Regiospecific synthesis of tetra-substituted furans

Article abstract of DOI:10.1080/00397911.2012.748076

α,β-Unsaturated acetylenic γ-hydroxyketones have been shown to react with ethyl acetoacetate in a Michael-addition fashion and subsequently undergo cyclization followed by dehydration to give substituted furans with a predictable regiospecificity. The yields were good to excellent. A mechanism for the transformation is proposed, and this mechanism explains why furan formation does not take place when the same unsaturated ketones are treated with α-methylated acetoacetate and diethyl malonate.

Full text of DOI:10.1080/00397911.2012.748076

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Regiospecific Synthesis of Tetra-
Substituted Furans
Leiv K. Sydnes a , Rustem Isanov a b , Myagmarsuren Sengee a
Francesco Livi c
&
a Department of Chemistry , University of Bergen , Bergen , Norway
b Department of Organic Chemistry , Kazan State Technological
University , Kazan , Russia
c Department of Organic Chemistry “A. Mangini,” , University of
Bologna , Bologna , Italy
Accepted author version posted online: 31 May 2013.Published
online: 25 Jul 2013.
To cite this article: Leiv K. Sydnes , Rustem Isanov , Myagmarsuren Sengee & Francesco Livi (2013)
Regiospecific Synthesis of Tetra-Substituted Furans, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 43:21, 2898-2905, DOI:
10.1080/00397911.2012.748076
To link to this article: http://dx.doi.org/10.1080/00397911.2012.748076
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Synthetic Communications1, 43: 2898–2905, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.748076
REGIOSPECIFIC SYNTHESIS OF TETRA-SUBSTITUTED
FURANS
Leiv K. Sydnes,1 Rustem Isanov,1,2 Myagmarsuren Sengee,1
and Francesco Livi3
1Department of Chemistry, University of Bergen, Bergen, Norway
2Department of Organic Chemistry, Kazan State Technological University,
Kazan, Russia
3Department of Organic Chemistry ‘‘A. Mangini,’’ University of Bologna,
Bologna, Italy
GRAPHICAL ABSTRACT
Abstract a,b-Unsaturated acetylenic c-hydroxyketones have been shown to react with ethyl
acetoacetate in a Michael-addition fashion and subsequently undergo cyclization followed
by dehydration to give substituted furans with a predictable regiospecificity. The yields were
good to excellent. A mechanism for the transformation is proposed, and this mechanism
explains why furan formation does not take place when the same unsaturated ketones are
treated with a-methylated acetoacetate and diethyl malonate.
[Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications1 for the following free supplemental resource: Full
experimental and spectral details.]
Keywords Conjugated acetylenic ketones; ketoesters; propargylic alcohols
INTRODUCTION
Furans constitute an important class of compounds in organic chemistry. This
five-membered heterocycle is found in a number of natural products and in some
pharmaceuticals,[1] and in recent years the furan moiety has been incorporated in
materials with promising commercial potential.[2,3] In addition furans are applied
as synthetic intermediates to prepare a number of interesting substructures including
unsaturated esters, 1,4-dicarbonyl compounds, and modified carbohydrates.[4] All
these applications have boosted interest in furan chemistry, and in recent years a
number of new synthetic methods of furans have been published.[5]
Received October 24, 2012.
Address correspondence to Leiv K. Sydnes, Department of Chemistry, University of Bergen,
´
Allegt. 41, NO-5007, Bergen, Norway. E-mail: leiv.sydnes@kj.uib.no
2898

TETRA-SUBSTITUTED FURANS
2899
Scheme 1. Synthesis of acetylenic ketones 3.
Some time ago we reported a simple synthesis of 3,3,4,4-tetraethoxybutyne
(TEB) (1),[6] which has made the preparation of propargylic alcohols 2 and 5-substi-
tuted 1,1-diethoxy-5-hydroxypent-3-yn-2-ones (3) straightforward (Scheme 1).[7–9]
Ketones 3 are highly functionalized, and by exploiting their reactive moieties in
a selective fashion synthesis of a range of interesting compounds has been
achieved.[8,10] Among the valuable transformations realized is conversion of esters
of 2 to tetra-substituted furans in a regiospecific manner by treatment with various
Gilman-type cuprates.[11] On this basis we envisaged that furans could also be made
under mild conditions by treating 3 with Michael donors that were not basic enough
to react with the hydroxyl group in the hydroxyketone. So far we have studied
reactions with the monoenolate of ethyl acetoacetate, which appears to convert 3
to substituted furans in good yields in a predictable fashion.
RESULTS AND DISCUSSION
The a,b-unsaturated acetylenic ketones used were synthesized from TEB as
outlined in Scheme 1, with an overall yield ranging from 63 to 81% (Table 1). In
the first step the bromomagnesium acetylide of 1 was applied when aliphatic alde-
hydes were reacted, but when benzaldehyde and p-tolylaldehyde were used it
appeared more favorable to use the lithium acetylide instead because this change pre-
vented significant by-product formation, mainly by the Cannizzaro reaction.[9] In the
second step all the substrates behaved basically in the same way and gave 3 in 90%
yield or better (Table 1).
Ketones 3 were reacted with 1 molar equivalent of the monoenolate of ethyl
acetoacetate by dropping ethanol solutions of the ketones to an ethanol solution
of a 1:1 mixture of the ester and its enolate. At room temperature the reaction
was slow (as indicated by thin-layer chromatography [TLC] analysis), but when
the reaction temperature was increased to 80 ꢀC all the starting material was con-
sumed at a reasonable rate and complete conversion was achieved after 3 h when

2900
L. K. SYDNES ET AL.
Table 1. Isolated yields of acetylenic alcohols 2 and ketones 3
R
Step 1 product=yield (%)
Step 2 product=yield (%)
H
Me
2a=90a
2b=85a
2c=80a
2d=82a
2e=81a
2f=70b
2g=75b
3a=93
3b=91
3c=95
3d=95
3e=90
3f=92
3g=92
n-C6H13
n-C7H15
i-Pr
Ph
p-MeC6H4
aEtMgBr as base.
bn-BuLi as base.
R ¼ H or alkyl and 5 h when R ¼ aryl (Table 2). The longer time required for the aro-
matic aldehydes probably reflects steric interactions in the ring-closure steps. In all
cases one product predominated, namely, the corresponding 5-substituted 2-methyl-
3-ethoxycarbonyl-4-(3,3-diethoxy-2-oxopropyl)furan (4) (Table 2). The workup was
straightforward and after purification by flash chromatography (hexanes=ethyl acet-
ate) furans 4 were obtained in excellent yields (Table 2).
A conceivable reaction mechanism for the furan formation is outlined in
Scheme 2. In the first step the acetylenic ketones react with the monoenolate of ethyl
acetoacetate (EAAꢁ) in a Michael fashion[12,13] and give intermediate A. This inter-
mediate is an enolate, which reacts either with ethyl acetoacetate furnishing its
monoenolate and the more stable intermediate B, or with B already formed, afford-
ing another molecule of B and a carbanion from B. The latter species is stabilized by
delocalization of the anion as indicated by some conceivable resonance forms (I–IV;
not exhaustive) in Scheme 2. Intermediate B contains three carbonyl groups, two
keto functions, and one ester moiety, which theoretically can react with a nucleophi-
lic site elsewhere in the molecule and lead to cyclization. The only nucleophile
present in B is the OH group (or the corresponding alkoxide), which apparently
reacts in a regiospecific fashion, attacking only the acetyl group originally belonging
Table 2. Preparation of furans 4
R
Furan
Reaction time (h)
Isolated yield (%)
H
Me
4a
4b
4c
4d
4e
4f
3
3
3
3
3
5
5
80
74
90
84
70
85
88
i-Pr
n-C6H13
n-C7H15
Ph
p-MeC6H4
4g

TETRA-SUBSTITUTED FURANS
2901
Scheme 2. Proposed mechanism for formation of furans 4.
to ethyl acetoacetate, and gives intermediate C and finally tetra-substituted furan 4,
perhaps via intermediate D.
On the basis of the mechanism outlined previously a prerequisite for furan for-
mation is dehydration. Consequently, if ethyl acetoacetate is replaced by a 2-substi-
tuted analog it is predicted that furan formation should not take place, and sure
enough, when 1,1-diethoxy-5-hydroxy-3-butyn-2-one (3a) was reacted with ethyl
2-methyl-3-oxobutanoate following the procedure for reactions with ethyl acetoace-
tate no furan was formed. In fact the course of reaction changed completely and
afforded two other heterocyclic compounds, 4,4-diethoxy-2-diethoxymethyl-2-hydro-
xytetrahydrofuran (5) and 1,1-diethoxy-3-(2-(diethoxy-methyl)-2-(3-hydroxyprop-
1-ynyl)-1,3-dioxolan-4-ylidene)propan-2-one (6) (Scheme 3), in 60% and 15% yield,
respectively. Chromatographic analyses of the latter compound indicate that it is
formed as a single isomer, and on the basis of the proton NMR spectrum it is con-
cluded that the compound has a Z configuration. Thus, both hydrogen atoms in
the allylic group exhibit a 1.6–1.8 Hz coupling to the olefinic proton, and such a
strong coupling is only compatible with a cis allylic coupling.[14]

2902
L. K. SYDNES ET AL.
Scheme 3. Reactions of 1,1-diethoxy-5-hydroxypent-3-yn-2-one (3a) with ethyl 2-methyl-3-oxobutanoate
and diethyl malonate.
The formation of compound 5 is easy to understand on the basis of a double
Michael addition of ethoxide=ethanol to 3a[10] followed by intramolecular alkoxide
attack of the keto moiety to give 5. How 6 is formed in detail is less evident, but con-
ceivably an intermolecular alkoxide attack of the carbonyl group in 3a (alkoxide
from 3a) occurs first and the resulting alkoxide then reacts by intramolecular attack
of the triple bond in a Michael fashion.
Finally, to try to force the hydroxyl group=alkoxide in intermediate B to attack
the ester group and form a lactone, 3a was also reacted with diethyl malonate, but
that hope was not met at all. The starting material was indeed consumed and one
product only was isolated, 1,3-dioxolane 6, but why this complete change in the
course of reaction takes place remains to be investigated.
In summary, we have developed an easy and regiospecific method for construc-
tion of tri- and tetra-substituted furans, which involves addition of ethyl acetoacetate
to c-hydroxylated a,b-unsaturated alkynones followed by intramolecular cyclization.
The ease with which the transformation occurs makes the method an attractive
alternative to other syntheses available for the preparation of such furans.[15]
EXPERIMENTAL
Synthesis of Propargylic Alcohols 2: General Procedure
By bromomagnesium acetylide. A THF solution of ethylmagnesium bro-
mide (7.3 mL, 3.0 M, 22 mmol) was added dropwise to TEB (1) (5.00 g, 22.0 mmol)
in anhydrous THF (150 mL) under nitrogen over 10 min at rt. The mixture was
refluxed for 2 h and cooled to 0 ꢀC before aldehyde (22.0 mmol) in anhydrous
THF (25 mL) was added dropwise. Stirring was continued at 20 ꢀC until all the
starting material was consumed (monitored by TLC).
By lithium acetylide. TEB (1) (5.00 g, 22.0 mmol) was dissolved in anhy-
drous THF (100 mL) under nitrogen and cooled to ꢁ78 ꢀC before a 1.6 M hexane
solution of n-butyllithium (15 mL, 22.0 mmol) was added dropwise. The mixture
was stirred at ꢁ78 ꢀC for 30 min and then slowly heated to 0 ꢀC before aldehyde

TETRA-SUBSTITUTED FURANS
2903
(22.0 mmol) in THF was added dropwise. The mixture was stirred at rt until all the
starting material was consumed (monitored by TLC).
Work-up for both procedures. The reaction mixture was cooled to 0 ꢀC and
quenched with saturated aqueous NH4Cl (100 mL). The phases were separated and
the aqueous phase was extracted (Et2O; 3 ꢂ 50 mL). The combined organic extracts
were dried (MgSO4) and concentrated. From the residue 2 was obtained pure by
flash chromatography (hexanes and ethyl acetate, 80:20).
Compounds 2a–2c, 2e, and 2f are described in the literature[9,11] whereas
1,1,2,2-tetraethoxydodec-3-yn-5-ol (2d) and 4,4,5,5-tetraethoxy-1-(p-methylphenyl)pent-
2-yn-1-ol (2g), isolated as yellowish liquids by flash chromatography, are new compounds.
Synthesis of 3 by Deketalization of 2: General Procedure
Propargylic alcohol 2 (15–5.5 mmol) was dissolved in 100 mL of a 7:3 mixture
of THF and H2O. p-Toluenesulfonic acid monohydrate (0.57 g, 3.0 mmol) was
added, and the mixture was refluxed for 2 h until all the starting material was con-
sumed (followed by TLC). Most of the THF was evaporated, and the residue was
mixed with a saturated aqueous solution of NaCl (50 mL) and CH2Cl2 (50 mL).
After thorough stirring, the phases were separated, the aqueous phase was extracted
(CH2Cl2; 3 ꢂ 20 mL), and the combined organic phases were washed with a saturated
aqueous solution of NaHCO3 (50 mL) and dried (MgSO4). Filtration and evapor-
ation gave the corresponding ketone essentially pure (1H NMR).
Compounds 3a–3c, 3e, and 3f are described in the literature[9,11] whereas
1,1-diethoxy-5-hydroxydodec-3-yn-2-one (3d) and 1,1-diethoxy-5-hydroxy-5-(4-
methylphenyl)pent-3-yn-2-one (3g), isolated as yellow liquids by flash chromato-
graphy, are new compounds.
Furan Synthesis Sample Procedure: Reaction of Ethyl Acetoacetate
with Conjugated c-Hydroxyalkynones (3) and Ethyl 4-(3,3-Diethoxy-
2-oxopropyl)-2-methylfuran-3-carboxylate (4a)
Ethyl acetoacetate (0.13 g, 1.0 mmol) was added to sodium ethoxide (0.034 g,
0.5 mmol) in ethanol (10 mL). The reaction mixture was stirred at rt for 30 min. Then
1,1-diethoxy-5-hydroxypent-3-yn-2-one (3a) (0.18 g, 1.0 mmol) in ethanol (5 mL) was
added dropwise and the mixture was stirred at 80 ꢀC for an additional 3 h before the
mixture was allowed to cool down to rt. Water (30 mL) and CH2Cl2 (30 mL) were
added and the phases separated. The aqueous phase was extracted (CH2Cl2;
3 ꢂ 30 mL) and the combined organic phases were dried (MgSO4), filtered, and con-
centrated. Flash chromatography (hexane=ethyl acetate; 90:10) gave 4a (0.24 g, 80%;
colorless liquid). IR (film): 2978 (s), 2932 (s), 2900 (s), 1738 (s), 1709 (s), 1611 (w),
1567 (m), 1429 (m), 1388 (m), 1366 (w), 1297 (s), 1273 (s), 1218 (m), 1205 (m),
1
1153 (m), 1093 (s), 1057 (s), 939 (w), 836 (w), 780 (m), 609 (w) cmꢁ1; H NMR
(CDCl3, 400 MHz): d 7.20 (s, 1H), 4.76 (s, 1H), 4.23 (q, J ¼ 7.2 Hz, 2H), 3.92 (s,
2H), 3.76–3.60 (m, 4H), 2.55 (s, 3H), 1.32–1.25 (m, 9H); 13C NMR (CDCl3,
100 MHz): d 203.2, 164.7, 160.6, 140.0þ, 118.5, 113.6, 102.9, 63.8, 60.5, 34.0, 15.8,
15.0, 14.8. HRMS calcd. for C15H23O6 [M þ H]þ 299.14946; found 299.14861.

2904
L. K. SYDNES ET AL.
Reaction with 2-Methylacetoacetate: Formation of 4,4-Diethoxy-2-
diethoxymethyl-2-hydroxytetrahydrofuran (5) and 1,1-Diethoxy-3-
(2-(diethoxymethyl)-2-(3-hydroxyprop-1-ynyl)-1,3-dioxolan-4-ylid-ene)
prop-an-2-one (6)
1,1-Diethoxy-5-hydroxy-6-methylhept-3-yn-2-one (3a) (0.50 g, 2.7 mmol)
reacted with ethyl 2-methylacetoacetate (0.39 g, 2.7 mmol) for 3 h at 80 ꢀC. Flash
chromatography (hexanes=ethyl acetate; 80:20) provided 5 (0.45 g, 60%; yellowish)
and 6 (0.15 g, 15%; yellow) as liquids.
Data for 5. IR (film): 3469 (m), 2975 (s), 2931 (w), 2883 (m), 1727 (m), 1608
(m), 1481 (m), 1444 (w), 1390 (m), 1368 (w), 1322 (m), 1285 (m), 1238 (m), 1158
(m), 1106 (m), 1052 (s), 976 (m), 908 (m), 870 (m), 820 (m), 790 (m), 765 (w) cmꢁ1
.
1H NMR (400 MHz, CDCl3): d 4.39 (s, 1H), 4.07 (d, J ¼ 9.1 Hz, 1H), 3.81–3.47 (m,
9H), 2.39 (d, J ¼ 13.2 Hz, 1H), 2.16 (d, J ¼ 13.4 Hz, 1H), 1.27–1.19 (m, 15H); 13C
NMR (100 MHz, CDCl3): d 108.8, 106.3, 104.1, 72.7, 65.5, 65.2, 59.0, 58.6, 42.5,
16.0. HRMS calcd. for C11H1904þ [M–OEt–H2O]þ 215.12833; found 215.12734.
Data for 6. IR (film): 3450 (s), 2977 (s), 2932 (s), 2882 (m), 2252 (m), 1736 (s),
1689 (s), 1605 (s), 1480 (m), 1444 (w), 1372 (s), 1322 (m), 1270 (m), 1237 (s), 1158
(m), 1097 (s), 1069 (s), 1039 (s), 969 (m), 900 (s), 864 (s), 831 (w), 759 (w), 763
1
(m), 633 (w), 606 (m) cmꢁ1. H NMR (400 MHz, CDCl3): d 6.26 (m, 1H, X part
of ABX system), 5.18 (dd, 1H, J ¼ 16.1 and 1.66 Hz, A part of ABX system), 5.06
(dd, 1H, J ¼ 16.1 and 1.74 Hz, B part of ABX system), 4.65 (s, 1H), 4.51 (s, 1H),
4.34 (s, 2H), 3.85–3.55 (m, 8H), 1.98 (bs, 1H), 1 28–1.23 (m, 12H); 13C NMR
(100 MHz, CDCl3): d 195.9, 171.0, 106.4, 103.7, 102.7, 94.5, 86.8, 79.2, 72.3, 66.3,
63.5, 61.1, 51.5, 15.8, 14.8. HRMS calcd. for C18H2908þ [M þ H]þ 373.18624; found
373.18581.
Reaction with Diethyl Malonate
Diethoxy-5-hydroxy-6-methylhept-3-yn-2-one (3a) (0.50 g, 2.7 mmol) reacted
with diethyl malonate (0.43 g, 2.7 mmol) for 3 h at 80 ꢀC. Flash chromatography
(hexanes = ethyl acetate; 80:20) provided 6 (0.50 g, 50%; yellowish liquid).
SUPPORTING INFORMATION
1
Full experimental detail and H and 13C NMR spectra can be found via the
Supplementary Content section of this article’s Web page.
ACKNOWLEDGMENTS
Financial support (to L. K. S.) from the Research Council of Norway, through
the Research Programme for Catalysis and Synthetic Organic Chemistry (KOSK),
and from the Munin Foundation is acknowledged with gratitude. Thanks are also
due to the Norwegian Centre for international studies (to I. R. and M. S.) and the
Erasmus Programme for scholarships (to F. L.) and to Kazan State Technological
University, Kazan, Russia (to I. R.) for financial support. Recording of mass spectra

TETRA-SUBSTITUTED FURANS
2905
by Terje Lygre and Egil Nodland and, more lately, Bjarte Holmelid, all at the
University of Bergen is highly appreciated.
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