Asymmetric rhodium-catalyzed hydrogenation meets gold-catalyzed cyclization: Enantioselective synthesis of 8-hydroxytetrahydroisoquinolines

Article abstract of DOI:10.1002/chem.200600192

Different furyl-substituted (Z)-dehydroamino acid derivatives were hydrogenated with the rhodium/Mandyphos(OMe)-system to give enantiomeric excesses between 80 and 98%. The absolute configuration of the newly formed stereogenic center was determined by an

Full text of DOI:10.1002/chem.200600192

DOI: 10.1002/chem.200600192
Asymmetric Rhodium-Catalyzed Hydrogenation Meets Gold-Catalyzed
Cyclization: Enantioselective Synthesis of 8-Hydroxytetrahydroisoquinolines
A. Stephen K. Hashmi,*[a] Patrick Haufe,[a] Christoph Schmid,[a]
Andreas Rivas Nass,[b] and Wolfgang Frey[a]
Abstract: Different furyl-substituted
(Z)-dehydroamino acid derivatives
were hydrogenated with the rhodium/
droisoquinolines by employing gold-
catalyzed arene synthesis as the key
step. During the latter reaction se-
quence, also including either a propar-
gylation or a reduction, a protection of
the hydroxy group, and a subsequent
propargylation, no racemization of the
stereogenic center was observed. With
very electron-rich furans, instead of the
8-hydroxytetrahydroquinolines as prod-
ucts, furans anellated to seven-mem-
Mandyphos
A
to
give
enantiomeric excesses between 80 and
98%. The absolute configuration of the
newly formed stereogenic center was
determined by anomalous diffraction
to be R. These chiral furyl alanines
were transferred into 8-hydroxytetrahy-
ꢀ
bered rings with exocyclic C C double
Keywords: asymmetric hydrogena-
bonds are formed under the same reac-
tion conditions.
tion
·
cyclization
·
gold
·
homogeneous catalysis · rhodium
Introduction
Chiral tetrahydroisoquinolines are useful intermediates in
the synthesis of many alkaloids and therefore interesting
compounds for the pharmaceutical industry. Currently, 55
examples of tetrahydrosioquinoline alkaloids are known to
possess antitumor activity, for example, quinocarcine 1 or
ecteinascidin 743 2 (Yondelis) which is being studied in a
phase III trial on ovarian cancer and evaluated in breast and
prostate cancer.[1,2]
Some of these compounds contain an 8-hydroxytetrahy-
droisoquinoline-substructure, which is not easy to assemble
by classical routes like the Bischler–Napieralski,[3] the Pom-
eranz–Fritsch,[4] or the Pictet–Spengler reaction.[5] All these
methods are based upon the principle of electrophilic aro-
matic substitution and so the positional selectivity of the
anellation step in the arene system is often problematic. In
particular, the positioning of OR groups ortho to the anel-
lated ring, as found in 1, is difficult to achieve. The classical
methods above, deliver the wrong constitutional isomer 4 as
the major product (Scheme 1).
Laschat et al. have already carried out a diastereoselective
synthesis towards 1, but without the OR group at the 8-posi-
tion of the tetrahydroisoquinoline system, as with this OR
group the wrong constitutional isomer was found in the
anellation step as shown in Scheme 1.[6] The new gold-cata-
lyzed phenol synthesis is probably the best tool to obtain
such 8-hydroxytetrahydroisoquinolines (Scheme 2).[7–9]
Starting from furfurals 12 with different substituents, (Z)-
dehydroamino acid esters 11 can be prepared and subse-
[a] Prof. Dr. A. S. K. Hashmi, Dipl.-Chem. P. Haufe,
Cand. Chem. C. Schmid, Dr. W. Frey+
Institut für Organische Chemie
Universität Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Fax : (+49)711-685-4321
E-mail: hashmi@hashmi.de
[b] Dr. A. Rivas Nass
Umicore AG & Co. KG
Precious Metals Chemistry
Rodenbacher Chaussee 4
63403 Hanau (Germany)
[+] Crystallographic investigation.
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
5376
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Chem. Eur. J. 2006, 12, 5376 – 5382

FULL PAPER
chiral ligands are tested with furyl-containing dehydroamino
acid derivatives.[13]
Results and Discussion
Diastereomerically pure (Z)-dehydroamino acid esters 15
for asymmetric hydrogenation were prepared by a Horner–
Wadsworth–Emmons protocol described by Schmidt et al.
(Table 1).[14]
Scheme 1. Problem of positional selectivity in the anellation step
(* shows the possible positions for an electrophilic attack).
Table 1. Generation of the (Z)-dehydroamino acid esters 15 (Cbz=car-
boxybenzyloxy).
Entry
1
13
a
R
Yield of 15 [%]
81
2
3
4
5
b
c
71
49
70
80
Scheme 2. Gold-catalyzed phenol synthesis.
d
e
quently hydrogenated with a chiral rhodium catalyst. Fur-
ther transformations to furyl alanines 10 and finally gold-
catalyzed phenol synthesis leads to the chiral, nonracemic
8-hydroxytetrahydroisoquinolines 9 (Scheme 3).
Unlike the acetamidocinnamic acid (ACA) or its methyl
ester (methyl acetamidocinnamate, MAC), which both pos-
sess a phenyl substituent and which are often used as test
substrates for asymmetric hydrogenations of olefins, the cor-
responding furyl-substituted dehydroamino acids cause
much more problems. The first attempt was published by
MelØndez et al. in 1982, in which a substrate/rhodium ratio
of 30 was used at 508C to achieve a 13% ee (ee=enantio-
meric excess).[10] Much better results were achieved by
Dçbler and Krause et al. in 1992. Depending on the sub-
strates and catalyst systems they used, enantiomeric excesses
between 67 and 90% ee could be obtained.[11] The first
highly selective asymmetric hydrogenation was performed
6
f
34
7
8
g
82
63
h
by Masquelin et al. at 408C with 1 mol% [Rh(cod)2]BF4 and
N
1 mol% MeDuphos, producing the product with about
The Horner–Wadsworth–Emmons protocol worked well
for most furfurals, except for the 3-(trifluoromethyl)phenyl-
99% ee.[12] So far, few examples are published in which new
substituted
derivative
13 f
(entry 6) for which only 34% of
the (Z)-dehydroamino acid
ester 15 f could be isolated by
crystallization (the residual
product remained in the mother
liquor). Initial hydrogenation
reactions were attempted with
compound 15a. Out of all
Scheme 3. Retrosynthesis of chiral 8-hydroxytetrahydroisoquinolines 9.
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5377

A. S. K. Hashmi et al.
tested catalyst precursors and ligands, only [Rh
A
For compound 15 f, both racemic and enantiomerically pure
single crystals for X-ray structure analysis could be obtained
(Figure 1).[15] Unfortunately, at this stage the determination
of the absolute configuration by anomalous diffraction was
impossible, as the enantiomerically pure 15 f lacked the nec-
essary heavy atom.
and MandyPhos(OMe) 22 gave satisfying results (Table 2).
ACHTREUNG
In the other cases either no hydrogenation was observed
As the efforts with enantiomerically pure 15 f had shown
(Figure 1), to determine the absolute configuration of the
newly formed stereogenic center of 16a by anomalous dif-
fraction, a heavy atom was prerequisite and therefore the
synthetically much more useful Cbz-protected (Cbz=carbo-
benzyloxy) 16a was converted to the brosyl-protected amine
23a (Bs=brosyl, 4-bromobenzenesulfonyl), which also facil-
itates crystallization. This was accomplished by reductive
cleavage of the Cbz group and subsequent protection with
brosyl chloride (Scheme 4). As expected, a suitable single
crystal could be obtained and from the X-ray structure
(anomalous diffraction). The configuration of the stereogen-
ic center at C1 was assigned as R (Figure 2). This assignment
was previously assumed by Miyazawa et al. for the Cbz-pro-
tected methyl esters of natural occurring and nonproteino-
genic amino acids on the basis of enantiomeric separation
by chiral HPLC spectroscopic analysis.[16]
Only chiral furyl alanines 16a–e were chosen to carry on
further transformations to the corresponding 8-hydroxyte-
trahydroisoquinolines. Proceeding to the gold-catalyzed re-
action, the brosyl-protected amine 23a was alkylated with
propargyl bromide in 85% yield. Gold catalysis of 24a with-
out a substituent at the 5-position of the furan with 5 mol%
Table 2. Optimisation of hydrogenation conditions.
of the Uson–Laguna salt[17] [m-Cl
(AuPPh3)2]BF4 in deuterat-
A
ed chloroform for 5 h furnished a mixture of the constitu-
tional isomers 25aa and ab (28 and 20%, respectively),
which is in accordance with earlier results (Scheme 5).[7]
Deprotection, brosylation, and propargylation of methyl-
substituted 16b lead to 24b in 52% overall yield. The fol-
Entry
Ligand (mol%)
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
17[a] (5)
18 (10)
18 (1)
47.5
18
18
21.5
25
82
23
16
5.5
–
–
3
–
7
4
10
3
18
98
43
3[d]
81
19[a] (10)
19[a] (1)
20[a] (10)
20[a] (1)
21[a] (5)
22[a] (1)
9[d]
19[d]
99
lowing conversion of 24b with 5 mol% [m-Cl(AuPPh3)2]BF4
A
in deuterated chloroform was complete in five minutes at
room temperature to give the single isomer 25b in 62% iso-
lated yield (Scheme 6).
15
97
To test which additional functional groups can be tolerat-
ed in the gold-catalyzed phenol synthesis, methyl ester 23b
was reduced with diisobutylaluminum hydride (DIBAL-H)
and alkylated again with propargyl bromide in 71% overall
yield. Reaction of alcohol 27b with 5 mol% AuCl3 in deu-
terated acetonitrile did not afford the corresponding 8-hy-
droxytetrahydroisoquinoline; however, intramolecular nu-
cleophilic attack of the hydroxy group onto the alkyne fur-
nished dehydromorpholine 28b in almost quantitative yield
after spontaneous isomerization of the double bond to the
thermodynamically more stable position. This proves that
the 6-exo-dig attack of the OH-oxygen is significantly faster
than the first step of the phenol synthesis. Addition of nucle-
ophiles to alkynes is quite common, but a selective monoad-
dition to deliver enol ethers has never been observed
before.[18] To drive the reaction to the 8-hydroxytetrahydro-
isoquinoline, the free hydroxy group was protected as the
tert-butyldimethylsilyl (TBDMS) ether 29b, which was then
[a] Catalyst precursor: [Rh(nbd)2]BF4, ligand/[Rh] 1:1. [b] Isolated yield.
A
[c] Determined by HPLC spectroscopy with a Chiralcel OD column.
1
[d] Conversion determined by H NMR spectroscopy.
(entry 1), the yield was very low (entries 3, 5, 6, and 8), or
the ee was low (entries 2, 4, and 7).
By using the optimized reaction conditions, most dehy-
droamino acid esters were reduced and isolated in good
yields and with good to excellent ee values (Table 3). In the
case of silyl-substituted compound 15e, just 0.1 mol% cata-
lyst was used leading to longer reaction times and a better
isolated yield, but slightly lower selectivity (entry 5). Only
two derivatives caused problems, the 3-(trifluoromethyl)-
phenyl-substituted derivative 15 f which was only reduced in
37% isolated yield (the remaining starting material could be
recovered, entry 6), and the 4-bromophenyl-substituted
compound 15g which did not react at all under these condi-
tions, presumably because of its very low solubility (entry 7).
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Synthesis of 8-Hydroxytetrahydroisoquinolines
FULL PAPER
Table 3. Effect of different substituents in the hydrogenation of 15.
converted cleanly to 30b with
1 mol% [m-Cl(AuPPh3)2]BF4 in
G
two hours (70% isolated yield)
(Scheme 7).
As both 8-hydroxytetrahy-
droisoquinolines 25b and 30b
had the common precursor 23b
(96% ee after hydrogenation),
we were interested as to wheth-
er the enantiomeric ratio
changed during the reaction se-
quences, and especially in the
gold-catalyzed step. Determina-
tion of both enantiomeric ex-
cesses showed that no signifi-
cant racemization had occurred
(94% ee for 25b, 96% ee for
30b). The 8-hydroxytetrahy-
droisoquinolines could be used
for further transformations, but
the brosyl group is difficult to
remove, therefore the same re-
action sequence was repeated
with the Cbz group. Furyl ala-
nines 16b–e were reduced with
Entry
1
16
a
Product
t [h]
5.5
Yield [%][a]
97
ee [%][b]
98
2
3
4
5
b
c
16.5
17.5
64
85
96
95
87
80
88
d
e
82
120
99[c]
6
f
18
37[d]
87
DIBAL-H,
protected
with
TBDMS, and alkylated with
propargylbromide and sodium
hydride as base. Reactions of
33b and c with 1 mol% [m-Cl-
[d,e]
7
8
g
68
–
–
A
hydroxytetrahydroisoquinolines
34b and c in 84 and 80% yields,
respectively (Scheme 8).
As the gold-catalyzed trans-
formations were carried out in
h
14.5
85
93
[a] Isolated yield. [b] The ee was determined by HPLC analysis with a Chiralcel OD column. [c] 0.1 mol% of
catalyst/ligand was used. [d] Recovered starting material. [e] Basically insoluble.
Figure 1. The lack of a heavy atom made the determination of the abso-
lute configuration of enantiomerically pure (ꢀ)-15 f impossible.
Scheme 4. Changing the protecting group for crystallographic determina-
tion of the absolute configuration by anomalous diffraction. Reaction
conditions: a) H2 (1 bar), Pd/C, MeOH, 38 h, 38%; b) BsCl, NEt3,
DMAP (cat.), CH2CH2, 08C!RT, overnight, 62%. DMAP=4-(dimeth-
Figure 2. Solid-state structure of 23a showing the stereogenic center (C1)
to be R configurated (anomalous diffraction).
ylamino)pyridine.
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5379

A. S. K. Hashmi et al.
er the phenol, the alkyne at-
tacked at the 3-position of the
furan to form the previously
unknown furan 36e anellated
to a seven-membered heterocy-
ꢀ
cle with an exocyclic C C
double bond in 84% isolated
yield (Scheme 9). A similar re-
Scheme 5. The gold-catalyzed conversion of 24a delivers two constitutional isomers 25aa and ab. Reaction
ꢁ
conditions: a) BrCH2C CH, Cs2CO3, acetone, overnight, 85%; b) [m-Cl
(28%), 25ab (20%).
A
action pathway was also ob-
served by Echavarren et al.;
however, a furan anellated to a six-membered ring was
formed.[20]
We also tried to alkylate the carbamates 31b, d, and e in
the presence of the free hydroxy group. Three equivalents
of sodium hydride and propargyl bromide in DMF afforded
the cyclic carbamates 37b, d, and e (Scheme 10). These com-
pounds were also substrates for the gold-catalyzed phenol
synthesis. The reactions of the cyclic carbamates proceeded
significantly slower than those of the corresponding open
chained derivatives. While conversion of 37b with 1 mol%
Scheme 6. Reaction sequence to 8-hydroxytetrahydroisoquinoline 25b.
Reaction conditions: a) Cyclohexene, Pd/C, MeOH, reflux; b) BsCl,
ꢁ
NEt3, DMAP (cat.), CH2Cl2, 08C!RT, ꢀ77%; c) BrCH2C CH, Cs2CO3,
acetone, RT, 67%; d) [m-Cl(AuPPh3)2]BF4, CDCl3, RT, 5 min, 62%.
AHCTREUNG
[m-Cl(AuPPh3)2]BF4 furnished
G
phenol 38b in 50% isolated
yield, compounds 37d and e
again afforded furans anellated
to seven-membered heterocy-
ꢀ
cles with an exocyclic C C
double bond in 31 (39d) and
37% (39e) yields, respectively
(Scheme 11).
Conclusion
The combination of asymmetric
rhodium-catalyzed hydrogena-
tion and gold-catalyzed cycliza-
tion offers a short and easy
access to chiral, nonracemic 8-
hydroxytetrahydroisoquinoline
Scheme 7. The unprotected alcohol leads to 28b, while the protected one delivers 30b. Reaction conditions:
ꢁ
a) DIBAL-H, THF, ꢀ788C!RT; b) BrCH2C CH, Cs2CO3, acetone, RT, ꢀ77%; c) AuCl3 (5 mol%), CD3CN,
RT, 99%; d) TBDMSCl, imidazole, DMF, RT, 66%; e) [m-Cl(AuPPh3)2]BF4, CDCl3, RT, 70%.
A
systems. No significant racemi-
zation was observed while car-
NMR tubes, the progress of the reactions could normally be
monitored by H NMR spectroscopy. However, for the Cbz-
rying out the reaction sequence. The absolute configuration
of the newly-formed stereogenic center was unambiguously
assigned by anomalous diffraction. Both ester and silyl-pro-
tected alcohol groups are tolerated at the g-position to the
alkyne moiety during cyclization. Even cyclic carbamates
are converted, but the reaction proceeds significantly
slower. However, depending on the employed substrates,
gold-catalysis affords interesting and so far unknown furans
1
protecting group this was difficult due to the high rotational
barrier of the carbamate. This was particularly the case for
33d and e for which the reaction progress could not be fol-
lowed by 1H NMR spectroscopy. Thus we decided to use
brosyl-protected compounds again, which were much easier
to observe spectroscopically by NMR. This time the Cbz
group was cleaved under transfer hydrogenation conditions
with cyclohexene as the hydrogen source. Subsequent pro-
tection and alkylation furnished the starting materials for
the gold-catalyzed transformations. Conversion of 29d with
1 mol% of the Schmidbaur–Bayler salt[19] [m-Cl-
ꢀ
anellated to seven-membered rings with an exocyclic C C
double bond, which themselves are interesting building
blocks and could also be used for further transformations.
(AuPMes3)2]BF4 (Mes=mesityl, 2,4,6-trimethylphenyl) af-
forded the tetrasubstituted phenol 30d in 37% isolated
yield. Under the same reaction conditions 29e did not deliv-
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Synthesis of 8-Hydroxytetrahydroisoquinolines
FULL PAPER
Scheme 8. Synthesis of the Cbz-protected substrates 33b–e and 34b–c. Reaction conditions: a) DIBAL-H, THF, ꢀ788C!RT; b) TBDMSCl, imidazole,
ꢁ
DMF, RT; c) BrCH2C CH, NaH, DMF, RT; d) [m-Cl
A
Scheme 9. Synthesis of 29d and e and their conversion to 30d and 36e. Reaction conditions: a) Cyclohexene, Pd/C, MeOH, reflux; b) BsCl, NEt3,
ꢁ
CH2Cl2, 08C!RT; c) BrCH2C CH, Cs2CO3, acetone, RT; d) [m-Cl
A
Scheme 10. Formation of cyclic carbamates 37b, d, and e during the prop-
ꢁ
argylation. Reaction conditions: a) BrCH2C CH, NaH, DMF, RT.
Experimental Section
Scheme 11. Formation of 8-hydroxytetrahydroisoquinoline 38b and furo-
[2,3-d]azepines 39d and e by gold-catalyzed conversion. Reaction condi-
tions: a) [m-Cl(AuPPh3)2]BF4 (1 mol%), CDCl3, RT.
Detailed reaction and catalysis conditions, as well as full characterization
of all unknown compounds are given in the Supporting Information.
AHCTREUNG
AHCTREUNG
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5381

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We thank Dr. Klaus Schçllkopf from Schering AG, Berlin (Germany) for
the determination of the enantiomeric excess of the 8-hydroxytetrahydro-
quinolines, Umicore AG & Co KG, Hanau (Germany) for donation of
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