Diastereoselective synthesis of diamino 1,2-diols from homochiral α-aminoacylsilanes

Article abstract of DOI:10.1055/s-2001-14644

We have developed a new synthetic access to stereodefined diamino 1,2-diols starting from homochiral α-aminoacylsilanes. A [3+2] cycloaddition with benzo nitrile oxide of the vinylated adducts and a reductive ring opening constitute key steps of the reaction sequence.

Full text of DOI:10.1055/s-2001-14644

LETTER
995
Diastereoselective Synthesis of Diamino 1,2-Diols from Homochiral
-Aminoacylsilanes
Bianca F. Bonini, Mauro Comes-Franchini, Mariafrancesca Fochi, Lodovico Lunazzi, Andrea Mazzanti, Germana
Mazzanti, Alfredo Ricci,* Greta Varchi
Dipartimento di Chimica Organica "A. Mangini", Università di Bologna, Viale Risorgimento 4, 40136-Bologna, Italy
Fax +39 51 2093654; E-mail: ricci@ms.fci.unibo.it
Received 01 January 2001
devised a convenient new entry to silylated allylic alco-
hols from acylsilanes by using the magnesium to cerium
transmetallation technique.
Abstract: We have developed a new synthetic access to stereode-
fined diamino 1,2-diols starting from homochiral -aminoacylsi-
lanes. A 3+2 cycloaddition with benzo nitrile oxide of the
vinylated adducts and a reductive ring opening constitute key steps
of the reaction sequence.
Treatment of 1 and 2 with the vinylmagnesium bromide/
CeCl3 complex, led (Scheme 1) to the formation of ad-
ducts 3 and 4 in good isolated yields and high diastereose-
lectivities (99% d.e.).8
Key words: -aminoacylsilanes, vinylation, dipolar cycloaddition,
aminopolyols
Stereodefined hydroxy amino alcohols are well recogn-
ised as key components for a variety of protease inhibitors
and other new generation pharmaceutics and as ligands
for asymmetric catalysis.1 We recently reported2 the stere-
oselective synthesis of functionalized amino alcohol units
and of statine analogs via three- and two-carbon elonga-
tions respectively, from -aminoacylsilanes. The use of
these synthetic equivalents of aldehydes, allows to
overcome2 the serious problems due to the easy racemiza-
tion and avoids the special precautions necessary for the
synthesis, handling and storing frequently encountered
with -amino aldehydes.3 In this paper we present a new
approach to the two-carbon homologation of acylsilanes
based on the delivery of a vinyl unit to a carbonyl group.
The reaction under study is aimed at the synthesis of ste-
reodefined aminopolyols possessing the core-unit present
in dihydroxypropylamine derivatives capable of highly
efficient renin4 and HIV-1 protease5 inhibitions (Fig.1).
Scheme 1
For introducing new hydroxy and amino functionalities in
the vinyl moiety we envisioned the 3+2 cycloaddition
with nitriloxides. Cycloadditions of this type are usually
regarded as mildly electrophilic in character. Therefore
( -hydroxyallyl)silanes in which the preferred location of
allylic substituents maximizes9 electron donation should
act, as shown by Curran,10 as efficient dipolarophiles.
When 3 and 4 were reacted with the in situ prepared benzo
nitrile oxide, isoxazolines 5 and 6 were obtained in 70%
and 55% yields. In contrast with the modest diastereomer-
ic excess (20%) observed in the case of 5, only one dias-
tereoisomer (99% d.e.) could be detected and isolated in
the case of 6. The diastereomeric couple 5a,b could be
Figure 1
Our approach started with the homochiral aminoacylsi-
lanes 1 and 2 derived from phenylalanine and isoleucine.2
Since the addition of vinylmagnesium and lithium orga-
nometallics to acylsilanes studied by Kuwajima6 gives
rise to extensive amounts of silyl enol ethers along with
the expected allylic alcohols, in a previous paper7 we
Scheme 2
Synlett 2001 SI, 995–998 ISSN 0936-5214 © Thieme Stuttgart · New York

996
B. F. Bonini et al.
LETTER
separated by column chromatography on silica gel and the use of NaBH4 in methanol in the presence of
characterized.11
NiCl2•6H2O. Exposure of 8 and 9 to this reagent at 30 °C
according to the literature15 led to heterocyclic ring open-
ing and reduction of the carbonyl function and afforded
after 65 h, the expected aminopolyols. These were directe-
ly converted to the corresponding bis-Boc derivatives: in
both cases one diastereoismer was found to largely prevail
which was isolated by column chromatography and char-
acterized.16 By this procedure N-Boc protected aminopo-
lyols 10 and 11 are produced in 35% and 30% overall
yields in three steps from oxazolidines 5a and 6 respec-
tively. Connectivity of diamino 1,2-diols 10 and 11 was
achieved by homonuclear and heteronuclear correlation.
The stereochemical assignment of the three unknown ste-
reogenic centers of 11 was attempted on the basis of 2D-
NOESY constraints and molecular modelling.17 The 15
observed constraints restricted the choice to two possible
solutions: in both cases the three unknown centres are ex-
pected to display the same chirality and either configura-
tion SSSSS or SSRRR, could be considered for 11 (Figure
2). To solve this ambiguity a single crystal X-ray
diffraction18 was performed. The configuration was found
to be SSSSS (Figure 2, left). Work is in progress to estab-
lish also the stereochemistry of compound 10.
The subsequent desilylation of the -hydroxysilane cy-
cloadducts carried out on a model compound, was disap-
pointing. Under the action of bases the -elimination of
silanol led to ring opening with generation of an enolic
form which upon rearrangement resulted in the formation
of the desilylated hydroxylamino derivative 7 (Scheme 3).
Scheme 3
To prevent the occurrence of the above mentioned Peter-
son elimination reaction from the -hydroxysilane
moiety12 in the presence of strongly basic reagents, isox-
azolines 5a, the major diastereoisomer, and 6 were sub-
jected to an oxidative desilylation. This reaction, although
already reported in the literature,13 has been however
scarcely investigated. Since the oxidation with pyridinium
dichromate (PDC) has found an useful and high yielding
application in the synthesis of acylsilanes from 1,1-disilyl
alcohols,13b we applied this procedure to the desilylation
of 5a and 6. Upon treatment with PDC at room tempera-
ture, these compounds underwent (Scheme 4) smooth des-
ilylative oxidation to give 8 and 9 in high yields and
virtually diastereomerically pure.14
In conclusion we have established the viability of a new
short route to diamino 1,2-diols starting from homochiral
-aminoacylsilanes. The importance of the target com-
pounds highlights the significance of this new synthetic
protocol in which in a single reductive-unfolding step,
three new functional groups are generated.
Acknowledgement
This work has been supported by University of Bologna (Funds for
Selected Research Topics, A.A. 1999-2001) and by the National
Project “Stereoselezione in Sintesi Organica. Metodologie ed Ap-
plicazioni 1999-2001”.
References and Notes
(1) Pastò, M.; Moyano, A.; Pericàs, M.A.; Riera, A. Tetrahedron:
Asymmetry 1996, 7, 243, and references therein.
(2) (a) Bonini, B.F.; Comes-Franchini, M.; Fochi, M.;
Gawronsky, J.; Mazzanti, G.; Ricci, A.; Varchi, G. Eur. J.
Org. Chem. 1999, 437; (b) Bonini, B.F.; Comes-Franchini,
M.; Fochi, M.; Laboroi, F.; Mazzanti G.; Ricci, A.; Varchi, G.
J. Org. Chem. 1999, 64, 8008.
(3) (a) Coy, D.H.; Hocart, S.J. Tetrahedron 1988, 44, 835;
(b) Jurczak, J.; Golebiowski Chem. Rev. 1989, 89, 149.
(4) Luly, J.R.; Maung, N.B.; Soderquist, J.; Fung, A.K.L.; Stein,
H.; Kelinert, H.D.; Marcotte, P.A.; Egan, D.A.; Bopp, B.;
Meirts, I.; Bolis, G.; Greer, J.; Perun, T.J.; Plattner, J. J. Med.
Chem. 1988, 31, 2264.
(5) Thaisrivongs, S. Int. Pat. Appl. WO 91/01327, 1991.
(6) Kato, M.; Mori, A.; Oshino, H.; Ende, J.; Kobayashi, K.;
Kuwajima, I. J. Am. Chem. Soc. 1984, 106, 1773.
Scheme 4 a) PDC, DMF, r.t., 18 h, 98% yield, 99% d.e. for both 8
and 9; b) NaBH4, NiCl2•6H2O, MeOH, 30 °C, 65 h; c) Boc2O, Me-
OH, Et3N, r.t., 4 days, 35% for 10 and 30% for 11.
(7) Bonini, B.F.; Comes-Franchini, M.; Fochi, M.; Mazzanti, G.;
Ricci, A.; Varchi, G. Synlett 2000, 1688.
The reductive ring opening of the oxazolidine ring aimed
at the formation of the new hydroxy and amino function-
alities was then performed. To avoid reaction conditions
able to promote an acidic proton abstraction, we exploited
(8) 3: Yellowish oil: [ ]D20 = -49.3 (c = 2.0, CHCl3); 1H NMR
(200 MHz, CDCl3) : 0.38 (s, 3H, Si-CH3), 0.40 (s, 3H, Si-
CH3), 1.24 (s, 9H, C(CH3)3), 1.46 (s, 1H, OH), 2.54 (dd, 1H,
J = 13.6, 10.4 Hz, CH2-Ph), 2.90 (dd, 1H, J = 13.6, 3.6 Hz,
Synlett 2001, SI, 995–998 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Diastereoselective Synthesis of Diamino 1,2-Diols
997
Figure 2 The two alternative structures of 11 obtained on the basis of 2D-NOESY constraints. (oxygens are represented as black, nitrogens
as grid)
CH2-Ph), 3.75-3.90 (m, 1H, CH-NH), 4.30 (d, 1H, J = 10.5
Hz, NH), 5.05-5.17 (m, 2H, CH2 = ), 5.93-6.06 (m, 1H,
CH = ), 7.10-7.70 (m, 10H, Ar); 13C NMR (50.3 MHz, CDCl3)
: -4.79, -4.65, 28.11, 36.69, 57.78, 75.72, 79.06, 111.70,
125.95, 127.76, 128.12, 129.13, 129.40, 133.01, 134.57,
139.00, 139.92, 155.98. (4): white solid; m.p. = 65-66 C.
[ ]D20 = -44.5 (c = 2.0, CHCl3); 1H NMR (300 MHz, CDCl3)
: 0.30 (s, 3H, SiMe), 0.33 (s, 3H, SiMe), 0.52 (t, 3H, J = 7.2
Hz, CH3CH2), 0.78-0.85 (m, 5H, CH3CH, CH2CH3), 1.34 (s,
9H, C(CH3)3), 1.40-1.70 (m, 2H, CHCH3, OH), 3.68.3,78 (dd,
1H, J = 10.4, 1.9 Hz, CHNH), 4.50 (bd, 1H, J = 10.7 Hz),
4.78-5.04 (m, 2H, CH2 = ), 5.88-6.05 (m, 1H, CH = ), 7.25-
7.55 (m, 5H, ArH); 13C NMR (75.46 MHz, CDCl3) : -4.97, -
4.34, 11.40, 17.94, 23.50, 28.34, 36.93, 59.68. 76.39, 78.78,
110.60, 127.67, 129.28, 134.60, 136.40, 141.25, 156.05.
(9) For discussions of such electronic effects of allylic
substituents see: (a) Eyer, M.; Seebach, D. J. Am. Chem. Soc.
1985, 107, 3601. (b) Danishefsky, S.J.; Larson, E.; Springer,
J.P. J. Am. Chem. Soc. 1985, 107, 1274.
(10) Curran, D.P.; Gothe, S.A. Tetrahedron 1988, 44, 3945.
(11) Representative procedure: preparation of (5a-b):
Triethylamine (0.27 mL, 1.90 mmol) and 3 (600 mg, 1.52
mmol) were dissolved in Et2O (15 mL). To this solution
benzohydroxymoyl chloride (300 mg, 1.90 mmol) in Et2O (10
mL) was slowly added (3.5 h) through a funnel. After the
addition was completed the reaction was stirred for 48 h. The
solvent was evaporated and the crude was purified by column
chromatography on silica gel (petroleum ether : Et2O/3:1)
affording a 560 mg (70%) of two diastereoisomers in the ratio
60:40. Major diastereoisomer (5a): Rf = 0.042 cm (petroleum
ether : Et2O = 3:1). [ ]D20 = +23.65 (c = 4.0, CHCl3); 1H
NMR (300 MHz, CDCl3) : 0.86 (s, 3H, Si- CH3), 0.90 (s, 3H,
Si-CH3), 1.50 (s, 9H, C(CH3)3), 2.96 (dd, 1H, J = 12.6, 10.5
Hz, Ph-CH2), 3.16 (s, 1H, OH), 3.32 (dd, 1H, J = 12.6, 3.1 Hz,
Ph-CH2), 3.42 (dd, 1H, J = 16.8, 10.5 Hz, CH2-CH-O), 3.84
(dd, 1H, J = 16.8, 9.4 Hz, CH2-CH-O), 4.10-4.25 (m, 1H, CH-
N), 4.50 (d, 1H, J = 9.4 Hz, NH), 5.34 (t, 1H, J = 10.5 Hz, CH-
O), 7.28-8.00 (m, 15H, Ar-H). 13C NMR (50.3 MHz, CDCl3)
: -2.83, -2.63, 28.02, 35.34, 36.83, 58.76, 73.72, 79.46,
85.47, 126.32, 126.64, 128.03, 128.33, 128.53, 128.83,
128.38, 129.54, 129.98, 134.77, 137.46, 138.21, 156.35,
157.62. HRMS (EI): m/z found: 517.2529 C30H27N2O4Si
requires 517.2523. Minor diastereoisomer (5b): Rf = 0.11 cm
(petroleum ether : Et2O/ 3:1). [ ]D20 = -107.63 (c = 4.0,
CHCl3); 1H NMR (300 MHz, CDCl3) : 0.52 (s, 3H, Si-CH3),
0.57 (s, 3H, Si-CH3), 1.24 (s 9H, C(CH3)3), 2.56 (dd, 1H,
J = 13.4, 8.6 Hz, CH2-Ph.), 3.00 (dd, 1H, J = 16.1, 10.1 Hz,
CH2-CH-O), 3.22 (dd, 1H, J = 13.4, 2.2 Hz, CH2- Ph), 3.30
(bs, 1H, OH), 3.38 (dd, 1H, J = 16.1, 11.2 Hz, CH2-CH- O),
4.26 (d, 1H, J = 9.0, NH), 4.38-4.44 (m, 1H, CH-N), 5.40 (t,
1H, J = 10.1 Hz, CH-O), 7.05-7.54 (m, 15 H, Ar-H); 13C NMR
(50.3 MHz, CDCl3) : -3.55, -3.30, 28.09, 36.33, 37.36, 57.72,
73.64, 79.43, 85.02, 126.18, 126.53, 128.07, 128.44, 128.58,
129.07, 129.44, 129.68, 129.99, 133.35, 134.66, 136.81,
138.37, 156.78, 157.77. 6: Rf = 0.8 cm (petroleum ether : Et2O
/ 3:1); [ ]D20 = -49.30 (c = 3.0, CHCl3); 1H NMR (CDCl3, 200
MHz), : 0.55 (s, 6H, SiMe2Ph), 0.71-0.92 (m, 8H, CH3CH2,
CH3CH), 1.35 (s, 9H, (CH3)3C), 1.55-1.75 (m, 1H, CHCH3),
2.12 (bs, 1H, OH), 3.16 (dd, 1H, J = 16.5 Hz, 9.5 Hz,
CH2CHO), 3.46 (dd, 1H, J = 16.5 Hz, 10.0 Hz, CH2CHO),
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B. F. Bonini et al.
LETTER
3.82 (m, 1H, CHNH), 4.76-4.98 (m, 2H, CH-O+NH), 7.19-
7.39 (m, 6H, ArH), 7.42-7.58 (m, 4H, ArH); 13C NMR
(CDCl3, 50.3 MHz), : -3.39, -2.61, 11.73, 18.90, 24.70,
28.26, 36.05, 36.55, 59.77, 74.20, 79.07, 85.69, 126.59,
127.71, 127.84, 128.47, 129.34, 134.67, 129.95, 137.24,
155.98, 157.50. HRMS (EI): m/z found: 496.2748
C28H40N2O4Si requires 496.2757.
1.35 (s, 9H, C(CH3)3), 1.70 (bs, 1H, OH), 2.10 (bs, 1H, OH),
1.40-1.50 (m, 2H, CH2CHPh), 2.40-2.50 (m, 2H, PhCH2),
3.10 (d, 1H, J = 9.9Hz, PhCH2CHNH), 3.60-3.65 (m, 1H,
PhCH2CHCH), 3.70-3.75 (m, 1H, PhCH2CH), 4.55-4.60 (m,
1H, PhCHCH2CHOH), 4.90-5.00 (m, 1H, PhCHNH), 5.10 (d,
1H, J = 10.1 Hz, PhCHNH), 7.00-7.40 (m, ArH, 10H); 13
NMR (50.3 MHz, CDCl3) : 28.33, 28.46, 36.68, 39.08,
52.14, 53.04, 67.11, 75.26, 77.27, 79.64, 127.73-129.53,
137.68, 142.06, 156.05, 156.76. HRMS (EI): m/z found:
C
(12) Hudrlik, P.F.; Peterson, D. J. Am. Chem. Soc. 1975, 97, 1464.
(13) (a) Kuwajima, I.; Abe, T.; Minami, N. Chem. Lett. 1976, 993.
(b) Fleming, I.; Gosh, U. J. Chem. Soc. Perkin Trans. I 1994,
257.
500.2894 C20H28N2O4 requires 500.2886. 11: White solid:
30% overall yield for three reaction steps. M.p. 184-186. °C;
[ ]D = -11.46 (c = 3.0, CHCl3); 1H NMR (200 MHz, CDCl3)
: 0.15-0.20 (m, 1H, CH3CH2), 0.60-0.80 (2S, 6H, CH3-
CH2+CH3CH), 1.00-1.10 (m, 1H, CH3CH2), 1.30-1.55 (m,
19H, 2Me3C, CH2CHPh), 1.85-1.95 (m, 1H, CHCH3), 1.95-
2.00 (m, 1H, NHCHCHOH), 2.35-2.40 (m, 1H, CH2CHPh),
2.80-2.85 (m, 1H, CH(OH)CHNH), 3.40-3.45 (m, 1H,
CHCHNH), 3.60-3.65 (m, 1H, CH2CHOH), 4.00-4.10 (d, 1H,
J = 9.8Hz, CHCHNH), 4.60 (s, 1H, CH2CHOH), 5.20-5.25
(m, 1H, PhCH), 6.10 (d, 1H, J = 10.0 Hz, PhCHNH), 7.10-
7.40 (m, 5H, ArH); 13C NMR (200 MHz, CDCl3) : 12.05,
16.65, 22.93, 38.74, 28.10, 28.29, 38.74, 52.37, 57.39, 67.06,
73.30, 79.34, 80.07, 126.09, 128.20, 128.43, 142.25, 155.77,
157.68. HRMS (EI): m/z found: 466.3052 C20H28N2O4
requires 466.3043.
(14) Representative procedure: preparation of 8: To a solution of
5a (503 mg, 0.77 mmol) in DMF (20 mL) pyridinium
dichromate (PDC) (1.74 g, 4.36 mmol) was added under Ar
and the mixture was stirred for 18 h. The reaction was
quenched with water, the aqueous phase extracted twice with
ethyl acetate, and the organic phases were combined, washed
twice with water and dried over MgSO4. The drying agent was
filtered and the solvent evaporated giving 8 (297 mg, 98%) as
a yellow oil. From 1H NMR the crude resulted pure enough for
being used in the next step without any purification. 8:
[ ]D = +125.8 (c = 3.0, CHCl3); 1H NMR (200 MHz, CDCl3)
: 1.31 (s, 9H, C(CH3)3, 2.83 (dd, 1H, J = 13.8, 8.9 Hz, CH2-
Ph), 3.21 (dd, 1H, J = 13.8, 4.8 Hz, CH2-Ph), 3.47 (dd, 1H,
J = 17.2, 11.6 Hz, CH2-CH-O), 3.76 (dd, 1H, J = 17.2, 6.0 Hz,
CH2-CH-O), 4.82 (dd, 1H, J = 13.4, 8.3 Hz, CH-NH), 5.08 (d,
1H, J = 6.5 Hz, NH), 5.32 (dd, 1H, J = 11.6, 6.0 Hz, CH-O),
7.15-7.69 (m, 10 H, Ar-H); 13C NMR (50.3 MHz, CDCl3) :
28.18, 36.28, 36.84, 58.04, 80.22, 83.15, 126.93, 136.35,
155.35, 156.93, 206.39. HRMS (EI): m/z found: 394.1894
C23H26N2O4 requires 394.1892. 9 (98% yield): Rf = 0.8
(CH2Cl2:MeOH / 10:1); [ ]D = +77.21 (c = 4.0, CHCl3); 1H
NMR (CDCl3, 200 MHz), : 0.69-1.11 (m, 8H,
(17) NMR spectra were recorded at 400 MHz in C6D6.
Homonuclear correlations were obtained by the gCOSY
sequence. Heteronuclear correlations were obtained by edited-
gHSQC sequence. 2D-NOESY spectra were recorded using
mixing times of 1.5 s and 2.0 s. Conformational search was
performed with PC-Spartan Pro v1.0.5, using the MMFF
Force Field, and then the conformational minima were refined
using PC-Model V7.5 and the MMX Force Field.
CH3CH2+CH3CH) 1.46 (s, 9H, (CH3)3C), 1.85-2.02 (m, 1H,
CHCH3), 3.58 (m, 2H, CH2-CH-O), 4.51 (dd, 1H, J = 8.7 Hz,
J = 5.2 Hz, CHNH), 5.01 (d, 1H, J = 8.7 Hz, NH), 5.12-5.30
(m, 1H, CH-O), 7.29-7.42 (m, 3H, ArH), 7.58-7.66 (m, 2H,
ArH); 13C NMR (CDCl3, 50 MHz), : 11.30, 16.26, 24.03,
28.17, 35.69, 37.75, 61.98, 79.87, 83.25, 126.6, 128.46,
128.72, 130.49, 155.70, 156.47, 207.52. HRMS (EI): m/z
found: 360.2052 C20H28N2O4 requires 360.2049.
(18) Crystal Data of 11: C25H42N2O6 (466.61), Orthorombic,
Space group P212121, Z = 4, a = 10.8904(6), b = 11.2626(6),
c = 22.5995(12) Å, V = 2771.9(3) Å3, Dc = 1.118 g cm-3,
F(000) = 1016, Mo = 0.079 cm-1, T = 298 K. Data were
collected using a graphite monochromated Mo-K X-
radiation ( = 0.71073 Å) in the range 3.25° < < 30.00°. Of
31346 reflections measured, 8104 were found to be
independent (Rint = 0.0847), 3670 of which were considered as
observed [F0 > 4 (F0)], and were used in the refinement of
302 parameters leading to a final R1 of 0.0546 and a Rall of
0.1141. The structure was solved by direct method and refined
by full-matrix least squares on F2, using SHELXTL 97
program packages. In refinements were used weights
according to the scheme w = [ 2 (Fo 2)+(0.0895 P)2+0.0000P
(15) Annunziata, R.; Cinquini, M.; Cozzi, F.; Gilardi, A.; Restelli,
A. J. Chem. Soc. Perkin Trans. 1 1985, 2289.
(16) Representative procedure: preparation of 10: To a solution
of 8 (297 mg, 0.75 mmol) in methanol (19.3 mL) stirred under
Ar, NiCl2 6 H2O (750 mg, 3.1 mmol) was added. The
temperature was lowered to 30 °C and NaBH4 (290 mg, 7.7
mmol) was added in three portions. The reaction mixture was
stirred at 30 °C for 65 h in a cryostat, the solvent was
evaporated and to the residue 25 mL of conc. NH4OH were
added. The aqueous phase was extracted twice with CH2Cl2
and the combined organic phases were dried over MgSO4,
filtered and the solvent evaporated under vacuum. The crude,
without any further purification, di-tert-butyl dicarbonate
(188 mg, 0.86 mmol) and triethylamine (0.13 mL, 0.99 mmol)
were sequentially dissolved in methanol (16 mL). The
reaction mixture was stirred under Ar 4 days at room
temperature. The solvent was evaporated and the crude
purified by column chromatography over silica gel (petroleum
ether: Et2O/ 1:1) giving 134 mg of the largely prevailing
diastereoisomer 10 as a white solid (overall yield for three
reaction steps 35%). 10: mp 200-203 °C; [ ]D = -13.3 (c = 1.5,
CHCl3); 1H NMR (200 MHz, CDCl3) : 1.20 (s, 9H, C(CH3)3),
]
-1 where P = (Fo 2)+2 Fc2)/3. The hydrogen atoms were
located by geometrical calculations and refined using a
"riding" method; wR2 was equal to 0.1447. The other atoms
were anisotropically refined except the two t-butyl groups that
were disordered and so splitted into two position and
isotropically refined. The goodness of fit parameters S was
0.846. The largest difference between peak and hole was
0.572 and -0.272 eÅ-3. Crystallographic data (excluding
structure factors and including selected torsion angles) have
been deposited with the Cambridge Crystallographic Data
Center. CCDC-160358.
Article Identifier:
1437-2096,E;2001,0,SI,0995,0998,ftx,en;Y03801ST.pdf
Synlett 2001, SI, 995–998 ISSN 0936-5214 © Thieme Stuttgart · New York