Enantioselective and Diastereoselective Additions of Allylic Stannanes to Aldehydes Promoted by a Chiral (Acyloxy)borane Catalyst

Article abstract of DOI:10.1021/jo980145c

A modified Yamamoto Lewis acid (CAB), prepared from the 2,6-dimethoxybenzoic ester of (R,R)- tartaric acid, and 1.5 equiv of BH₃-THF was employed in additions of crotyltributyltin (6) and allyltributyltin (9) to representative achiral aldehydes in the presence of 2 equiv of (CF₃CO)₂O. The crotyltin additions proceeded with good to excellent diastereoselectivity and enantioselectivity affording the syn adducts 7a-e of 70-90% ee as major products (78:22-92:8). Addition of allylstannane 9 to cyclohexanecarboxaldehyde (1a) afforded the (R)-adduct of only 55% ee. In contrast, the use of Keek's BINOL catalyst gave 10, the allyl adduct of 1a, of 87% ee. However, addition of the crotylstannane to 1a with this catalyst led to a 65:35 mixture of syn and anti adducts 7a and 8a of 95% and 49% ee. Additions of crotylstannane 6 to (R)- and (S)-2-methyl-3-ODPS- propanal [(R)-11 and (S)-11] promoted by the modified CAB Lewis acid afforded the syn,syn and syn,anti products 12 and 14 in large predominance (98:2 and 90:10), indicative of effective complex control in the transition state. The results are consistent with the Corey H-bonded aldehyde transition-state proposal.

Full text of DOI:10.1021/jo980145c

J . Org. Chem. 1998, 63, 4381-4384
4381
En a n tioselective a n d Dia ster eoselective Ad d ition s of Allylic
Sta n n a n es to Ald eh yd es P r om oted by a Ch ir a l (Acyloxy)bor a n e
Ca ta lyst
J ames A. Marshall* and Michael R. Palovich
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901
Received J anuary 27, 1998
A modified Yamamoto Lewis acid (CAB), prepared from the 2,6-dimethoxybenzoic ester of (R,R)-
tartaric acid, and 1.5 equiv of BH3‚THF was employed in additions of crotyltributyltin (6) and
allyltributyltin (9) to representative achiral aldehydes in the presence of 2 equiv of (CF3CO)2O.
The crotyltin additions proceeded with good to excellent diastereoselectivity and enantioselectivity
affording the syn adducts 7a -e of 70-90% ee as major products (78:22-92:8). Addition of
allylstannane 9 to cyclohexanecarboxaldehyde (1a ) afforded the (R)-adduct of only 55% ee. In
contrast, the use of Keck’s BINOL catalyst gave 10, the allyl adduct of 1a , of 87% ee. However,
addition of the crotylstannane to 1a with this catalyst led to a 65:35 mixture of syn and anti adducts
7a and 8a of 95% and 49% ee. Additions of crotylstannane 6 to (R)- and (S)-2-methyl-3-ODPS-
propanal [(R)-11 and (S)-11] promoted by the modified CAB Lewis acid afforded the syn,syn and
syn,anti products 12 and 14 in large predominance (98:2 and 90:10), indicative of effective complex
control in the transition state. The results are consistent with the Corey H-bonded aldehyde
transition-state proposal.
Ta ble 1. Op tim iza tion of Cr otylsta n n a n e Ad d ition s to
Cycloh exa n eca r boxa ld eh yd e in th e P r esen ce of CAB 5
a n d (CF 3CO)2O
Some years ago, we reported on additions of allylic
stannane 2 to various aldehydes in the presence of the
chiral (acyloxy)borane (CAB) 5, first described by Yama-
moto (eq 1).1,2 In the interim, a number of studies on
4, equiv
BH3:4
6:1a
yield, %
7a :8a
ee, % (7a )
1.0
1.0
0.5
0.5
0.2
0.5
1:1
1:1
1:1
1.5:1
1.5:1
1.5:1
1:1
62
73
55
70
<50
70
90:10
93:7
92:8
92:8
91:9
93:7
96a
90a
91a
93a
NDc
91d
1:1b
1:1b
1:1b
1:1b
2:1b
a
b
Determined by GC analysis. Stannane added over 3 h by
syringe pump. c Not determined; sample contained impurities.
d
Estimated from the optical rotation of entry 3.
additions of allylic and allenylstannanes catalyzed by
metal complexes of BINOL and BINAP have appeared.3
Results with crotylstannanes were reported in only one
of these.4 In that investigation, addition of cis- and trans-
crotyltributyltin to methyl glyoxalate afforded mixtures
of syn and anti adducts of modest ee in relatively low
yield (38% and 53%, respectively).
In the course of developmental work on the synthesis
of polypropionate natural products, we had occasion to
examine the use of chiral catalysts for additions of
crotylstannanes to aldehydes. We were especially inter-
ested in employing the Keck binaphthol catalyst5 for
these additions, but our previous experience suggested
that such reactions might be too slow for practical
applications, especially with hindered branched alde-
hydes. Accordingly, we decided to conduct a limited
survey of the CAB and Keck BINOL methodology with
crotyltributyltin. We began by examining a series of
additions to cyclohexanecarboxaldehyde (1a ) with CAB
5 as the promoter/catalyst for the purpose of optimizing
reaction conditions (Table 1). We had previously discov-
ered that trifluoroacetic anhydride improved reaction
efficiency, although best results were obtained with a
stoichiometric amount of the (acyloxy)borane 5.1 In the
present study, we identified several modifications that
allowed the use of 50 mol % of the tartrate precursor 4.
These include (1) slow addition of the crotylstannanes
by means of a syringe pump and (2) the use of a 50%
excess of BH3‚THF. In all cases, the syn adduct 7a of at
least 90% ee predominated by 9:1 or better.
(1) Marshall, J . A.; Tang, Y. Synlett 1992, 653.
(2) Furuta, K.; Mouri, M.; Yamamoto, H. Synlett 1991, 561.
(3) For a recent review of these additions, see: Marshall, J . A.
Chemtracts-Org. Chem. 1996, 9, 280.
(4) Aoki, S.; Mikami, L.; Terada, M.; Nakai, T. Tetrahedron 1993,
49, 1783.
We next proceeded to compare these results with
additions performed with Keck’s BINOL-Ti(O-i-Pr)4
(5) Keck, G. E.; Tarbet, K. H.; Geraci, L. S. J . Am. Chem. Soc. 1993,
115, 8467. Keck, G. E.; Geraci, L. S. Tetrahedron Lett. 1993, 34, 7827.
S0022-3263(98)00145-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/11/1998

4382 J . Org. Chem., Vol. 63, No. 13, 1998
Marshall and Palovich
Ta ble 2. Com p a r a tive BINOL- a n d CAB-P r om oted
Ad d ition s of Sta n n a n es 6 a n d 9 to
Cycloh exa n eca r boxa ld eh yd e
R
cat.
yield, %
7 (ee):8 (ee)
Me (6)
Me (6)
H (9)
BINOLa
CAB
18
71
53
42
65 (95):35 (49)
93 (93):7 (80)
10 (87)
BINOLa
CABb
H (9)
10 (55)
a
20 mol % (M)-BINOL, 10 mol % Ti(O-i-Pr)4, CH2Cl2, 4 A MS,
b
-20 °C, 70 h. Same as Table 3.
F igu r e 1. Chem 3D representation of a possible CAB complex
with aldehydes (R)- and (S)-11.
Ta ble 3. CAB-P r om oted Ad d ition of Cr otyl Sta n n a n e 6
to Ach ir a l Ald eh yd es
Assignment of configuration to adducts 7a -e is based
on our previous findings (eq 1).1 Independent verification
1
came from H NMR analysis of the O-methylmandelate
derivatives of adducts 7a ,b,e.7 Furthermore, the optical
rotation of 7a was equal and opposite to that reported
for the enantiomer.8
R1
yield,a
%
7:8b,c
ee 7,a
%
As an initial application of the CAB/crotylstannane
methodology for assemblage of polypropionate subunits,
we examined additions to the nonracemic R-methyl
â-ODPS aldehydes (R)- and (S)-11 (eq 2, DPS ) Ph2-t-
BuSi). Keck and Abbott have carried out the BF3-
promoted addition of crotylstannane 6 to racemic alde-
hyde 11.6 They obtained a 90:10 mixture of syn,syn (rac-
12) and syn,anti (rac-14) products. Roush and co-workers
obtained a roughly 2.5:1 mixture of syn,syn (ent-12) and
syn,anti (14) products along with small amounts of the
corresponding anti adducts upon addition of their (R,R)-
tartrate (Z)-crotylboronate to aldehyde (S)-11.9 The (S,S)
enantiomer gave a 7:1 mixture favoring 14 over ent-12.
We repeated the BF3 reaction with aldehyde (R)-11 and
obtained a 90:10 mixture of adducts 12 and ent-14.
Aldehydes (R)- and (S)-11 proved less reactive than 1a -e
in the CAB-promoted additions. No reaction was ob-
served at -78 °C under the conditions described in Table
3. When the reaction temperature was increased to -10
°C after an initial period at -78 °C, the adduct 12 from
aldehyde (R)-11 was secured as a 98:2 mixture of dia-
stereomers 12/ent-14 in 53% yield. An increase in the
amount of CAB employed to a full equivalent improved
the yield to 68%.
CAB-promoted addition of crotylstannane 6 to alde-
hyde (S)-11 afforded a 90:10 mixture of adducts favoring
the syn,anti diastereomer 14. This adduct was the minor
isomer from the BF3-promoted addition of crotylstannane
6 to aldehyde (S)-11. Approximate ratios of diastereo-
mers could be estimated from the 1H NMR spectra of
reaction mixtures. A more exact ratio was secured by
GC analysis of the derived diols 13 and 15 (TBAF). Care
was taken to avoid separation of the diastereomeric diols
during removal of the silyl byproducts on silica gel.
On the basis of the results summarized in eq 2, we
conclude that the CAB-promoted additions are strongly
c-C6H11 (1a )
C6H13 (1b)
(E)-PrCHdCH (1c)
C6H13CtC (1d )
DPSOCH2CH2 (1e)
70
74
71
70d
73
92:8 (97:3)
91
92
89
71
70e
91:9 (88:12)
78:22 (86:14)
79:21 (79:21)
88:12 (94:6)e
a
Ratio of tartrate:BH3:stannane:aldehyde:(CF3CO)2O ) 0.5:
b
0.75:1.0:1.0:2.0. Determined by GC analysis on an R-DEX or
â-DEX cyclodextrin GC column. c Ratios in parentheses are from
d
additions employing BF3‚OEt2 as the Lewis acid. The reaction
was complete after 6 h. e The analysis was performed on the diol
after cleavage of the DPS ether.
catalyst (Table 2).5 Parallel experiments were conducted
with cyclohexanecarboxaldehyde (1a ) and the crotyl- and
allylstannanes 6 and 9. The latter reaction was reported
to proceed in 95% yield to afford adduct 10 of 92% ee.5
Under nonoptimized conditions, we obtained 10 of 87%
ee in 53% yield after a reaction time of 70 h. The addition
of crotylstannane 6 afforded a 65:35 mixture of syn and
anti adducts in less than 20% yield under these reaction
conditions. With the CAB catalyst, this addition gave
adduct 7a of 93% ee along with 8a of 80% ee as a 92:8
mixture in 71% yield after a reaction time of 10 h.
Addition of allylstannane 9 to aldehyde 1a by this
procedure was less enantioselective, affording adduct 10
of 55% ee in 35% yield.
In view of these findings, we decided to continue our
studies with the CAB catalyst 5, crotylstannane 6, and
various aldehydes under the optimized conditions (Table
3). In all cases, syn/anti product ratios were comparable
to or only slightly lower than those obtained from
additions promoted by BF3‚OEt2. TLC analysis of reac-
tions in progress indicated that mixtures of alcohols and
the derived trifluoroacetates were formed. The esters
were cleaved in the workup by brief treatment with
methanolic K2CO3. Those reactions that produced the
smallest amounts of trifluoroacetates afforded adducts
of lowest ee and vice versa. Although quantification of
this phenomenon was beyond the scope of this study,
these observations suggest that trifluoroacetic anhydride
is assisting catalyst turnover.
(7) Trost, B. M.; Belletire, J . L.; Godleski, S.; McDougal, P. G.;
Balkovec, J . M.; Baldwin, J . J .; Christy, M. E.; Ponticello, G. S.; Varga,
S. L.; Springer, J . D. J . Org. Chem. 1986, 51, 2370.
(8) Roush, W. R.; Ando, K.: Powers, D. B.; Palkowitz, A. D.;
Halterman, R. L. J . Am. Chem. Soc. 1990, 112, 6339.
(9) Roush, W. R.; Palkowitz, A. D.; Ando, K. J . Am. Chem. Soc. 1990,
112, 6339.
(6) Keck, G. E.; Abbott, D. E. Tetrahedron Lett. 1984, 25, 1883.

Additions of Allylic Stannanes to Aldehydes
J . Org. Chem., Vol. 63, No. 13, 1998 4383
mmol) in propionitrile (0.40 mL) at 0 °C was added 1.0 M
BH3‚THF in THF (0.310 mL, 0.310 mmol).1,2 After 1 h, the
reaction was cooled to -78 °C, and cyclohexanecarboxaldehyde
(1a ) (0.050 mL, 0.413 mmol) was added followed by trifluoro-
acetic anhydride (0.120 mL, 0.826 mmol). After 5 min,
crotyltributyltin (0.140 g, 0.413 mmol) in propionitrile (1 mL)
was added over a period of 3 h with a syringe pump. After 7
h, the reaction was quenched with a saturated NaHCO3
solution (1 mL) and extracted with ether. The combined
organic extracts were washed with brine, dried over anhydrous
MgSO4, and concentrated under reduced pressure to give a
yellow oil. This was diluted with MeOH (5 mL), and a catalytic
amount of K2CO3 was added. After 1 h at room temperature,
the mixture was quenched with water and extracted with
ether. The combined organic extracts were washed with water
and brine, dried over anhydrous MgSO4, and concentrated
under reduced pressure to give a yellow oil. The crude product
was chromatographed on silica gel (5% ethyl acetate in
hexanes) to give 49.5 mg (71%) of alcohols 7a /8a as a 92:8
mixture of diastereomers of 93 and 80% ee, respectively (GC
analysis, R-DEX column): [R]D ) -28.1 (c 1.52, CHCl3) [lit.8
[R]D ) +28.0 (c 0.61, CHCl3) for the enantiomer]; 1H NMR (300
MHz, CDCl3) δ 5.89-5.72 (m, 1H), 5.17-4.98 (m, 2H), 3.20
(dd, J ) 5.9, 5.5 Hz, 1H), 2.41 (m, 1H), 2.01-0.81 (m, 11H),
1.00 (d, J ) 6.6 Hz, 3H).
complex controlled, essentially overriding the intrinsic
facial preference of the aldehyde substrate. These find-
B. BINOL-Ti(O-i-P r )4 Ca ta lyst. A solution of (R)-
BINOL (24.0 mg, 0.083 mmol), Ti(O-i-Pr)4 (0.012 mL, 0.041
mmol), and trifluoromethanesulfonic acid (0.004 mL, 0.041
mmol) in CH2Cl2 (2.5 mL) in the presence of 4 Å molecular
sieves (0.2 g) was heated at reflux for 1 h.5 The mixture was
cooled to room temperature, and aldehyde 1a (0.050 mL, 0.41
mmol) was added. After 30 min, the mixture was cooled to
-78 °C, and crotyltributyltin (0.43 g, 1.24 mmol) was added.
After 10 min, the mixture was warmed to -20 °C. After 73 h,
the reaction was quenched with saturated NaHCO3 (5 mL) and
extracted with CH2Cl2. The combined organic extracts were
washed with brine, dried over anhydrous MgSO4, and concen-
trated under reduced pressure to give a yellow oil. The crude
product was chromatographed on silica gel (5% ethyl acetate
in hexanes) to give 14.0 mg (18%) of alcohols 7a /8a as a 65:35
mixture of diastereomers of 95 and 49% ee, respectively (GC
analysis, R-DEX column).
ings are in accord with the H-bonded transition states
proposed by Corey as an explanation for enantioselective
additions to certain aldehyde-Lewis acid complexes
(Figure 1).10 A syn-selective addition to such a complex
would account for the observed stereochemical prefer-
ences.
(S)-1-Cycloh exyl-3-bu ten -1-ol (10).5,11 A. CAB Ca ta -
lyst. The procedure for 7a was followed with tartrate 4 (0.058
g, 0.10 mmol), BH3‚THF (0.31 mL, 0.31 mmol), aldehyde 1a
(0.050 mL, 0.41 mmol), trifluoroacetic anhydride (0.12 mL, 0.83
mmol), and allyltributyltin 9 (0.13 g, 0.41 mmol) in propioni-
trile (0.5 mL) at -78 °C for 10 h to give 22 mg (35%) of alcohol
10 of 55% ee (R-DEX column): 1H NMR (300 MHz, CDCl3) δ
5.92-5.76 (m, 1H), 5.20-5.07 (m, 2H), 3.39 (m, 1H), 2.39-
2.27 (m, 1H), 2.20-2.00 (m, 1H), 1.92-1.58 (m, 5H), 1.44-
0.95 (m, 6H).
B. BINOL-Ti(O-i-P r )4 Ca ta lyst. The procedure for 7a
was followed with (R)-BINOL (25.0 mg, 0.089 mmol), Ti(O-i-
Pr)4 (0.013 mL, 0.045 mmol), trifluoromethanesulfonic acid
(0.004 mL, 0.045 mmol), aldehyde 1a (0.050 mL, 0.41 mmol),
crotyltributyltin (0.42 g, 0.13 mmol), and molecular sieves (0.2
g) in CH2Cl2 (2.5 mL) at -20 °C for 70 h to give 36 mg (53%)
of alcohol 10 of 87% ee (R-DEX column).
Our results indicate that the modified CAB Lewis acid
gives better syn/anti selectivity than the Keck BINOL/
Ti catalyst5 in additions of crotyltributyltin to aldehydes.
However, allylations are more enantioselective with the
Keck catalyst. Additions to the CAB-aldehyde complex,
as pictured in Figure 1, are relatively insensitive to
Felkin-Ahn control in the case of the R-chiral aldehydes
(R)- and (S)-11. The diminished selectivity of additions
to alkynal 1d may result from the smaller size of the
alkynyl substituent, leading to lower syn/anti product
ratios, and the greater electrophilicity of the aldehyde,
favoring reactions via non-H-bonded complexes. At
present, we are unable to account for the relatively lower
selectivity observed for the â-ODPS aldehyde 1e. Roush
has also noted that â-oxygen substituents can lower the
enantioselectivity of additions of chiral crotylboronates
to aldehydes.9
(2S,3R,4S)-2,4-Dim et h yl-5-h exen e-1,3-d iol (13).9 A.
CAB-P r om oted Ad d ition . The standard procedure was
followed with tartrate 4 (92.0 mg, 0.327 mmol), BH3‚THF
(0.330 mL, 0.327 mmol), aldehyde (R)-11 (0.107 g, 0.327 mmol),
trifluoroacetic anhydride (0.092 mL, 0.65 mmol), and crotyl-
tributyltin (0.110 g, 0.327 mmol) in propionitrile (1.3 mL) at
-78 °C for 10 h and then -10 °C for an additional 12 h to
give 85.2 mg (68%) of alcohol as a yellow oil: 1H NMR (300
MHz, CDCl3) δ 7.77-7.60 (m, 1H), 7.49-7.31 (m, 6H), 5.60
(m, 1H), 5.00 (m, 2H), 3.80-3.58 (m, 3H), 2.29 (m, 1H), 1.79
(m, 1H), 1.10 (d, J ) 6.6 Hz, 3H), 1.06 (s, 9H), 0.96 (d, J ) 7.0
Hz, 3H).
Exp er im en ta l Section
GC analyses were performed on R-DEX or â-DEX cyclodex-
trin columns with temperature programming starting at 100
°C with a ramp of 1°/min.
(1S,2S)-(-)-1-Cycloh exyl-2-m eth yl-3-bu ten -1-ol (7a ). A.
CAB Ca ta lyst. To a solution of tartrate 4 (58.0 mg, 0.206
(10) Corey, E. J .; Rohde, J . J .; Fischer, A.; Azimioara, M. D.
Tetrahedron Lett. 1997, 38, 37. Corey, E. J .; Barnes-Seeman, D.; Lee,
T. W. Tetrahedron Lett. 1997, 38, 1699.
(11) Roush, W. R.; Hoong, L. K.; Palmer, M. A. J .; Park, J . C. J .
Am. Chem. Soc. 1990, 112, 6339.

4384 J . Org. Chem., Vol. 63, No. 13, 1998
Marshall and Palovich
The above alcohol (58.3 mg, 0.152 mmol) in THF (1.0 mL)
at 0 °C was treated with 1 M TBAF in THF (0.23 mL, 0.23
mmol), and the mixture was allowed to warm to room tem-
perature. After 1.5 h, the mixture was concentrated under
reduced pressure, and the residue was chromatographed on
silica gel (50% ethyl acetate in hexanes) to give 20.1 mg (92%)
of the diol as a 98:2 mixture of syn,syn (13) and syn,anti (ent-
15) diastereomers as determined by GC on an R-DEX col-
umn: 1H NMR (300 MHz, CDCl3) δ 5.64 (m, 1H), 5.04 (m, 2H),
3.80-3.54 (m, 3H), 2.32 (m, 1H), 1.84 (m, 1H), 1.10 (d, J ) 6.6
Hz, 3H), 0.96 (d, J ) 7.0 Hz, 3H).
B. BF 3‚OEt2 P r om oted Ad d ition . The standard proce-
dure was followed with aldehyde (R)-11 (57.5 mg, 0.176 mmol),
crotyltributyltin (0.073 g, 0.211 mmol), and BF3‚OEt2 (0.045
mL, 0.352 mmol) in CH2Cl2 (1.0 mL) at -78 °C for 6 h to give
48.2 mg (71%) of alcohol adducts.
mmol), BH3‚THF (0.385 mL, 0.385 mmol), aldehyde (S)-11
(0.126 g, 0.385 mmol), trifluoroacetic anhydride (0.110 mL,
0.770 mmol), and crotyltributyltin (0.132 g, 0.385 mmol) in
propionitrile (1.5 mL) at -78 °C for 10 h and then -10 °C for
an additional 12 h to give 95.8 mg (65%) of alcohol 14 as a
yellow oil: 1H NMR (300 MHz, CDCl3) δ 7.73-7.61 (m, 4H),
7.50-7.33 (m, 6H), 5.86 (m, 1H), 5.03 (m, 2H), 3.87-3.42 (m,
3H), 2.34 (m, 1H), 1.83 (m, 1H), 1.06 (s, 12H), 0.91 (d, J ) 7.0
Hz, 3H).
The alcohol (70.0 mg, 0.183 mmol) and TBAF (0.27 mL, 0.27
mmol) in THF (1.0 mL) were stirred for 1 h to give 25.6 mg
(97%) of diol as a 90:10 mixture of syn,anti (15) and syn,syn
(ent-13) diastereomers: 1H NMR (300 MHz, CDCl3) δ 5.86 (m,
1H), 5.13 (m, 2H), 3.80-3.43 (m, 3H), 2.46 (m, 1H), 1.85 (m,
1H), 1.03 (d, J ) 6.6 Hz, 3H), 0.90 (d, J ) 7.0 Hz, 3H).
The above alcohol (46.0 mg, 0.12 mmol) and TBAF (0.20 mL,
0.18 mmol) in THF (1.0 mL) were stirred for 2 h to give 15.9
mg (92%) of diol as a 90:10 mixture of syn,syn (13) and syn,anti
(ent-15) diastereomers.
(2R,3S,4R)-2,4-Dim eth yl-5-h exen e-1,3-diol (en t-13). The
standard procedure was followed with aldehyde (S)-11 (80 mg,
0.24 mmol), crotyltributyltin (0.100 g, 0.294 mmol), and
BF3‚OEt2 (0.062 mL, 0.49 mmol) in CH2Cl2 (2.0 mL) at -78
°C for 3 h to give 71 mg (75%) of alcohol adducts.
Ack n ow led gm en t. This work was supported by
research grants from the National Science Foundation
(CHE 9220166) and the National Institutes of Health
(AI31422). M.P. was the recipient of an NIH Postdoc-
toral Fellowship.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
1
cedures for 7b-e and racemic-7a -e and H NMR spectra and
GC traces for all compounds and derivatives (42 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, and
can be ordered from the ACS; see any current masthead page
for ordering information.
The above alcohol (71.0 mg, 0.18 mmol) and TBAF (0.27 mL,
0.18 mmol) in THF (1.0 mL) were stirred for 1 h to give 24 mg
(90%) of a 90:10 mixture of syn,syn and syn,anti diastereomers
ent-13 and 15.
(2R,3R,4S)-2,4-Dim eth yl-5-h exen e-1,3-d iol (15).9 The
standard procedure was followed with tartrate 4 (0.110 g, 0.385
J O980145C