TiCl4-promoted addition of nucleophiles to open chain α-amidoalkylphenyl sulfones

Article abstract of DOI:10.1016/j.tetlet.2005.07.031

Linear α-amidoalkylphenyl sulfones are converted into the corresponding N-acyliminium ions by treatment with TiCl₄ at low temperature and then made to react with different nucleophiles such as allyltrimethylsilane, silyl ketene acetal, anisole

Full text of DOI:10.1016/j.tetlet.2005.07.031

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5999–6003
TiCl4-promoted addition of nucleophiles to open chain
a-amidoalkylphenyl sulfones
Marino Petrini* and Elisabetta Torregiani
`
Dipartimento di Scienze Chimiche, Universita di Camerino, via S. Agostino, 1. I-62032 Camerino, Italy
Received 27 May 2005; revised 6 July 2005; accepted 8 July 2005
Available online 26 July 2005
Abstract—Linear a-amidoalkylphenyl sulfones are converted into the corresponding N-acyliminium ions by treatment with TiCl4 at
low temperature and then made to react with different nucleophiles such as allyltrimethylsilane, silyl ketene acetal, anisole and
thiophene.
Ó 2005 Elsevier Ltd. All rights reserved.
The addition of nucleophilic reagents to N-substituted
imines represents one of the most exploited routes to
amino derivatives.1 The poor electrophilicity of simple
N-alkyl and N-aryl imines often dictates the utilization
of strong nucleophiles that having a consistent basic
character, may lead to some undesired side reactions.
Weakly nucleophilic reagents are sparely reactive
towards these azomethine compounds unless the imino
group is activated by the addition of Lewis acids.2 A
consistent increase in the electrophilicity of the car-
bon–nitrogen double bond can be achieved by moving
to iminium ions. These unstable, reactive cations are
the electrophilic reagents involved in the Mannich reac-
tion, a very popular procedure mainly used for the syn-
thesis of b-dialkylamino carbonyl derivatives.3 Finally,
N-acyliminium ions 2 are the most electrophilic sub-
strates among iminium ions4 although they are too
unstable to be prepared and stored in whatever extent.5
Therefore, they may be formed in situ from suitable pre-
cursors 1 by means of Lewis acid-mediated elimination
of the leaving group X (Scheme 1).
Preparation of compounds 1 depends on the nature of
the nitrogen substituents as well as on the nature of
the leaving group X employed. Direct coupling of an
amide (carbamate) with an aldehyde in the presence of
HX (MX) is usually the most used procedure to access
derivatives 1.6,7 Compounds 1 are isolated and purified
before their utilization for the generation of the corre-
sponding N-acyliminium ions 2. Alternatively, in suit-
able conditions amido compound 1 may represent a
transient intermediate in the formation of N-acylimin-
ium ion 2 that is promptly made to react with a nucleo-
phile already present in the reaction mixture. This
strategy is the basis of the three-component coupling
synthesis of homoallylamines that is realized by reaction
of a carbamate with an aldehyde in the presence of allyl-
trimethylsilane, promoted by different acid catalysts.8 A
biscarbamate 1 (R2 = H, X = NHCOR3) is a probable
intermediate for this process that works efficiently with
the allyl and propargyl nucleophiles, but has found only
a narrow application with other systems.9 Among the
various precursors of N-acyliminium ions, a-amido sulf-
ones 1 (X = PhSO2) are surely compounds of great
importance since they are mostly stable solids that can
be prepared by a simple three-components coupling in
the presence of benzenesulfinic acid or its salt.10 Elimi-
nation of benzenesulfinate anion in basic conditions
leads to the formation of the corresponding N-acylimine
that upon reaction with nucleophilic reagents leads to
the final addition products.11 We have recently demon-
strated that chiral sulfones react with Lewis acids giving
O
O
X
Lewis acid (-X-)
+X-
R1
R
N
R2
R1
R
N
R2
2
1
Scheme 1.
Keywords: N-Acyliminium ions; b-Amino esters; Homoallylamines;
Lewis acids; Sulfones.
*
Corresponding author. Tel.: +39 0737 402253; fax: +39 0737
402297; e-mail: marino.petrini@unicam.it
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.07.031

6000
M. Petrini, E. Torregiani / Tetrahedron Letters 46 (2005) 5999–6003
a
Table 1. Addition of 8 to 6 promoted by TiCl4
O
R
SO2Ph
R1
O
Nu
O
Nu-X
8
TiCl4
Entry Sulfone Nucleophile Productsb
Yield
(%)c
R1
N
H
R
N
H
R1
R
N
Z
CH2Cl2
-78°C
NHCbz
6
9-11
7
1
2
3
4
5
6
7
6a
6b
6c
6d
6e
6f
8a
8a
8a
8a
8a
8a
8a
80
85
89
70
83
86
77
Z = H or Lewis acid
9a
NHCbz
6a: R = OBn; R1= Me2CHCH2
6b: R = OBn; R1= n-C7H15
6c: R = OBn; R1= PhCH2CH2
6d: R = OBn; R1= (E)-C5H11CH=CH(CH2)2
6e: R = OBn; R1= BnO(CH2)4
6f: R = Ph; R1= PhCH2CH2
6g: R = Cl(CH2)2; R1= PhCH2CH2
6h: R = FCH2; R1= PhCH2CH2
6i: R = CH2=CH; R1= PhCH2CH2
8a: CH2=CHCH2SiMe3
8b: MeOPh
8c: thiophene
C7H15
9b
NHCbz
Ph
9c
NHCbz
C H
5
11
9d
9e
9f
Scheme 2.
NHCbz
BnO
Ph
the corresponding N-acyliminium ions that in the pres-
ence of nucleophiles afford the corresponding adducts
with a variable degree of diastereoselectivity depending
on the nature of the heterocyclic ring system.12 The same
procedure is also effective in the reaction of N-unsubsti-
tuted a-amidoalkylphenyl sulfones 6 that in the presence
of TiCl4 at low temperature lead to N-acyliminium ions
7, which react with allyltrimethylsilane 8a and electron-
rich aromatics 8b–c giving addition products 9–11
(Scheme 2).13
NHCOPh
O
HN
Cl
6g
Ph
Ph
9g
O
F
HN
8
9
6h
6i
8a
8a
80
81
The real structure of the N-acyliminium ion 7 involved
in this reaction is obviously unknown; the nitrogen atom
could be linked to a hydrogen or to the Lewis acid since
two equivalents of TiCl4 are employed in the reaction.14
Other Lewis acids tested for this reaction, such as SnCl4,
BF3 Æ Et2O and TMSOTf were less effective in promoting
the conversion of sulfones 6 into the intermediate imin-
ium ion 7. As displayed in Table 1, N-carbobenzoxy
sulfones 6a–e gave an efficient addition with nucleo-
philes 8 (Table 1, entries 1–5 and 10–12) while any
attempt to use the parent N-tert-butoxycarbamoyl sulf-
ones only led to disappointing results. Functionalized
amides can be used to prepare amido sulfones 6 contain-
ing halogen atoms (Table 1, entries 7 and 8) and unsatu-
rations (Table 1, entry 9), all these compounds were
efficiently allylated in our conditions.
9h
O
HN
Ph
9i
NHCbz
10
11
12
6a
6b
6a
8b
8b
8c
80
81
78
OMe
OMe
10a
NHCbz
C H
7
15
10b
NHCbz
Silyl ketene acetal 8e is also effective as a nucleophile in
the reaction with sulfones 6a–c giving N-carbobenzoxy-
b-amino acid esters 12 (Scheme 3).15 This procedure
represents a complementary approach to the synthesis
of this class of important biologically active
compounds16 since b-amino esters are usually prepared
from sulfones 6 via the corresponding N-acylimine using
metal enolates.11h,17
S
11
a All reactions were carried out at À78 °C in CH2Cl2, in the presence of
TiCl4 (2 equiv) and the nucleophile (1.5 equiv).
b All products were identified on the basis of their IR and NMR
spectra.
c Yields of pure products isolated by column chromatography.
Next, we were intrigued by the chemical behaviour of
bisamido sulfone 13 obtained from glutaraldehyde and
butanamide that has been previously used as a substrate
for the preparation of diamino derivatives through a
reduction process (Scheme 4).18
Monoallylation of bisamido sulfone 13 gave poor results
since a mixture of mono, bis and other by-products aris-
ing from unreacted 13 were obtained. Bisallylation of

M. Petrini, E. Torregiani / Tetrahedron Letters 46 (2005) 5999–6003
6001
TiCl4 (2 eq.), CH2Cl2,
OSiMe3
PhO2S
C7H15
O
CbzHN
R1
O
H
N
TiCl4 (2 eq.)
-78 C, then 8a (1.5 eq.)
C7H15
SO2Ph
6
+
OEt
(twice)
N
H
CH2Cl2, -78 C
OEt
8e
O
79%
15
12a: R1= Me2CHCH2, 77%
12b: R1= n-C7H15, 81%
O
H
N
12c: R1= PhCH2CH2, 73%
C7H15
C7H15
N
H
O
Scheme 3.
16
Scheme 5.
TiCl4 (2 eq.),
SO2Ph
O
SO2Ph
O
CH2Cl2, -78 C,
then 8a (1.5 eq.)
N
H
NH
N
H
NH
(twice)
O
ing a further amount of TICl4 (2 equiv) and silane 8a
(1.5 equiv).19 Asimilar behaviour is displayed by bis-
amido sulfone 15 obtained from adipamide and octanal
that can be bisallylated to compound 16 in the usual way
(Scheme 5).
O
13
77%
14
Scheme 4.
sulfone 13 efficiently occurs by doubling the amount of
the reagents employed giving compound 14 in good
yield. In order to obtain higher yields of bisallyl deriva-
tive 14 it is advisable to add the reagents in two distinct
steps, namely 2 equiv TiCl4+1.5 equiv 8a and then add-
Anisole and thiophene that efficiently add to a-amido
sulfones 6 are also capable of affording the correspond-
ing bis adducts 17–20 by reaction with bisamido sulf-
ones 13 and 15 as displayed in Table 2.
Table 2. Addition of 8 to 13 and 15a
Entry
Sulfone
Nucleophile
Productsb
OMe
Yield (%)c
OMe
O
O
1
13
8b
70
N
H
NH
O
17
18
S
S
2
3
4
13
15
15
8c
8b
8c
71
68
72
N
H
NH
O
MeO
C7H15
NH
O
HN
C7H15
O
OMe
S
19
O
C7H15
NH
HN
C7H15
S
O
20
a All reactions were carried out at À78 °C in CH2Cl2, in the presence of TiCl4 (2 equiv) and the nucleophile (1.5 equiv). The addition of the reactants
was repeated twice.
b All products were identified on the basis of their IR and NMR spectra.
c Yields of pure products isolated by column chromatography.

6002
M. Petrini, E. Torregiani / Tetrahedron Letters 46 (2005) 5999–6003
9. Electron-rich aromatics and methyl aryl ketones can be
used as nucleophiles for such additions but only with
amido compounds 1 obtained from non-enolisable alde-
hydes: (a) Xu, L.-W.; Xia, C.-G.; Li, L. J. Org. Chem.
2004, 68, 8482–8484; (b) Bensel, N.; Pevere, V.; Desmur,
J. R.; Wagner, A.; Mioskowski, C. Tetrahedron Lett. 1999,
40, 879–882.
10. (a) Engberts, J. B. F. N.; Strating, J. Rec. Trav. Chim.
Pays-Bas 1965, 84, 942–950; (b) Pearson, W. H.; Lind-
beck, A. C.; Kampf, J. W. J. Am. Chem. Soc. 1993, 115,
2622–2636.
In conclusion, a-amido sulfones 6 can be easily con-
verted into the corresponding N-acyliminium ions 7 by
reaction with TiCl4 at À78 °C. These reactive iminium
ions can add weak nucleophiles such as allyltrimethyl-
silane 8a, silyl ketene acetal 8e or electron-rich aromatics
(anisole 8b and thiophene 8c) giving the corresponding
adducts 9–12. The same procedure can be extended to
bisamido sulfones 13 and 15 that in usual conditions
lead to bis adducts 14 and 16 and 17–20.
11. For some very recent applications see: (a) Mennen, S. M.;
Gipson, J. D.; Kim, Y. R.; Miller, S. J. J. Am. Chem. Soc.
2005, 127, 1654–1655; (b) Zhang, H.-L.; Liu, H.; Cui, X.;
Mi, A.-Q.; Jiang, Y.-Z.; Gong, L.-Z. Synlett 2005, 615–
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13. General procedure for nucleophilic additions on a-amido
sulfones 6. Sulfone 6 (2 mmol) was dissolved in CH2Cl2
(20 mL), and the solution was cooled at À78 °C. TiCl4
(4 mmol) was then added dropwise in 15 min and the
temperature was kept at À78 °C for 30 min. The nucleo-
phile (3 mmol) dissolved in CH2Cl2 (10 mL) was then
added dropwise and after 1 h at À78 °C the temperature
was slowly raised to 0 °C. The reaction mixture was then
diluted with CH2Cl2 (20 mL), washed with brine and the
organic phase was dried over MgSO4. After removal of the
solvent at reduced pressure the additional product
obtained was purified by column chromatography.
Selected data of compounds: 9d: Oil. IR (cmÀ1, neat):
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1
3310, 1683. H NMR (300 MHz, CDCl3) d: 0.89 (t, 3H,
J. 2003, 9, 4286–4294.
J = 6.2 Hz); 1.11–1.79 (m, 10H); 1.81–2.16 (m, 2H); 2.18–
2.39 (m, 2H); 3.72–3.80 (m, 1H); 4.61 (d, 1H, J = 9.1 Hz);
5.02–5.20 (m, 4H); 5.24–5.48 (m, 2H); 5.66–5.92 (m, 1H);
7.22–7.41 (m, 5H). 13C NMR (75 MHz, CDCl3) d: 14.1,
23.2, 28.3, 30.0, 32.7, 33.3, 36.4, 39.6, 49.6, 69.6, 118.1,
127.3, 127.5, 127.7 130.2, 131.5, 134.4, 136.9, 156.9.
Compound 10b: Oil. IR (cmÀ1, neat): 3318, 1688. 1H
NMR (300 MHz, CDCl3) d: 0.88 (t, 3H, J = 7.1 Hz); 1.20–
1.42 (m, 10H); 1.90–2.08 (m, 2H); 3.67–3.80 (m, 1H); 3.78
(s, 3H); 4.95–5.03 (m, 1H); 5.05–5.15 (m, 2H), 5.43 (d, 1H,
J = 8.9 Hz); 6.83 (d, 2H, J = 9.0 Hz); 7.15 (d, 2H,
J = 9.0 Hz); 7.22–7.38 (m, 5H). 13C NMR (75 MHz,
CDCl3) d: 14.3, 22.9, 28.2, 29.4, 29.832.1, 36.3, 55.4, 55.8,
66.2, 113.8, 127.4, 128.1, 128.4, 128.6, 137.7, 138.2, 157.1,
157.9. Compound 12a : Oil. IR (cmÀ1, neat): 3339, 1728.
1H NMR (300 MHz, CDCl3) d: 0.83 (d, 3H, J = 6.2 Hz);
0.89 (d, 3H, J = 6.2 Hz); 1.20 (t, 3H, J = 7.0 Hz); 1.22–1.32
7. (a) Zaugg, H. E. Synthesis 1984, 85–110, and 181–212; (b)
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Rene, L.; Dau, M. J. Org. Chem. 2002, 67, 5408–5411; (e)
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Mann, A. Tetrahedron Lett. 2005, 46, 1897–1900; (b)
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M. Petrini, E. Torregiani / Tetrahedron Letters 46 (2005) 5999–6003
6003
(m, 2H); 1.40–1.58 (m, 1H); 2.33–2.41 (m, 2H); 4.07 (q, 2H,
J = 7.0 Hz); 4.10–4.15 (m, 1H); 4.96–5.10 (m, 2H); 6.15 (d,
1H, J = 10.2 Hz); 7.30–7.43 (m, 5H). 13C NMR (75 MHz,
CDCl3) d: 14.5, 22.8, 24.1, 41.8, 44.0, 48.8, 60.1, 66.5,
128.0, 128.2, 128.6, 137.1, 155.8, 175.1.
19. General procedure for nucleophilic additions on bisamido
sulfones 13, 15. Bisamido sulfone 13 or 15 (2 mmol)
was dissolved in CH2Cl2 (20 mL), and the solution was
cooled at À78 °C. TiCl4 (4 mmol) was then added
dropwise in 15 min and the temperature was kept at
À78 °C for 20 min. The nucleophile (3 mmol) dissolved in
CH2Cl2 (6 mL) was then added dropwise at the same
temperature. After 10 min more TiCl4 (4 mmol) was
added followed after 20 min by the nucleophile (3 mmol)
in CH2Cl2 (6 mL). Stirring was continued for further
30 min at À78 °C and then the temperature was slowly
raised to 0 °C. The reaction mixture was then diluted
with CH2Cl2 (20 mL), washed with brine and the organic
phase was dried over MgSO4. After removal of the
solvent at reduced pressure the addition product
obtained was purified by column chromatography.
Selected data of compound 16: Oil. IR (cmÀ1, neat):
14. (a) Harding, K. E.; Coleman, M. T.; Liu, L. T. Tetra-
hedron Lett. 1991, 32, 3795–3798; (b) Harding, K. E.;
Davis, K. S. Tetrahedron Lett. 1988, 29, 1891–1894.
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¨
Tetrahedron 1988, 44, 5403–5414.
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Ed.; Wiley-VCH: New York, 1997.
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67, 4530–4535; (b) Mecozzi, T.; Petrini, M. Tetrahedron
Lett. 2000, 41, 2709–2712; Ketene diethyl acetal reacts
under basic conditions (Et3N, CH2Cl2, reflux) with a-
formyl sulfones prepared from non-enolisable aldehydes
giving b-formamido esters: (c) Rossen, K.; Jakubec, P.;
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1821.
1
3330, 1690. H NMR (300 MHz, CDCl3) d: 0.88 (t, 6H,
J = 6.2 Hz); 1.05–1.90 (m, 32H); 2.20–2.40 (m, 4H); 3.98–
4.08 (m, 2H); 5.01–5.21 (m, 6H); 5.68–5.95 (m, 2H). 13C
NMR (75 MHz CDCl3) d: 14.2, 22.4, 24.6, 25.5, 26.7,
29.1, 29.6, 31.8, 31.9, 35.2, 69.1, 129.2, 134.1, 172.5.
18. Mataloni, M.; Petrini, M.; Profeta, R. Synlett 2003, 1129–
1132.