Diastereoselective oxidation of β-hydroxysulfides with TBHP: A comparative study of titanocenes and Ti(Oi-Pr)4 as catalysts

Article abstract of DOI:10.1016/S0040-4020(02)00686-5

Titanocene dichlorides and Ti(Oi-Pr)₄ have been used as catalysts for the oxidation with t-butyl hydroperoxide of racemic β-hydroxysulfides having a stereogenic carbon centre in α- or β- position with respect to the sulfur atom. β-Hydroxysulfides were oxidized in high yields to the corresponding sulfoxides, titanocenes being more chemo- and generally more diastereoselective than Ti(Oi-Pr)₄. A divergent stereochemical outcome has been observed when using titanocenes or Ti(Oi-Pr)₄ in the oxidation of protected β-hydroxy sulfides.

Full text of DOI:10.1016/S0040-4020(02)00686-5

Tetrahedron 58 (2002) 6679–6683
Diastereoselective oxidation of b-hydroxysulfides with TBHP:
a comparative study of titanocenes and Ti(Oi-Pr)4 as catalysts
*
Giorgio Della Sala, Stefania Labano, Alessandra Lattanzi and Arrigo Scettri
*
`
Dipartimento di Chimica, Universita degli Studi di Salerno, 84081 Baronissi, Salerno, Italy
Received 12 April 2002; revised 29 May 2002; accepted 20 June 2002
Abstract—Titanocene dichlorides and Ti(Oi-Pr)4 have been used as catalysts for the oxidation with t-butyl hydroperoxide of racemic
b-hydroxysulfides having a stereogenic carbon centre in a- or b- position with respect to the sulfur atom. b-Hydroxysulfides were oxidized in
high yields to the corresponding sulfoxides, titanocenes being more chemo- and generally more diastereoselective than Ti(Oi-Pr)4. A
divergent stereochemical outcome has been observed when using titanocenes or Ti(Oi-Pr)4 in the oxidation of protected b-hydroxy
sulfides. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
acidities, coordinating properties as well as steric factors of
these Ti(IV) catalysts. Furthermore, there have been no
reports on the use of Ti(Oi-Pr)4 as catalyst for this oxidation.
The main employment of metallocenes as catalysts in
organic synthesis concerns C–C bond forming reactions.1
Only some examples of their use, with little success, have
been reported in the epoxidation of alkenes.2 Indeed,
recently we have found that metallocenes are useful
catalysts in various oxidative processes such as the
epoxidation of allylic alcohols,3 the oxidative cyclization
of bishomoallylic alcohols4 and the sulfoxidation of
prochiral sulfides.5
2. Results and discussion
Two titanocene dichlorides: (bis-cyclopentadienyl titanium
b-Hydroxysulfoxides can be prepared via aldol-type reac-
tions with variable diastereoselectivity,6 via reduction of
b-ketosulfoxides with generally high diastereoisomeric
ratio.7 As regards metal-catalyzed oxidative processes
only two methods have been reported in the literature:
VO(acac)2/t-butyl hydroperoxide (TBHP)8 and Ti(Oi-Pr)4/
(þ)-DET/TBHP9 systems. In the vanadium catalyzed
transformation the starting compounds were optically pure
products, while in the modified Sharpless Ti(IV) system
both the diastereo- and the enatioselectivity of the oxidation
were studied.
On the grounds of literature findings and on the basis of the
encouraging results on the catalytic activity of titanocenes,
we decided to examine the employment of titanocene
dichlorides and Ti(Oi-Pr)4 as promoters in the oxidation of
racemic b-hydroxysulfides using TBHP as oxidant. We
thought it would have been interesting to compare the
activity of the well-known Ti(Oi-Pr)4 with the titanocene
dichlorides, especially on the basis of the different Lewis
Figure 1.
Keywords: diastereoselection; oxidation; titanium and compounds;
b-hydroxysulfoxides.
*
Corresponding authors. Tel.: þ39-89-965-374; fax: þ39-89-965-296;
Scheme 1.
e-mail: lattanzi@unisa.it
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00686-5

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G. D. Sala et al. / Tetrahedron 58 (2002) 6679–6683
Table 1. Ti(IV)-catalyzed oxidation of 1 by 2/TBHP systems
Entry
Catalyst (mol%)
1
R
R1
R2
T (8C)
t (h)
Yield (%)a
drb 3/4
1
2
2a (1)
2a (1)
2b (5)
2c (5)
2a (1)
2b (5)
2c (5)
2a (1)
2b (5)
2c (5)
1a
1a
1a
1a
1b
1b
1b
1c
1c
1c
p-Tol
p-Tol
p-Tol
p-Tol
Me
Me
Me
Me
Me
Me
Me
Me
Me
Ph
Ph
Ph
H
H
H
H
H
H
H
H
Ph
Ph
Ph
0
25
29
29
29
29
29
29
24
24
24
75c
81
57
78e
97
85
95
88
95
80
58/42
57/43
62/38
60/40
59/41
74/26
65/35
88/12
81/19
81/19
220
220
220
220
220
220
220
220
220
3
4d
5
6
7d
8
9
10d
H
H
Me
˚
Molar ratio employed: 1a/TBHP/1/1.2. 4 A MS were added (3.5 g/mmol of titanocene catalyst).
Isolated yield of both sulfoxide diastereoisomers.
a
b
The diastereoisomeric ratios were determined by 1H NMR analysis.
18% of the corresponding sulfone was detected.
˚
In this reaction activated 4 A MS were not added.
6% of the corresponding sulfone was detected.
c
d
e
dichloride) Cp2TiCl2 2a and [(^)ethylen-bis-(4,5,6,7-tetra-
hydro-1-indenyl)dichlorotitanium(IV)] rac-(EBTHI)2TiCl2
2b (Fig. 1) were chosen and studied in catalytic loadings
(1–5 mol%) in the presence of activated 4 A MS in the
oxidation of compounds 1a–c (Scheme 1).
to a similar level of the diastereoselectivity obtained in the
oxidation of 1a (anti/syn 59/41).
10
˚
Titanocene 2b improved the anti/syn ratio of the product
(entry 6), which was isolated in very good yield, again
proving to be the more diastereoselective (anti/syn 74/26)
catalyst among the three and when using Ti(Oi-Pr)4 the
sulfoxide was isolated in high yield but with lower
stereoselecivity (entry 7). Surprisingly, with the Ti(Oi-Pr)4/
(þ)-DET/TBHP9 system, 1b was oxidized, under the same
reaction conditions (CH2Cl2 at 2208C), but with stoichio-
metric metal loadings, in very poor yield (20% yield) and
with a comparable diastereisomeric ratio (anti/syn 68/32).
The presence of the optically pure ligand [(þ)-DET]
lowered the reaction rate and did not produce any positive
influence on the diastereo- and the enantioselectivity of the
process; in fact the two diastereoisomers proved to be almost
racemic products.
Moreover, a comparative study of their efficiency was
carried out using Ti(Oi-Pr)4 2c as promoter in the same
process (Scheme 1, Table 1). Di Furia and Modena9
reported that almost no diastereoselectivity is achievable
when the asymmetric carbon atom is located in the b
position with respect to the sulfur atom, while a pronounced
improvement is observed when the stereogenic carbon
centre is placed a to the sulfur atom.
Compound 1a was initially subjected to the optimized
conditions utilized in the oxidation of 2-substituted
1,3-dithianes and 1,3-dithiolanes,5b employing Cp2TiCl2
10
˚
in 1 mol% amount at 08C in the presence of 4 A MS; the
corresponding sulfoxide11 was isolated in good yield and
with poor anti/syn ratio. The overoxidation to the corre-
sponding sulfone was found to take place in these conditions
in 18% yield. In order to improve the chemoselectivity, the
reaction was carried out at the lower temperature of 2208C
and under these conditions, the sulfoxide was isolated in
higher yield (81%) and with comparable diastereoisomeric
ratio (entry 2).
When the stereogenic carbon centre was placed a to the
sulfur atom, as in compound 1c, the diastereoselectivity
markedly increased, confirming that the stereochemical
recognition is amplified.9 Cp2TiCl2 was the best catalyst in
terms of the diastereoselective ratio observed (anti/syn 88/
12) (entry 8) and the product15 was recovered in high yield.
2b and 2c proved to be very efficient (especially the
titanocene 2b, entry 9) but less diastereoselective than 2a
(entries 9 and 10).
When employing catalyst 2b, the product was obtained in
lower yield,12 but with slightly better diastereoselectivity
(entry 3). Ti(Oi-Pr)4 turned out to be more a reactive
catalyst13 (entry 4); the sulfoxide was produced with
lower chemoselectivity (sulfone was detected at the
end of the reaction) and with a comparable level of
diastereoselectivity.
Then, we thought it would be interesting to study the
sulfoxidation of the hydroxy protected sulfides 5, in order to
investigate if the presence of the hydroxy group could
influence the diastereoselectivity of the oxidation (Scheme
2, Table 2). Compounds 1, protected as t-butyldimethylsilyl
ethers 5, were less reactive than 1 and so 5 mol% of
Cp2TiCl2 catalyst was employed.16
It has to be noted that the optically pure compound [(S)-1-p-
tolylsulfanyl propan-2-ol 1a] was converted under the same
reaction conditions,8 using VO(acac)2/TBHP at 2208C in
CH2Cl2, to the sulfoxide in a modest 51% yield and with a
slightly better anti/syn 66/34 ratio.
5a was oxidized by titanocene 2a in very good yield (Table
2, entry 1), with a low anti/syn ratio, but the opposite
preference for the syn diastereoisomer 6a17 was observed
with respect to the reaction carried out on the unprotected
b-hydroxysulfoxide 1a (Table 1, entry 1).
When the methyl and aryl groups were inverted in the
starting material 1b, a complete conversion to the final
sulfoxide14 was observed with Cp2TiCl2 (entry 5), leading
The more sterically demanding catalyst 2b reacted very
slowly, even at 08C, with almost no diastereoselectivity

G. D. Sala et al. / Tetrahedron 58 (2002) 6679–6683
6681
oxidation was rather different from Ti(Oi-Pr)4/
(þ)-DET/TBHP system;9 in fact, in the oxidation of
protected b-hydroxysulfides with the chiral Ti(IV) catalyst,
irrespective to the protecting group employed, a lower anti/
syn ratio with respect to the oxidation of the unprotected
b-hydroxysulfides was always found.
In conclusion, we have discovered a synthetically useful
catalytic activity of titanocenes and set up a methodology of
diastereoselective oxidation of b-hydroxysulfides, which
compares favorably with the other available metal-
catalyzed procedures.8,9 Catalytic loadings of titanocene
dichlorides are needed (1–5 mol%) to chemoselectively
produce the final b-hydroxysulfoxides in high yields. The
diastereoselectivity reached is comparable and in some
cases better than the previous metal-catalyzed oxidations
and showed to be dependent on the structure of the starting
sulfide and position of the stereogenic carbon centre. A
reversed stereochemical preference for the syn products has
been observed in the oxidation of hydroxy protected
sulfides. From this study it emerged that Ti(Oi-Pr)4, without
the presence of any chiral ligand, is a suitable catalyst for
the diastereoselective oxidation of b-hydroxysulfides, being
more reactive but less chemoselective than titanocenes.
The stereochemical preference for the anti sulfoxides is
maintained in the oxidation of unprotected and protected
b-hydroxysulfides.
Scheme 2.
Table 2. Ti(IV)-catalyzed oxidation of 5 by 2/TBHP systems at 2208C
Entry Catalyst
(mol%)
5
R
R1 R2 t (h) Yield (%)a drb 6/7
1
2
3
4
5
6
7
8
9
10
2a (5)
2b (5)
2b (5)
2c (5)
2a (5)
2b (5)
2c (5)
2a (5)
2b (5)
2c (5)
5a p-Tol Me
5a p-Tol Me
5a p-Tol Me
5a p-Tol Me
5b Me
5b Me
5b Me
5c Me
5c Me
5c Me
H
H
H
H
H
H
H
Ph
Ph
Ph
30
48
24
24
24
48
5
24
24
5
80
34
44/56
53/47
54/46
72/28
46/54
49/51
58/42
77/23
79/21
85/15
45c
70d,e
89
Ph
Ph
Ph
H
H
H
64
85d,f
83
48
72d,e
˚
Molar ratio employed: 5/TBHP/1/1.2. 4 A MS were added (3.5 g/mmol of
titanocene catalyst).
a
Isolated yield of both sulfoxide diastereoisomers.
The diastereoisomeric ratios were determined by H NMR analysis; the
b
1
syn and anti diastereoisomers were identified after desilylation by
comparison with the spectroscopic data of 3/4.
This reaction was carried out at 08C.
In this reaction activated 4 A MS were not added.
8% of sulfone was detected.
10% of sulfone was detected.
c
d
e
f
3. Experimental
3.1. General
˚
(entries 2 and 3). Ti(Oi-Pr)4 2c showed to be less
chemoselective, but a good diastereoisomeric ratio (anti/syn
72/28) was achieved, better than the one determined on the
unprotected sulfoxides 3/4a (Table 1, entry 4).
All reactions were carried out under a dry nitrogen
atmosphere. Glassware was flame-dried (0.05 Torr) before
use. Catalysts 2a and 2c were purchased from Aldrich and
used without further purification. Titanocene 2b was
purchased from Boulder Scientific. Dichloromethane was
used freshly distilled. Reactions were monitored by thin
layer chromatography (TLC) on Merck silica gel plates
(0.25 mm) and visualized by UV light or by 10% H2SO4/
ethanol spray test. Flash chromatography was performed on
Merck silica gel (60, particle size: 0.040–0.063 mm).
NMR spectra were recorded in CDCl3 solutions on a Bruker
DRX 400 spectrometer (400 MHz) at room temperature.
Chemical shifts are reported relative to the residual solvent
peak (CHCl3: dH¼7.26). EIMS spectra were performed on
Polaris Trace 2000 spectrometer. IR spectra were recorded
with a Bruker Vector 22 spectrophotometer.
In the case of compound 5b titanocenes 2a and 2b (entries 5
and 6) gave a slight preference for the syn diastereoisomer,
while 2c was found to be a more reactive catalyst furnishing
a not negligible amount of sulfone and maintaining the anti
preference for the oxidation.
In the last example, catalysts 2a and 2b, furnished the
protected sulfoxides 6/7c with reduced anti/syn ratio
(entries 8 and 9) compared to the results obtained in entries
8 and 9 in Table 1. On the contrary, with catalyst 2c, the
product was synthesized with a diastereoselective ratio
(anti/syn 85/15) higher than the one measured for the 3/4
mixture (Table 1, entry 10).
Yields of products 3/4 and 6/7 refer to isolated pure
compounds. Compound 1a was synthesized according to
Ref. 8; compounds 1b and 1c were synthesized according to
Ref. 9b.
From these findings, a different stereochemical outcome
emerged in the Ti(IV) catalyzed oxidation of protected
sulfides 5 with respect to unprotected sulfides 1.
anti/syn ratios of b-hydroxysulfoxides 3/4 were determined
by integrating the characteristic peaks of the two dia-
stereomers in the H NMR spectra of the crude reaction
mixtures.
In the oxidation of sulfides 5 with titanocenes 2a and 2b, a
general preference for the syn diastereoisomer or a
decreased anti/syn ratio was observed in the product.
Ti(Oi-Pr)4 maintained the anti selectivity in both oxidations
and furnished in some cases a better anti/syn ratio for the
final protected sulfoxides with respect to 3/4 products. This
stereochemical outcome for the Ti(Oi-Pr)4 catalyzed
1
3.2. General procedure for the silylation of 1
To dry CH2Cl2 (9 mL) under argon atmosphere at 08C was

6682
G. D. Sala et al. / Tetrahedron 58 (2002) 6679–6683
added 1 (6.4 mmol), tert-butyldimethylsilyl chloride
(1.26 g, 8.4 mmol) and imidazole (572 mg, 8.4 mmol),
with stirring. The mixture was stirred until completion of
the reaction by TLC monitoring. Then, water (9 mL) was
added to the reaction mixture and stirring was maintained
for a few minutes. The reaction mixture was extracted with
CH2Cl2 (3£20 mL) and the organic phase was dried with
MgSO4 and after filtration the solvent was removed in
vacuo.
CHCl3 to CHCl3/CH3OH 96/4 mixtures, giving compounds
6/7a–c in yields reported in Table 2.
3.3.1. (syn/anti ) tert-Butyl-(2-methanesulfinyl-1-phenyl-
ethoxy)-dimethyl-silane 6b/7b. Colorless oil. 1H NMR
(400 MHz, CDCl3) d 20.15 (s, 3H, syn ), 20.14 (s, 3H,
anti ), 0.07 (s, 3H, syn ), 0.11 (s, 3H, anti ), 0.88 (s, 9H, syn ),
0.89 (s, 9H, anti ), 2.58 (s, 3H, anti ), 2.60 (s, 3H, syn ), 2.87
(dd, 1H, J¼12.8, 2.4 Hz, anti ), 2.97 (dd, 1H, J¼12.8,
6.8 Hz, syn ), 2.97 (dd, 1H, J¼12.8, 10.6 Hz, anti ), 3.27 (dd,
1H, J¼12.8, 6.1 Hz, syn ), 5.12 (dd, 1H, J¼6.8, 6.1 Hz,
syn ), 5.21 (dd, 1H, J¼10.6, 2.4 Hz, anti ), 7.27–7.40 (m,
5H). IR nmax (KBr): 2928, 1471, 1250, 1105, 1028,
735 cm21. Anal. Calcd for C15H26O2SSi: C, 60.35; H,
8.78, S, 10.74. Found: C, 60.21; H, 8.67, S, 10.66.
3.2.1. tert-Butyl-dimethyl-(1-methyl-2-p-tolylsulfanyl-
ethoxy)-silane 5a. Purification by column chromatography
on silica gel (petrol/diethyl ether 98/2) gave 5a in 72% yield
as colorless oil. 1H NMR (400 MHz, CDCl3) d 0.02 (s, 3H),
0.04 (s, 3H), 0.87 (s, 9H), 1.24 (d, 3H, J¼6.1 Hz), 2.31 (s,
3H), 2.84 (dd, 1H, J¼12.9, 6.8 Hz), 3.02 (dd, 1H, J¼12.9,
5.4 Hz), 3.92 (m, 1H), 7.08 (d, 2H, J¼8.2 Hz), 7.25 (d, 2H,
J¼8.2 Hz). 13C NMR (100 MHz, CDCl3) d 24.8, 24.7,
18.0, 20.9, 23.0, 25.8, 43.3, 67.9, 129.5, 129.6, 133.3, 135.6.
IR nmax (KBr): 2930, 1478, 1235, 1060, 830 cm21. Anal.
Calcd for C16H28OSSi: C, 64.80; H, 9.52, S, 10.81. Found:
C, 64.61; H, 9.43, S, 10.74.
3.3.2. (anti/syn ) tert-Butyl-(2-methanesulfinyl-2-phenyl-
ethoxy)-dimethyl-silane 6c/7c. Colorless oil. 1H NMR
(400 MHz, CDCl3) d 20.04 (s, 3H, anti), 20.02 (s, 3H,
anti ), 0.03 (s, 3H, syn ), 0.07 (s, 3H, syn ), 0.82 (s, 9H, anti ),
0.86 (s, 3H, syn ), 2.15 (s, 3H, syn ), 2.28 (s, 3H, anti ), 3.55
(dd, 1H, J¼9.4, 5.0 Hz, syn ), 3.67 (dd, 1H, J¼5.2, 3.8 Hz,
anti ), 4.02 (dd, 1H, J¼10.8, 5.0 Hz, syn ), 4.12 (dd, 1H,
J¼10.6, 3.1 Hz, anti ), 4.30 (m, 1H, overlapped, syn ), 4.34
(dd, 1H, J¼10.5, 5.1 Hz, anti ), 7.26–7.42 (m, 10H). 13C
NMR (400 MHz, CDCl3) d 25.8, 25.8, 18.1, 25.7, 35.4,
36.7, 61.1, 61.6, 68.6, 72.1, 128.4, 128.5, 128.7, 129.0,
129.4, 133.9. IR nmax (KBr): 2928, 1471, 1256, 1109, 1034,
837, 778 cm21. EIMS m/z: 299 (Mþ1, 12), 235 (24), 179
(52), 177 (33), 161 (29), 121 (21), 105 (27), 75 (44), 73
(100). Anal. Calcd for C15H26O2SSi: C, 60.35; H, 8.78, S,
10.74. Found: C, 60.20; H, 8.68, S, 10.65.
3.2.2. tert-Butyl-dimethyl-(2-methylsulfanyl-1-phenyl-
ethoxy)-silane 5b. Purification by column chromatography
on silica gel (petrol/diethyl ether 96/4) gave 5b in 71% yield
1
as colorless oil. H NMR (400 MHz, CDCl3) d 20.11 (s,
3H), 0.07 (s, 3H), 0.88 (s, 9H), 2.03 (s, 3H), 2.68 (dd, 1H,
J¼13.4, 5.3 Hz), 2.83 (dd, 1H, J¼13.4, 7.3 Hz), 4.78
(dd, 1H, J¼7.3, 5.3 Hz), 7.22–7.36 (5H, m). 13C NMR
(100 MHz, CDCl3) d 24.9, 24.7, 16.6, 18.2, 25.8, 44.8,
75.1, 126.0, 127.4, 128.1, 144.0. IR nmax (KBr): 2929, 1476,
1237, 1065, 730 cm21. Anal. Calcd for C15H26OSSi: C,
63.77; H, 9.28, S, 11.35. Found: C, 63.59; H, 9.18, S, 11.43.
3.4. Deprotection of silylated 6/7
3.2.3. tert-Butyl-dimethyl-(2-methylsulfanyl-2-phenyl-
ethoxy)-silane 5c. Purification by column chromatography
on silica gel (petrol/diethyl ether 98/2) gave 5c in 88% yield
To dry THF (3 mL) under argon atmosphere at rt were
added: the 6/7 mixture (0.420 mmol), then TBAF
(0.840 mL, 0.840 mmol, 1 M in THF solution), with
stirring. The mixture was stirred until TLC monitoring
showed the reaction to be complete. Then, the reaction was
quenched with water and the mixture was extracted with
CH2Cl2 (3£40 mL). The organic phase was dried with
MgSO4 and the solvent removed in vacuo. The 1H NMR of
the crude mixtures matched with the reported spectral data
of anti/syn 3/4 isomers.
1
as colorless oil. H NMR (400 MHz, CDCl3) d 20.07 (s,
3H), 20.02 (s, 3H), 0.83 (s, 9H), 1.98 (s, 3H), 3.29–3.82
(m, 3H), 7.21–7.35 (m, 5H). 13C NMR (100 MHz, CDCl3) d
25.5, 14.6, 18.2, 25.8, 53.9, 67.2, 127.1, 128.2, 140.1. IR
n
max (KBr): 2929, 1468, 1252, 1072, 770 cm21. Anal. Calcd
for C15H26OSSi: C, 63.77; H, 9.28, S, 11.35. Found: C,
63.64; H, 9.20, S, 11.40.
3.3. General procedure for the oxidation of 1 and 5
catalyzed by 2
Acknowledgments
To dry CH2Cl2 (4 mL) under argon atmosphere at rt was
˚
added: activated 4 A MS (3.5 g/mmol of titanocene
`
Ministero dell’Universita e Ricerca Scientifica e Tecno-
logica (MIUR) is gratefully acknowledged for financial
support.
catalyst), 2a, 2b or 2c (1 or 5 mol%) and TBHP (220 mL,
1.2 mmol, 5.5 M solution in decane), with stirring. Stirring
was maintained for 30 min, then the temperature was
lowered to the reported value in Tables 1 and 2, sulfide 1 and
5 (1 mmol) was added and stirring was maintained for the
time reported in Tables 1 and 2. At the end of the reaction,
the mixture was filtered to remove molecular sieves and
extracted with a saturated solution of Na2SO3. The organic
phase was then treated with MgSO4 and the solvent
removed in vacuo. When using 2c no molecular sieves
were added. The crude reaction mixtures were purified by
silica gel column chromatography eluting starting with
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See Ref. 9b.
15. Spectral data of compounds 3/4c matched with the literature:
Alcudia, F.; Brunet, E.; Sanchez, F. An. Quim. 1979, 75, 162.
16. It has been reported that b-hydroxysulfides are more reactive
compared to simple sulfides in the VO(acac)2/TBHP catalyzed
sulfoxidation. The hydroxy group of the sulfide might
be involved in the transition state thus facilitating the
1,2-vanadium shift process accompanying the peroxide
oxygen transfer to the sulfur atom. See Bortolini, O.; Di
Furia, F.; Modena, G. J. Mol. Catal. 1983, 19, 319.
17. Spectral data of compounds 6/7a matched with the literature:
Kita, Y.; Shibata, N.; Yoshida, N.; Fujita, S. J. Chem. Soc.,
Perkins Trans. 1 1994, 22, 3335.
7. (a) Annunziata, R.; Cinquini, M.; Cozzi, F. J. Chem. Soc.,
´
Perkin Trans. 1 1979, 1687. (b) Solladie, G.; Demailly, G.;
´
Greck, C. J. Org. Chem. 1985, 50, 1552. (c) Solladie, G.;
Frechou, C.; Demailly, G.; Greck, C. J. Org. Chem. 1986, 51,
1912.
8. Breitschuh, R.; Seebach, D. Synthesis 1992, 1170.
9. (a) Conte, V.; Di Furia, F.; Licini, G.; Modena, G. Tetrahedron
Lett. 1989, 30, 4859. (b) Conte, V.; Di Furia, F.; Licini, G.;
Modena, G.; Sbampato, G.; Valle, G. Tetrahedron: Asymmetry
1991, 2, 257.