Synthesis, structure and chiroptical spectra of the bicyclic α-diketones, imides and dithioimides related to santenone

Article abstract of DOI:10.1016/S0957-4166(00)00283-4

Isomeric α-diketones, imides and dithioimides related to santenone were prepared in a multistep synthesis from (+)-camphor and their CD spectra compared with those of analogous chromophoric systems related to camphor. In the case of conformationally rigid α-diketones the methyl substituents at C-7, lying on the symmetry plane of the chromophore, exert only a weak contribution to the n-π* Cotton effects. In contrast, the Cotton effect magnitudes of the anhydrides, imides and dithioimides are significantly affected by the substituents at C-8. The steric interaction of these chromophores with the syn-methyl group at C-8, leading to the chromophore distortion, was confirmed by X-ray crystallographic studies. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00283-4

Tetrahedron: Asymmetry 11 (2000) 3113±3122
Synthesis, structure and chiroptical spectra of the bicyclic
a-diketones, imides and dithioimides related to santenone
Tadeusz Po•onski,a,* Maria J. Milewskaa and Maria Gdaniecb
a
Â
Department of Chemistry, Technical University, 80-952 Gdansk, Poland
Faculty of Chemistry, A. Mickiewicz University, 60-780 Poznan, Poland
b
Â
Received 5 June 2000; accepted 18 July 2000
Abstract
Isomeric a-diketones, imides and dithioimides related to santenone were prepared in a multistep synthesis
from (+)-camphor and their CD spectra compared with those of analogous chromophoric systems related
to camphor. In the case of conformationally rigid a-diketones the methyl substituents at C-7, lying on the
symmetry plane of the chromophore, exert only a weak contribution to the n±p* Cotton e€ects. In contrast,
the Cotton e€ect magnitudes of the anhydrides, imides and dithioimides are signi®cantly a€ected by the
substituents at C-8. The steric interaction of these chromophores with the syn-methyl group at C-8, leading
to the chromophore distortion, was con®rmed by X-ray crystallographic studies. # 2000 Elsevier Science
Ltd. All rights reserved.
1. Introduction
Santenones 1a,b, the isomeric ketones related to camphor, have been isolated from Indian
sandalwood (Santalum album).1 A close structural relationship between the a- and b-stereo-
isomers (1a and 1b, respectively) results in serious diculties in the preparation of their derivatives
in a pure form. Furthermore, there are con¯icting reports on their con®guration at C-7.1 3
However, the optically active ketones 1a,b can be considered as useful substrates for the synthesis
of the model chromophoric systems suitable for spectroscopic studies.
Recently, we have been involved in the chiroptical studies of a-diketones,4 cyclic anhydrides,
imides5 and dithioimides.6 In this paper we report the preparation and the CD spectra of santenone
related compounds 2a,b±5a,b containing the above chromophores. Our aim was to compare their
chiroptical properties with those of the analogous compounds 2c±5c derived from camphor. The
systems 2a,b,c±5a,b,c contain methyl substituents at C-7 or C-8 located on the symmetry plane of
the chromophores.4 6 Because this plane is also a nodal surface in the corresponding sector rules,
* Corresponding author. E-mail: tadpol@chem.pg.gda.pl
0957-4166/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00283-4

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T. Po•onski et al. / Tetrahedron: Asymmetry 11 (2000) 3113±3122
3114
used for a prediction of the Cotton e€ect (CE) sign, the methyl substituents are expected to exert
only a negligible contribution to the CE sign and magnitude of the conformationally rigid
compounds. On the other hand, the steric e€ect of the substituents may in¯uence geometry of the
¯exible chromophores and thus change not only the intensity of the CD bands but also their sign.
2. Results and discussion
The syntheses of the compounds 2a,b±5a,b (Scheme 1) were based on p-hydroxycamphor 6 as a
substrate, that can be prepared in several steps from (+)-camphor.7 A mixture of a- and b-santenones
1a,b was obtained by decarbonylation of the corresponding aldehyde 7 in an inert atmosphere.
This method gives much better yields than the autooxidation of 7 reported by Ishidate and Sano2
and later by Nishimitsu.3 A signi®cant steric e€ect of the C-7 syn-substituent facilitates di€erentia-
tion of the a- and b-stereoisomers; the hydrogen peroxide oxidation of the a-diketone mixture 2
to anhydride 3 results ®nally in the acid 8 formed by immediate hydrolysis of the a-anhydride 3a,
whereas the b-anhydride 3b remains unchanged in the reaction conditions. Imides 4a,b were
converted to dithioimides 5a,b by reaction with Lawesson's reagent in boiling toluene. The
con®guration at C-8 was con®rmed by the X-ray crystallographic structures of the anhydrides 3a
and 3b (Fig. 1).
The chromophores studied are composed of two carbonyl or thiocarbonyl subunits and their
interaction lifts degeneracy between two `non-bonding' ns and na levels being a combinations of
the oxygen 2p or sulphur 3p atomic orbitals.4 6,8 In consequence, two n±p* electronic transitions
can be observed in the visible or near UV region. Because of the opposite symmetry of the
corresponding excited states, a sequence of the negative and positive Cotton e€ects (CEs) is
expected in the CD spectra of these chromophoric systems (Table 1). According to our earlier
studies4 6 the sign of the long-wavelength CE for the planar a-diketone, anhydride, imide and
dithioimide chromophores can be predicted by the antioctant sector rule (the sector signs are
opposite to those known for ketones). Since the only source of dissymmetry in the molecules
2a,b,c±5a,b,c is the methyl group at C-1 and because the substituents at C-7 or C-8 are located on
the nodal plane, the negative CE sign is predicted for the lowest energy n±p* band (Fig. 2).

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T. Po•onski et al. / Tetrahedron: Asymmetry 11 (2000) 3113±3122
3115
Scheme 1.
Figure 1. Molecular structures of anhydrides 3a and 3b
Indeed, in the case of the a-diketones 2a±c two oppositely signed CEs showing a pronounced ®ne
structure are detected at ca. 480 and 290 nm (Fig. 3). The long-wavelength negative CD band is
only slightly in¯uenced by the substituents at C-7, whereas some di€erences observed in the
magnitude of the second CE are apparently due to its overlap with the neighboring strong p±p*
transition.

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T. Po•onski et al. / Tetrahedron: Asymmetry 11 (2000) 3113±3122
Table 1
Circular dichroism data
Figure 2. The octant projections of a-diketones 2a and 2b. The arrow shows direction of projection
Two well-resolved n±p* bands can also be detected in the dithioimide spectra near 470 and 420
nm. In contrast to a-diketones, the corresponding CEs in 5a±c are strongly in¯uenced by the
substituents at C-8 (Fig. 4). The santenone related dithioimides 5a,b exhibit much weaker CE
magnitudes in the region of both n±p* transitions than those of the camphor related compound
5c. However, the shape of the CD curves of all of three dithioimides 5a±c is generally similar.
Furthermore, these compounds show bisignate CEs in the region of the long-wavelength
absorption band and their intensities are in¯uenced by the solvent polarity. Two overlapping n±p*
CEs in the CD spectra of the parent imides 4a±c are shifted to much shorter wavelengths, i.e. 260

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T. Po•onski et al. / Tetrahedron: Asymmetry 11 (2000) 3113±3122
3117
Figure 3. Circular dichroism (CD) spectra of a-diketones 2a and 2b in cyclohexane
and 230 nm but their magnitudes are also strongly dependent on the substituents at C-8 and the
solvent changes. Such behavior is usually indicative of conformational equilibria in solution as
shown by a temperature dependence of the spectra of 5c.9 Obviously, the shifts in conformational
equilibria caused by substituents and solvents may in¯uence the CE magnitude and sign.
The above results prove that in the case of rigid systems like a-diketones 2a±c the substituents
located on the nodal surface give only a weak contribution to the CE. Similar results have been
reported by Lightner and Toan for substituted adamantanones.10 However, in the case of
compounds with ¯exible chromophores like 3a,b,c±5a,b,c the substituents may indirectly a€ect
the CE magnitude or sign by steric interactions with the imide or dithioimide moieties; particu-
larly, the syn-methyl group at C-8 may interfere with the chromophores changing their geometry.
It is known that even a slight deformation of a chromophore may cause its inherent chirality that
considerably a€ects the CE magnitude.11 The ¯exibility of the anhydride, imide and dithioimide
functions has been already evidenced4 6 and was further con®rmed by the X-ray crystallographic
structures of 3a,b and 5a,b (Figs. 1 and 5). Owing to a steric interaction between the syn-methyl
group at C-8 in 3b and the anhydride chromophore, the oxygen atom is located 0.101(2) A below
the plane formed by the carbonyl groups. A very similar geometry has been also reported for
camphoric anhydride12a and N-substituted camphorimide.12b On the other hand, in the absence
of this kind of interaction in 3a, the anhydride group is nearly planar in this compound [the
oxygen atom is deviated only by 0.012(2) A from the plane formed by the neighboring carbonyls].
Though crystal structures of 5a,b reveal only small distortions of the dithioimide chromophore,
however, the MNDO calculations, reported earlier,6 suggest a possibility of much stronger
deviations of this chromophore in a solution. In addition, it has been shown that a speci®c

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T. Po•onski et al. / Tetrahedron: Asymmetry 11 (2000) 3113±3122
Figure 4. Circular dichroism (CD) spectra of dithioimides 5a and 5b in cyclohexane
solvent-solute interaction is possible for ketones containing axially oriented b-substituents.13
Assuming that the syn-methyl group at C-8 has an analogous orientation to the carbonyl or
thiocarbonyl units in 3b±5b its e€ect may also be responsible for a strong solvent dependence of
their CD spectra.
3. Experimental
CD spectra were recorded on a JASCO J-715 dichrograph. UV±vis measurements were per-
1
formed on a Beckman 3600 spectrophotometer. H and 13C NMR spectra were obtained with
Bruker MSL-300 and WP-200 spectrometers at 300 and 50 MHz, respectively. The deuteriated
solvents were used as an internal lock for 1H and 13C NMR. FT-IR absorptions were taken with
a Bruker IFS66 spectrometer. Speci®c rotations were measured on a Rudolph Autopol II digital
polarimeter.

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T. Po•onski et al. / Tetrahedron: Asymmetry 11 (2000) 3113±3122
3119
Figure 5. Molecular structures of dithioimides 5a and 5b
3.1. (1R,4R,7R)-7-Formyl-1,7-dimethylbicyclo[2.2.1]heptan-2-one 7
Pyridinium chlorochromate (PCC) (12.9 g, 60 mmol) was added to a stirred solution of
p-hydroxycamphor 67 (5.0 g, 30 mmol) in methylene chloride (100 mL). After stirring for 2 h at
room temperature pentane (100 mL) was added to the reaction mixture. The organic layer was
decolorized with silica gel, evaporated to dryness to obtain of 4.2 g of the crude product that was
used for the next step. An analytically pure sample of the aldehyde was prepared by column
chromatography on silica gel using benzene as the eluent; monosemicarbazone m.p. 198ꢀC (lit.14
21
D
m.p. 202ꢀC); ‰ꢀŠ +40 (c 1, CHCl3); IR (KBr): 2877, 2819, 1747, 1721 cm^1; 1H NMR (CDCl3) ꢁ
9.73 (s, 1H), 2.61 (t, J=4.4 Hz, 1H), 2.39 (dt, J=19.6 and 3.9 Hz, 1H), 1.98 (m, 3H), 1.84 (m,
1H), 1.52 (m, 2H), 1.18 (s, 3H), 1.00 (s, 3H); 13C NMR (CDCl3) ꢁ 215.5, 202.4, 60.2, 57.2, 42.2,
39.3, 29.2, 26.4, 11.7, 9.7.
3.2. (1R,4S,7R)-1,7-Dimethylbicyclo[2.2.1]heptan-2,3-dione 2a
Aldehyde 7 (4.0 g, 24 mmol) and 10% palladium on charcoal (0.3 g) were heated at 180ꢀC for
12 h under nitrogen. The product was dissolved in pentane, ®ltered and the ®ltrate was evaporated
to dryness. The resulted mixture of santenones 1a,b (3.1 g) was mixed with selenium dioxide (2.8
g, 25 mmol), dissolved in acetic acid (10 mL) and re¯uxed for 18 h. After cooling the reaction
mixture was ®ltered and evaporated to dryness. The residue was dissolved in toluene and washed
with water. The organic layer was dried (MgSO4), the solvent evaporated at reduced pressure and
the residue was crystallized from hexane to obtain 0.85 g of a-diketone 2a; m.p. 97±99ꢀC (lit.3
m.p. 97±98ꢀC); ‰ꢀŠ22 ^111 (c 1.3, CCl4) [lit.3 ‰ꢀŠ15 ^38.5 (c 1, EtOH) which, according to Nishimitsu,
D
D
rose to +10 after 50 h and remained constant thereafter3,15]; IR (KBr): 1777, 1757 cm^1; 1H NMR
(CDCl3) ꢁ 2.73 (d, J=5.1 Hz, 1H), 2.18 (m, 1H), 2.01 (q, J=6.6 Hz, 1H), 1.81 (m, 1H), 1.68 (m,
1H), 1.55 (m, 1H), 1.16 (s, 3H), 1.06 (d, J=6.6 Hz, 3H); 13C NMR (CDCl3) ꢁ 203.9, 201.9, 56.0,
52.7, 39.9, 27.4, 22.5, 11.2, 8.3.

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T. Po•onski et al. / Tetrahedron: Asymmetry 11 (2000) 3113±3122
3120
3.3. (1R,4S,7S)-1,7-Dimethylbicyclo[2.2.1]heptan-2,3-dione 2b
a-Diketone 2b crystallizes upon concentration and cooling of the mother liquor from the
above; m.p. 62.5±63ꢀC (lit.3 m.p. 59±60ꢀC); ‰ꢀŠD22^155 (c 2, CCl4) [lit.3 ‰ꢀŠ ^110.3 (c 1, EtOH)
15
D
which, according to Nishimitsu, changes to ^9.5 after 60 h and remained constant thereafter3,15];
1
IR (KBr): 1770, 1756 cm^1; H NMR (CDCl3) ꢁ 2.90 (dd, J=5.2 and 1.5 Hz, 1H), 2.27 (dq,
J=7.3 and 1.6 Hz, 1H), 2.15 (m, 1H), 1.88 (m, 1H), 1.68 (m, 2H), 1.20 (s, 3H), 0.93 (d, J=7.3 Hz,
3H); 13C NMR (CDCl3) ꢁ 205.4, 203.5, 56.7, 54.4, 43.7, 31.5, 23.5, 12.2.
3.4. (1R,4S,8RS)-1,8-Dimethyl-3-oxabicyclo[3.2.1]octan-2,4-dione 3b
The mixture of santenones 1a,b (3.1 g) was mixed with selenium dioxide (2.8 g, 25 mmol),
dissolved in acetic acid (10 mL) and re¯uxed for 18 h. The reaction mixture was ®ltered,
concentrated to ca. 3 mL and 30% hydrogen peroxide (1.5 mL) was added. After cooling to 0ꢀC
the product was ®ltered, dried and crystallized from toluene±hexane; yield 0.65 g; m.p. 109±111ꢀC
(lit.3 m.p. 110ꢀC); ‰ꢀŠ2D1+6.4 (c 1.8, C6H6) [lit.3 ‰ꢀŠ1D5+10.3 (c 1, EtOH)]; IR (KBr): 1806, 1769 cm^1;
1H NMR (CDCl3) ꢁ 3.11 (dd, J=6.8 and 4.0 Hz, 1H), 2.25±2.10 (complex m, 2H), 2.00±1.85
(complex m, 2H), 1.34 (s, 3H), 1.06 (d, J=7.1 Hz, 3H); 13C NMR (CDCl3) ꢁ 171.8, 170.0, 50.3,
49.2, 43.8, 34.9, 25.4, 18.3, 11.3.
3.5. (1R,4S,8R)-1,8-Dimethyl-3-oxabicyclo[3.2.1]octan-2,4-dione 3a
The reaction mixture from the above was evaporated to dryness. The residue was dissolved in
acetic anhydride (5 mL), re¯uxed for 10 min and the solvent was evaporated at reduced pressure.
The product was crystallized from toluene±hexane; yield 0.87 g; m.p. 127ꢀC (lit.3 m.p. 126±
21
D
15
D
127ꢀC); ‰ꢀŠ ^17.5 (c 2, C6H6) [lit.3 ‰ꢀŠ ^16.7 (c 1, EtOH)]; IR (KBr): 1815, 1769 cm^1; 1H NMR
(CDCl3) ꢁ 2.95 (d, J=7.2 Hz, 1H), 2.35±2.25 (complex m, 2H), 2.10±1.90 (complex m, 3H), 1.35
(s, 3 H), 0.98 (d, J=7.0 Hz, 3H); 13C NMR (CDCl3) ꢁ 172.8, 170.0, 50.9, 49.2, 40.8, 31.8, 25.2,
16.4, 12.4.
3.6. (1R,4S,8R)-1,8-Dimethyl-3-azabicyclo[3.2.1]octan-2,4-dione 4a
Anhydride 3a (0.34 g, 2 mmol) was dissolved in 25% aqueous ammonia (3 mL) and evaporated
to dryness. The residue was heated at 180ꢀC for 0.5 h, dissolved in chloroform, decolorized with
silica gel, ®ltered and the solvent was evaporated. The product was crystallized from toluene±
21
D
hexane; yield 0.27 g; m.p. 156±158ꢀC; ‰ꢀŠ ^8.7 (c 3.3, CHCl3); IR (KBr): 3204 (br), 3088, 1719,
1
1705 cm^1; H NMR (CDCl3) ꢁ 7.78 (br s, 1H), 2.76 (dd, J=7.2 and 1.5 Hz, 1H), 2.32 (m, 1H),
2.26 (q, J=7.0 Hz, 1H), 1.90 (m, 3H), 1.28 (s, 3H), 0.96 (d, J=7.0 Hz, 3H); 13C NMR (CDCl3) ꢁ
178.9, 176.9, 51.4, 42.8, 32.5, 25.9, 15.5, 13.2. Anal. calcd for C9H13NO2 (167): C, 64.65; H, 7.83;
N, 8.38. Found: C, 64.58; H, 7.86; N, 8.31.
3.7. (1R,4S,8S)-1,8-Dimethyl-3-azabicyclo[3.2.1]octan-2,4-dione 4b
Imide 3b was obtained from anhydride 3b as above; m.p. 147±148ꢀC (from toluene±hexane);
23
D
s, 1H), 2.91 (dd, J=4.4 and 2.1 Hz, 1H), 2.20±1.75 (complex m, 5H), 1.26 (s, 3H), 1.02 (d, J=7.0
‰ꢀŠ +12.5 (c 2, CHCl3); IR (KBr): 3209 (br), 3091, 1722, 1698 cm^1; 1H NMR (CDCl3) ꢁ 8.06 (br

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T. Po•onski et al. / Tetrahedron: Asymmetry 11 (2000) 3113±3122
3121
Hz, 3H); 13C NMR (CDCl3) ꢁ 177.5, 176.4, 50.6, 50.4, 45.5, 35.7, 26.2, 17.5, 10.7. Anal. calcd for
C9H13NO2 (167): C, 64.65; H, 7.83; N, 8.38. Found: C, 64.68; H, 7.68; N, 8.22.
3.8. (1R,4S,8R)-1,8-Dimethyl-3-azabicyclo[3.2.1]octan-2,4-dithione 5a
A mixture of imide 4a (0.25 g, 1.5 mmol) and Lawesson's reagent (0.70 g, 1.7 mmol) in toluene
(5 mL) was re¯uxed for 12 h. The toluene was evaporated at reduced pressure and the residue was
chromatographed on silica gel using toluene as the eluent. After collection of the yellow fraction
and evaporation of the solvent, the product was crystallized from heptane; yield 0.25 g; m.p.
22
D
69ꢀC; ‰ꢀŠ ^45.4 (c 1.4, C6H6); IR (KBr): 3161 (br), 1488, 1313, 1119, 1044 cm^1; H NMR
1
(CDCl3) ꢁ 10.54 (br s, 1H), 3.40 (dd, J=7.2 and 1.2 Hz, 1H), 2.38 (m, 1H), 2.26 (q, J=7.0 Hz,
1H), 1.96 (m, 3H), 1.45 (s, 3H), 1.00 (d, J=7.0 Hz, 3H); 13C NMR (CDCl3) ꢁ 215.5, 210.8, 61.0,
57.7, 43.8, 36.4, 29.8, 20.6, 14.4. Anal. calcd for C9H13NS2 (199): C, 54.23; H, 6.57; N, 7.03; S,
32.17. Found: C, 54.18; H, 6.65; N, 6.91; S, 32.25.
3.9. (1R,4S,8S)-1,8-Dimethyl-3-azabicyclo[3.2.1]octan-2,4-dithione 5b
23
Compound 5b was obtained from imide 4b as above; m.p. 82ꢀC (from heptane); ‰ꢀŠ +31 (c 1,
D
1
C6H6); IR (KBr): 3191 (br), 1478, 1346, 1118, 1055 cm^1; H NMR (CDCl3) ꢁ 10.48 (br s, 1H),
3.56 (m, 1H), 2.30±1.90 (complex m, 5H), 1.44 (s, 3H), 0.98 (d, J=7.0 Hz, 3H); 13C NMR
(CDCl3) ꢁ 213.3, 209.3, 59.9, 56.2, 46.6, 39.7, 30.2, 23.1, 10.2. Anal. calcd for C9H13NS2 (199): C,
54.23; H, 6.57; N, 7.03; S, 32.17. Found: C, 54.32; H, 6.57; N, 6.93; S, 32.30.
3.10. X-Ray crystal structure analysis
Di€raction data were obtained on a Kuma KM-4 di€ractometer with graphite mono-
chromated Cu Ka radiation (l=1.54178 A). The structures were solved by direct methods with
the program SHELXS-86.16 Full matrix least-squares re®nement was carried out with SHELXL-
97.17
Crystal data for C9H12O3 3a: monoclinic, P21, a=7.414(2), b=6.455(1), c=8.996(2) A,
ꢂ=97.99(2)ꢀ, V=426.4(2) A3, Z=2, Dcalcd=1.310 g cm^3, T=293(2) K, R1=0.0377,
wR2=0.1106 for 1408 re¯ections with I>2ꢃ(I) [R1=0.0403, wR2=0.1126 for all 1469 independent
re¯ections].
Crystal data for C9H12O3 3b: monoclinic, P21, a=6.773(1), b=7.334(1), c=8.741(2) A,
ꢂ=97.91(3)ꢀ, V=430.1(1) A3, Z=2, Dcalcd=1.299 g cm^3, T=293(2) K, R1=0.0352,
wR2=0.1006 for 1588 re¯ections with I>2ꢃ(I) [R1=0.0364, wR2=0.1023 for all 1631 independent
re¯ections].
Crystal data for C9H13NS2 5a: monoclinic, P21, a=12.306(2), b=8.6780(9), c=9.9470(7) A,
ꢂ=105.600(8)ꢀ, V=1023.1(2) A3, Z=4, Dcalcd=1.294 g cm^3, T=293(2) K, R1=0.0559,
wR2=0.1476 for 2984 re¯ections with I>2ꢃ(I) [R1=0.0616, wR2=0.1511 for all 3221 independent
re¯ections].
Crystal data for C9H13NS2 5b: monoclinic, P212121, a=7.805(2), b=10.782(2), c=28.899(5) A,
V=2011.2(8) A3, Z=8, Dcalcd=1.317 g cm^3, T=293(2) K, R1=0.0492, wR2=0.1260 for 3062
re¯ections with I>2ꢃ(I) [R1=0.0554, wR2=0.1289 for all 3336 independent re¯ections].
The atomic coordinates for the reported crystal structures are available on request from the
Director of the Cambridge Crystallographic Data Centre, University Chemical Laboratory,

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T. Po•onski et al. / Tetrahedron: Asymmetry 11 (2000) 3113±3122
3122
Lens®eld Road, Cambridge CB2 1EW, UK. Any request should be accompanied by a full
literature citation.
Acknowledgements
We are indebted to Dr. J. Frelek (IChO PAN, Warsaw) for CD measurements with use of her
JASCO J-715 instrument. The ®nancial support from the Committee of Scienti®c Research is
gratefully acknowledged.
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