Stereoselective intramolecular cycloadditions of homochiral nitrilimines: Synthesis of enantiopure (6S)-substituted-2,3,3a,4,5,6-hexahydro-furo[3,4-c] pyrazoles

Article abstract of DOI:10.1016/j.tetasy.2004.02.002

Starting from (S)-ethyl lactate and (S)-ethyl mandelate the homochiral hydrazonoyl chlorides 4b-d have been synthesised. Their base treatment promoted the in situ generation of the corresponding nitrilimines 5b-d, which gave the enantiopure title compound

Full text of DOI:10.1016/j.tetasy.2004.02.002

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 1127–1131
Stereoselective intramolecular cycloadditions of homochiral
nitrilimines: synthesis of enantiopure (6S)-substituted-
2,3,3a,4,5,6-hexahydro-furo[3,4-c]pyrazoles
Lara De Benassuti, Luisa Garanti and Giorgio Molteni*
Universitꢀa degli Studi di Milano, Dipartimento di Chimica Organica e Industriale, Via Golgi 19, 20133 Milano, Italy
Received 14 January 2004; accepted 4 February 2004
Abstract—Starting from (S)-ethyl lactate and (S)-ethyl mandelate the homochiral hydrazonoyl chlorides 4b–d have been synthes-
ised. Their base treatment promoted the in situ generation of the corresponding nitrilimines 5b–d, which gave the enantiopure title
compounds with good overall yields and diastereoselectivities.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Stereoselective 1,3-dipolar cycloadditions represents one
of the most productive fields of modern synthetic
organic chemistry.1 As a result, the behaviour of a large
array of 1,3-dipolar functionalities have been exploited
in the construction of a number of enantiopure five-
membered heterocycles.2 However, despite the utility of
enantiopure 4,5-dihydropyrazole derivatives in organic
synthesis3 and some interesting applications of the
related products,4 the stereoselective cycloadditions of
nitrilimines have scarcely been investigated. The first
case of asymmetric induction in intramolecular nitril-
imine cycloaddition was reported by our laboratory,5
and up until now, this approach to fused bi- or tricyclic
4,5-dihydropyrazoles has received little attention.6 This
lack of experimental data led us to investigate in more
detail the behaviour in terms of stereoselectivity of nit-
rilimines 5b–d in which the stereocentre is placed inside
the tether joining the reactive groups. Furthermore,
having recognised that the synthesis of enantiopure
molecules from simple starting materials is a valuable
target,7 we developed the inexpensive, commercially
available (S)-ethyl lactate and (S)-ethyl mandelate as
suitable starting chiral units.
Our synthetic sequence starts from the known ethyl
(2S)-substituted-2-allyloxyacetates 1a–d8 as the chiral
building blocks. Their basic hydrolysis followed by
treatment with thionyl chloride gave the corresponding
(2S)-substituted-2-allyloxyacetyl chlorides 2a–d, which
were used as crude materials without isolation. Acyl
hydrazide intermediates 3a–d were obtained pure from
the latter by treatment with phenylhydrazine. Hydr-
azonoyl chlorides 4a–d were synthesised from acyl
hydrazides 3a–d via treatment with triphenylphosphine
and carbon tetrachloride according to the method
originally proposed by Wolkoff9 (Scheme 1). The in situ
generation of labile intermediates 5a–d was accom-
plished by refluxing the corresponding hydrazonoyl
chlorides 4a–d with an excess of triethylamine (5 equiv)
in dry toluene, thus following the classic HuisgenÕs nit-
rilimine cycloaddition protocol.10 Reaction times, over-
all product yields and diastereoisomeric ratios are
summarised in Table 1. In the case of unsubstituted 5a,
simple crystallisation of the crude gave the racemic
mixture 6a and 7a in analytically pure form with 96%
overall yield. Spectral data of these cycloadducts are
fully consistent with those reported for similar furo[3,4-
c]pyrazoles.11
The diastereoisomeric cycloadducts 6b–d and 7b–d were
obtained chemically and enantiomerically pure through
silica gel column chromatography. Their isolated yields
ranged from fair to good, with some tarry material being
formed. The absolute configurations of the newly
* Corresponding author. Tel.: +39-02-50314141; fax: +39-02-503141-
15; e-mail: giorgio.molteni@unimi.it
0957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.02.002

1128
L. De Benassuti et al. / Tetrahedron: Asymmetry 15 (2004) 1127–1131
R
O
R
O
R
O
O
O
PhNH NH
O
1) aq. NaOH/THF
2) SOCl2, 40°C
Ph NHNH2
toluene, r.t.
EtO
Cl
1
1
R
1
R
R
1
2
3
R
R
O
PhNH N
Cl
Ph
N
R
N
Ph3P, CCl4
MeCN, r.t.
Et3N
toluene,
O
∆
1
1
R
4
5
R
R
N
N
Ph
N
1
+
Ph
N
O
O
H
R
1
H
R
6
7
Entry
R
a
b
c
d
H
H
Me
H
Me
Me
Ph
H
1
R
Scheme 1.
Table 1. Intramolecular cycloadditions of nitrilimines 5a
for 7d,13 thus justifying the observed mutual NOE
enhancements. As far as diastereoselection is concerned,
it can be inferred that: (i) the relative configurations of
the newly formed stereocentres of cycloadducts 6c and
7c is anti as a consequence of the concerted cycloaddi-
tion mechanism, and (ii) the ratio 6b–d/7b–d encom-
passes the range between 70:30 and 100:0 depending
upon the size and shape of R and R1. These results may
account for the proposed transition states A and B (Fig.
2). Due to the inner disposition of R, transition state B
should be the less stable one giving minor 7b–d; the
intervention of such a transition state is made impervi-
ous in the case of the bulky phenyl pendant. Hence, the
preferred formation of major 6b–d finds rationalisation.
Entry
R
R1
Time (h)
6 and 7 (%)b 6/7c
a
b
c
H
H
3
4
8
5
96
74
65
39
50:50
Me
Me
Ph
H
86:14
70:30
100:0
Me
H
d
a In refluxing toluene.
b Overall yields.
c Determined from 1H NMR analysis of reaction crudes.
formed stereocentres of minor 7b–d were determined
unambiguously by the mutual NOE enhancements
reported in Figure 1. It is apparent that the observed
NOE effects can be operative only in the case of the
depicted stereochemical arrangement. Furthermore,
AM112 calculated distances between HA and the average
H
R1
H
H
ꢁ
ꢁ
ꢁ
O
position of HB were 2.79 A for 7b, 2.81 A for 7c, 2.73 A
C
Ar
N
N
H
R
C
Ar
N
N
H
O
H
H
R
HB
HB
HB
N
N
R1
H
Ph
Ph
N
N
A
B
O
O
HB
HB
1
R
1
R
HA
HA
8.5%
6b-d
7b-d
1
1
=
7b: R
=
H, 8.0%; 7c: R
Me, 4.8%
7d
Figure 2. Proposed transition states for the formation of cycloadducts
6b–d and 7b–d.
Figure 1. NOE enhancements for cycloadducts 6 and 7.

L. De Benassuti et al. / Tetrahedron: Asymmetry 15 (2004) 1127–1131
1129
3. Conclusion
toluene (25 mL) at room temperature. After 15 min
phenylhydrazine (0.81 g, 7.5 mmol) was added and the
mixture stirred at room temperature for 4 h. Water
(50 mL) was added to reaction mixture, the organic layer
separated, dried over sodium sulfate and evaporated
under reduced pressure. The dark red residue was
chromatographed on a silica gel column with ethyl
acetate–hexane 4:1 giving acyl hydrazides 3.
In conclusion, the favourable entropic effects, which
work in the formation of the furo[3,4-c]pyrazole skele-
ton, makes the intramolecular cycloaddition of nitril-
imines 5a–d very efficient and hence valuable on a
preparative scale.
4.2.1. Compound 3a. 0.73 g, 47%. White powder; mp
70 ꢁC (from diisopropyl ether); IR (nujol) 3310, 3240,
1650 cmÀ1; 1H NMR (CDCl3) d 4.08 (2H, d, J 4.6), 4.12
(2H, s), 5.21–5.93 (3H, m), 6.08 (1H, br d, J 4.2), 6.8–7.3
(5H, m), 8.22 (1H, br s); MS m=z 206 (Mþ). Anal. Calcd
for C11H14N2O2: C, 64.06; H, 6.84; N, 13.58. Found: C,
64.10; H, 6.87; N, 13.62.
4. Experimental section
€
Melting points were determined with a Buchi apparatus
in open tubes and are uncorrected. IR spectra were
recorded with a Perkin–Elmer 1725 X spectrophoto-
meter. Mass spectra were determined with a VG-70EQ
apparatus. 1H NMR (300 MHz) spectra were taken with
a Bruker AMX 300 instrument (in CDCl3 solutions at
room temperature). Chemical shifts are given as ppm
from tetramethylsilane and J values are given in Hz.
NOE experiments were performed by setting the fol-
lowing parameters: relaxation 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 Per-
kin–Elmer 241 polarimeter at the sodium D-line at
25 ꢁC.
4.2.2. Compound 3b. 0.99 g, 60%. White powder; mp
25
D
61 ꢁC (from diisopropyl ether); ½aŠ ¼ )16.6 (c 0.16,
CHCl3); IR (nujol) 3310, 3230, 1650 cmÀ1
;
1H NMR
(CDCl3) d 1.48 (3H, d, J 6.5), 4.10 (1H, q, J 6.5), 4.12–
4.20 (2H, m), 5.12–5.90 (3H, m), 6.07 (1H, br d, J 4.3),
6.8–7.3 (5H, m), 8.30 (1H, br s); MS m=z 220 (Mþ).
Anal. Calcd for C12H16N2O2: C, 65.43; H, 7.32; N,
12.72. Found: C, 65.40; H, 7.36; N, 12.77.
4.1. Synthesis of ethyl (2S)-but-2-enyloxylactate 1c
4.2.3. Compound 3c. 0.76 g, 43%. White powder; mp
25
D
58 ꢁC (from diisopropyl ether); ½aŠ ¼ )18.3 (c 0.75,
Ag2O (2.32 g, 10.0 mmol) was added portionwise, under
vigorous stirring, to a solution of ethyl (2S)-lactate
(1.18 g, 10.0 mmol) and 1-bromobut-2-ene (1.69 g,
12.5 mmol) in anhydrous diethyl ether (25 mL) at room
temperature. The mixture was refluxed for 3 h, then
cooled and filtered over Celite. The organic layer was
dried over sodium sulfate, the solvent evaporated and
CHCl3); IR (nujol) 3300, 3225, 1660 cmÀ1
;
1H NMR
(CDCl3) d 1.02 (3H, d, J 6.4), 1.74 (3H, d, J 5.0), 4.08
(2H, d, J 4.7), 4.16 (1H, q, J 6.4), 5.15–5.85 (2H, m),
6.05 (1H, br d, J 4.2), 6.8–7.3 (5H, m), 8.25 (1H, br s);
MS m=z 234 (Mþ). Anal. Calcd for C13H18N2O2: C,
61.52; H, 7.74; N, 11.96. Found: C, 61.55; H, 7.76; N,
12.03.
the residue distilled in vacuo giving 1c (1.53 g, 89%); pale
25
D
yellow oil. ½aŠ ¼ )21.8 (c 0.16, CHCl3); IR (neat)
1
1745 cmÀ1; H NMR d 1.35 (3H, t, J 4.1), 1.45 (3H, t,
J 6.4), 1.70 (3H, d, J 7.1), 4.16 (2H, q, J 6.4), 5.08 (2H,
d, J 5.2), 5.25–5.90 (3H, m). MS m=z: 172 (Mþ). Anal.
Calcd for C9H16O3: C, 62.80; H, 9.36. Found: C, 62.85;
H, 9.40.
4.2.4. Compound 3d. 0.70 g, 33%. White powder; mp
25
D
85 ꢁC (from diisopropyl ether); ½aŠ ¼ )4.3 (c 1.12,
CHCl3); IR (nujol) 3340, 3230, 1650 cmÀ1
;
1H NMR
(CDCl3) d 4.16 (2H, d, J 4.8), 4.95 (1H, s), 5.25–5.94
(3H, m), 6.02 (1H, br d, J 4.6), 6.8–7.5 (10H, m), 8.40
(1H, br s); MS m=z 282 (Mþ). Anal. Calcd for
C17H18N2O2: C, 72.32; H, 6.43; N, 9.92. Found: C,
72.28; H, 6.45; N, 9.88.
4.2. Synthesis of acyl hydrazides 3
A solution of the appropriate 2-allyloxyacetate 1
(7.5 mmol) in tetrahydrofuran (75 mL) was treated with
2 M aqueous sodium hydroxide (75 mL) and stirred at
room temperature for 3 h. Aqueous 6 M hydrochloric
acid was added until 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 pressure. Thionyl chloride (0.89 g, 7.5 mmol)
was added dropwise 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 giving crude 2-allyloxyacetyl chlorides 2 as
dark brown oils. Freshly distilled triethylamine (1.52 g,
15.0 mmol) was added dropwise to a solution of 2 in dry
4.3. Synthesis of acyl hydrazonoyl chlorides 4
A
solution of the appropriate acyl hydrazide 3
(4.5 mmol) in dry acetonitrile (45 mL) and carbon tet-
rachloride (3.45 g, 22.5 mmol) was treated with triphe-
nylphosphine (5.90 g, 75 mmol) and stirred at room
temperature for 12 h. Brine (25 mL) was added to reac-
tion mixture, the organic layer separated, dried over
sodium sulfate and evaporated under reduced pressure.
The dark red residue was chromatographed on a silica
gel column with ethyl acetate–hexane 2:1 giving hydr-
azonoyl chlorides 4.

1130
L. De Benassuti et al. / Tetrahedron: Asymmetry 15 (2004) 1127–1131
4.3.1. Compound 4a. 0.74 g, 73%. Yellow oil; IR (neat)
3330 cmÀ1; 1H NMR (CDCl3) d 4.10 (2H, d, J 5.7), 4.33
(2H, s), 5.20–5.94 (3H, m), 6.9–7.3 (5H, m), 7.80 (1H, br
s); MS m=z 224 (Mþ). Anal. Calcd for C11H13ClN2O: C,
58.80; H, 5.83; N, 12.47. Found: C, 58.76; H, 5.80; N,
12.54.
(3H, d, J 6.7), 3.24 (1H, dd, J 8.2, 6.7), 3.54 (1H, dd, J
8.0, 7.3), 3.70–3.82 (1H, m), 4.12 (1H, dd, J 8.2, 8.0),
4.32 (1H, dd, J 8.0, 7.7), 4.64 (1H, q, J 6.7), 6.8–7.3 (5H,
m); MS m=z 202 (Mþ). Anal. Calcd for C12H14N2O: C,
71.26; H, 6.98; N, 13.85. Found: C, 71.30; H, 7.02; N,
13.81.
25
D
4.3.2. Compound 4b. 0.47 g, 44%. Pale yellow oil; ½aŠ
¼
4.4.3. Major cycloadduct 6c. 0.25 g, 46%. Pale yellow
25
+13.1 (c 0.23, CHCl3); IR (neat) 3325 cmÀ1; H NMR
(CDCl3) d 1.47 (3H, d, J 6.4), 3.98 (2H, dddd, J 13.8,
7.4, 2.4, 1.8), 4.40 (1H, q, J 6.4), 5.18–5.93 (3H, m), 6.9–
7.3 (5H, m), 7.80 (1H, br s); MS m=z 238 (Mþ). Anal.
Calcd for C12H15ClN2O: C, 60.38; H, 6.33; N, 11.74.
Found: C, 60.43; H, 6.30; N, 11.80.
powder; mp 75 ꢁC (from diisopropyl ether); ½aŠ
¼
1
D
1
)18.3 (c 0.31, CHCl3); IR (nujol) 1600 cmÀ1; H NMR
(CDCl3) d 1.45 (3H, d, J 6.5), 1.70 (3H, d, J 6.5), 3.56
(1H, dd, J 8.3, 6.6), 3.81–4.02 (2H, m), 4.28 (1H, dd, J
8.3, 8.1), 4.58 (1H, q, J 6.5), 6.8–7.4 (5H, m); MS m=z
216 (Mþ). Anal. Calcd for C13H16N2O: C, 72.19; H,
7.46; N, 12.95. Found: C, 72.14; H, 7.44; N, 13.00.
25
4.3.3. Compound 4c. 0.51 g, 44%. Pale yellow oil; ½aŠ
¼
D
+96.7 (c 0.75, CHCl3); IR (neat) 3330 cmÀ1; H NMR
(CDCl3) d 1.45 (3H, d, J 6.5), 1.70 (3H, d, J 5.9), 3.78–
4.15 (2H, m), 5.02 (1H, q, J 6.5), 5.50–5.89 (2H, m), 6.9–
7.3 (5H, m), 7.70 (1H, br s); MS m=z 252 (Mþ). Anal.
Calcd for C13H17ClN2O: C, 61.78; H, 6.78; N, 11.08.
Found: C, 61.81; H, 6.81; N, 11.12.
4.4.4. Major cycloadduct 6d. 0.26 g, 39%. Pale yellow
1
25
powder; mp 88 ꢁC (from diisopropyl ether); ½aŠ ¼ )39.7
(c 0.21, CHCl3); IR (nujol) 1610 cmÀ1
;
1DH NMR
(CDCl3) d 3.39 (1H, dd, J 11.5, 9.2), 3.65 (1H, s), 3.76
(1H, dd, J 9.5, 7.9), 3.85–4.03 (1H, m), 4.20 (1H, dd, J
9.2, 9.0), 4.52 (1H, q, J 7.9, 7.7), 6.9–7.5 (10H, m); MS
m=z 264 (Mþ). Anal. Calcd for C17H16N2O: C, 77.25; H,
6.10; N, 10.60. Found: C, 77.28; H, 6.07; N, 10.66.
4.3.4. Compound 4d. 0.88 g, 67%. Pale yellow powder;
25
mp 63 ꢁC (from methanol); ½aŠ ¼ +54.6 (c 0.80,
D
CHCl3); IR (nujol) 3340 cmÀ1; 1H NMR (CDCl3) d 4.14
(2H, dddd, J 14.0, 7.5, 2.6, 1.8), 5.20–6.02 (3H, m), 5.35
(1H, s), 6.9–7.5 (10H, m), 7.85 (1H, br s); MS m=z 300
(Mþ). Anal. Calcd for C17H17ClN2O: C, 67.88; H, 5.70;
N, 9.31. Found: C, 67.93; H, 5.73; N, 9.26.
4.4.5. Minor cycloadduct 7b. 52 mg, 10%. Pale yellow
25
powder; mp 55 ꢁC (from diisopropyl ether); ½aŠ ¼ +12.7
(c 0.28, CHCl3); IR (nujol) 1610 cmÀ1
;
1DH NMR
(CDCl3) d 1.40 (3H, d, J 6.7), 3.27 (1H, dd, J 8.2, 6.7),
3.63 (1H, dd, J 8.0, 7.3), 3.70–3.84 (1H, m), 4.13 (1H,
dd, J 8.2, 8.0), 4.24 (1H, dd, J 8.0, 7.8), 4.68 (1H, q, J
6.7), 6.8–7.3 (5H, m); MS m=z 202 (Mþ). Anal. Calcd for
C12H14N2O: C, 71.26; H, 6.98; N, 13.85. Found: C,
71.28; H, 6.96; N, 13.80.
4.4. Intramolecular cycloaddition of hydrazonoyl chlor-
ides 4
A solution of the appropriate hydrazonoyl chloride 4
(2.5 mmol) in dry toluene (125 mL) was treated with
triethylamine (1.01 g, 10.0 mmol) and refluxed for the
time indicated in Table 1. The crude was evaporated
under reduced pressure and the residue then chroma-
tographed on a silica gel column with ethyl acetate–
hexane 2:1. The first fractions contained the major
cycloadduct 6, while further elution gave minor cy-
cloadduct 7.
4.4.6. Minor cycloadduct 7c. 0.11 g, 20%. Pale yellow
25
powder; mp 67 ꢁC (from diisopropyl ether); ½aŠ ¼ +11.9
(c 0.51, CHCl3); IR (nujol) 1600 cmÀ1
;
1DH NMR
(CDCl3) d 1.41 (3H, d, J 6.5), 1.69 (3H, d, J 6.4), 3.51
(1H, dd, J 8.3, 6.6), 3.80–3.94 (2H, m), 4.32 (1H, dd, J
8.3, 8.1), 4.63 (1H, q, J 6.4), 6.8–7.3 (5H, m); MS m=z
216 (Mþ). Anal. Calcd for C13H16N2O: C, 72.19; H,
7.46; N, 12.95. Found: C, 72.13; H, 7.42; N, 12.89.
4.4.1. Racemic cycloadduct(s) 6a and 7a. 0.45 g, 96%.
White powder; mp 66 ꢁC (from diisopropyl ether–
1
methanol); IR (nujol) 1605 cmÀ1; H NMR (CDCl3) d
Acknowledgements
3.30 (1H, dd, J 8.0, 6.6), 3.58 (1H, dd, J 6.7, 5.5), 3.75–
3.90 (1H, m), 4.18 (1H, dd, J 8.0, 7.8), 4.34 (1H, dd, J
6.7, 6.5), 4.40 (1H, d, J 9.1), 4.52 (1H, d, J 9.1), 6.8–7.3
(5H, m); MS m=z 188 (Mþ). Anal. Calcd for C11H12N2O:
C, 70.19; H, 6.43; N, 14.88. Found: C, 70.23; H, 6.39; N,
14.93.
Thanks are due to MURST and CNR for financial
support.
References and notes
4.4.2. Major cycloadduct 6b. 0.32 g, 64%. White powder;
1. (a) Cycloaddition Reactions in Organic Synthesis; Koba-
yashi, S., Jørgensen, K. A., Eds.; Wiley-WCH: Weinheim,
2002; (b) Synthetic Applications of 1,3-Dipolar Cycloaddi-
25
D
mp 62 ꢁC (from diisopropyl ether); ½aŠ ¼ )24.7 (c 0.56,
CHCl3); IR (nujol) 1610 cmÀ1; 1H NMR (CDCl3) d 1.42

L. De Benassuti et al. / Tetrahedron: Asymmetry 15 (2004) 1127–1131
1131
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