Gold(I)-catalyzed furan-yne cyclizations involving 1,2-rearrangement: Efficient synthesis of functionalized 1-naphthols and its application to the synthesis of wailupemycin g

Article abstract of DOI:10.1002/chem.201403113

Gold-catalyzed cascade cyclization/1,2-rearrangement of 1-(2-furanyl)phenyl propargyl alcohols has been developed, which provides a rapid and efficient access to multisubstituted 1-naphthols bearing an enal or enone moiety with high stereoselectivity. The (Z)- or (E)-stereochemistry can be easily controlled by choosing protected- or non-protected substrates. The utility of the methodology has been illustrated in the first total synthesis of wailupemycin G.

Full text of DOI:10.1002/chem.201403113

DOI: 10.1002/chem.201403113
Communication
&
Synthetic Methods
Gold(I)-Catalyzed Furan-yne Cyclizations Involving
1
1
,2-Rearrangement: Efficient Synthesis of Functionalized
-Naphthols and Its Application to the Synthesis of
Wailupemycin G
Yifeng Chen, Lu Wang, Ning Sun, Xin Xie, Xiaobo Zhou, Haoyi Chen, Yuxue Li,* and
[a]
Yuanhong Liu*
ynes bearing a propargylic alcohol moiety such as 1 were used
Abstract: Gold-catalyzed cascade cyclization/1,2-rear-
rangement of 1-(2-furanyl)phenyl propargyl alcohols has
been developed, which provides a rapid and efficient
access to multisubstituted 1-naphthols bearing an enal or
enone moiety with high stereoselectivity. The (Z)- or (E)-
stereochemistry can be easily controlled by choosing pro-
tected- or non-protected substrates. The utility of the
methodology has been illustrated in the first total synthe-
sis of wailupemycin G.
as the substrates, the reaction could be terminated by a 1,2-re-
2
arrangement reaction, resulting in migration of R group to vi-
cinal carbon with simultaneous opening of the furan ring
[6]
[
Scheme 1, Eq. (2)]. In this paper, we report gold-catalyzed cy-
cloisomerizations of 1-(2-furanyl)phenyl propargyl alcohols in-
volving 1,2-rearrangement, which provide 1-naphthols bearing
enal or enone functionalities with high levels of E-stereoselec-
tivity. In the present reactions, there is no need to protect the
hydroxy group in substrates 1, thus providing a straightforward
Rearrangement reactions, especially those proceed
via 1,2-migration of a CÀC or CÀH bond driven by
the vicinal electrophilic carbon center such as carbo-
cations, are well-established reaction processes for
[
1]
the assembly of functionalized products. Incorpora-
tion of a 1,2-rearrangement step into the transition-
metal-catalyzed cascade reactions is highly attractive
for the rapid construction of architecturally complex
[
2]
structures from easily available starting materials.
Recently, we have developed a series of gold-cata-
[
3–5]
lyzed cascade reactions of furan-ynes
via endo-
type cyclizations leading to aromatic compounds
such as benzenes, phenanthrenes and fulvenes con-
taining enal or enone functionalities with excellent
[
3]
Scheme 1. Transformations of furan-ynes in the presence of gold catalyst.
stereoselectivity. For example, in the case of gold(I)-
catalyzed cycloisomerization of furan-ynes to protect-
ed 1-naphthol derivatives [Scheme 1, Eq. (1)], the re-
access to 1-naphthols. In addition, the use of its protected de-
action proceeds via the formation of a cationic intermediate
a followed by ring-opening of the furan ring and aromatization
via elimination of a proton at the benzylic position to furnish
rivatives afforded the a,b-unsaturated carbonyl products with
excellent Z-stereoselectivity. The methodology is also applied
successfully to the first total synthesis of wailupemycin G.
Our initial investigation focused on the cyclizations of 1-(2-
[
3d]
the final products.
We have now found that when furan-
(
furan-2-yl)phenyl)-3-phenylprop-2-yn-1-ol 1a, which was easily
prepared in two steps from 2-bromobenzaldehyde via Stille
coupling with 2-tributylstannylfuran followed by acetylide ad-
dition. To our delight, the desired cyclization occurred efficient-
ly, and the best result was achieved by employing 2 mol%
[
a] Dr. Y. Chen, Dr. L. Wang, N. Sun, X. Xie, Dr. X. Zhou, H. Chen, Prof. Y. Li,
Prof. Y. Liu
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
Johnphos(MeCN)AuSbF (A) as the catalyst (for detailed optimi-
6
3
45 Lingling Lu, Shanghai 200032 (P.R. China)
zation studies, see the Supporting Information). Thus, treat-
ment of 1a with 2 mol% of catalyst A in DCE at room temper-
ature for 25 min produced 1-naphthol 2a bearing an (E)-enal
functionality in 93% yield as a single olefin isomer (Table 1).
Fax: (+86)021-64166128
E-mail: yhliu@mail.sioc.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403113.
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Communication
According to our previous study, the (E)-enal was formed
through the double bond isomerization of the initially generat-
ed (Z)-enal, promoted by gold catalyst. High yields of 2a (95–
groups were all compatible, leading to the desired 1-naphthols
in generally excellent yields. For example, aryl alkynes with
electron-withdrawing (p-Cl) or electron-donating (p-MeO)
groups underwent the reaction smoothly, furnishing the ex-
pected products 2b and 2c in 88 and 98% yields, respectively.
The presence of a bulky 1-naphthyl substituent does not inter-
fere with the cyclization, and the desired 2d was obtained in
95% yield. Thienyl group was well tolerated, producing 2e in
96% yield. A range of alkyl-substituted alkynes such as n-butyl
or cyclopropyl-substituted one were efficiently transformed
into 1-naphthols 2 f and 2g with excellent yields. Interestingly,
a cyclohexenyl-substituted alkyne afforded a naphthyl-fused
polycyclic compound 3 in 53% yield, indicating a further
alkene-enal cyclization of the initially formed product occurred
during the reaction process. Substitution of the parent
phenyl ring with fluorine was also compatible for this
transformation, providing the corresponding product 2i
in 95% yield.
98%) were also obtained in solvents of DCM or toluene within
3
0 min. 1-Naphthol scaffolds are commonly occurring features
[
7,8]
in pharmaceuticals and natural products.
Our method
allows the facile assembly of multisubstituted 1-naphthols
under mild reaction conditions.
With the optimized reaction conditions in hand, we exam-
ined the scope of this rearrangement reaction using 2 mol% of
catalyst A as the catalyst. As shown in Table 1, the method is
applicable to a wide range of suitably substituted furan-ynes.
The reaction proved to be quite general with respect to sub-
stituents at the alkyne terminus, since aryl, heteroaryl and alkyl
Table 1. Formation of 1-naphthols via gold-catalyzed cyclizations of furan-ynes.
These initial results encouraged us to investigate the
possible 1,2-migration of various groups other than hy-
drogen, which would lead to an additional substitution
at C-2 position of the 1-naphthol products. Thus a varie-
ty of tertiary propargylic alcohol substrates were pre-
pared to probe the gold-catalyzed cycloisomerizations.
To our delight, our method showed great efficiency for
these 1,2-migrations. Methyl, phenyl and 2-furanyl
groups all migrate smoothly, leading to multisubstituted
[
a]
[a]
Substrate
Product
Substrate
Product
1-naphthols 2j–2p in 81–94% yields. A thienyl group
could also undergo the migration reaction,
however, with a lower product yield of 2q (59%). Inter-
estingly, when the migrating group is an alkynyl group,
the initially formed naphthol spontaneously cyclized via
a tandem nucleophilic addition of the OH group, giving
rise to naphtha[1,2-b]furan 4 in 71% yield. In this case,
5
mol% of the catalyst and higher reaction tempera-
tures (708C) were required to achieve the full conver-
sion. Substrate 1s with a substituted furan ring was also
suitable to deliver (E)-enone 2s in 98% yield. The struc-
tures of compounds 2i, 2k, 3 and 4 were unambigu-
[9]
ously confirmed by X-ray crystallographic analysis.
Recently we developed a gold-catalyzed cascade re-
action of 1,6-diynyl carbonates, which is initiated by 3,3-
[10]
rearrangement of the propargyl carbonate moiety.
We envisioned that a carbonate derivative of substrate
might produce different types of products via 3,3-rear-
1
rangement. However, we found that 1-naphthyl carbo-
nates 6a–c with a (Z)-enal moiety were formed from
carbonates 5a–c in 82–91% yield catalyzed by 5 mol%
of catalyst A (Scheme 2). The result indicated that the
furan-yne cyclization occurred more rapidly than 3,3-re-
arrangement of the propargyl carbonate moiety. Sub-
strate 5d protected by TBS group also afforded (Z)-enal
[11]
6
d as a major geometric isomer in 85% yield. The
above results indicated that the stereochemistry of the
enal double bond can be easily controlled by choosing
[12]
protected- or non-protected substrates. Interestingly,
[a] Yield of isolated product. [b] 5 mol% catalyst A, 708C. E/Z = 33:1.
treatment of 6d (Z/E 50:1) with 5 mol% catalyst A in
Chem. Eur. J. 2014, 20, 12015 – 12019
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Communication
nished the desired product 9 in 81% yield with high stereose-
lectivity (E/Z 53:1). Protection of the naphthol OH group fol-
lowed by oxidative cleavage of the double bond using KMnO4/
acetone afforded aldehyde 11 in 53% overall yield. Next, we
need to construct an a-pyrone moiety. Inspired by the recent
report of gold-catalyzed synthesis of 4-hydroxy-2-pyrones de-
[15]
veloped by Fꢁrstner and co-workers 11 was transferred to b-
keto ester 13 via first conversion of 11 to acetylenic ester 12
followed by the reaction with (2-tert-butoxy-2-oxoethyl)lithium.
Gold-catalyzed cyclization of 13 in the presence of 5 mol%
XphosAuNTf in MeNO indeed afforded the desired pyrone 14
Scheme 2. Gold catalyzed cycloisomerizations of furan-ynes 5.
2
2
DCE at 808C for 3 h resulted in Z/E isomerization to afford (E)-
in 78% yield. Removal of the Bn protecting group in 14 deliv-
ered wailupemycin G, whose NMR data is consistent with that
[
13]
6
d (E/Z 50:1) in 75% yield. In the absence of gold catalyst,
[
14a]
no isomerization occurred at 808C in DCE for 3 h. The results
indicated that gold(I) was capable of catalyzing the isomeriza-
tion reaction.
reported by Moore et al.
The structure of wailupemycin G
[9]
was further confirmed by X-ray crystal analysis. The method
presented here can be used for the assembly of a library of un-
natural wailupemycin G analogues by tuning the substitution
To demonstrate the synthetic utility of this new approach to
diverse-oriented synthesis of 1-naphthols, total synthesis of
wailupemycin G was performed. Waillupemycin G is an a-
pyrone-containing metabolite produced by the marine bacteri-
[14d]
pattern of the starting furan-ynes 8.
To disclose the reaction mechanism, density functional
[16]
theory (DFT) studies have been performed with GAUSSIAN09
[
14]
[17]
[18]
um Streptomyces maritimus, which was first elucidated by
program using the PBE1PBE method. For C, H, O and P,
[
14a]
Moore et al.
during the study of the mutational analysis of
the 6-311+G** basis set was used; and, for Au, the SDD basis
[19]
the enterocin favorskii biosynthetic rearrangement. The bioac-
tivity of this natural product has not been fully evaluated, pos-
sibly due to the limited availability. So far, no total synthesis of
this natural product has been reported. As shown in Scheme 3,
benzyl-protected phenol substrate 8 was first constructed from
set with Effective Core Potential (ECP) was used. Geometry
optimization was performed in dichloroethane using the
[20]
SMD
method. Harmonic vibration frequency calculations
were carried out and the optimized structures are all shown to
be either minima (with no imaginary frequency) or transition
states (with one imaginary frequency).
2-bromo-6-hydroxybenzaldehyde 7 over three steps. Gratify-
ingly, exposure of furan-yne 8 to catalyst A in DCE at 808C fur-
Firstly, in the cyclization step, the C2/C3 carbon atom of the
furan ring attacks the triple bond. Several possible attacking
+
modes were explored with substrate 1a and the Me PAu cat-
3
alyst, three transition states for C2 and C3 attacking were lo-
[13]
cated (Scheme 4 and Figure 1). The most favorable transition
Scheme 3. Total synthesis of wailupemycin G. a) i) 2-tributylstannylfuran, cat.
[
Pd
nyl)lithium, THF, 98%; b) 808C, 81%; E/Z 53:1; c) BnBr, K
6%; E/Z 41:1; d) KMnO , acetone, 62%; e) i) CBr , PPh , CH
THF, À788C then ClCO Me, 89%; f) LDA, tert-butyl acetate, THF, then 12,
6%; 13 was a keto/enol mixture with a ratio of 3.7:1; g) 78%; h) cat. Pd/C,
, in EtOAc, 94%.
2
(dba)
3
]/PPh
3
, toluene, 94%; ii) BnBr, K
2
CO
3
, acetone, 85%; iii) (phenylethy-
CO , NaI, acetone,
Cl , 86%; ii) BuLi,
2
3
8
4
4
3
2
2
Scheme 4. Calculated reaction pathways for substrate 1a. The selected
2
bond lengths are in angstroms, and the relative free energies in dichloro-
À1
7
H
ethane (DGsol) are in kcalmol . Calculated at the PBE1PBE/6-311+G**/SDD
2
level.
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Communication
In summary, we have developed a novel gold-catalyzed cyc-
lization of furan-ynes bearing a propargylic alcohol moiety by
incorporation of a 1,2-rearrangement step into the cascade.
The reaction provides a rapid and efficient access to multisub-
stituted 1-naphthols bearing an enal or enone moiety at the C-
4
position with high stereoselectivity. Notably, the stereochem-
istry can be controlled by choosing protected- or non-protect-
ed substrates. The utility of this method has been illustrated
by the first total synthesis of wailupemycin G.
Acknowledgements
We thank the National Natural Science Foundation of China
(
Grant Nos. 21125210, 21121062, 21372244, 21172248), Chinese
Academy of Science, and the Major State Basic Research Devel-
opment Program (Grant No. 2011CB808700) for financial sup-
port.
Keywords: 1,2-migration · 1-naphthols · furans · gold ·
homogeneous catalysis
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Received: April 16, 2014
Published online on July 30, 2014
Chem. Eur. J. 2014, 20, 12015 – 12019
www.chemeurj.org
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