Effect of protic additives in Cu-catalysed asymmetric Diels-Alder cycloadditions of doubly activated dienophiles: Towards the synthesis of magellanine-type: Lycopodium alkaloids

Article abstract of DOI:10.1039/c7cc06367a

The pronounced beneficial effect of a precise amount of protic additive in an enantioselective Cu-catalysed Diels-Alder reaction is reported. This reaction, which employs a cyclic alkylidene β-ketoester as a dienophile, represents one of the first example

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Effect of protic additives in Cu-catalysed asymmetric Diels-Alder
cycloadditions of doubly activated dienophiles: towards the
synthesis of magellanine-type Lycopodium alkaloids†
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
c
DOI: 10.1039/x0xx00000x
Vincent N. G. Lindsay,‡ Rebecca A. Murphy,‡ and Richmond Sarpong*
www.rsc.org/
The pronounced beneficial effect of a precise amount of protic chiral Lewis or protic acid, the instability of cyclic alkylidene
additive in an enantioselective Cu-catalysed Diels-Alder reaction is ketoesters towards such catalysts has significantly impeded
reported. This reaction, which employs a cyclic alkylidene their use in these reactions. In addition to this inherent
β-
β
-
ketoester as a dienophile, represents one of the first examples of reactivity issue, the suppression of background (non-catalysed)
a transformation where these extremely versatile, though highly reactions can also be challenging since the presence of geminal
unstable reaction partners participate effectively in catalytic electron-withdrawing groups makes these dienophiles
asymmetric cycloaddition with
cycloadduct was used as an intermediate towards the synthesis of enable the development of a general catalytic asymmetric
magellanine-type Lycopodium alkaloids featuring a Stille cross- reaction with cyclic alkylidene -ketoesters, it is therefore
coupling of a highly congested enol triflate and a unique critical to identify a catalyst which activates the dienophile
a functionalised diene. The particularly reactive, even at low temperatures. In order to
β
Meinwald rearrangement / cyclopropanation sequence.
enough to outcompete the background reaction, while
minimising its acid-catalysed decomposition. Since these
dienophiles are capable of rigid two-point binding with
bidentate Lewis acids, one particularly attractive class of chiral
complexes that may be employed in their activation are Cu-
bis(oxazolines) (Cu-BOX), which have been extensively used in
Alkylidene β-ketoesters feature prominently in the
synthesis of complex natural products, due in part to their
effectiveness as substrates for the formation of quaternary
1
,2,3
stereocenters.
In addition to being extremely reactive
3
Michael acceptors in simple conjugate additions and formal
5
+2 cycloadditions, their potential as dienophiles in Diels-
,4
8
asymmetric Lewis acid catalysis. While this catalyst system
2
was previously found to be suitable in terms of activating
Alder cycloadditions has been widely exploited, because of the
ubiquity of the resulting hydrindane core in various classes of
alkylidene β-ketoesters, the resulting asymmetric induction
has remained low due to inefficient chiral projection on the
2
7
a
natural products when 5-membered cyclic analogs are used.
distal reactive site of the dienophile (Figure 1, A).
While many racemic and diastereoselective examples of Diels-
Alder cycloadditions using alkylidene
β-ketoesters as
dienophiles have been reported, successful catalytic
asymmetric variants remain scarce and typically lack generality
6
with respect to the structure of the diene and dienophile.
Indeed, this subset of dienophiles is notoriously unstable to
protic and Lewis acids, heat, and even simple dissolution in
certain solvents, which has been attributed to their high
1
,7
propensity to polymerise following tautomerisation. Since Figure 1 Different stereoinduction models for Cu-catalysed enantioselective Diels-
enantioinduction in catalytic asymmetric Diels-Alder reactions Alder using cyclic alkylidene
most often stems from the activation of the dienophile with a
β-ketoesters
An elegant solution to this challenge was recently reported by
6
d
Nakada and co-workers, where the ester moiety was
replaced by an imide to afford a complex in which the
substituents on the chiral oxazoline groups and the alkene of
the dienophile are more proximal (Figure 1,
B). This
relationship can be inferred on the basis of stereoinduction
models of N-acryloyloxazolidin-2-ones commonly used in
9
asymmetric Diels-Alder reactions. In this model, the rigidity of
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the system is achieved by intramolecular hydrogen bonding the rate of the reaction was highly dependent on
between the imide NH and the carbonyl of the enone moiety. concentration, and full suppression of the non-catalysed
While this affords excellent asymmetric induction in Diels- (‘background’) reaction could be achieved under dilute
Alder cycloadditions when the appropriate chiral bis(oxazoline) conditions at 0 °C (Scheme 1b).
is employed, the synthesis of this dienophile and the additional
With this information in hand, we evaluated the effect of
steps required to install/cleave the imide auxiliary significantly solvent and protic additives on the efficiency of a potential
impedes its practicality and general use in complex molecule stereoselective variant of the transformation (Table 1).
2
e
synthesis. In order to address this unresolved issue, we
envisioned that similar rigidification might be achieved directly
from the ester if an intermolecular hydrogen bond could be
established using a catalytic amount of a protic additive (R-OH,
Table 1 Effect of solvent and protic additives on the enantioselective Diels-Alder of
doubly activated dienophile 2.
Figure 1, C). Key to this hypothesis would be the requirement
that the remaining binding site in the resulting complex is
occupied by an added Lewis basic ligand (L) to disfavor
complexation as found in A. Here, we report our studies
towards the establishment of such a model in a catalytic Entry
b
dr
c
er
Solvent
Additive
Yield
a
(x mol%)
(%)
asymmetric Diels-Alder cycloaddition using a cyclic alkylidene
1
CH
2
2
Cl
2
2
-
<5
51
19
36
35
<5
39
19
<5
41
48
53
44
57:43
58:42
79:21
86:14
87:13
66:34
86:14
86:14
60:40
88:12
89:11
90:10
89:11
51:49
51:49
63:37
80:20
82:18
49:51
79:21
78:22
50:50
82:18
83:17
86:14
83:17
β-ketoester as dienophile in the presence of protic additives.
Such additives were found to both accelerate the reaction and
2
3
4
5
6
7
8
9
CH
Cl
H
2
O (10)
O (10)
O (10)
O (10)
EtOAc
TBME
H
2
significantly improve the observed stereoselectivity, in
H
2
accordance with a model such as
C. The hydrindenone
Et
2
Et
2
Et
2
Et
2
O
O
O
O
H
2
cycloadduct was employed as a key intermediate towards a
-
-
synthesis of magellanine-type Lycopodium alkaloids. Given the
2
versatility of the resulting hydrindenone products, these
d
H
2
O (20)
initial forays into a general catalytic asymmetric Diels-Alder
2
Et O
H2O (100)
cycloaddition using cyclic alkylidene -ketoesters should find 10
broad applicability in the synthesis of complex natural 11
β
Et
2
Et
2
Et
2
Et
2
O
MeOH (10)
MeOH (10)
MeOH (10)
i-PrOH (10)
e
O:Tol
O:Tol
O:Tol
e,f
e
products.
Our studies began with the evaluation of various non-
catalysed reaction conditions between silyloxy diene and
dienophile
background reaction would be suppressed (Scheme 1). As
alluded to previously, dienophiles such as are extremely
12
1
3
a
1
Determined by NMR analysis of the crude mixture using 1,3,5-
b
2
in order to identify a suitable medium where the trimethoxybenzene as internal standard. Diastereomeric ratio determined by
c
NMR analysis of the crude mixture. Enantiomeric ratio of major diastereomer
d
drawn determined by HPLC using a chiral stationary phase. Cu(OTf)
2
and t-
2
2
BuBOX were stirred in Et O at rt for 16 h prior to the reaction (pre-complexation).
reactive due to the presence of geminal electron-withdrawing
groups, and so not surprisingly, complete conversion to the
e
f
Et
2
O:Toluene ratio = 1:1. Pure E isomer (E:Z >99:1) of diene 1 was used (reaction
time: 16 h).
corresponding product (3) was observed when the reaction
was performed neat at room temperature for 20 hours Preliminary studies indicated that the nature of the
(Scheme 1a).
bis(oxazoline) ligand and the Cu salt counterion used had a
significant impact on the stereoselectivity of the reaction, with
1
0 mol% Cu(OTf)2 and 12 mol% t-Bu-BOX, a common
8
combination in asymmetric Lewis acid catalysis, affording the
1
0,11
best initial results.
A striking feature of these reactions that
we quickly observed was that the presence of water (e.g. using
Cu(OTf)2 stored open to the atmosphere), significantly
accelerated the reaction. Indeed, very little conversion was
observed after 3 h when carefully dried Cu(OTf) was used in
2
the absence of protic additives (see entries 1 and 6), while full
conversion was observed in the presence of 10 mol% H O for
2
most solvents evaluated (entries 2–5). While CH Cl as solvent
2
2
Scheme 1 Identification of racemic conditions (a) and evaluation of background
afforded virtually no enantiocontrol (entry 2), the use of
slightly Lewis basic solvents afforded increased
reaction upon dilution (b)
enantioselectivity, with Et O leading to the best enantiomeric
2
With an eye towards applications in complex molecule
synthesis, it should be noted that this reaction could be scaled
ratio (entries 3–5). Notably, this observation is in accordance
with our proposed model (see Figure 1,
Lewis basic ligand (L) is necessary to avoid bidentate
complexation as in which would lead to poor
C), suggesting that a
1
0
up to produce 25 grams of
3 with similar efficiency. The
major diastereomer was identified by X-Ray diffraction analysis
A
,
of a single crystal to be the ester-endo product. We found that
enantioinduction because of inefficient relay of chiral
2
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information. It should be noted that strictly anhydrous membered
conditions led to a comparable yield when t-Bu-BOX and rearrangement, which in turn could be transformed to a
Cu(OTf) were pre-complexed for 16 h, albeit affording lower cyclopropylketone if an appropriate -leaving group was
enantiocontrol (entry 7). This observation suggests that the present. Considering the strain associated with such an
rate acceleration observed in the presence of H O might be intermediate, a subsequent vinylcyclopropane-cyclopentene
due to a faster ligand complexation event. Interestingly, using rearrangement would directly lead to the functionalised
ring
via
a
stereospecific
Meinwald
1
5
2
γ
2
1
6
more than 10 mol% H
effect on the efficiency of the reaction. Presumably, over- sequence, the primary hydroxyl was transformed into a
complexation to the chiral Cu complex occurs in this case nucleofuge using Ts O and the TBS-protected secondary
2 2
O (1:1 Cu:H O ratio) had a detrimental tetracyclic core of magellaninone. To set the stage for this
2
precluding binding of the dienophile (entries 8–9). Other protic alcohol group was readily converted into the corresponding
additives such as MeOH and i-PrOH were also found to have a ketone via a deprotection / oxidation sequence. Preliminary
similar effect, with MeOH affording a slightly better yield results indicated that the basicity of the pyridine needed to be
(compare entry 5 vs 10 and entry 11 vs 13). Lowering the further tempered in order for the Meinwald rearrangement to
polarity of the solvent by adding toluene improved both the take place, due to the requirement of a strong Lewis acid.
yield and the stereoselectivity, and using the pure E isomer of Therefore, the pyridine moiety was converted to the
diene
minor Z isomer was either reacting with poor stereocontrol or oxidation, which gave
affording predominantly the opposite enantiomer (entries 11– optimisation identified Mg(OTf) as the ideal Lewis acid for the
1
led to increased enantioselectivity, suggesting that the corresponding N-oxide using a methyltrioxorhenium-catalysed
6
in good overall yield. Extensive
2
1
0
1
2). It should be noted that in all cases presented in Table 1, Meinwald rearrangement. At the high temperatures required
the main side-products of the reaction were identified as for this transformation, the direct formation of
resulting from the acid-catalysed decomposition of diene (3- cyclopropylketone was observed in quantitative yield.
methylbutenal isomers) or of dienophile , with the former Presumably, arises from as outlined in Scheme 3. This
side-reaction being mainly responsible for the moderate yields homoallylic epoxidation Meinwald rearrangement
observed. cyclisation sequence is unprecedented and constitutes an
These optimised conditions readily translated to a larger expedient approach to the stereospecific formation of
scale, affording 1.75 mmol of enantioenriched hydrindenone cyclopropylketones from homoallylic alcohols. While the
Scheme 2). It was found that in order to conduct the vinylcyclopropane-cyclopentene rearrangement proved
enantioselective cycloaddition reproducibly on scale, it was unsuccessful directly from we anticipate that an
preferable to reduce the amount of dienophile used to 2 intermediate such as pyridinium , readily obtained by a
methylation sequence, may
1
7
2
7
6
/
/
3
(
7,
2
8
equivalents and increase the catalyst loading to 15 mol%.
chemoselective reduction /
provide an opportunity to rapidly access magellaninone
following a final pyridinium reduction. Alternatively, partial
pyridinium reduction followed by a similar rearrangement may
lead to the realisation of this goal by avoiding the enthalpic
cost of dearomatisation that is inherent in the possible
Scheme 2 Preparation of hydrindenone 3 on a larger scale
vinylcyclopropane-cyclopentene rearrangement of 8. Efforts to
effect the conversion of
3 obtained from this enantioselective Diels- ongoing studies.
8
to magellaninone are the subject of
Hydrindenone
Alder reaction constitutes a framework commonly found in
many biologically relevant natural products including
2
alkaloids. We envisioned that such a highly functionalised
cycloadduct bearing three contiguous stereocenters could
serve as an intermediate towards the synthesis of
magellaninone, a tetracyclic Lycopodium alkaloid belonging to
12 13
,
the fawcettimine class (Scheme 3).
Key to our synthetic
strategy was masking the basic piperidine moiety present in
the natural product as a pyridine. Towards this end,
hydrindenone
3 was converted to the corresponding vinyl
triflate, which was then employed in a Stille cross-coupling
1
4
based on conditions developed by Corey. This coupling
efficiently led to pyridine , which contains all of the carbon
atoms present in magellaninone. Reduction of the ester group
in to the primary hydroxyl and directed VO(acac) -catalysed
homoallylic epoxidation led to epoxide in good yield, the
4
4
2
5
stereochemistry of which was unambiguously confirmed by X-
Ray crystallographic analysis. We envisioned using the epoxide
moiety in
5 to introduce the requisite ketone group in the five-
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5
6
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a
β
and the development of a stereoselective synthesis of a
functionalised hydrindenone commonly used in the synthesis
2
of complex molecules relevant to natural product synthesis.
The utility of the hydrindenone cycloadduct was demonstrated
in its application towards the synthesis of magellaninone. Key
to the synthetic studies is a robust Stille cross-coupling of a
highly congested enol triflate and a unique Meinwald
rearrangement / cyclopropanation sequence. These studies
should provide the basis for achieving higher levels of
7
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Conflicts of interest
Am. Chem. Soc., 1999, 121, 7559; (d) E. J. Corey, N. Imai and
H. Y. Zhang, J. Am. Chem. Soc., 1991, 113, 728.
See Supporting Information for details.
Eleven (11) chiral ligands were evaluated in preliminary
studies. See Supporting Information.
There are no conflicts to declare.
1
1
0
1
Notes and references
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Graphical Abstract:
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