Enantioselective synthesis of (+)-5,6-exo-(isopropylidendioxy)-2-phenylsulfonyl-7-oxabicyclo[2.2.1]hept-2-ene

Article abstract of DOI:10.1016/S0957-4166(99)00217-7

The title compound (+)-1 has been synthesized in four steps from furan and dienophile (+)-3 via an asymmetric Diels-Alder reaction.

Full text of DOI:10.1016/S0957-4166(99)00217-7

Pergamon
Tetrahedron: Asymmetry 10 (1999) 2237–2242
Enantioselective synthesis of (+)-5,6-exo-(isopropylidendioxy)-
2-phenylsulfonyl-7-oxabicyclo[2.2.1]hept-2-ene
Odón Arjona,∗ Fátima Iradier, Rocío Medel and Joaquín Plumet ∗
Departamento de Química Orgánica I, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain
Received 18 May 1999; accepted 1 June 1999
Abstract
The title compound (+)-1 has been synthesized in four steps from furan and dienophile (+)-3 via an asymmetric
Diels–Alder reaction. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
In several previous reports, we have described the use of 7-oxanorbornenic sulfone 1 as a starting
material for the synthesis of several interesting compounds such as the aminocyclitol fragment of the
antitumoral alkaloid pancratistatin,1 the antibiotic rancynamycin III,2 the naturally occurring inositol
derivative (+)-pinitol3 and a modified A ring of vitamin D3.4 Synthesis of enantiomerically pure 1 has
been achieved by resolution of its alcohol precursor camphanoyl ester derivative 2 (Fig. 1). The absolute
configuration of compound 1 has been established by chemical correlation3 and X-ray analysis of the
camphanoyl derivative 2b.4 Nevertheless, tedious separations of diastereomeric mixtures are necessary
in both cases.
Thus, we envisaged the use of a dienophilic sulfone attached to a chiral auxiliary in order to achieve
an enantioselective Diels–Alder reaction using furan as the 4π component.5 It should be pointed out that
Figure 1.
∗
Corresponding author. E-mail: plumety@eucmax.sim.ucm.es
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00217-7

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enantioselective Diels–Alder reactions of furan and chiral dienophilic sulfones have not been, to the best
of our knowledge, previously described.
For this purpose, we selected the sulfinyl-activated sulfone 3,6 previously described as a diastereomeric
mixture by De Lucchi et al.7 Chromatographic (SiO2, Hex:AcOEt 50:1) separation of the diastereomeric
mixture of 3 gave pure (+)-3.8
2. Results and discussion
Reaction of (+)-3 with furan as solvent in the presence of ZnI2 as catalyst afforded adduct (+)-
4 (d.e.=99%) in 91% isolated yield (Scheme 1). All attempts to eliminate the chiral auxiliary using
the conditions described by De Lucchi et al.7 in the case of the related cyclopentadiene adduct were
unsuccesful. For instance, using DBU in CH2Cl2, variable amounts (37–50%) of epimeric trans-sulfone
(−)-5 were obtained. The same compound (−)-5 was obtained with t-BuOK/THF (60%). Finally, pyrolytic
elimination (toluene, reflux) performed on (−)-5 afforded an unidentified aromatic compound in 73%
isolated yield.
Scheme 1.
Due to the difficulty in removing the chiral auxiliary at this stage, we decided to transform (+)-4 into
protected exo-diol (+)-7 using standard conditions (Scheme 2).9 Reaction of (+)-7 with KOH in CH3CN
gave sulfone (+)-1b10 in 63% isolated yield together with 33% of epimeric sulfoxide (−)-8. Compound
(−)-8 can also be obtained from (+)-7 by reaction with DBU in CH2Cl2 in 90% isolated yield. Pyrolytic
elimination (xylene, reflux) performed on (−)-8 also gave (+)-1b in 40% isolated yield.
Scheme 2. Reagents and conditions: (i) OsO4, 4-methylmorpholine N-oxide, NaHCO3, 90%; (ii) 2,2-dimethoxypropane,
p-TsOH, 100%; (iii) KOH, CH3CN, 63%; (iv) DBU, CH2Cl2, 90%; (v) xylene, reflux, 40%; 46% of compound (−)-8 was
recovered
In summary, an efficient method for the synthesis of sulfone (+)-1b, a useful chiral building block for
the synthesis of a variety of natural products, has been described using as the key step the enantioselective
Diels–Alder reaction of dienophile (+)-3 and furan.

O. Arjona et al. / Tetrahedron: Asymmetry 10 (1999) 2237–2242
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3. Experimental
3.1. General methods
All air-sensitive reactions were carried out under a positive pressure of dry argon using freshly distilled
solvents under anhydrous conditions. Reagents and solvents were handled by using standard syringe
techniques. Tetrahydrofuran (THF) was distilled from sodium and benzophenone; dichloromethane,
acetonitrile and xylene from CaH2. Flash chromatography was performed using Merck 230–400 mesh
silica gel. Analytical TLC was carried out on 0.20 mm Merck precoated silica gel plates (60F-254).
Melting points were determined on a Büchi 512 apparatus and are uncorrected. Infrared spectra were
1
recorded on a Perkin–Elmer 781 spectrophotometer. H NMR and 13C NMR spectra were recorded on
Bruker AM-300 or Varian VXR-300S instruments using CDCl3 as solvent. The following abbreviations
are used to describe peak patterns when appropriate: br, broad; s, singlet; d, doublet; t, triplet; m,
multiplet. Optical rotations were measured on a Perkin–Elmer 241 polarimeter. Elemental analyses were
performed at the Universidad Complutense de Madrid.
3.2. (+)-(1R,4S,5S,6R)-6-endo-[1-(S)-Isobornyl-10-sulfinyl]-5-endo-phenylsulfonyl-7-oxabicyclo-
[2.2.1]hept-2-ene, (+)-4
To a solution of (+)-3 (244 mg, 0.67 mmol) in furan (13.4 ml), ZnI2 (85 mg, 0.27 mmol) was added.
The mixture was stirred protected from light for 7 days. Solvent was removed in vacuo and the crude
product was purified by column chromatography (CH2Cl2:AcOEt, 10:1) to afford adduct (+)-4 (291 mg,
91%) as a colorless oil. [α]D +22.2 (c 0.01, CHCl3). 1H NMR (CDCl3, 300 MHz): δ 0.90 (s, 3H, Me),
1.15 (s, 3H, Me), 1.24–1.28 (m, 1H, CH), 1.42–1.49 (m, 1H, CH2), 1.72–1.89 (m, 5H, CH2), 3.30 (d,
1H, J=12.7 Hz, CH2SO), 3.67 (d, 1H, J=12.7 Hz, CH2SO), 3.91 (d, 1H, J=2.9 Hz, CHOH), 3.99 (br s,
1H, OH), 4.03 (dd, 1H, J=3.4, 8.8 Hz, H-6), 4.19 (dd, 1H, J=4.4, 8.8 Hz, H-5), 4.64 (dd, 1H, J=1.0, 3.4
Hz, H-1), 5.40 (dd, 1H, J=1.0, 3.4 Hz, H-4), 6.75 (dd, 1H, J=1.5, 5.9 Hz, H-2), 6.99 (dd, 1H, J=1.5, 5.9
Hz, H-3), 7.66 (t, 2H, J=6.8 Hz, SO2Ph), 7.76 (t, 1H, J=7.3 Hz, SO2Ph), 7.92 (d, 2H, J=7.3 Hz, SO2Ph).
13C NMR (CDCl3, 75 MHz): δ 20.0, 20.5, 27.1, 30.3, 38.5, 45.1, 48.2, 51.6, 55.5, 66.5, 67.0, 79.9, 82.5,
97.3, 127.8, 129.9, 134.6, 134.9, 136.7, 140.7. IR (CHCl3): ν 3676, 3018, 1425, 1163 cm−1. Anal. calcd
for C22H28O5S2: C, 60.55; H, 6.42. Found: C, 60.49; H, 6.38.
3.3. (−)-(1R,4S,5R,6R)-6-endo-[1-(S)-Isobornyl-10-sulfinyl]-5-exo-phenylsulfonyl-7-oxabicyclo-
[2.2.1]hept-2-ene, (−)-5
To a solution of (+)-4 (30 mg, 0.07 mmol) in THF (0.7 ml) cooled to −78°C, t-BuOK (18 mg, 0.14
mmol) was added. The mixture was stirred for 30 min. The reaction was quenched with distilled water and
extracted with AcOEt. Organic layers were dried over MgSO4 and solvent was evaporated under reduced
pressure. After purification by column chromatography (hexane:AcOEt, 5:1), (−)-5 was obtained as a
1
colorless oil (18 mg, 60%). [α]D −69.3 (c 0.01, CHCl3). H NMR (CDCl3, 300 MHz): δ 0.90 (s, 3H,
Me), 1.17 (s, 3H, Me), 1.26 (s, 1H, CH), 1.41–1.54 (m, 2H, CH2), 1.70–1.83 (m, 4H, CH2), 2.97 (d, 1H,
J=4.1 Hz, H-5), 3.13 (d, 1H, J=13.4 Hz, CH2SO), 3.41 (d, 1H, J=13.4 Hz, CH2SO), 3.72 (d, 1H, J=3.2
Hz, OH), 3.84 (t, 1H, J=4.1 Hz, H-6), 3.96 (ddd, 1H, J=3.4, 4.4, 7.8 Hz, CHOH), 5.28 (dd, 1H, J=1.2,
4.4 Hz, H-1), 5.31 (dd, 1H, J=1.0, 1.9 Hz, H-4), 6.52 (dd, 1H, J=1.7, 5.9 Hz, H-2), 6.77 (dd, 1H, J=1.7,
5.9 Hz, H-3), 7.65 (t, 2H, J=7.6 Hz, SO2Ph), 7.73 (t, 1H, J=7.1 Hz, SO2Ph), 7.95 (d, 2H, J=6.8 Hz,
SO2Ph). 13C NMR (CDCl3, 75 MHz): δ 19.9, 20.4, 27.1, 30.9, 38.6, 44.9, 48.3, 51.4, 53.5, 62.4, 65.5,

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O. Arjona et al. / Tetrahedron: Asymmetry 10 (1999) 2237–2242
76.9, 80.2, 81.3, 129.2, 129.6, 134.7, 135.5, 136.7, 137.1. IR (CHCl3): ν 3394, 2926, 1308, 1134 cm−1.
Anal. calcd for C22H28O5S2: C, 60.55; H, 6.42. Found: C, 60.61; H, 6.50.
3.4. (+)-(1R,2S,3R,4S,5S,6R)-6-endo-[1-(S)-Isobornyl-10-sulfinyl]-5-endo-phenylsulfonyl-7-
oxabicyclo[2.2.1]heptane-2,3-diol, (+)-6
A solution of (+)-4 (282 mg, 0.65 mmol), 4-methylmorpholine N-oxide (114 mg, 0.95 mmol) and
OsO4 (2.5% t-BuOH) (0.06 ml, 4.4×10−3 mmol) in THF (1 ml) was added dropwise to a solution of
NaHCO3 (55 mg, 0.65 mmol) in t-BuOH:H2O 4:1 (5.2 ml of t-BuOH, 1.3 ml of H2O). The reaction
mixture was stirred at room temperature for 16 h and then excess 40% aqueous NaHSO3 solution was
added. Stirring was continued for 30–45 min, the reaction mixture was diluted with a large volume of
AcOEt and extracted. The organic layer was dried over MgSO4, evaporated in vacuo and purified by
column chromatography (hexane:AcOEt, 1:2) to afford (+)-6 (274 mg, 90%) as a colorless oil. [α]D
+30.0 (c 2.6, CHCl3). 1H NMR (CDCl3, 300 MHz): δ 0.91 (s, 3H, Me), 1.13 (s, 3H, Me), 1.49–1.55 (m,
1H, CH), 1.74–1.88 (m, 5H, CH2), 2.12 (br s, 1H, OH), 3.33 (d, 1H, J=12.7 Hz, CH2SO), 3.76 (d, 2H,
J=12.7 Hz, CH2SO, OH), 3.87 (dd, 2H, J=4.9, 10.7 Hz, H-6, CHOH), 3.92 (d, 1H, J=4.9 Hz, H-1), 3.97
(dd, 1H, J=5.4, 10.7 Hz, H-5), 4.03 (br s, 1H, OH), 4.73 (br s, 1H, OH), 4.76 (d, 1H, J=4.9 Hz, H-4), 4.83
(d, 1H, J=5.6 Hz, H-2), 5.01 (t, 1H, J=5.6 Hz, H-3), 7.66 (t, 2H, J=7.8 Hz, SO2Ph), 7.76 (t, 1H, J=7.3
Hz, SO2Ph), 7.93 (d, 2H, J=7.6 Hz, SO2Ph). 13C NMR (CDCl3, 75 MHz): δ 14.1, 19.9, 20.4, 27.0, 30.3,
38.6, 45.0, 48.3, 51.6, 55.9, 61.2, 64.9, 69.6, 70.5, 84.1, 86.0, 127.7, 130.0, 134.8, 138.5. IR (CHCl3): ν
3676, 3404, 1364, 1151 cm−1. Anal. calcd for C22H30O7S2: C, 56.17; H, 6.38. Found: C, 56.22; H, 6.27.
3.5. (+)-(1R,2S,3R,4S,5S,6R)-6-endo-[1-(S)-Isobornyl-10-sulfinyl]-2,3-exo-(isopropylidendioxy)-5-
endo-phenylsulfonyl-7-oxabicyclo[2.2.1]heptane, (+)-7
To a solution of diol (+)-6 (213 mg, 0.45 mmol) in acetone (4.5 ml), p-TsOH (catalytic amounts)
and 2,2-dimethoxypropane (0.22 ml, 1.81 mmol) were added. After 1 h of stirring, a saturated aqueous
solution of NaHCO3 was added. The mixture was extracted with AcOEt, the organic layer was dried
over MgSO4, filtered and solvent was eliminated in vacuo. The crude product was purified by column
chromatography on silica gel (hexane:AcOEt, 1:1) to produce (+)-7 (229 mg, 100%) as a colorless oil.
[α]D +20.3 (c 0.01, CHCl3). 1H NMR (CDCl3, 300 MHz): δ 0.90 (s, 3H, Me), 1.15 (s, 3H, Me), 1.25 (s,
1H, CH), 1.35 (s, 3H, Me), 1.42 (s, 3H, Me), 1.74–1.84 (m, 6H, CH2), 3.31 (d, 1H, J=12.7 Hz, CH2SO),
3.79 (d, 1H, J=12.7 Hz, CH2SO), 3.84 (br s, 1H, CHOH), 3.88 (dd, 1H, J=4.6, 10.7 Hz, H-6), 3.94 (d,
1H, J=5.1 Hz, H-1), 3.98 (d, 1H, J=3.9 Hz, OH), 4.02–4.06 (m, 1H, H-5), 4.82 (d, 1H, J=5.1 Hz, H-4),
5.20 (d, 1H, J=5.6 Hz, H-2 or H-3), 5.35 (d, 1H, J=5.4 Hz, H-2 or H-3), 7.67 (t, 2H, J=7.1 Hz, SO2Ph),
7.76 (t, 1H, J=7.3 Hz, SO2Ph), 7.93 (d, 2H, J=7.1 Hz, SO2Ph). 13C NMR (CDCl3, 75 MHz): δ 8.5, 19.9,
20.0, 20.5, 25.0, 25.5, 27.1, 30.4, 38.6, 45.1, 48.3, 51.7, 56.7, 60.8, 64.8, 77.1, 79.1, 81.1, 83.4, 111.5,
127.7, 130.1, 135.0, 138.7. IR (CHCl3): ν 3350, 1425, 1024 cm−1. Anal. calcd for C25H34O7S2: C, 58.82;
H, 6.67. Found: C, 58.75; H, 6.72.
3.6. (−)-(1R,2S,3R,4S,5R,6R)-6-endo-[1-(S)-Isobornyl-10-sulfinyl]-2,3-exo-(isopropylidendioxy)-5-
exo-phenylsulfonyl-7-oxabicyclo[2.2.1]heptane, (−)-8
To a solution of (+)-7 (17 mg, 0.03 mmol) in CH2Cl2 (0.33 ml), DBU (0.01 ml, 0.07 mmol) was added.
The reaction mixture was stirred at room temperature for 30 min, then quenched with 0.5N aqueous HCl
solution and extracted with CH2Cl2. The organic layer was dried over MgSO4 and solvent was evaporated

O. Arjona et al. / Tetrahedron: Asymmetry 10 (1999) 2237–2242
2241
under reduced pressure to afford (−)-8 (15 mg, 90%) as a colorless oil. [α]D −78.2 (c 0.03, CHCl3). 1H
NMR (CDCl3, 300 MHz): δ 0.89 (s, 3H, Me), 0.97 (s, 1H, CH), 1.15 (s, 3H, Me), 1.26 (s, 3H, Me), 1.43
(s, 3H, Me), 1.60 (s, 1H, CH2), 1.74–1.86 (m, 5H, CH2), 2.90 (d, 1H, J=13.2 Hz, CH2SO), 3.26 (d, 1H,
J=6.1 Hz, H-6), 3.46 (d, 1H, J=12.9 Hz, CH2SO), 3.52 (d, 1H, J=6.1 Hz, H-5), 3.56 (d, 1H, J=3.9 Hz,
OH), 3.97–4.09 (m, 1H, CHOH), 4.23 (d, 1H, J=5.6 Hz, H-2 or H-3), 4.73 (s, 1H, H-4), 4.85 (d, 1H,
J=4.9 Hz, H-1), 5.12 (d, 1H, J=5.6 Hz, H-2 or H-3), 7.64 (t, 2H, J=7.3 Hz, SO2Ph), 7.76 (t, 1H, J=7.6
Hz, SO2Ph), 7.94 (d, 2H, J=7.3 Hz, SO2Ph). 13C NMR (CDCl3, 75 MHz): δ 19.9, 20.4, 25.2, 25.6, 27.1,
30.9, 38.6, 44.9, 48.4, 51.5, 58.6, 63.2, 63.7, 76.9, 78.9, 81.0, 81.1, 81.5, 112.6, 129.0, 129.8, 134.8,
136.7. IR (CHCl3): ν 3352, 1425, 1022 cm−1. Anal. calcd for C25H34O7S2: C, 58.82; H, 6.67. Found: C,
58.90; H, 6.61.
3.7. (+)-(1S,4S,5S,6R)-5,6-exo-(Isopropylidendioxy)-2-phenylsulfonyl-7-oxabicyclo[2.2.1]hept-2-ene,
(+)-1b
3.7.1. Procedure A
To a solution of (+)-7 (50 mg, 0.1 mmol) in CH3CN (1 ml), KOH (37 mg, 0.6 mmol) was added.
The mixture was stirred at 40°C for 7 h. The reaction was quenched with distilled water and extracted
with AcOEt. The organic layer was dried over MgSO4 and solvent was evaporated in vacuo. The crude
product was purified by column chromatography (hexane:AcOEt, 5:1) to afford (+)-1b (19 mg, 63%) as
a white solid and (−)-8 (16 mg, 33%) as a colorless oil.
3.7.2. Procedure B
A solution of (−)-8 (41 mg, 0.08 mmol) in xylene (0.16 ml) was heated at reflux for 3 h to afford, after
purification by column chromatography (hexane:AcOEt, 5:1), (+)-1b (10 mg, 40%) as a white solid. 46%
1
(19 mg) of (−)-8 was recovered. [α]D +32.0 (c 0.01, CHCl3). Mp: 112–114°C. H NMR (CDCl3, 300
MHz): δ 1.30 (s, 3H, Me), 1.45 (s, 3H, Me), 4.44 (d, 1H, J=5.1 Hz, H-5), 4.55 (d, 1H, J=5.1 Hz, H-6),
4.77 (d, 1H, J=0.6 Hz, H-1), 4.94 (dd, 1H, J=0.6, 2.0 Hz, H-4), 7.05 (d, 1H, J=2.0 Hz, H-3), 7.58 (t, 2H,
J=7.0 Hz, SO2Ph), 7.66 (t, 1H, J=7.5 Hz, SO2Ph), 7.90 (d, 2H, J=7.5 Hz, SO2Ph). 13C NMR (CDCl3,
75 MHz): δ 25.6, 31.5, 79.2, 79.5, 81.2, 83.1, 116.5, 127.9, 129.6, 134.3, 138.4, 143.3, 150.4. IR (KBr):
ν 3050, 1520, 1420 cm−1. Anal. calcd for C15H16O5S: C, 58.44; H, 5.20. Found: C, 58.40; H, 5.13.
Acknowledgements
This research was supported by D.G.I.C.Y.T. (Ministerio de Educación y Cultura, grant no. PB96-
0641). We also thank Universidad Complutense de Madrid and Ministerio de Educación y Cultura for
two predoctoral fellowships to F.I. and R.M., respectively.
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