A new convenient synthesis of ethenyl ethers

Article abstract of DOI:10.1055/s-2001-18757

Addition-elimination of an alcohol to 1,2-bis (phenylsulfonyl)ethylene afforded β-alkoxyvinyl sulfones which were submitted to reductive elimination with 6% sodium amalgam to produce the corresponding ethenyl ethers in high yields and purity.

Full text of DOI:10.1055/s-2001-18757

1962
LETTER
A New Convenient Synthesis of Ethenyl Ethers
A
N
ew Conven
l
e
E
n
thenyl Ether
a
s
Cabianca,a Florence Chéry,a Patrick Rollin,*a Sergio Cossu,b Ottorino De Lucchi*b
a
ICOA-UMR 6005 / Université d'Orléans, B.P. 6759-F-45067 Orléans Cedex 2, France
Fax +33(2)38494579; E-mail: Patrick.Rollin@univ-orleans.fr
b
Dipartimento di Chimica, Università Ca' Foscari di Venezia, Dorsoduro2137, I-30123 Venezia, Italy
Fax +39(041)2578517; E-mail: delucchi@unive.it
Received 1 August 2001
The reaction sequence, which has been used for the prep-
aration of ethenyl ethers 2a-i and the yields obtained are
shown in the Scheme.
Abstract: Addition-elimination of an alcohol to 1,2-bis(phenylsul-
fonyl)ethylene afforded -alkoxyvinyl sulfones which were submit-
ted to reductive elimination with 6% sodium amalgam to produce
the corresponding ethenyl ethers in high yields and purity.
Key words: -alkoxyvinylsulfones, ethenyl ethers, bis(phenylsul-
fonyl)ethylene, reductive desulfonylation
Ethenyl ethers (ROCH=CH2) are widespread compounds
in organic synthesis as they display a high and diversified
reactivity. Indeed, beside being used as dienophiles in ev-
ery type of cycloaddition (hetero [4+2],1 inverse-electron-
demand [4+2],2 photochemical or dipolar [2+2],3 dipolar
[3+2]4 and cation radical5 cycloadditions), they can partic-
ipate as substrates in many reactions such as metathesis,6
Claisen rearrangements,7 addition of organometallics,8
metallation at the -carbon,9 etc.10
Ethenyl ethers are usually obtained either through Reppe
vinylation11 directly from the alcohol and acetylene at
high pressures and temperatures12 or through mercury(II)
acetate mediated trans-etherification of ethyl vinylether13
or high temperature base-induced elimination reactions.14
Even though the synthesis of simple ethenyl ethers is no
more a challenge and direct vinylation is performed indus-
trially in multi-thousand tons scale, at the laboratory scale
all actual methods present drawbacks and disadvantages
especially because of the hazard of handling acetylene at
the drastic reaction conditions which are needed when
dealing with reluctant, hindered alcohols. We now report
a new alternative synthetic protocol for the preparation of
ethenyl ethers which can be applied to highly hindered
substrates and which is based on the use of the safe and
crystalline bis(phenylsulfonyl)ethylene (BPSE)15 via a
rather mild substitution-reductive desulfonylation reac-
tion sequence. In other words, BPSE is utilized as a sub-
stitute for acetylene as it was in other instances to replace
acetylene in cycloaddition reactions.15 This methodology
has been applied to several alcohols with a special atten-
tion to chiral enantiopure alcohols because of prospect-
able work in asymmetric synthesis.16
Scheme
Primary, secondary as well as tertiary and phenolic (with
poorer yields, though) hydroxyl groups are represented. It
should be emphasized that even strongly hindered alco-
hols such as fenchol and norborneol efficiently afford the
respective -alkoxyvinyl sulfones 1f and 1g.17 As an im-
provement over the formerly reported reaction proce-
dure,18 it was noticed that when the reaction of an alcohol
with BPSE was carried out in THF with LiHMDS19 in-
stead of NaH, the addition-elimination product 1 was pro-
duced exclusively and no formation of the
-
phenylsulfonyl acetal 3 resulting from a second alkoxide
addition to 1 was observed (Figure).
Synlett 2001, No. 12, 30 11 2001. Article Identifier:
1437-2096,E;2001,0,12,1962,1964,ftx,en;G17001ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

LETTER
Figure
A New Convenient Synthesis of Ethenyl Ethers
1963
References
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The configuration of the -alkoxyvinyl sulfones 1a-i is
trans in any case, i.e. they possess the same configuration
as the starting BPSE, thus confirming the stereospecificity
of the reaction.
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The reductive desulfonylation occurred under mild reac-
tion conditions using the standard sodium amalgam re-
duction of arylsulfonyl derivatives in buffered (NaH2PO4)
methanolic solution.20 After completion, the crude reac-
tion mixture was filtered over Celite and washed with
dichloromethane, then extracted with the same solvent
and washed with 5% aqueous NaHCO3. Careful evapora-
tion of the organic phase afforded the crude enol ether,
which was highly pure in all cases, with the exception of
2c where a comparable amount of cyclopropyldiphenyl-
methane was also formed. In this latter case, the high sta-
bility of the cyclopropyldiphenylmethyl radical may be
responsible for such an observation. It should be noted
that the intrinsic reactivity of enol ethers towards water
even under slightly acidic conditions would inevitably re-
sult in hydrolytic cleavage of the =HC–O– bond. Actual-
ly, purification by silica gel chromatography proved
detrimental as it resulted in very poor recovery of the de-
sired vinyl ether.
Other reducing agents previously reported to promote re-
ductive desulfonylation - lithium naphthalenide,21 magne-
sium in methanol22 - were also tested but led to inferior
results.
In conclusion, the above procedure represents a novel pre-
parative method for the production of parent vinyl ethers.
The methodology - which avoids the use of acidic reaction
conditions - is of general utility and constitutes a valuable
alternative to the existing intrinsically hazardous and/or
noxious methodologies. The case of enol ether 2i, which
has been recently reported from vinylation of cholesterol
with gaseous acetylene at high pressure and tempera-
ture,12 is illustrative of the mildness of the method and of
its potentiality. Extension of our methodology to more
complex molecules such as carbohydrates is currently in-
vestigated in our laboratory.
(11) Reppe, W. Liebigs Ann. Chem. 1956, 601, 81.
(12) Trofimov, B. A.; Vasil'tsov, A.; Schmidt, E.; Zaitsev, A. B.;
Mikhaleva, A. I.; Afonin, A. V. Synthesis 2000, 1521.
(13) See for example: Palani, N.; Chadha, A.; Balasubramanian,
K. J. Org. Chem. 1998, 63, 5318.
(14) Dulcere, J.-P.; Rodriguez, J. Synthesis 1993, 399.
(15) Pasquato, L.; De Lucchi, O. Encyclopedia of Reagents for
Organic Synthesis, Vol. 1; Paquette, L. A., Ed.; Wiley:
Chichester, 1995, 547.
(16) It should be noted that the enantiopure ethenyl ethers chosen
by us carry the chiral auxiliary at the minimum distance from
the reactive site. References 1–10 have been selected from
the vast recent literature about uses of ethenyl ethers in
asymmetric synthesis.
Aknowledgement
E.C. thanks Ca’ Foscari University for granting her traineeship at
the University of Orleans. Work partially supported by MURST
(Rome) within the national project ’Stereoselezione in Sintesi Orga-
nica. Metodologie e Applicazioni’.
Synlett 2001, No. 12, 1962–1964 ISSN 0936-5214 © Thieme Stuttgart · New York

1964
E. Cabianca et al.
LETTER
(17) General Procedure for the Synthesis of b-Alkoxy Vinyl
Sulfones (1a-i): To a stirred reaction mixture containing 0.1
g (0.32 mmol) of (E)-BPSE and the alcohol (0.32 mmol) in
anhydrous THF (5 mL) was added 0.32 mL (0.32 mmol) of
LiHMDS (1 M solution in hexane) at –78 °C. The mixture
was allowed to stir overnight, then quenched with water,
extracted with EtOAc and dried over MgSO4. Evaporation of
the solvents left a white solid which was purified by flash
chromatography (petroleum ether or toluene–EtOAc). The
physical properties of the products matched the reported
data (Ref. 18)or agreed with the proposed structure. For
example: (E)-1-(2-Bornyloxy)-2-phenylsulfonyl
0.44 mmol of precursor 1a-i) and phosphate buffer NaH2PO4
(4 g, 0.44 mmol). The reaction mixture was allowed to stir at
room temperature while monitoring the consumption of the
sulfone by TLC (petroleum ether). After completion
(typically 1-2 h) the reaction mixture was filtered over Celite
and washed with CH2Cl2. The organic phase was washed
with 5% aqueous NaHCO3, dried over anhydrous K2CO3 and
cautiously concentrated under vacuum to give the vinyl ether
whose NMR spectrum showed high purity grade. The
physical properties of known compounds matched the
reported data: 2b (Ref. 23), 2e and 2f (Ref 23, 24), 2g (Ref
25), 2i (Ref 12). Further examples: (Adamantan-1-yl)methyl
ethenyl ether(2a): Colourless oil; 1H NMR (250 MHz,
CDCl3): = 6.48 (dd, 1 H, H-1, J = 6.8, 14.2 Hz), 4.13 (dd,
1 H, H-2eE, J = 1.7, 14.2 Hz), 3.92 (dd, 1 H, H-2Z, J = 1.7,
6.8 Hz), 3.23 (s, 2 H, CH2O), 1.98 (m, 3 H, H-3,5,7), 1.76–
ethylene(1f): Colourless solid; mp 108 °C; [ ]D20 = –60 (c =
1.1, CHCl3); 1H NMR (250 MHz, CDCl3): = 7.89–7.87 (m,
2 H, Hortho), 7.60–7.48 (m, 4 H, Hmeta, Hpara, H-1), 5.64 (d, 1
H, H-2, Jvic = 12.1 Hz), 4.14 (m, 1 H, CHO), 2.25 (m, 1 H),
1.91 (m, 1 H), 1.72 (m, 2 H), 1.24 (m, 2 H), 1.06 (dd, 1 H, H-
3e, J = 3.2, 13.6 Hz), 0.87 (s, 6 H, CH3), 0.85 (s, 3 H, CH3);
13C NMR (62.89 MHz, CDCl3): = 161.3 (C-1), 143.0
(CIVar), 132.9 (Cpara), 129.4 (Cmeta), 127.1 (Cortho), 106.9 (C-
2), 89.0 (CHO), 49.8, 48.2 (CIV), 45.1 (CH), 36.3, 28.0, 26.8
(CH2), 19.8, 19.0, 13.7 (CH3); IR (KBr): 1609 cm-1 (C=C);
MS m/z (IS): 321 [M+H]+, 338 [M+NH4]+, 343 [M+Na]+.
(E)-1-(2-Fenchyloxy)-2-phenylsulfonyl ethylene (1g):
Colourless solid; mp 67 °C; [ ]D20 = +9 (c = 1.26, CHCl3);
1H NMR (250 MHz, CDCl3): = 7.88–7.85 (m, 2 H, Hortho),
7.58–7.48 (m, 4 H, Hmeta, Hpara, H-1), 5.76 (d, 1 H, H-2, Jvic
= 11.9 Hz), 3.55 (s, 1 H, CHO), 1.70–1.66 (m, 3 H), 1.54–
1.45 (m, 2 H), 1.21–1.17 (m, 1 H), 1.08–1.02 (m, 7 H), 0.82
(s, 3 H, CH3); 13C NMR (62.89 MHz, CDCl3): = 162.5 (C-
1), 143.0 (CIVar), 132.9 (Cpara), 129.4 (Cmeta), 127.0 (Cortho),
107.0 (C-2), 97.8 (CHO), 49.5 (CIV), 48.5 (CH), 41.4 (CH2),
40.3 (CIV), 31.0 (CH3), 26.3, 26.0 (CH2), 20.8, 19.6 (CH3);
IR (KBr): 1624, 1602 cm-1 (C=C); MS m/z (IS): 321 [M +
H]+, 338 [M + NH4]+, 343 [M + Na]+.
1.63 (m, 6 H, CH2-4,6,10), 1.56 (m, 6 H, CH2-2,8,9); 13
C
NMR (62.89 MHz, CDCl3): = 153.0 (C-1), 85.9 (C-2), 78.8
(CH2O), 39.8 (C-2,8,9), 37.5 (C-4,6,10), 33.8 (CIV), 28.5 (C-
3,5,7); IR (film): 1651, 1645 cm-1 (C=C); MS m/z (EI): 149
[M–OCH=CH2]+, 192 M+. (Cyclopropyldiphenyl)methyl
ethenyl ether (2c): Obtained as a ca. 1: 1 mixture with
cyclopropyldiphenylmethane (Ref. 26); 1H NMR (250 MHz,
CDCl3): = 7.41–7.17 (m, 10 H, Har), 6.27 (dd, 1 H, H-1, J
= 6.2, 13.6 Hz), 4.58 (dd, 1 H, H-2E, J = 0.6, 13.6 Hz), 4.01
(dd, 1 H, H-2Z, J = 0.6, 6.2 Hz), 1.65 (m, 1 H), 0.62 (m, 2
Hcis), 0.30 (m, 2 Htrans); 13C NMR (62.89 MHz, CDCl3):
147.8 (C-1), 143.7 (CIVar), 128.6-126.2 (3Car), 91.3 (C-2),
86.3 (CIV), 19.4 (CH), 2.5 (CH2); IR (film): 1631 cm-1
=
(C=C); MS m/z (EI): 207 [M–OCH=CH2]+ .
.
(21) Beau, J.-M.; Sinaÿ, P. Tetrahedron Lett. 1985, 26, 6185.
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(19) Meek, J. S.; Fowler, J. S. J. Org. Chem. 1968, 33, 985.
(20) General Procedure for the Synthesis of the Ethenyl Ethers
2a-i: To a solution of the -alkoxyvinyl sulfone in anhydrous
MeOH (100 mg/5 mL) were added 6% Na/Hg amalgam (8 g,
Synlett 2001, No. 12, 1962–1964 ISSN 0936-5214 © Thieme Stuttgart · New York