Investigating direct access to 2-oxospiro[4.5]decanones via 6π-electrocyclisation

Article abstract of DOI:10.1002/ejoc.200700367

The 2-oxospiro[4.5]decan-1-one (or spiro-γ-lactone) structural motif is contained within a number of natural products, for example, the clerodane family of diterpenes. Methods to construct this structural motif are somewhat limited and usually involve mul

Full text of DOI:10.1002/ejoc.200700367

FULL PAPER
DOI: 10.1002/ejoc.200700367
Investigating Direct Access to 2-Oxospiro[4.5]decanones via 6π-
Electrocyclisation
Rebecca H. Pouwer,[a] Heiko Schill,[a] Craig M. Williams,*[a] and Paul V. Bernhardt[a]
Keywords: 2-Oxospiro[4.5]decan-1-one / Spirolactones / Microwave irradiation / Electrocyclic reactions / Calculations
The 2-oxospiro[4.5]decan-1-one (or spiro-γ-lactone) struc-
tural motif is contained within a number of natural products,
for example, the clerodane family of diterpenes. Methods to
construct this structural motif are somewhat limited and usu-
ally involve multiple functional group interconversions. A
novel synthetic approach to this system utilising 6π-electro-
cyclisation has been identified and associated density func-
tional calculations performed. Details of the scope, limita-
tions, and ramifications of this methodology are discussed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
The 2-oxospiro[4.5]decan-1-one (1) structural motif ex- al.[15] in which allenic lactones 5 and 7 were treated with
ists within a number of natural product groups [e.g. canan- diene 6 affording the 2-oxospiro[4.5]decan-1-ones 4 and 8
gone (2),[1] teusalvin A (3)[2]], of which the clerodanes[3] (i.e. (Scheme 1).
3) are a major contributor (Figure 1).
Scheme 1. Diels–Alder approach to the 2-oxospiro[4.5]decan-1-one
structural motif.
With these examples in mind, it seemed reasonable that
a 6π-electrocyclisation approach to the six-membered ring
(i.e. 2-oxospiro[4.5]decan-1-ones of type 4 and 8) was wor-
thy of investigation. For example, the lactone trienes 9 and
10, via thermal or photochemical conditions, could be
transformed into cyclised adducts 11 (12) and 13 (14),
which then have the potential to undergo [1,5]-hydrogen
shifts, affording the dienes 15 (16) and 17 (18). An ad-
ditional advantage to this approach would be the relative
stereocontrol of the products as described by the Wood-
ward–Hoffmann principles[17–19] (Scheme 2).
To test the viability of this method a suite of compounds
of type 9 were synthesised in good to excellent yields by
Sonogashira coupling[20–22] of the corresponding alkyne,
followed by partial hydrogenation to give the cis-alkenes
28–31, 38–40 (Scheme 3), as well as 45 and 46 (Scheme 4).
The partial hydrogenation of these systems was at times
capricious, however, neither diimide or boron mediated re-
duction gave satisfactory results. Furthermore, all attempts
to reduce 27 to 31 failed. Though direct synthesis of the
aromatic dienes and trienes through Stille coupling is an
appealing route, it was found that the triflate 23 did not
react with vinyl stannanes at temperatures low enough to
Figure 1. Examples of the 2-oxospiro[4.5]decan-1-one structural
motif in nature.
Considering the usual mode of 2-oxospiro[4.5]decan-1-
one construction is to assemble the lactone portion at a
later stage,[4–14] which often involves multiple steps, we con-
templated the approach of initial γ-lactone inclusion (i.e.
construction of the six-membered ring at the later stage).
Although this approach presents limitations it does open
additional avenues for constructing the 2-oxospiro[4.5]-
decan-1-one structural motif (1) directly. In this regard we
are only aware of two examples that fit this criterion.[15–16]
Of note is the Diels–Alder approach reported by Jung et
[a] School of Molecular and Microbial Sciences, University of
Queensland,
Brisbane, 4072, Queensland, Australia
Fax: +61-7-3365-4299
E-mail: c.williams3@uq.edu.au
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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FULL PAPER
Scheme 4. Synthesis of trienes 45 and 46.
Scheme 2. Electrocyclisation approach to the 2-oxospiro[4.5]decan-
1-one structural motif.
Scheme 5. Synthesis of triene 48 via Luche reduction and partial
hydrogenation.
Surprisingly, all of the aromatic substrates failed to un-
dergo cyclisation when exposed to either light (broad spec-
trum, 254 and 300 nm, with and without I2), conventional
heating (up to 220 °C) or microwave irradiation (up to
220 °C) (toluene, diphenyl ether, toluene/SiO2). Even flash
vacuum pyrolysis (up to 600 °C) failed. The only observable
products were due to partial double bond isomerisation.
This lack of success was somewhat intriguing as there is
literature precedent on related systems.[24–26]
Success, however, was found in the cyclisation of the non-
aromatic triene substrates 38, 40, 45, 46, and 48 (Table 1).
The optimal conditions for cyclisation were found to be mi-
crowave irradiation in toluene at 300 W with a maximum
reaction temperature of 160 °C, which gave cyclised product
in good yield. For reasons unknown, the benzoyl-substi-
tuted 39 failed to react. Triene 45, however, cyclised at only
100 °C to give 52 in excellent yield, the structure of which
was confirmed by X-ray crystallography (Figure 2). We at-
Scheme 3. Construction of trienes 28–31 and 38–40 by Sonoga-
shira/partial hydrogenation sequence.
avoid significant isomerisation of the cis-alkene. Luche re- tribute this reactivity to a lowering of the transition state
duction[23] of the enone 43 to give 47, followed by partial through a capto-dative effect,[27] induced by the electron-
hydrogenation gave allylic alcohol 48 in good yield withdrawing effect of the ketone substituent with respect to
(Scheme 5).
the β-carbon.
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Direct Access to 2-Oxospiro[4.5]decanones via 6π-Electrocyclisation
Table 1. 6π Electrocyclisation of trienes 38–40, 45–46, and 48.
Lowering the temperature to 190 °C using dimethylform-
amide as solvent gave rise to all three products 52, 56 and
57. The unexpected formation of 57 can most likely be at-
tributed to dimethylformamide slowly decomposing at high
temperature affording dimethylamine which catalysed the
isomerisation.
Scheme 6. Thermal and base-catalysed rearrangements of 52.
In an attempt to obtain the opposite relative stereochem-
istry, the trienes 38, 39, and 45 were subjected to photo-
chemical irradiation. Suprisingly, when irradiated at 300 nm
only the products of a [1,7]-hydrogen shift were obtained,
i.e. 58, 59, and 60, respectively (Scheme 7). Irradiation at
254 nm gave a gross mixture of products resulting from iso-
merisation of the double bonds.
[a] Reaction performed at 100 °C. [b] Non-isolated yield calculated
by 1H NMR spectroscopy. [c] Isolated from attempted purification
using silica gel and neutral Al2O3 chromatography.
Scheme 7. Photochemical irradiation of trienes 38, 39, and 45.
Attempts to synthesise the trans-lactones fell to a similar
fate. While the Sonogashira couplings of triflate 61 with
acetylenes 32 and 33 proceeded in good yield to give 62
and 63 respectively, partial hydrogenation did not afford the
desired triene, but the products of [1,7]-hydrogen shift, 58
Figure 2. Molecular structure of compounds 45 and 52 (30% pro-
bablity ellipsoids).
Adding further to the utility of this reaction, it was found and 59, respectively (Scheme 8).
that a rearrangement of 52 could be driven at higher tem-
This was also the case in the partial hydrogenation of
peratures (Scheme 6). If the temperature was elevated to 66 (Scheme 9). Although [1,7]-hydrogen shifts are known to
220 °C (MW) using dimethylformamide as solvent the [1,5]- occur in preference to 6π-electrocyclisation[29] it is also
hydrogen shift product 56 could be obtained along with the known that subjection to high temperatures drives cycli-
isomeric diene 57 (1:1.5, 96%), which was separable using sation,[30] however, subjecting 58 to increased temperatures
silver nitrate impregnated silica gel chromatography.[28] failed to afford any cyclised material.
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R. H. Pouwer, H. Schill, C. M. Williams, P. V. Bernhardt
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Scheme 8. Attempted synthesis of trans-lactones from substrates 62
and 63.
Figure 3. Calculated energy profiles for the cyclisation reactions of
compounds 28–30 (top) and for the saturated series 38, 40, and
45 (bottom). Enthalpy values in italics refer to the photochemical
pathway.
The calculated enthalpies of activation (∆∆H) for the
electrocyclisations were predicted to range from 33.0 to
37.7 kcal/mol for thermally induced processes and to be
Scheme 9. Attempted synthesis of trans-lactones from substrate 66.
To add further understanding to these results, a compu- 34.9–41.1 kcal/mol for photochemical induction for the aro-
tational study was undertaken in which heats of formation matic compounds 28–30. The reaction enthalpies leading to
of starting materials, the products of the electrocyclisation the electrocyclisation products 70–72 were predicted to be
and the [1,5]-hydrogen shifts, and the transition states for endothermic by ca. 10–20 kcal/mol owing to the lack of the
these transformations were examined. Thus, unrestricted aromatic stabilisation. For the photochemical processes,[35]
geometry optimisations including full frequency calcula- the predicted cyclisation barriers are ca. 2–4 kcal/mol
tions were performed at the B3LYP/6-31g(d) level of higher than for the thermal processes. Calculations per-
theory.[31] Although other functionals (viz. MPW1K) per- formed on the hexatrienes 38, 40, and 45 on the other hand
form better in the calculation of barrier heights for pericy- predicted exothermic cyclisation processes and significantly
clic reactions they give only poor results for reaction en- lower activation energies for the thermal pathway (23.2–
thalpies.[32–33] The enthalpies of formation were calculated 24.1 kcal/mol) and the photochemical induced processes
using an atomisation scheme at standard conditions (25 °C, (26.3–26.9 kcal/mol).
1 atm) using unscaled frequencies.[34] Calculations have
Additionally, the enthalpies of the transition states for
been carried out for both the thermal and the photochemi- the [1,5]-shifts of the thermal cyclisation products have been
cal pathways,[35] results of which are summarised in Fig- calculated, to compare the activation enthalpy with experi-
ure 3. Detailed results as well as structural representations mentally determined values for unsubsituted 1,3-cyclohexa-
and atomic coordinates for all calculated structures can be diene (∆∆H = 39.0 kcal/mol for interconversions of 2-d-1,3-
found in the Supporting Information.
cyclohexadiene;[36] similar values have been obtained for
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Direct Access to 2-Oxospiro[4.5]decanones via 6π-Electrocyclisation
1,4-d2-1,3-cyclohexadiene[37] and by calculations[38]), which
is considerably higher than that for the 6π-electrocyclisation
tion (2ϫ10 mL) and then dried with magnesium sulfate. The sol-
vent was removed in vacuo and the residue was purified by silica
gel column chromatography (1:1 petroleum ether/diethyl ether) to
give the title compound 35 as a white powder (106 mg, 73 %
yield). The powder was recrystallised from dichloromethane/
petroleum ether to give colourless needles. M.p. 44–47 °C. 1H
NMR (300 MHz, CDCl3): δ = 1.50–1.64 (m, 4 H), 1.98 (t, J =
1.8 Hz, 3 H), 2.05–2.20 (m, 4 H), 2.85–2.95 (m, 2 H), 4.22–4.27 (m,
2 H), 6.22–6.25 (m, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
21.3, 22.1, 22.8, 25.8, 27.8, 28.7, 63.9, 85.9, 102.4, 120.8, 127.2,
of the unsubstituted (Z)-hexa-1,3,5-triene (∆∆H
=
29.1 kcal/mol determined experimentally;[39] calculations on
various levels of theory agreed with this[32]). However, in
the aromatic series 28–30, these activation enthalpies are
predicted to be much smaller (23.8–24.4 kcal/mol) owing to
the energetically unfavourable cyclisation products 67–69
which already possess rather high ground state energies. For
the saturated compounds 38, 40, and 45, the activation en- 128.7, 137.5, 168.4 ppm. MS (EI): m/z (%) = 217 (15), 216 (100)
[M+], 215 (20), 210 (14), 187 (14), 173 (20), 157 (19), 145 (24), 143
thalpies (∆∆H of 35.1–37.3 kcal/mol) are in line with those
(28), 131 (18), 130 (15), 129 (38), 128 (36), 117 (23), 115 (39), 105
for the unsubstituted parent system.
(23), 93 (14), 91 (53), 80 (17), 79 (32), 78 (19), 77 (47). HRMS (EI):
Although the activation energies for the [1,5]-shifts of the
saturated system are predicted to be significantly higher
than those for the aromatic system, these processes are still
calcd. for C14H16O2: (M+) 216.1150, found 216.1149.
Typical Procedure for Partial Hydrogenation (Compounds 28–30)
considered feasible, since the absolute values of the en- (Z)-3-[(Z)-4-(4-Isopropoxyphenyl)but-3-en-2-ylidene]dihydrofuran-
2(3H)-one (28): Lindlar’s catalyst (30 mg, 5% palladium poisoned
with lead) was added to a solution of 24 (250 mg, 0.93 mmol) in
freshly distilled ethyl acetate (10 mL) and the vessel was flushed
with hydrogen gas and stirred for 1.5 h. The mixture was filtered
through Celite®, the solvent was removed in vacuo, and the residue
was purified by silica gel column chromatography (1:1 petroleum
ether/diethyl ether) to give the title compound 28 as a pale yellow
powder (213 mg, 85% yield). The powder was recrystallised from
diethyl ether/petroleum ether to give large pale yellow needles. M.p.
thalphies for TS2 barely differ from those for TS1 in this
series. Thus, when the system is provided with enough en-
ergy to cross the first barrier (TS1) it should suffice for the
second transition state. For the aromatic compounds 28–
30, however, the second barrier height is only lower due
to the non-stabilised electrocyclisation products 70–72. The
energy to cross TS1 constitutes the highest barrier in these
systems and the height of this barriers renders the reaction
of the aromatic systems impossible.
1
54–55 °C. H NMR (400 MHz, CDCl3): δ = 1.31 (d, J = 6.1 Hz, 6
In conclusion, it has been found that 2-oxospiro[4.5]- H), 1.77 (br. s, 3 H), 2.89–2.94 (m, 2 H), 4.33–4.37 (m, 2 H), 4.52
(sept, J = 6.1 Hz, 1 H), 6.63 (d, J = 12.2 Hz, 1 H), 6.78 (d, J =
8.7 Hz, 2 H), 7.07 (d, J = 12.2 Hz, 1 H), 7.10 (d, J = 8.8 Hz, 2 H)
ppm. 13C NMR (101 MHz, CDCl3): δ = 21.5, 22.0, 27.9, 64.5, 69.8,
115.3, 121.7, 127.4, 129.7, 130.3, 132.8, 147.9, 157.5, 169.8 ppm.
MS (EI): m/z (%) = 273 (19), 272 (94) [M+], 230 (36), 215 (15), 202
(100), 185 (25), 184 (25), 172 (18), 171 (81), 157 (40), 153 (11), 141
(13), 128 (23), 115 (20), 107 (20), 77 (18). HRMS (EI): calcd. for
C17H20O3 (M+) 272.1412, found 272.1406.
decanones can be constructed by 6π-electrocyclisation,
however, only certain hexatrienes are amenable to this pro-
cess, which concur with theoretical predictions.
Experimental Section
1H and 13C NMR spectra were recorded with a Bruker AV300
(300.13 MHz; 75.47 MHz), AV400 (400.13 MHz; 100.62 MHz) and
a DRX500 (500.13 MHz; 125.77 MHz) in deuteriochloroform
(CDCl3) or hexadeuteriobenzene (C6D6) unless otherwise stated.
Coupling constants are given in Hz and chemical shifts are ex-
pressed as δ values in ppm. High and low resolution EI mass spec-
troscopic data were obtained with a KRATOS MS 25 RFA. Elec-
trospray negative and positive ion mass spectra were measured in
methanol solutions with a Finigan Mat 900 XL-Trap. Microanaly-
ses were performed by the University of Queensland Microanalyti-
cal Service. Column chromatography was undertaken on silica gel
(Flash Silica gel 230–400 mesh), with distilled solvents. Tetra-
hydrofuran was freshly distilled from a sodium/benzophenone still.
Melting points were determined on a Fischer Johns Melting Point
apparatus and are uncorrected.
Typical Procedure for Partial Hydrogenation (Compounds 38–40,
48, and 58–59)
(Z)-3-[(Z)-4-Cyclohexenylbut-3-en-2-ylidene]dihydrofuran-2(3H)-
one (38): 10% palladium on carbon (4 mg) and 1 drop of pyridine
was added to the dienyne 35 (40 mg, 0.19 mmol) in distilled ethyl
acetate (2 mL). The vigorously stirring mixture was flushed with
hydrogen gas and stirred for 1 h. The mixture was filtered, and the
solvent removed in vacuo. The residue was subjected to the same
reaction conditions a further 3 times after which time the ratio
between starting material and desired product was approximately
1:1 by 1H NMR spectroscopy. The residue was purified by silica
gel column chromatography (2:1 petroleum ether/diethyl ether) to
give the title compound 38 as a colourless oil (19 mg, 48% yield,
92:8 Z/E). 1H NMR (400 MHz, C6D6): δ = 1.33–1.52 (m, 4 H),
1.67 (dt, J = 1.7, 0.6 Hz, 3 H), 1.86–2.00 (m, 6 H), 3.50–3.55 (m,
2 H), 5.58 (m, 1 H), 5.97 (d, J = 11.9 Hz, 1 H), 7.35 (d, J = 11.9 Hz,
1 H) ppm. 13C NMR (101 MHz, C6D6): δ = 21.2, 22.3, 22.9, 26.0,
27.6, 28.5, 63.6, 122.0, 127.3, 128.6, 136.0, 136.2, 146.5, 168.9 ppm.
MS (EI): m/z (%) = 218 (47) [M+], 203 (87), 190 (31), 185 (15), 173
(16), 172 (100), 159 (20), 146 (17), 145 (92), 144 (23), 131 (54), 130
(14), 129 (32), 128 (21), 118 (31), 117 (36), 115 (23), 105 (28), 91
(45), 77 (24). HRMS (ESI, pos.): calcd. for C14H18NaO2 [M +
Na]+ 241.1204, found 241.1201.
Typical Sonogashira Procedure (Compounds 24–27, 35–37, and 62–
63)
(Z)-3-(4-Cyclohexenylbut-3-yn-2-ylidene)dihydrofuran-2(3H)-one
(35): A solution of triflate 23[40] (175 mg, 0.67 mmol) and 1-ethyn-
ylcyclohexene (32) (100 mg, 0.94 mmol) was added to copper io-
dide (13 mg, 0.07 mmol) and tetrakis(triphenylphosphane)palla-
dium (39 mg, 0.04 mmol) in anhydrous tetrahydrofuran (6 mL) and
anhydrous triethylamine (0.29 mL, 2.03 mmol) by cannula under
argon. The reaction was degassed (freeze-thawed) and stirred for
1.5 h, after which time the reaction was diluted with brine (20 mL)
and extracted with diethyl ether (3ϫ20 mL). The combined or-
ganic phase was washed with saturated ammonium chloride solu-
Typical Sonogashira Procedure (Compounds 43–44, and 66)
(Z)-3-[4-(6-Oxocyclohexenyl)but-3-yn-2-ylidene]dihydrofuran-
2(3H)-one (43): Diisopropylamine (0.43 mL, 3.07 mmol) was added
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to a solution of the enyne 42 (210 mg, 1.54 mmol), 2-iodocyclohex-
2-enone[41] (230 mg, 1.04 mmol), PdCl2(Ph3P)2 (37 mg, 0.05 mmol),
and copper iodide (20 mg, 0.11 mmol) in anhydrous tetra-
hydrofuran (10 mL) at 0 °C. The reaction was stirred at 0 °C for
30 min, and then diluted with diethyl ether (20 mL), and the or-
ganic phase was washed with 1  HCl solution (1ϫ5 mL), and
brine (1ϫ5 mL), and then dried with magnesium sulfate. The resi-
due was purified by silica gel column chromatography (14:1 diethyl
ether/petroleum ether) to give the title compound 43 as a white
powder (191 mg, 80% yield). The powder was recrystallised from
dichloromethane/petroleum ether to give large colourless needles.
Acknowledgments
The authors thank The University of Queensland for financial sup-
port.
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M.p. 133–135 °C. H NMR (400 MHz, CDCl3): δ = 2.00–2.07 (m,
2 H), 2.08 (t, J = 1.8 Hz, 3 H), 2.46–2.52 (m, 4 H), 2.95–3.01 (m,
2 H), 4.30–4.34 (m, 2 H), 7.48 (t, J = 4.5 Hz, 1 H) ppm. 13C NMR
(101 MHz, CDCl3): δ = 22.2, 22.7, 26.6, 27.8, 38.0, 64.1, 90.3, 94.5,
125.1, 128.1, 128.9, 156.4, 168.3, 195.2 ppm. MS (EI): m/z (%) =
230 (24) [M+·], 207 (14), 84 (22). HRMS (ESI, pos.): calcd. for
C14H14NaO3 [M + Na]+ 253.0841, found 253.0835. C14H14O3
(230.25): calcd. C 73.03, H 6.13; found C 72.95, H 6.17.
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Typical Procedure for Partial Hydrogenation (Compounds 45–46,
and 60)
(Z)-3-[(Z)-4-(6-Oxocyclohexenyl)but-3-en-2-ylidene]dihydrofuran-
2(3H)-one (45): Palladium on barium sulfate (5 mg) in distilled
methanol (1 mL) was purged with hydrogen, and the solution be-
came black. Dienyne 43 (20 mg, 0.09 mmol) in pyridine (2 mL) was
added and the reaction was stirred for 10 min, after which time the
solution was filtered through celite. The solvent was removed in
vacuo and the residue was subjected to the same conditions. The
residue was purified by silica gel column chromatography (5:1 di-
ethyl ether/petroleum ether) to give the title compound 45 as a
white powder (16 mg, 80% yield), which was recrystallised from
dichloromethane/petroleum ether to give small colourless needles.
M.p. 95–98 °C. 1H NMR (400 MHz, CDCl3): δ = 1.87 (dt, J = 1.7,
0.6 Hz, 3 H), 1.98–2.05 (m, 2 H), 2.38–2.43 (m, 2 H), 2.45–2.50 (m,
2 H), 2.86–2.93 (m, 2 H), 4.32–4.36 (m, 2 H), 6.35–6.41 (m, 1 H),
6.75–6.80 (m, 1 H), 7.14–7.21 (m, 1 H) ppm. 13C NMR (101 MHz,
CDCl3): δ = 21.3, 22.6, 26.6, 27.9, 38.3, 64.5, 122.5, 128.1, 130.5,
137.2, 146.7, 149.2, 169.6, 198.1 ppm. MS (ESI, pos.): m/z = 255
[M + Na]+. HRMS (ESI, pos.): calcd. for C14H16NaO3 [M + Na]
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11164–11165.
+
255.0997, found 255.0994.
Typical Procedure for Cyclisation (Compounds 49, 51–55)
2Ј-Methyl-4,5,5Ј,6Ј,7Ј,8Ј,8Јa-heptahydrospiro[furan-3(2H),1Ј(7ЈH)-
naphthalene]-2-one (49): A solution of triene 38 (19 mg, 0.09 mmol)
in dry toluene (0.5 mL) was heated under microwave irradiation for
20 min (maximum temperature 160 °C, 300 W). The solvent was
removed in vacuo and the residue was purified by silica gel column
chromatography (2:1 petroleum ether/diethyl ether) to give the title
compound 49 as a colourless oil (10 mg, 53% yield). 1H NMR
(500 MHz, CDCl3): δ = 1.24–1.37 (m, 3 H), 1.68–1.76 (m, 2 H),
1.76 (d, J = 1.1 Hz, 3 H), 1.81–1.86 (m, 1 H), 1.90 (ddd, J = 13.9,
9.7, 7.8 Hz, 1 H), 2.12–2.21 (m, 1 H), 2.43 (bd, J = 16.8 Hz, 1 H),
2.72 (ddd, J = 13.6, 8.8, 4.6 Hz, 1 H), 2.78–2.84 (m, 1 H), 4.14 (m,
1 H), 4.29–4.35 (m, 1 H), 5.58–5.62 (m, 1 H), 5.68 (bd, J = 5.6 Hz,
1 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 18.8, 24.6 (2 C), 25.4,
27.8, 32.0, 42.8, 52.2, 66.3, 118.8, 121.3, 133.7, 136.1, 181.4 ppm.
MS (ESI): m/z = 241 [M + Na]+. HRMS (ESI, pos.): calcd. for
C14H18NaO2: [M + Na]+ 241.1204, found 241.1203.
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Received: April 24, 2007
Published Online: July 24, 2007
Eur. J. Org. Chem. 2007, 4699–4705
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4705