Synthesis of stereodefined substituted cycloalkenes by a one-pot catalytic boronation-allylation-metathesis sequence

Article abstract of DOI:10.1002/adsc.200800324

Stereodefined cyclohexene and cyclopentene derivatives were prepared bythe coupling of allylic alcohols and other allylic precursors with unsaturated aldehydes. These reactions are based on a multicatalytic one-pot approach involving palladium pincer complex-catalyzed boronation, allylation and ring-closing metathesis reactions. This reaction sequence can be performed in an operationally simple procedure affording the cycloalkene products in high overall yields and excellent regio- and stereoselectivities. The presented procedure has a broad synthetic scope and high functional group tolerance, which allows the synthesis of bicyclic lactone and spirane skeletons and various substitution patterns including hydroxy, silyl, vinyl, allyl, and sulfonyl groups. The studied catalytic one-pot reactions involve up to three individual processes performed by up to four acid-and transition metal-catalyzed events.

Full text of DOI:10.1002/adsc.200800324

FULL PAPERS
DOI: 10.1002/adsc.200800324
Synthesis of Stereodefined Substituted Cycloalkenes by a One-Pot
Catalytic Boronation–Allylation–Metathesis Sequence
Nicklas Selandera and Kµlmµn J. Szabóa,*
a
Department of Organic Chemistry; Stockholm University, Arrhenius Laboratory; 106 91 Stockholm, Sweden
Fax : +46-8-15-4908; e-mail: kalman@organ.su.se
Received: May23, 2008; Published online: August 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800324.
Abstract: Stereodefined cyclohexene and cyclopen- scope and high functional group tolerance, which
tene derivatives were prepared bythe coupling of al-
allows the synthesis of bicyclic lactone and spirane
lylic alcohols and other allylic precursors with unsa- skeletons and various substitution patterns including
turated aldehydes. These reactions are based on a hydroxy, silyl, vinyl, allyl, and sulfonyl groups. The
multicatalytic one-pot approach involving palladium studied catalytic one-pot reactions involve up to
pincer complex-catalyzed boronation, allylation and three individual processes performed byup to four
ring-closing metathesis reactions. This reaction se- acid- and transition metal-catalyzed events.
quence can be performed in an operationallysimple
procedure affording the cycloalkene products in high Keywords: allylation; boronation; homogenous catal-
overall yields and excellent regio- and stereoselectiv- ysis; metathesis; palladium pincer complexes; stereo-
ities. The presented procedure has a broad synthetic selectivity
Introduction
Results and Discussion
One-pot, multi-step catalytic reactions represent one of Now, we have found (Figure 1 and Table 1) that by
the most important approaches for development of appropriate choice of the aldehyde (2a) or acetal (2b)
sustainable and cost-efficient synthetic procedures.[1–4] component, stereodefined dienes and trienes can be
In this approach, the transition metal-catalyzed pro- generated, which undergo ring-closing metathesis[23]
[5]
cesses ensure a high level of atom economy, while (RCM) in a one-pot sequence with the catalytic boro-
the performance of several cooperative processes shar- nation[12] and the allylation reaction. In this way, com-
ing the same solvent and reaction vessel ensures that plex regio- and stereodefined cyclohexene and cyclo-
energy- and labor-intensive purification processes can pentene derivatives (7a–j) can be prepared from
be avoided. Our recent efforts[6–8] have been focused simple precursors in an operationallysimple and effi-
on the design of such one-pot, multi-step reactions in- cient one-pot reaction. In this study, we disclose our
volving the palladium- and iridium-catalyzed genera- results on the synthetic scope of this process, discuss
tion of organoboronates. Organoboronates[9] are often the possibilities of the selective ring-closing metathe-
stabile compounds, however, manyfunctionalized allyl-
sis of intermediate dienes and trienes as well as fur-
boronates are unstable and cannot be isolated without ther reactions triggered by the allylation and cycliza-
considerable purification losses.[10–12] On the other tion processes.
hand, allylboronates are remarkably air- and moisture-
stable in solution, which allows their in situ application
in various multi-step one-pot reactions.[6–8,13–22]
Synthetic Scope
In previous studies, we have shown[6–8] that allylbor-
onates generated in situ from allylic alcohols (such as In a typical experiment (Figure 1), the boronation and
1a–f), acetates (such as 1g, h) and other precursors allylation reactions were performed in a single opera-
(such as 1i) bypalladium-catalyzed processes readily
tional step bymixing allyl precursor 1, aldehyde 2a,
react with aldehyde, acetal, ketone and imine sub- diboronate reagent 4, catalytic amounts (5 mol%) of
strates in a one-pot sequence.
pincer complex catalyst 3 and para-toluenesulfonic
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Nicklas Selander and Kµlmµn J. Szabó
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Figure 1. One-pot approach for synthesis of cycloalkenes (7) from allylic substrates (1) and aldehydes (2) using palladium
(3) and ruthenium (8) catalysts.
acid (p-TsOH) in a mixture of DMSO and methanol
(1:1). After the boronation reaction was completed
(508C/16–36 h), Hoveyda–Grubbs catalyst[23,24] 8 in di-
chloromethane was added to the reaction mixture of
the coupling reaction (without removal of the solvent)
and the metathesis reaction was furnished in 20 h.
Addition of 8 in the beginning of the one-pot process
led to inhibition of the RCM reaction, and therefore
unsatisfactorytermination ( 6!7) of the one-pot se-
quence. On the other hand, the sequential treatment
does not involve anyinconvenience and byusing it,
the interference between the catalyst actions can be
completelyavoided.
Figure 2. Selective formation of cyclohexenyl derivative 7c
via RCM of triene 6c.
Simple primary( 1a) and secondaryalcohols ( 1b)
reacted smoothly, providing syn-substituted hydroxy-
cyclohexene derivatives 7a and 7b as single stereoiso-
mers (entries 1 and 2). These reactions proceeded
with relativelyhigh overall iyeld (68%) considering
Using 2b as aldehyde precursor, another cyclic
the fact that the one-pot sequence involves at least diene (7d) can be prepared via triene 6d (entry4). In
three individual chemical processes. The reaction of this process, highly reactive acrylaldehyde was used as
dienyl alcohol 1c with 2a or 2b is particularlyinterest-
ing from a selectivitypoint of view. In the coupling
the coupling component. This aldehyde was generated
in situ from acetal 2b, which was added after comple-
reaction of 1c and 2a (entry3) the in situ formed in- tion of boronation of 1c. Acetal 2b was smoothlyhy-
termediate product 6c is a triene (Figure 2). This com- drolyzed[8] in the reaction mixture in the presence of
pound mayundergo RCM to either syn-substituted catalytic amounts of p-TsOH and water. Thus, the
cyclohexene derivative 7c or anti-substituted cyclo- overall reaction is a quadruple catalytic system, in
heptene derivative 9. However, the RCM reaction led which p-TsOH catalyzes two reactions: hydrolysis of
to exclusive formation of the six-membered ring prod- 2b and the boronation[12] of allylic alcohol 1c, while
uct 7c. It is well-known[25] that formation of six-mem- palladium pincer complex 3 catalyzes the boronation
bered rings is faster with RCM than formation of process[12] and ruthenium catalyst 8 the RCM reac-
seven-membered rings, however, the anti-sterochemis- tion. There is a theoretical possibilitythat triene 6d
try of the vinyl and hydroxy substituents (9) is expect- cyclizes to the cyclobutene derivative 10, however,
ed to be thermodynamically more stabilizing than the the expected product in this process is anti-substituted
syn interaction of the allyl and hydroxy groups in the cyclopentene derivative 7d, which was obtained in
six-membered ring product 7c. Apparently, the stereo- 63% overall yield as the only product (Figure 3). The
selectivityhas much less influence on the rate of the
RCM reaction than the ring size of the product.
above methods represent
(entry3) or a complementary (entry4) approach to
a
useful alternative
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Synthesis of Stereodefined Substituted Cycloalkenes
FULL PAPERS
Table 1. Coupling of allylic substrates with aldehyde 2a and
acetal 2b followed byring closing metathesis in a one-pot
sequence.[a]
Figure 3. One-pot, multi-step synthesis of cylopentene deriv-
ative 7d via RCM of triene 6d.
the carbometallation procedure of cyclic dienes re-
[26,27]
ported bySingleton and co-workers.
Carboxyfunctionalized allylic alcohol 1d (entry5)
undergoes boronation and coupling reaction to form
6e, which is followed byimmediate lactonization pro-
viding 6e’. The RCM process of 6e’ can be smoothly
performed affording bicyclic lactone 7e, which is a
useful building block in natural product synthesis.[28,29]
Together with the lactonization, this one-pot proce-
dure (1d!7e) involves four discrete reactions
(Figure 4).
Cyclohexene derivatives with quaternary carbons
are important synthetic intermediates in the prepara-
tion of steroids and related bioactive species.[30] Start-
ing from allylic alcohols 1e, f (entries 6 and 7), these
structural motifs can easilybe created bythe present-
ed one-pot reaction (Figure 1). The presence of a qua-
ternarycarbon does not affect the regiochemistryof
the allylation reaction, which affords the branched
product selectively. The RCM reaction also proceeds
smoothlyin the presence of the quaternarycarbon to
give dimethylcyclohexenol 7f and bicyclic spirane 7g.
Usually, the preferred substrates of the catalytic
boronation reactions are inexpensive allylic alco-
hols.[6,12] However, there are a few exceptions, when
the allylic alcohols are unstable under the applied re-
action conditions (entries 8 and 9) or when the requi-
site alcohols are more difficult to access than other
derivatives (entry10). Compound 1g (and the corre-
[a]
Unless otherwise stated, substrate 1 and 2a were dis-
solved in a DMSO/MeOH mixture in the presence of the
diboronic reagent 4a (1.2 equiv.), palladium catalyst 3a (5
mol%), p-TsOH (10 mol%) and water (8.0 equiv.). After
stirring at 508C for 16–36 h, a DCM solution of rutheni-
um catalyst 8 (5 mol%) was added and the solution was
refluxed for another 20 h.
Isolated yield of the overall process.
RCM was performed in only2–4 h.
Acetal 2b was added after 16 h.
The boronation reaction was conducted without addition
[b]
[c]
[d]
[e]
of p-TsOH and water.
This product was obtained after hydrolysis.
Figure 4. Synthesis of bicyclic lactone 7e in a one-pot se-
quence.
[f]
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Nicklas Selander and Kµlmµn J. Szabó
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Figure 5. Efficient synthesis of dihydroxycyclohexene derivative 7i from 1h and 2a.
sponding allylic alcohol) is highly unstable under precursor 6g because of free rotation of the carbon-
acidic conditions. Even in the presence of 5 mol% of carbon bonds. The final product 7i was isolated after
p-TsOH, 1g undergoes rapid Peterson elimination to acetate hydrolysis of 7i’.
give butadiene. Therefore, the boronation had to be
carried out under neutral conditions to obtain the cor- lylic alcohol and acetate precursors. For example, acti-
responding silylated allylboronate,[10] which readilyre-
vated vinylcyclopropane derivative 1i readilyunder-
The one-pot reactions do not necessarilyrequire al-
acted with 2a under one-pot conditions. Subsequent went the boronation, allylation followed by RCM re-
addition of catalytic amounts of 8 to the reaction mix- action providing 7j. The excellent selectivityof the
ture of the coupling reaction resulted in 7h. Because cooperative processes ensures that the bulkysulfone
of the mild, neutral reaction conditions of this one- substituent and the hydroxy group have a syn geome-
pot reaction of three discrete processes, the desilyla- tryin the single diastereomeric product ( 7j) of the re-
tion reaction could be completelyavoided.
action sequence.
Dihydroxycyclohexene derivative 7i is an important
intermediate in synthesis of carbosugars and sugar
mimics.[31–34] A straightforward stereoselective synthe- Mechanistic Aspects
sis of 7i can easilybe performed using the presented
one-pot procedure (entry9, Figure 5). The synthesis
The overall one-pot reaction is based on the coopera-
can be achieved using easilyavailable diacetate 1h, tive action of palladium-catalyzed borylation, subse-
which is first boronated and then coupled with 2a af- quent allylation and ruthenium-catalyzed metathesis
fording coupling product 6g. In 6g, the acetate func- reaction (Figure 6). The optimized conditions of the
tionality is located on the allylic hydroxy group. How- boronation reactions involve application of pincer
ever, after one-pot RCM reaction of 6g, in the cy- complex catalysts 3a or 3b, boronate source 4a or 4b
clized product (7i’) the acetate group has partiallymi-
in a mixture of DMSO and methanol.[6,10,12] The boro-
grated to the homoallylic hydroxy group. One possi- nation reaction is accelerated byapplication of p-
ble reason for the acetate migration can be the close TsOH (5 mol%),[12] however, the reaction can also be
vicinityof the syn hydroxy groups in 7i’, which is im- carried out under neutral conditions (e.g., entry8) [10]
posed bythe six-membered ring framework. This
close vicinityis probablynot realized in the acyclic
when diboronic acid 4b is used as boronate source.
On the other hand, when 4a is used as a boronate
Figure 6. Mechanistic overview of the one-pot sequence.
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Synthesis of Stereodefined Substituted Cycloalkenes
FULL PAPERS
olution mass data were obtained using the ESI technique.
The applied chemicals were obtained from commercial sour-
ces or synthesized according to literature procedures: 1d,[42]
1g,[43] 1i,[44] 3a[6] and 3b.[45]
source, addition of p-TsOH is required for the effi-
cient boronation process. As 4a is less expensive and
more accessible, than 4b, we used this boronate
source whenever it was possible together with catalyt-
ic amounts of p-TsOH.
The coupling reaction between allylboronate 5 and
aldehyde 2, providing 6 proceeds smoothlyunder the
reaction conditions of the boronation. The high dia-
stereoselectivityof the allylation arises from the com-
pact, highlyordered, six-membered ring TS of the
process.[35] The allylation process is probably also fa-
cilitated in the presence of catalytic amounts of a
Brønsted acid, such as p-TsOH.[36,37] Most importantly,
the metathesis reaction performed byaddition of 8 in
dichloromethane could be performed smoothlyin the
presence of the solvents (DMSO, MeOH), catalysts (3
General Procedure
Substrates 1 (0.15 mmol) and 2a (2.0 equiv.) were dissolved
in a DMSO/MeOH (0.2/0.2 mL) mixture followed byaddi-
tion of the diboronic reagent 4a (1.2 equiv.), palladium cata-
lyst 3a (5 mol%), p-toluenesulfonic acid (10 mol%) and
water (8.0 equiv.). The mixture was stirred at 508C for 20 h.
Subsequently, ruthenium catalyst 8 (5 mol%) in dichlorome-
thane (20 mL) was added, and this mixture was refluxed
under argon for 20 h. Thereafter, the reaction mixture was
quenched bywater and the phases separated. After evapora-
and p-TsOH) and waste products (water, boric acid tion of the organic phase, product 7 was purified bysilica
gel chromatography.
and its esters) of the boronation reaction. This is due
to the exceptional robustness of catalyst 8, which
allows performance of RCM reactions as the termi-
nating catalytic step of one-pot multi-step reactions.[38]
The overall one-pot procedure involves typically
three individual reactions (borylation, allylation and
metathesis) incorporating three catalytic events: pal-
ladium (3) and a sulfonic acid (p-TsOH) catalyzed
boronation and ruthenium (8) catalyzed RCM reac-
tion.
2-[(Benzyloxy)methyl]-3-cyclohexen-1-ol (7a): This com-
pound was prepared according to the above general proce-
dure from 1a and 2a, affording 7a in 68% yield. The NMR
data obtained for 7a are in agreement with the previously
1
reported[6] values. H NMR (CDCl3): d=7.32 (m, 5H), 5.78
(m, 1H), 5.43 (m, 1H), 4.55 (s, 2H), 4.15 (m, 1H), 3.62 (m,
2H), 2.99 (d, J=5.0 Hz, 1H), 2.65 (m, 1H), 2.21 (m, 1H),
2.05 (m, 1H), 1.79 (m, 2H); 13C NMR (CDCl3): d=137.8,
129.0, 128.5, 127.8, 127.7, 124.8, 73.4, 71.5, 68.1, 40.2, 27.9,
22.4;
HR-MS
(ESI):
m/z=241.1202,
calcd.
for
[C14H18O2+Na]+: 241.1199.
2-Pentyl-3-cyclohexen-1-ol (7b): This compound was pre-
pared according to the above general procedure from 1b
and 2a except that the borylation based allylation was con-
ducted for 36 h, affording 7b in 68% yield. The NMR data
obtained for 7b are in agreement with the previouslyreport-
ed[6] values. 1H NMR (CDCl3): d=5.69 (m, 1H), 5.47 (m,
1H), 4.00 (m, 1H), 2.17 (m, 2H), 2.05 (m, 1H), 1.89 (m,
1H), 1.68 (m, 1H), 1.54 (m, 1H), 1.35 (m, 8H), 0.89 (t, J=
6.6 Hz, 3H); 13C NMR (CDCl3): d=128.9, 126.5, 67.6, 40.1,
Conclusions
In this study, we have shown that allylic alcohols and
other allylic substrates can be transformed to stereo-
defined functionalized cyclohexene and cyclopentene
derivatives in a multicatalytic one-pot sequence. This
transformation is based on palladium pincer complex
based boronation of the allylic precursors, which is 32.1, 31.0, 28.5, 26.7, 22.6, 21.4, 14.1; HR-MS (ESI): m/z=
191.1405, calcd. for [C11H20O+Na]+: 191.1406.
2-Allyl-3-cyclohexen-1-ol (7c): This compound was pre-
pared according to the above general procedure from 1c
and 2a, except that the ring-closing metathesis was conduct-
followed byhighlydiastereoselective coupling with
unsaturated aldehydes and terminated by ring-closing
metathesis. The procedure is operationallysimple,
providing the corresponding products in a high overall
yield (63–82%) with excellent diastereo- and regiose-
lectivity. The presented method has a broad synthetic
scope including functionalized allylic alcohols, ace-
tates and vinylcyclopropane substrates. The overall
process mayinvolve up to four individual reactions
ed for 2 h, affording 7c in 78% yield. The NMR data ob-
tained for 7c are in agreement with the previouslyreport-
1
ed[26] values. H NMR (CDCl3): d=5.88 (dddd, J=7.0, 7.0,
10.3, 17.0 Hz, 1H), 5.72 (m, 1H), 5.47 (m, 1H), 5.11 (d, J=
17.0 Hz, 1H), 5.06 (d, J=10.3 Hz, 1H), 4.01 (m, 1H), 2.17
(m, 5H), 1.79 (m, 2H), 1.51 (br, 1H); 13C NMR (CDCl3):
(entries 4 and 5), in which three or four catalytic d=137.0, 128.0, 126.9, 116.3, 67.6, 40.0, 35.6, 28.4, 21.3; HR-
MS (ESI): m/z=161.0937, calcd. for [C9H14O+Na]+:
161.0929.
5-Vinyl-2-cyclopenten-1-ol (7d): This compound was pre-
pared according to the above general procedure from 1c
and 2b, except that 2b was added after 16 h. The mixture
was thereafter stirred for another 6 h. The ring-closing meta-
events take place. The products are useful building
blocks in advanced organic synthesis and natural
product synthesis.[26–34,39–41]
Experimental Section
thesis was conducted for 4 h, affording 7d in 63% yield.
1H NMR (CDCl3): d=5.88 (m, 1H), 5.86 (ddd, J=7.9, 10.3,
1
The H NMR and 13C NMR spectra were recorded in CDCl3
(internal standard: 7.26 ppm, 1H; 77.0 ppm, 13C) at room
temperature using a 400 MHz NMR spectrometer. High res-
17.3 Hz, 1H), 5.76 (m, 1H), 5.11 (d, J=17.3 Hz, 1H), 5.02
(d, J=10.3 Hz, 1H), 4.59 (m, 1H), 2.62 (m, 2H), 2.15 (m,
1H), 2.03 (br, 1H); 13C NMR (CDCl3): d=140.2, 133.1,
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Nicklas Selander and Kµlmµn J. Szabó
FULL PAPERS
133.0, 114.7, 82.8, 52.9, 37.3; HR-MS (ESI): m/z=133.0620,
calcd. for [C7H10O+Na]+: 133.0624.
7-OxabicycloACHTREUNG[4.3.0]-2-nonen-8-one (7e): This compound
2-[2,2-Di(phenylsulfonyl)ethyl]-3-cyclohexen-1-ol
(7j):
This compound was prepared according to the above gener-
al procedure from 1i and 2a, except that instead of 3a, cata-
lyst 3b was used, affording 7j in 80% yield. 1H NMR
(CDCl3): d=7.96 (m, 4H), 7.70 (m, 2H), 7.58 (m, 4H), 5.68
(m, 1H), 5.33 (m, 1H), 4.98 (dd, J=3.6, 7.3 Hz, 1H), 4.02
(m, 1H), 2.63 (m, 1H), 2.43 (m, 1H), 2.09 (m, 3H), 1.95 (d,
J=6.0 Hz, 1H), 1.70 (m, 2H); 13C NMR (CDCl3): d=137.8,
137.6, 134.6, 134.5, 129.7, 129.6, 129.1, 129.0, 128.1, 126.8,
81.5, 67.8, 38.0, 27.3, 26.6, 22.6; HR-MS (ESI): m/z=
429.0798, calcd. for [C20H22O5S2 +Na]+: 429.0801.
was prepared according to the above general procedure
from 1d and 2a, except that instead of 3a, catalyst 3b was
used, affording 7e in 68% yield. The NMR data obtained
[28]
for 7e are in agreement with the previouslyreported
1
values. H NMR (CDCl3): d=5.88 (m, 1H), 5.47 (m, 1H),
4.76 (ddd, J=2.7, 5.6, 5.6 Hz, 1H), 3.00 (m, 1H), 2.76 (m,
1H), 2.30 (m, 1H), 2.10 (m, 3H), 1.78 (m, 1H); 13C NMR
(CDCl3): d=176.8, 128.8, 125.6, 78.1, 35.8, 34.5, 24.6, 19.2;
HR-MS (ESI): m/z=161.0574, calcd. for [C8H10O2+Na]+:
161.0573.
2,2-Dimethyl-3-cyclohexen-1-ol (7f): This compound was Acknowledgements
prepared according to the above general procedure from 1e
and 2a, affording 7f in 82% yield. 1H NMR (CDCl3): d=
5.51 (m, 1H), 5.36 (m, 1H), 3.57 (m, 1H), 2.10 (m, 2H),
1.73 (m, 2H), 1.44 (br, 1H), 1.05 (s, 3H), 0.98 (s, 3H);
13C NMR (CDCl3): d=136.2, 124.0, 75.2, 36.7, 28.1, 26.9,
23.8, 22.7; HR-MS (ESI): m/z=149.0936, calcd. for
[C8H14O+Na]+: 149.0937.
This work was mainly supported by the Swedish Natural Sci-
ence Research Council (VR). The authors are indebted to the
Alice and Knut Wallenberg Foundation for funding a UPLC
instrument.
SpiroACHTREUNG[5.5]undec-4-en-1-ol (7g): This compound was pre-
References
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1
2a, affording 7g in 72% yield. H NMR (CDCl3): d=5.71 (d,
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(m, 1H), 2.11 (m, 2H), 1.79 (m, 2H), 1.43 (m, 11H);
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26.2, 25.4, 22.3, 21.6, 21.3; HR-MS (ESI): m/z=189.1251,
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2-[(1,1,1-Trimethylsilyl)methyl]-3-cyclohexen-1-ol
(7h):
This compound was prepared according to the above gener-
al procedure from 1g and 2a, except that 3b was used as cat-
alyst and 4b was employed as boronate source. In addition,
this reaction was performed in the absence of p-TsOH and
added water conducting the boronation step for 16 h, afford-
1
ing 7h in 63% yield. H NMR (CDCl3): d=5.62 (m, 1H),
5.47 (m, 1H), 3.86 (m, 1H), 2.37 (m, 1H), 2.10 (m, 2H),
1.76 (m, 2H), 1.51 (br, 1H), 0.66 (m, 2H), 0.05 (s, 9H);
13C NMR (CDCl3): d=130.6, 125.9, 69.6, 36.7, 27.8, 21.9,
17.8, ꢀ0.7; HR-MS (ESI): m/z=207.1176, calcd. for
[C10H20OSi+Na]+: 207.1176.
[11] S. Sebelius, V. J. Olsson, O. A. Wallner, K. J. Szabó, J.
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3-Cyclohexen-1,2-diol (7i): This compound was prepared
bya modified version of the above general procedure. Thus,
1h (0.15 mmol) was stirred with diboronic reagent 4b
(1.2 equiv.), molecular sieves (4 Š, 15 mg) and palladium
catalyst 3b (5 mol%), in DMSO (0.4 mL) for 36 h at
408C.[10] Thereafter, aldehyde 2a was added and the stirring
continued for another 24 h. To this mixture ruthenium cata-
lyst 8 (5 mol%) in dichloromethane (20 mL) was added and
refluxed under argon for 20 h. Thereafter, the reaction mix-
ture was quenched bywater and the phases separated. After
evaporation of the organic phase 7i’ was purified bysilica
gel chromatography. After hydrolysis of 7i’ (by20 mol%
Na2CO3 in MeOH at room temperature for 4 h followed by
filtration through a plug of silica), diol 7i was obtained in
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d=5.89 (m, 1H), 5.74 (m, 1H), 4.14 (m, 1H), 3.83 (m, 1H),
2.28 (br, 2H), 2.14 (m, 2H), 1.76 (m, 2H); 13C NMR
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