Reactions of imines with 2,2-difluoro-2-fluorosulfonylacetyl fluoride

Article abstract of DOI:10.1016/j.jfluchem.2010.07.004

Selected N-alkyl ketoimines having at least one methyl group at the imino carbon were reacted in a Stork enamine reaction with 2,2-difluoro-2- fluorosulfonylacetyl fluoride to yield enaminones carrying one fluorosulfonyldifluoromethylene function. In the

Full text of DOI:10.1016/j.jfluchem.2010.07.004

Journal of Fluorine Chemistry 131 (2010) 996–999
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Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Reactions of imines with 2,2-difluoro-2-fluorosulfonylacetyl fluoride
Katja Vlasov a, Nataliya Kalinovich b, Enno Lork a, Gerd-Volker Ro¨schenthaler b,
*
a Institute of Inorganic & Physical Chemistry, Leobener Strasse, University of Bremen, 28334 Bremen, Germany
b School of Engineering and Science, Jacobs University Bremen gGmbH, Campus Ring 1, 28759 Bremen, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
Selected N-alkyl ketoimines having at least one methyl group at the imino carbon were reacted in a Stork
enamine reaction with 2,2-difluoro-2-fluorosulfonylacetyl fluoride to yield enaminones carrying one
fluorosulfonyldifluoromethylene function. In the case of N-tertbutyl methyl aldimine two of these
groups were present in a triketone derivative. Taking a ethylene bis(imine) also a ring closure reaction
was observed, proven by X-ray structure analysis.
Received 30 May 2010
Received in revised form 5 July 2010
Accepted 8 July 2010
Available online 15 July 2010
ß 2010 Elsevier B.V. All rights reserved.
Keywords:
Ketoimines
Aldimine
2,2-Difluoro-2-fluorosulfonylacetyl fluoride
Enaminones
1. Introduction
fluoride 2 to give the HF adducts of the corresponding fluorinated
enaminones (aminovinyl ketones). Hydrogen fluoride was removed
Enamino ketones are important synthetic precursors, e.g. in
heterocyclic chemistry, and their synthesis is well-documented [1–
4]. Fluorinated b-enamino ketones have attracted attention mainly
due to theirpossibleuse in synthesis ofpharmacologically important
heterocycles [5,6]. These compounds were synthesized by regiose-
lective condensation of fluoroalkyl-b-diketones using amines [7],
the partial hydrolysis of 1,3-diimines to imidoyl chlorides and
enolisable ketones [8] or by C-perfluoroalkylation of stable enamines
[9,10]. Recently, a novel method for preparing fluorinated enamino
ketones and diketones was introduced using perfluoroalkanoyl
fluorides [11,12]. Commercially available 2,2-difluoro-2-fluorosul-
fonylacetyl fluoride has found many applications in organofluorine
chemistry. This reagent was used as a sourcefor difluorocarbeneand,
as an effective trifluoromethylating reagent [13,14]. 2,2-Difluoro-2-
fluorosulfonylacetyl fluoride as acylating reagent in the case of
amines and alcohols has been already described. [15–17] To the best
of our knowledge the reactions of 2,2-difluoro-2-fluorosulfonylace-
tylfluoridewithimineshavenot beenstudied. Theaimofthisworkis
the synthesis of new aminovinyl ketones in the reaction of imines
with 2,2-difluoro-2-fluorosulfonylacetyl fluoride.
easily by extraction with sodium hydrogen carbonate solution to
give the compounds 3a–g in good to high yields (Scheme 1). The
reactions proceeded at ambient temperature in 3–8 h. All the
compounds obtained are non-moisture sensitive colorless oils or
solids. It is notable that the aminovinyl ketones 3a–h were obtained
exclusively as the Z-isomers as a result of the formation an
intramolecular hydrogen NHꢀ ꢀ ꢀO bond. The formation of the latter
was proved by X-ray studies of compound 3f (Fig. 1). The X-ray data
show intramolecular hydrogen bonding N(1)Hꢀ ꢀ ꢀO(1)C with 271.7
pm (3f) giving rise to almost planar six-membered ring systems. The
respective distances in the enamino ketone system between the
atoms N(1)–C(7) 132.2(3), C(7)–C(12) 140.4(3), C(12)–C(13)
139.7(3) pm are between a single and a double bond [20] indicating
the expected delocalization with sp2-hybridised carbon atoms. The
O–C bond length O(1)–C(13) 124.2(3) pm of 3f is slightly longer than
a typical C55O double bond [20].
The reaction of 1h with two equivalents of 2,2-difluoro-2-
fluorosulfonylacetyl fluoride 2 under the same conditions men-
tioned above led to the corresponding product 4 h containing two
alkanoyl fragments in good yield (Scheme 2).
When the diimine 1i was reacted with two equivalents of 2, the
mixture of two colorless products 5i and 6i in a 1:1 ratio (100%
conversion by 19F NMR) was obtained (Scheme 3).
2. Results and discussion
Obviously, two successive Stork reactions took place; the first to
generate the intermediate A1, which is attacked by a second
molecule of 2 to yield the linear product 5i or to produce the cyclic
compound 6i via an intramolecular attack from the amine nitrogen
in the tautomer A2 to the imine carbon under formation of a 2,2-
dimethylimidazolidine where subsequently the N–H fragment
Selected imines 1a–i reacted in a Stork enamine reaction [18,19]
with equimolar amount of 2,2-difluoro-2-fluorosulfonylacetyl
* Corresponding author. Tel.: +49 0 421 2003138.
E-mail address: g.roeschenthaler@jacobs-university.de (G.-V. Ro¨schenthaler).
0022-1139/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2010.07.004

K. Vlasov et al. / Journal of Fluorine Chemistry 131 (2010) 996–999
997
Scheme 2.
Scheme 1.
Fig. 2. Molecular structure of 5i. Selected bond lengths (pm) and angles (8): N(1)–
C(2)132.4(3), C(2)–C(4) 140.6(3), C(4)–C(5) 138.9(3), O(1)–C(5) 124.9(2), S(1)–F(3)
146.93(18), S(1)–O(2) 143.03(17), S(1)–O(3) 140.9(17);O(1)–C(5)–C(4) 128.63(19),
C(5)–C(4)–C(2) 122.5218, N(1)–C(2)–C(4) 120.89(18), O(1)–C(5)–C(6) 115.2(17).
Fig. 3. Molecular structure of 6i. Selected bond lengths (pm) and angels (8): N(1)–
C(2) 133.2(3), N(1)–C(1) 147.4(3), N(1)–C(8) 149.8(3), N(2)–C(11) 133.3(3), C(7)–
C(1) 150.7(4), C(2)–C(4) 139.4(3), C(4)–C(5) 140.2(4), O(1)–C(5) 123.6(3), S(1)–F(3)
155.4(2), S(1)–O(2) 140.1(2), S(1)–O(3) 140.5(2);O(1)–C(5)–C(4) 132.3(2), C(5)–
C(4)–C(2) 124.9(2), N(1)–C(2)–C(4) 123.7(2), O(1)–C(5)–C(6) 111.8(2).
Fig. 1. Molecular structure of 3f. Selected bond lengths (pm) and angles (8): N(1)–
C(7)132.2(3), C(7)–C(12) 140.4(3), C(12)–C(13) 139.7(3), O(1)–C(13) 124.2(3),
S(1)–F(3) 156.28(15), S(1)–O(2) 139.71(19), S(1)–O(3) 140.2(2);O(1)–C(13)–C(12)
129.2(2), C(13)–C(12)–C(7) 122.2(2), N(1)–C(7)–C(12) 120.9(2), O(1)–C(13)–C(14)
116.08(19).
generated reacts with 2 giving rise to the amide 6i via abstraction
of HF.
Similar to compound 3f, the X-ray analysis of 5i (Fig. 2) shows a
planar six-membered ring with distances among atoms (N(1)–
C(2)132.4(3), C(2)–C(4) 140.6(3), C(4)–C(5) 138.9(3) pm), which
indicates a delocalized double bond system. An intramolecular
hydrogen bonding N(1)Hꢀ ꢀ ꢀO(1)C with 271.9 pm was observed.
The structure of compound 6i was confirmed by X-ray analysis.
Scheme 3.

998
K. Vlasov et al. / Journal of Fluorine Chemistry 131 (2010) 996–999
The 1H, 19F, 13C NMR data of the products 3a–h, 4h, 5i, 6i (Fig. 3)
are in accord with the proposed structures. The CF2 group of the
new compounds was observed in 13C NMR as a triplet of doublets
Bu]+ (47); HRMS for [M]+ (C14H16F3NO3S): calculated 335.0803,
found 335.0801.
in the
d
C = 115.2–116 region, the C55O group appears as a triplet in
4.1.3. (Z)-4-(Benzylamino)-1,1,5,5,5-pentafluoro-2-oxopent-3-ene-
the value about
d
C = 173.
1-sulfonyl fluoride 3c (78%)
Colorless crystals; mp. 31 8C; 1H NMR (CDCl3):
d
4.73 (d,
J = 5.8 Hz, 2H), 6.10 (s, 1H), 7.32–7.35 (m, 2 H), 7.39–7.43 (m, 3H),
11.0 (bs, 1H, NH); 13C NMR (CDCl3):
50.0 (s), 87.3 (s), 115.2 (dt,
3. Conclusion
d
J = 303.2 Hz, J = 27.9 Hz, CF2), 119.3 (q, J = 278.9 Hz), 128.1 (s), 129.4
(s), 129.8 (s), 134.8 (s), 154.8 (q, J = 33.9 Hz), 178.1 (t, J = 22.6 Hz,
We have studied the reaction of imines with 2,2-difluoro-2-
fluorosulfonylacetyl fluoride as an acylating reagent. It has been
shown that the corresponding aminovinyl ketones 3a–h exist
C55O); 19F NMR (CDCl3):
d
= 39.7 (s, 1F), ꢁ68.3 (s, 3F), ꢁ106.0(s, 2F);
MS (EI) m/z (%): 361 [M]+ (10), 278 [M-SO2F]+ (50); HRMS for [M]+
exclusively as Z-isomers as
a result of the formation an
(C12H9F6NO3S): calculated 361.0207, found 361.0219.
intramolecular hydrogen NHꢀ ꢀ ꢀO bond. In the case of the aldimine
1h an enamine dione is formed. Reacting the diimine 1i with 2
surprisingly a ring closure product was observed.
4.1.4. (Z)-4-(Cyclohexylamino)-1,1-difluoro-2-oxo-4-phenylbut-3-
ene-1-sulfonyl fluoride 3d (40%)
Yellow crystals, mp. 55 8C; 1H NMR (CDCl3):
d 1.20–1.24 (m, 3
4. Experimental
H), 1.41–1.45 (m, 4H), 1.53–1.58 (m, 4H), 3.40–3.48 (m, 1H), 5.48
(s, 1H), 7.23–7.25 (m, 2H), 7.51–7.56 (m, 3H), 11.40 (bs, 1H, NH);
The 1H (200.13 MHz), 13C (50.32 MHz) and 19F (188.31 MHz)
NMR spectra were recorded on a Bruker DPX-200 spectrometer
using CDCl3 as solvent and TMS or CCl3F as internal standards. MS
(EI) and HRMS spectra were obtained on a Varian MAT CH7A
instrument at 70 eV. The X-ray structural study was carried out
173(2) K on a Siemens P4 diffractometer using graphite mono-
13C NMR (CDCl3):
d
23.9 (s), 24.8 (s), 33.6 (s), 53.9 (s), 91.1 (s), 115.5
(td, J = 302.4 Hz, J = 25.5 Hz, CF2), 126.9 (s), 128.8 (s), 130.6 (s),
133.4 (s), 170.1 (s), 172.4 (t, J = 21.2 Hz, C55O); 19F NMR (CDCl3):
d
38.8 (s, 1F), ꢁ104.2 (s, 2F); MS (EI) m/z (%): 361 [M]+ (27), 278 [M-
SO2F]+ (94), 228 [M-CF2SO2F]+ (100); HRMS for [M]+
(C16H18F3NO3S): calculated 361.0960, found 361.0959.
chromated Mo
Ka radiation (l = 71.073 pm) and the low
temperature device LT2. All reactions were carried out under
the inert atmosphere (nitrogen) and monitored by 19F NMR
spectroscopy. The ketimines have been prepared according to
literature procedures. N-(1-Methylethyliden)-2-propanamin 1a
[21], 2-methyl-N-(1-phenylethyliden)-2-propanamin 1b [22], N-
(1-phenylethyliden)propan-2-amin 1e [22], N-ethyliden-2-propa-
namin 1g [23], N-ethyliden-2-methyl-2-propanamin 1h [23],
ethylen-bis(2-imino-propan) 1i [24].
4.1.5. (Z)-1,1-Difluoro-4-(isopropylamino)-2-oxo-4-phenylbut-3-
ene-1-sulfonyl fluoride 3e (67%)
Colorless oil, 1H NMR (CDCl3):
d
1.27 (d, J = 6.3 Hz, 6H), 3.76–
3.82 (m, 1H), 5.47 (s, 1H), 7.34–7.40 (m, 2H), 7.50–7.54 (m, 3H),
11.26 (bs, 1H, NH); 13C NMR (CDCl3):
24.1 (s), 48.0 (s), 91.5 (s),
d
116.3 (td, J = 302.3 Hz, J = 25.6 Hz, CF2), 127.4 (s), 129.5 (s), 131.1
(s), 133.9 (s), 170.8 (s), 173.2 (t, J = 21.2 Hz, C55O); 19F NMR (CDCl3):
d
43.9 (s, 1F), ꢁ99.1 (s, 2F); MS (EI) m/z (%): 321 [M]+ (22), 238 [M-
4.1. Reactions of 1a–i with 2,2-difluoro-2-fluorosulfonylacetyl
fluoride
SO2F]+ (42), 188 [M-CF2SO2F]+ (100); HRMS for [M]+
(C13H14F3NO3S): calculated 321.0647, found 321.0642.
General procedure: 2,2-Difluoro-2-fluorosulfonylacetyl fluoride
4.1.6. (Z)-4-(tert-Butylamino)-4-(4-chlorophenyl)-1,1-difluoro-2-
2
was condensed to
a
solution of ketimine in anhydrous
oxobut-3-ene-1-sulfonyl fluoride 3f (54%)
diethylether at ꢁ196 8C. After stirring for several hours at room
temperature the reaction mixture was washed twice with a
saturated NaHCO3 solution. The organic layer was separated, dried
over Na2SO4, all volatiles removed in vacuum and the residue was
purified by SiO2 gel column chromatography or crystallization to
give the corresponding products 3a–h, 4h, 5i, 6i.
Colorless crystals; mp. 76 8C; 1H NMR (CDCl3):
d
1.26 (s, 9H),
5.32 (s, 1H), 7.20–7.32 (m, 2H), 7. 36–7.43 (m, 2H), 11.63 (bs, 1H,
NH); 13C NMR (CDCl3):
31.8 (s), 56.7 (s), 93.4 (s), 116.1 (td,
J = 302.3 Hz, J = 25.7 Hz, CF2), 129.2 (s), 133.8 (s), 136.8 (s), 170.8
d
2
(s), 172.8 (t, JCF = 21.4 Hz, C55O); 19F NMR (CDCl3):
d 38.9 (s, 1F),
ꢁ104.5 (s, 2F); MS (EI) m/z (%) = 369 [M]+ (15), 286 [M-SO2F]+ (30),
236 [M-CF2SO2F]+ (90), 180 [M-t-Bu]+ (100), 57 [t-Bu]+ (80); HRMS
for [M]+ (C14H15ClF3NO3S): calculated 369.0413, found 369.0426.
4.1.1. (Z)-1,1-Difluoro-4-(isopropylamino)-2-oxopent-3-ene-1-
sulfonyl fluoride 3a (82%)
Colorless oil; 1H NMR (CDCl3):
d
1.28 (d, J = 6.1 Hz, 6H), 2.15 (s,
3H), 3.86 (sep, J = 6.1 Hz, 1H), 5.37 (s, 1H), 11.21 (bs, 1H, NH); 13
NMR (CDCl3): 19.8 (s, CH3), 24.1 (s, CH3), 48 (s, CH), 93.7 (s, CH),
115.8 (td, J = 302.5 Hz, J = 24.9 Hz, CF2), 171.9 (s, C), 172.4 (t,
J = 22.3 Hz, C55O); 19F NMR (CDCl3):
4.1.7. (Z)-1,1-Difluoro-4-(isopropylamino)-2-oxobut-3-ene-1-
C
sulfonyl fluoride 3g (76%)
d
Colorless oil; 1H NMR (CDCl3):
d
1.35 (d, J = 6.8 Hz, 6H), 3.65
(sep, J = 6.8 Hz, 1H), 5.49 (d, J = 6.8 Hz, 1H), 7.29 (dd, J = 13.8 Hz,
J = 6.8 Hz, 1H), 10.47 (s, 1H, NH); 13C NMR (CDCl3):
23.7 (s), 52.2
(s), 88.2 (s), 115.6 (td, J = 302.2 Hz, J = 25.6 Hz, CF2), 157.1 (s), 175.2
d
38.4 (s, 1F, SF), ꢁ104.4 (s, 2F,
d
CF2); MS (EI) m/z (%): 259 [M]+(15), 178 [M-SO2F]+ (10), 126 [M-
CF2SO2F]+ (100); HRMS for [M]+ (C8H12F3NO3S): calculated
259.0490, found 259.0489.
(t, 2JCF = 21.9 Hz, C55O); 19F NMR (CDCl3):
d
38.9 (s, 1F), ꢁ104.8 (s,
2F); MS (EI) m/z (%): 245 [M]+ (35), 112 [M-CF2SO2F]+ (100); HRMS
for [M]+ (C7H10F3NO3S): calculated 245.0334, found 245.0336.
4.1.2. (Z)-4-(tert-Butylamino)-1,1-difluoro-2-oxo-4-phenylbut-3-
ene-1-sulfonyl fluoride 3b (55%)
4.1.8. (Z)-4-(tert-Butylamino)-1,1-difluoro-2-oxobut-3-ene-1-
Colorless crystals; mp. 88 8C; 1H NMR (CDCl3):
d
1.26 (s, 9H),
5.37 (s, 1H), 7.32–7.36 (m, 2H), 7.45–7.47 (m, 3H), 11.7 (s, 1H, NH);
13C NMR (CDCl3):
31.8 (s, CH3), 56.7 (s, C), 93.5 (s, CH), 116.1 (td,
sulfonyl fluoride 3h (62%)
Colorless crystals; mp. 65 8C; 1H NMR (CDCl3):
d
1.24 (s, 9H),
5.51 (dt, J = 6.9 Hz, J = 1.3 Hz, 1H), 7.38 (dd, J = 14.3 Hz, J = 6.9 Hz,
1H, CH), 10.81 (bs, 1H, NH); 13C NMR (CDCl3):
29.6 (s), 54.4 (s),
87.8 (s), 115.6 (td, J = 302.1 Hz, J = 25.8 Hz, CF2), 154.5 (s), 174.5 (t,
J = 21.6 Hz, C55O); 19F NMR (CDCl3):
43.1 (s, 1F), ꢁ100.5 (s, 2F);
MS (EI) m/z (%) = 259 [M]+ (40), 126 [M-CF2–SO2F]+ (55), 70 [M-t-
d
J = 302.3 Hz, J = 25.5 Hz, CF2), 127.8 (s, CH), 128.8 (s, CH), 130.5 (s,
CH), 135.4 (s, CH), 172.1 (s, C), 172.6 (t, J = 22.9 Hz, C55O); 19F NMR
d
(CDCl3):
d
= 43.9 (s, 1F), ꢁ99.3 (s, 2F); MS (EI) m/z (%): 335 [M]+
d
(15), 202 [M-CF2–SO2F]+ (60), 146 [M-t-Bu, –CF2SO2F]+ (100), 57 [t-

K. Vlasov et al. / Journal of Fluorine Chemistry 131 (2010) 996–999
999
Bu, –CF2SO2F]+ (100), 57 [t-Bu]+(90); HRMS for [M]+
(C8H12F3NO3S): calculated 259.0490, found 259.0488.
Crystallographic Data Centre as supplementary publication nos.
CCDC 775065 (3f), CCDC 775066 (5i) and CCDC 775067 (6i). Copies
of the data can be obtained, free of charge, on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44 1223 336033 or
e-mail: deposit@ccdc.cam.ac.uk).
4.1.9. 3-((tert-Butylamino)methylene)-1,1,5,5-tetrafluoro-2,4-
dioxopentane-1,5-disulfonyl difluoride 4h (45%)
Colorless crystals; mp. 120 8C; 1H NMR (CD3CN):
d
1.50 (s, 9H),
8.23 (d, J = 15.3 Hz, 1H), 11.11 (bs, 1H, NH); 13C NMR (CD3CN):
28.4
(s), 58.8 (s), 101.1 (s), 118.7 (td, J = 312.1 Hz, J = 33.4 Hz, CF2), 157.1
d
Acknowledgments
(s), 177.1 (t, J = 22.2 Hz, C55O); 19F NMR (CDCl3):
d40.2 (s, 1F), 39.7 (s,
This work was supported by the Deutsche Forschungsge-
meinschaft (Ro362/34-1). We thank Dr. L.G. Sprague, E.I. Du Pont
de Nemours and Company, Wilmington (DE, USA) for the generous
gift of 2,2-difluoro-2-fluorosulfonylacetyl fluoride.
1F), ꢁ96.8 (s, 2F), ꢁ97.6 (s, 2F); MS: (EI) m/z (%): 419 [M]+ (15), 336
+
[M-SO2F] (5), 286 [M-CF2SO2F]+ (75), 57 [t-Bu]+ (100); HRMS for
[M]+ (C10H11F6NO6S2): calculated 418.9932, found 418.9928.
4.1.10. (3Z,30Z)-4,40-(Ethane-1,2-diylbis(azanediyl))bis(1,1-difluoro-
References
2-oxopent-3-ene-1-sulfonyl fluoride) 5i (46%)
Colorless crystals; mp. 135 8C; 1H NMR (CD3CN):
d
2.12 (s, 6H),
3.65–3.81 (m, 4H), 5.53 (s, 2H), 11.16 (bs, 2H, NH); 13C NMR
(CD3CN): 19.7(s), 44.4 (s), 91.4 (s), 117.5 (td, J = 302 Hz,
J = 25.7 Hz, CF2), 172.5 (t, JCF = 25.6 Hz, C55O), 173.9 (s); 19F
NMR (CD3CN):
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d
2
d
42.7 (s, 1F), ꢁ99.1 (s, 2F, CF2); MS (EI) m/z (%) 460
[M]+ (15), 377 [M-SO2F]+ (20), 327 [M-CF2SO2F]+ (60), 299 [M-CO–
CF2SO2F]+ (40); HRMS for [M]+ (C12H14F6N2O3S2): calculated
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4.1.11. (3Z,30Z)-4,40-(Ethane-1,2-diylbis(azanediyl))bis(1,1-difluoro-
2-oxopent-3-ene-1-sulfonyl fluoride) 6i (42%)
Colorless crystals; mp. 143 8C; 1H NMR (CD3CN):
d
1.94 (s, 6H),
2.67 (s, 3H), 3.86–4.05 (m, 4H), 5.45 (s, 1H); 13C NMR (CD3CN):
d
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603.
20.9 (s), 21.3(s), 26.4 (s), 42.4 (s), 43.9 (s), 83.5 (s), 94.4 (s), 117.5
(td, J = 302 Hz, J = 25.7 Hz, CF2), 123.7 (td, J = 305 Hz, J = 27.2 Hz,
CF2) 150.9 (t, 2JCF = 23.5 Hz, C55O), 171.6 (s), 173.4 (t, 2JCF = 25.6 Hz,
C55O); 19F NMR (CD3CN):
d
37.2 (s, 1F), 38.2 (s, 1F), ꢁ103.5 (s, 2F),
ꢁ104.4 (s, 2F); MS (EI) m/z (%) = 460 [M]+ (20), 405 [M-NH–
C(CH3)55CH]+ (100), 327 [M-CF2SO2F]+ (55), 299 [M-CO–CF2SO2F]+
(40). HRMS for [M]+ (C12H14F6N2O3S2): calculated 460.0197, found
460.0195.
[19] M.W. Buxton, R.H. Mobbs, D.E.M. Wotton, J. Fluorine Chem. 2 (1972) 231.
[20] A.J.C. Wilson, International Tables for Crystallography, vol. C, Kluwer Academic
Publishers, Doldrecht, 1995, pp. 685.
[21] E.A. Mistryukov, I.K. Korshevetz, Synthesis (1984) 947.
[22] H. Weingarten, J.P. Chupp, W.A. White, J. Org. Chem. 32 (1967) 3246.
[23] K.N. Campbell, A.H. Sommers, B.K. Campbell, J. Am. Chem. Soc. 66 (1943) 82.
[24] M. Clericuzio, G. Alagona, C. Ghio, P. Salvadori, J. Am. Chem. Soc. 119 (1997)
1059.
4.2. X-ray crystal structure determination of 3f, 5i and 6i
Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the Cambridge