Enantioselective radical methods for lactone synthesis: Use of unprotected haloalcohols as radical precursors

Article abstract of DOI:10.1055/s-2005-865319

We have developed an efficient free radical method for the synthesis of enantioenriched 6- and 7-membered lactones in one synthetic operation. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-865319

1528
PAPER
Enantioselective Radical Methods for Lactone Synthesis: Use of Unprotected
Haloalcohols as Radical Precursors
E
nantioselective Radic
u
a
l
Methods
f
k
or
L
actone
S
u
ynthesis nd P. Sibi,* Miguel A. Guerrero
Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105, USA
Fax +1(701)2311057; E-mail: Mukund.Sibi@ndsu.edu
Received 14 March 2005
Dedicated to Bernd Giese in appreciation of his seminal contributions to free radical chemistry
shown that imide templates are quite effective in radical
Abstract: We have developed an efficient free radical method for
the synthesis of enantioenriched 6- and 7-membered lactones in one
synthetic operation.
chemistry.4 However, radical addition to 2a was also not
effective (entries 2 and 3). More encouraging results were
obtained with imide 3a providing the lactone directly in
30% yield (entry 5). Cinnamates are generally less reac-
tive in conjugate radical additions. Changing the b-sub-
stituent to a dihydrocinnamyl moiety (3b) improved
reactivity and gave the lactone product in good yield (en-
tries 6 and 7). 3,5-Dimethylpyrazole has proven to be an
effective achiral template in a variety of reactions.5 This
template proved to be ideal in that radical addition to 4a
and 4b proceeded well giving the lactones 5a and 5b, re-
spectively in good yields (entries 9–15).
Key words: radical reactions, chiral Lewis acid, lactones, conju-
gate additions, enantioselective reactions
Free radical chemistry provides an opportunity to explore
reactions wherein reactive functional groups can be uti-
lized without protection.1 In the context of developing
new routes to lactones, we have explored the enantiose-
lective addition of radical precursors containing hydroxyl
groups to a,b-unsaturated carboxylic acid derivatives
(Equation 1, A → B).2 The route is flexible and can pro-
vide access to functionalized lactones depending on the
substitution in the radical precursor or the acceptor. In this
work we demonstrate that functionalized 6- and 7-mem-
bered lactones can be synthesized in a single operation in
good yield and good to high enantioselectivity.
O
OH
O
I
O
Z
R
Lewis acid, Et3B, O2
–78 °C, CH2Cl2
R
5a,b
1–4
O
O
O
N
N
N
H
N
H
O
N
O
O
Enantioselectivity?
Chiral Lewis acid
OH
O
1
2
3
4
Z
R
B
I
( )n
( )n
R
b R =
a R =
A
C
Equation 1
Scheme 1
We began our work with the identification of an appropri-
ate acceptor that would undergo conjugate addition effec-
tively using iodoethanol as the radical precursor
(Scheme 1).3 At the outset, we were concerned with the
relatively low nucleophilicity of the primary radical de-
rived from iodoethanol and surmised that we would need
a very good acceptor with additional Lewis acid activa-
tion. To this end, we screened combinations of different
acceptors and Lewis acids and these results are shown in
Table 1. For these reactions we employed five equivalents
of iodoethanol, one equivalent of the Lewis acid, three
equivalents of tin hydride, and triethylborane/oxygen as
the radical initiator. Radical addition to oxazolidinone
cinnamate 1a using ytterbium triflate as a Lewis acid did
not give any product (Table 1, entry 1). We have recently
Our next task was the identification of an optimal chiral
Lewis acid system for the enantioselective radical addi-
tion to 3b and 4b (Scheme 2).6 These experiments would
allow us to assess the impact on reactivity and/or selectiv-
ity due to the presence of the Lewis basic hydroxyl group
in the radical precursor and its potential interaction with
the Lewis acid. Our group has demonstrated several chiral
Lewis acid systems that are effective in enantioselective
radical reactions and thus they were reagents of choice for
additions to 3b and 4b.7 Radical addition to 3b using
Mg(ClO4)2 and ligand 6 (1 equiv) gave the lactone 5b in
modest yield and enantioselectivity (Table 2, entry 1).
Other chiral Lewis acids (data not shown) did not show
any improvement in selectivity and only ytterbium triflate
in combination with 8 was effective (entries 2 and 3).
Compound 4b containing a pyrazole template was
screened next and it showed superior characteristics with
respect to both yield and enantioselectivity. Of the several
chiral Lewis acids examined (entries 4–16), magnesium
SYNTHESIS 2005, No. 9, pp 1528–1532
x
x
.
x
x
.2
0
0
5
Advanced online publication: 18.04.2005
DOI: 10.1055/s-2005-865319; Art ID: C02205SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Enantioselective Radical Methods for Lactone Synthesis
1529
Table 2 Effect of Lewis Acids on Enantioselectivity
Table 1 Identification of an Optimal Acceptor for Reactions with
Iodoethanol
Entry Substrate Lewis acid
Ligand Yield (%)a ee (%)b
Entry
1
Substrate
1a
Lewis acid
Yb(OTf)3
MgBr2
Product
5a
Yield (%)a
<3
1
2
3b
3b
3b
4b
4b
4b
4b
4b
4b
4b
4b
4b
4b
4b
4b
4b
Mg(ClO4)2
Yb(OTf)3
Sm(OTf)3
Zn(OTf)2
Zn(NTf2)2
MgI2
6
8
48
54
48
40
40
33
58
50
53
40
50
59
30
38
NR
NR
40
10
<3
55
10
55
32
85
44
4
2
2a
5a
NR
<3
3
8
3
2a
Yb(OTf)3
MgBr2
5a
4
6
4
3a
5a
NR
30
5
6
5
3a
Yb(OTf)3
Mg(ClO4)2
Yb(OTf)3
MgBr2
5a
6
6
6
3b
5b
5b
5a
60
7
Mg(OTf)2
Mg(NTf2)2
Mg(ClO4)2
Zn(OTf)2
Mg(NTf2)2
Yb(OTf)3
Ni(ClO4)2
Ni(ClO4)2
Sc(OTf)3
Sc(OTf)3
6
7
3b
61
8
6
8
4a
NR
68
9
6
9
4a
Yb(OTf)3
Mg(NTf2)2
Mg(ClO4)2
MgBr2
5a
10
11
12
13
14
15
16
7
10
11
12
13
14
4a
5a
48
7
<3
<3
49
88
–
4a
5a
56
8
4b
5b
5b
5b
<3
9
4b
Mg(ClO4)2
Yb(OTf)3
61
9
4b
50
10
a Isolated yield after column chromatography. NR = no reaction.
11
–
a Isolated yield after column chromatography.
b Determined by chiral HPLC or chiral GC.
triflimide and ligand 6 (85% ee, entry 8) and nickel per-
chlorate and ligand 9 (88% ee, entry 14) gave the highest
selectivity. Of these two, magnesium triflimide was more
efficient with respect to yield. These experiments demon-
strate that lactones can be synthesized in one synthetic op-
eration in modest to good yield and high
enantioselectivity. Furthermore, the high selectivity ob-
tained using a small primary radical without any protec-
tion of the hydroxyl group is noteworthy.
sults are tabulated in Table 3. For these reactions, the
chiral Lewis acid derived from Mg(NTf2)2/ligand 6 was
used. The conjugate radical addition followed by cycliza-
tion leading to lactones 5a–g was generally highly selec-
tive with ee values ranging from 79–85% (entries 1–7),
except for reaction with 4g, which gave 48%ee (entry 7).
The chemical efficiency of the reaction was dependent on
the substrate. This underscores the low reactivity of the
primary radical and high yields are obtained with electron
poor substrate (4e, 81%, entry 5) or a small alkyl group
(4g, 85%, entry 7) only. Substrates 4a–d gave only mod-
est yields (entries 1–4). Compounds 4f–g gave good
yields.
The synthesis of d-lactones with different groups at the 4-
position was investigated next (Equation 2) and these re-
OH
O
I
O
Chiral Lewis acid (100%)
Et3B/O2, CH2Cl2, –78 °C
O
Z
3b, 4b
5b
OH
I
O
O
O
O
N
Mg(NTf2)2/Ligand 6 (1 equiv)
O
N
O
Ph
Ph
O
N
R
N
N
Et3B/O2, CH2Cl2,
–78 °C
N
N
R
OH
Et
O
O
4a–g
5a–g
6
8
7
Equation 2
O
O
N
O
We have also assessed few different radical precursors in
lactone synthesis and these results are shown in Figure 1.
Addition of the radical derived from 1-iodo-2-methyl-2-
propanol to 4g and 4e gave lactones 12 and 13, respective-
ly, in modest yield and selectivity. Addition of a primary
O
O
N
N
N
N
R
9
R
10 R = Ph
11 R = i-Pr
Ph
Ph
Scheme 2
Synthesis 2005, No. 9, 1528–1532 © Thieme Stuttgart · New York

1530
M. P. Sibi, M. A. Guerrero
Table 3 Radical Addition to Different Substrates
Entry Substrate R Product Yield Config. ee
PAPER
the addition of ethyl radical to 4a (Scheme 3). The ethyl
addition product 17 was obtained in good yield and low
selectivity.11 The absolute stereochemistry for the product
was established as S by hydrolysis to the known pentanoic
acid.12 Thus the addition of ethyl and hydroxyethyl radi-
cals occurs from the same face. The large difference in the
level of enantioselectivity between the two radicals sug-
gests that the hydroxy group could influence the addition
through potential coordination to the Lewis acid. This
conclusion is corroborated by the match/mismatch in se-
lectivity observed with the chiral alcohols (see Figure 1,
data for 14). The higher selectivity observed for the seven-
membered lactones (Figure 1, 15 and 16) is also instruc-
tive in that tether length could play a role in determining
the level of enantioselection.
(%)a
48
50
40
52
81
68
85
(%)b
1
2
3
4
5
6
7
Ph (4a)
5a
5b
5c
5d
5e
5f
S
85
PhCH2CH2 (4b)
2-Furyl (4c)
Rc
85
79
2-Thiophenyl (4d)
4-CF3Ph (4e)
Cyclohexyl (4f)
PMPOCH2 (4g)
80
S
S
80
81
5g
48
a Isolated yield after column chromatography.
b Determined by chiral HPLC or chiral GC.
c Note the change in priority.
OH
I
O
O
Mg(NTf2)2/Ligand 6 (1 equiv)
N
O
N
Ph
Ph
Et3B/O2, CH2Cl2,
–78 °C
Ph
radical derived from enantiopure (S)-3-chloro-1-iodo-2-
propanol8 to 4e gave lactone 14 as a mixture of diastereo-
mers in a ratio of 9:1. The 80% ee in this reaction is simi-
lar to that observed for iodoethanol addition to 4e (see
entry 5, Table 2). An analogous experiment using the
enantiomer of ligand 6 gave a mixture of diastereomeric
lactones in a ratio of 1:2. These experiment suggest that
selectivity is optimal when the ligand and the reagent are
matched. We have also examined iodopropanol as the rad-
ical precursor. Addition of the radical derived from
1-iodo-3-propanol to 4b and 4e furnished the 7-membered
lactones 15 and 16, respectively in high yield and selectiv-
ity. It is interesting to note that the 7-membered lactone
formation proceeds in higher yields and selectivity as
compared to the 6-membered ring analogs.
4a
5a
48% yield
85% ee
I
O
O
Mg(NTf2)2/Ligand 6 (1 equiv)
N
N
N
N
Ph
Et3B/O2, CH2Cl2,
–78 °C
68% yield
10% ee
17
4a
Scheme 3
In conclusion we have developed an efficient free radical
methodology for the synthesis of 6-and 7-membered lac-
tones. The use of precursors containing unprotected reac-
tive functional groups highlights the power of free radical
methods in the enantioselective synthesis of chiral lac-
tones. The new methodology holds promise for the intro-
duction of a wide variety of substituents on to the lactone
ring. The application of the new methodology to the syn-
thesis of natural products is underway in our laboratory.
O
O
O
O
O
O
OPMP
Cl
CF3
14
12
13
CF3
53% yield
28% ee
71% yield
40% ee
Ligand 6 64% yield
9:1 dr
Enant. 6 65% yield
1:2 dr
Conjugate Radical Additions; General Procedure
Under N2, a mixture of Lewis acid (0.10 mmol) and ligand (0.10
mmol) in CH2Cl2 (2 mL) was stirred at r.t. for 45 min. Pyrazole (4a;
0.10 mmol) (in 1 mL CH2Cl2) was added and the mixture was al-
lowed to stir for an additional 30 min and then cooled to –78 °C. The
reaction was then initiated by sequential addition of 2-iodo ethanol
(or other radical precursors) (1 mmol), n-Bu3SnH (0.5 mmol), Et3B
(0.5 mmol 1 M solution in hexanes) and oxygen (introduced via sy-
ringe). The reaction was monitored by TLC (50% EtOAc in hexane)
and when judged complete was quenched with silica gel, concen-
trated, washed with hexanes and extracted with Et2O. The Et2O ex-
tract was concentrated over silica gel and purified by silica gel
chromatography (hexane–EtOAc) to give the products. The enanti-
omeric excess of the product was determined by chiral GC or
HPLC.
O
O
O
O
15
CF3
16
68% yield
92% ee
64% yield
87% ee
Figure 1
The absolute stereochemistry of lactones 5a, 5b, 5e and 5f
have been reported in the literature.9 Comparison of the
sign of rotation for 5a, 5b, 5e, and 5f from our experi-
ments with those in the literature establishes that the S
products are formed (see Table 3).10 We also investigated
Compound 5a
Ee estimated by chiral GC analysis; column bETA Dex 225 (0.25
cm × 30cm), Temp = 165 °C, tR (minor) = 68 min, tR (major) = 74
min, with 85% ee; [a]D25 +3.5 (c = 0.34, CHCl3).
Synthesis 2005, No. 9, 1528–1532 © Thieme Stuttgart · New York

PAPER
Enantioselective Radical Methods for Lactone Synthesis
(2) For lactone synthesis using radical intermediates, see:
1531
IR (neat): 2921, 1720 cm–1.
(a) Tsunoi, S.; Ryu, I.; Okuda, T.; Tanaka, M.; Komatsu, M.;
Sonoda, N. J. Am. Chem. Soc. 1998, 120, 8692. (b) Tsunoi,
S.; Ryu, I.; Sonoda, N. J. Am. Chem. Soc. 1994, 116, 5473.
(c) Kreimerman, S.; Ryu, I.; Minakata, S.; Komatsu, M. Org.
Lett. 2000, 2, 389. (d) Castle, K.; Hau, C.-S.; Sweeney, J.
B.; Tindall, C. Org. Lett. 2003, 5, 757. (e) Molander, G. A.;
St. Jean, D. J. Jr. J. Org. Chem. 2002, 67, 3861. (f) Clive,
D. L. J.; Zhang, J.; Subedi, R.; Bouetard, V.; Hiebert, S.;
Ewanuk, R. J. Org. Chem. 2001, 66, 1233. (g) Berlin, S.;
Ericsson, C.; Engman, L. Org. Lett. 2002, 4, 3. (h) Miyabe,
H.; Fujii, K.; Goto, T.; Naito, T. Org. Lett. 2000, 2, 4071.
(i) Kim, K.; Okamoto, S.; Sato, F. Org. Lett. 2001, 3, 67.
(j) Fischer, J.; Reynolds, A. J.; Sharp, L. A.; Sherburn, M. S.
Org. Lett. 2004, 6, 1345. (k) Fukuzawa, S.-I.; Seki, K.;
Tatsuzawa, M.; Mutoh, K. J. Am. Chem. Soc. 1997, 119,
1482. (l) Kerrigan, N. J.; Upadhyay, T.; Procter, D. J.
Tetrahedron Lett. 2004, 45, 9087. (m) Kerrigan, N. J.;
Hutchison, P. C.; Heightman, T. D.; Procter, D. J. Org.
Biomol. Chem. 2004, 2, 2476. (n) Fang, X.; Xia, H.; Yu, H.;
Dong, X.; Chen, M.; Wang, Q.; Tao, F.; Li, C. J. Org. Chem.
2002, 67, 8481.
1H NMR (500 MHz, CDCl3): d = 7.36 (t, J = 7.0 Hz, 2 H), 7.27 (t,
J = 7.5 Hz, 1 H), 7.21 (d, J = 7.0 Hz, 2 H), 4.51 (ddd, J = 11.0, 4.5,
4.0 Hz, 1 H), 4.39 (ddd, J = 11.5, 10.5, 3.5 Hz, 1 H), 3.28–3.20 (m,
1 H), 2.92 (ddd, J = 17.5, 6.0, 1.5 Hz, 1 H), 2.64 (dd, J = 17.5, 10.5
Hz, 1 H), 2.21–2.15 (m, 1 H), 2.1–2.0 (m, 1 H).
13C NMR (125 MHz, CDCl3): d = 170.8, 142.9, 129.2, 127.4, 126.7,
68.8, 37.7, 37.6, 30.5.
Compound 5b
Yield: 50%; colorless oil.
Ee was estimated by chiral HPLC (254 nm, 25 °C) [chiralcel OD
column (0.46 cm × 25 cm) (from Daicel Chemical Ind., Ltd), hex-
ane–i-PrOH (90:10), flow rate = 0.8 mL/min], tR (major) = 36 min,
tR (minor) = 48 min, with 85% ee; [a]D25 +14.5 (c = 0.48, CHCl3).
IR (neat): 2919, 1738, 1254, 1080 cm–1.
1H NMR (500 MHz, CDCl3): d = 7.29 (t, J = 7.5 Hz, 2 H), 7.20 (t,
J = 7.5 Hz, 1 H), 7.16 (d, J = 7.5 Hz, 2 H), 4.41 (ddd, J = 11.0, 4.5,
4.0 Hz, 1 H), 4.24 (td, J = 11.0, 4.0 Hz, 1 H), 2.73 (ddd, J = 17.5,
6.0, 1.5 Hz, 1 H), 2.66 (t, J = 8.0 Hz, 2 H), 2.19 (dd, J = 17.5, 10.5
Hz, 1 H), 2.01–1.95 (m, 2 H), 1.74–1.66 (m, 2 H), 1.61–1.54 (m, 1
H).
(3) For selected examples on the use of achiral templates in
enantioselective transformations from other laboratories,
see: (a) Matsunaga, S.; Kinoshita, T.; Okada, S.; Harada, S.;
Shibasaki, M. J. Am. Chem. Soc. 2004, 126, 7559.
(b) Corminboeuf, O.; Renaud, P. Org. Lett. 2002, 4, 1731.
(c) Evans, D. A.; Miller, S. J.; Lectka, T.; von Matt, P. J. Am.
Chem. Soc. 1999, 121, 7559. (d) Jensen, K. B.; Gothelf, K.
V.; Hazell, R. G.; Jørgensen, K. A. J. Org. Chem. 1997, 62,
2471. (e) Kanemasa, S.; Kanai, T. J. Am. Chem. Soc. 2000,
122, 10710. (f) Palomo, C.; Oiarbide, M.; Garcia, J. M.;
Gonzalez, A.; Arceo, E. J. Am. Chem. Soc. 2003, 125,
13942. (g) For work from our laboratory see the following:
Sibi, M. P.; Sausker, J. B. J. Am. Chem. Soc. 2002, 124, 984.
(h) Sibi, M. P.; Shay, J. J.; Liu, M.; Jasperse, C. P. J. Am.
Chem. Soc. 1998, 120, 6615. (i) Sibi, M. P.; Liu, M. Org.
Lett. 2000, 2, 3393. (j) Sibi, M. P.; Prabagaran, N. Synlett
2004, 2421.
13C NMR (125 MHz, CDCl3): d = 171.3, 141.4, 128.7, 128.4, 126.3,
68.6, 38.0, 36.6, 32.8, 31.0, 29.0.
HMRS (ESI): m/z calcd for C13H16O2Na: 227.1048; found:
227.1029.
Compound 16
Yield: 64%; white solid; mp 109 °C.
Ee was estimated by chiral HPLC (254 nm, 25 °C) [chiralcel OD-H
column (0.46 cm × 25 cm) (from Daicel Chemical Ind., Ltd), hex-
ane–i-PrOH (90:10), flow rate = 1.0 mL/min], tR (major) = 17 min,
tR (minor) = 20 min, with 87% ee; [a]D25 –41.5 (c = 0.8, CHCl3).
IR (neat): 2925, 1730, 1326, 1180, 1111, 1025 cm–1.
1H NMR (500 MHz, CDCl3): d = 7.58 (d, J = 8.5 Hz, 2 H), 7.30 (d,
J = 8.5 Hz, 2 H), 4.43–4.36 (m, 1 H), 4.31 (dd, J = 13, 10.5 Hz, 1
H), 3.07 (dd, J = 12.0, 12.2 Hz, 1 H), 3.01 (ddd, J = 12.0, 12.2, 2.5
Hz, 1 H), 2.81 (dd, J = 12.0, 1.0 Hz, 1 H), 2.15–2.06 (m, 2 H), 2.00–
1.90 (m, 1 H), 1.85–1.75 (m, 1 H).
(4) Sibi, M. P.; Petrovic, G.; Zimmerman, J. J. Am. Chem. Soc.
2005, 127, 2390.
(5) Sibi, M. P.; Shay, J. J.; Ji, J. Tetrahedron Lett. 1997, 38,
5955.
13C NMR (125 MHz, CDCl3): d = 174.1, 149.5, 129.5 (q, J = 32 Hz),
126.9, 126 (q, J = 3 Hz), 124 (q, J = 272 Hz), 69.2, 41.3, 40.7, 37.7,
29.1.
HMRS (ESI): m/z calcd for C13H14F3O2 (M + H+): 259.0946; found:
259.0944.
(6) For reviews on stereoselective radical chemistry, see:
(a) Renaud, P.; Gèrster, M. Angew. Chem. Int. Ed. 1998, 37,
2563. (b) Sibi, M. P.; Porter, N. A. Acc. Chem. Res. 1999,
32, 163. (c) Sibi, M. P.; Manyem, S.; Zimmerman, J. Chem.
Rev. 2003, 103, 3263. (d) Bar, G.; Parsons, A. F. Chem. Soc.
Rev. 2003, 32, 251. (e) Sibi, M. P.; Manyem, S. Tetrahedron
2000, 56, 8303.
(7) Enantioselective conjugate radical reactions: (a) Sibi, M. P.;
Ji, J.; Wu, J. H.; Gurtler, S.; Porter, N. A. J. Am. Chem. Soc.
1996, 118, 9200. (b) Sibi, M. P.; Ji, J. J. Org. Chem. 1997,
62, 3800. (c) Sibi, M. P.; Chen, J. J. Am. Chem. Soc. 2001,
123, 9472. (d) Sibi, M. P.; Manyem, S. Org. Lett. 2002, 4,
2929. (e) Sibi, M. P.; Petrovic, G. Tetrahedron: Asymmetry
2003, 15, 2879. (f) Sibi, M. P.; Zimmerman, J.; Rheault, T.
R. Angew. Chem. Int. Ed. 2003, 42, 4521. (g) Iserloh, U.;
Curran, D. P.; Kanemasa, S. Tetrahedron: Asymmetry 1999,
10, 2417. (h) Murakata, M.; Tsutsui, H.; Hoshino, O. Org.
Lett. 2001, 3, 299. (i) Sibi, M. P.; He, L. Org. Lett. 2004, 6,
1749.
Acknowledgment
We thank the National Institutes of Health (NIGMS-54656) for fi-
nancial support. We also thank Conacyt-Mexico for providing a fel-
lowship to M. Guerrero.
References
(1) (a) Radicals in Organic Synthesis, Vol. 1; Renaud, P.; Sibi,
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Organic Synthesis, Vol. 2; Renaud, P.; Sibi, M. P., Eds.;
Wiley-VCH: Weinheim, 2001.
(8) The compound was prepared from optically pure
epichlorohydrin.
Synthesis 2005, No. 9, 1528–1532 © Thieme Stuttgart · New York

1532
M. P. Sibi, M. A. Guerrero
PAPER
(10) Note the change in priority for compound 5b. The radical
reaction occurs from the same face for all compounds.
(11) This reaction proceeds with 39% ee when magnesium
bromide is used as the Lewis acid. See ref. 5.
(12) (a) Elzner, S.; Maas, S.; Engel, S.; Kunz, H. Synthesis 2004,
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Tetrahedron: Asymmetry 2002, 13, 1915.
(9) Compound 5a and 5e: (a) Takaya, Y.; Senda, T.;
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Asymmetry 1999, 10, 4047. (b) Irwin, A. J.; Jones, J. B. J.
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Synthesis 2005, No. 9, 1528–1532 © Thieme Stuttgart · New York