A flexible synthetic route to isoxazolidine β-proline analogs

Article abstract of DOI:10.1016/j.tet.2008.12.062

The function and higher order structure of β-proline oligomers have been relatively unexplored compared to other foldamer classes due in part to synthetic challenges associated with substituted monomers. Here we report a flexible synthesis of di- and tris

Full text of DOI:10.1016/j.tet.2008.12.062

Tetrahedron 65 (2009) 3305–3313
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
A flexible synthetic route to isoxazolidine b-proline analogs
*
Sara J. Buhrlage, Bin Chen, Anna K. Mapp
Department of Chemistry, University of Michigan, Ann Arbor, MI, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 October 2008
Received in revised form 15 December 2008
Accepted 18 December 2008
Available online 25 December 2008
The function and higher order structure of
pared to other foldamer classes due in part to synthetic challenges associated with substituted mono-
mers. Here we report a flexible synthesis of di- and trisubstituted isoxazolidine -proline monomers that
enables access to a wide range of densely functionalized analogs suitably protected for solid-phase
synthetic protocols. This strategy utilizes chiral isoxazolines as key intermediates and can readily provide
single stereoisomers of the target molecules.
b-proline oligomers have been relatively unexplored com-
b
We dedicate this paper to Professor Justin
Du Bois in honor of the 2007 Tetrahedron
Young Investigator Award in recognition of
his many important contributions to Or-
ganic Chemistry
Ó 2008 Published by Elsevier Ltd.
1. Introduction
recently demonstrated that b-amino acid foldamers can assemble
into higher order structures, a critical step toward proteomimetics
Nonnatural molecules that mimic key protein structural motifs
that mimic the structure and function of natural proteins.7–12
such as helices and
b
-turns have found widespread application as
One class of
oligomers composed of
meric units have been utilized in a number of applications.13–16
Early solution studies of unfunctionalized -proline oligomers (4 or
b
-peptides that has been less studied includes
catalysts, sensors, materials, and as inhibitors of protein–protein
interactions.1–4 A key advantage of these so-called ‘foldamers’
compared to proteins is their often enhanced structural and
proteolytic stability relative to their natural counterparts. Fol-
b-prolines (Fig. 1); however, the mono-
b
more subunits) suggested the existence of multiple rotameric
damers composed of
b-amino acids, for example (Fig. 1), can
states, precluding thorough structural characterization.17,18 How-
adopt stable and predictable secondary structures, are resistant to
proteolytic degradation, and can be more densely functionalized
ever, the introduction of two substituents at C2 of the
leads to a strong preference for the Z-amide bond isomer.19 Indeed,
CD analysis of the homooligomers of disubstituted -prolines in-
b-proline (3)
than their natural
a
-amino acid counterparts.5,6 Further, it was
b
dicated the adoption of regular structure for the pentameric and
hexameric species.19 Although an exciting advance, a major road-
block for further implementing these more substituted b-proline
HN
NH2
O
OH
oligomers is the synthetic difficulties associated with selectively
installing a range of functional groups adjacent to the nitrogen and
along the backbone. The standard approach employs 4-hydroxyl-
proline as the starting material and uses alkylation to install a sec-
ond functional group adjacent to the nitrogen.19,20 This inherently
limits the diversity of functional groups that can be installed as one
is derived from the carboxylic acid moiety. Further, the in-
corporation of additional functional groups within the backbone,
OH
O
1
2
R
N
P
O
N
O
3
R2
R1
OH
2
OH
H
R3
4
O
O
a substitution pattern important for other
b-amino acid classes,
3
would require a significantly more complicated synthetic route.
To address this need, we hypothesized that a flexible synthetic
strategy could be developed for densely functionalized b-proline
Figure 1. Structural classes of
monomer; oligomers of 2 populate multiple rotamer states; 3: disubstituted
oligomers of 3 adopt regular structure; 4: trisubstituted isoxazolidine
b
-amino acids. 1:
b
-amino acid skeleton; 2:
b
b
-proline
-proline;
b-proline analog.
analogs in which one carbon has been replaced by an oxygen (4,
Fig. 1), a substitution that should confer advantageous aqueous
solubility properties while minimally perturbing the amide bond in
the context of oligomers. An additional advantage is that the
* Corresponding author.
E-mail address: amapp@umich.edu (A.K. Mapp).
0040-4020/$ – see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tet.2008.12.062

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S.J. Buhrlage et al. / Tetrahedron 65 (2009) 3305–3313
synthetic strategy employs chiral isoxazolines as the key inter-
mediates; isoxazolines are readily accessible as single stereoiso-
mers and as such have been employed as building blocks for other
i. tBuOCl
tol
N
O
5
-78 °C, 1 h
R1
OTBS
5
4
H
ii. 6, EtMgBr, tBuOH
tol
0 °C to rt, 12 h
-amino acid classes and complex targets such as polyketides.21–29
OR
b
7a: R = H; R1 = iBu (88%)
Toward isoxazolidine
b
-prolines, a 1,3-dipolar cycloaddition using
7b: R = H; R1 = Et (79%)
TBSOTf
Et3N
THF
the conditions of Kanemasa and Carreira establishes the stereo-
chemistry at C5 of isoxazoline 7 and introduces the first substituent
adjacent to the nitrogen (Fig. 2).30,31 Additional substitution at C4
of the isoxazoline ring of 7 can be incorporated by using a di-
substituted double bond in the cycloaddition and/or through
a separate alkylation step post-cycloaddition.28 Selective nucleo-
philic attack at the C]N bond within 7 then provides the addi-
tional C3 substituent in 8.28,29 Finally, oxidative cleavage of the C5
diol moiety would provide the requisite carboxylic acid at this
position (9). Here we describe the development and implementa-
10a: R = TBS; R1 = iBu (85%)
10b: R = TBS; R1 = Et (95%)
Scheme 1. Synthesis of the key isoxazoline intermediates.
Table 1
Nucleophile addition to isoxazoline 10
tion of this synthetic strategy for a range of b-proline analogs, in-
cluding those with substitution patterns not accessible by previous
methodologies.
HN
R2
O
H
BF3•OEt2, R2-M
10
OTBS
OTBS
R1
tol, -78 °C, 4-7 h
11-16
2. Results and discussion
Entry
R1
R2–M
Yielda (%)
drb
11
12
13
14
15
16
iBu
iBu
iBu
iBu
iBu
Et
BnMgBr
PhLi
allylMgCl
MeLi
thienylLi
2-MethylallylMgCl
80
60
76
64
73
79
10:1
7:1
8:1
7:1
20:1
8:1
2.1. Disubstituted isoxazolidine
b-prolines
The first step in the synthesis of disubstituted isoxazolidine
b
-
prolines is a 1,3-dipolar cycloaddition between a nitrile oxide and
a chiral allylic alcohol derived from glycidol (Scheme 1).30,31–33
Consistent with previous reports, the cycloaddition proceeds in
excellent yield with either an isobutyl (5a) or ethyl (5b) R1 sub-
stituent.28,29 Subsequent protection of the secondary alcohol as
a silyl ether produced isoxazoline 10 in excellent yield.
Installation of the second functional group at C3 was accom-
plished by the combination of isoxazoline 10 with either a stabi-
lized Grignard reagent (11, 13, and 16) or an organolithium reagent
(12, 14, 15) in the presence of BF3/Et2O (Table 1). In all examples,
the major diastereomer isolated results from addition to the re face
of the C]N bond, with good (7:1) to excellent (>20:1) di-
astereomeric ratios observed. The relative stereochemistry was
assigned by NOE analysis. In this way, a variety of alkyl, aryl, and
heteroaryl functional groups can be positioned at C3. Protection of
the secondary alcohol prior to this reaction is not a requirement,
but the isolation and purification of the product are facilitated.
a
Yield of the major diastereomer.
b
Diastereomeric ratios determined by 1H NMR spectral integration of the crude
product mixture.
produced an intermediate aldehyde; due to instability of the al-
dehyde, this intermediate was immediately oxidized to the more
stable carboxylic acid. This was accomplished by treatment with
sodium perchlorate to cleanly produce the desired isoxazolidine
b
-proline analogs shown (17–22) in good overall yields (over the
four steps shown) and with no detectable epimerization.
A key feature of this synthetic approach to -prolines is that
b
diverse functionality can be introduced at multiple stages, from
choice of oxime in the cycloaddition step to late-stage modification
of the C3 and N2 side chains. As a representative example, we
prepared an amphipathic isoxazoldine b-proline, 26, bearing a hy-
drophobic (isobutyl) and polar (hydroxyl) functional group at the
carbon adjacent to the nitrogen (Scheme 2). From 13, the nitrogen
was protected with an Fmoc group in 92% yield. The double bond of
isoxazolidine 23 was oxidatively cleaved and protected as the MOM
ether; the MOM group is compatible with Fmoc solid-phase pep-
tide synthesis. Subsequently the silyl ethers were selectively re-
moved by treatment with 20% HCl in MeOH. In the final step the
pendant diol of 25 was oxidatively cleaved to yield isoxazolidine
Conversion of isoxazolidines 11–16 to the
b-proline analogs
required only protection at N2 followed by unmasking of the C5
diol moiety and oxidative cleavage to the carboxylic acid (Fig. 3).
Protection of the ring nitrogen with a group compatible with solid-
phase synthesis was anticipated to be a challenging step. Due to
significant steric hindrance and concomitant reduced nucleophi-
licity of the nitrogen, we have previously found that alkyl groups
can only be added to that position using microwave-accelerated
conditions.34 However, it was possible to protect the nitrogen with
a BOC, Fmoc or Cbz group, all commonly used in solid-phase pep-
tide synthesis, without requiring forcing conditions. Installation of
the Cbz group consistently provided the highest yields and is
shown in Figure 3. The TBS groups were then removed in situ by
addition of a fluoride source (TBAF). The resulting diols were oxi-
datively cleaved in a two-step procedure in which a NaIO4 cleavage
b-proline 26 (Scheme 2). This monomer is suitable for use in solid-
phase synthesis.
2.2. Synthesis of trisubstituted isoxazolidine b-prolines
As described earlier, we envisioned that one advantage of our
synthetic strategy would be that it could be used to access
OH
N
P
P
R1
H
N
O
N
O
H
N
O
R2
R1
R2
R1
3
5
5
5
OH
4
R1
OTBS
+
OH
H
H
R3 OH
8
R3
9
O
R3 OH
7
OTBS
OH
6
Figure 2. Isoxazoline strategy for the synthesis of di- and trisubstituted isoxazolidine b-prolines.

S.J. Buhrlage et al. / Tetrahedron 65 (2009) 3305–3313
3307
i. NaIO4
i. CBzCl, Na2CO3
CH3CN/H2O
rt, 1-2 h
Cbz
CBz
THF/H2O
N
O
H
N
O
H
R2
R1
0 °C to rt, 12 h
11-16
R2
R1
OH
ii. NaClO2, KH2PO4,
2-methyl-2-butene
t-BuOH/H2O
OH
ii. TBAF
OH
O
THF
0 °C to rt, 1-3h
17-22
rt, 12h
CBz
N
CBz
CBz
N
O
H
N
O
O
OH
OH
O
OH
H
H
O
O
17
18
19
42%
52%
40%
CBz
CBz
CBz
S
N
O
H
N
O
H
N
O
H
OH
OH
OH
O
O
O
20
21
22
45%
55%
42%
Figure 3. Conversion of isoxazolidines 11–16 to b-prolines.
Fmoc
1. OsO4, NMO
t-BuOH/THF/H2O
rt, 6h
Fmoc
HN
O
H
N
O
H
FmocCl
N
O
H
sat. aq. Na2CO3
OTBS
OTBS
HO
OTBS
OTBS
OTBS
OTBS
THF
2. NaIO4
0 °C to rt, 12H
92%
CH3CN/H2O
0 °C, 1h
3. NaBH4
MeOH
13
23
24
0 °C, 30 min
1. 20% HCl/MeOH
2. NaIO4
CH3CN/H2O
0 °C, 1h
3. NaClO2, KH2PO4,
2-methyl-2-butene
Fmoc
Fmoc
N
O
H
N
O
MOMCl, iPr2NEt
CH2Cl2, rt, 12h
MOMO
OH
MOMO
OTBS
OTBS
H
O
t-BuOH/H2O
25
26
78%
rt, 12h
53%
Scheme 2. Synthesis of amphipathic isoxazolidine b-proline 26.
trisubstituted
b
-proline analogs. Although not expected to strongly
assemblies. A C4 substituent can be introduced at the cycloaddition
stage through the use of allylic alcohols containing disubstituted
double bonds or via an alkylation reaction with the isoxazolidine.
We elected the alkylation strategy for this study as it offers a more
impact the conformational preferences of oligomers, functional
groups at C4 of the isoxazolidine ring could be useful for inter-
oligomer interactions, facilitating the formation of higher order
S
Li
or
HN
R2
R
O
H
N
O
H
N
O
H
BF3 OEt2,
BnBr, LDA
OTBS
OTBS
OTBS
Bn OTBS
OTBS
OTBS
THF
-78 C, 3 h
4
MgCl
tol
Bn
-78 C, 4-7 h
10b
27
28-30
HN
O
H
HN
O
H
HN
O
H
S
OTBS
OTBS
OTBS
Bn OTBS
OTBS
OTBS
Bn
28
Bn
30
29
55% yield; d.r. >20:1
81% yield; d.r. 1.2:1
Figure 4. Nucleophile addition to C4 substituted isoxazolidines.

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S.J. Buhrlage et al. / Tetrahedron 65 (2009) 3305–3313
convergent synthesis (Fig. 4). Deprotonation of isoxazolidine 10b
with LDA and the addition of benzyl bromide proceeded in good
yield (62%) to provide 27 as a single stereoisomer. Nucleophilic
attack at the C]N bond proceeded with either an organolithium
reagent (28) or a stabilized Grignard (29, 30) reagent. The C4 and C5
substituents exert opposing steric effects in this reaction. In the
case of 28, the additional steric hindrance leads to an attenuated
yield (55%) relative to the earlier example (Table 1, 15), although
only a single diastereomer was observed. In the second example,
a better overall yield was obtained of a 1.2:1 mixture of di-
astereomers. In all cases, the relative stereochemistry was verified
by NOESY carried out on N2-protected derivatives.
stirred for 2 h with continued cooling at which time TLC analysis
indicated complete consumption of starting material. In a separate
flask, chiral allylic alcohol 632,33 (1.7 g, 8.6 mmol, 1.3 equiv) in
toluene (66 mL) was cooled in an ice–H2O bath. To this solution
was added tBuOH (2.1 mL, 22 mmol, 3.3 equiv) followed by
dropwise addition of EtMgBr (10 mL of a 2.0 M solution in Et2O,
20 mmol, 3.0 equiv) and the solution was stirred for 1 h with
continued cooling. The solution of hydroximinoyl chloride was
then transferred via cannula to the allylic alcohol solution and the
mixture allowed to slowly warm to ambient temperature and
stirred for 15 h. Satd NH4Cl (10 mL) was added to the reaction
mixture followed by further dilution with H2O (40 mL). The or-
ganic and aqueous layers were separated and the aqueous
extracted with CH2Cl2 (3ꢀ40 mL). The combined organic extracts
were washed with brine (1ꢀ30 mL), dried over MgSO4, filtered,
and concentrated. Purification by flash chromatography yielded
1.4 g of isoxazoline 7b as a colorless oil in 79% yield as a single
stereoisomer. IR: 3365, 2929, 2857, 1115, 1059, 837 cmꢁ1; 1H NMR:
Conversion of isoxazolidines 28–30 to
b-proline analogs oc-
curred using the same reaction sequences as outlined in Figure 3.
Despite the additional steric hindrance in these isoxazolidines, Cbz
protection proceeded without incident as did the deprotection/
oxidative cleavage sequence. Purification of the acids was, however,
facilitated by in situ protection as the methyl ester. Although only
three examples are presented here, these results suggest that the
strategy should be utilizable for a wider range of substrates by
variation of the oxime substituent in the initial cycloaddition, al-
ternative alkylating agents, and the organometallic reagents used
in Table 1.
d
0.06 (s, 6H), 0.88 (s, 9H), 1.15 (t, 3H, J¼7.33), 2.32–2.57 (m, 2H),
2.95–2.98 (m, 2H), 3.56–3.73 (m, 3H), 4.62 (ddd, 1H, J¼11.4, 7.7,
2.6); 13C NMR:
d
ꢁ5.5, ꢁ5.5, 10.8, 18.2, 21.2, 25.8, 38.9, 63.9, 73.0,
79.2, 160.6; HRMS (ESI) calcd for [C13H27NO3SiþNa]þ: 296.1658,
found: 296.1655.
3. Conclusions
4.3. 5-[1,2-Bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-3-
ethyl-4,5-dihydro-isoxazole (10b)
Here we have described the preparation of isoxazolidine
b-
proline analogs functionalized with a variety of substituents. This
was accomplished via a flexible synthetic route employing simple
building blocks (oximes, allylic alcohols, organometallic nucleo-
philes) that should enable selective installation of wider range of
functional groups than previously available. In addition, this
To a solution of isoxazoline 7b (1.4 g, 5.1 mmol, 1.0 equiv) in THF
(26 mL) cooled in an ice–H2O bath were added DMAP (63 mg,
0.51 mmol, 0.10 equiv) and Et3N (1.6 mL, 11 mmol, 2.2 equiv).
TBSOTf (2.6 mL, 11 mmol, 2.2 equiv) was then added dropwise and
the solution slowly warmed to ambient temperature. Upon stirring
for 2 h, TLC analysis indicated complete consumption of starting
material. The mixture was again cooled in an ice–H2O bath and
diluted with satd NH4Cl (15 mL). The solution was extracted with
Et2O (3ꢀ15 mL) and the combined organic extracts were washed
with brine (1ꢀ20 mL), dried over MgSO4, filtered, and concentrated
in vacuo. Purification of the crude product by flash chromatography
(95:5 hexanes/EtOAc) yielded 1.9 g of isoxazoline 10b in 95% yield
strategy provides access to either enantiomer of a given b-proline
monomer depending upon the choice of chiral allylic alcohol used
in the initial cycloaddition step. We have previously described the
biological activity of related isoxazolidines as transcriptional ac-
tivators; given the likely structural preferences of oligomers of
isoxazolidine b-prolines such as the ones shown here, transcrip-
tional activators with enhanced function should now be accessi-
ble.34–36
as a colorless oil. IR: 2929, 2856, 1462, 1254, 1090, 835 cmꢁ1
;
1H
NMR: 0.04 (s, 3H), 0.04 (s, 3H), 0.06 (s, 3H), 0.07 (s, 3H), 0.85 (s,
d
4. Experimental section
4.1. General
9H), 0.87 (s, 9H), 1.14 (t, 3H, J¼7.3), 2.25–2.37 (m, 2H), 2.88 (d, 2H,
J¼9.3), 3.56 (dd, 1H, J¼9.0, 1.7), 3.62–3.69 (m, 2H), 4.59–4.63 (m,
1H); 13C NMR:
d
ꢁ5.5, ꢁ4.9, ꢁ4.4, 10.7, 18.0, 18.2, 21.3, 25.7, 25.8,
38.2, 64.2, 74.2, 80.1, 159.4; HRMS (ESI) calcd for
Unless otherwise noted, starting materials were obtained from
commercial suppliers and used without further purification.
CH2Cl2, THF, benzene, and toluene were dried by passage through
activated alumina columns and degassed by stirring under a dry N2
atmosphere.37 All reactions involving air- or moisture-sensitive
compounds were performed under a dry N2 atmosphere. BF3$OEt2,
[C19H41NO3Si2þNa]þ: 410.2523, found: 410.2517.
4.4. 4-Benzyl-5-[1,2-bis-(tert-butyl-dimethyl-silanyloxy)-
ethyl]-3-ethyl-4,5-dihydro-isoxazole (27)
iPr2NH (760
mL, 5.4 mmol, 1.5 equiv) was added to 25 mL THF
i
Et3N, Pr2NH, and MeOH were distilled from CaH2. Purification by
and the solution was cooled in an ice–H2O bath. To the solution was
added n-BuLi (3.4 mL of a 1.7 M solution in hexanes, 5.6 mmol,
1.6 equiv). The mixture was stirred for 10 min and transferred to
a dry ice–acetone bath. A solution of isoxazoline 10b (1.4 g,
3.6 mmol, 1.0 equiv) in 5 mL THF was added to the solution drop-
wise over 15 min and the mixture was stirred for 1 h with contin-
ued cooling. BnBr (1.4 mL, 11 mmol, 3.0 equiv) in 5 mL THF was
subsequently added and the mixture continued to stir while being
cooled until complete by TLC analysis (3 h). The mixture was di-
luted with satd aq NH4Cl (10 mL) and extracted with Et2O
(3ꢀ20 mL). The combined organic extracts were washed with brine
(20 mL), dried over MgSO4, filtered, and concentrated in vacuo. The
crude product was purified by flash chromatography (98:2 hex-
anes/EtOAc) to yield 1.1 g of isoxazoline 27 as a colorless oil in 62%
flash column chromatography was carried out with E. Merck Silica
Gel 60 (230–400 mesh) according to the procedure of Still et al.38
1H and 13C NMR spectra were recorded in CDCl3 at 500 MHz and
125 MHz, respectively, unless otherwise specified. IR spectra were
measured as thin films on NaCl plates. Compounds 7a, 10a, and 11
have been reported previously.35
4.2. 2-(tert-Butyl-dimethyl-silanyloxy)-1-[(3-ethyl-4,5-
dihydro-isoxazol-5-yl)]-ethanol (7b)
To a solution of oxime 5, in which R¼ethyl (480 mg, 6.6 mmol,
1.0 equiv), in toluene (33 mL) cooled in a dry ice–acetone bath and
shielded from light was added tBuOCl (790
mL, 6.6 mmol,
1.0 equiv) dropwise over 20 min. The resulting mixture was
yield. IR: 2929, 2856, 1462, 1254, 1090, 835 cmꢁ1; 1H NMR:
d
ꢁ0.04

S.J. Buhrlage et al. / Tetrahedron 65 (2009) 3305–3313
3309
(s, 6H), ꢁ0.02 (s, 3H), ꢁ0.02 (s, 3H), 0.82 (s, 9H), 0.83 (s, 9H), 1.14 (t,
3H, J¼7.3), 2.10–2.20 (m, 1H), 2.34–2.43 (m, 1H), 2.65 (dd, 1H, J¼9.9,
13.6), 3.01 (dd, 1H, J¼5.5, 13.6), 3.17–3.21 (m, 1H), 3.35–3.45 (m,
2H), 3.52 (dd, 1H, J¼6.9, 9.9), 4.33 (dd, 1H, J¼2.9, 5.9), 7.13–7.15 (m,
12 as a colorless oil in 60% yield (major diastereomer, dr 7:1). IR:
2954, 2929, 2858, 1472, 1255 cmꢁ1; 1H NMR:
d
ꢁ0.01 (d, 6H, J¼5.2),
0.09 (d, 6H, J¼6.5), 0.66 (d, 3H, J¼6.6), 0.76 (d, 3H, J¼6.6), 0.81 (s,
9H), 0.91 (s, 9H), 1.22 (br s, 1H), 1.52 (br s, 1H), 2.00 (br s, 1H), 2.03–
2.24 (m, 1H), 2.74 (br s, 1H), 3.58–3.60 (m, 2H), 3.65–3.70 (m, 1H),
4.02–4.06 (m, 1H), 5.45 (br s, 1H), 7.16–7.22 (m, 1H), 7.29–7.31 (m,
2H), 7.19–7.30 (m, 3H); 13C NMR:
d
ꢁ5.5, ꢁ5.5, ꢁ4.8, ꢁ4.3, 10.7, 18.0,
18.3, 20.1, 25.8, 25.9, 37.6, 52.1, 63.7, 74.2, 76.7, 77.0, 77.3, 84.6, 126.7,
128.7,128.9, 138.1, 161.5; HRMS (ESI) calcd for [C26H47NO3Si2þNa]þ:
500.2992, found: 500.2999.
2H), 7.49–7.51 (m, 2H); 13C NMR (100 MHz):
23.7, 24.4, 25.1, 26.2, 46.5, 49.1, 64.9, 71.3, 75.9, 78.6, 126.5, 126.7,
128.2, 144.2; HRMS (ESI) calcd for [C27H51NO3Si2þNa]þ: 516.3305,
found: 516.3302.
d
ꢁ5.2, ꢁ4.1, 18.3, 18.6,
4.5. General procedure for nucleophile addition to
isoxazolines
4.5.2. 3-Allyl-5-[1,2-bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-3-
isobutyl-isoxazolidine (13)
To a solution of isoxazoline 10a or 10b (1.0 equiv) in toluene
(final concentration 0.1–0.15 M) cooled in a dry ice–acetone bath
was added BF3$OEt2 (3.0 equiv) dropwise, and the resulting mix-
ture was stirred with continued cooling for 30 min. A solution of
organolithium reagent or Grignard reagent (6.0–8.0 equiv) was
then added dropwise. The reaction mixture was allowed to stir
with continued cooling until the starting material had been con-
sumed as ascertained by TLC analysis, typically within 4 h. Satu-
rated NaHCO3 was added slowly to consume any remaining
organolithium reagent or Grignard reagent, the solution was
transferred to an ice–H2O bath, and the mixture was extracted
with EtOAc. The organic extracts were combined, washed with
H2O and brine, dried over Na2SO4, and concentrated in vacuo. The
major and minor diastereomers were separated chromatographi-
cally and the major diastereomer was used in subsequent steps. In
the case of the addition of methylallylmagnesium chloride to 27,
a 1.2:1 mixture of diastereomers was obtained and each was in-
dividually carried forward as depicted in Figure 5. The di-
astereomeric ratios were determined by spectral integration of
the crude product mixture. However, in the case of 12–15, there
was significant spectral overlap and it was thus difficult to accu-
rately ascertain diastereoselectivity at this stage. In these in-
stances, the diastereoselectivity was assessed following protection
of N2 (Fig. 3).
Prepared following the general procedure with addition of
allylmagnesium chloride (5 mL of a 2.0 M solution in THF, 10 mmol,
7.2 equiv) to 10a (580 mg, 1.4 mmol, 1.0 equiv). Purification of the
crude product mixture by flash column chromatography (15:1
hexanes/EtOAc and 2% NH4OH) provided 490 mg of isoxazolidine
13 as a colorless oil in 76% yield (major diastereomer, dr 8:1). IR:
2955, 2930, 2858, 1472, 1255 cmꢁ1; 1H NMR:
d
ꢁ0.02 (d, 6H, J¼1.8),
0.05 (br s, 6H), 0.84 (s, 9H), 0.86 (s, 9H), 0.88 (s, 3H), 0.90 (s, 3H),
1.30–1.44 (m, 2H), 1.75–1.82 (m, 2H), 2.13–2.19 (m, 2H), 2.30–2.36
(m, 1H), 3.53–3.57 (m, 2H), 3.66–3.68 (m, 1H), 4.23 (br s, 1H), 5.02–
5.06 (m, 2H), 5.77–5.89 (m, 1H); 13C NMR (100 MHz):
d
ꢁ5.2, ꢁ4.4,
ꢁ4.1, 18.2, 18.5, 24.2, 24.4, 24.5, 26.1, 40.1, 42.5, 43.7, 64.5, 67.6, 76.0,
78.9, 117.3, 135.4; HRMS (ESI) calcd for [C24H51NO3Si2þNa]þ:
480.3305, found: 480.3304.
4.5.3. 5-[1,2-Bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-3-isobutyl-
3-methyl-isoxazolidine (14)
Prepared following the general procedure with addition of
MeLi (3.1 mL of a 1.6 M solution in ether, 5.0 mmol, 7.2 equiv) to
10a (290 mg, 0.7 mmol, 1.0 equiv). Purification of the crude
product mixture by flash column chromatography (9:1 hexanes/
EtOAc and 2% NH4OH) provided 190 mg of isoxazolidine 14 as
a colorless oil in 64% yield (major diastereomer, dr 7:1). IR: 2954,
2929, 2858, 1472, 1255 cmꢁ1
;
1H NMR:
d
0.01 (d, 6H, J¼1.5), 0.05
(s, 6H), 0.85 (s, 9H), 0.86 (s, 9H), 0.89 (d, 3H, J¼6.6), 0.90 (d, 3H,
J¼6.6), 1.13 (s, 3H), 1.30–1.35 (m, 1H), 1.39–1.44 (m, 1H), 1.77–1.86
(m, 2H), 2.01–2.10 (m, 1H), 3.54–3.57 (m, 2H), 3.66–3.69 (m, 1H),
4.5.1. 5-[1,2-Bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-3-isobutyl-
3-phenyl-isoxazolidine (12)
Prepared following the general procedure with addition of PhLi
4.30 (br s, 1H), 5.17 (br s, 1H); 13C NMR (100 MHz):
d
ꢁ5.2, ꢁ4.3,
(2.3 mL of
a 1.9 M solution in cyclohexane/ether, 4.3 mmol,
ꢁ4.2, 18.2, 18.6, 23.7, 24.2, 24.5, 25.2, 26.2, 44.5, 46.5, 64.6, 65.1,
75.9, 79.2; HRMS (ESI) calcd for [C22H49NO3Si2þH]þ: 432.3329,
found: 432.3322.
7.2 equiv) to 10a (250 mg, 0.6 mmol, 1.0 equiv). Purification of the
crude product mixture by flash column chromatography (9:1
hexanes/EtOAc and 2% NH4OH) provided 180 mg of isoxazolidine
i. NaIO4
CH3CN/H2O
CBzCl, Na CO
i.
2
3
CBz
R2
rt, 1-2 h
CBz
R2
THF/H2O
N
O
H
N
O
H
0 °C to rt, 12 h
ii. NaClO2, KH2PO4,
2-methyl-2-butene
t-BuOH/H2O
OMe
28-30
OTBS
OTBS
ii.
THF pyr
THF
0 °C to rt, 1-3h
R1
R1
Bn
Bn
O
rt, 12h
31-33
iii. TMSCHN2
benzene/MeOH
rt, 5 min
CBz
CBz
CBz
O
H
N
S
O
H
N
N
O
H
OMe
OMe
OMe
O
O
O
31
61%
32
54%
33
56%
Figure 5. Generation of trisubstituted b-prolines.

3310
S.J. Buhrlage et al. / Tetrahedron 65 (2009) 3305–3313
4.5.4. 5-[1,2-Bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-3-isobutyl-
3-thiophen-isoxazolidine (15)
2.36 (d, 1H, J¼13.2), 2.47–2.53 (m, 1H), 2.56–2.64 (m, 2H), 2.97 (dd,
1H, J¼12.5, 2.9), 3.34 (dd, 1H, J¼9.5, 5.1), 3.58 (dd, 1H, J¼9.5, 8.1),
4.06 (d, 1H, J¼6.6), 4.75 (s, 1H), 4.87–4.88 (m, 1H), 5.34 (br s, 1H),
2-Thienyllithium (4.3 mL of a 1 M solution in THF, 4.3 mmol,
7.2 equiv) was added to isoxazoline 10a (250 mg, 0.6 mmol,
1.0 equiv). Purification of the crude product mixture by flash col-
umn chromatography (10:1 hexanes/EtOAc and 2% NH4OH) pro-
vided 220 mg of isoxazolidine 15 as a colorless oil in 73% yield
(major diastereomer, dr 20:1). IR: 2954, 2929, 2858, 1472,
7.12–7.27 (m, 5H); 13C NMR:
d
ꢁ5.5, ꢁ5.4, ꢁ4.8, ꢁ4.4, 9.1, 17.9, 18.3,
24.9, 25.9, 26.0, 26.1, 36.7, 37.6, 53.7, 64.1, 68.5, 75.7, 76.7, 77.0, 77.3,
85.0, 113.9, 126.3, 128.6, 128.8, 140.3, 143.9; HRMS (ESI) calcd for
[C30H55NO3Si2þNa]þ: 556.3618, found: 556.3615.
1255 cmꢁ1; 1H NMR:
d
0.02 (d, 6H, J¼3.3), 0.12 (d, 6H, J¼3.0), 0.83–
4.5.7.2. 4-Benzyl-5-[1,2-bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-
3-ethyl-3-(2-methyl-allyl)-isoxazolidine (30). IR: 2953, 2928, 2857,
0.85 (m, 6H), 0.86 (s, 9H), 0.93 (s, 9H), 1.40–1.55 (m, 1H), 1.64–1.86
(m, 2H), 2.25–2.29 (m, 1H), 2.75–2.90 (m, 1H), 3.59–3.65 (m, 2H),
3.70–3.74 (m, 1H), 4.29 (br s, 1H), 5.65 (br s, 1H), 6.92–6.96 (m, 2H),
1471, 1254, 1093, 836 cmꢁ1; 1H NMR:
d
ꢁ0.15 (s, 3H), ꢁ0.07 (s, 3H),
ꢁ0.01 (s, 3H), 0.00 (s, 3H), 0.84 (s, 9H), 0.89 (s, 9H), 1.03 (t, 3H,
J¼7.7), 1.51–1.59 (m, 1H), 1.64–1.73 (m, 1H), 1.89 (s, 3H), 2.28 (d, 1H,
J¼13.2), 2.33 (d, 1H, J¼13.3), 2.51–2.68 (m, 3H), 3.02 (dd, 1H, J¼12.5,
2.2), 3.37 (dd, 1H, J¼9.5, 5.1), 3.49–3.53 (m, 1H), 4.12 (d, 1H, J¼6.59),
4.91 (s, 1H), 5.02 (s, 1H), 7.16–7.22 (m, 3H), 7.25–7.28 (m, 2H); 13C
7.17–7.19 (m, 1H); 13C NMR (100 MHz):
d
ꢁ5.2, ꢁ5.2, ꢁ4.3, ꢁ4.1,
18.2, 18.6, 23.9, 24.0, 25.3, 26.2, 46.1, 48.8, 64.8, 70.4, 76.1, 79.4,
123.9, 124.3, 126.7; HRMS (ESI) calcd for [C25H49NO3SSi2þH]þ:
500.3050, found: 500.3055.
NMR:
d
ꢁ5.5, ꢁ5.4, ꢁ4.7, ꢁ4.4, 8.1, 17.9, 18.3, 24.3, 24.9, 25.9, 25.9,
4.5.5. 5-[1,2-Bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-3-ethyl-3-
(2-methyl-allyl)-isoxazolidine (16)
35.3, 40.5, 53.6, 64.0, 67.8, 75.1, 84.2, 115.9,126.2, 128.6, 128.9, 140.3,
141.5; HRMS (ESI) calcd for [C30H55NO3Si2þNa]þ: 556.3618, found:
556.3615.
Compound 16 was prepared following the general procedure
using isoxazoline 10b (800 mg, 2.1 mmol, 1.0 equiv) and the nu-
cleophile 2-methylallylmagnesium chloride (30 mL of a 0.5 M so-
lution in THF, 15 mmol, 7.2 equiv). Purification of the crude product
by flash chromatography (95:5 hexanes/EtOAc) yielded 730 mg of
the major diastereomer (dr 8:1) as a clear oil in 79% yield. 1H NMR:
4.6. General procedure for Cbz protection, TBS deprotection,
and oxidative cleavage
A solution of isoxazolidine (1.0 equiv) in THF (final concentra-
tion of 0.3 M) was cooled in an ice–H2O bath. To the stirring solu-
tion were added CbzCl (3.0 equiv) and satd Na2CO3 (final
concentration of 0.3 M) and the biphasic reaction mixture was
stirred at rt for 12 h. The reaction mixture was then partitioned
between H2O and EtOAc, and the aqueous layer was extracted 3ꢀ
with EtOAc. The combined organic extracts were dried over Na2SO4
and concentrated in vacuo. The crude mixture was passed through
a short plug of SiO2 (15:1 hexanes/EtOAc) and concentrated. The
residue was dissolved in THF, cooled in an ice–H2O bath, and
4 equiv TBAF (1 M in THF) or HF$Py (3.0 equiv) was added drop-
wise. HF$Py was used for the deprotection of isoxazolidines con-
taining a C4 substituent (28–30) as the deprotection with TBAF
proceeded very slowly. After 1 h, the ice–H2O bath was removed
and the progress of the reaction was monitored by TLC analysis.
Upon complete consumption of starting material, the reaction was
then diluted with H2O and extracted 3ꢀ with Et2O or EtOAc. The
organic extracts were combined, washed with H2O and brine, dried
over Na2SO4, filtered, and concentrated in vacuo. The crude diol was
then used in the subsequent oxidative cleavage reactions after
passing through a plug of silica to eliminate the nonpolar impurities
(1:9 MeOH/CH2Cl2).
d
0.03 (s, 3H), 0.03 (s, 3H), 0.06 (s, 6H), 0.86 (s, 9H), 0.86 (s, 9H), 0.95
(apparent triplet, 3H, J¼7.3), 1.35 (dd, 1H, J¼13.2, 7.3), 1.62–1.69 (m,
1H), 1.82 (s, 3H), 1.84 (dd, 1H, J¼13.2, 7.3), 2.05–2.15 (m, 2H), 2.26–
2.29 (m, 1H), 3.56–3.59 (m, 2H), 3.67–3.71 (m, 1H), 4.21 (br s, 1H),
4.71 (s, 1H), 4.83 (s, 1H); 13C NMR:
d
ꢁ5.4, ꢁ5.4, ꢁ4.6, ꢁ4.4, 9.5, 18.0,
18.3, 24.4, 25.6, 25.9, 41.6, 41.7, 42.2, 64.6, 67.2, 68.1, 75.4, 113.9,
114.7; HRMS (ESI) calcd for [C23H49NO3Si2þNa]þ: 466.3149, found:
466.3145.
4.5.6. 4-Benzyl-5-[1,2-bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-
3-ethyl-3-thiophen-2-yl-isoxazolidine (28)
Prepared following the general procedure by additional of 2-
thienyllithium (3.9 mL of a 1 M solution, 3.9 mmol, 7.2 equiv) to
isoxazoline 27 (260 mg, 0.54 mmol, 1.0 equiv). Purification of the
crude product by flash chromatography (98:2 hexanes/EtOAc) gave
170 mg of 28 as a clear oil in 55% yield as a single diastereomer. 1H
NMR:
d
ꢁ0.08 (s, 3H), ꢁ0.02 (s, 3H), 0.00 (s, 3H), 0.04 (s, 3H), 0.89 (s,
9H), 0.89 (s, 9H), 0.96 (t, 3H, J¼6.8), 1.82 (br s, 1H), 1.96 (t, 1H,
J¼12.0), 2.54 (br s, 1H), 2.62–2.66 (m, 1H), 2.78–2.80 (m, 2H), 3.44
(dd, 1H, J¼9.8, 5.4), 3.59 (t, 1H, J¼9.0), 4.16 (d, 1H, J¼7.3), 7.01–7.03
(m,1H), 7.05–7.06 (m, 3H), 7.16–7.19 (m, 1H), 7.22–7.25 (m, 2H), 7.36
(d, 1H, J¼4.9); 13C NMR:
ꢁ5.5, ꢁ5.4, ꢁ4.6, ꢁ4.5, 9.5, 17.9, 18.3, 25.9,
d
To a stirred solution of crude diol (1.0 equiv) dissolved in tBuOH/
THF/H2O (10:3:1, final concentration 0.2 M)y was added NaIO4
(1.2 equiv). The reaction mixture was stirred at rt until the starting
material was fully consumed as ascertained by TLC analysis. Two
methods were effective for isolation of the aldehyde at this stage. In
the first method, the reaction mixture was diluted with 5 mL CHCl3
and filtered through a layer of Celite. Alternatively, the mixture was
diluted with H2O and extracted 3ꢀ with Et2O; the combined or-
ganic extracts were washed with brine, dried over MgSO4, and fil-
tered. Concentration in vacuo of the combined filtrate from either
procedure then provided a crude oil that was immediately dis-
25.9, 30.8, 37.1, 56.7, 64.0, 72.1, 75.2, 84.2, 124.5, 124.7, 125.7, 126.3,
128.5,
128.9,
139.7,
144.5;
HRMS
(ESI)
calcd
for
[C30H51NO3SSi2þNa]þ: 584.3026, found: 584.3032.
4.5.7. Compounds 29 and 30
Prepared according to the general procedure by addition of 2-
methylallylmagnesium chloride (12 mL of a 0.5 M solution in THF,
6.0 mmol, 7.2 equiv) to isoxazoline 27 (400 mg, 0.83 mmol,
1.0 equiv). A 1.2:1 diastereomeric ratio of products was determined
by spectral integration of the crude product mixture. Purification by
flash chromatography (98:2 hexanes/EtOAc) yielded 190 mg of di-
astereomer 29 and 160 mg of diastereomer 30, each as colorless
oils, for a combined yield of 81%.
t
solved in BuOH (0.1 M) and 2-methyl-2-butene (47 equiv). A so-
lution of NaClO2 (9.2 equiv) and KH2PO4 (7.0 equiv) in H2O was
added dropwise to give a pale yellow mixture. The reaction mixture
was stirred at rt until TLC analysis indicated complete consumption
of starting material, typically within 3 h. The mixture was diluted
4.5.7.1. 4-Benzyl-5-[1,2-bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-
3-ethyl-3-(2-methyl-allyl)-isoxazolidine (29). IR: 2928, 2857, 1471,
1254, 1093, 836 cmꢁ1; 1H NMR:
d
ꢁ0.17 (s, 3H), ꢁ0.13 (s, 3H), ꢁ0.06
(s, 3H), ꢁ0.04 (s, 3H), 0.81 (s, 9H), 0.84 (s, 9H), 1.08 (t, 3H, J¼7.7),
1.28–1.36 (m,1H),1.70–1.79 (m,1H),1.91 (s, 3H), 2.18 (d,1H, J¼13.2),
y
A 1:1 mixture of CH3CN/H2O can also be used.

S.J. Buhrlage et al. / Tetrahedron 65 (2009) 3305–3313
3311
with satd NaHCO3 and EtOAc. The aqueous layer was then extracted
with EtOAc and the combined organic extracts were dried over
Na2SO4, filtered, and concentrated to provide the crude product
mixture. The compounds were purified using flash chromatogra-
phy. Isoxazolidines containing a substituent at the C4 position were
isolated as the methyl ester. The methyl ester was generated by
treatment of the crude acid (1.0 equiv) in benzene (0.05 M) and
MeOH (0.15 M) with TMSCHN2 (10 equiv). The resulting mixture
was stirred for 10 min, at which time TLC analysis indicated com-
plete consumption of starting material. The solvent was removed
by rotary evaporation and the product purified by flash
chromatography.
23.8, 24.9, 46.2, 46.5, 68.3, 69.6, 77.2, 124.4, 124.7, 126.4, 128.3,
128.3, 128.4, 135.5, 147.8; HRMS (ESI) calcd for [C20H23NO5SþNa]þ:
412.1195, found: 412.1201.
4.6.5. (3S,5R)-3-Ethyl-3-(2-methyl-allyl)-isoxazolidine-2,5-
dicarboxylic acid 2-benzyl ester (22)
Compound 22 was prepared according to the general procedure
from isoxazolidine 16 (44 mg, 0.10 mmol, 1.0 equiv). Purification by
flash chromatography (95:5 CH2Cl2/MeOHþ2% acetic acid) of the
crude product yielded 14 mg of 22 as a colorless oil in 42% yield.
IR: 2927, 2363, 1696, 1559, 1456 cmꢁ1 1H NMR (CD3OD):
; d 0.89
(t, 3H, J¼7.3), 1.74 (s, 3H), 1.78–1.85 (m, 1H), 1.97–2.04 (m, 1H), 2.37
(d, 1H, J¼13.7), 2.50 (dd, 1H, J¼12.7, 7.3), 2.69 (d, 1H, J¼13.7), 2.82
(dd,1H, J¼12.9, 8.1), 4.53–4.56 (m,1H), 4.82 (s,1H), 4.95 (s,1H), 5.22
4.6.1. 3-Benzyl-3-isobutyl-isoxazolidine-2,5-dicarboxylic acid 2-
benzyl ester (17)
(s, 2H), 7.33–7.41 (m, 3H), 7.43–7.45 (m 2H); 13C NMR:
d 8.6, 24.2,
Prepared following the general procedure on 0.05 mmol scale.
Purification of the crude product mixture by flash column chro-
matography (1:9 MeOH/CH2Cl2 and 2% acetic acid) provided 8.3 mg
of acid 17 as a colorless oil in 42% overall yield over two steps. IR:
3388, 2956, 2929, 2870, 1694, 1455, 1350 cmꢁ1; 1H NMR (CD3OD):
32.0, 42.3, 45.7, 68.5, 69.5, 85.7,117.2,129.5,129.6,129.7,143.2,166.7;
HRMS (ESI) calcd for [C18H23NO5þNa]þ: 356.1474, found: 356.1472.
4.6.6. 4-Benzyl-3-ethyl-3-thiophen-2-yl-isoxazolidine-2,5-
dicarboxylic acid 2-benzyl ester 5-methyl ester (31)
d
0.85 (d, 3H, J¼6.3), 0.92 (d, 3H, J¼6.2), 1.74–1.81 (m, 2H), 1.82–1.90
Isoxazolidine 28 (67 mg, 0.12 mmol, 1.0 equiv) was treated
according to the general procedure to yield 34 mg of 31 as a color-
less oil in 61% yield. IR: 2950, 1710, 1454, 1327, 697 cmꢁ1; 1H NMR
(m, 1H), 2.46–2.54 (m, 1H), 2.66–2.73 (m, 1H), one set of benzylic
CH2 buried under CD3OD around 3.30, 3.60 (br s, 1H), 5.11–5.24 (m,
2H, rotamers), 7.04–7.06 (m, 2H), 7.16–7.17 (m, 3H), 7.30–7.40 (m,
(CD3OD):
d
0.93 (t, 3H, J¼7.2), 2.09 (d, 1H, J¼14.3, 9.6), 2.14 (dd, 1H,
5H); 13C NMR (100 MHz) (CD3OD):
d
22.7, 24.3, 41.3, 42.3, 45.4, 67.3,
J¼14.1, 7.0), 2.40–2.49 (m, 1H), 2.54 (dd, 1H, J¼14.5, 5.5), 3.26–3.33
(m, 4H), 4.41 (d, 1H, J¼9.4), 4.94 (s, 2H), 6.91 (dd, 1H, J¼3.9, 1.2),
6.93–6.96 (m, 3H), 7.00–7.02 (m, 2H), 7.07–7.09 (m, 1H), 7.11–7.14
68.7, 75.5, 126.7, 128.1, 128.3, 128.4, 128.5, 130.5, 136.3, 136.6, 153.0;
HRMS (ESI) calcd for [C23H27NO5þNa]þ: 420.1787, found: 420.1782.
(m, 2H), 7.15–7.18 (m, 3H), 7.31 (dd, 1H, J¼5.1, 1.2); 13C NMR:
d 7.7,
4.6.2. 3-Isobutyl-3-phenyl-isoxazolidine-2,5-dicarboxylic acid 2-
benzyl ester (18)
29.2, 34.1, 52.3, 53.7, 67.5, 71.2, 78.9, 124.8, 125.0, 126.7, 126.8, 128.0,
128.0, 128.3, 128.4, 128.8, 137.4, 145.2, 169.5; HRMS (ESI) calcd for
[C26H27NO5SþNa]þ: 488.1508, found: 488.1507.
Prepared following the general procedure on 0.12 mmol scale.
Purification of the crude product mixture by flash column chro-
matography (1:9 MeOH/CH2Cl2 and 2% acetic acid) provided 24 mg
of acid 18 as a colorless oil in 52% overall yield from 12. IR: 3388,
4.6.7. 4-Benzyl-3-ethyl-3-(2-methyl-allyl)-isoxazolidine-2,5-
dicarboxylic acid 2-benzyl ester 5-methyl ester (32)
Isoxazolidine 29 (43 mg, 0.08 mmol, 1.0 equiv) was treated
according to the general procedure to yield 19 mg of 32 as a color-
less oil in 54% yield. IR: 3468, 2923, 2852, 1695, 1455, 1348,
2956, 2929, 2870, 1700, 1447, 1346 cmꢁ1; 1H NMR (CD3OD):
d
0.84
(d, 3H, J¼6.6), 0.94 (d, 3H, J¼6.6), 1.81–1.89 (m, 1H), 2.18–2.25 (m,
2H), 2.64–2.70 (m, 1H), 2.81–2.87 (m, 1H), 4.49–4.52 (m, 1H), 5.02–
5.03 (m, 2H, rotamers), 7.11–7.15 (m, 2H), 7.21–7.28 (m, 8H); 13C
1115 cmꢁ1; 1H NMR (CD3OD):
d
0.93 (t, 3H, J¼7.3),1.65–1.72 (m,1H),
NMR (100 MHz) (CD3OD):
d
23.2, 24.2, 24.4, 29.5, 44.6, 53.6, 67.1,
1.76 (s, 3H), 2.31–2.38 (m, 1H), 2.41 (d, 1H, J¼14.2), 2.66 (d, 1H,
J¼13.7), 2.93 (dd, 1H, J¼13.9, 10.0), 3.02 (dd, 1H, J¼13.7, 5.4), 3.19–
3.23 (m, 1H), 3.40 (s, 3H), 4.44 (d, 1H, J¼9.3) (two protons buried
under MeOH), 5.11 (d,1H, J¼12.2), 5.19 (d,1H, J¼12.2), 7.22–7.25 (m,
3H), 7.30–7.43 (m, 7H); HRMS (ESI) calcd for [C26H31NO5þNa]þ:
460.2100, found: 460.2093.
70.8, 125.0, 127.0, 128.0, 128.1, 128.2, 128.3, 136.1, 145.1, 153.7; HRMS
(ESI) calcd for [C22H25NO5þNa]þ: 406.1630, found: 406.1638.
4.6.3. 3-Isobutyl-3-methyl-isoxazolidine-2,5-dicarboxylic acid 2-
benzyl ester (20)
Prepared following the general procedure on 0.06 mmol scale.
Purification of the crude product mixture by flash column chro-
matography (1:9 MeOH/CH2Cl2 and 2% acetic acid) provided 8.7 mg
of acid 20 as a colorless oil in 45% overall yield from 14. IR: 3388,
4.6.8. 4-Benzyl-3-ethyl-3-(2-methyl-allyl)-isoxazolidine-2,5-
dicarboxylic acid 2-benzyl ester 5-methyl ester (33)
Isoxazolidine 30 (37 mg, 0.07 mmol, 1.0 equiv) was treated
according to the general procedure to yield 17 mg of 33 as a color-
less oil in 56% yield. IR: 3469, 2924, 2852, 1695, 1455, 1348,
2956, 2929, 2870, 1698, 1447, 1346 cmꢁ1; 1H NMR (CD3OD):
d 0.89
(d, 3H, J¼6.3), 0.94 (d, 3H, J¼6.6), 1.43 (s, 3H), 1.69–1.74 (m, 2H),
1.82–1.85 (m, 1H), 2.49–2.53 (m, 1H), 2.60–2.64 (m, 1H), 4.68 (t, 1H,
J¼7.7), 5.18 (s, 2H), 7.31–7.42 (m, 5H); 13C NMR (100 MHz) (CD3OD)
1115 cmꢁ1
;
1H NMR:
d
0.95 (t, 3H, J¼7.6), 1.67–1.74 (m, 4H), 2.02–
2.09 (m, 1H), 2.24 (d, 1H, J¼14.6), 2.67 (dd, 1H, J¼13.7, 10.7), 2.93
(dd, 1H, J¼13.7, 4.9), 3.03 (d, 1H, J¼14.6), 3.26 (s, 3H), 3.31–3.36 (m,
1H), 4.28 (d, 1H, J¼9.8), 4.72 (s, 1H), 4.95 (s, 1H), 5.21 (s, 2H), 7.11–
(some aromatic peaks overlap):
d 22.8, 23.8, 23.9, 24.6, 45.0, 46.5,
66.0, 67.1, 74.7, 128.1, 128.3, 136.3, 154.2, 173.5; HRMS (ESI) calcd for
[C17H23NO5þNa]þ: 344.1474, found: 344.1467.
7.38 (m, 10H); 13C NMR:
d 10.1, 22.5, 23.2, 29.0, 33.7, 37.0, 44.9, 54.5,
68.5, 70.7, 81.2, 117.1, 127.9, 129.7, 129.7, 130.3, 141.9, 163.9; HRMS
4.6.4. 3-Isobutyl-3-thiophen-2-yl-isoxazolidine-2,5-dicarboxylic
acid 2-benzyl ester (21)
(ESI) calcd for [C26H31NO5þNa]þ: 460.2100, found: 460.2098.
Prepared following the general procedure on 0.05 mmol scale.
Purification of the crude product mixture by flash column chro-
matography (1:9 MeOH/CH2Cl2 and 2% acetic acid) provided
10.7 mg of acid 21 as a colorless oil in 55% overall yield from 15. IR:
4.7. (9H-Fluoren-9-yl)methyl 3-allyl-3-isobutyl-5-
(2,2,3,3,8,8,9,9-octamethyl-4,7-dioxa-3,8-disiladecan-5-
yl)isoxazolidine-2-carboxylate (23)
3401, 2956, 2929, 1705, 1400, 1329 cmꢁ1; 1H NMR (CD3OD):
d
0.86
To a stirred solution of 13 (73, mg, 0.16 mmol, 1.0 equiv) in THF
(s, 3H), 0.88 (s, 3H), 1.73–1.84 (m, 1H), 2.17–2.34 (m, 2H), 2.88–2.99
(m, 2H), 5.14 (s, 2H), 6.87–6.89 (m, 1H), 6.97–6.99 (m, 1H), 7.17–7.19
(530
0.64 mmol, 4.0 equiv) followed immediately by satd Na2CO3
(530 L) and the mixture was allowed to slowly warm to rt. Upon
mL) cooled in an ice–H2O bath was added FmocCl (170 mg,
(m, 2H), 7.24–7.27 (m, 4H); 13C NMR (100 MHz) (CD3OD):
d
23.1,
m

3312
S.J. Buhrlage et al. / Tetrahedron 65 (2009) 3305–3313
stirring for 12 h TLC analysis indicated complete consumption of
starting material. The reaction mixture was diluted with H2O
(5 mL) and extracted with Et2O (3ꢀ5 mL). The combined organic
extracts were washed with brine (10 mL), dried over Na2SO4, fil-
tered, and concentrated. The crude mixture was purified by flash
chromatography (96:4 hexanes/EtOAc) to yield 100 mg of 23 as
ꢁ5.41, ꢁ4.7, ꢁ4.6, 18.2, 18.3, 23.2, 24.1, 25.1, 25.8, 25.9, 37.2, 40.8,
45.7, 47.1, 55.3, 64.1, 65.3, 66.8, 66.9, 73.6, 76.7, 77.0, 77.3, 79.9, 96.5,
119.9, 125.1, 127.1, 127.6, 127.6, 141.3, 141.3, 144.0, 152.2; HRMS (ESI)
calcd for [C40H65NO7Si2þNa]þ: 750.4197, found: 750.4181.
4.9. 2-(((9H-Fluoren-9-yl)methoxy)carbonyl)-3-isobutyl-3-(2-
(methoxymethoxy)ethyl)isoxazolidine-5-carboxylic acid (26)
a colorless oil in 92% yield. 1H NMR:
d 0.03 (s, 3H), 0.04 (s, 3H), 0.08
(s, 3H), 0.10 (s, 3H), 0.10 (s, 3H), 0.86 (s, 18H), 1.24–1.28 (m, 1H),
1.47–1.50 (m, 1H), 1.56–1.61 (m, 1H), 1.66–1.73 (m, 1H), 2.12–2.23
(m, 2H), 2.54 (br s, 1H), 3.59–3.66 (m, 2H), 3.68–3.71 (m, 1H), 4.09–
4.13 (m, 1H), 4.24 (apparent triple, 1H, J¼7.08), 4.39–4.43 (m, 1H),
4.47–4.51 (m, 1H), 4.97–5.04 (m, 2H), 5.61–5.71 (m, 1H), 7.27 (m,
2H), 7.36 (m, 2H), 7.63 (dd, 2H, J¼2.9, 7.3), 7.73 (d, 2H, J¼7.8); 13C
A 40% solution of HCl in MeOH (2 mL) was cooled in an ice–H2O
bath and then added to a solution of 25 (58 mg, 0.08 mmol,
1.0 equiv) in 2 mL MeOH that was also cooled in an ice–H2O bath.
Upon stirring 30 min ESI-MS analysis of the reaction mixture in-
dicated complete consumption of starting material. The reaction
mixture was diluted with H2O (5 mL) and extracted with EtOAc
(3ꢀ5 mL). The combined organic extracts were washed with H2O
(10 mL) and brine (10 mL), dried over Na2SO4, filtered, and con-
NMR (100 MHz):
d
ꢁ5.5, ꢁ5.4, 4.7, 4.6, 18.2, 18.3, 23.1, 24.1, 25.0,
25.8, 25.9, 39.9, 42.2, 45.5, 47.1, 65.2, 66.8, 67.6, 73.5, 76.7, 77.0, 77.3,
79.7, 118.6, 119.8, 125.2, 127.1, 127.6, 133.6, 141.3, 144.1, 144.1, 152.1;
HRMS (ESI) calcd for [C39H61NO5Si2þNa]þ: 702.3986, found:
702.4001.
centrated. The crude material was dissolved in CH3CN (400
mL) and
H2O (400 L), cooled in an ice–H2O bath, and NaIO4 (21 mg,
m
0.08 mmol, 1.2 equiv) was added to the solution. Upon stirring for
1 h TLC analysis indicated complete consumption of starting ma-
terial. The reaction mixture was diluted with H2O (5 mL) and
extracted with Et2O (3ꢀ5 mL). The combined organic extracts were
washed with brine, dried over Na2SO4, filtered, and concentrated.
4.8. (9H-Fluoren-9-yl)methyl 3-allyl-3-isobutyl-5-((R)-
2,2,3,3,8,8,9,9-octamethyl-4,7-dioxa-3,8-disiladecan-5-
yl)isoxazolidine-2-carboxylate (25)
t
To a stirred solution of 23 in BuOH (7.5 mL), THF (2.0 mL), and
The crude material was dissolved in tBuOH (400
mL) and 2-methyl-
H2O (500 mL) cooled in an ice–H2O bath were added NMO (150 mg,
2-butene was added to the solution. A solution of KH2PO4 (76 mg,
0.56 mmol, 7.0 equiv) and NaOCl2 (67 mg, 0.745 mmol, 0.2 equiv) in
H2O (230 mL) was then added dropwise to the isoxazolidine solu-
tion. The reaction mixture was stirred for 12 h at which point TLC
analysis indicated complete consumption of starting material. The
reaction mixture was diluted with H2O (5 mL) and extracted with
EtOAC (3ꢀ5 mL). The combined organic extracts were washed with
brine, dried over Na2SO4, filtered, and concentrated in vacuo. The
crude material was purified by flash chromatography (elution in
80:20 CH2Cl2/MeOH) to yield 21 mg of 26 as a colorless oil in 53%
1.3 mmol, 1.2 equiv) and OsO4 (1.0 mL of a 2.5 wt % solution in
tBuOH). The reaction mixture was stirred for 6 h at which point TLC
analysis indicated complete consumption of starting material.
Na2SO3 (25 mg) was then added and the solution was stirred for
1 h. The mixture was then diluted with H2O (10 mL) and extracted
with EtOAc (3ꢀ20 mL). The combined organic extracts were
washed with brine (25 mL), dried over Na2SO4, filtered, and con-
centrated. The crude material was dissolved in CH3CN (5.5 mL) and
H2O (0.5 mL), cooled in an ice–H2O bath, and NaIO4 (280 mg) was
added to the solution. The mixture was stirred for 1 h at which
point TLC analysis indicated complete consumption of starting
material. The reaction mixture was diluted with H2O (10 mL) and
extracted with Et2O (3ꢀ15 mL). The combined organic extracts
were washed with brine, dried over Na2SO4, filtered, and concen-
trated. The crude material was then dissolved in 11 mL MeOH and
cooled in an ice–H2O bath. NaBH4 was added to the solution and
after 30 min TLC analysis indicated complete consumption of
starting material. The reaction mixture was diluted with H2O
(10 mL) and extracted with Et2O (3ꢀ20 mL). The combined organic
extracts were washed with brine, dried over Na2SO4, filtered, and
concentrated. The crude material was purified by flash chroma-
tography (elution in 60:40 hexanes/EtOAC) to give 450 mg of
product as a colorless oil in 60% yield. To a solution of product
(500 mg, 0.73 mmol, 1.0 equiv) in CH2Cl2 (7.3 mL) cooled in an ice–
yield. 1H NMR (CD3OD):
d
1.57 (d, 3H, J¼6.6), 1.64 (d, 3H, J¼6.6),
1.86–1.94 (m, 1H), 2.04–2.20 (m, 2H), 2.24–2.40 (m, 2H), 3.44 (dd,
1H, J¼5.9,12.9), 3.66 (dd,1H, J¼8.6, 12.5), 4.26 (s, 3H), 5.24–5.26 (m,
1H), 5.39–5.44 (m, 3H), 5.76 (dd,1H, J¼3.3,10.7), 5.92 (dd,1H, J¼3.7,
10.7), 7.28–7.33 (m, 2H), 7.36–7.40 (m, 2H), 7.60 (dd, 2H, J¼4.9, 6.8),
7.80 (dd, 2H, J¼2.7, 7.4); HRMS (ESI) calcd for [C27H33NO7þNa]þ:
506.2155, found: 506.2147.
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by dropwise addition of MOMCl (170
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