[3 + 2] Cycloreversion of Bicyclo[m.3.0]alkan-3-on-2-yl-1-oxonium Ylides to Alkenyloxyketenes. Stereospecific Aspect

Article abstract of DOI:10.1021/jo0301806

Rhodium(II)-catalyzed intramolecular reaction of diazoketones 1 bearing a cyclic ethereal moiety transiently formed bicyclo[m.3.0]octan-3-one-1-oxonium-2-ylides (2), which underwent sigmatropic and stereospecific [3 + 2] cycloreversion reaction to form alkenyloxyketenes 3. The ketenes were efficiently trapped by methanol to form the corresponding esters 4. Mechanistic studies revealed that the size of ethereal ring can be variable at least from THF to the THP, oxepane, and oxocane moiety, i.e., m = 3-6. On the other hand, the size of the ylide ring containing the carbonyl unit is limited to a five-membered ring. The cycloreversion was found to be stereospecific as was proven by the reactions of diastereoisomeric pairs bearing a methyl group at the bond-cleaving position. From threo isomers 7, (E)-alkenyloxyacetates 15 were exclusively formed (77-84%), whereas from erythro isomers 8, (Z)-isomers 16 were formed (80-88%). Mechanism of the cleavage from diazoacetonyl-substituted cyclic ethers to alkenyloxyketenes via bicyclic oxonium ylides was analyzed on the basis of calculations employing the hybrid density functional B3LYP and the highly correlated quadratic configuration interaction QCISD method to reveal that the concerted [3 + 2] cycloreversion is the key step of this reaction.

Full text of DOI:10.1021/jo0301806

[3 + 2] Cyclor ever sion of Bicyclo[m .3.0]a lk a n -3-on -2-yl-1-oxon iu m
Ylid es to Alk en yloxyk eten es. Ster eosp ecific Asp ect
Akira Oku,* Yuichi Sawada, Marc Schroeder, Ichiro Higashikubo, Tomohiro Yoshida, and
Shigeji Ohki
Department of Chemistry and Materials Technology, Kyoto Institute of Technology,
Matsugasaki, Sakyo, Kyoto 606-8585, J apan
oku@ipc.kit.ac.jp
Received May 27, 2003
Rhodium(II)-catalyzed intramolecular reaction of diazoketones 1 bearing a cyclic ethereal moiety
transiently formed bicyclo[m.3.0]octan-3-one-1-oxonium-2-ylides (2), which underwent sigmatropic
and stereospecific [3 + 2] cycloreversion reaction to form alkenyloxyketenes 3. The ketenes were
efficiently trapped by methanol to form the corresponding esters 4. Mechanistic studies revealed
that the size of ethereal ring can be variable at least from THF to the THP, oxepane, and oxocane
moiety, i.e., m ) 3-6. On the other hand, the size of the ylide ring containing the carbonyl unit is
limited to a five-membered ring. The cycloreversion was found to be stereospecific as was proven
by the reactions of diastereoisomeric pairs bearing a methyl group at the bond-cleaving position.
From threo isomers 7, (E)-alkenyloxyacetates 15 were exclusively formed (77-84%), whereas from
erythro isomers 8, (Z)-isomers 16 were formed (80-88%). Mechanism of the cleavage from
diazoacetonyl-substituted cyclic ethers to alkenyloxyketenes via bicyclic oxonium ylides was analyzed
on the basis of calculations employing the hybrid density functional B3LYP and the highly correlated
quadratic configuration interaction QCISD method to reveal that the concerted [3 + 2] cycloreversion
is the key step of this reaction.
In tr od u ction
the presence of relatively strong acids,5b we found that
with a weakly acidic nucleophile such as MeOH, ylide 2
underwent a sigmatropic cleavage of the bicyclic ring,
namely [3 + 2] cycloreversion reaction. Products were
alkenyloxyacetates 4 that were unequivocally formed
from 1 via ketene intermediates 3 (Scheme 1).5b
In contrast to a number of studies on [2 + 2] cyclo-
addition reactions of ketenes with alkenes,6 a related
reverse process such as [3 + 2] cycloreversion of bicyclic
oxonium ylides to oxyketenes and olefins are yet un-
known except our preliminary report.5b In this regard,
our primary concern was how to control this reaction by
structural factors, e.g., length of tethering side chain,4a
ring size,7 stereochemistry at the bond-cleaving site, and
polar factors of attacking reagents.8 The second concern
was the mechanism of the reaction pathway and the
structure of transient species involved therein.
Ethereal oxonium ylides are highly reactive and short-
lived intermediates compared with other onium ylides.1
While they possess appreciable potential for synthetic
use, systematic utilization for synthesis has been limited2
due to their transient lifetime. The advantage of in-
tramolecular formation of oxonium ylides was previously
reported by Kirmse,3 and we also reported4 novel reac-
tions to control the high reactivity of bicyclooxonium
ylides by the design of intramolecular pathways.3 In
particular, we were intrigued by the behavior of bicyclo-
[3.3.0]octan-3-one-1-oxonium-2-ylide (2), which was tran-
siently formed in the transition-metal-catalyzed reaction
of THF-substituted diazoacetone 1.5a In contrast to the
ring-enlargement reaction that exclusively took place in
* To whom correspondence should be addressed. Present address:
Research Institute for Production Development, 15 Morimotocho,
Shimogamo, Sakyo-ku, Kyoto 606-0805, J apan.
Resu lts a n d Discu ssion
(1) (a) Padwa, A.; Fryxell, G. E.; Zhi, L. J . Am. Chem. Soc. 1990,
112, 3100. (b) Padwa, A.; Hornbuckle, S. F. Chem. Rev. 1991, 91, 263.
(c) Ye, T.; Mckervey, M. A. Chem. Rev. 1994, 94, 1091. (d) Padwa, A.;
Weingarten, M. D. Chem. Rev. 1996, 96, 223. (e) Naito, I.; Oku, A.;
Ohtani, N.; Fujiwara, Y.; Tanimoto, Y. J . Chem. Soc., Perkin Trans. 2
1996, 725. (f) Tomioka, H.; Kobayashi, N.; Murata, S.; Ohtawa, T. J .
Am. Chem. Soc. 1991, 113, 8771.
(2) (a) Thijis, L.; Zwanenburg, B. Tetrahedron 1980, 36, 2145. (b)
Pirrung, M. C.; Werner, J . A. J . Am. Chem. Soc. 1986, 108, 6060. (c)
Clark, J . S.; Krowiak, S. A.; Street, L. J . Tetrahedron Lett. 1993, 34,
4385.
P r ep a r a tion of Dia zok eton es. Diazoketones bearing
cyclic ethers with varying ring sizes were prepared from
the corresponding acid chlorides by the treatment with
(5) (a) Roskamp, E. J .; J ohnson, C. R. J . Am. Chem. Soc. 1986, 108,
6062. (b) Oku, A.; Ohki, S.; Yoshida, T.; Kimura, K. J . Chem. Soc.,
Chem. Commun. 1996, 1077.
(6) (a) Snider, B. B. Chem. Rev. 1988, 88, 793. (b) Snider, B. B.;
Hui, R. A. J . Org. Chem. 1985, 50, 5167. (c) Allen, A. D.; Ma, J .;
McAllister, M. A.; Tidwell, T. T. Acc. Chem. Res. 1995, 28, 265. (d)
Hyatt, J . A.; Raynolds, P. W. Org. React. 1994, 45, 159.
(7) Oku, A.; Numata, M. J . Org. Chem. 2000, 65, 1899.
(3) Kirmse, W. J . Am. Chem. Soc. 1989, 111, 1465.
(4) (a) Oku, A.; Murai, N.; Baird, J . J . Org. Chem. 1997, 62, 2123.
(b) Mori, T.; Taniguchi, M.; Suzuki, F.; Doi, H.; Oku, A. J . Chem. Soc.,
Perkin Trans. 1 1998, 3623
(8) DoMinh, T.; Strausz, O. P. J . Am. Chem. Soc. 1970, 92, 1766.
10.1021/jo0301806 CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/24/2004
J . Org. Chem. 2004, 69, 1331-1336
1331

Oku et al.
SCHEME 1. Rh (II)-Ca ta lyzed Decom p osition of
Dia zok eton e 1
SCHEME 2. P r ep a r a tion of Dia zok eton es 1, 7, a n d
8
diazomethane. General preparative procedures are sum-
marized in Scheme 2.
Cyclor ever sion of Dia zoa ceton yl-Su bstitu ted Cy-
clic Eth er s w ith Differ en t Rin g Sizes. First of all, the
effect of ring sizes of ethereal moiety was examined to
demonstrate the scope of the cycloreversion reaction.
Results in Table 1 indicate that diazoacetones bearing
not only a tetrahydrofuryl moiety but also tetrahydro-
pyranyl, oxepanyl, and furthermore oxocanyl moieties all
reacted smoothly to afford the cycloreversion products
under the Rh(II)-catalyzed reaction conditions. This
indicates that the size of cyclic ethers is basically not a
limiting factor of the cycloreversion, but a smaller ring
size7 seems to be better for facilitating the ring-cleavage
process due to kinetic reason. In contrast, the size of
transiently formed ylide ring that contains a carbonyl
group is strictly limited to the five-membered furanone
ring.
TABLE 1. Effects of Rin g Size a n d Su bstitu en t Y on th e
Cyclor ever sion
As the size of ethereal ring became larger, the yield of
product decreased to some extent. One reason for this
trend seems to be the gain of stability of bicyclic ylide
due to a smaller strain than that of bicyclo[3.3.0] system.
This is because the strain facilitates the two-bond cleav-
age process, the essential requisite of the cycloreversion,
more than the central one-bond cleavage leading to the
ring-enlargement.
In addition to the strain, the lifetime of oxonium ylide
seems to be another key factor. In general, ethereal ylides
are extremely short-living.1e Nevertheless, one can expect
that substitution of an additional electron-withdrawing
group on the diazomethyl carbon will elongate the
lifetime of the ylides.6 Indeed, as presumed, diazocarbonyl
precursor 1f bearing an ethoxycarbonyl group on the
diazomethyl group improved the yield of cycloreversion
products from 40 (4b) to 85% (4f) (Table 1).
entry
substrate
n
Y
yield of 4 (%)
1
2
3
4
1a
1b
1c
1d
1e
1f
1
2
3
4
1
2
2
H
H
H
H
4a (88)
4b (40)
4c (67)
4d (51)
4e (82)
4f (85)
4g (90)
5
COOEt
COOEt
COOEt
6
7a
1f
a
PhOH was used instead of MeOH. Product 4g was the
corresponding Ph ester.
did not give the ester product nor ring-enlargement
product at all, thus ruling out this possibility.
As an alternative route to the ketene formation, [2 +
2] cycloreversion of bicyclo[4.2.0]octan-7-one (6),6a which
can be formed by the 1,2-shift reaction of ylide 2, was
examined (Scheme 3). Treatment of 6 with Rh2(OAc)4 in
methylene chloride under the same conditions, however,
Cyclor ever sion of Dia ster eoisom er ic R-Meth yl-
Su bstitu ted Dia zoa ceton yl Cyclic Eth er s. One can
assume from the mechanistic examination of the cyclo-
reversion on prototype ylide 2a that the cleavage of two
rings may occur either in a single step or in a very fast
1332 J . Org. Chem., Vol. 69, No. 4, 2004

Cycloreversion of Bicyclo[m.3.0]alkan-3-on-2-yl-1-oxonium Ylides
TABLE 2. Ster eosp ecificity of th e Cyclor ever sion
SCHEME 3. Exclu sion of P ossible [2 + 2]
Cyclor ever sion of 6
entry
substrate
n
Y
product yield (%)
1
2
3
4
threo-7a
erythro-8a
threo-7b
1
1
2
2
H
H
15a (E) (84)
16a (Z) (88)
15b (E) (87)
16b (Z) (80)
SCHEME 4. Ster eosp ecific [3 + 2] Cyclor ever sion
of Bicyclo[3.(n + 2).0]oxon iu m Ylid e to
Alk en yloxyk eten e
COOEt
COOEt
erythro-8b
reospecificity will be lost. Similarly, with erythro isomer
8, ylide 10 will also prefer cis-fused configuration to the
trans, in which the methyl group keeps a syn (or cis)
position to the vicinal C-C bond. Therefore, a synchro-
nous bond-cleavage will lead to the formation of a
Z-double bond.
To verify a general profile of the above-predicted
stereospecific cycloreversion by chemical means, we
prepared two pairs of diastereoisomeric diazoketones 7a ,
8a bearing a THF ring and 7b, 8b bearing a THP ring,
all having a methyl group at the 3-position of diazo-
acetonyl side chain. Results of the Rh(II)-catalyzed
decomposition of the pairs in the presence of methanol
are summarized in Table 2.
Trapping of the primary product alkenyloxyketenes 13
and 14 (n ) 1, 2) by MeOH proceeded smoothly to afford
the corresponding methyl esters in good to high yields,
i.e., 15a , 16a , 15b, 16b from 7a , 8a , 7b, 8b in 84, 88, 87,
80%, respectively. As expected from the molecular frame-
work examination described above (Scheme 4), stereo-
chemistry of the alkenyl groups in the product structures
was stereospecifically controlled by the diastereoisomer-
ism of starting diazoketones 7 and 8. Thus, E-olefins 15
were exclusively formed from threo isomers 7 whereas
Z-olefins 16 from erythro isomers 8. The Z-configuration
of 16a was identified by the NOESY observable between
two olefin protons9 and, consequently, 15a was assigned
to the E-isomer. The stereochemistry of 15b and 16b was
assigned on the basis of their J values, 15 and 11 Hz,
respectively.9 Diazoketones bearing much larger rings
such as oxepane and oxocane rings are also assumed to
react stereospecifically on the same basis.
Diazoketones having a longer tethering chain than
diazoacetonyl were found unsuccessful in cycloreversion
because the corresponding ylide rings are larger than five
so that one-step [3 + 2] cycloreversion mechanism cannot
work.
R ea ct ion of (3,3-Dim et h yl-1-d ia zoa cet on -3-yl)-
Su bstitu ted Tetr a h yd r ofu r a n . It has been well stud-
ied that alkoxyketenes10 are labile, having lifetimes
shorter than milliseconds. Alkenyloxyketene 3 which is
assumed to be a key intermediate in the present cyclo-
reversion is an analogue of this family and, therefore,
was expected to be identifiable spectroscopically. How-
stepwise manner. If these are the only two possible cases,
one can verify the mechanism by choosing a diastereo-
isomeric pair of methyl-substituted diazoketone 7 and 8
as the model, and the stereospecific outcome of the
product structure will give the answer to this question.
According to the presumed one-step cycloreversion
mechanism as depicted in Scheme 4, ring-cleavage of the
bicyclo[3.(n + 2).0]-1-oxonium-4-methyl-3-oxo-2-ylide rings
take place to afford either E- or Z-alkenyloxyketene
depending on the threo and erythro diastereoisomerism
of the starting diazoketone, respectively. Taking threo
isomer 7 for example, the ylide will prefer cis-fused
configuration 9 to the trans-fused one 11. In this isomer
9, the methyl group always keeps an anti (or trans)
position to the vicinal C-C bond that once composed the
ether ring. Here, if a concerted [two single-bond cleavage]-
[two double-bond formation] takes place, it will result in
the formation of E-configuration at the newly borne
olefinic bond. If the ring-fission takes place slowly in a
stepwise manner via a biradical or zwitterion, the ste-
(9) Configuration of isomer 15a , although its olefin protons had
identical d values and it was impossible to measure its NOESY, was
assumed to E after determining 16a to Z. For 15b and 16b, although
d values of two olefin protons in each isomer were almost identical
again being impossible to measure NOESY, irradiation of allylic
methylene protons showed a clear difference in the olefinic vicinal J
values between two isomers.
(10) Tidwell, T. P. Ketenes; Wiley: New York, 1995.
J . Org. Chem, Vol. 69, No. 4, 2004 1333

Oku et al.
SCHEME 5. P r ep a r a tion a n d Rea ction of
Dia zok eton e 19
SCHEME 6. P la u sible Rou te fr om 19 to P r od u cts
22, 23, 29, a n d 30
SCHEME 7. Ba sic Rea ction Mod el for Ca lcu la tion
ever, attempted FT-IR detection was unsuccessful. Snider
reported6a that alkenyloxyketenes underwent intra-
molecular [2 + 2] cyclization reaction to form bicyclic
ketones. Therefore, expecting that this type of reaction
would afford a chemical proof for the intermediacy of
alkenyloxyketene, we prepared both (3,3-dimethyl-1-
diazoacetonyl-3-yl)-substituted tetrahydrofuran 19 and
alkenyloxyacetic acid 20 as plausible precursors of alk-
enyloxyketene 21 (Scheme 5, eqs 1 and 2, respectively).
Indeed, from 20, bicyclo[4.1.1]-2-oxaoctan-7-one 22 was
obtained though in 10%6a,b (eq 2). In parallel, the Rh(II)-
catalyzed reaction of 19 that was carried out in methyl-
ene chloride in the presence of MeOH afforded mainly
ester 23 (33%) besides minor amounts of 29 and 30 (5%
each), indicating that the corresponding ylide 2h was
formed to some extent. In contrast, a similar reaction of
19 in benzene without MeOH gave 29 (9%) and 30 (52%)
but expected 22 was not found (Scheme 6). Minor product
29 can be formed most plausibly by the intramolecular
stepwise C-H insertion of the intermediate carbene or
carbenoid. However, the formation of major product 30,
the structure of which was confirmed by HMBC spectral
analysis, cannot be well accounted for yet. One of possible
routes is hypothetically shown in Scheme 6 in which an
intermolecular [2 + 2] cycloaddition of ketene and iso-
propylidenetetrahydrofuran 32 is proposed, the latter
may be derivable from oxonium ylide 31(Scheme 6).
ester 4 via the [3 + 2] cycloreversion of ylide 2 by means
of theoretical calculations employing the hybrid density
functional B3LYP11 and the highly correlated quadratic
configuration interaction QCISD method.12,13 The B3LYP
theory was employed to model the entire reaction path.
The tetrahydrofuryl-substituted diazo compound 24 (see
Scheme 7) was used as the model because it has less
conformers compared with other compounds bearing
larger cyclic substituents.
In 24 () 1a ), the ethereal oxygen atom and the
diazocarbon have a short interatomic distance of 3.322
Å (see Figure 1). The cyclization after loss of nitrogen
leads to bicyclic oxonium ylide 25 () 2a ), a process that
is endothermic by 5.7 kcal‚mol-1. The formation of three
different conformers of 25 (Grel ) +5.7, +6.4, +12.314),
(11) (a) Becke, A. D. J . Chem. Phys. 1993, 98, 1372. (b) Becke, A. D.
J . Chem. Phys. 1993, 96, 5648.
(12) Pople, J . A.; Head-Gordon, M.; Raghavachari, K. J . Chem. Phys.
1987,87, 5968.
(13) Quantum calculations were carried out using the Gaussian98
package of programs. Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.;
Scuseria, G. E.; Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.;
Montgomery, J . A., J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.;
Millam, J . M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.;
Tomasi, J .; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli,
C.; Adamo, C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P.
Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu,
G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen,
W.; Wong, M. W.; Andres, J . L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J . A. Gaussian 98, revision A.5; Gaussian, Inc.:
Pittsburgh, PA, 1998.
Th eor etica l Ca lcu la tion s
Supportive information for the stereospecific nature of
the present cycloreversion was also available from high-
level theoretical calculations.
We have investigated the mechanism of the transfor-
mation of diazocarbonyl compound 1 to ketene 3 and
(14) All energies are from B3LYP/6-311+G(d,p) and given as Gibbs’
free energies relative to [1a + N2 + CH3OH] in kcal‚mol-1
1334 J . Org. Chem., Vol. 69, No. 4, 2004

Cycloreversion of Bicyclo[m.3.0]alkan-3-on-2-yl-1-oxonium Ylides
SCHEME 8. Mod els to Sh ow Ster eosp ecificity of
th e Rea ction
F IGURE 1. Conformations and relative energies of interme-
diates. All structures and Gibbs’ energies are from the B3LYP/
6-311-G(d,p) level.
stable intermediate linking 25 and 27 were found.
Therefore, a two-step reaction seems to be unfavorable
and, as a consequence, the present reaction is a one-step
reaction passing through a planar transition state and
can be defined as a pericyclic reaction, a novel example
of a reverse [3 + 2] cycloaddition, namely a [3 + 2]
cycloreversion.
In the course of the reaction from 25 via TS26 to 27,
O(1) loses its positive charge (-0.01 f -0.11f -0.10 e)
and C(2) its negative character (-0.42 f -0.35 f -0.21
e).17 The formation of ketene 27 from 25 is exothermic
by 23.0 kcal‚mol-1 and the conformational conversion
from “U”-shaped isomer 27 to a linear isomer releases
further 1.5 kcal‚mol-1, rendering the reverse reaction
cycloaddition unfavorable.
ascribable to anti-anti, syn-anti, and syn-syn isomers,
respectively, on the B3LYP15 of 25 can be rationalized
by a syn-attack of one of the lone-pairs of ethereal oxygen
atom O(1) because an anti-attack seems disfavored
because of the formation of highly strained trans-fused
bicyclic ring.
As a result, atoms O(1) and C(5), which are incorpo-
rated in the cyclic ethereal substituent, become more
positive by 0.2 and 0.3 e, respectively, whereas the oxygen
O(9) of the carbonyl unit becomes more negative by 0.1
e. The two bond lengths of O(1)-C(5) (1.566 Å) and O(1)-
C(8) (1.470 Å) are 6 to 8% longer than in 24a . The
tricoordinate carbanionic atom C(2) is less planar than
expected (Σ bond angles: 348°) and possesses an elec-
tronic charge density of -0.2 e. The carbonyl oxygen O(9)
is only slightly more negative than in 24 (3%) and C(3)-
O(9) bond (1.230 Å) is a pure double bond. All these
properties reveal that the C(2)-C(3)-O(9) unit is an
R-keto carbanion rather than an enolate.
As suggested by the experiments, bicyclic ylide 25 can
undergo a ring-opening reaction to form oxyketene 27
() 3a ). The low-energy transition state16 TS26 (only ∆G
) +2.5 kcal/mol above 25 at the B3LYP level) was found,
which connects two minimum structures 25 and 27
directly. In TS26, the main interatomic distances are as
follows: O(1)-C(5) ) 2.024 Å, C(3)-C(4) ) 1.720 Å,
C(2)-C(3) ) 1.370 Å, C(4)-C(5) ) 1.420 Å and O(1)-
C(2) ) 1.400 Å, revealing that the reaction from 25 to
27 is a concerted process in which two bonds [C(1)-C(5)
and C(3)-C(4)] are cleaved while two double-bonds are
formed [C(2)dC(3) and C(4)dC(5)]. Additionally, the
opening of five-membered ring shows almost perfect
planarity with a maximum deviation from planarity of
0.008 Å and a sum of dihedral angles of 0.025°. Despite
a careful search, neither a second transition state nor a
In the final step, the exothermic (∆G ) -30.1 kcal/
mol) addition of one molecule of methanol to ketene 27
yields the unsaturated ester of type 28. The overall loss
of potential Gibbs’ free energy from 24 to 28 is 47.4
kcal‚mol-1
.
To understand more easily the selective formation of
either Z or E double-bond in the esters 4 as found in the
experiment (see above), we carried out calculations on
the transformation of diastereoisomeric methyl-substi-
tuted derivatives, i.e., from 25B () 9a ) to 27B () 13a )
and 25C () 10a ) to 27C () 14a ) (Scheme 8). Concerning
the energy profile for the reaction of ylides (25B and 25C)
to the oxyketenes (27B and 27C), no great deviations in
comparison with the unsubstituted derivatives (25 to 27)
were found. Only the formation of 27B is less exothermic
due to its higher stability in that the substituents are in
anti positions.
The molecular structures 27B and 27C (see Scheme
8) clearly indicate that due to the concerted nature of
the reaction (TS26B and TS26C) the stereochemical
information is retained (stereospecific) and, thus, a E
C(4)-C(5) alkenyl double bond (eventually coincides the
C(4), C(5) numbers of 25 in Scheme 7) of 27B is formed
from threo-25B () 9a ) and a Z double bond in 27C is
formed from erythro-25C () 9a ). A two-step process,
(15) Calculations on the B3LYP -level were performed with the
6-311+G(d,p) basis set.
(16) An IRC calculation (implemented in Gaussian98) proved that
2a and 4a are connected via TS26.
(17) All charges are carbon-centered Mulliken charges.
J . Org. Chem, Vol. 69, No. 4, 2004 1335

Oku et al.
about 7 kcal‚mol-1 higher and the reaction is less
exothermic by 5-6 kcal‚mol-1. Apparently, highly cor-
related calculation theories have to be employed to
describe the sensitive oxonium ylide structure correctly.
TABLE 3. Ca lcu la ted En er gies for th e Tr a n sfor m a tion
of Oxon iu m Ylid es 25, 25B, a n d 25C to Oxyk eten es 27,
27B, a n d 27C (Gibbs’ F r ee En er gy (Ha r tr ees) a n d
Rela tive En er gies (k ca l/m ol) Below )a
diazoketone oxonium ylide 25
unsubst. 24:
TS26
oxyketene 27
Con clu sion
B3LYP
-423.093804
-423.089883 -423.130503
+2.46 -23.03
-421.707273 -421.7501544
+9.44 -17.47
(0.0
Both studies on (1) experimental verification of ster-
eochemical profile and (2) theoretical calculations have
proven that the ring-cleavage rearrangement reaction
reported here is a hitherto unknown concerted [3 + 2]
cycloreversion process and, hence, realizing a highly
stereospecific formation of Z- or E-double bond in the
alkenyloxyesters 15 and 16.
QCISD
-421.722315
(0.0
th r eo-24B:
B3LYP
-462.394323
(0.0
-460.880082
(0.0
-462.389003 -462.434218
+3.34 -25.03
-460.863491 -460.908203
+10.41 17.65
QCISD
er yth r o-24C:
B3LYP
-462.390848
(0.0
-460.876384
(0.0
-462.386653 -462.432014
+2.63 -25.83
-460.861308 -460.905480
+9.46 -18.26
Ack n ow led gm en t. Support by a Grant-in-Aid for
Scientific Research on Priority Area “Dynamic Control
of Stereochemistry” from the Ministry of Education,
Science, Sports and Culture is acknowledged. We are
also indebted to Computer Network Service at the
University of Kaiserslautern, Germany, for theoretical
calculations.
QCISD
a
B3LYP ) B3LYP/6-311+G(d,p); QCISD ) QCISD/6-31+G(d)//
B3LYP/6-311+G(d,p).
which would allow a rotation around C(4)-C(5) bond can
thus be clearly excluded.
Employing the more sophisticated but time-expensive
QCISD method, we found (see Table 3) that our calcula-
tions on the B3LYP level are valid for mainly all
structures but underestimate the stability of oxonium
ylide structures 25, 25B, and 25C. The more precise
activation barrier (on the QCISD/6-31+G(d) level) is
Su p p or tin g In for m a tion Ava ila ble: General methods;
detailed experimental procedures for products 1a -d and 19;
spectroscopic data for 1a -f, 4c-g, 5c,d , 7a ,b, 8a ,b, 15b,
16a ,b, 19, 23, 29, and 30. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O0301806
1336 J . Org. Chem., Vol. 69, No. 4, 2004