Intramolecular chromium(II)-catalyzed pinacol cross coupling of 2-methylene-α,ω-dicarbonyls

Article abstract of DOI:10.1055/s-2004-822895

Using only 10% of CrCl₂ as catalyst, manganese-powder as reducing agent and TMSC1 as scavenger, 2-methylene-α,ω-dialdehydes and -ketones can be coupled to form cyclic diols diastereoselectively. The diastereomeric excess strongly depends on the ring size and the substituents of the ω-carbonyl group. The greater the ring size the higher diastereoselectivities are observed. In all cases cis-diols are preferentially formed.

Full text of DOI:10.1055/s-2004-822895

1054
LETTER
Intramolecular Chromium(II)-Catalyzed Pinacol Cross Coupling of
2-Methylene-a,w-dicarbonyls1
I
ntramolecular Chr
l
omium
r
(II)-cata
i
lyze
d
P
c
inacol Cro
h
ss Coupling Groth,* Marc Jung, Till Vogel
Fachbereich Chemie, Universität Konstanz, Fach M-720, Universitätsstr. 10, 78457 Konstanz, Germany
Fax +49(7531)884155; E-mail: Ulrich.Groth@uni-konstanz.de
Received 11 February 2004
aldehydes to form 1,2-diols diastereoselectively.10 The
great advantage of this method is the decrease of the
amount of chromium chloride to only 10% by using Fürst-
ner’s redox system11 and the extention of Takai’s method
Abstract: Using only 10% of CrCl2 as catalyst, manganese-powder
as reducing agent and TMSCl as scavenger, 2-methylene-a,w-di-
aldehydes and -ketones can be coupled to form cyclic diols dia-
stereoselectively. The diastereomeric excess strongly depends on
the ring size and the substituents of the w-carbonyl group. The to acroleins and sterically demanding substrates. We now
greater the ring size the higher diastereoselectivities are observed.
In all cases cis-diols are preferentially formed.
report an intramolecular coupling to form cyclic diols
from small to mid-sized rings (Scheme 1).
Key words: chromium, catalysis, cross-coupling, cyclizations,
pinacols
Table 1 Cyclization of Dialdehydes 1 (R = H)
Entry
1
n
1
Pinacol 2
Yield (%)a cis/transb de (%)
60
59:41
18
Many pharmacologically active natural products in-
clude 1,2-diol substructures occurring in cyclic systems.
1,2-Diols can be generated in general by bishydroxylation
of olefinic double bonds2 or reductive coupling of carbo-
nyl compounds.3 The last method plays an important role
in the synthesis of HIV-protease inhibitors4 and natural
products5 such as Taxol6 and Cotylenol7 and their deri-
vates. For their synthesis this reaction has to be performed
in a diastereoselective fashion.
However, most reported catalytic pinacol couplings8 are
limited to highly reactive aldehydes and even more im-
portant only symmetrically substituted pinacols can be
obtained in intermolecular processes.
OH
OH
OH
2
3
2
3
55
75
71:29
80:20
42
60
OH
OH
OH
4
5
3
7
65
–
92:8
–
84
–
OH
OH
Previously, a Cr(II)-mediated pinacol-type cross coupling
between a,b-unsaturated ketones and aldehydes has been
reported.9 The diastereoselectivity of this overstoichio-
metric process highly depends on the reaction tempera-
ture. Moreover, 4 equivalents of chromium chloride were
necessary and no intramolecular example was given.
OH
OH
a Yields refer to isolated products.
b Diastereomeric ratios were determined by isolation and/or NMR-
spectroscopy.
Recently, we reported a chromium catalyzed pinacol cross
coupling of a,b-unsaturated carbonyl compounds and
1) 4.0 equiv TMS-Cl
4.0 equiv Mn
O
OH
OH
OH
0.2 equiv CrCl2, DMF
R
+
R = H, alkyl
H
R
n
2) 4.0 equiv Bu4NF, THF
O
OH
R
n
n
1
trans-2
cis-2
Scheme 1 Chromium(II)-catalyzed cross coupling of 2-methylene-a,w-dicarbonyl compounds
SYNLETT 2004, No. 6, pp 1054–1058
0
6.
0
5.
2
0
0
4
Advanced online publication: 25.03.2004
DOI: 10.1055/s-2004-822895; Art ID: G04404ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Intramolecular Chromium(II)-Catalyzed Pinacol Cross Coupling
1055
Application of the reported coupling conditions10 to 2-me-
thylene-a,w-dialdehydes 1 predominantly produces cis-
diols 2. The diastereoselectivity strongly depends on the
ring size. While 6-membered rings are formed with only
18% de, diastereoselectivities increase up to 84% de for 8-
membered rings (Table 1).
Table 2 Cross-Coupling Between 2-Methylene-3-methylbutanal
with Various Ketones
1) 10% CrCl2
4 equiv Mn
4 equiv TMSCl
DMF
O
O
+
R1
R2
However, a 12-membered ring could not be obtained. In
this case the reactive centers are too far from each other so
that intermolecular couplings as well as homo couplings
and other side reactions become dominant.
We were able to show10 that substituted vinylketones as
well as acroleins can be coupled with aliphatic aldehydes
in good yields and with high selectivities.
2) 4 equiv TBAF
THF
3
4
OH
OH
R1
R2
+
OH
OH
5
6
However, when ketones are used instead of aldehydes
yields decrease dramatically. In an attempt to cross couple
2-isopropylpropenal 3 (which reacts with aliphatic alde-
hydes in good yields up to 96%) with certain aromatic and
aliphatic ketones such as 4 (Table 2), only 15% to 27% of
the tertiary alcohol 5 could be obtained. Nevertheless the
symmetrically substituted pinacol 6 resulting from homo
coupling of acrolein 3 could be isolated in 35% to 63%
yield with a rac/meso-ratio of 61:39 (22% de). Although
the concentration of ketone is much higher than of
acrolein, the acrolein itself acts as carbonyl compound to
form the homo pinacol.
Entry
Ketone 4
Pinacol 5 (%)a
1
2
3
4
5
Acetone
19
24
27
15
–
Cyclohexanone
Mesityl oxide
Acetophenone
p-NO2-acetophenone
a Yields refer to isolated products.
However, via the intramolecular pathway, we were able
to obtain tertiary cyclic alcohols in high yields and with
excellent diastereoselectivities. Starting from keto-
acroleins 7 the corresponding cyclic pinacols 8 are
obtained generally in better yields than in the case of the
dialdehydes 1. Some representative results are listed in
Table 3.
Again, the diastereoselectivity depends on the ring size.
Changing the ring size from n = 1 (entry 1) to n = 2 (en-
try 2) a dramatic increase in the diastereoselectivity from
10% to 86% is observed. But also the substituent of the
keto group is important. Change of the substituent from
Table 3 Cyclization of Keto-acroleins 7
1) 10% CrCl2
4 equiv Mn
4 equiv TMSCl
DMF
O
OH
OH
O
R
n
2) 4 equiv TBAF
THF
R
n
7
8
Entry
1
n
1
R
Pinacol 8
Yield (%)
82
cis/trans
de
10
Me
55:45
OH
OH
2
3
2
2
Me
75
30
93:7
86
OH
OH
i-Pr
>99:1
>98
OH
OH
a Yields refer to isolated products.
b Diastereoeric ratios were determined by isolation and/or NMR-spectroscopy.
Synlett 2004, No. 6, 1054–1058 © Thieme Stuttgart · New York

1056
U. Groth et al.
LETTER
R = H (Table 1, entry 1) to R = Me (Table 3, entry 1) tetrahydrofuran leads to a-methylene dialdehydes 1 in
results in a slight decrease in diastereoselectivity from 55% to 65% yield.
18% to 10% de for the 6-membered ring but a remarkable
increase from 42% (Table 1, entry 2) to 86% de (Table 3,
entry 2) for the 7-membered ring, respectively.
In case of the ketones the monoprotected aldehydes 10 are
treated with a Grignard reagent to form the secondary
alcohols 12 which are oxidized using Dess–Martin
With R = i-Pr (Table 3, entry 3) the cis-diol is formed ex- periodinane15 to ketones 13. Deprotection followed by
clusively with more than 98% de. Unfortunately the yield Mannich reaction as described above leads to the corre-
is significantly lower (30%). The main product in this case sponding keto-acroleins 7. Under the chosen conditions
is again the symmetrically substituted pinacol from the only the aldehyde reacts with the Eschenmoser salt
homo coupling of the acrolein, which could be isolated in leaving the keto-group unchanged.
54% yield. The reactivity of the keto group is lowered by
the sterically demanding and electron donating residue
R = i-Pr so that the disfavored intermolecular homo
Determination of Relative Configurations
coupling gains in importance.
The relative configurations of the cyclic diols were deter-
mined by NOE-spectroscopy of the corresponding ace-
tonides 18 (Scheme 3) and in the case of the 6-membered
ring by comparison of the spectroscopic data with a refer-
ence substance obtained as shown in Scheme 4.
In conclusion, the chromium(II)-catalyzed pinacol type
cross coupling serves as an excellent method for the
cyclization of 2-methylene-dicarbonyl compounds. In
contrast to the intermolecular way even ketones can be
coupled in good yields and high diastereoselectivities.
The cis/trans-ratio strongly depends on the ring-size and
the steric demand at the keto group. In all of these cases
the cis-diol is preferred.
NOE
H
H
O
O
CH3
CH3
O
R
CH3
CH3
trans-18
cis-18
NOE
Application of this method to the total synthesis of
Cotylenol is presently under investigation in our group.
O
R
n
n
NOE
Preparation of Starting Materials
Scheme 3 Determination of relative configurations by NOE-spec-
troscopy.
Precursors for the cyclization can be easily obtained from
cycloolefins in three steps for the aldehydes and five steps
for the ketones, respectively (Scheme 2).
O
O
O
O
O
O
OH
OH
O
O
2)
3)
1)
O
O
O
O
2)
3)
1)
cis-17
15
cis-16
14
O
O
O
O
n
n
n
n
5)
4)
9
1
10
11
4)
OH
6)
OH
OH
cis-2
(R=H, n=1)
O
O
O
O
O
5)
6), 7)
R
R
R
n
O
O
cis-18
(R=H, n=1)
O
O
n
12
n
7
13
Scheme 4 Preparation of reference substance. 1) HOCH2CH2OH,
PTSA, toluene, reflux; 2) OsO4, NMO, H2O, acetone; 3) PTSA, ace-
tone, r.t.; 4) Cp2Ti(CH2)(Cl)AlMe2, THF, 0 °C to r.t.; 5) O3, CH2Cl2,
MeOH, Me2S; 6) PPTS, 2,2-dimethoxypropane, acetone, r.t.
Scheme 2 Syntheses of the starting materials 1 and 7. 1) a) O3,
CH2Cl2, MeOH, –78 °C; b) PTSA, –78 °C to r.t.; c) NaHCO3; d)
Me2S; 2) Me2NCH2Cl, Et3N, CH2Cl2, r.t.; 3) HCl, THF, r.t.; 4)
RMgX, Et2O, 0 °C to r.t.; 5) DMP, CH2Cl2, pyridine, r.t.; 6) HCl,
THF, r.t.; 7) Me2NCH2Cl, Et3N, CH2Cl2, r.t.
2-Cyclohexen-1-one (14) was protected to 15 and cis-hy-
Cycloolefins 9 are converted to the monoprotected alde- droxylated using 1% osmium tetroxide and N-methyl-
hydes 10 in a one-pot reaction by treatment with ozone in morpholin-N-oxide as cooxidans.16 Treatment of cis-16
dichloromethane and methanol according to a procedure with catalytic amounts of p-toluenesulfonic acid in ace-
published by Schreiber et al.12 in 80% to 95% yield. Man- tone led directly to the acetonide cis-17. The cis-configu-
nich type reaction13 of 10 with Eschenmoser-chloride14 ration was validated by NOE-spectroscopy. The ketone
followed by deprotection with hydrochloric acid in cis-17 can be converted to the allylic alcohol cis-18 with
Synlett 2004, No. 6, 1054–1058 © Thieme Stuttgart · New York

LETTER
Intramolecular Chromium(II)-Catalyzed Pinacol Cross Coupling
1057
Tebbe’s reagent.17 Compound cis-18 is also available by
treatment of pinacol cis-2 with pyridinium tosylate and
2,2-dimethoxypropane in acetone and can be converted
into cis-17 by ozonolysis.
CH2), 2.92 (br, 2 H, 2 OH), 4.07 [s, 1 H, CH(OH)], 4.96, 5.17 (s,
2 H, CH2). 13C (100 MHz, CDCl3): d = 21.4, 27.4, 33.3, 40.9 (CH2),
26.4 (CH3), 74.5 [C(OH)], 80.0 [CH(OH)], 112.5 (CH2), 148.1 (C).
MS (EI, 70 eV): m/z (%) = 156 (1) [M+], 138 (9) [M+ – H2O], 123
(7), 109 (63), 95 (51), 43 (100). IR (CDCl3): 3554, 3429, 2935,
2862, 1446, 1375, 1252 cm–1. Anal. Calcd for C9H16O2 (156.12): C,
69.19; H, 10.32. Found: C, 68.90; H, 10.51.
General Coupling Procedure
All experiments were carried out under an argon atmosphere using
Schlenk techniques.
Hexahydro-2,2,3a-trimethyl-8-methylene-3aH-cyclohep-
ta[d][1,3]dioxole [acetonide of 8 (R = CH3, n = 2)]: cis-diol: H
1
(400 MHz, CDCl3): d = 1.31 (s, 3 H, CH3), 1.39, 1.58 (s, 6 H,
2 CH3), 1.18–1.77 (m, 6 H, 3 CH2), 2.13 (dt, J1 = 3.9 Hz, J2 = 12.9
Hz, 1 H, CH2), 2.45 (dt, J1 = 4.7 Hz, J2 = 12.5 Hz, 1 H, CH2), 4.39
[s, 1 H, CH(OR)], 4.93, 4.94 (s, 2 H, CH2). 13C (100 MHz, CDCl3):
d = 22.8 (CH3), 26.2, 31.6, 31.8, 40.4 (CH2), 27.2, 28.6 (CH3), 83.4
[C(OR)], 88.0 [CH(OR)], 107.3 [C(OR)2], 114.1 (CH2), 147.0 (C).
trans-Diol: 1H (400 MHz, CDCl3): d = 1.24 (s, 3 H, CH3), 1.28, 1.35
(s, 6 H, 2 CH3), 1.18–1.77 (m, 6 H, 3 CH2), 2.13 (dt, J1 = 3.9 Hz,
J2 = 12.9 Hz, 1 H, CH2), 2.45 (dt, J1 = 4.7 Hz, J2 = 12.5 Hz, 1 H,
CH2), 4.43 [s, 1 H, CH(OR)], 4.86, 5.17 (s, 2 H, CH2). 13C (100
MHz, CDCl3): d = 22.8 (CH3), 24.7, 31.6, 31.8, 40.4 (CH2), 27.2,
28.6 (CH3), 77.3 [C(OR)], 82.5 [CH(OR)], 107.3 [C(OR)2], 109.1
(CH2), 147.0 (C).
4 mmol (220 mg) of manganese powder and 0.2 mmol (25 mg) of
chromium dichloride were suspended in 8 mL of dry DMF. To the
green mixture 4 mmol (435 mg, 0.51 mL) of TMSCl were added.
The solution was allowed to stir at r.t. for 15 min. A solution of
1 mmol of the dicarbonyl compound in 2 mL of dry DMF was added
over a period of 11 h via a syringe pump. After additional 4 h 20 mL
of H2O were added. The extraction with four 30 mL portions of
Et2O followed by drying with MgSO4 and evaporation of the sol-
vent afforded a colorless oil consisting of silylated pinacol and trac-
es of DMF. For desilylation the oil was dissolved in 10 mL of THF
and 4 mL of a 1 M solution of TBAF in THF were added. After 1 h
20 mL of H2O were added and the diol was extracted with four
30 mL portions of Et2O. After drying with MgSO4 and evaporation
the crude product was purified by column chromatography using
40 g of silica-gel with petroleum ether/EtOAc = 1:1 affording the
cis-diol as a colorless oil and the trans-diol as colorless crystals.
Acknowledgment
The authors are grateful to the Fond der Chemischen Industrie and
the EU-Commission, Directorate XII, for financial support. We are
thankful to the Merck KGaA and the Wacker GmbH for generous
gifts of reagents. M. J. thanks the Landesgraduiertenförderung
Baden-Württemberg for a PhD fellowship.
For acetalization the diol was dissolved in 1 mL of acetone and
1 mL of 2,2-dimethoxypropane and a small amount of pyridinium
tosylate was added. After 2 h approximately 0.5 g of silica-gel were
added and the solvent was removed in vacuo. Purification by col-
umn chromatography using 4 g of silica-gel with petroleum ether/
EtOAc = 9:1 afforded the acetonide as a colorless oil.
References
Representative spectroscopic data:
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MHz, CDCl3): d = 1.33–1.37, 1.56–1.60, 1.71–1.78, 1.89–1.96,
2.22–2.27 (m, 6 H, 3 CH2), 2.09 (br, 1 H, OH), 2.35 (br, 1 H, OH),
3.79, 4.08 [br, 2 H, 2 × CH(OH)], 4.83, 4.91 (s, 2 H, CH2). 13C (100
MHz, CDCl3): d = 22.60, 29.84, 31.49 (CH2), 72.08, 74.61
(CHOH), 109.9 (CH2), 147.5 (C); trans-Diol: 1H (400 MHz,
CDCl3): d = 1.33–1.37, 1.56–1.60, 1.71–1.78, 1.89–1.96, 2.22–2.27
(m, 6 H, 3 CH2), 2.09 (br, 1 H, OH), 2.35 (br, 1 H, OH), 3.79, 3.31
[br, 2 H, 2 × CH(OH)], 4.76, 4.96 (s, 2 H, CH2). 13C (100 MHz,
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106.4 (CH2), 147.7 (C). MS (EI, 70 eV): m/z (%) = 128 (2) [M+],
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9.44. Found: C, 65.12; H, 9.53.
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1
acetonide of 2 (n = 1)]: cis-diol: H (400 MHz, CDCl3): d = 1.37,
1.51 (s, 6 H, 2 CH3), 1.21–1.83 (m, 4 H, 2 CH2), 2.04–2.13, 2.30–
2.38 (m, 2 H, CH2), 4.23 [q, J = 5.0 Hz, 1 H, CH(OR)], 4.45 [d,
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cis-diol: 1H (400 MHz, CDCl3): d = 1.26 (s, 3 H, CH3), 1.49–1.77
(m, 6 H, 3 CH2), 2.10–2.19, 2.49–2.55 (m, 2 H, CH2), 2.92 (br, 2 H,
2 OH), 3.97 [s, 1 H, CH(OH)], 4.96, 5.02 (s, 2 H, CH2). 13C (100
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[C(OH)], 81.0 [CH(OH)], 113.6 (CH2), 148.7 (C). MS (EI, 70 eV):
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LETTER
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