Concise synthesis of tetrahydropyrans by a tandem oxa-michael/tsuji-trost reaction

Article abstract of DOI:10.1002/anie.201003304

A novel domino sequence facilitates the rapid assembly of polysubstituted tetrahydropyrans. The one-pot relay process generates up to three new stereogenic centers, including a tetrasubstituted carbon center, in a highly concise and convergent fashion from simple starting materials. Copyright

Full text of DOI:10.1002/anie.201003304

Communications
DOI: 10.1002/anie.201003304
Domino Reactions
Concise Synthesis of Tetrahydropyrans by a Tandem Oxa-Michael/
Tsuji–Trost Reaction**
Liang Wang, Pengfei Li, and Dirk Menche*
Dedicated to Professor G. Helmchen on the occasion of his 70th birthday
Metal complexes have been successfully applied to a broad
range of organic transformations and occupy a central
position in preparative organic chemistry. In recent years,
the notion of combining several metal-mediated processes in
relay-type domino sequences has been attracting attention.[1,2]
Through the combination of several synthetic transformations
in a one-pot fashion, domino reactions efficiently transform
simple starting materials into products of structural complex-
ity. Surprisingly, oxa-Michael reactions have not been devel-
oped for such purposes, despite their obvious potential for
heterocycle synthesis, presumably because of the inherent
instability of the generated enolates towards eliminations or
retro processes.[3] Herein, we report the design and develop-
Scheme 1. Three-step tandem concept for the synthesis of tetrahydro-
pyrans.
ment of a conceptually novel cascade reaction based on an
oxa-Michael addition and an allylic substitution,[4] and
successfully implement this concept for the highly concise
synthesis of polysubstituted tetrahydropyrans.
then be generated (step 2), which would finally be trapped in
an intramolecular fashion through an allylic substitution
reaction, generating the desired THP motif in a highly direct
fashion (step 3). Notably, three new stereogenic centers are
assembled in this process, demonstrating a high increase in
structural complexity from very simple starting materials. It
should also be noted that the synthetic design is highly
convergent and flexible and may be readily adapted to various
other substrates enabling direct access to a broad range of
heterocycles.
Based on initial experiments with different Michael
acceptors,[19] nitroolefins were selected for further develop-
ment. The coupling of alcohol 6[20] and nitroolefin 7[21] was
studied in more detail (Table 1). Gratifyingly, after we had
evaluated reagents (bases, catalysts, ligands) and parameters
(temperature, solvent) our synthetic strategy could be suc-
cessfully implemented to generate the desired THP motif 8.
The best conditions included catalytic amounts of [{Pd-
(allyl)Cl}2] with PPh3 in combination with LiHMDS as th base
(Table 1, entry 10).[22] The absence of PPh3 resulted in lower
yields (Table 1, entries 8 and 10) and alternative ligands led to
lower selectivities [P(iOPr)3, P(OEt)3: Table 1, entries 11 and
12] or conversions (dppf, dppp, dppe, dppb: Table 1,
entries 13 and 14, footnote [h]). Methyl and tert-butyl car-
bonate proved to be the best leaving groups of those
evaluated (Table 1, entries 5–8). While in principle it might
be possible to run the reaction with the tert-butoxy-substi-
tuted substrate using only catalytic amounts of base, only low
degrees of conversion were observed in such cases (Table 1,
entries 8 and 9). Encouragingly, out of the eight possible
products only two major (8a, 8b) and one minor isomer (8c)
formed, suggesting a high degree of conformational bias in
this sequential process (see below). The major products differ
Substituted tetrahydropyrans (THPs) are prevalent con-
stitutional chemotypes and underlying structural motifs in
numerous natural products, registered drugs, and bioactive
synthons.[5] Various strategies for the construction of such
systems have been reported,[6] including cyclizations involving
oxocarbenium ions[7] and epoxides,[8] hetero-Diels–Alder
reactions,[9,10] Prins cyclizations,[11] intramolecular nucleo-
philic reactions,[12] Michael reactions,[13] reductions of cyclic
hemiacetals,[14] cyclizations involving nonactivated double
bonds,[15] and one-pot procedures based on alkene–alkyne
couplings followed by ether formation.[16] Inspired by present
targets in our group in combination with certain limitations of
these existing methods, we desired a more direct and concise
sequence for THP synthesis. As shown in Scheme 1, our
synthetic concept is based on a three-step sequential process
involving an oxa-Michael addition[3,17] and a Tsuji–Trost
coupling.[18] Accordingly, the readily available homoallylic
alcohol 1 should first add to the suitably acceptor-substituted
alkene 2 giving enolate 3 (step 1). A p-allyl complex 4 would
[*] L. Wang, Dr. P. Li, Prof. Dr. D. Menche
University of Heidelberg, Department of Organic Chemistry
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)6221-54-4205
E-mail: dirk.menche@oci.uni-heidelberg.de
[**] Generous financial support from the Deutsche Forschungsge-
meinschaft (SFB 623 “Molekulare Katalysatoren: Struktur und
Funktionsdesign”) and the Wild-Stiftung is gratefully acknowledged.
We thank Prof. G. Helmchen for helpful discussions and sugges-
tions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003304.
9270
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Table 1: Tandem oxa-Michael/Tsuji–Trost reaction.[a],[23]
Table 2: Domino reaction generating the tetrasubstituted THP 11a.[a,23]
Entry
R
[PdLn]/base[b]
Yield d.r.
Entry Substrate; [PdLn]/base[b]
Yield
[%][c]
d.r.
[%][c] 8a/8b/8c[d]
11a/11b/11c[d]
1
2
3
4
5
6
7
8
9
OMe [{Pd(allyl)Cl}2]/LiOtBu
OMe [Pd2(dba)3]/LiOtBu
OMe [Pd2(dba)3]/LiHMDS
OMe [{Pd(allyl)Cl}2]/LiHMDS
OMe [{Pd(allyl)Cl}2]/LiHMDS
PMB [{Pd(allyl)Cl}2]/LiHMDS
24
14
17
39
71
43
44
47
17
6.9:9.1:1
10.4:12.3:1
12:11.7:1
4.8:3.4:1
10.3:8:1
14.4:10.2:1
1.4:1.3:1
2.1:1.8:1
n.d.[f]
1
2
3
R=Me; [Pd2(dba)3] (5 mol%)/PPh3
(20 mol%)/LiHMDS (1.5 equiv)
R=Me; [Pd2(dba)3] (5 mol%)/PPh3
(20 mol%)/KOtBu (1.5 equiv)
R=tBu; [Pd2(dba)3] (5 mol%)/PPh3
(20 mol%)/LiOtBu (1.5 equiv)
47
63
78
5.8:2.5:1
4.4:1.4:1
5.2:1.4:1
tBu
[{Pd(allyl)Cl}2]/LiHMDS
[a] The reactions were carried out on a 0.2 mmol scale with 0.3 to
0.4 mmol of nitroolefin in 3 mL of THF. [b] Commercially available
solutions of the bases in THF were used (Sigma-Aldrich). [c] Yield of
isolated product. [d] Ratio was determined by H NMR analysis of the
crude product.
OtBu [{Pd(allyl)Cl}2]/LiHMDS
OtBu [{Pd(allyl)Cl}2]/LiHMDS
(10 mol%)[e]
OtBu [{Pd(allyl)Cl}2]/PPh3 (20 mol%)/
LiHMDS
1
10
11
12
13
14
62
1.6:1 :
<0.05
OtBu [{Pd(allyl)Cl}2]/P(iOPr)3 (20 mol%)/ 52
18.4:13.5:1[g]
LiHMDS
temperature (Table 2, entry 3). Optimum yields for the
coupling of nitroolefin 10 were obtained with LiOtBu rather
than LiHMDS and KOtBu (Table 2, entries 1–3), while
LiHMDS was shown to be optimal for the reaction of
nitroolefins lacking a b substituent of (see Table 3).
OtBu [{Pd(allyl)Cl}2]/P(OEt)3 (20 mol%)/ 59
9.4:6.9:1[g]
13.0:8.8:1.4
11.2:6.9:1
LiHMDS
OtBu [{Pd(allyl)Cl}2]/dppf (10 mol%)/
LiHMDS
35
25
OtBu [{Pd(allyl)Cl}2]/dppp (10 mol%)/
LiHMDS[h]
As shown in Scheme 2, various further THPs (11–15) were
readily obtained by this domino process. In all cases,
preparatively useful yields and selectivities resulted, without
the need to adjust the reaction conditions to specific
substrates. Any minor diastereomer could be readily removed
by column chromatography, which underlines the efficiency
of the overall process. The selectivities obtained for the
generation of THPs bearing a sterically less hindered
substituent at C-6 (e.g. 12 and 11) were slightly better than
that of the original system. In contrast, only little selectivity
was observed for the corresponding nitrostyrenes: the (5-
epi,6-epi) isomers 16 and 17 were obtained as main products,
suggesting that slight structural changes at C-6 have a critical
influence on the stereoselectivity at this position.
[a] The reactions were carried out in 2.5 mL THF with 0.2 mmol
homoallylic alcohol 6, 2 mmol nitroolefin, 0.3 mmol base, and
0.01 mmol (5 mol%) catalyst. [b] Commercially available solutions of
the bases in THF were used (Sigma-Aldrich). [c] Yield of isolated
product. [d] Ratio was determined by 1H NMR analysis of the crude
product. [e] Low degrees of conversion were also observed with other
bases (NEt3, DBU, LiOtBu). [f] n.d.: not determined. [g] Formation of
two further diastereomers was observed in yields similar to that of 8c.
[h] Similar results were obtained with dppe and dppb. dba=dibenzyl-
ideneacetone, DBU=1,8-diazabicyclo[5.4.0]undec-7-ene, dppb=1,4-
bis(diphenylphosphanyl)butane, dppe=1,2-bis(diphenylphosphanyl)-
ethane, dppf=1,1’-bis(diphenylphosphanyl)ferrocene, dppp=1,3-bis-
(diphenylphosphanyl)propane, HMDS=1,1,1,3,3,3-hexamethyldisilaza-
nide, PMB=para-methoxybenzyl.
only in the configuration of the isopropyl-bearing center (C-
6), while the phenyl and the vinyl substituent are in equatorial
positions and the nitro group resides in an axial position.[23]
We then directed our efforts toward increasing the
stereoselectivity of this process. In a rational to enhance the
influence of the substituents vicinal to the nitro group on the
generation of the new stereogenic center in the b position, the
a-methyl group of 7 was removed. Accordingly, desmethylni-
troolefin 10 was evaluated, and indeed this concept proved
successful. The desired THP 11a was now obtained as the
main product with good selectivities (Table 2), considering
the stereochemical complexity of this process. In contrast to
the previous results, formation of two other side products
(11b and 11c) was observed, which underlines the important
stereochemical influence of the a substituent. Optimized
conditions for the selective generation of tetrasubstituted
THP 11a required a slight excess of nitroolefin 10 (1.5 to
2 equiv), catalytic amounts of [Pd2(dba)3] (5 mol%) and PPh3
(20 mol%) and conducting the reaction in THF at room
Scheme 2. Selected tetrasubstituted THPs prepared by our
method.[23,24]
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The scope of this tandem process was then expanded to
the stereoselective generation of THPs in which C-5 is
tetrasubstituted. As shown in Table 3, the a-substituted
Table 3: Assembly of THPs with a tetrasubstituted carbon center.[a,23]
Entry
R
[PdLn]/base
Yield [%][b] d.r. 19a/19b[c]
1
2
3
4
OMe [{Pd(allyl)Cl}2]/KOtBu
16
OMe [{Pd(allyl)Cl}2]/NaHMDS 12
2.5:1
4.4:1
8.8:1
8.8:1
Scheme 4. Mechanistic rationale for the stereochemical outcome.
OMe [{Pd(allyl)Cl}2]/LiHMDS
OtBu [{Pd(allyl)Cl}2]/LiHMDS
17
62
[a] The reactions were carried out on a 0.2 mmol scale with 0.4 mmol of
nitroolefin in 2.5 mL of THF. [b] Yield of isolated product. [c] Ratio was
determined by H NMR analysis of the crude product.
the observed stereochemical outcome. Generation of the
axial configuration at C-5, in turn, may be explained by
chelation of the metal counterion to the ether oxygen and the
nitro group, which would be more favorable with an axial
nitro group, as in 26. Alternatively, also minimization of
dipole–dipole interactions of the nitronate with the p-allyl
complex would be more favorable for 26 than for 27. The
intermediate chelate complex 26 may also rationalize the
observed selectivity at C-6, as this substituent would reside in
a pseudoequatorial position.
In conclusion, we have designed, developed, and imple-
mented a conceptually novel domino process for the highly
concise synthesis of polysubstituted tetrahydropyrans from
simple starting materials. Notably, the starting homoallyl
alcohols are readily available—also in enantiopure form
through asymmetric allylation methodology—adding to the
scope and potential of this reaction. Mechanistically, the
procedure is based on a sequential oxa-Michael/Tsuji–Trost
reaction and generates up to three new stereogenic centers in
in a one-pot process. It may also be successfully applied for
the stereoselective synthesis of tetrasubstituted carbon cen-
ters bearing a nitro group. The heterocyclic products bear two
functional handles (NO2, alkene), which may be further
elaborated, adding to the synthetic usefulness of the process.
It is expected that this novel domino concept will be further
explored and applied to the synthesis of functional molecules.
1
nitroolefin 18 cyclizes with good selectivity to give tetrahy-
dropyran 19a. As before, the major product bears the nitro
group in an axial position. The best results were obtained with
2 equiv nitroolefin, 1.5 equiv LiHMDS and the dimeric
catalyst [{Pd(allyl)Cl}2]. Alternative bases (Table 3, entries 1
and 2) and leaving groups (Table 3, entry 3) resulted in lower
degrees of synthetic efficiency.
As shown in Scheme 3, the method was readily applicable
to various trisubstituted THPs, without the need to adjust the
reaction conditions to specific substrates. In all cases, the
products were obtained with good selectivities, considering
the number of possible stereoisomeric products.
Experimental Section
Representative procedure (THP 11; Table 2, entry 3): A solution of
homoallylic alcohol 9 (R = tBu; 56 mg, 0.2 mmol) and nitroolefin 10
(44 mg, 0.38 mmol) in 1 mL anhydrous THF was treated at À788C
with a suspension of [Pd2(dba)3]·CHCl3 (10 mg, 0.01 mmol, 5 mol%)
and PPh3 (10 mg, 0.04 mmol) in 1.5 mL anhydrous THFand a solution
of lithium tert-butoxide (1m in THF, 0.3 mL, 1.5 equiv, as commer-
cially supplied from Sigma-Aldrich). The mixture was then warmed to
room temperature and stirred until the alcohol was complete
consumed (ca. 2 h). After cooling to À788C,[26] the reaction was
stopped by addition of sat. aq. NH4Cl solution. After warming to
room temperature, the mixture was extracted three times with ethyl
acetate. The combined organic phases were washed with brine, dried
(MgSO4), and filtered. Evaporation of the solvent in vacuo and
purification of the residue by column chromatography on silica gel
(petroleum ether/ethyl acetate = 60:1 to 30:1) afforded the diaste-
reomers 11a, 11b, and 11c (d.r. 5.2:1.4:1, 78%) as viscous oils.
Scheme 3. Synthesis of THPs with a tetrasubstituted carbon
center.[23,25]
Mechanistically, the observed selectivities and yields of
these domino reactions may be explained by a reversible oxa-
Michael reaction. As shown in Scheme 4, this first addition
may not be stereodiscriminating. However, intermediate 27
may be reversibly transformed into the presumably more
favorable diastereomer 26 leading to the major product 11a
together with minor amounts of 11b. The observed stereose-
lectivity may arise from a Zimmerman—Traxler-type tran-
sition state, in which the substituents at C-2 and C-4
substituents are in equatorial position, in agreement with
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Chemie
Received: May 31, 2010
[15] For an example, see: P. A. Clarke, M. Grist, M. Ebden, C.
Revised: September 9, 2010
Published online: October 26, 2010
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[17] a) I. C. Stewart, R. G. Bergman, D. F. Toste, J. Am. Chem. Soc.
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[19] Initial evaluations with alternative electron-withdrawing groups,
including esters, b-dicarbonyl compounds or sulphonates were
less promising.
Keywords: allylic substitution · domino reactions · heterocycles ·
.
homogeneous catalysis · oxa-Michael reactions
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[20] Homoallylic alcohols of type 6 are readily available in two steps
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Synth. Catal. 2002, 344, 634. For full details see the Supporting
Information.
[21] All nitroolefins were readily obtained by base-catalyzed con-
densation of nitroalkanes with the corresponding aldehydes: A.
Duursma, A. J. Minnaard, B. L. Feringa, Tetrahedron 2002, 58,
5773. For full details see the Supporting Information.
[22] Alternative bases included KHMDS, NaHMDS, and LiOtBu.
[23] In all cases, the stereochemistry was assigned by NMR methods
involving the nuclear Overhauser effect (NOE) .
[6] For a review, see: P. A. Clarke, S. Santos, Eur. J. Org. Chem.
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[24] Typical reaction conditions: A solution of the corresponding
homoallylic alcohol (1 equiv) and the nitroolefin (1.5 equiv) in
THF was treated with [Pd2(dba)3] (5 mol%), PPh3 (20 mol%),
and LiOtBu (1.5 equiv) at À788C; the reaction mixture was
stirred at room temperature until conversion was complete
(ca. 2 h). For full details see the Supporting Information.
[25] Typical reaction conditions: A solution of the corresponding
homoallylic alcohol (1 equiv) and the nitroolefin (2 equiv) in
THF was treated with [{Pd(allyl)Cl}2] (10 mol%), PPh3
(30 mol%) and LiHMDS (1.5 equiv) at À788C; the reaction
mixture was stirred at room temperature until conversion was
complete ( ꢀ 2 h). For full details see the Supporting Informa-
tion.
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[26] For convenience, the aqueous workup may also be conducted at
08C giving the products with similar yields.
[14] See e.g.: L. Boulard, S. Bouz, J. Cossy, X. Franck, B. Figadere,
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