Palladium-catalyzed O-allylation of α-hydroxy carbonyl compounds

Article abstract of DOI:10.1002/adsc.200505361

α-Hydroxy carbonyl compounds undergo smooth O-allylation using allylic carbonates and Pd(0) catalysts. This method has significant advantages over other O-allylation methods as it provides a solution to several problems previously observed for this synthe

Full text of DOI:10.1002/adsc.200505361

FULL PAPERS
DOI: 10.1002/adsc.200505361
Palladium-Catalyzed O-Allylation of a-Hydroxy Carbonyl
Compounds
Bernd Schmidt,a, b,* Stefan Navea
a
Universität Dortmund, Fachbereich Chemie, Organische Chemie, Otto-Hahn-Strasse 6, D-44227 Dortmund, Germany
Fax: (þ49)-231-755-5363, e-mail: bernd.schmidt@uni-dortmund.de
b
New address: Universität Potsdam, Institut für Chemie, Organische Chemie II, Karl-Liebknecht-Strasse 24–25,
14476 Golm, Germany
E-mail: berschmi@rz.uni-potsdam.de
Received: September 15, 2005; Accepted: December 16, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de or from the author.
Abstract: a-Hydroxy carbonyl compounds undergo Keywords: alcohols; allylation; allylic compounds;
smooth O-allylation using allylic carbonates and homogeneous catalysis; palladium
Pd(0) catalysts. This method has significant advantag-
es over other O-allylation methods as it provides a
solution to several problems previously observed for
this synthetic transformation.
Introduction
Although apparently trivial, the first step of this se-
quence may cause significant problems. For instance, al-
Over the past few years we have demonstrated that a- lylation of the corresponding sodium alkoxides with all-
hydroxy carbonyl compounds[1] are versatile starting yl bromide or iodide requires strongly basic conditions
materials for the olefin metathesis-based synthesis of and leads to immediate racemization if, e.g., (S)-ethyl
functionalized oxacycles.[2–7] Key features of our strat- lactate is employed in the reaction.[2,20] In the case of
egy are i) a selective O-allylation of the corresponding mandelates not only racemization occurs, but a subse-
a-hydroxy carbonyl compound 1, ii) the diastereoselec- quent Wittig rearrangement of the primary O-allylation
tive addition of vinyl- or allylmetal compounds to the re- product is observed which gives a carbinol in prepara-
sulting O-allyl ethers 2, iii) the Ru-catalyzed ring closing tively useful yield (Scheme 2).[4] The same phenomenon
metathesis of dienes 3,[8,9] which yields dihydropyrans was observed for a-hydroxy ketones such as benzoin.[7]
(n¼0) or oxepins (n¼1) 4, respectively (Scheme 1). Racemization and undesired Wittig rearrangements
Oxacycles 4 have great potential in the de novo synthesis can be avoided by using trichloroacetimidates as allylat-
of carbohydrates or related compounds,[10,11] which has ing agent[21] or allyl bromide in the presence of excess sil-
been demonstrated by us[5,12–14] and others.[15–19]
ver oxide.[22,23] We have successfully used the latter
method for a-hydroxy esters.[2,3,5] However, application
of these conditions to aromatic a-hydroxy ketones[24]
turned out to give less satisfactory results, because these
ꢀ
substrates partially undergo an oxidative C C bond
cleavage under these conditions, resulting in the forma-
tion of allyl benzoate (8) as a by-product. For 3-hydroxy-
propiophenone (6) and substituted derivatives this side
reaction becomes a serious limitation, as approximately
50% of the starting material are converted via this path-
way (Scheme 2).[7]
This observation, together with the economical (over-
stoichiometric amounts of expensive Ag2O are re-
quired) and ecological (generation of heavy metal-con-
taining waste in large quantities) drawbacks of this
Scheme 1. Synthesis of functionalized oxacycles from a-hy-
droxy carbonyl compounds based on olefin metathesis.
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Bernd Schmidt, Stefan Nave
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on the efficiency of the reaction.[40] For instance, quanti-
tative allylation of a secondary alcohol can be achieved
with only 1.5 equivalents of allyl tert-butyl carbonate
(9c), while more than 6.9 equivalents of the correspond-
ing methyl derivative (9b) are required. As allyl ethyl
carbonate (9a)[43] is more conveniently available than
the corresponding tert-butyl derivative 9c, we wanted
to use this allylating agent for preparative purposes
whenever possible, although it had to be assumed that
the reactivity of 9a is similar to the reactivity of allyl
methyl carbonate (9b).
We started our investigation with a comparative study
of various palladium catalysts. Under standardized con-
ditions, benzoin (10f) and two equivalents of allyl ethyl
Scheme 2. Side reactions of established allylation procedures.
method led us to evaluate the potential ofa Pd-catalyzed carbonate (9a) were heated in refluxing THFin the pres-
allylation process for this transformation. The Pd-cata- ence of 2.5 mol % Pd precatalyst. In the case of dimeric
lyzed allylation has been widely applied to C-nucleo- precatalysts, 1.25 mol % were used. After one hour, the
ꢀ
philes and is one of the most important C C bond-form- reaction was stopped and the crude reaction mixture
ing reactions.[25] In this light it is somewhat surprising was analyzed by NMR-spectroscopy (Scheme 3).
that many examples describing the Pd-catalyzed allyla-
As can be seen from the results summarized in Ta-
tion of O-nucleophiles are limited to carboxylic acids ble 1, less than 5% conversion is obtained with the cata-
and their salts,[26] phenols,[27–29] metal alkoxides[30–32] lyst/ligand combination PdCl2/PPh3. The two dimeric
and intramolecular Pd-catalyzed decarboxylation of al- precatalysts Pd2(dba)3 ·CHCl3 and [Pd(h3-C3H5)Cl]2
lyloxy carbonates.[33–35] Tsujiꢁs statement that “alcohols give, in combination with PPh3 as a ligand, better results,
are poor O-nucleophiles, and thePd-catalyzed allylation however, in the latter case unidentified by-products are
of alcohols to form allyl alkyl ethers is somewhat slug- observed in small amounts. It has previously been re-
gish”[36] might explain why this reaction has not been ported by Zumpe and Kazmaier that this precatalyst
as thoroughly explored for aliphatic alcohols.[37–42] In can be used for the selective O-allylation of Boc-protect-
this contribution conditions are described that allow ed serine methyl ester.[38] The modest activity of [Pd(h3-
the facile and selective allylation of several hydroxy car- C3H5)Cl]2 in the allylation of 10f might be explained by
bonyl compounds using Pd catalysis, as well as some oth- the fact that a sterically more hindered secondary rather
er substrates that have been found to be problematic un- than a primary alcohol is present. The best results for the
der established conditions.
allylation of 10f were obtained with Pd(OAc)2 in combi-
nation with PPh3, or with [Pd(PPh3)4] without additional
ligand. In both cases, approximately 80% of 10f were
converted to 11f after one hour, without noticeable for-
mation of by-products.
Results and Discussion
In their pioneering study, Sinou et al. described the use
of allyl ethyl carbonate (9a) and a Pd(0) catalyst for
the allylation of carbohydrates.[37] One important result
of this work is that the allylation of anomeric hydroxy
groups is much more facile, which has been attributed
to their higher acidity. More recently, Haight et al. dis-
covered, for primary, secondary and tertiary alcohols
bearing no further functional groups, that the leaving
Scheme 3. Pd-catalyzed allylation of benzoin with allyl ethyl
group of the allyl carbonate exerts a strong influence carbonate.
Table 1. Evaluation of precatalysts for the allylation of benzoin (10f) with allyl ethyl carbonate (9a) (Scheme 3).
Entry
Precatalyst [mol %]
Ligand [mol %]
Ratio 11f:10f[a]
1
2
3
PdCl2 (2.5)
Pd(OAc)2 (2.5)
Pd2(dba)3 ·CHCl3 (1.25)
h3-C3H5)Cl]2 (1.25)
[Pd(PPh3)4] (2.5)
PPh3 (12.5)
PPh3 (12.5)
PPh3 (3.1)
PPh3 (3.1)
–
1: 20
4: 1
1: 2
1: 1.2
4: 1
4[Pd(
5
[a]
Ratio was analyzed by 1H NMR spectroscopy of the crude reaction mixture after one hour of reaction time (for details, see
Experimental Section).
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Table 2. Pd-catalyzed allylation of a-hydroxy carbonyl com-
With these results in hand, we started to evaluate the
substrate scope of this allylation procedure for several
other a-hydroxy carbonyl compounds. Application of
the conditions found for the allylation of benzoin to
enantiomerically pure S-ethyl lactate (10a) and S-meth-
yl mandelate (10b) gave the allyl ethers S-11a and S-11b,
respectively, in enantiomerically pure form. Confirma-
tion of the stereochemical integrity of this allylation pro-
tocol was achieved for 10a by comparison of the [a]D val-
ues with data from the literature.[44] Allyl ether 11b had
previously only been described in racemic form, which
made it necessary to determine the enantiomeric excess
of S-11b using HPLC on a chiral stationary phase. After
comparison with racemic 11b, the ee was determined to
be >98%, indicating that no racemization occurred. Ex-
amples 11c, d, e illustrate that the rapid assembly of sub-
strates for ring closing metathesis reactions is possible
using the Pd-catalyzed allylation reaction. Precursors
10c[45] and 10e[46] have been prepared in one step by ad-
dition of allyl- or allenylmetal compounds to ethyl
glyoxalate. Ester 10d is available in one step from di-
methyl malate via a highly diastereoselective enolate al-
lylation, using a method described by Seebach et al.[47]
The ratio of diastereomers observed for 10d is not al-
tered during the O-allylation. The relative configuration
assigned to 11d was confirmed by conversion of this di-
pounds.[a]
ene to oxepine 12 using a ring closing metathesis reac-
[48]
¼
tion catalyzed by [RuCl2(PCy3)2 CHPh]. Indicative
for the cis-configuration of both ester substituents is a
small value for the coupling constant JH-2,H-3 of 3.6 Hz
3
(Scheme 4).
[a]
Compounds 11f and 11g are examples for the allyla-
tion of aromatic a-hydroxy ketones. Ketone 10g was ob-
tained from cinnamyl acetate together with the regioiso-
meric by-product using Plietkerꢁs ketohydroxylation
Conditions: 9a (2.0 equivs.), THF, Pd(PPh3)4 (2.5 mol %),
658C.
protocol.[49] Removal of the by-product was not possible problems with established allylation protocols. Results
at this stage, but was achieved after allylation by careful are summarized in Table 3.
column chromatography. This might explain why 11g
Compound 10h, for instance, undergoes a retro-aldol
was obtained in only mediocre yield. Neither 10f nor reaction upon attempted deprotonation of the OH
ꢀ
10g undergo any C C bond cleavage or other undesired group with NaH. On the other hand, no conversion is ob-
side reactions during the allylation process, which is, as served with the silver oxide/allyl bromide method. Un-
discussed in the introduction, a serious problem if the sil- der the conditions described here, allyl ether 11h is ob-
ver oxide-allyl bromide method is applied to these sub- tained in good yield. The same observations as for 10h
strates. The results obtained for the allylation of a-hy- have also been made for diol 10i: decomposition occurs
droxy carbonyl compounds are summarized in Table 2.
under basic conditions, whereas silver oxide/allyl bro-
With these results in hand, we evaluated the Pd-cata- mide has no effect at all. Using Pd-catalysis, selective
lyzed allylation of other substrates that have caused mono-allylation to 11i was observed in excellent yield.
Triene 11j was required for a parallel study dealing
with selectivity aspects of RCM reactions. We suspected
that conversion of 10j into the sodium alkoxide under
strongly basic conditions might lead to inter- or intramo-
lecular scrambling of the silyl group, an effect that has
previously been observed by us in other cases.[50] Under
the conditions described here, 11j was obtained exclu-
sively in good yield. However, allylation of epoxide
10k (obtained as a single diastereomer from 10j under
the conditions of a Sharpless epoxidation) results in
Scheme 4. RCM of diene 11d to oxepine 13.
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Bernd Schmidt, Stefan Nave
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Table 3. Pd-catalyzed allylation of other secondary alco-
hols.[a]
Scheme 6. V-catalyzed epoxidation of 14.
visible. Pd-catalyzed allylic substitutions involving silyl
ethers as pronuclecophiles have precedence in the liter-
ature.[51,52] However, such reactions normally involve the
presence of nucleophiles such as fluorides or chlorides
which induce desilylation, leading to an alkoxide which
is the actual nucleophile in the allylation reaction. In the
present case, ethoxide liberated from the allyl carbonate
might be the nucleophile responsible for desilylation.
Alternatively, ethoxide might act as a base which de-
protonates the secondary alcohol in 10k and induces in-
tramolecular transfer of the silyl group. Although it is
likely that other silyl ethers show a lower tendency to-
wards migration, we did not investigate this issue further
but chose a benzyl ether as OH protecting group. The re-
quired precursor 10l was prepared from the mono-ben-
zyl ether of 1,5-hexadiene-3,4-diol (14)[53] using a vana-
dium-catalyzed epoxidation. Under these conditions,
[a]
Conditions: 9a (2.0 equivs.), THF, Pd(PPh3)4 (2.5 mol %),
658C.
the formation of two isomers 11k and 11k’ in a 2:1 ratio. 10l was obtained as a 2:1 mixture of diastereomers
It was not possible to decide on the basis of routine (Scheme 6).
NMR spectra whether 11k and 11k’ are diastereomers
It has previously been reported that 10l can be ob-
or regioisomers, resulting from a scrambling of the tained from 14 in diastereomerically pure form under
TBS group prior to allylation. To answer this question, Sharpless conditions.[54,55] Submitting 10l as a 2:1 mix-
the 2:1 mixture of 11k and 11k’ was subjected to ring ture of diastereomers to the conditions of the Pd-cata-
closing metathesis. The products were identified as dihy- lyzed allylation gives, as expected, the allyl ether 11l in
dropyran 13k and dihydrofuran 13k’, which leads to the good yield and an unaltered ratio of diastereomers.
assignment of the structures depicted in Table 3 and
Scheme 5 to 11k and 11k’.
This result suggests that scrambling of the silyl group Conclusion
also occurs during the allylation of 10j, however, due
to the C2-symmetry of the parent diol this effect is not The results presented in this contribution clearly dem-
onstrate that Pd-catalyzed O-allylation has a strong po-
tential beyond the more acidic O-nucleophiles such as
carboxylates and phenols. We have applied this reaction
successfully to a variety of a-hydroxy carbonyl com-
pounds, which normally require expensive reagents in
over-stoichiometric amounts. An extension to some oth-
er substrates that are difficult to allylate under establish-
ed conditions is also described. Application of this meth-
od to the solution of synthetic problems associated with
the metathesis-based synthesis of oxacycles is currently
underway in our laboratory.
Scheme 5. RCM of the mixture of 11k and 11k’.
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O-Allylation of a-Hydroxy Carbonyl Compounds
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ue reported in the literature: [a]2D5: ꢀ77.78 (c 2.21, MeOH);
ref.[44] [a]2D5:ꢀ70.78 (c 2.68, MeOH).
Experimental Section
S-(þ)-O-Allyl methyl mandelate (11b): Obtained from S-
(þ)-methyl mandelate (10b) (415 mg, 2.5 mmol) as a colorless
liquid; yield: 430 mg (83%). Spectroscopic data are identical to
those reported in the literature.[3] [a]D25: þ95.38 (c 1.59,
MeOH). The enantiomeric excess of 11b was determined by
HPLC analysis (Daicel Chiralcel OD, eluent: heptane/2-prop-
anol, 98:2, flow 0.7 mL/min, 208C) to be>98% after compar-
ison with racemic 11b.
General Remarks
Experiments were conducted in dry reaction vessels under an
atmosphere of dry argon. Solvents were purified by standard
procedures. 1H NMR spectra were recorded on a Bruker
Avance DRX 400, Bruker Avance DRX 500, or Varian Inova
600 in CDCl3 or benzene-d6. Chemical shifts (d) are reported
in ppm relative to TMS with CHCl3 (dH ¼7.24ppm, dC ¼
77.0 ppm) or C6D5H (dH ¼7.18 ppm, dC ¼128.0 ppm) as inter-
nal standard. Coupling constants (J) are given in Hertz. In
13C NMR spectra the number of coupled protons was analyzed
by APT or DEPT experiments and is denoted by a number in
parentheses following the dC value. IR spectra were recorded
as films on NaCl or KBr plates. The peak intensities are defined
as strong (s), medium (m) or weak (w). Mass spectra were ob-
tained at 70 eV. Optical purities were determined by HPLC us-
ing a HP-LC-1050 system equipped with a Daicel Chiralcel OD
column. The following compounds were prepared according to
procedures described in the literature: 10c,[45] 10d,[47] 10e,[46]
10g,[49] 10h,[56] 10i,[3] 10j,[57] Pd2(dba)3 ·CHCl3,[58] [Pd(h3-
C3H5)Cl]2,[59] and [Pd(PPh3)4].[58]
Rac-2-Allyloxypent-4-enoic acid ethyl ester (11c): Ob-
tained from 10c[45] (144 mg, 1.0 mmol) as a colorless liquid;
yield: 158 mg (84%).
(2R,3S)-2-Allyl-3-allyloxysuccinic acid dimethyl ester
(11d): Obtained from 10d[47] (101 mg, 0.5 mmol) as a colorless
liquid; yield: 120 mg (99%).
Rac-2-Allyloxypent-4-ynoic acid ethyl ester (11e): Ob-
tained from 10e[46] (3.43 g, 24.1 mmol) as a colorless liquid;
yield: 3.91 g (89%).
rac-Benzoin allyl ether (11f): Obtained from rac-benzoin
(10f) (424 mg, 2.0 mmol) as a colorless liquid; yield: 444 mg
(88%). Spectroscopic data are identical to those reported in
the literature.[7]
rac-Acetic acid 2-allyloxy-3-oxo-3-phenyl-propyl ester
(11g): Obtained from 10g[49] (104mg, 0.5 mmol) as a colorless
liquid; yield: 65 mg (52%).
Rac-3-Allyloxypent-4-enoic acid ethyl ester (11h): Ob-
tained from 10h[56] (72 mg, 0.50 mmol) as a colorless liquid;
yield: 70 mg (76%).
3-(Allyloxyphenylmethyl)-penta-1,4-dien-3-ol (11i): Ob-
tained from 10i[3] (380 mg, 2.0 mmol) as acolorless liquid; yield:
430 mg (93%). Spectroscopic data are identical to those report-
ed in the literature.[3]
(3R,4R)-3-Allyloxy-4-(tert-butyldimethylsilanyloxy)-1,5-
hexadiene (11j): Obtained from 10j[57] (228 mg, 1 mmol) as a
colorless liquid; yield: 200 mg (74%).
Screening Experiments for Different Pd-Precatalysts
Compound 10f (0.2 mmol) and carbonate 9a (0.4mmol) were
dissolved in THF (2.0 mL). The appropriate Pd-precatalyst
and the ligand were added, and all samples were simultaneous-
ly heated in closed vials in a stainless steel heating block for one
hour. After this time, the reactions were quenched by filtration
through a short pad of silica, followed by elution with ether. Af-
ter evaporation of all volatiles, the ratio 11f:10f was determined
from the 1H NMR-spectrum by comparison of the integrals of
¼
the ortho protons of the C( O)C6H5 moiety, which are base-
line-separated at 500 MHz. Alternatively, the integral of the
singlet observed for the proton CH(Oallyl) of 11f can be com-
pared with the integral of the OH signal of 10f. In both cases
comparable values for the 11f:10f ratio are obtained. GC anal-
ysis turned out to be unreliable due to partial decomposition of
pure 11f under GC conditions.
(2R,3R,4R)-1,2-Epoxy-4-(tert-
Butyldimethylsilanyloxy)-hex-5-en-3-ol (10k)
The title compound was prepared in analogy to aliterature pro-
cedure:[60] To a solution of Ti(O-i-Pr)4 (2.81 mL, 9.5 mmol) in
DCM (45 mL) in a Schlenk tube was added l-(þ)-diethyl tar-
trate (1.89 mL, 11.0 mmol) at ꢀ308C. After 15 min of stirring,
a solution of 10j[57] (1.80 g, 7.9 mmol) in DCM (5 mL), followed
by a solution of tert-butyl hydroperoxide in toluene (2.85 M,
14.2 mmol, 5.0 mL) was added. The reaction mixture was
then stored in a freezer at ꢀ308C for 15 days, after which an-
other 10.0 mL of tert-butyl hydroperoxide in toluene (2.85 M,
28.4mmol) were added. After 3 more days at ꢀ308C, FeSO4
(5.0 g) was dissolved in an aqueous solution of tartaric acid
(15 wt %, 47 mL) and added to the reaction mixture. The mix-
ture was allowed to warm to room temperature and filtered
through a pad of celite, extracted twice with dichloromethane,
and washed with brine. After drying over MgSO4, filtration,
evaporation of the solvents and flash chromatography (silica,
cyclohexane:MTBE 5:1) 10k (yield: 1.38 g, 72%) was obtained
as a colorless liquid in diastereomerically pure form (NMR),
along with unreacted starting material (135 mg, 8%).
General Procedure for the Pd-Catalyzed Allylation
In a flame-dried Schlenk tube, the alcohol (1 mmol) was dis-
solved in 5 mL of dry THF. In a second Schlenk tube, a solution
of allyl ethyl carbonate (260 mg, 2.0 mmol) and Pd(PPh3)4
(28.8 mg, 2.5 mol %) in dry THF (5 mL) was prepared and add-
ed to the substrate solution via a Teflon cannula. The reaction
mixture was heated to reflux until TLC indicated complete
consumption of the starting material (approximately 3 to 4
hours). The reaction mixture was allowed to cool to ambient
temperature and then filtered through a short pad of silica, fol-
lowed by washing with MTBE. After evaporation of the sol-
vents, the crude product was purified by flash chromatography.
S-(ꢀ)-O-Allyl ethyl lactate (11a): Obtained from S-(ꢀ)-eth-
yl lactate (10a) (591 mg, 5 mmol) as a colorless liquid; yield:
681 mg (86%). Spectroscopic data are identical to those report-
ed in the literature.[3] Optical rotation compares well to the val-
Alternatively, to a solution of 10j[57] (460 mg, 2.0 mmol) in
toluene (10 mL) was added vanadyl acetylacetonate (11 mg,
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Bernd Schmidt, Stefan Nave
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2 mol %) and tert-butyl hydroperoxide (0.45 mL, 5.5 M in dec-
ane, 2.5 mmol). After heating the mixture to reflux for 3 hours,
additional tert-butyl hydroperoxide (0.2 mL, 5.5 M in decane,
1.1 mmol) was added and heating was continued for 0.5 hours.
After cooling to ambient temperature, 10 mL of water were
added, and the mixture was extracted three times with ethyl
acetate. The combined organic phases were washed with satu-
rated Na2CO3 solution, dried over MgSO4, filtered and the sol-
vents removed under vacuum. After column chromatography,
10k was obtained as a colorless oil as a 4:1 ratio of diastereom-
ers (NMR); yield: 250 mg (52%).
mixture was stirred at ambient temperature until the starting
material was fully consumed (2.5 h), as indicated by TLC.
The reaction mixture was filtered over a short pad of silica
and washed with ether. The solvent was evaporated, and the
residue was purified by flash chromatography on silica using
cyclohexane/MTBE (5:1) as eluent; yield: 353 mg (82%).
Acknowledgements
Generous financial support by the Deutsche Forschungsge-
meinschaft and the Chemistry department of the University of
´
Dortmund is gratefully acknowledged. We thank Nina Mes-
zaros and Christa Nettelbeck for excellent technical assistance.
Attempted Synthesis of [1-(Allyloxyoxiranylmethyl)-
allyloxy]-tert-butyl(dimethyl)silane (11k)
Following the general procedure, 10k (489 mg, 2.0 mmol) was
converted to an inseparable 2:1 mixture of the title compound
11k and its isomer 11k’; combined yield: 547 mg (96%).
Characteristic NMR data for major isomer 11k: 13C NMR
(CDCl3, 125 MHz): d¼79.5 (1), 74.2 (1), 72.5 (2), 51.0 (1),
44.6 (2).
References
[1] G. M. Coppola, H. F. Schuster, a-Hydroxy acids in enan-
tioselective synthesis, Wiley-VCH, Weinheim, 1997.
[2] B. Schmidt, H. Wildemann, Synlett 1999, 1591–1593.
[3] B. Schmidt, H. Wildemann, J. Org. Chem. 2000, 65,
5817–5822.
[4] B. Schmidt, H. Wildemann, J. Chem. Soc. Perkin Trans. 1
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[5] B. Schmidt, H. Wildemann, Eur. J. Org. Chem. 2000,
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Characteristic NMR-data for major isomer 11k’: 13C NMR
(CDCl3, 125 MHz): d¼82.5 (1), 72.3 (1), 69.8 (2), 51.8 (1),
44.0 (2).
Subjecting the mixture to the conditions of ring closing
metathesis reaction gives a mixture of dihydropyran 13k and
dihydrofuran 13k’: To a solution of the inseparable mixture
of 11k and 11k’ (284mg, 1.0 mmol) in CH 2Cl2 was added
[RuCl2(PCy3)2 CHPh][48] (25 mg, 3.0 mol %). After complete
¼
conversion of the starting materials, the solvent was removed
under vacuum to give 13k and 13k’ as an inseparable mixture
which was analyzed by NMR spectroscopy without further pu-
rification. Mass of crude product: 260 mg, approximately
100%.
[10] A. Kirschning, M. Jesberger, K.-U. Schçning, Synthesis
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[11] R. Roy, S. K. Das, Chem. Commun. 2000, 519–529.
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(2R,3R,4R)-1,2-Epoxy-4-(benzyloxy)-hex-5-en-3-ol
[(2R,3R,4R)-10 l] and (2S,3R,4R)-1,2-Epoxy-4-
(benzyloxy)-hex-5-en-3-ol [(2S,3R,4R)-10l][54]
To a solution of 14[53] (1021 mg, 5.0 mmol) in toluene (25 mL)
was added vanadyl acetylacetonate (53 mg, 4mol %) and
tert-butyl hydroperoxide (2.6 mL, 2.85 M in toluene,
7.5 mmol). The mixture was heated to reflux for 2 hours. After
cooling to ambient temperature, 10 mL of water were added,
and the mixture was extracted three times with ethyl acetate.
The combined organic phases were washed with saturated
Na2CO3 solution, dried over MgSO4, filtered and the solvents
removed under vacuum. After column chromatography, 10l
was obtained as a colorless oil in a 2:1 ratio of diastereomers
(NMR); yield: 665 mg (60%).
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¼
536
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