Polyol synthesis through hydrocarbon oxidation: De novo synthesis of L-galactose

Article abstract of DOI:10.1002/anie.200603321

(Chemical Equation Presented) Carbohydrates from hydrocarbons : A hydrocarbon oxidation strategy for the synthesis of chiral polyols is validated by the enantioselective, de novo synthesis of differentially protected L-galactose (see scheme, TBS = tert-bu

Full text of DOI:10.1002/anie.200603321

Angewandte
Chemie
ꢀ
C H Oxidation
DOI: 10.1002/anie.200603321
Polyol Synthesis through Hydrocarbon Oxidation:
De Novo Synthesis of l-Galactose**
Dustin J. Covell, Nicolaas A. Vermeulen,
Nathan A. Labenz, and M. Christina White*
ꢀ
Direct oxidation of C H bonds has the potential to emerge as
a powerful approach for introducing oxygen and nitrogen
functionality in the synthesis of complex molecules.[1] Func-
tional-group manipulations (FGMs) that are required for
carrying oxygenated functionalities throughout a synthetic
route may be avoided by installing them directly into the
hydrocarbon framework.[2] For a hydrocarbon oxidation
ꢀ
strategy to reach its full potential, C H-oxidation reactions
with high levels of chemo-, regio- and stereoselectivity must
be identified and developed. We recently described a DMSO-
promoted, Pd(OAc)2-catalyzed allylic oxidation reaction that
furnishes E allylic acetates from a-olefins with high regio- and
stereoselectivity, and with outstanding tolerance of functional
groups.[3] Herein, we describe advances in this methodology
ꢀ
for the oxidation of C H bonds which enabled the develop-
ment of a new hydrocarbon oxidation strategy for the rapid
assembly of polyol frameworks. This strategy has been
validated in an enantioselective, de novo synthesis of differ-
entially protected l-galactose from a commercial, achiral
starting material in which all new oxygen functionality has
ꢀ
=
been installed through oxidation reactions of C H and C C
bonds.
A short hydrocarbon oxidation strategy for the synthesis
of chiral polyols is presented in Scheme 1. Key to the
efficiency of this strategy is the rapid access to chiral (E)-2-
butene-1,4-diols such as 2, which may be directly elaborated
to polyol structures through asymmetric dihydroxylation
(AD). Compounds like 2 are particularly attractive building
[*] D. J. Covell, N. A. Vermeulen, Prof. M. C. White
Department ofChemistry, Roger Adams Laboratory
University ofIllinois
Urbana, IL 61801 (USA)
Fax: (+1)217-244-8024
E-mail: white@scs.uiuc.edu
N. A. Labenz
Harvard College
Cambridge, MA 02138 (USA)
[**] Financial support was provided by the Henry Dreyfus Foundation,
the University ofIllinois (UIUC), the Merck Research Laboratories,
and the NIH/NIGMS (GM076153). D.J.C. gratefully acknowledges
the UIUC for Roger Adams and C. S. Marvel fellowships. We thank
Prof. E. N. Jacobsen (Harvard University) and Dr. C. P. Stevenson
(Merck) for a gift of oligomeric (R,R)-[(salen)CoIII]OTfcatalyst and
helpful advice on the Payne rearrangement, respectively. We thank
Johnson Matthey for a generous gift of Pd(OAc)2. We are also
grateful to M. S. Chen for checking our experimental procedure for
the allylic oxidation reaction (Table 1, entry 7).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 8217 –8220
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8217

Communications
Table 1: Investigation into the linear allylic oxidation reaction to form the
hexose precursor 6.
Entry
DMSO/CH2Cl2
Molarity [m]
Quinone
(Q)
Acid 5
(equiv)
Yield [%][c]
Scheme 1. A hydrocarbon oxidation strategy for the streamlined
construction ofpolyols.
1
2
3
4
0.33
0.33
0.33
0.6
1.0
2.0
2.0
2.0
3.0
BQ
BQ
15
15
15
10
5
3
3
3
1.5
23[d]
45
55
66
PhBQ
PhBQ
PhBQ
PhBQ
PhBQ
PhBQ
PhBQ
blocks because of their dense functionalization, dissonant
relationship between the oxygen atoms,[4] and the ease with
which they can be further elaborated through the established
oxidation chemistry of olefins. Compounds similar to 2 have
been routinely employed as intermediates to install a diverse
range of structures in the synthesis of natural products; for
example, five- and six-membered mono- and polycyclic
ethers,[5] epoxy alcohols,[6] and, most extensively, contiguous
polyol structures.[7] Notably, compounds analogous to 2 have
been used as intermediates in several stereodivergent syn-
theses of the hexoses, an important class of polyols
(Scheme 2).[8a,9] State-of-the-art syntheses of 2 based on
5
67
75
6[b]
7[b]
8
71[e]
63[f]
50
9
[a] DMSO/CH2Cl2 (3.2:1). [b] Linear to branched allylic ester and
E/Z ratios were determined by HPLC for the material obtained from
entries 6 and 7 on comparison with branched or acetonide-free E and Z
standard compounds: linear/branched>300:1, E/Z=30:1 and 36:1 (for
entries 6 and 7, respectively). [c] Yield ofisolated product rfom the
reactions carried out on a 1 mmol scale (4, 262 mg). Yields and
selectivities represent an average ofat least 2 runs. [d] With no DIPEA
added. [e] [Pd(CH3CN)4](BF4)2 (10 mol%), 13% of 4 was recovered.
[f] Pd(OAc)2 (5 mol%). Bn=benzyl, DIPEA=N,N-diisopropylethyl-
amine, MS=molecular sieves.
in the DMSO/PdII-promoted linear allylic oxidation reaction
to form the hexose precursor 6, if the challenges associated
with high acid loadings and low yields were resolved (15 equiv
of 5, 23% yield, Table 1, entry 1). We were encouraged by the
observation that significant amounts of the a-olefin starting
material remained at the end of the reaction, thus suggesting
that the acid-labile acetonide functionality was tolerant of
these conditions. The addition of DIPEA, a noncoordinating
base additive, effected a significant increase in yield (45%,
Table 1, entry 2). Although the exact role of the base is
currently unclear, we hypothesize that it increases concen-
trations of the benzoate anion. The yield was also increased
on switching oxidants from benzoquinone (BQ) to phenyl-
benzoquinone (PhBQ, 55%, Table 1, entry 3). Finally, on
increasing the reaction concentration to 2.0m, we achieved
further increases in yields, and were able to use fewer
equivalents of carboxylic acid (2.0m, 3 equiv 5, 75% yield,
Table 1, entry 6).
The linear allylic oxidation reaction is exceptionally
stereo- and regioselective, with the major product formed
being the linear E isomer (linear/branched > 300:1, E/Z =
30!36:1, entries 6 and 7, Table 1). The reaction is also
preparatively convenient, with all reactions carried out in an
air atmosphere with no precautions taken to exclude moisture
or O2. Significantly, with these newly developed conditions,
the catalyst loading may be decreased to 5 mol% with only a
minor decrease in yield (63%, Table 1, entry 8). Moreover,
useful yields are obtained from fragment couplings with
1.5 equivalents of carboxylic acid at 3.0m concentrations of a-
olefin (50%, Table 1, entry 9). The only by-product we
Scheme 2. Chiral (E)-2-butene-1,4-diols as key intermediates in
stereodivergent hexose syntheses.
Wittig-type olefinations[10] or cross-metathesis reactions[11]
suffer from lengthy sequences, in part because of the difficulty
in accessing enantioenriched a-hydroxyaldehyde and a-
hydroxy olefin starting materials. Alternatively, 2 may be
synthesized directly from protected chiral homoallylic alco-
hols such as 3 through the DMSO/PdII-promoted allylic
oxidation by using the strategy presented in Scheme 1.[2,3a]
Homoallylic alcohols 3 are readily accessible from the
asymmetric allylation of aldehydes[12,13] or the regioselective
vinylation of chiral epoxides (Scheme 1).[14]
4-Methoxybenzoate derivatives of chiral (E)-2-butene-
1,4-diols 2 are unique among allylic alcohol derivatives in
their ability to undergo AD with excellent reagent-controlled
diastereoselectivity and minimal acyl transfer.[8] We recently
demonstrated that the DMSO/PdII-promoted linear allylic
oxidation of protected chiral homoallylic alcohols with acetic
acid furnishes the acetate derivatives of 2 with excellent regio-
and stereoselectivities and with no erosion of optical purity.[2]
To avoid FGMs, we set out to identify conditions for allylic
oxidation of a-olefins using p-anisic acid (5) to directly
generate 4-methoxybenzoate derivatives of 2 from a-olefins
(Scheme 1). As shown in Table 1, preliminary studies with a-
olefin 4 suggested that acid 5 may be a competent nucleophile
8218
www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 8217 –8220

Angewandte
Chemie
observed in the reaction using Pd(OAc)2 as the PdII source
was allylic acetate, which can be eliminated by using
[Pd(CH3CN)4](BF4)2 (Table 1, entry 7).
We set out to test the efficiency of this overall strategy for
polyol construction in the context of a short, de novo syn-
thesis of the differentially protected l-galactose (ꢀ)-11
(Scheme 3). Bulk chemical (Z)-2-butene-1,4-diol (7) was
10 mol% Pd(OAc)2 in DMSO under the same conditions,
(+)-6 was obtained in 75% yield with around 10% of the
allylic acetate product that was arduous to separate by silica
gel chromatography. Asymmetric dihydroxylation of (+)-6
proceeded smoothly to give the polyoxygenated (ꢀ)-10 in
96% yield and greater than 20:1 d.r. (1H NMR).[18] Protection
of the diol (ꢀ)-10 as the silyl ether, followed by removal of the
p-methoxybenzoate ester with DIBAL, Swern oxidation of
the resulting primary alcohol, and removal of the isopropyl-
idene ketal group with Zn(NO3)·6H2O[19] gave the differ-
entially protected l-galactose (ꢀ)-11 in 74% yield (over
4 steps).[20] The enantioselective, de novo synthesis of (ꢀ)-11
thus proceeded in a total of 10 linear steps with 20% overall
yield from the commercially available 7.
Several stereodivergent, de novo syntheses of the hexoses
from 7 have employed chiral (E)-2-butene-1,4-diols analo-
ꢀ
gous to 6 as intermediates. The C H-oxidation route to 6
(5 steps, 28% overall yield) compares favorably, in number of
steps and overall yield, with the Wittig olefination routes
previously reported (for example: 11 steps, 18% overall
yield,[9] and 9 steps, 16% overall yield[8a]). The strategy
developed herein, along with these previous syntheses,
provides access to hexose stereoisomers that are complemen-
tary to those obtained through aldol-based approaches.[21]
In summary, we have reported a mild and efficient
hydrocarbon oxidation strategy for the preparation of chiral
polyols, a process validated by an efficient, enantioselective
synthesis of the differentially protected l-galactose (ꢀ)-11.
The synthesis used a highly regio- and stereoselective linear
Scheme 3. Total synthesis of differentially protected l-galactose (ꢀ)-11
(see the Supporting Information for full experimental details).
a) mCPBA, CH2Cl2, 08C!RT (74%); b) oligomeric (R,R)-[(salen)CoIII]-
OTf(0.05 mol%), CH 3CN; then 2-methoxypropene, p-TsOH·H2O
=
(1 mol%), 08C; c) CuBr (10 mol%), CH2 CHMgBr, THF, ꢀ408C;
d) NaH, TBAI (10 mol%), DMF, 08C, BnBr, 08C!RT, (54%, 3 steps
from 8, 99% ee); e) [Pd(CH3CN)4](BF4)2 (10 mol%), PhBQ (2 equiv), 5
(3 equiv), DIPEA (0.5 equiv), 4-Š MS, DMSO/CH2Cl2 (3:1), 418C, 72h
(71%, linear/branched>300:1, E/Z=97:3); f) K2OsO4·2H2O
(1 mol%), (DHQD)2PHAL (5 mol%), K3Fe(CN)6, K2CO3,MeSO2NH2,
tBuOH/H2O, 08C (96%, >20:1 d.r.); g) TBSOTf, 2,6-lutidine, CH2Cl2,
08C!RT, (90%); h) DIBAL, CH2Cl2, ꢀ788C, (98%); i) (COCl)2,
DMSO, NEt3, ꢀ658C, (90%), j) Zn(NO3)2·6H2O, CH3CN, 508C, (93%).
DHQD=dihydroquinidine, DIBAL= diisobutylaluminum hydride,
DMF=N,N-dimethylformamide, PHAL=phthalazine, TBAI=tetra-n-
butylammonium iodide, TBS=tert-butyldimethylsilyl, Ts=toluene-4-
sulfonyl.
ꢀ
allylic C H-oxidation reaction that generated 4-methoxyben-
zoate derivatives of chiral (e)-2-butene-1,4-diols directly from
readily available chiral homoallylic alcohols and carboxylic
acids. We anticipate that the simple and mild nature of this
transformation (that is 2!3, Scheme 1) will render it useful
for the synthesis of polyoxygenated motifs in complex
molecules.
Experimental Section
Representative procedure for the Pd(OAc)2-catalyzed linear allylic
ꢀ
C H oxidation of (ꢀ)-4 to (+)-6: Catalyst Pd(OAc)2 (22.4 mg,
epoxidized with m-chloroperbenzoic acid (mCPBA) to give
the meso-epoxide 8 in 74% yield. The meso-epoxide 8 was
then desymmetrized by using the enantioselective Payne
rearrangement with oligomeric (R,R)-[(salen)CoIII]OTf cata-
lyst (salen = N,N’-bis(salicylidene)ethylenediamine, Tf = tri-
fluoromethanesulfonyl) and subsequently ketalized in situ to
give the chiral epoxyketal (S,S)-9.[15,16] Regioselective opening
at the terminal position of the epoxyketal with vinylcuprate
and subsequent protection of the intermediate alcohol gave
the benzyl ether protected homoallylic alcohol (ꢀ)-4 in 54%
0.1 mmol, 10 mol%), PhBQ (368 mg, 2.0 mmol, 2 equiv), p-anisic
acid (456 mg, 3.0 mmol, 3 equiv), 4-Š MS (200 mg), (ꢀ)-4 (262 mg,
1 mmol, 1 equiv), DMSO (0.380 mL), CH2Cl2 (0.120 mL), DIPEA
(0.087 mL, 0.5 mmol, 0.5 equiv), and a teflon stir bar were added
sequentially to a borosilicate vial (40 mL). The vial was then capped
and the reaction mixture stirred at 418C for 72 h. Care was taken to
keep all reagents off of the walls of the vial and in maintaining the
temperature between 408C and 438C. After 72 h, the reaction was
quenched with saturated aqueous NH4Cl solution (1 mL), stirred for
30 minutes, and then transferred to a separating funnel, and the vial
rinsed with ethyl acetate (10 mL). The mixture was diluted with
hexanes (40 mL; causing significant precipitation of phenyldihydro-
quinone as a black solid that was later removed by filtration). The
organic phase was washed with H2O (50 mL) and aq Na2CO3 solution
(5%, 2 ” 50 mL). The organic layer was dried (MgSO4), filtered, and
concentrated in vacuo. Subsequent transfers were performed using
Et2O to minimize transfer of phenyldihydroquinone. Purification by
column chromatography (silica gel, 30% Et2O in hexanes) gave (+)-6
as an amber oil (309 mg, 75% average yield of 2 runs). The ratios of
the linear/branched products (> 300:1) were determined by HPLC on
overall yield (3 steps, 99% ee).[14,15b] Linear allylic C H
ꢀ
oxidation of (ꢀ)-4 using 10 mol% [Pd(CH3CN)4](BF4)2 in
DMSO under the optimized conditions (2m, PhBQ, 50 mol%
DIPEA) with 3 equivalents of p-anisic acid 5 furnished the 4-
methoxybenzoate-derived (E)-2-butene-1,4-diol (+)-6 in
71% yield (along with 13% recovered (ꢀ)-4) as essentially
one isomer (linear/branched = > 300:1; E/Z = 36:1) and with
no erosion of enantiopurity.[17] Alternatively, on using
Angew. Chem. Int. Ed. 2006, 45, 8217 –8220
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8219

Communications
comparison with the branched-product standard[22] (Agilent Zobrax
Eclipse XDB-C8 column, 35% iPrOH in H2O, 1 mLminꢀ1
A. W. M. Lee, S. Masamune, L. A. Reed III, K. B. Sharpless, F. J.
Walker, Tetrahedron 1990, 46, 245.
,
tR(linear) = 15.7 min, tR(branched, two diastereomers) = 18.0 and
19.0 min). E/Z ratios (30:1) were determined by HPLC using the E
and Z diols without the acetonide group (Symmetry C-18 column,
[10] For example, see: K. Tsuboi, Y. Ichikawa, A. Naganawa, M.
Isobe, M. Ubukata, K. Isono, Tetrahedron 1997, 53, 5083.
[11] a) A. K. Chatterjee, T.-L. Choi, D. P. Sanders, R. H. Grubbs, J.
Am. Chem. Soc. 2003, 125, 11360; b) V. A. Keller, I. Kim, S. D.
Burke, Org. Lett. 2005, 7, 737.
[12] For syntheses of chiral (E)-2-buten-1,4-diols with good to
moderate enantioselectivities from allylation with chiral (E)-g-
(dimethylphenylsilyl)allylboronate reagents followed by epox-
idation/Petersen elimination, see: a) W. R. Roush, P. T. Grover,
Tetrahedron Lett. 1990, 31, 7567; b) W. R. Roush, A. N. Pinchuk,
G. C. Micalizio, Tetrahedron Lett. 2000, 41, 9413.
[13] For representative asymmetric allylation reactions, see: a) G.
Xia, H. Yamamoto, J. Am. Chem. Soc. 2006, 128, 2554; b) S. E.
Denmark, J. Fu, M. J. Lawler, J. Org. Chem. 2006, 71, 1523; c) K.
Kubota, J. L. Leighton, Angew. Chem. 2003, 115, 976; Angew.
Chem. Int. Ed. 2003, 42, 946; d) U. S. Racherla, H. C. Brown, J.
Org. Chem. 1991, 56, 401; e) W. R. Roush, A. E. Walts, L. K.
Hoong, J. Am. Chem. Soc. 1985, 107, 8186.
40% CH3CN in H2O, 1.0 mLminꢀ1
, tR(E) = 10.1 min, tR(Z) =
11.3 min). The linear acetate product (32 mg, 10%) was also
formed in this procedure and was difficult to separate from (+)-6.
Representative procedure for the [Pd(CH3CN)4](BF4)2-catalyzed
ꢀ
linear allylic C H oxidation of (ꢀ)-4 to (+)-6: Catalyst
[Pd(CH3CN)4](BF4)2 (44.4 mg, 0.1 mmol, 10 mol%), PhBQ (368 mg,
2.0 mmol, 2 equiv), p-anisic acid (456 mg, 3.0 mmol, 3 equiv), 4-Š MS
(200 mg), DMSO (0.380 mL), CH2Cl2 (0.120 mL), DIPEA (0.122 mL,
0.7 mmol, 0.7 equiv), and a teflon stir bar were added sequentially to a
borosilicate vial (40 mL). The vial was then capped and the reaction
mixture stirred at 418C for 1 h. The reaction was cooled to room
temperature and (ꢀ)-4 (262 mg, 1 mmol, 1 equiv) was added. The vial
was capped and stirred at 418C for a further 72 h. Care was taken to
keep all reagents off of the walls of the vial and in maintaining the
temperature between 408C and 438C. The workup and isolation
procedures were identical to those described on using Pd(OAc)2, to
give (+)-6 (71% average yield of 2 runs). Approximately 13% of (ꢀ)-
4 was also recovered. Ratios of linear/branched and E/Z products
were determined as described above and found to be similar to those
determined for Pd(OAc)2 (linear/branched > 300:1, E/Z = 36:1).
[14] C. Mukai, J. S. Kim, M. Uchiyama, S. Sakamoto, M. Hanaoka, J.
Chem. Soc. Perkin Trans. 1 1998, 2903.
[15] a) D. E. White, E. N. Jacobsen, Tetrahedron: Asymmetry 2003,
14, 3633; b) C. P. Stevenson, PhD thesis, Harvard University
(USA) 2005; c) J. M. Ready, E. N. Jacobsen, Angew. Chem. 2002,
114, 1432; Angew. Chem. Int. Ed. 2002, 41, 1374; d) M. H. Wu,
K. B. Hansen, E. N. Jacobsen, Angew. Chem. 1999, 111, 2167;
Angew. Chem. Int. Ed. 1999, 38, 2012.
Received: August 14, 2006
Published online: November 16, 2006
[16] This reaction was optimized using commercially available (R,R)-
[(salen)CoIII]OAc (2 mol%) to give the desired chiral epoxyke-
tal in 50% overall yield (95% ee). Lower yields may be
attributed to epoxide opening by MeOH during the ketalization
step with higher catalyst loadings of the monomeric catalyst. See
the Supporting Information for details.
ꢀ
Keywords: allylic oxidation · C H activation · carbohydrates ·
enantioselectivity · polyols
.
[17] The diastereomer which corresponds to racemization of the
allylic center was independently synthesized and was not
detected in the HPLC analysis of the products (see the
Supporting Information for details).
[18] H. C. Kolb, M. S. VanNieuwenhze, K. B. Sharpless, Chem. Rev.
1994, 94, 2483.
[19] S. Vijayasaradhi, J. Singh, I. Aidhen, Synlett 2000, 110.
[20] The silyl groups were removed and (ꢀ)-11 was peracetylated to
give 1,2,3,6-O-tetraacetyl-4-O-benzyl-l-galactopyranose, the
spectroscopic data of which was found to correspond to those
reported for the known compound (see the Supporting Infor-
mation for details): L. Ermolenko, N. A. Sasaki, J. Org. Chem.
2006, 71, 693.
[21] A. B. Northrup, D. W. C. MacMillan, Science 2004, 305, 1752.
[22] a) M. S. Chen, N. Prabagaran, N. Labenz, M. C. White, J. Am.
Chem. Soc. 2005, 127, 6970; b) K. J. Fraunhoffer, N. Prabagaran,
L. E. Sirois, M. C. White, J. Am. Chem. Soc. 2006, 128, 9032.
[1] a) P. A. Wender, M. K. Hilinski, A. V. W. Mayweg, Org. Lett.
2005, 7, 79; b) Q. Zhang, J. O. Rich, I. C. Cotterill, D. P.
Pantaleone, P. C. Michels, J. Am. Chem. Soc. 2005, 127, 7286;
c) A. Hinman, J. Du Bois, J. Am. Chem. Soc. 2003, 125, 11510;
d) B. D. Dangel, K. Godula, S. W. Youn, B. Sezen, D. Sames, J.
Am. Chem. Soc. 2002, 124, 11856; e) R. Breslow, S. Baldwin, T.
Flechter, P. Kalicky, S. Liu, W. Washburn, J. Am. Chem. Soc.
1973, 95, 3251.
[2] K. J. Fraunhoffer, D. A. Bachovchin, M. C. White, Org. Lett.
2005, 7, 223.
[3] a) M. S. Chen, M. C. White, J. Am. Chem. Soc. 2004, 126, 1346;
for a related N,N-dimethylacetamide system, see: b) T. Mitsu-
dome, T. Umetani, N. Nosaka, K. Mori, T. Mizugaki, K. Ebitani,
K. Kaneda, Angew. Chem. 2006, 118, 495; Angew. Chem. Int. Ed.
2006, 45, 481.
[4] For an explanation of the term dissonant relationship, see: D. A.
Evans, G. C. Andrews, Acc. Chem. Res. 1974, 7, 147.
[5] For example, see: a) L. Jiang, S. D. Burke, Org. Lett. 2002, 4,
3411; b) A. B. Smith III, Q. Lin, V. A. Doughty, L. Zhuang,
M. D. McBriar, J. K. Kerns, C. S. Brook, N. Murase, K.
Nakayama, Angew. Chem. 2001, 113, 202; Angew. Chem. Int.
Ed. 2001, 40, 196.
[6] For example, see: M. T. Crimmins, F. Caussanel, J. Am. Chem.
Soc. 2006, 128, 3128.
[7] For example, see: a) B. M. Trost, S. T. Wrobleski, J. D. Chisholm,
P. E. Harrington, M. Jung, J. Am. Chem. Soc. 2005, 127, 13589;
b) N. Ikemoto, S. L. Schreiber, J. Am. Chem. Soc. 1992, 114,
2524.
[8] a) A. Guzman-Perez, E. J. Corey, Tetrahedron Lett. 1997, 38,
5941; b) E. J. Corey, A. Guzman-Perez, M. C. Noe, J. Am. Chem.
Soc. 1995, 117, 10805.
[9] a) S. Y. Ko, A. W. M. Lee, S. Masamune, L. A. Reed III, K. B.
Sharpless, F. J. Walker, Science 1983, 220, 949; b) S. K. Ko,
8220 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 8217 –8220