
Scheme 5. Application of the palladium-catalyzed allylic alkylation to
the synthesis of 8 and 9. DBU=1,8-diazabicyclo[5.4.0]undec-7-ene,
DMSO=dimethylsulfoxide, [HG]-II=Hoveyda–Grubbs second genera-
tion catalyst, HMDS=hexamethyldisilazide.
Experimental Section
Representative procedure for the palladium-catalyzed decarboxyla-
tive asymmetric allylic alkylation of 1a: to a solution of allyl dienol
carbonate 1a (0.2 mmol) in NMP (1 mL) at ꢀ208 was slowly added
a highly stereospecific microwave-assisted Cope rearrange-
ment (toluene, 1808C, 1 h, quant.) before a diastereoselective
conjugate addition of nitromethane, subsequent cross-meta-
thesis-mediated side-chain elongation, and a final hydro-
genation over Pd/C eventually afforded the two natural
product precursors in roughly 18% overall yield. To complete
the syntheses, the latter were finally engaged in a modified
Kornblum oxidation[26] (NaNO2, AcOH, 458C) to afford the
desired natural products 8 and 9 in 70 and 75% yield,
respectively.
a
premixed solution of [Pd2(dba)3·CHCl3] CHCl3 (0.01 mmol,
5 mol%) and (R,R)-DACH-phenyl Trost ligand (0.02 mmol,
10 mol%) in NMP (1 mL). The resulting reaction mixture was then
stirred at the same temperature until complete conversion of the
starting material (reaction monitored by TLC analysis). A saturated
aqueous solution of brine (20 mL) was then added and the aqueous
phase was extracted with AcOEt (3 ꢁ 2 mL). The combined organic
layers were washed with brine (2 mL), dried over anhydrous MgSO4,
filtered, and concentrated under reduced pressure. The crude residue
was finally purified by flash column chromatography over silica gel to
afford pure furanone 2a (80%) as a colorless oil.
Gratifyingly, the spectroscopic and physical data of 8 and
9 were consistent with the ones reported in the literature for
20
(ꢀ)-nephrosteranic acid {½aꢁD ¼ꢀ13.8 (c = 0.5, CHCl3); lit.:
22
½aꢁD ¼ꢀ27.7 (c = 0.9, CHCl3)}[24i] and (ꢀ)-roccellaric acid
Received: August 8, 2012
Published online: December 7, 2012
20
22
{½aꢁD ¼ꢀ14.9 (c = 0.45, CHCl3); lit.: ½aꢁD ¼ꢀ26.0 (c = 0.5,
CHCl3)},[24i] respectively, thus confirming that the use of the
(R,R)-DACH-phenyl Trost ligand leads preferentially to the
formation of the (S)-butenolides.
Keywords: alkylation · asymmetric catalysis · heterocycles ·
.
natural products · palladium
The selectivity of the palladium-catalyzed asymmetric
allylic alkylation could have also been predicted using the
model proposed by Trost et al. (Figure 1)[7] in which the
nucleophile should approach the p-allylpalladium/L1 com-
plex from its Si face (Figure 1a) to avoid the disfavored steric
interaction between the “wall” of the ligand and the
substrateꢀs furan ring (Figure 1b). Hence, if this assumption
is correct, the S-configured enantiomer should be formed
predominantly during the course of the reaction.
In summary, we have developed an extremely mild and
particularly efficient method for the enantioselective syn-
thesis of butenolides bearing an all-carbon a-quaternary
stereogenic center. Additionally, we showed that these
butenolides could undergo a variety of synthetic transforma-
tions and thus serve as valuable intermediates for the
synthesis of useful heterocyclic building blocks including b-
quaternary butyrolactones, as well as b-quaternary- and g-
tertiary furanones. Finally, we also demonstrated the synthetic
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