D. A. Lanfranchi et al.
Cis acetates did, however, distinguish themselves from their
trans counterparts regarding the splitting data of methyl C7 (δ
position). Indeed, ꢀꢀδN of methyl C7 of cis acetates were by
far higher than those of trans acetates (respectively 6.46–8.46 vs
1.74–2.71).
width (SW) of 12 500 Hz (250 ppm); CPD mode decoupling; and
digital resolution 0.763 Hz per point. The number of accumulated
scans ranged from 500 to 1000 for pure and reagent-added
compounds. For enantiomeric differentiation, zero filling was
applied before Fourier transformation.
A 0.5 M solution of ( )substrate in CDCl3 (having TMS as internal
reference) was prepared, and (+)Yb(hfc)3 was added portionwise
(five portions) by little equimolar increments until we reached
a molar ratio [Yb(hfc)3]/[substrate] around 0.25. The LSR was
dissolved by shaking and spectra were recorded at 25 ◦C after
dissolution. Each time when an increment of Yb(hfc)3 had been
added, we reported for each carbon the variation obtained on
a plot bearing in abscissa [Yb(hfc)3]/[substrate] (ranging from 0
to 0.25), and the ordinate in ppm (ꢀδ for LIS and ꢀꢀδ for the
magnitude of splitting between two enantiotopic peaks).
The carvomenthone group (Fig.3, Table 4)
Carvomenthone-like ketones (substituted on C2), carvone 16 and
trans-dihydrocarvone 19, had as much as five carbons without
splittings:C8, C9andC10oftheisopropylenegroup, C5(δ position)
and C4 or C6 (γ position) (Fig. 3, Table 4). Conversely, cis-carveol
17, its acetate 18 and trans-dihydrocarveol 20 presented the
opposite behaviour showing practically all carbon signals split.
Two signals of trans-dihydrocarvyl acetate 18 remained unsplit.
Allcarvomenthone-typecompounds(16–21)exhibitedaꢀꢀδN
of methyl C7 (Cδ) superior to the ꢀꢀδN of cyclohexanic carbons C4
andC6(bothinγ position),exceptinonecase.Indeed,surprisingly,
no splitting was observed for the signal of methyl carbon C7 of
trans-dihydrocarveol 18.
Acknowledgements
The authors are grateful to the Collectivite´ Territoriale de Corse
(CTC) for financial support.
Regarding the magnitude of splitting along the isopropylene
group of carvomenthone-like compounds, we observed, as
expected, higher splitting values for methine C8 than methyls References
C9 and C10. It should be kept in mind, as mentioned earlier,
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Experimental
Materials
The
CLSR,
tris[3(heptafluoropropylhydroxymethylene)-(+)-
camphorato] ytterbium(III), known as Yb(hfc)3, was obtained from
Aldrich and stored in a dissicator over silica gel. Chloroform-d
was obtained from Euriso-top. Menthone 1, carvone 16, and
trans-dihydrocarveol 20 were purchased from Fluka, and menthol
2, neomenthol 3, isomenthol 7, piperitone 11, from Extrasynthese.
Other compounds have been synthesised: all acetates (4, 5, 9,
10, 14, 15, 18, 21) by acetylation (AcCl/Et3N/CH2Cl2) of the
correspondingalcohols, isomenthone6andtrans-dihydrocarvone
19 by oxidation (K2Cr2O7/dilute H2SO4/Et2O) of isomenthol 7
and trans-dihydrocarveol 20, respectively, neoisomenthol by
reduction (LiAlH4/Et2O) of isomenthone 6, trans- and cis-piperitol
(12, 13) by reduction of piperitone 11, cis-carveol 17 by reduction
of carvone 16.
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Spectra
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R. E. Lenkinski,
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a Bruker AC 200 Fourier transform spectrometer operating at
50.323 MHz for 13C, equipped with a 5 mm probe with all
shifts referred to internal tetramethylsilane (TMS). Spectra were
recordedwiththefollowingparameters:pulsewidth(PW)3 µs (flip
angle 45◦);acquisitiontime1.3 sfora32 Kdatatablewithaspectral
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c
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2008, 46, 1188–1194