Synthesis of α-hydroxy ketones from terpene aldehydes

Article abstract of DOI:10.1081/SCC-200025618

Unsymmetrically substituted α-hydroxy ketones possessing isoprenoid units within the molecule were synthesised from commercially available terpene aldehydes. The synthesis was applicable to α,β-unsaturated aldehydes and afforded the respective α-hydroxy-m

Full text of DOI:10.1081/SCC-200025618

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Synthesis of α‐Hydroxy
Ketones from Terpene
Aldehydes
Martin Schueler a , Holger Zorn b , Alexandra M. Z.
Slawin a & Ralf G. Berger b
a University of St Andrews , North Haugh, St
Andrews, Scotland
b Institute of Food Chemistry , University of
Hannover , Wunstorfer Str. 14, Hannover, D‐30453,
Germany
Published online: 10 Jan 2011.
To cite this article: Martin Schueler , Holger Zorn , Alexandra M. Z. Slawin & Ralf
G. Berger (2004) Synthesis of α‐Hydroxy Ketones from Terpene Aldehydes, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 34:14, 2591-2600, DOI: 10.1081/SCC-200025618
To link to this article: http://dx.doi.org/10.1081/SCC-200025618
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 14, pp. 2591–2600, 2004
Synthesis of a-Hydroxy Ketones from
Terpene Aldehydes
Martin Schueler,1 Holger Zorn,2 Alexandra M. Z. Slawin,1
and Ralf G. Berger2,
*
1University of St Andrews, North Haugh,
St Andrews, Scotland
2Institute of Food Chemistry, University of Hannover,
Hannover, Germany
ABSTRACT
Unsymmetrically substituted a-hydroxy ketones possessing isoprenoid
units within the molecule were synthesised from commercially available
terpene aldehydes. The synthesis was applicable to a,b-unsaturated alde-
hydes and afforded the respective a-hydroxy-methyl ketones in overall
good yield. Tautomerisation of these products did not occur under the
conditions of reaction but was observed upon heating. Diastereoisomeric
a-hydroxy ketones have been resolved, and X-ray analysis on one of these
*
Correspondence: Ralf G. Berger, Institute of Food Chemistry, University of
Hannover, Wunstorfer Str. 14, Hannover, D-30453, Germany; E-mail: rg.berger@
lci.uni-hannover.de.
2591
DOI: 10.1080/SCC-200025618
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

2592
Schueler et al.
analogues allowed for the elucidation of the stereochemistry after conver-
sion into its camphanate ester.
Key Words: Acyloins; a-Hydroxy ketones; Terpene aldehydes; Cyano-
hydrin trimethylsilyl ethers; Copper salt catalysis; Nitriles; Grignard
reaction; Camphanates; Tautomerisation.
INTRODUCTION
a-Hydroxy ketones (acyloins) are of general interest as building blocks in
organic synthesis,[1] and many of them have been found to display biological
activity.[2–4] Several chemical methods were developed to synthesise both
symmetrical and unsymmetrical acyloins. The most efficient routes to such
compounds involve the “umpolung” reactions via cyanohydrins or dithio-
ketals, which have been reviewed extensively.[5–7] Alternatively, unsymme-
trical aliphatic and aromatic acyloins were obtained by nucleophilic
addition of an organometallic reagent to the nitrile group of cyanohydrins.[8]
These compounds, which in turn are useful intermediates for the synthesis of
b-amino alcohols and carboxylic acid derivatives, are obtained readily from
their respective aldehydes, and methods for their stereoselective formation
are available.[9] We herein report on the application of this methodology to
a,b-unsaturated aldehydes, which gave rise to some novel acyloins. The
products containing various isoprenoid units within the molecule were used
as reference substances for a related biosynthetic project[10] and are being
evaluated as precursors in the synthesis of novel odour active pyrazines.[11]
RESULTS AND DISCUSSION
Cyanohydrin trimethylsilyl ethers are useful intermediates in the syn-
thesis of acyloins, since protection of the hydroxyl group allows for a nucleo-
philic attack of an organometallic reagent at the nitrile functionality. The
protected cyanohydrins are obtained readily by the direct conversion of an
aldehyde with trimethylsilyl cyanide in the presence of Lewis acids or
bases. Under basic conditions, the reaction presumably proceeds via a hyper-
valent transition state of the silicon reagent, in which the nucleophilicity of
the cyanide ion is increased strongly.[12] We have applied this reaction to
a,b-unsaturated aldehydes 1 using a catalytic amount of triethylamine for
activation. As expected, 1,2-addition of trimethylcyanide prevails in the reac-
tion and in all cases, the respective cyanohydrin trimethylsilyl ethers 2 were
obtained in essentially quantitative yields.

Synthesis of a-Hydroxy Ketones from Terpene Aldehydes
2593
Scheme 1. Reagents and conditions: (i) 1.1 eq. TMSCN, 10 mol% Et3N, CH2Cl2,
2 hr, 258C, 95–98%; (ii) 1.1 eq. CH3MgBr, Et2O, 6 hr, reflux; and (iii) 4 N HCl,
1–2 hr, 258C, 80–90%.
The cyanohydrin trimethylsilyl ethers were subsequently subjected to a
Grignard reaction to achieve a nucleophilic addition of methylmagnesium
bromide onto the nitrile group of 2. In contrast to lithiumorganic species
which yield tertiary alcohols when reacted with aldehydes,[8,13] the Grignard
reagent does not facilitate a second attack on the ketimine salt 3. Thus,
acyloins 4 were obtained after acidic hydrolysis of the intermediate ketimine
salt. However, the yield of the reaction generally remained low, even at
elevated temperatures. Furthermore, the formation of significant amounts of
a byproduct, identified as the deprotected cyanohydrin, became a major
drawback in the reaction as its separation from the acyloins 4 by flash
chromatography was time-consuming.
Clearly, these byproducts were formed upon hydrolysis of unreacted
cyanohydrin trimethylsilyl ethers 2, and thus result from the low reactivity
of the nitrile to nucleophilic attack by the Grignard reagent. In the course of
our investigation, we found that the reaction was rendered synthetically
useful if copper(I) bromide was added to the reaction mixture. This allowed
for complete conversion of the cyanohydrin trimethylsilyl ethers 2 and gave
rise to the ketimine salts 3 in a comparably short reaction time. Formation
of the aforementioned byproduct was not observed under these conditions
and the acyloins 4a–f were obtained in overall good yields (Table 1).
It has been reported previously[14] that copper(I) salts enhance strongly
the reactivity of bulky Grignard reagents to nitriles. The origin of this acti-
vation is not clear but may be explained by an increased electrophilicity of
the nitrile carbon due to copper coordination.
Transfer of an acyl unit onto an aldehyde results in the formation of a new
chiral centre. When optically active aldehydes 1a–c and f were used as sub-
strates, the diastereoisomeric acyloins 4a–c and f were formed in a 1 : 1 ratio.
Conversion of neral and geranial gave racemic mixtures of acyloins 4d and
4e, respectively. Apart from the citronellal derived compounds (1R)-4f and
(1S)-4f, the diastereoisomers could be separated by flash chromatography
on silica gel. In order to determine their absolute configuration, the acyloins
were converted into their corresponding camphanates (Fig. 1).

2594
Schueler et al.
Table 1. Isoprenoid acyloins obtained from terpene aldehydes and copper(I) bromide
catalysis.
Isolated
yield (%)
Entry
A
Aldehyde
Acyloin
(þ)-Perillaaldehyde
88
B
C
(2)-Perillaaldehyde
(2)-Myrtenal
85
89
D
Neral
82
E
F
Geranial
84
90
(þ/2)-Citronellal
The compound 6b, derived from the acyloins 4a, gave crystals suitable
for x-ray analysis, and thus the stereochemistry of both diastereoisomers of
4a was unambiguously assigned (Fig. 2).
The absolute configuration of the acyloins (1R)-4b and (1S)-4b was
resolved as well, since (1S)-4b is the mirror image to (1R)-4a, and similarly,
Figure 1. Camphanates 6a and 6b of diastereoisomeric acyloins (1R)-4a and (1S)-4a
derived from (1S)-(2)-camphanoyl chloride.

Synthesis of a-Hydroxy Ketones from Terpene Aldehydes
2595
Figure 2. Ball and stick drawing of the x-ray generated molecular structure of
the camphanate 6b. The hydroxyl substituted carbon C(13) is referred to as the C(1)
of acyloins 4a.
(1R)-4b to (1S)-4a. Hence, these sets of acyloins represent enantiomers and
therefore NMR and MS data of all four stereoisomers could be assigned.
Although the diastereoisomeric acyloins (1R)-4c and (1S)-4c were separated
readily by flash chromatography, these compounds and their respective cam-
phanate esters did not give crystals suitable for x-ray spectroscopy.
Nevertheless, we feel justified to infer the stereochemistry of these
diastereoisomers on the basis of their optical rotation, since structurally
related acyloins with a negative sign of optical rotation have been reported
to have an R-configuration.[15–18]
In relation to our biosynthetic project, it was of interest to evaluate the
stability of the newly formed stereogenic centre and the liability of these com-
pounds towards tautomerisation. An attempt to purify acyloin 4f by distillation
led to the formation of its tautomer 5f (Sch. 2).
Moreover, when the specified acyloins were analysed by means of
GC–MS, mixtures of both tautomers were observed for the aliphatic acyloins
4d–f. Although the tautomers 5a–f could not be detected in acidic or basic
solution by means of NMR, the formation of 5d–f indicates that a thermal
Scheme 2. Thermal tautomerisation of 1-hydroxy-2-oxopropanes 4d–f results in the
formation of their corresponding 2-hydroxy-1-oxopropanes 5d–f.

2596
Schueler et al.
tautomerisation occurs readily for aliphatic acyloins at high temperatures. The
degree of tautomerisation was found to be highly dependent on the nature of
the substrate, and the absence of the alicyclic tautomers 4a–c may possibly be
attributed to the limits of GC–MS detection.
EXPERIMENTAL
Reagents and solvents were purchased from Fluka and Merck. All solvents
˚
were dried prior to use and kept over 4 A molecular sieves. NMR spectra were
1
recorded with either a Bruker AV-400 (400.13 MHz H; 100.61MHz 13C) or
1
a Varian Gemini 300 MHz (299.98 H; 75.43 13C) spectrometer. GC–MS
analyses were performed on an Agilent 6890 gas chromatograph (split-injec-
tion) connected to an Agilent 5973 mass-selective detector equipped with an
Optima 5-MS column (Macherey&Nagel; 30m ꢀ 0.25mm; ID, 0.25mm). IR
spectra were obtained using a Perkin Elmer Paragon 1000 FT-IR instrument.
Optical rotations were determined on an AA-1000 polarimeter (Optical Activity
Ltd). A GallenKamp GRIFFIN MPA350.BM2.5 melting point apparatus was
used to provide melting points, which are uncorrected.
General Procedure for the Synthesis of Acyloins
To a solution of the aldehyde (5 mmol) in dry dichloromethane (10 mL)
was added trimethylsilyl cyanide (0.75 mL, 6 mmol) via syringe at 08C.
Triethylamine (0.07 mL, 0.5 mmol) was added to the reaction mixture,
which was then stirred at room temperature for 2 hr under nitrogen. The
solvent was removed under reduced pressure and the residue dissolved in
dry diethylether (20 mL). This mixture was added to a preformed solution
of copper bromide (90 mg, 0.6 mmol) and 3 M methylmagnesium bromide sol-
ution in diethylether (2 mL, 6 mmol) under a static atmosphere of nitrogen.
The reaction mixture was refluxed for 6 hr, then cooled to room temperature,
and made up in ice-water (10 mL). Hydrochloric acid (10 mol%, 10 mL) was
added slowly and the mixture was stirred vigorously for 1–2 hr. The phases
were separated and the water layer extracted with EtOAc (3 ꢀ 10 mL). The
combined organic phases were washed with water, saturated sodium bicarbon-
ate solution, and saturated sodium chloride solution before being dried over
sodium sulphate. The solvent was removed under reduced pressure and the
product was purified by flash chromatography (hexane/ethylacetate 4 : 1).
(1R)-1-Hydroxy-1-[(40R)-40-isopropenyl-1-cyclohexen-1-yl]-2-propa-
1
3
none, (R)-4a. H-NMR (400 MHz, CDCl3): d 5.96 (t, J ¼ 2.3, 1H, C(20)–
H), 4.73 (m, 1H, C(80)–H), 4.71 (m, 1H, C(80)–H), 4.52 (s, 1H, C(1)–H),

Synthesis of a-Hydroxy Ketones from Terpene Aldehydes
2597
3
3.82 (d, J ¼ 4.36, 1H, OH), 2.18 (s, 3H, C(3)–H3), 2.24–1.60 (m, 7H,
C(30)–H2, C(40)–H, C(50)–H2, C(60)–H2), 1.73 (s, 3H, C(9)–H3). 13C-
NMR d (100 MHz, CDCl3): 208.4 (C55O), 149.2 C(70), 109.0 C(80), 135.0
C(10), 129.4 C(20), 82.2 C(1), 41.22 C(40), 30.82 C(60), 27.18 C(50), 24.71
†
C(3), 23.16 C(30), 20.62 C(90). MS (EI, 70 eV): m/z 194 (Mþ , 3), 151
(100), 149 (46), 45 (10), 43 (72). IR n (film, cm21): 3447 (br), 1713. The
analytical data matched those of enantiomeric (S)-4b.
(1S)-1-Hydroxy-1-[(40R)-40-isopropenyl-1-cyclohexen-1-yl)-2-propa-
1
none, (S)-4a. H-NMR d (400 MHz, CDCl3): 5.95 (m, 1H, C(20)–H), 4.74
3
(m, 1H, C(10)–H), 4.68 (m, 1H, C(10)–H); 4.51 (d, J ¼ 4.45, 1H, C(1)–
3
H), 3.77 (d, J ¼ 4.36, 1H, OH), 2.15 (s, 3H, C(3)–H3), 2.24–1.60 (m, 7H,
C(30)–H2, C(40)–H, C(50)–H2, C(60)–H2), 1.73 (s, 3H, C(30)–H3). 13C-NMR
d (100 MHz, CDCl3) 208.6 C(2), 149.0 C(70), 108.9 C(80), 134.8 C(10), 128.8
C(20), 82.5 C(1), 40.14 C(40), 30.57 C(60), 26.98 C(50), 24.84 C(3), 23.08
†
C(30), 20.85 C(90). MS (EI, 70 eV): m/z 194 (Mþ , 3), 151 (100), 149 (40),
45 (12), 43 (90). IR n (film, cm21): 3460 (br), 1713. The analytical data
matched those of enantiomeric (R)-4b.
(1R,10R)-1-(6,6-Dimethyl-bicyclo[3.1.1]hept-2-en-2-yl)-1-hydroxy-2-pro-
1
panone, (1R)-4c. [a]D ¼ 2 2918 (CHCl3, c ¼ 0.520). H-NMR d (400 MHz,
CDCl3): 5.75 (s, 1H, C(30)–H), 4.53 (d, 3J ¼ 3.6, 1H, C(1)–H), 3.66
3
3
(d, J ¼ 4.2, 1H, OH), 2.42–2.37 (m, 1H, C(70)–H), 2.35 (t, J ¼ 2.9, 2H,
C(40), 2.16 (s, 3H, C(3)–H3), 2.14–2.09 (m, 1H, C(50)–H), 1.92 (td, J ¼ 5.5,
J ¼ 1.5, 1H, C(10)–H), 1.25 (s, 3H, C(60)–CH3), 1.05 (d, 2J ¼ 8.7, 1H, C(70)–
H), 0.85 (s, 3H, C(60)–CH3). 13C-NMR d (100 MHz, CDCl3): 208.1 (C55O),
144.5 C(20), 124.8 C(30), 80.7 C(1), 41.28 C(10), 40.78 C(50), 38.09 C(60),
32.11 C(70), 31.67 C(40), 25.96 C(90), 25.04 C(3), 21.06 C(80). MS (EI, 70 eV):
†
m/z 194 (Mþ , 0.5), 151 (13), 149 (2), 45 (10), 43 (100). IR n (film, cm21):
3474 (br), 1716.
(1S,10R)-1-(6,6-Dimethyl-bicyclo[3.1.1]hept-2-en-2-yl)-1-hydroxy-2-pro-
1
panone, (1S)-4c. [a]D ¼ þ 2978 (CHCl3, c ¼ 0.520). H-NMR d (400 MHz,
CDCl3): 5.75 (s, 1H, C(30)–H), 4.60 (d, 3J ¼ 3.2, 1H, C(1)–H), 3.75
3
2
(d, J ¼ 4.56, 1H, OH), 2.49–2.43 (m, 1H, C(70)–H), 2.35 (dt, J ¼ 18.2,
3J ¼ 2.46, 2H, C(40), 2.18 (s, 3H, C(3)–H3), 2.15–2.10 (m, 1H, C(50)–H),
2.14–2.06 (m, J ¼ 1.5, 1H, C(10)–H), 1.28 (s, 3H, C(60)–CH3), 1.22
2
(d, J ¼ 8.76, 1H, C(70)–H), 0.82 (s, 3H, C(60)–CH3). 13C-NMR d (100 MHz,
CDCl3): 207.4 (C55O); 144.6 C(20), 124.3 C(30), 81.0 C(1), 41.66 C(10), 40.51
C(50), 37.54 C(60), 32.01 C(70), 31.47 C(40), 26.01 C(90), 25.34 C(3), 21.35
†
C(80). MS (EI, 70 eV): m/z 194 (Mþ , 0.5), 151 (13), 149 (2), 45 (10), 43
(100). IR n (film, cm21): 3456 (br), 1716.
(Z)-3-Hydroxy-5,9-dimethyl-deca-4,8-dien-2-one, (R,S)-4d. A commer-
cially available mixture of neral and geranial was used as starting material in the
synthesis, and the E- and Z-isomers 4d and 4e were then separated by flash

2598
Schueler et al.
chromatography on silica gel. 1H-NMR d (400MHz, CDCl3): 5.75 (m, 3J ¼ 7.0,
1H, C(8)–H), 4.95 (d, 3J ¼ 10.0, 1H, C(4)–H), 4.75 (d, 3J ¼ 10.0, 1H, C(3)–
H), 3.58 (d, J ¼ 2.9, 1H, OH), 2.2 (m, 2H, C(6)–H), 2.13 (m, 2H, C(7)–H), 2.1
4
4
(s, 3H, C(1)–H3), 1.75 (d, J ¼ 1.4, 3H, C(5)–CH3), 1.63 (d, J ¼ 1.0, 3H,
C(10)–H3), 1.57 (s, 3H, C(9)–CH3). 13C-NMR d (100MHz, CDCl3): 208.4
C(2), 143.8 C(5), 132.5 C(9), 123.4 C(8), 121.8 C(4), 74.3 C(3), 32.5 C(6),
26.4 C(7), 25.7 C(10), 25.1 (1), 23.4 C(5)–CH3, 17.6 C(9)–CH3. MS (EI,
†
70eV): m/z 196 (Mþ , 0.5), 153 (4), 151 (36), 69 (100), 45 (10), 43 (12).
(E)-3-Hydroxy-5,9-dimethyl-deca-4,8-dien-2-one, (R,S)-4e. 1H-NMR
3
4
d (400 MHz, CDCl3): 5.03 (m, 1H, C(8)–H), 5.0 (dq, J ¼ 9.40, J ¼ 1.3,
3
3
1H, C(4)–H), 4.8 (dd, J ¼ 9.4, 3.4, 1H, C(3)–H), 3.7 (d, J ¼ 4.1, 1H,
OH), 2.13 (s, 3H, C(1)–H3), 2.12–2.05 (m, 4H, C(7)–H2, C(6)–H2), 1.83
(d, 4J ¼ 1.4, 3H, C(5)–CH3), 1.65 (d, J ¼ 1.0, 3H, C(10)–H3), 1.58 (s, 3H,
4
C(9)–CH3). 13C-NMR d (100 MHz, CDCl3): 208.3 C(2), 143.0 C(5), 131.9
C(9), 123.5 C(8), 121.6 C(4), 74.8 C(3), 39.6 C(6), 26.1 C(7), 25.6 C(10),
†
24.9 (1), 17.6 C(9)–CH3, 16.9 C(5)–CH3. MS (EI, 70 eV): m/z 196 (Mþ
0.5), 153 (32), 151 (29), 69 (100), 45 (9), 43 (39).
,
1
3-Hydroxy-5,9-dimethyl-dec-8-en-2-one, (R,S)-4f. H-NMR d (300 MHz,
CDCl3): 5.12, 5.08 (m, 1H, C(8)–H), 4.2 (m, 1H, C(3)–H), 2.2 (s, 3H, C(1)–H3),
2.0 (m, 2H, C(7)–H2), 1.8 (m, 2H, C(4)–H), 1.68 (m, 3H, C(9)–CH3), 1.62, 1.60
(s, 3H, C(10)–H3), 1.5–1.1 (m, 3H, C(3)–H, C(6)–H2), 1.0, 0.96 (d, J ¼ 6.6 Hz,
3H, C(5)–CH3). 13C-NMR d (75 MHz, CDCl3): 210.4 (C55O), 131.8 C(9),
124.9, 124.8 C(8), 75.5, 75.2 C(3), 41.5, 41.3 C(7), 38.3, 38.1 C(4), 29.8, 29.5
C(5), 25.7 C(1), 25.8, 25.7 C(6), 25.2, 25.1 C(5)–CH3, 20.3 C(10), 17.6
†
C(10)–CH3. MS (EI, 70eV): m/z 198 (Mþ , 3), 155 (3), 153 (5), 69 (100), 45
(24), 43 (85). IR n (film, cm21): 3404 (br), 1703.
(Z)-2-Hydroxy-5,9-dimethyl-deca-4,8-dien-3-one, (R,S)-5d. MS (EI,
†
70 eV): m/z 196 (Mþ , 0.2), 153 (1), 151 (40), 69 (100), 45 (13), 43 (12).
(E)-2-Hydroxy-5,9-dimethyl-deca-4,8-dien-3-one, (R,S)-5e. MS (EI,
†
70 eV): m/z 196 (Mþ , 0.5), 153 (3), 151 (23), 69 (100), 45 (5).
2-Hydroxy-5,9-dimethyl-dec-8-en-3-one, (R,S)-5f. MS (EI, 70 eV):
†
m/z 198 (Mþ , 2), 155 (1), 153 (3), 69 (100), 45 (53), 43 (48).
Synthesis of Camphanate Esters
To a solution of acyloin 4a (110 mg, 0.57 mmol) in pyridine (1 mL) was
added (S)-(2)-camphanoyl chloride (125 mg, 1 mmol) at 2208C. The reac-
tion mixture was stirred at 08C for 8 hr and the resulting suspension was
then diluted with ether (1 mL). This was quenched into ice-water (2 mL),
the layers were separated, and the organic phase washed with saturated
aqueous solution of sodium bicarbonate, water, and brine. The solvent was

Synthesis of a-Hydroxy Ketones from Terpene Aldehydes
2599
removed under reduced pressure and the diastereoisomers were separated by
flash chromatography (hexane/ethylacetate, 9 : 1).
(1S)-Camphanate ester 6a. Yield 68%. M.p.: 868C. 1H-NMR d
(300 MHz, CDCl3): 6.04 (d, 1H, C(15)–H), 5.52 (s, 1H, C(13)–H), 4.75,
(m, 1H, C(23)–H), 4.69 (m, 1H, C(23)–H), 2.5–1.4 (m, 11H, C(5)–H2,
C(6)–H2, C(16)–H2, C(17)–H, C(18)–H2, C(19)–H2), 2.16 (s, 3 H,
C(21)–H3), 1.73 (s, 3 H, C(24)–H3); 1.15 (s, 3 H, C(7)–CH3), 1.12 (s, 3 H,
†
C(4)–CH3), 1.03 (s, 3 H, C(7)–CH3). MS (EI, 70 eV): m/z 374 (Mþ , 1).
IR n (film, cm21): 1791, 1748, 1725.
1
(1S)-Camphanate ester 6b. M.p. 888C. H NMR d (300MHz, CDCl3):
6.04 (d, 1H, C(15)–H), 5.52 (s, 1H, C(13)–H), 4.75, (m, 1H, C(23)–H), 4.69
(m, 1H, C(23)–H), 2.5–1.4 (m, 11H, C(5)–H2, C(6)–H2, C(16)–H2, C(17)–
H, C(18)–H2, C(19)–H2), 2.16 (s, 3H, C(21)–H3), 1.73 (s, 3H, C(24)–H3),
1.15 (s, 3H, C(7)–CH3), 1.12 (s, 3H, C(4)–CH3), 1.03 (s, 3H, C(7)–CH3).
†
MS (EI, 70eV) m/z 374 (Mþ , 5). IR n (film, cm21): 1790, 1748, 1726.
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1. Hannessian, S. Total Synthesis of Natural Products: The Chiron
Approach; Pergamon Press: New York, 1983; Chap. 2.
2. Wallace, O.B.; Smith, D.; Deshpande, M.S.; Polson, C.; Felsenstein, K.M.
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Received in Poland February 23, 2004