Lewis Acid Catalyzed Synthesis of N,O-Heterocycles
COMMUNICATION
employed and that the heterocyclic product 3 can be trans-
formed to heterocyclic product 4 through AuI-catalyzed hy-
dration of the alkyne moiety of compound 3. On the basis
of 1H NMR spectroscopic tracing experiments under the
AuI-catalyzed standard reaction conditions, we confirmed
that, as the reaction is proceeding, 2l is initially transformed
to the corresponding alkynyl product 3l within 2 h, indicat-
ing that the reaction rate for the formation of 3l is very fast
(see Supporting Information for details). After 24 h, the
alkyne-moiety-containing compound 3l was completely
transformed to the corresponding carbonyl-group-containing
product 4l through AuI-catalyzed hydration of the alkyne
moiety of compound 3l. When H218O was added to the reac-
tion system, product 4a-18O with 57% 18O content was
formed in 72% yield, further indicating that external H2O is
involved in this reaction (Scheme 3c).
bond can be further activated by the regenerated AuI to ex-
clusively afford the final carbonyl-group-containing product
4 through alkyne hydration via the olefinic gold species Bꢀ,
presumably owing to a steric effect (Scheme 4, path b).[23]
In conclusion, we have established two efficient Lewis
acid catalyzed reaction systems to construct five-membered
N,O-heterocyclic products containing an alkyne moiety or a
carbonyl group from sulfonamide-substituted VDCP-di-
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a significant role to effect the reaction outcomes. Further ef-
forts regarding the scope and mechanistic details are in
progress.
On the basis of the above experiments, plausible reaction
mechanisms for the formation of 3 and 4 catalyzed by Yb-
Experimental Section
ACHTUNGTRENNUNG
(OTf)3 and the AuI complex are proposed in Scheme 4 to ra-
General remarks: 1H and 13C NMR spectra were recorded at 400 (or 300)
and 100 (or 75) MHz, respectively. Mass and HRMS spectra were record-
ed by ESI (or MALDI) method. Organic solvents used were dried by
standard methods, when necessary. Satisfactory CHN microanalyses were
obtained with an analyzer. Commercially obtained reagents were used
without further purification. All reactions were monitored by TLC with
silica gel coated plates. Flash column chromatography was carried out
using silica gel at increased pressure.
General procedure for the preparation of sulfonamide-substituted
VDCP-diesters 2: Hydrazine hydrate (85% , 0.2 mL) was added to a so-
lution of phthalimide-substituted VDCP-diester 1 (2.0 mmol) in THF
(20 mL). The reaction mixture was stirred at room temperature for
30 min and during this time white solids appeared. The mixture was fil-
tered to remove the precipitates and the filter cake was washed with
Et2O. The filtrate was concentrated to afford the crude alkoxylamine-
substituted VDCP-diester, which was applied to the next transformation
without further purification. Et3N (2.5 mmol) and subsequently the corre-
sponding sulfonyl chloride (2.0 mmol) were added to a chilled solution of
alkoxylamine-substituted VDCP-diester (2.0 mmol) in dry CH2Cl2
(20 mL). The reaction mixture was then stirred at room temperature.
After the substrate was consumed, the solvent was removed under re-
duced pressure and the residue was purified by silica gel flash column
chromatography to give the desired sulfonamide-substituted VDCP-die-
ster 2.
General procedure for the YbACHTUNTRGNEUNG(OTf)3-catalyzed domino intramolecular
hydroamination and ring-opening of sulfonamide-substituted 1,1-vinylide-
necyclopropanediesters: Under an argon atmosphere, sulfonamide-substi-
Scheme 4. A plausible reaction mechanism.
tuted VDCP-diester 2 (0.1 mmol) and YbACTHNUTRGNENUG(OTf)3 (10 mol%) were added
to a Schlenk tube, and then toluene (1.0 mL) was added. The mixture
was stirred at room temperature (208C) for 2 d. After the substrate was
consumed, the solvent was removed under reduced pressure and the resi-
due was purified by silica gel flash column chromatography to afford the
desired product 3.
tionalize the reaction outcomes. YbACTHNUTRGENUN(G OTf)3 acts as the hard
Lewis acid to initiate the domino reaction through coordina-
tion with the two carbonyl oxygen atoms of 2 (Scheme 4,
path a) and the AuI complex acts the soft Lewis acid to ini-
tiate the domino reaction through coordination with the
allene moiety (Scheme 4, path b). In path a, the common in-
tramolecular hydroamination along with the ring-opening of
cyclopropane[8,22] takes place to give intermediate A, which
gives the corresponding heterocyclic product 3 through
proton transfer. In path b, the common intramolecular hy-
droamination takes place to give intermediate A’, which af-
fords the corresponding heterocyclic product 3 through a
ring-opening of cyclopropane, elimination of the AuI com-
General procedure for the Au-catalyzed domino intramolecular hydro-
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2
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added to a Schlenk tube, and then toluene (1.0 mL) was added. The mix-
ture was stirred at room temperature (208C) for 5 min and AgOTf
(5 mol%) was added to the reaction system, followed by the addition of
H2O (0.1 mmol, 1.0 equiv). After the substrate was consumed, the solvent
was removed under reduced pressure and the residue was purified by
silica gel flash column chromatography to afford the desired product 4.
Sulfonamide-substituted VDCP-diester 2a: A light yellow oil. 1H NMR
(CDCl3, 400 MHz,): d=2.32 (dd, J=8.0 Hz, 4.8 Hz, 1H; CH2), 2.35–2.42
(m, 1H), 2.46–2.56 (m, 2H), 3.74 (s, 3H; OCH3), 3.75 (s, 3H; OCH3),
À
plex, and a proton transfer sequence. Herein, the C C triple
Chem. Eur. J. 2011, 17, 13160 – 13165
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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