Electrocyclization reactions of annulated 1,2-diazahepta-2,4-dien-6-ynyl anions

Article abstract of DOI:10.1055/s-2007-991071

Hydrazones 12a,b,f furnish annulated indenone derivatives (6-azafulvenes) 13a-d upon deprotonation with t-BuOK in DMF at 50°C. In contrast, deprotonation of a similar N-methyl hydrazone 12d unexpectedly leads to the isoquinoline derivative 15. Mechanisms

Full text of DOI:10.1055/s-2007-991071

LETTER
2733
Electrocyclization Reactions of Annulated 1,2-Diazahepta-2,4-dien-6-ynyl
Anions
E
V
lectrocyclization
o
Reactions of
l
Annul
oa ted 1,2-Dia
d
y
ien-6-ynyl
A
m
nions yr Lyaskovskyy, Roland Fröhlich, Ernst-Ulrich Würthwein*
Organisch Chemisches Institut, Westfälische Wilhelms-Universität and International NRW Graduate School of Chemistry, Münster,
Corrensstraße 40, 48149 Münster, Germany
Fax +49(251)8339772; E-mail: wurthwe@uni-muenster.de
Received 10 July 2007
undergo exothermic cyclization reactions to form the
heterocyclic analogues 8 and 10. In contrast, systems
with the electronegative heteroatom in an odd position
Abstract: Hydrazones 12a,b,f furnish annulated indenone deriva-
tives (6-azafulvenes) 13a–d upon deprotonation with t-BuOK in
DMF at 50 °C. In contrast, deprotonation of a similar N-methyl
5a
hydrazone 12d unexpectedly leads to the isoquinoline derivative 15. (anions 3 and 5) have low relative energies and therefore
Mechanisms for both transformations are proposed, supported by cyclizations are predicted to be endothermic. Qualitative-
quantum chemical density functional theory (DFT) and ab initio
calculations.
ly, the systems are low in energy, if the electronegative
heteroatom is located in a position with a big HOMO co-
Key words: anionic electrocyclic reactions, indenes, heterocycles, efficient, and they are high in energy, if the electronega-
hydrazones, quinolines, 6-azafulvenes
tive heteroatom is placed in a nodal position of the
HOMO. Various examples for such conversions can be
5
,6
found in the literature. In this context, we became espe-
In recent years, anionic electrocyclization reactions in- cially interested to study derivatives of 1,2-diazahepta-
volving C≡C bonds have emerged as efficient methods for 2,4-dien-6-ynyl anions 2, which have one nitrogen atom
the construction of different carbo- and heterocyclic com- in a even position, favorable for cyclizations, and another
1
pounds. Recently, we have reported on our explorations one in an unfavorable odd position. We wanted to learn,
of annulated 2-azahepa-2,4-dien-6-ynyl anions
1
how chemistry behaves in such ambiguous systems and
(
Figure 1). They were recognized as valuable intermedi- what kind of products may be formed.
2
2c,3
ates for the synthesis of aniline or azepine derivatives,
depending on the reaction conditions applied. Now, we
became interested in the related 1,2-diazahepta-2,4-dien-
6
(
previously, in contrast to simpler 1,2-diazaallyl anions,
which have been well investigated, mostly in anionic
[
H
N
N
[
[
+14.11]
–18.87]
-ynyl anions 2 and their application in synthesis
Figure 1). These extended systems have not been studied
H
3
4
7
N
N
4
3+2]-cycloaddition reactions. In this letter we present
8
our first results.
N
N
[
+3.39]
5
9
N
R
N
R
N
R
N
N
[
–15.27]
R
1
2
6
10
Figure 1
Scheme 1 Calculated heats of reactions for the electrocyclization
reaction of azaheptatrienyl anions to azacycloheptadienyl anions
[kcal/mol](MP2/6-31+G*//6-31+G*)
Earlier, we have demonstrated on the basis of quantum
chemical ab initio calculations that for anionic hetero-
polyenyl systems the nature and position of the hetero- The annulated 1,2-diazahepta-2,4-dien-6-ynyl anions 2
atom within the chain mainly determine the relative were prepared by deprotonation of the corresponding
5
energy of such systems. Thus, for azaheptatrienyl anions hydrazones 12a–g (Table 1), which were synthesized in
2
3
–6 (Scheme 1), high relative energy has to be expected, one step by condensation of alkynyl aldehydes 11 with
if the electronegative heteroatom is located at an even phenyl- or methylhydrazine in the presence of molecular
7
position within the chain (anions 4 and 6). Such systems sieves (Scheme 2).
The deprotonation of the N-phenyl hydrazones 12a,b,f,
bearing an alkyl substituent at the C≡C moiety, was
carried out in DMF at 50 °C using t-BuOK as base and led
to the indenone hydrazones (6-azafulvenes) 13a,c,d
SYNLETT 2007, No. 17, pp 2733–2737
Advanced online publication: 25.09.2007
DOI: 10.1055/s-2007-991071; Art ID: G24007ST
1
6
.1
0
.
2
0
0
7
©
Georg Thieme Verlag Stuttgart · New York

2734
V. Lyaskovskyy et al.
LETTER
H
N
A possible mechanism, proposed for this reaction, is
represented in Scheme 4. Deprotonation of the starting
hydrazone by t-BuOK leads to the 1,2-diazaallyl anion 2a,
a rather versatile nucleophile, which may attack the C≡C
bond utilizing either carbon atom C3 or the nitrogen at-
oms N1 or N2, resulting in an electrocyclic process to pro-
duce the five-membered carbocycle or the six- or seven-
membered heterocycles (exo- or endo–dig cyclization). In
our experiments, exclusively the cyclization reaction to
H
O
CH2Cl2, 20 °C , 16 h
R1
MS
R1
N
+
N
H2N
R2
R2
1
1
12a–g
Scheme 2
8
(
Scheme 3) in moderate yields after aqueous workup. In form the five-membered ring 14 (Scheme 4) was ob-
a similar fashion, quenching the reaction mixture with served, and not the reaction involving the nitrogen atoms,
methyl iodide resulted in the methylated compound 13b. leading to six- or seven-membered rings. Finally, the in-
It is noteworthy that the yields of the crude products were termediate 14 is stabilized by protonation with subsequent
much higher (60–70%), but column chromatographic pu- tautomerism to form the 6-azafulvene derivatives 13a–d
rification led to some decomposition of the 6-azafulvene as reaction products.
derivatives 13a–d. We were able to characterize the mo-
lecular structure of the picrate of 13d, a pyridinium salt,
by X-ray diffraction. Such cyclization reactions forming
Ph
N
Ph
N
Ph
N
9
N
H
N
N
five-membered rings are especially interesting in compar-
2
,3
ison to our previous investigations of similar anions (1),
t-BuOK
where either six- or seven-membered ring systems were
obtained.
R
R
1
2a,b,f
2a
R
N
Ph
Ph
Ph
H
Ph
N
N
N
N
N
N
N
H O
or MeI
N
t-BuOK
0 °C
2
E
N
Ph
H
5
H
R
R
R
R
DMF
X
X
R
E = H or Me
3a–d
H
14
H
1
2a,b,f
13a–d
1
12a, 13a X = CH, R = n-Bu, E = H;
13b, X = CH, R = n-Bu, E = Me;
12b, 13c X = CH, R = CH2OMe, E = H;
12f, 13d X = N, R = n-Bu, E = H
Scheme 4 Deprotonation of hydrazone 12b and atomic numbering
(IUPAC) of the isoquinoline 15
Scheme 3
Table 1 Structures and Yields of Compounds 12a–g and 13a–d
H
N
8
1
N
Me
t-BuOK
20 °C
2
H O
N
DMF
6
4a
n-Bu
Structure
Compound, yield
4
n-Bu
1
2
H
N
12a, R = Ph, R = n-Bu, 80%
12d
15, 47%
1
2
1
2b, R = Ph, R = CH OMe, 82%
2
R1
1
2
N
N
12c, R = R = Ph, 85%
Scheme 5
1
2
1
1
2d, R = Me, R = n-Bu, 86%
1
2
2e, R = Me, R = CH OMe, 80%
2
R2
It is interesting to note that the deprotonation of the
corresponding phenylethynyl-substituted hydrazone 12c
afforded no cyclization product under the same condi-
tions, as only starting material was re-isolated.
1
2
H
N
12f, R = Ph, R = n-Bu, 91%
1
2
1
2g, R = Me, R = n-Bu, 80%
R1
N
The cyclization of the related methylethinyl hydrazone
12d gave a rather unexpected result. Thus, deprotonation
of 12d using t-BuOK in DMF at room temperature led to
R2
1
2
3
R1
13a, R = Ph, R = n-Bu, R = H, 25%
1
2
3
1
3b, R = Ph, R = n-Bu, R = Me, 28%
N
the formation of the isoquinoline derivative 15
N
N
R3
R2
1
2
3
13c, R = Ph, R = CH OMe, R = H, 50%
10
2
(
Scheme 5). Its structure was identified using 2D NMR
spectroscopy (all NMR data were in agreement with
11
literature data and are summarized in Table 2).
1
2
3
R1
13d, R = Ph, R = n-Bu, R = H, 25%
The surprising removal of the terminal NMe moiety,
stemming from the hydrazone, encouraged us to investi-
gate, whether it occurs during the aqueous workup or
takes place in the reaction mixture before the addition of
the electrophile. Quenching of the crude reaction mixture
N
R3
R2
N
Synlett 2007, No. 17, 2733–2737 © Thieme Stuttgart · New York

LETTER
Electrocyclization Reactions of Annulated 1,2-Diazahepta-2,4-dien-6-ynyl Anions
2735
Table 2 Selected NMR Data (ppm) for Compound 15
1
3
4
4a
5
6
7
8
8a
–
1H NMR (d)
13C NMR (d)
9.22
151.9
–
7.48
118.0
–
7.76
126.0
7.66
130.2
7.54
126.3
7.94
127.5
155.6
136.5
127.0
with methyl iodide (instead of water) led to a colorless The proposed next step might be considered as an S 2-
N
1
3
solid, insoluble in diethyl ether, which was identified as type substitution reaction at the exocyclic nitrogen atom.
the iodomethylate of isoquinoline 15 (15·MeI). From this It requires the attack of a nucleophilic species (e.g. t-
result it appears that the removal of the NMe moiety most BuOK) on the NHMe nitrogen atom with subsequent
probably occurs before the aqueous workup.
fission of the N–N bond and loss of an amine moiety. A
number of examples of similar removals of NR fragments
from quinolinium N-imines are described in the literature.
However, such reactions take place under quite different
The possible mechanism of this process is represented in
Scheme 6. We suggest that the deprotonation of hydra-
zone 12d by t-BuOK leads to the annulated 1,2-diaza-
hepta-2,4-dien-6-ynyl anion 2b. Apparently, the anionic
1
4
conditions, e.g. as photochemical process, as thermal
1
5
2
reaction (refluxing in ethylene glycol), or in the presence
character at the sp -hybridized nitrogen atom N2 of inter-
1
6
of reductive agents (Zn–AcOH).
mediate 2b is sufficiently pronounced to allow the attack
at the C≡C bond. The subsequent electrocyclization reac- To gain additional insight into the electrocyclization
1
2
tion leads to the zwitterionic intermediate 16a, which in processes, we have carried out quantum chemical calcula-
turn may be stabilized by protonation (e.g. from t-BuOH) tions [SCS-MP2/6-311G(d,p)//B3LYP/6-311G(d,p), in-
1
7,18
furnishing the intermediate 17. It is remarkable, that cluding zero point correction ] of the cyclization step
3
neither the sp -hybridized nitrogen atom N1 nor the of some model systems of type 2c (Me instead of n-Bu as
carbon atom C3 are involved in the cyclization.
a substituent) involving three possible ring-closure path-
ways after the initial deprotonation. The counterions were
not considered in these model calculations. The results are
represented in Figure 2.
Me
N
Me
N
Me
N
N
H
N
.
.
N
According to the results presented in Figure 2, quantum
chemical calculations favor the formation of the six-mem-
bered ring intermediate of type 16b thermodynamically
by ca. 5 kcal/mol over both competing five- (18, red
curve) and seven-membered (19, blue curve) ring inter-
mediates. Kinetically, the formation of the seven-mem-
bered ring system 19 (TS2) is predicted to be slightly
preferred (ca. 1 kcal/mol) over the cyclization to the six-
membered ring intermediate 16b (TS1). This indicates
that the first reaction step might be understood as a
n-Bu
n-Bu
2
b
n-Bu
Nu
1
2d
1
6a
Me
N
:
H+
Me
N
2
N
H
15
+
H
Nu
n-Bu
1
7
Scheme 6
Figure 2 Energy diagrams for three possible cyclization reactions of anions of type 2c [kcal/mol][SCS-MP2/6-311G(d,p)//B3LYP/6-
11G(d,p), including zero point correction]
3
Synlett 2007, No. 17, 2733–2737 © Thieme Stuttgart · New York

2736
V. Lyaskovskyy et al.
LETTER
thermodynamically and, to a considerable degree, also
kinetically controlled process. Interestingly, the calcula-
tions predict the formation of the five-membered ring sys-
tem 18 to be kinetically less favorable for the N-methyl
hydrazone. However, from N-phenyl hydrazones 12a,b,f,
five-membered rings 13a–d were obtained experimental-
ly. Metal template effects may further contribute to ex-
plain the observed reaction products.
(4) (a) Kauffmann, T.; Berger, D.; Scheerer, B.; Woltermann, A.
Chem. Ber. 1977, 110, 3034. (b) Kauffmann, T.; Berger, D.;
Scheerer, B.; Woltermann, A. Angew. Chem., Int. Ed. Engl.
1
970, 9, 961. (c) Kauffmann, T.; Beißner, G.; Köppelmann,
E.; Kuhlmann, D.; Schott, A.; Schrecken, H. Angew. Chem.,
Int. Ed. Engl. 1968, 7, 131. (d) Kauffmann, T.; Ban, L.;
Kuhlmann, D. Angew. Chem., Int. Ed. Engl. 1967, 6, 256.
(e) Kauffmann, T.; Hansen, J.; Kosel, C.; Schoeneck, W.
Ann. 1962, 656, 103. (f) Baldwin, J. E.; Adlington, R. M.;
Newington, I. M. J. Chem. Soc., Chem. Commun. 1986, 176.
5) (a) Klötgen, S.; Fröhlich, R.; Würthwein, E.-U. Tetrahedron
5
In view of our earlier studies, these calculations indicate
(
that the stabilizing effect of the nitrogen atom in an odd
position of the 1,2-diazahepta-2,4-dien-6-ynyl anions 2 is
more pronounced than the destabilization of the nitrogen
in an even position, since the anionic open-chain inter-
mediate is calculated to be slightly lower in energy than its
cyclic isomers. However, experimentally, irreversible ad-
ditions of electrophiles open new synthetic options for the
preparation of five- and six-membered rings as shown.
1
996, 52, 14801. (b) Klötgen, S.; Würthwein, E.-U.
Tetrahedron Lett. 1995, 36, 7065.
(6) (a) Hunter, D. H.; Steiner, R. P. Can. J. Chem. 1975, 53,
55. (b) Kloosterziel, H.; Van Drunen, J. A. A. Recl. Trav.
3
Chim. Pays-Bas 1970, 89, 667.
(
7) Preparation of 1-[2-(Hex-1-ynyl)benzylidene]-2-phenyl-
hydrazine (12a): A mixture of 2-(1-hexynyl)benz-aldehyde
(0.93 g, 5.00 mmol) and phenylhydrazine (0.54 g, 5.00
mmol) in anhyd CH Cl (20 mL) was stirred in the presence
2
2
of molecular sieves (4 Å) at r.t. for 16 h. Molecular sieves
were removed by filtration through Celite and washed with
CH Cl (20 mL). The solvent was removed in vacuo to
afford the hydrazone 12a (1.10 g, 4.00 mmol, 80%) as a
yellow oil. The obtained hydrazone was pure enough to be
used without further purification. FTIR (film): 3313 (br, w),
The deprotonation of similar hydrazones 12e,g under the
same reaction conditions resulted in a mixture of different
substances. Their separation by column chromatography
was unsuccessful.
2
2
In conclusion, electrocyclization reactions of the hitherto
unknown annulated 1,2-diazahepta-2,4-dien-6-ynyl an-
ions 2 were investigated. Their chemical fate after depro-
tonation was recognized to be quite sensitive to the nature
of the substituents attached to the conjugated chain. Thus,
deprotonation of N-phenyl hydrazones 12a,b,f led to inde-
none derivatives 13a–d in moderate yields. In contrast,
deprotonation of N-methyl hydrazone 12d afforded an un-
expected product, the isoquinoline derivative 15, which
was isolated in 47% yield. Mechanisms to explain the two
transformations were proposed. In contrast to our earlier
2
931 (s), 2871 (s), 2862 (m), 2225 (m), 1575 (m), 1494 (s),
–
1 1
1448 (m), 1257 (s), 1145 (m) cm . H NMR (400 MHz,
CDCl ): d = 0.98 (t, J = 7.3 Hz, 3 H), 1.47–1.68 (m, 4 H),
3
2.48 (t, J = 7.4 Hz, 2 H), 6.87 (tt, J = 7.3, 1.2 Hz, 1 H), 7.11–
7.14 (m, 2 H), 7.19 (dd, J = 7.4, 1.4 Hz, 1 H), 7.25–7.30 (m,
3 H), 7.38 (dm, J = 7.8 Hz, 1 H), 7.72 (br s, 1 H, NH), 8.04
1
3
(
dm, J = 8.0 Hz, 1 H), 8.19 (s, 1 H, CH=N). C NMR (100
MHz, CDCl ): d = 13.7, 19.4, 22.2, 30.9, 78.1, 95.7, 112.8,
3
1
1
2
8
20.2, 122.4, 124.5, 127.8, 127.9, 129.3, 132.6, 136.0,
36.1, 144.6. HRMS (ESI): m/z calcd for C H N H:
1
9
20
2
77.1705; found: 277.1699. Anal. Calcd for C H N : C,
1
9
20
2
2.57; H, 7.29; N, 10.14. Found: C, 82.87; H, 7.48; N, 9.57.
2
,3
investigations on similar monoaza derivatives 1, the
formation of seven-membered heterocycles was not
observed in the present experiments.
(8) Preparation of 1-(2-Butyl-1H-inden-1-ylidene)-2-
phenylhydrazine (13a): Hydrazone 12a (0.28 g, 1.00
mmol) was added under argon to the solution of t-BuOK
(
0.22 g, 2.00 mmol) in DMF (5 mL) and the mixture was
stirred at 50 °C until all starting hydrazone had disappeared
(approx. 5 h, TLC monitoring). H O (20 mL) was added and
Acknowledgment
2
the mixture was extracted with Et O (3 × 20 mL). The
2
The authors thank the NRW Graduate School of Chemistry
combined organic extracts were dried with MgSO and the
4
(
Münster), the Sonderforschungsbereich 424 (Deutsche For-
schungsgemeinschaft) and the Fonds der Chemischen Industrie
Frankfurt) for financial support of this work.
residue was purified by flash chromatography (Et O–
2
pentane–Et N, 0.5:10:0.2) giving 13a (0.07 g, 0.25 mmol,
3
(
2
1
1
5%) as a yellowish oil. FTIR (film): 2956 (s), 2927 (s),
600 (s), 1566 (s), 1504 (s), 1494 (s), 1255 (s), 1207 (s),
–
1 1
168 (s), 1151 (w), 1122 (m), 1093 (w) cm . H NMR (500
References and Notes
MHz, CDCl ): d = 1.00 (t, J = 7.1 Hz, 3 H), 1.43–1.52 (m, 2
3
H), 1.68–1.75 (m, 2 H), 2.66–2.69 (m, 2 H), 6.56 (s, 1 H),
(
1) (a) Zeni, G.; Larock, R. C. Chem. Rev. 2004, 104, 2285.
7
.03 (tt, J = 7.3, 1.0 Hz, 1 H), 7.17–7.40 (m, 7 H), 7.68 (d,
(b) Zeni, G.; Larock, R. C. Chem. Rev. 2006, 106, 4644.
(c) Cacchi, S.; Fabrizi, G. Chem. Rev. 2005, 105, 2873.
(d) Harvey, D. F.; Sigano, D. M. Chem. Rev. 1996, 96, 271.
(e) Jäger, V.; Viehe, H. G. In Houben–Weyl, Methoden der
1
3
J = 7.2 Hz, 1 H), 8.92 (s, 1 H). C NMR (125 MHz, CDCl3):
d = 14.1, 22.7, 26.2, 31.4, 113.6, 121.0, 121.8, 122.7, 124.9,
126.1, 127.4, 128.8, 129.4, 143.9, 144.0, 144.1, 144.3.
HRMS (ESI): m/z calcd for C H N H: 277.1705; found:
organischen Chemie, Vol. 5/2a; Jäger, V., Ed.; Thieme:
Stuttgart, 1977, 883–908.
19 20
2
277.1699. Anal. Calcd for C H N : C, 82.57; H, 7.29; N,
19 20 2
10.14. Found: C, 82.64; H, 7.54; N, 9.62.
(
2) (a) Sagar, P.; Fröhlich, R.; Würthwein, E.-U. Angew. Chem.
Int. Ed. 2004, 42, 5694. (b) Lyaskovskyy, V.; Fröhlich, R.;
Würthwein, E.-U. Synthesis 2007, 2135. (c) Lyaskovskyy,
V.; Fröhlich, R.; Würthwein, E.-U. Chem. Eur. J. 2007, 13,
(
9) CCDC 658782 contains the supplementary crystallographic
data of picrate 13d·2,4,6-(O N) C H (OH). These data can
2
3
6
2
be obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html [or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
3
113.
(3) Lyaskovskyy, V.; Bergander, K.; Fröhlich, R.; Würthwein,
+44(1223)336033, E-mail: deposit@ccdc.cam.ac.uk].
E.-U. Org. Lett. 2007, 9, 1049.
Synlett 2007, No. 17, 2733–2737 © Thieme Stuttgart · New York

LETTER
Electrocyclization Reactions of Annulated 1,2-Diazahepta-2,4-dien-6-ynyl Anions
2737
(
10) Preparation of 3-Butylisoquinoline (15): Hydrazone 12d
0.21 g, 1.00 mmol) was added under argon to the solution
(15) Ghosh, C. K.; Pal, C.; Maiti, J.; Sarkar, M. J. Chem. Soc.,
Perkin Trans. 1 1988, 1489.
(
of t-BuOK (0.22 g, 2.00 mmol) in DMF (5 mL) and the
mixture was stirred at r.t. until all starting hydrazone had
(16) Burgos, C.; Delgado, F.; García-Navío, J. F.; Izquierdo, M.
L.; Alvarez-Builla, J. Tetrahedron 1995, 51, 8649.
disappeared (approx. 0.5 h, TLC monitoring). H O (20 mL)
(17) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria,
G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.
Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;
Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi,
M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.;
Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V.
G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.;
Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision C.02; Gaussian, Inc.: Wallingford CT, 2004.
(b) Details of the quantum chemical calculations may be
obtained from E.-U. Würthwein upon request.
2
was added and the mixture was extracted with Et O (3 × 20
2
mL). The combined organic extracts were dried with MgSO4
and the residue was purified by flash chromatography (tert-
butyl methyl ether) giving 15 (0.09 g, 0.47 mmol, 47%) as
a yellow oil. All spectroscopic and physical data were in
accordance with published data (see ref. 11).
(
11) (a) Korivi, R. P.; Cheng, C.-H. Org. Lett. 2005, 7, 5179.
(b) Flippin, L. A.; Muchowski, J. M. J. Org. Chem. 1993, 58,
2
631.
(
12) Bast, K.; Behrens, M.; Durst, T.; Gashey, R.; Huisgen, R.;
Schiffer, R.; Temme, R. Eur. J. Org. Chem. 1998, 379.
13) (a) Van Gunsteren, W. E.; Liu, H.; Muller-Plathe, E. Chem.
Eur. J. 1996, 2, 191. (b) Bühl, M.; Schaefer, H. F. III J. Am.
Chem. Soc. 1993, 115, 9143. (c) Geerke, D. P.; Thiel, S.;
Thiel, W.; van Gunsteren, W. F. J. Chem. Theory Comput.
(
2007, 3, 1499. (d) Yi, R.; Basch, H.; Hoz, S. J. Org. Chem.
2002, 67, 5891. (e) Ren, Y.; Chu, S.-Y. J. Phys. Chem. A
2004, 108, 7079. (f) Ulbrich, R.; Famulok, M.; Bosold, F.;
Boche, G. Tetrahedron Lett. 1990, 31, 1689.
g) Yamamoto, F.; Oae, S. Bull. Chem. Soc. Jpn. 1975, 48,
7. (h) Erdik, E.; Ay, M. Chem. Rev. 1989, 89, 1947.
i) Beak, P.; Li, J. J. Am. Chem. Soc. 1991, 113, 2796.
14) Snieckus, V.; Kan, G. J. Chem. Soc., Chem. Commun. 1970,
72.
(
7
(
(
(18) Grimme, S. J. Chem. Phys. 2003, 118, 9095.
1
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