Stereoselective intramolecular cycloadditions of homochiral nitrilimines as a source of enantiopure 2,3,3a,4,5,6-hexahydro-pyrrolo[3,4-c]pyrazoles

Article abstract of DOI:10.1016/j.tetasy.2006.02.018

Starting from the methyl esters of glycine, l-alanine, l-phenylalanine and (S)-2-phenylglycine, we developed a synthetic route to the title compounds in the enantiopure form by means of a stereoselective intramolecular 1,3-dipolar cycloaddition of homochi

Full text of DOI:10.1016/j.tetasy.2006.02.018

Tetrahedron: Asymmetry 17 (2006) 842–845
Stereoselective intramolecular cycloadditions
of homochiral nitrilimines as a source of enantiopure
2,3,3a,4,5,6-hexahydro-pyrrolo[3,4-c]pyrazoles
Lara De Benassuti, Paola Del Buttero and Giorgio Molteni*
`
Universita degli Studi di Milano, Dipartimento di Chimica Organica e Industriale, via Golgi 19, 20133 Milano, Italy
Received 26 January 2006; accepted 16 February 2006
Abstract—Starting from the methyl esters of glycine, L-alanine, L-phenylalanine and (S)-2-phenylglycine, we developed a synthetic route
to the title compounds in the enantiopure form by means of a stereoselective intramolecular 1,3-dipolar cycloaddition of homochiral
nitrilimines 5a–d. Fair to good overall product yields and high cycloaddition diastereoselectivity were obtained.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
starting stereocentre and the new one was responsible for
the disappointing outcome, which was observed. To cir-
cumvent this drawback, we planned the synthesis of novel
nitrilimines 5b–d in which the stereocentre is placed inside
the tether joining the reactive groups. We herein report
the behaviour of labile intermediates 5b–d leading to enan-
tiopure 2,3,3a,4,5,6-hexahydro-6-substituted-pyrrolo[3,4-c]-
pyrazole derivatives 6 and 7. Furthermore, with the
synthesis of enantiopure molecules from simple starting
materials being a valuable target,8 we herein report the
inexpensive, commercially available L-alanine, L-phenylala-
nine and (S)-2-phenylglycine methyl esters as suitable start-
ing chiral units.
Intramolecular 1,3-dipolar cycloadditions often constitute
the key step of synthetic routes leading to complex cyclic
as well as open-chain molecules.1 The stereoselective ver-
sion of these reactions allow the synthesis of enantiopure
products, thus representing one of the most promising
fields of contemporary organic chemistry.2 Within this pic-
ture, stereoselective nitrone and nitrile oxide intramolecu-
lar cycloadditions play a major role in the construction
of a number of enantiopure five-membered heterocycles.3
Despite the utility of enantiopure 4,5-dihydropyrazole
derivatives in organic synthesis,4 the field of nitrilimine
stereoselective cycloadditions occupy a less prominent
position. In fact, this approach to fused bi- or tricyclic
4,5-dihydropyrazoles has received little attention, although
it has been reviewed quite recently.5
2. Results and discussion
N,N-bis-Allylamino acetates 1a–d9,10 were first submitted to
basic hydrolysis followed by treatment with thionyl chlo-
ride. The corresponding N,N-bis-allylaminoacetyl chlorides
2a–d were obtained as crude materials, which were used
without isolation. Treatment of the latter with phenylhydr-
azine afforded acyl hydrazines 3a–d, which were recovered
as analytically pure compounds after chromatographic
purification and subsequent crystallisation. Hydrazonoyl
chlorides 4a–d were obtained as crude oils by treating the
appropriate acyl hydrazine precursor with triphenylphos-
phine and carbon tetrachloride, according to the original
method by Wolkoff (Scheme 1).11 The generation of nitrili-
mines 5a–d was accomplished, in situ, by refluxing a
The construction of pyrrolo[3,4-c]pyrazole skeleton is one
of the first successes of the intramolecular nitrilimine cyclo-
addition methodology.6 In a previous paper from our lab-
oratory,7 we first described the synthesis of the above
skeleton in the enantiopure form by means of stereoselec-
tive cycloaddition of a homochiral nitrilimine bearing a
pendant L-alanine benzylester as the chiral auxiliary. How-
ever, since the control of the absolute stereochemistry was
modest, we reasoned that the large distance between the
*
Corresponding author. Tel.: +39 02 51314141; fax: +39 02 50314139;
e-mail: giorgio.molteni@unimi.it
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.02.018

L. De Benassuti et al. / Tetrahedron: Asymmetry 17 (2006) 842–845
843
O
O
O
R
R
R
MeO
NH
Cl
PhNH
1) aq. NaOH/THF
2) SOCl2, 40 oC
PhNHNH2
toluene, r.t.
N
N
N
1
2
3
Cl
R
N
N
Ph
R
PhNH
N
Ph3P, CCl4
MeCN, r.t.
Et3N
N
N
toluene, Δ
4
5
R
R
N
N
+
Ph
N
N
Ph
N
N
H
H
6
7
Entry
R
a
b
c
d
H
Me
CH2Ph
Ph
Scheme 1.
solution of the corresponding crude hydrazonoyl chlorides
in the presence of a large excess of triethylamine (5 equiv)
in dry toluene. Cycloaddition products, reaction times,
product yields and diastereoisomeric ratios are shown in
Table 1. It can be seen that unsubstituted 5a gave a racemic
mixture of cycloadducts 6a and 7a with 89% overall yield
(Table 1, entry a). Spectral data of the latter cycloadducts
agrees perfectly with that reported for similar pyrrolo[3,4-c]-
pyrazoles.7 The diastereoisomeric cycloadducts 6b–d and
7b were obtained as analytically and enantiomerically pure
solids through silica gel column chromatography followed
by crystallisation, while in the case of minor 7c, a very small
amount of isolated product precluded its full characterisa-
tion. As can be inferred from Table 1, cycloaddition yields
ranged from fair to good, since some tarry material was
always formed. The absolute configurations of the newly
formed stereocentre of minor 7b,c were determined, unam-
biguously, by the mutual NOE enhancements reported in
Figure 1. It is apparent that the observed NOE effects can
arise only for the stereochemical arrangement depicted. In
order to further substantiate these NOE effects, the dis-
H
R
N
Ph
N
HB
Allyl
N
HB
HA
7b: R = H, 6.6%; 7c: R = Ph, 7.2%
Figure 1. NOE enhancement for cycloadducts 7b and c.
tances between HA and the average position of HB were cal-
culated within the AM 112 semi-empirical method. Values
13
˚
˚
of 2.86 A for 7b and 2.78 A for 7c were found, which
match the observed mutual NOE enhancements. As far as
diastereoselection is concerned, we were pleased to find that
the ratio 6/7 encompasses the range between 85:15 and
100:0, depending upon the size and shape of R. This high
cycloaddition stereoselectivity can be rationalised on the
basis of the proposed transition states A and B (Fig. 2).
Due to the inner disposition of R, transition state B should
be the more energetic one giving minor compounds 7b,c; the
intervention of such a transition state is made impervious in
the case of the bulky phenyl pendant (Table 1, entry d).
Hence, the preferred formation of major compounds 6b–d
finds rationalisation.
Table 1. Intramolecular cycloadditions of nitrilimines 5a
Entry
R
Time (h)
6 + 7 (%)b
6:7c
a
b
c
H
Me
CH2Ph
Ph
3
4
3
6
89
50
59
35
—
85:15
>95:5
100:0
3. Conclusions
d
a In refluxing toluene.
b Overall yields from acyl hydrazines 3.
c Determined from 1H NMR analysis of reaction crudes.
The ready availability of a-amino acid methyl esters as
valuable starting chiral building blocks allow the highly
stereoselective synthesis of a variety of enantiopure

844
L. De Benassuti et al. / Tetrahedron: Asymmetry 17 (2006) 842–845
Allyl
1 M aqueous sodium hydroxide (65 mL) and stirred at
H
H
H
H
room temperature for 4 h. Aqueous 6 M hydrochloric acid
was added to pH 1 and the mixture extracted twice with
ethyl acetate (2 · 100 mL). The organic layer was dried
over sodium sulfate and evaporated under reduced pres-
sure. Thionyl chloride (0.77 g, 6.5 mmol) was added drop-
wise to the oily residue under vigorous stirring, and the
mixture warmed to 40 ꢁC for 2 h. The excess of thionyl
chloride was removed by in vacuo distillation to give crude
2-allyloxyacetyl chlorides 2 as dark brown oils. Freshly dis-
tilled triethylamine (1.31 g, 13.0 mmol) and phenylhydr-
azine (0.70 g, 6.5 mmol) were added dropwise to a
solution of 2 in dry toluene (20 mL) at room temperature
and the mixture was stirred at room temperature for 4 h.
Water (50 mL) was added to reaction mixture, the organic
layer was separated, dried over sodium sulfate and evapo-
rated under reduced pressure. The dark red residue was
chromatographed on a silica gel column with ethyl ace-
tate–hexane 4:1 giving acyl hydrazines 3.
N
C
N
Ar
N
H
R
N
C
Ar
N
H
N
R
Allyl
H
H
A
B
(2S
,4
R)-6
(2S,4S)-7
Figure 2. Proposed transition states for the formation of cycloadducts
6b–d and 7b,c.
6-substituted-pyrrolo[3,4-c]pyrazoles via intramolecular
nitrilimine cycloaddition. The resulting cycloadducts still
possess an ethylenic double bond readily susceptible to fur-
ther functionalisations, thus enhancing their potential as
synthetic intermediates.
Compound 3a: 0.64 g, 40%. Pale yellow powder; mp 63 ꢁC
(from diisopropyl ether); IR (nujol) 3300, 3240, 1680 cmꢀ1
;
1H NMR (CDCl3) d 3.28 (4H, d, J 12.8), 3.31 (2H, s), 5.28–
5.31 (4H, m), 5.89–6.01 (2H, m), 6.08 (1H, br s), 6.9–7.3
(5H, m), 8.82 (1H, br s); MS m/z 245 (M+). Anal. Calcd
for C14H19N3O: C, 68.54; H, 7.81; N, 17.13. Found: C,
68.51; H, 7.78; N, 17.19.
4. Experimental
Melting points were determined with a Buchi apparatus in
¨
open tubes and are uncorrected. IR spectra were recorded
with a Perkin–Elmer 1725 X spectrophotometer. Mass
1
spectra were determined with a VG-70EQ apparatus. H
Compound 3b: 0.57 g, 34%. Pale yellow powder; mp 57 ꢁC
NMR (300 MHz) spectra were taken with a Bruker
AMX 300 instrument (in CDCl3 solutions at room temper-
ature). Chemical shifts are given as ppm from tetrameth-
ylsilane and J values are given in Hz. NOE experiments
were performed by setting the following parameters: relaxa-
tion delay (d1) 2 s, irradiation power (dl2) 74 dB and total
irradiation time (for each signal) 1.8 s. Optical rotations
were recorded on a Perkin–Elmer 241 polarimeter at the
sodium D-line.
25
(from diisopropyl ether); ½aꢁD ¼ ꢀ16:4 (c 0.24, CH2Cl2); IR
1
(nujol) 3310, 3250, 1670 cmꢀ1; H NMR (CDCl3) d 1.52
(3H, d, J 7.3), 3.03 (2H, dd, J 13.4, 7.1), 3.13 (2H, dd, J
13.4, 5.0), 3.70 (1H, q, J 7.3), 5.08–5.12 (4H, m), 5.80–
5.90 (2H, m), 6.03 (1H, br d, J 9.0), 6.9–7.1 (5H, m), 8.90
(1H, br s); MS m/z 259 (M+). Anal. Calcd for
C15H21N3O: C, 69.47; H, 8.16; N, 16.20. Found: C,
69.51; H, 8.19; N, 16.26.
Compounds 1a10 and 1b,c9 were prepared according to the
literature procedures.
Compound 3c: 0.61 g, 28%. Yellow powder; mp 59 ꢁC
25
(from diisopropyl ether); ½aꢁD ¼ ꢀ9:4 (c 0.15, CH2Cl2);
IR (nujol) 3310, 3260, 1670 cmꢀ1 1H NMR (CDCl3) d
;
2.96 (2H, dd, J 13.4, 4.8), 3.16 (2H, dd, J 13.4, 7.2), 3.44
(1H, dd, J 15.6, 2.7), 3.48 (1H, dd, J 15.6, 9.8), 3.80
(1H, dd, J 9.8, 2.7), 5.25–5.28 (4H, m), 5.82–5.94 (2H,
m), 6.08 (1H, br d, J 8.4), 6.9–7.2 (10H, m), 8.84 (1H, br
s); MS m/z 335 (M+). Anal. Calcd for C21H25N3O: C,
75.19; H, 7.51; N, 12.53. Found: C, 75.23; H, 7.47; N,
12.58.
4.1. Synthesis of N,N-bis-allyl-(S)-(ꢀ)-2-phenylglycine
methyl ester 1d
A solution of (S)-(ꢀ)-2-phenylglycine methyl ester (2.01 g,
10.0 mmol), allyl bromide (3.03 g, 25.0 mmol) and triethyl-
amine (1.01 g, 10.0 mmol) in anhydrous toluene (25 mL)
was warmed to 75 ꢁC for 5 h. The mixture was then cooled
and filtered over Celite. The organic layer was dried over
sodium sulfate, the solvent evaporated and the residue dis-
Compound 3d: 0.65 g, 31%. Pale yellow powder; mp 67 ꢁC
25
(from diisopropyl ether); ½aꢁD ¼ ꢀ12:2 (c 0.21, CH2Cl2); IR
tilled in vacuo to give 1d (2.25 g, 92%); as a pale yellow oil;
1
(nujol) 3300, 3240, 1680 cmꢀ1; H NMR (CDCl3) d 3.02
25
bp 146 ꢁC (5 mmHg); ½aꢁD ¼ ꢀ23:7 (c 0.26, CH2Cl2); IR
1
(neat) 1735 cmꢀ1; H NMR d: 3.20 (3H, d, J 10.7), 3.78
(2H, dd, J 13.6, 7.3), 3.44 (2H, dd, J 13.6, 5.9), 4.64 (1H,
s), 5.18–5.21 (4H, m), 5.80–5.93 (2H, m), 6.08 (1H, br s),
6.9–7.3 (10H, m), 8.89 (1H, br s); MS m/z 321 (M+). Anal.
Calcd for C20H23N3O: C, 74.73; H, 7.21; N, 13.07. Found:
C, 74.70; H, 7.18; N, 13.14.
(3H, s), 4.67 (1H, s), 5.10–5.25 (4H, m), 5.80–5.94 (2H,
m), 7.2–7.4 (5H, m). MS m/z: 245 (M+). Anal. Calcd for
C15H19NO2: C, 73.44; H, 7.81; N, 5.71. Found: C, 73.50;
H, 7.84; N, 5.76.
4.2. Synthesis of acyl hydrazines 3
4.3. Intramolecular cycloaddition of nitrilimines 5
A solution of the appropriate N,N-bis-allylaminoacetate 1
A solution of the appropriate acyl hydrazine 3 (4.0 mmol)
(6.5 mmol) in tetrahydrofuran (65 mL) was treated with
in dry acetonitrile (40 mL) and carbon tetrachloride

L. De Benassuti et al. / Tetrahedron: Asymmetry 17 (2006) 842–845
845
(3.08 g, 20.0 mmol) was treated with triphenylphosphine
(4.72 g, 60 mmol) and stirred at room temperature for
12 h. Brine (20 mL) was then added to reaction mixture.
The organic layer was separated, dried over sodium sulfate
and evaporated under reduced pressure. The dark red res-
idue contained crude hydrazonoyl chlorides 4 whose IR
spectra exhibited the typical N–H stretch band at 3230–
3250 cmꢀ1. A solution of crude hydrazonoyl chloride 4
(4.0 mmol) in dry toluene (200 mL) was treated with trieth-
ylamine (2.02 g, 20.0 mmol) and refluxed for the time indi-
cated in Table 1. The crude was evaporated under reduced
pressure, and then the residue was chromatographed on a
silica gel column with ethyl acetate–hexane 2:1. The first
fractions contained major cycloadduct 6, further elution
gave minor cycloadduct 7.
(M+). Anal. Calcd for C20H21N3: C, 79.17; H, 6.98; N,
13.85. Found: C, 79.22; H, 7.02; N, 13.93.
Minor cycloadduct 7b: 71 mg, 7.4%. Pale yellow powder;
25
mp 59 ꢁC (from diisopropyl ether); ½aꢁD ¼ þ112:7 (c 0.58,
1
CHCl3); IR (nujol) 1605 cmꢀ1; H NMR (CDCl3) d 1.29
(3H, d, J 6.5), 2.84 (1H, dd, J 11.4, 8.5), 3.16 (2H, dd, J
12.2, 8.9), 3.29 (1H, q, J 6.5), 3.40–3.70 (3H, m), 4.17
(1H, dd, J 9.2, 8.5), 5.20–5.30 (2H, m), 5.90–6.04 (1H,
m), 6.9–7.2 (5H, m); 13C NMR (CDCl3) d 20.2 (q), 34.8
(d), 44.6 (t), 45.4 (t), 47.1 (t), 48.8 (t), 110.2 (t), 117.8 (d),
126.3 (d), 128.0 (d), 130.3 (d), 131.6 (s), 142.0 (s); MS
m/z 241 (M+). Anal. Calcd for C15H19N3: C, 74.65; H,
7.94; N, 17.41. Found: C, 74.72; H, 7.98; N, 17.50.
25
Major cycloadduct 7c: 38 mg, 2%. Pale yellow oil; ½aꢁD
¼
1
Racemic cycloadducts 6a and 7a: 0.81 g, 89%. White
powder; mp 74 ꢁC (from diisopropyl ether–methanol); IR
þ16:7 (c 0.11, CHCl3); H NMR (CDCl3) d 2.11 (1H, dd,
J 16.8, 7.0), 2.32 (1H, dd, J 16.8, 6.0), 2.40 (1H, dd, J
10.5, 8.5), 3.32 (2H, dd, J 12.5, 8.5), 3.50 (1H, dd J 7.0,
6.0), 3.70–3.90 (3H, m), 4.21 (1H, dd, J 9.0, 8.5), 5.10–
5.30 (2H, m), 5.80–5.90 (1H, m), 6.9–7.4 (10H, m); 13C
NMR (CDCl3) d 20.4 (q), 38.1 (d), 44.0 (t), 46.7 (t), 47.0
(t), 50.2 (t), 110.9 (t), 118.0–127.0, 130.8 (s), 134.7 (d),
142.9 (s); MS m/z 317 (M+).
1
(nujol) 1600 cmꢀ1; H NMR (CDCl3) d 2.28 (1H, dd, J
10.2, 8.6), 3.17 (2H, dd, J 12.3, 8.8), 3.20–3.90 (5H, m),
4.16 (1H, dd, J 9.1, 8.6), 5.10–5.25 (2H, m), 5.85–
6.00 (1H, m), 6.9–7.2 (5H, m); 13C NMR (CDCl3) d
38.3 (d), 44.1 (t), 46.7 (t), 47.1 (t), 48.5 (t), 113.6 (t),
119.1 (d), 129.2 (d), 129.8 (d), 131.4 (s), 133.7 (d), 145.3
(s); d MS m/z 227 (M+). Anal. Calcd for C14H17N3: C,
73.98; H, 7.54; N, 18.48. Found: C, 74.02; H, 7.51; N,
18.56.
Acknowledgements
Thanks are due to MURST for financial support.
Major cycloadduct 6b: 0.40 g, 42%. Pale yellow powder;
25
mp 68 ꢁC (from diisopropyl ether); ½aꢁD ¼ ꢀ106:0 (c 0.43,
1
CHCl3); IR (nujol) 1600 cmꢀ1; H NMR (CDCl3) d 1.37
References
(3H, d, J 6.5), 2.24 (1H, dd, J 10.0, 8.6), 3.18 (2H, dd, J
12.2, 8.9), 3.37 (1H, q, J 6.5), 3.40–3.72 (3H, m), 4.13
(1H, dd, J 9.1, 8.6), 5.20–5.30 (2H, m), 5.85–5.96 (1H,
m), 6.9–7.2 (5H, m); 13C NMR (CDCl3) d 21.7 (q), 38.8
(d), 46.3 (t), 47.4 (t), 47.8 (t), 50.3 (t), 111.6 (t), 119.0 (d),
125.4 (d), 128.8 (d), 133.0 (d), 134.9 (s), 143.1 (s); MS
m/z 241 (M+). Anal. Calcd for C15H19N3: C, 74.65; H,
7.94; N, 17.41. Found: C, 74.70; H, 7.97; N, 17.49.
1. (a) Wade, P. A. In Comprehensive Organic Synthesis; Trost,
B., Ed.; Pergamon Press: New York, 1992; pp 1111–1168; (b)
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3. (a) Gothelf, K. V.; Jørgensen, K. A. Chem. Rev 1998, 98, 863;
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1480.
Major cycloadduct 6c: 0.72 g, 57%. Pale yellow powder;
25
mp 57 ꢁC (from diisopropyl ether); ½aꢁD ¼ ꢀ83:2 (c 0.51,
1
CHCl3); IR (nujol) 1605 cmꢀ1; H NMR (CDCl3) d 2.18
(1H, dd, J 16.8, 7.0), 2.26 (1H, dd, J 16.8, 5.6), 2.34 (1H,
dd, J 10.5, 8.6), 3.21 (2H, dd, J 12.2, 8.6), 3.41 (1H, dd J
7.0, 5.6), 3.50–3.80 (3H, m), 4.13 (1H, dd, J 9.2, 8.6),
5.12–5.28 (2H, m), 5.83–5.96 (1H, m), 6.9–7.4 (10H, m);
13C NMR (CDCl3) d 19.7 (q), 37.4 (d), 44.3 (t), 46.4 (t),
47.1 (t), 48.9 (t), 112.2 (t), 119.0–127.0, 130.3 (d), 131.6
(s), 140.7 (s); MS m/z 317 (M+). Anal. Calcd for
C21H23N3: C, 79.46; H, 7.30; N, 13.24. Found: C, 79.50;
H, 7.32; N, 13.33.
Major cycloadduct 6d: 0.42 g, 35%. Pale yellow powder;
25
9. Yang, Q.; Xiao, W.-J.; Yu, Z. Org. Lett. 2005, 7, 871.
10. Murzina, L. T.; Baires, S. V.; Nekhoroshkov, V. M.; Ivanov,
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12. Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. P.
J. Am. Chem. Soc. 1985, 107, 3902.
mp 78 ꢁC (from diisopropyl ether); ½aꢁD ¼ ꢀ76:1 (c 0.29
1
CHCl3); IR (nujol) 1600 cmꢀ1; H NMR (CDCl3) d 2.30
(1H, dd, J 10.4, 8.7), 3.20 (2H, dd, J 12.0, 8.9), 3.43 (1H,
s), 3.50–3.75 (3H, m), 4.15 (1H, dd, J 9.2, 8.7), 5.10–5.24
(2H, m), 5.80–5.93 (1H, m), 6.9–7.4 (10H, m); 13C NMR
(CDCl3) d 42.2 (t), 46.5 (t), 47.0 (t), 52.4 (t), 108.7 (t),
119.0–130.0, 134.6 (s), 137.2 (d), 141.4 (s); MS m/z 303
13. As implemented in the Hyperchem 7.04 Professional package
of programs. Hypercube, 2002.