Intramolecular addition of carbon radicals to aldehydes: synthesis of enantiopure tetrahydrofuran-3-ols

Article abstract of DOI:10.1016/j.tet.2007.04.047

A simple and efficient substrate-controlled asymmetric synthesis of enantiopure tetrahydrofuran-3-ols by a 5-exo-trig radical cyclization is described. This cyclization occurs when a δ-carbon radical adds intramolecularly to the carbonyl group of an aldeh

Full text of DOI:10.1016/j.tet.2007.04.047

Tetrahedron 63 (2007) 5482–5489
Intramolecular addition of carbon radicals to aldehydes: synthesis
of enantiopure tetrahydrofuran-3-ols
*
*
Marcello Tiecco, Lorenzo Testaferri, Francesca Marini, Silvia Sternativo, Claudio Santi,
Luana Bagnoli and Andrea Temperini
Dipartimento di Chimica e Tecnologia del Farmaco, Sezione di Chimica Organica, Universitaꢀ di Perugia, 06123 Perugia, Italy
Received 22 January 2007; revised 26 March 2007; accepted 12 April 2007
Available online 20 April 2007
Abstract—A simple and efficient substrate-controlled asymmetric synthesis of enantiopure tetrahydrofuran-3-ols by a 5-exo-trig radical
cyclization is described. This cyclization occurs when a d-carbon radical adds intramolecularly to the carbonyl group of an aldehyde. The
d-carbon radicals can be efficiently produced from the tin hydride mediated deselenenylation of 5-phenylseleno-3-oxapentanals, which
were easily prepared starting from commercially available enantiopure epoxides or chlorohydrins.
Ó 2007 Elsevier Ltd. All rights reserved.
PhSe
R
1. Introduction
a)
R
R
R
R
R
X
X
X
X
Several selenium-based methodologies to effect convenient
syntheses of a wide variety of heterocyclic compounds
have been developed in recent years.1 The cyclization reac-
tions of alkenes containing internal nucleophiles, promoted
by electrophilic selenium reagents, have emerged as a highly
efficient method to prepare these compounds in a regio and
diastereoselective manner. Of particular importance are
the asymmetric versions of these reactions, either reagent-
controlled or substrate-controlled, from which enantio-
merically enriched or enantiopure heterocycles can be ob-
tained.1,2 Alternatively, the synthesis of small or medium
sized heterocycles can be effected by ring closure reactions,
which occur by intramolecular nucleophilic substitution by
oxygen, nitrogen or sulfur nucleophiles, after transformation
of the organoselenium group into a good leaving-group, like
a selenonium ion or a selenone.3 A further interesting method
is represented by the radical cyclization reactions, which take
place when a carbon radical intramolecularly adds to a car-
bon–carbon double or triple bond. The carbon radicals are
generally produced by the tin hydrides mediated deseleneny-
lation of properly substituted selenides. This latter approach
has been successfully employed by Engman for the prepara-
tion of racemic tetrahydrofurans and pyrrolidines in high
yields and with a good level of regio and diastereocontrol
(Scheme 1, route a).4 Other cyclizations involving intramo-
lecular radical addition to conjugated carbon–carbon double
O
OH
PhSe
R
O
O
b)
R
R
X
X
X
X
X
3
1
2
5
O
4
Scheme 1.
bonds, allenes or carbon–nitrogen multiple bonds have found
useful applications in the stereoselective synthesis of simple
heterocycles or of more complex natural and biologically
active compounds.5 Similar cyclizations by addition to
carbon–oxygen double bonds (Scheme 1, route b) are
much less common. In fact, even if the addition of the carbon
radical to a carbonyl group is faster than that to a carbon–
carbon double bond the reaction is reversible and it is not al-
ways easy to trap the cyclic alkoxy radical before it suffers
the b-scission to the open chain carbon radical. The b-scis-
sion reactions of the cyclic alkoxy radicals are particularly
favored in the cases of poorly substituted five-membered
rings.6 Simple racemic cyclopentanols were prepared from
u-formyl halides by rapid trapping of the cyclic alkoxy rad-
ical intermediate using efficient hydrogen donors such as or-
ganosilanes,6c or radical quenchers, such as Et3B.6d A radical
cyclization reaction by addition to a carbon–oxygen double
bond has been employed to effect the stereoselective
Keywords: Asymmetric synthesis; Organoselenium compounds; Radical
cyclizations; Tetrahydrofuranols.
* Corresponding authors. Tel.: +39 075 585 5100; fax: +39 075 585 5116;
e-mail: tiecco@unipg.it
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.04.047

M. Tiecco et al. / Tetrahedron 63 (2007) 5482–5489
5483
synthesis of optically active perhydrofuro[2,3-b]furanols.7
Our continuous interest in the development of new synthetic
methodologies based on the use of versatile organoselenium
compounds induced us to investigate the tin hydrides medi-
ated deselenenylation of enantiopure 5-phenylseleno-3-
oxapentanals 1 (Scheme 1, path b, X¼O) to produce the
corresponding d-carbon radicals 2 and the cyclic alkoxy rad-
icals 3 and to study the competition between the b-scission to
afford radicals 2 or 4 and the hydrogen abstraction to give 5
as a function of the structure of compounds 1. It would be
thus possible to develop a new synthetic method for a conve-
nient production of enantiopure tetrahydrofuran-3-ols. Ster-
eocontrolled approaches8 to these compounds are in fact of
considerable importance because tetrahydrofuran-3-ols can
be employed as useful reaction intermediates for the prepara-
tion of a wide range of biologically active compounds as for
instance isonucleosides.9
to afford the two diastereomeric alkoxy ester 6 (in the ratio
of 72:28), which were separated by column chromatography.
The major isomer was then transformed into the correspond-
ing aldehyde 7 by controlled reduction with DIBALH. This
compound was then submitted to the radical cyclization.
For this purpose it was treated with a slight excess of tributyl-
tin hydride in the presence of a catalytic amount of AIBN in
refluxing benzene. The main product of this reaction was
identified as the open chain ether 11. The desired tetrahydro-
furan-3-ol 12 was only present in very small amounts. This
result can be explained assuming that the b-scission of the
alkoxy radical 9 to afford the carbon radical 10 is faster
than the hydrogen abstraction from the tributyltin hydride
to give the tetrahydrofuran-3-ol 12. The observed final prod-
uct 11 is then formed by hydrogen abstraction. Very likely
the b-scission is favored by the formation of an a-alkoxy sec-
ondary carbon radical. Similar results were also observed
when this reaction sequence was carried out starting from
1-octene. If the above reported interpretation is correct
then one can expect that starting from a 5-phenylseleno-3-
oxapentanal that has no substituents in the a-position, the
rate of the b-scission of the cyclic alkoxy radical should be
decreased thus favoring the formation of the tetrahydrofu-
ran-3-ols. The cyclization reaction was therefore effected
starting from the d-phenylseleno aldehyde 16.
2. Results and discussion
The first experiments were carried out on (2S)-2-[1-phenyl-
2-(phenylseleno)ethoxy]propanal 7 (Scheme 2). This com-
pound was prepared in two steps starting from styrene and
isopropyl (S)-lactate. The first step consisted in the alkoxy-
selenenylation of styrene promoted by the electrophilic
phenylselenenyl triflate. The addition of the electrophile
produced a seleniranium intermediate, which was regiospe-
cifically trapped by the hydroxy group of the hydroxyester
This was obtained in four steps starting from the commer-
cially available (R)-glycidol. As reported in Scheme 3 the
single steps proceeded with good to excellent yields. The
i-PrO
H
PhSe
Ph
O
O
O
PhSe
H
Oi-Pr
HO
Ph
a
b
+
CH3
Ph
Ph
CH3
Ph
CH3
O
O
O
H3C
O
6
8
7
O
O
d.r. = 72:28
H
c
H
CH3
Ph
O
O
10
CH3
O
11
Ph
CH3
c
O
9
OH
CH3
a) PhSeCl, AgOTf, CH2Cl2, 60%
b) DIBALH, toluene, -78 °C, 90%
c) Bu3SnH, AIBN, C6H6, 38%
Ph
O
12
Scheme 2.
MeO
O
H
PhSe
O
H
PhSe
O
O
O
PhSe
O
e
d
c
a
OH
O
O
OH
O
OR
OTBDPS
OTBDPS
OTBDPS
OTBDPS
17
16
13 R = H
15
18
b
14 R = TBDPS
a: PhSeSePh, NaBH4, EtOH, 93%
b: Imidazole, TBDPSCl, DMF, 98%
c: NaH, C6H6, 1h;
OH
OH
OH
OH
f
15-crown-5, BrCH2CO2Me, 40 °C, 74%
d: DIBALH, toluene, -78 °C, 98%
O
O
O
O
OH
OH
OTBDPS
OTBDPS
e: Bu3SnH, AIBN, C6H6, 57%,19:20 = 53:47
f: Bu4NF, THF, 21 80%; 22 86%
19
20
22
21
[α]D = +29.3
[α]D = +14.3
Scheme 3.

5484
M. Tiecco et al. / Tetrahedron 63 (2007) 5482–5489
regioselective ring opening of the epoxide by the phenylse-
lenium anion, generated in situ by treatment of the diphenyl
diselenide with sodium borohydride in ethanol, afforded the
dihydroxyselenide 13. The primary alcohol was selectively
protected and the resulting hydroxyselenide 14 was trans-
formed into the corresponding alkoxyester 15 by treatment
with methyl bromoacetate. Several experiments were carried
out in order to find the best experimental conditions to effect
this reaction. Good reaction yields were finally obtained by
working in the presence of 15-crown-5 in benzene at 40 ꢀC.
The controlled reduction with DIBALH generated the de-
sired aldehyde 16, which was submitted to radical cycliza-
tion under the usual reaction conditions. In agreement with
expectations no products derived from the b-scission of
the cyclic alkoxy radical intermediate 18 were present in
the reaction mixture. As indicated in Scheme 3 the only
products formed from this reaction in 57% yield and in the
ratio of 53:47 were the two enantiopure diastereomeric tetra-
hydrofuranols 19 and 20 derived from the hydrogen abstrac-
tion by the radical 18. The lack of diastereoselectivity was
not surprising since similar results were already observed
in the case of other 5-exo-cyclizations involving the carbonyl
group.6b The two diastereoisomers were easily separated by
flash column chromatography and their physical and spec-
troscopical properties could thus be measured. With the
aim of optimizing the reaction yield and the diastereoselec-
tivity of the cyclization step, the reaction was repeated under
different experimental conditions using Bu3SnH or other hy-
drogen donors in refluxing toluene (Table 1). The best chem-
ical yields and diastereoselectivities were obtained by using
2 equiv of Bu3SnH in one portion (entry 1). As expected,
a slow addition of Bu3SnH by a syringe pump (entry 2)
did not produce any increase in the yield of the cyclization
and only favored the re-opening of the cyclic alkoxyl radi-
cal.10 The attempt to improve the cyclization step by using
a larger excess of Bu3SnH (entry 3) in order to facilitate
the trapping of the cyclic alkoxy radical intermediate failed
because of the formation of a consistent amount (25%) of the
1-silyl protected 2-(2-hydroxyethoxy)propan-1-ol deriving
from the reduction of the carbonyl group by the stannyl rad-
ical. Different hydrogen donors such as Ph3SnH (entry 4) or
tris-trimethylsilylsilane (entry 5) gave similar or lower
yields of cyclic products, respectively.
The structural assignment of compounds 19 and 20 was
effected after conversion into the known compounds 21
and 22 by treatment with tetrabutylammonium fluoride9a
(Scheme 3). The tetrahydrofuranols synthesized with the
presently described protocol are usually prepared from car-
bohydrates through multi-steps sequences and are employed
as intermediates for further synthetic transformations.
As an example, compound 20 or similar derivatives possess-
ing different protecting groups have been employed in the
preparation of interesting antiviral agents such as isodide-
oxynucleosides, in which the natural or modified base is
attached to a non-anomeric carbon atom,9a,11 or to synthe-
size C-isodideoxynucleosides.12 These types of compounds
have attracted great interest as anti-HIV agents because of
their high stability toward acids and of their resistance to en-
zymatic deamination.9,11,12 The strongly active antiviral
isoddA and other examples of isodideoxynucleosides and
C-isodideoxynucleosides are indicated in Scheme 4.
The presently described procedure was then applied to the
aldehyde 25 from which it was expected to produce the
two enantiopure phenyl substituted tetrahydrofuranols 26
and 27. As indicated in Scheme 5 in the present case the hy-
droxyselenide 23 was not produced by the opening of the
corresponding (R)-styrene oxide since it is known that this
reaction is not regioselective.13 Compound 23 was instead
prepared in good yields by the nucleophilic substitution of
the commercially available (R)-2-chloro-1-phenylethanol
with sodium phenyl selenolate. The conversion of 23 into
24 and the successive reduction to 25 were carried out as de-
scribed above and proceeded in good yields (see Scheme 5).
The deselenenylation and the cyclization of the resulting car-
bon radical eventually afforded an almost equimolecular
mixture of the two enantiomerically pure diastereomeric 5-
phenyltetrahydrofuran-3-ols 26 and 27.14 These were con-
verted into the corresponding acetates 28 and 29, which
were easily separated by medium pressure liquid chromato-
graphy. NOESY experiments confirmed the proposed struc-
tures reported in Scheme 5. In the case of compound 28
a strong dipolar interaction was in fact observed between
the proton in the 3-position and the proton in the 5-position.
A similar interaction was not observed in the case of the dia-
stereoisomer 29. A similar synthetic sequence was also re-
peated starting from the (S)-2-chloro-1-phenylethanol from
which the enantiomeric 5-phenyltetrahydrofuran-3-yl ace-
tates ent-28 and ent-29 were obtained. The four isomeric ac-
etates are reported in Scheme 5 together with their measured
optical rotations. HPLC analyses on a chiral column con-
firmed that the enantiomeric purities of compounds 28, 29,
Table 1. Cyclization of 16 under different experimental conditions
Entry
H-donor
Yield (%)
19:20
1
2
3
4
5
Bu3SnHa
Bu3SnHb
Bu3SnHc
Ph3SnHa
63
47
37
57
35
63:37
60:40
57:43
60:40
60:40
(Me3Si)3SiHa
a
Hydrogen donor of 2 equiv was added in one portion.
Bu3SnH of 1.5 equiv was added by a syringe pump in 2 h.
Hydrogen donor 4 equiv was added in one portion.
b
c
O
NH2
N
NH2
HO
N
S
N
Base
H
HO
HO
NH2
N
O
N
O
O
HO
N
N
N
N
O
H
H
H
H
H
H
Base = Cyt, Hyp, Gua,
Thy, Ura
H
Scheme 4.

M. Tiecco et al. / Tetrahedron 63 (2007) 5482–5489
5485
OH
X
OH
O
Cl
PhSe
PhSe
Ph
b
a
d
+
Ph
Ph
Ph
Ph OH
Ph OH
23
O
O
26
O
27
24 X = OMe
25 X = H
c
e
OAc
OAc
+
a: PhSeSePh, NaBH4, THF-HMPA, 40 °C, 90%.
Ph
O
O
b: NaH, C6H6, 1h,15-Crown-5, BrCH2CO2Me, 40 °C, 75%.
c: DIBALH, toluene, -78 °C, 80%.
29
28
d: Bu3SnH, AIBN, refluxing toluene, 55%, 26:27 = 43:57.
e: AcCl, pyridine,THF, 88%.
[α]D = -23.8
[α]D = -4.8
OAc
OAc
Cl
Ph
+
O
Ph
O
ent-28
Ph OH
ent-29
[α]D = +24.1
[α]D = +4.0
Scheme 5.
ent-28, and ent-29 are identical to those of the starting chlo-
rohydrins, indicating that no loss of the enantiomeric purity
occurred during the various synthetic steps.
are given. HPLC analyses were performed on an HP 1100 in-
strument equipped with a chiral column and an UV detector.
Optical rotations were measured with a Jasco DIP-1000 dig-
ital polarimeter. Elemental analyses were carried out on
a Carlo Erba 1106 Elemental Analyzer. Commercially avail-
able chiral compounds possessed enantiomeric excesses
equal or greater than 98%.
3. Conclusions
In conclusion the tin hydride mediated radical 5-exo-trig
cyclizations of 5-phenylseleno-3-oxapentanals can be effi-
ciently employed for the synthesis of enantiopure tetrahy-
drofuran-3-ols. The starting aldehydes are easily prepared
in few steps from commercially available enantiomerically
enriched epoxides or chlorohydrins. The results of the exper-
iments here described allowed the appropriate structural fea-
tures of the starting aldehydes to be determined in order to
obtain good yields of the tetrahydrofuran-3-ols and to mini-
mize the formation of other products deriving from the com-
petitive b-scission of the initially formed cyclic alkoxy
radical intermediate. It was observed that these latter com-
pounds are the major reaction products when an alkyl
substituent is present in the a-position of the starting alde-
hyde. Experiments effected to find the best reagents and
the best experimental conditions to have a successful
hydrogen abstraction reaction by the cyclic alkoxy radical
intermediate were also illustrated. Finally it is worth men-
tioning that the tetrahydrofuran-3-ols here obtained starting
from the (R)-glycidol are useful intermediates in the prepa-
ration of antiviral agents.
4.1.1. Isopropyl (2S)-2-[1-phenyl-2-(phenylseleno)eth-
oxy]propanoate (6). PhSeCl (0.19 g, 1 mmol) was dissolved
in CH2Cl2 (8 mL) at 0 ꢀC and silver trifluoromethanesulfo-
nate (0.26 g, 1 mmol) was added. After 10 min, styrene
(0.26 g, 2.5 mmol) and (S)-isopropyl lactate (0.16 g,
1.2 mmol) were added at ꢁ78 ꢀC. The reaction temperature
was left to gradually reach ꢁ50 ꢀC over 30 min. The result-
ing white suspension was stirred for 1 h and then worked
up by treatment with water (30 mL). The mixturewas filtered
through Celite and extracted with CH2Cl2 (3ꢂ15 mL). The
organic layer was dried over Na2SO4 and evaporated. Crude
compound 1 was a 72:28 mixture of two diastereoisomers
1
(determined by H NMR), which were separated by flash
chromatography (light petroleum/dichloromethane 80:20
to 60:40 as eluant). Chemical yields and physical and spec-
tral data of the two compounds are reported below.
Major isomer: 43% yield (0.16 g); oil; Rf: 0.27 (light petro-
leum/dichloromethane 40:60); [a]2D1 ꢁ73.8 (c 1.5, CHCl3);
1H NMR: d¼7.50–7.45 (m, 2H; SePh), 7.38–7.25 (m, 5H;
3
Ph), 7.22–7.17 (m, 3H; SePh), 5.10 (sept, J¼6.3 Hz, 1H;
3
OCHMe2), 4.67 (dd, J¼7.3, 6.3 Hz, 1H; OCHPh), 3.81
4. Experimental
4.1. General
(q, 3J¼6.9 Hz, 1H; OCHMe), 3.48 (dd, 2J¼12.0 Hz,
3J¼7.3 Hz, 1H; CHaHb), 3.16 (dd, 2J¼12.0 Hz, 3J¼6.3 Hz,
3
1H; CHaHb), 1.34 (d, J¼6.9 Hz, 3H; OCHMe), 1.27 (d,
3J¼6.3 Hz, 6H; OCHMe2); 13C NMR: d¼172.6, 140.4,
132.2 (2C), 130.9, 128.9 (2C), 128.5 (2C), 128.3, 127.0
(2C), 126.5, 80.6, 72.3, 68.3, 34.9, 21.7 (2C), 18.8; MS
(70 eV, EI) m/z (%): 392 (23) [M+], 261 (15), 221 (59),
179 (100), 157 (23), 107 (48), 77 (11). Anal. Calcd for
C20H24O3Se: C, 61.38; H, 6.18. Found: C, 61.27; H, 6.15.
1
New compounds were characterized by H, 13C NMR, and
1
mass spectra. H and 13C NMR spectra were recorded on
a Bruker Advance-DRX 400 instrument (at 400 and
100.62 MHz, respectively) with CDCl3 as solvent and
TMS as internal reference. GC–MS analyses were carried
out with an HP 6890 gas chromatograph (HP-5MS capillary
column, 30 m i.d., 0.25 mm, film 0.25 mm) equipped with an
HP 5973 Mass Selective Detector; for the ions containing se-
lenium only the peaks arising from the selenium-80 isotope
Minor isomer: 17% yield (67 mg); oil; slightly impure; Rf:
0.20 (light petroleum/dichloromethane 40:60); H NMR:
1
d¼7.53–7.48 (m, 2H; SePh), 7.40–7.22 (m, 8H; Ph, SePh),

5486
M. Tiecco et al. / Tetrahedron 63 (2007) 5482–5489
3
3
4.84 (sept, J¼6.3 Hz, 1H; OCHMe2), 4.59 (dd, J¼8.1,
dried over Na2SO4 and evaporated under reduced pressure.
The residue was purified by flash chromatography (dichloro-
methane/MeOH 99:1 to 98:2 as eluant) to afford 13 in
93% yield (2.6 g); Rf: 0.36 (dichloromethane/MeOH
90:10); ([a]2D3 ꢁ21.7 (c 2.7, EtOH)). Spectral data of this
compound are in agreement with those already reported in
the literature.15
3
5.4 Hz, 1H; OCHPh), 4.0 (q, J¼6.8 Hz, 1H; OCHMe),
2
3
3.41 (dd, J¼12.6 Hz, J¼8.1 Hz, 1H; CHaHb), 3.16 (dd,
2J¼12.6 Hz, J¼5.4 Hz, 1H; CHaHb), 1.42 (d, J¼6.8 Hz,
3
3
3
3H; OCHMe), 1.10 (d, J¼6.3 Hz, 3H; OCHMe2), 1.08 (d,
3J¼6.3 Hz, 3H; OCHMe2); 13C NMR: d¼172.2, 140.7,
132.4 (2C), 130.6, 129.0 (2C), 128.3 (2C), 128.1, 126.9
(2C), 126.8, 81.7, 74.4, 68.1, 35.0, 21.4 (2C), 18.1; MS
(70 eV, EI) m/z (%): 392 (11) [M+], 261 (11), 221 (63),
179 (100), 157 (21), 107 (51), 77 (11).
4.1.5. (2R)-1-{[tert-Butyl(diphenyl)silyl]oxy}-3-(phenyl-
seleno)propan-2-ol (14). The diol 13 (2.6 g, 11.2 mmol)
was selectively protected by treatment with imidazole
(1.14 g, 16.8 mmol) and tert-butyl(diphenyl)silyl chloride
(TBDPSCl, 4.0 g, 16.8 mmol) in DMF (25 mL) at room tem-
perature. After 4 h the reaction was poured into H2O
(50 mL) and extracted with diethyl ether (3ꢂ30 mL). The
ethereal layer was washed with 20 mL of a 7% aqueous so-
lution of HCl and with 20 mL of water, dried over Na2SO4,
and evaporated under reduced pressure. The crude product
was purified by flash chromatography (light petroleum/di-
chloromethane 70:30 to 50:50 as eluant). Chemical yield
and physical and spectral data of 14 are reported below.
4.1.2. (2S)-2-[1-Phenyl-2-(phenylseleno)ethoxy]propanal
(7). The ester 6 (0.13 g, 1 mmol) was dissolved in toluene
(8 mL) and treated at ꢁ78 ꢀC with 1.1 equiv of DIBALH
(1.5 M in toluene) under N2. After 3 h 20 mL of a 7% aque-
ous solution of HCl was added and the mixture was stirred at
room temperature for 3 h. The reaction mixture was ex-
tracted with CH2Cl2 (3ꢂ15 mL), dried over Na2SO4, and
evaporated under reduced pressure. The residue was used
without further purification. Chemical yield and physical
and spectral data of 7 are reported below.
1
3
Yield 98% (0.33 g); oil; H NMR: d¼9.64 (d, J¼2.1 Hz,
Yield 98% (5.16 g); oil; Rf: 0.30 (light petroleum/dichloro-
methane 40:60); 1H NMR: d¼7.70–7.18 (m, 15H; Ph,
1H; CH]O), 7.40–7.35 (m, 2H; SePh), 7.25–7.05 (m, 8H;
3
2
SePh, Ph), 4.48 (dd, J¼8.7, 4.6 Hz, 1H; OCHPh), 3.56
SePh), 3.90–3.82 (m, 1H; CHOH), 3.75 (dd, J¼10.3 Hz,
3
2
2
3
(dq, J¼7.0, 2.1 Hz, 1H; OCHMe), 3.28 (dd, J¼12.6 Hz,
3J¼8.7 Hz, 1H; CHaHb), 2.99 (dd, 2J¼12.6 Hz, 3J¼4.6 Hz,
1H; CHaHb), 1.06 (d, 3J¼7.0 Hz, 3H; OCHMe); 13C NMR:
d¼204.0, 140.4, 132.4 (2C), 130.5, 129.0 (2C), 128.6
(2C), 128.4, 127.0, 126.5 (2C), 81.6, 78.0, 36.0, 16.0; MS
(70 eV, EI) m/z (%): 334 (42) [M+], 261 (18), 183 (42),
163 (100), 107 (23), 91 (55), 77 (28).
3J¼4.6 Hz, 1H; CHaHbCOSi), 3.72 (dd, J¼10.3 Hz, J¼
5.6 Hz, 1H; CHaHbCHO), 3.15 (dd, 2J¼12.7 Hz, 3J¼
5.7 Hz, 1H; CHaHbSePh), 3.04 (dd, 2J¼12.7 Hz,
3J¼6.9 Hz, 1H; CHaHbCHO), 2.68 (br s, 1H; OH), 1.08 (s,
9H; CMe3). 13C NMR: d¼135.5 (4C), 132.9 (2C), 132.5
(2C), 129.8 (3C), 129.1 (2C), 127.8 (4C), 127.0, 70.8, 66.5,
31.5, 26.8 (3C), 19.2. Anal. Calcd for C25H30O2SeSi: C,
63.95; H, 6.44. Found: C, 63.90; H, 6.42.
4.1.3. 3-Ethoxy-3-phenylpropanal (11). Compound 11 was
obtained by treatment of 7 (0.1 g, 0.3 mmol) with Bu3SnH
and AIBN in benzene under the experimental conditions de-
scribed in the typical procedure for the cyclization reaction.
The crude product was purified by flash chromatography
(light petroleum/dichloromethane 70:30 to 50:50). Chemi-
cal yield and physical and spectral data of 11 are reported
below.
4.1.6. Methyl ({(1R)-2-{[tert-butyl(diphenyl)silyl]oxy}-1-
[(phenylseleno)methyl]ethyl}oxy)acetate (15). A solution
of 14 (1.88 g, 4 mmol) in dry benzene (15 mL), was treated
with NaH (0.11 g, 4.4 mmol) and the reaction mixture was
stirred for 1 h at room temperature. 15-Crown-5 (0.88 mL,
4.4 mmol) and methyl bromoacetate (0.4 mL, 4.2 mmol)
were added and the reaction mixture was heated at 40 ꢀC
overnight. The reaction was quenched with water (30 mL)
and extracted with CH2Cl2 (3ꢂ15 mL). The organic phase
was dried with Na2SO4, filtered, and evaporated under re-
duced pressure. The residue was purified by flash chromato-
graphy (light petroleum to light petroleum/diethyl ether
90:10 as eluant). Chemical yield and physical and spectral
data of 15 are reported below.
Yield 38% (20 mg); oil; Rf: 0.40 (light petroleum/dichloro-
methane 20:80); H NMR: d¼9.81 (dd, J¼2.5, 1.6 Hz,
1
3
3
1H; CH]O), 7.48–7.30 (m, 5H; Ph), 4.82 (dd, J¼9.0,
2
3
4.3 Hz, 1H; CHOEt), 3.45 (dq, J¼9.3 Hz, J¼7.0 Hz, 1H;
CH3CHaHbO), 3.35 (dq, 2J¼9.3 Hz, 3J¼7.0 Hz, 1H;
CH3CHaHbO), 2.93 (ddd, 2J¼16.4 Hz, 3J¼9.0, 2.5 Hz, 1H;
CHaHbCH]O), 2.62 (ddd, 2J¼16.4 Hz, 3J¼4.3, 1.6 Hz,
1H; CHaHbCH]O), 1.18 (t, 3J¼7.0 Hz, 3H; Me); MS
(70 eV, EI) m/z (%): 178 (17) [M+], 149 (21), 135 (100),
107 (86), 105 (55), 104 (27), 79 (67), 42 (77). Anal. Calcd
for C11H14O2: C, 74.13; H, 7.92. Found: C, 74.47; H, 8.03.
Yield 74% (1.60 g); oil; Rf: 0.60 (light petroleum/diethyl
ether 70:30); [a]D19 ꢁ14.1 (c 2.9, CHCl3); 1H NMR:
d¼7.70–7.64 (m, 4H; Ph), 7.58–7.18 (m, 11H; Ph, SePh),
4.22 (d, 2J¼17.8 Hz, 1H; CHaHbCO2Me), 4.18 (d,
2J¼17.8 Hz, 1H; CHaHbCO2Me), 3.87 (dd, 2J¼10.8 Hz,
3J¼5.5 Hz, 1H; CHaHbO), 3.81 (dd, 2J¼10.8 Hz, 3J¼
5.0 Hz, 1H; CHaHbO), 3.72 (s, 3H; OMe), 3.74–3.66 (m,
4.1.4. (2R)-3-(Phenylseleno)propane-1,2-diol (13). To a so-
lution of diphenyl diselenide (1.87 g, 6 mmol) in EtOH
(20 mL), NaBH4 (0.68 g, 18 mmol) was added at room tem-
perature in small portions. The reaction mixture was warmed
to 40 ꢀC for 30 min, then (R)-glycidol (0.9 g, 12 mmol) was
added to the resulting white suspension. The reaction was
monitored by TLC and worked up after 6 h. The reaction
was poured into a saturated NH4Cl solution (30 mL) and ex-
tracted with diethyl ether (3ꢂ20 mL). The ethereal layer was
2
3
1H; CHO), 3.24 (dd, J¼12.8 Hz, J¼5.8 Hz, 1H; CHaHb-
2
3
SePh), 3.16 (dd, J¼12.8 Hz, J¼6.3 Hz, 1H; CHaHbO),
1.07 (s, 9H; CMe3); 13C NMR: d¼170.7, 135.5 (2C),
135.4 (2C), 133.0, 132.9, 132.2 (2C), 130.4, 129.7 (2C),
129.0 (2C), 127.7 (4C), 126.4, 80.6, 68.3, 65.1, 51.7, 28.9,
26.7 (3C), 19.1. Anal. Calcd for C28H34O4SeSi: C, 62.09;
H, 6.35. Found: C, 61.79; H, 6.63.

M. Tiecco et al. / Tetrahedron 63 (2007) 5482–5489
5487
4.1.7. ({(1R)-2-{[tert-Butyl(diphenyl)silyl]oxy}-1-[(phe-
nylseleno)methyl]ethyl}oxy)acetaldehyde (16). Com-
pound 16 (2.27 g, 4.4 mmol) was prepared according to
the procedure described for 7. In this case also the aldehyde
was used without further purification. Chemical yield and
physical and spectral data of 16 are reported below.
Anal. Calcd for C21H28O3Si: C, 70.74; H, 7.92. Found: C,
70.83; H, 7.78.
4.1.9. Deprotection of tetrahydrofuranols 19 and 20.
Bu4NF$3H2O (0.14 g, 0.45 mmol) was added to a solution
of 19 or 20 (0.1 g, 0.3 mmol) in THF (4 mL) at room temper-
ature and the resulting mixture was stirred overnight. After
addition of 0.1 mL of water and 2 g of silica gel the mixture
was evaporated to dryness and purified by flash chromato-
graphy (CH2Cl2/MeOH 98:2 to 80:20). Spectral data of
compounds 21 and 22 reported below are in agreement
with those described in the literature.9a
Yield 98%; oil; 1H NMR: d¼9.66 (t, 3J¼1.0 Hz, 1H;
CH]O), 7.68–7.62 (m, 4H; Ph), 7.5–7.34 (m, 7H; Ph,
SePh), 7.30–7.17 (m, 4H; Ph), 4.13 (dd, 2J¼17.8 Hz,
3J¼1.0 Hz, 1H; CHaHbCH]O), 4.08 (dd, 2J¼17.8 Hz, 3J¼
1.0 Hz, 1H; CHaHbCH]O), 3.85 (dd, 2J¼10.9 Hz, 3J¼
5.6 Hz, 1H; CHaHbOSi); 3.80 (dd, 2J¼10.9 Hz, 3J¼4.8 Hz,
3
1H; CHaHbOSi), 3.65 (dddd, J¼6.9, 5.6, 5.2, 4.8 Hz, 1H;
4.1.9.1. (3S,5S)-5-(Hydroxymethyl)tetrahydrofuran-
3-ol (21). Yield 80% (28 mg); Rf: 0.43 (dichloromethane/
methanol 90:10); oil; [a]2D1 +29.3 (c 1.2, CHCl3); 1H NMR:
d¼4.42–4.36 (m, 1H; CHO), 4.25 (dq, 3J¼9.8, 2.5 Hz, 1H;
CHO), 3.21 (dd, 2J¼13.0 Hz, 3J¼5.2 Hz, 1H; CHaHbSePh),
3.12 (dd, 2J¼13.0 Hz, 3J¼6.9 Hz, 1H; CHaHbSePh), 1.08 (s,
9H; CMe3); 13C NMR: d¼201.1, 135.6 (2C), 135.5 (2C),
133.1, 133.0, 132.5 (2C), 130.2, 129.9 (2C), 129.2 (2C),
127.8 (4C), 126.7, 81.4, 76.3, 61.9, 28.8, 26.8 (3C), 18.8.
2
3
CHOSi), 3.98 (dd, J¼9.7 Hz, J¼1.7 Hz, 1H; CHaHbO),
2
3
3.92 (dd, J¼11.7 Hz, J¼2.5 Hz, 1H; CHaHbOSi), 3.75
2
3
2
(dd, J¼9.7 Hz, J¼3.0 Hz, 1H; CHaHbO), 3.61 (dd, J¼
3
4.1.8. Cyclization reaction of 16: typical procedure. The
aldehyde 16 (0.26 g, 0.5 mmol) was dissolved in toluene
(5 mL) and Bu3SnH (0.27 mL, 1 mmol) was added in one
portion together with a catalytic amount of AIBN. The reac-
tion mixture was refluxed under N2 for 2 h. The progress of
the reaction was monitored by TLC. The solvent was re-
moved under reduced pressure. The crude mixture of 19
and 20 (dr¼53:47) was separated by flash chromatography
(light petroleum/diethyl ether 70:30 to 55:45 as eluant). Sim-
ilar experimental conditions were employed for the reactions
effected with other solvents or hydrogen donors (see Table 1).
11.7 Hz, J¼2.5 Hz, 1H; CHaHbOSi), 2.87 (br s, 2H; OH),
2.32 (ddd, 2J¼14.1 Hz, 3J¼9.8, 5.7 Hz, 1H; CHCHaHbCH),
1.99–1.87 (m, 1H; CHCHaHbCH). 13C NMR: d¼78.4, 76.5,
71.4, 63.2, 36.3. Anal. Calcd for C5H10O3: C, 50.84; H, 8.53.
Found: C, 50.99; H, 8.44.
4.1.9.2. (3R,5S)-5-(Hydroxymethyl)tetrahydrofuran-
3-ol (22). Yield 86% (30 mg); oil; Rf: 0.29 (dichlorome-
thane/methanol 90:10); [a]2D4 +14.3 (c 1.3, CHCl3); 1H
3
NMR: d¼4.56 (dddd, J¼4.8, 4.0, 2.0, 1.6 Hz, 1H; CHO),
3
4.33 (dddd, J¼9.1, 6.6, 5.4, 3.0 Hz, 1H; CHOSi), 3.97
(dd, 2J¼9.7 Hz, 3J¼4.0 Hz, 1H; CHaHbO), 3.81 (dd,
2J¼9.7 Hz, 3J¼1.6 Hz, 1H; CHaHbO), 3.78 (dd, 2J¼
11.8 Hz, 3J¼3.0 Hz, 1H; CHaHbOSi), 3.53 (dd, 2J¼
4.1.8.1. (3S,5S)-5-({[tert-Butyl(diphenyl)silyl]oxy}me-
thyl)tetrahydrofuran-3-ol (19). Yield 40% (72 mg); Rf:
0.27 (light petroleum/diethyl ether 40:60); oil; [a]2D2 +16.6
3
11.8 Hz, J¼5.4 Hz, 1H; CHaHbOSi), 2.10 (br s, 2H; OH),
1
2
3
4
(c 4.7, CHCl3); H NMR: d¼7.78–7.66 (m, 4H; Ph), 7.49–
1.96 (dddd, J¼13.3 Hz, J¼6.6, 2.2 Hz, J¼1.0 Hz, 1H;
CHCHaHbCH), 1.92 (ddd, 2J¼13.3 Hz, 3J¼9.1, 4.8 Hz,
1H; CHCHaHbCH); 13C NMR: d¼78.3, 75.5, 72.6, 64.2,
36.7. Anal. Calcd for C5H10O3: C, 50.84; H, 8.53. Found:
C, 50.97; H, 8.50.
7.36 (m, 6H; Ph), 4.37–4.30 (m, 1H; CHOH), 4.17 (dq,
2
3J¼10.4, 2.5 Hz, 1H; CHOSi), 4.10 (d, J¼10.6 Hz, 1H;
2
3
OH), 4.02 (dd, J¼9.3 Hz, J¼4.7 Hz, 1H; CHaHbO), 3.88
(dd, 2J¼11.1 Hz, 3J¼2.5 Hz, 1H; CHaHbOSi), 3.71 (dd,
2J¼9.3 Hz, 3J¼3.0 Hz, 1H; CHaHbO), 3.54 (dd, 2J¼
11.1 Hz, 3J¼2.1 Hz, 1H; CHaHbOSi), 2.31 (ddd, 2J¼
13.8 Hz, 3J¼10.4, 5.4 Hz, 1H; CHCHaHbCH), 2.07–2.0
(m, 1H; CHCHaHbCH), 1.09 (s, 9H; CMe3); 13C NMR:
d¼135.6 (2C), 135.5 (2C), 132.4, 132.1, 129.9, 129.8,
127.7 (4C), 77.8, 76.7, 71.6, 66.1, 36.7, 26.7 (3C), 19.0.
Anal. Calcd for C21H28O3Si: C, 70.74; H, 7.92. Found: C,
70.86; H, 8.01.
4.1.10. (1R)-1-Phenyl-2-(phenylseleno)ethanol (23). The
b-hydroxyalkyl phenyl selenide 23 was prepared by SN2
displacement of (R)-2-chloro-1-phenylethanol by sodium
phenyl selenolate in THF/HMPA (90% yield), as previously
described in the literature.3b,13
4.1.11. Methyl {[(1R)-1-phenyl-2-(phenylseleno)ethyl]-
oxy}acetate (24). Compound 24 (0.45 g, 1.28 mmol) was
prepared as described for 15. The crude product was purified
by medium pressure liquid chromatography with a LiChro-
prep Si 60 (40–63 mm, 310ꢂ25 mm i.d., Merck) and a FMI
LAB pump model QSY (light petroleum/diethyl ether 90:10
as eluant). Chemical yield and physical and spectral data of
24 are reported below.
4.1.8.2. (3R,5S)-5-({[tert-Butyl(diphenyl)silyl]oxy}me-
thyl)tetrahydrofuran-3-ol (20). Yield 23% (42 mg); Rf:
0.18 (light petroleum/diethyl ether 40:60); oil; [a]2D5 +7.2
1
(c 5.6, CHCl3); H NMR: d¼7.74–7.68 (m, 4H; Ph), 7.49–
7.38 (m, 6H; Ph), 4.62–4.52 (m, 1H; CHO), 4.35 (ddt,
3J¼8.6, 6.7, 4.3 Hz, 1H; CHOSi), 3.98 (dd, 2J¼9.6 Hz,
3J¼3.8 Hz, 1H; CHaHbO), 3.82 (dt, 2J¼9.6 Hz, 3J¼1.4 Hz,
1H; CHaHbO), 3.78 (dd, 2J¼10.8 Hz, 3J¼4.3 Hz, 1H;
CHaHbOSi), 3.69 (dd, 2J¼10.8 Hz, 3J¼4.3 Hz, 1H;
Yield 75%; Rf: 0.53 (light petroleum/diethyl ether 70:30);
1
[a]2D5 ꢁ49.0 (c 2.3, CHCl3); H NMR: d¼7.28–7.18 (m,
2
CHaHbOSi), 2.35 (br s, 1H; OH), 2.06 (ddd, J¼13.4 Hz,
2H; SePh), 7.13–7.00 (m, 5H; Ph), 7.00–6.90 (m, 3H;
SePh), 4.36 (dd, J¼7.7, 5.8 Hz, 1H; CHO), 3.76 (d, J¼
3J¼8.6, 5.2 Hz, 1H; CHCHaHbCH), 1.98 (ddt, 2J¼
3
2
3
13.4 Hz, J¼6.7, 1.5 Hz, 1H; CHCHaHbCH), 1.09 (s, 9H;
16.3 Hz, 1H; CHaHbO), 3.63 (d, 2J¼16.3 Hz, 1H; CHaHbO),
CMe3); 13C NMR: d¼135.6 (4C), 133.4 (2C), 129.6 (2C),
3.42 (s, 3H; OMe), 3.18 (dd, J¼12.3 Hz, J¼7.7 Hz, 1H;
2
3
127.6 (4C), 78.3, 75.7, 72.7, 65.9, 37.2, 26.8 (3C), 19.2.
CHaHbSePh), 2.90 (dd, 2J¼12.3 Hz, 3J¼5.8 Hz, 1H;

5488
M. Tiecco et al. / Tetrahedron 63 (2007) 5482–5489
CHaHbSePh); 13C NMR: d¼170.5, 139.6, 132.5 (2C), 130.5,
129.0 (2C), 128.6 (2C), 128.5, 127.0 (2C), 126.8, 81.9, 66.0,
51.8, 34.7; MS (70 eV, EI) m/z (%): 350 (16) [M+], 179
(100), 157 (10), 121 (54), 103 (12), 77 (12). Anal. Calcd
for C17H18O3Se: C, 58.46; H, 5.19. Found: C, 58.35; H, 5.38.
(100), 91 (17), 77 (20). Anal. Calcd for C12H14O3: C,
69.88; H, 6.84. Found: C, 69.94; H, 6.70.
4.1.13.3. (3R,5R)-5-Phenyltetrahydrofuran-3-yl ace-
tate (ent-28). Yield 35%; oil; [a]2D3 +24.1 (c 1.0, CHCl3);
HPLC analysis: tR 7.8 min. NMR and MS spectra are iden-
tical to those of compound 28. Anal. Calcd for C12H14O3
(206.2): C, 69.88; H, 6.84. Found: C, 69.90; H, 6.78.
4.1.12. {[(1R)-1-Phenyl-2-(phenylseleno)ethyl]oxy}ace-
taldehyde (25). Compound 25 (0.32 g, 1 mmol) was pre-
pared according to the procedure described for 7. The
crude product was purified by column chromatography
(light petroleum/diethyl ether 90:10 to 75:25 as eluants).
Chemical yield and physical and spectral data of 25 are re-
ported below.
4.1.13.4. (3R,5S)-5-Phenyltetrahydrofuran-3-yl ace-
tate (29). Yield 49%; Rf: 0.44 (light petroleum/diethyl ether
70:30); [a]2D5 ꢁ4.8 (c 2.7, CHCl3); HPLC analysis: tR
1
13.3 min; H NMR: d¼7.37–7.25 (m, 5H; Ph), 5.43–5.39
3
(m, 1H; CHOAc), 5.08 (dd, J¼10.2, 5.6 Hz, 1H; CHO),
Yield 80%; oil; Rf: 0.20 (light petroleum/diethyl ether 70:30);
[a]1D9 ꢁ56.4 (c 2.4, CHCl3); 1H NMR: d¼9.71 (t, 3J¼1.0 Hz,
1H; CH]O), 7.70–7.50 (m, 2H; SePh), 7.50–7.10 (m, 8H;
Ph, SePh), 4.60 (dd, 3J¼8.3, 5.2 Hz, 1H; CHO), 4.03
4.36 (dd, 2J¼10.5 Hz, 3J¼4.9 Hz, 1H; CHaHbO), 3.95
2
3
4
(ddd, J¼10.5 Hz, J¼2.0 Hz, J¼0.8 Hz, 1H; CHaHbO),
2.43 (ddt, 2J¼13.8 Hz, 3J¼5.6 Hz, 3J¼4J¼0.8 Hz, 1H;
CHCHaHbCH), 2.12 (s, 3H; MeCO), 2.05 (ddd, 2J¼
2
3
3
(dd, J¼17.7 Hz, J¼1.0 Hz, 1H; OCHaHbCH]O), 3.93
13.8 Hz, J¼10.2, 5.9 Hz, 1H; CHCHaHbCH); 13C NMR:
2
3
(dd, J¼17.7 Hz, J¼1.0 Hz, 1H; OCHaHbCH]O), 3.49
d¼170.7, 141.4, 128.4 (2C), 127.6, 125.7 (2C), 79.7, 75.3,
73.7, 41.2, 21.1; MS (70 eV, EI) m/z (%): 206 (3) [M+],
146 (41), 145 (25), 117 (18), 106 (12), 105 (100), 91 (13),
77 (15). Anal. Calcd for C12H14O3: C, 69.88; H, 6.84. Found:
C, 69.60; H, 6.91.
2
3
(dd, J¼12.5 Hz, J¼8.3 Hz, 1H; CHaHbSePh), 3.20 (dd,
3
2J¼12.5 Hz, J¼5.2 Hz, 1H; CHaHbSePh); 13C NMR: d¼
200.6, 139.6, 132.6 (2C), 130.3, 129.1 (2C), 128.7 (2C),
128.6, 127.0, 126.8 (2C), 82.8, 74.5, 34.8; MS (70 eV, EI)
m/z (%): 320 (31) [M+], 183 (12), 157 (20), 149 (100), 105
(11), 104 (17), 103 (16), 91 (92), 78 (15), 77 (23). Anal. Calcd
for C16H16O2Se: C, 60.19; H, 5.05. Found: C, 60.25; H, 4.95.
4.1.13.5. (3S,5R)-5-Phenyltetrahydrofuran-3-yl ace-
tate (ent-29). Yield 44%; [a]2D5 +4.0 (c 1.3, CHCl3); HPLC
analysis: tR 9.9 min. NMR and MS spectra are identical to
those of compound 29. Anal. Calcd for C12H14O3: C,
69.88; H, 6.84. Found: C, 69.99; H, 6.93.
4.1.13. 5-Phenyltetrahydrofuran-3-ols and their corre-
sponding acetates. Compounds 26 and 27 were obtained
from the aldehyde 25 by radical cyclization reaction effected
under the reaction conditions described in the typical proce-
dure. The mixture of diastereoisomers could be separated af-
ter conversion into the corresponding acetates 28 and 29 by
treatment with acetyl chloride and pyridine in THF at 0 ꢀC
(flash chromatography, light petroleum/diethyl ether 90:10
as eluant). An identical sequence effected on (S)-2-chloro-
1-phenylethanol gave compounds ent-28 and ent-29. The
enantiomeric purity of compounds 28, 29, ent-28, and
ent-29 was identical to that of the starting products as
determined by chiral HPLC (Chiralcel OD-H column
(250ꢂ4 mm i.d.), eluant: i-PrOH/hexane 10:90, flow rate:
1.0 mL/min, UV detection at 210 nm). Chemical yields
and physical and spectral data of these compounds are
reported below.
Acknowledgements
Financial support from MIUR, National Projects PRIN2003,
PRIN2005 and FIRB2001 and Consorzio CINMPIS is grate-
fully acknowledged.
References and notes
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4.1.13.1. 5-(3S,5S)-5-Phenyltetrahydrofuran-3-ol (26)
and (3R,5S)-5-phenyltetrahydrofuran-3-ol (27).14 Yield
55%; mixture of diastereoisomers, cis/trans¼43:57; oil.
4.1.13.2. (3S,5S)-5-Phenyltetrahydrofuran-3-yl ace-
tate (28). Yield 39%; oil; Rf: 0.36 (light petroleum/diethyl
ether 70:30); [a]2D3 ꢁ23.8 (c 1.4, CHCl3); HPLC analysis:
tR 9.6 min; 1H NMR: d¼7.14–6.97 (m, 5H; Ph), 5.07
(dddd, 3J¼7.7, 4.8, 3.3, 1.8 Hz, 1H; CHOAc), 4.58 (t,
2
3
3J¼7.7 Hz, 1H; CHO), 3.87 (ddd, J¼10.6 Hz, J¼1.8 Hz,
4J¼1.0 Hz, 1H; CHaHbO), 3.68 (dd, 2J¼10.6 Hz, 3J¼
2
3
4.8 Hz, 1H; CHaHbO), 2.46 (dt, J¼13.9 Hz, J¼7.7 Hz,
2
1H; CHCHaHbCH), 1.72 (s, 3H; MeCO), 1.69 (dddd, J¼
3
4
13.9 Hz, J¼7.7, 3.3 Hz, J¼1.0 Hz, 1H; CHCHaHbCH);
13C NMR: d¼170.9, 141.6, 128.4 (2C), 127.6, 126.0 (2C),
80.3, 75.2, 73.3, 40.7, 21.0; MS (70 eV, EI) m/z (%): 206
(1) [M+], 146 (63), 145 (41), 117 (34), 106 (12), 105
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