Solid-phase tandem radical addition-cyclization reaction: Triethylborane-induced reaction of oxime ethers anchored to polymer support

Article abstract of DOI:10.1248/cpb.52.842

Tandem radical addition-cyclization of oxime ethers anchored to polymer support was studied. The reaction of oxime ethers with stannyl radical proceeded effectively by the use of triethylborane as a radical initiator. The alkyl radical addition-cyclization reactions of oxime ether connected with α,β-unsaturated carbonyl group proceeded under iodine atom-transfer reaction conditions to give the functionalized azacycles via two carbon-carbon bonds-forming process.

Full text of DOI:10.1248/cpb.52.842

842
Chem. Pharm. Bull. 52(7) 842—847 (2004)
Vol. 52, No. 7
Solid-Phase Tandem Radical Addition-Cyclization Reaction:
Triethylborane-Induced Reaction of Oxime Ethers Anchored to Polymer
Support
Hideto MIYABE,1) Hirotaka TANAKA, and Takeaki NAITO*
Kobe Pharmaceutical University; Motoyamakita, Higashinada-ku, Kobe 658–8558, Japan.
Received April 8, 2004; accepted May 6, 2004; published online May 10, 2004
Tandem radical addition-cyclization of oxime ethers anchored to polymer support was studied. The reaction
of oxime ethers with stannyl radical proceeded effectively by the use of triethylborane as a radical initiator. The
alkyl radical addition-cyclization reactions of oxime ether connected with a,b-unsaturated carbonyl group pro-
ceeded under iodine atom-transfer reaction conditions to give the functionalized azacycles via two carbon–car-
bon bonds-forming process.
Key words radical reaction; solid-phase; oxime ether; cyclization; triethylborane
Solid-phase radical reactions have been developed for an bonyl group and the heteroatom radical addition-cyclization
important carbon–carbon bond-forming method on solid sup- reaction of oxime ethers having an olefin moiety (Fig. 1).37—
port under mild reaction conditions.2—11) We have recently 40) Based on these results, we have studied the tandem radical
demonstrated that triethylborane has the potential to induce addition-cyclization of oxime ethers (TYPE 1 and TYPE 2)
the intermolecular radical reactions on solid support.12,13) anchored to polymer support.16,17) In this paper, we report the
Moreover, the employment of triethylborane and its related radical addition-cyclization reaction of oxime ether (TYPE
radical initiator such as diethylzinc at low reaction tempera- 1). To enhance the reactivity of resin-bound substrates, we
ture would facilitate the control of stereochemistry in solid- introduced a temporary spacer generated from glutaric anhy-
phase reactions.14,15) As a part of our program directed to- dride.41)
ward the solid-phase radical reactions, the development of
solid-phase multi bonds-forming reactions has been the new Results and Discussion
focus of our efforts. We report here in detail the triethylbo-
rane-induced tandem radical addition-cyclization reaction of port is shown in Chart 1. The reaction of a-chloroacetal-
oxime ethers anchored to polymer support.16,17)
doxime ether 1, which was prepared from chloroacetaldehyde
Preparation of oxime ether 4 anchored to a polymer sup-
Free radical-mediated cyclization has been developed as a and O-benzylhydroxyamine hydrochloride,42) with allyl-
powerful method for preparing various types of cyclic com- amine gave the secondary amine 2 in 76% yield. The amine
pounds via carbon–carbon bond-forming processes.18—29) 2 was treated with glutaric anhydride to give the oxime ether
Our laboratory is interested in developing the effective and 3 as an E/Z mixture concerning oxime ether moiety in a 3 : 2
convenient methods for the synthesis of highly functional- ratio. In our recent studies on the radical reaction of oxime
ized cyclic compounds. For this purpose, we have studied the ethers, we have observed no remarkable effect of the geome-
radical reaction using oxime ether group as a radical accep- try of the starting oxime ether group on either the chemical
tor.30—36) Particularly, strategies involving the radical addi- yield or stereoselectivity by employing geometrically pure E
tion-cyclization reactions offer the advantage of the forma- and Z-isomers.32) Thus, oxime ether 3 was subjected to the
tion of multiple carbon–carbon and carbon–heteroatom following reactions without the separation of E/Z-isomers.
bonds in a single operation.
Wang resin purchased from Novabiochem was used. The
We recently reported the carbon radical addition–cycliza- oxime ether 3 having the spacer moiety was attached to
tion of oxime ethers connected with the a,b-unsaturated car- Wang resin by treatment with DCC in the presence of DMAP
in CH2Cl2 at 20 °C for 12 h to give the Wang resin-bound
oxime ether 4 in ca. 95% loading level. The loading level of
the Wang resin-bound oxime ether 4 was determined to be
0.81 mmol/g by quantification of nitrogen by elemental
Fig. 1
Chart 1
* To whom correspondence should be addressed. e-mail: taknaito@kobepharma-u.ac.jp
© 2004 Pharmaceutical Society of Japan

July 2004
843
Chart 2
Chart 3
Table 1. Solid-Phase Radical Reaction of Oxime Ether 4
Entry
XH
Solvent T (°C) Initiator Product Yield (%)a)
(entry 5). Recently, Ryu and Komatsu reported that diethyl-
zinc-air system can serve as an initiator of tin hydride-medi-
ated radical reaction as well as triethylborane.44—46) In order
to test the viability of diethylzinc on a solid support, we also
investigated the reaction of oxime ether 4 using a commer-
cially available 1.0 M solution of diethylzinc. Diethylzinc also
worked as a radical initiator at high temperature to give a
26% yield of the product 6a (entry 6). We also investigated
the reaction using different radical precursors (entries 7, 8).
The use of the less reactive triethylsilane did not give the de-
sired product, probably due to the predominant formation of
the ethylated product as a result of the competitive intermole-
cular addition of an ethyl radical, generated from triethylbo-
rane, to oxime ether group (entry 7).47) Moderate chemical
yield of 6b was also observed in the addition-cyclization of
oxime ether 4 using more reactive tris(trimethylsilyl)silane
(entry 8). The cyclic products 6a and 6b were obtained as a
diastereomeric mixture, although these stereostructures have
not been determined.
To survey the scope and limitations of the triethylborane-
induced radical addition-cyclization, we next investigated the
reaction of different oxime ethers 9 and 13 anchored to poly-
mer support (Chart 4). The reaction of alkyne-tethered oxime
ethers such as 9 with a stannyl radical has been studied in so-
lution phase.48—50) Additionally, we recently developed the
reaction between oxime ether group and alkoxycarbonyl-sta-
bilized radicals in solution phase.37—39) Thus, the solid-phase
reaction of oxime ether 13 connected with the a,b-unsatu-
rated carbonyl group is the new focus of our efforts.17) The
reaction of a-chloroacetaldoxime ether 1 with propargyl-
amine gave the secondary amine 7 in 77% yield. The amine
7 was treated with glutaric anhydride to give the oxime ether
8, which was attached to Wang resin by treatment with DCC
in the presence of DMAP in CH2Cl2 at 20 °C for 12 h to give
the Wang resin-bound oxime ether 9. Wang resin-bound
oxime ether 13 was easily prepared from a-chloroacetal-
doxime ether 1 as shown in Chart 4.
1
2
3
4
5
6
7
8
Bu3SnH
Bu3SnH
Bu3SnH
Bu3SnH
Bu3SnH
Bu3SnH
Et3SiH
Toluene
Toluene
(CH2Cl)2
CH2Cl2
Toluene
Toluene
Toluene
80
80
80
35
80
80
80
80
AIBN
Et3B
Et3B
6a
6a
6a
6a
6a
6a
47
64
13
16
Trace
26
ND
50
Et3B
9-BBN
Et2Zn
Et3B
(TMS)3SiH Toluene
Et3B
6b
a) Yields are for the isolated products after purification by preparative TLC.
analysis.
At first, we examined the stannyl radical addition-cycliza-
tion reaction of oxime ether 4 (Chart 2). The reaction of 4
with Bu3SnH was carried out in toluene at 80 °C by using
AIBN (1.0 eqꢀ3) (Table 1, entry 1). After the reaction mix-
ture was stirred at 80 °C for 20 h, the resin was then filtered
and washed successively with CH2Cl2, AcOEt followed by
MeOH, and the subsequent cleavage of the resin by treatment
with TFA/CH2Cl2 (1 : 5, v/v) gave the crude product 6a. Pu-
rification of 6a was accomplished by preparative TLC
(MeOH/CHCl3 1 : 20, v/v) to afford the product 6a in 47%
isolated yield. In the solid-phase reactions, the often-tedious
workup to remove excess reagents from reaction mixture was
eliminated simply by washing of the resin with solvents. We
next examined the reaction by using triethylborane as a radi-
cal initiator. Triethylborane worked well at high temperature
without interference of the polystyrene skeleton of the resin
(entry 2). In contrast to the reaction using AIBN, the reaction
using triethylborane proceeded smoothly. To a flask contain-
ing oxime ether 4 and Bu3SnH in toluene was added three
times a commercially available 1.0 M solution of triethylbo-
rane (13 eq) in hexane at 80 °C, and the reaction mixture was
stirred at 80 °C for 8 h. The cyclic product 6a was obtained in
64% isolated yield after cleavage of the resin.
The rationale of the triethylborane-induced reaction path-
way is that the stannyl radical added to the olefin moiety of 4
to form intermediate alkyl radical A, which attacked in-
tramolecularly the triethylborane-activated oxime ether group
as in 5-exo-trig radical cyclization to form the benzyl–
oxyaminyl radical B (Chart 3). The benzyloxyaminyl radical
B was trapped by triethylborane as a radical terminator to
give the product C and an ethyl radical, although another
possible route to the product 6a from B via the reaction with
Bu3SnH would not be rigorously excluded. In regard to the
solvent effect, the reaction in (CH2Cl)2 or CH2Cl2 gave poor
yields of the product 6a (entries 3, 4). The use of 9-BBN43)
as a radical initiator was less effective for the reaction of 4
At first, we examined the stannyl radical addition-cycliza-
tion of oxime ether 9 having propargyl group (Chart 5).
Treatment of 9 with triethylborane in the presence of
Bu3SnH gave the cyclic product 15 in 77% isolated yield,
after cleavage of the resin. The cyclic product 15 was stable
under acidic conditions. Treatment of the cyclic product 15
with hydrogen chloride in ethanol did not give the pro-
todestannylated product.40) We next investigated of oxime
ether 13 having an electron-deficient carbon–carbon double
bond. As expected, the stannyl radical added to an electron-
deficient carbon–carbon double bond as well as an isolated

844
Vol. 52, No. 7
Chart 6
Table 2. Tandem Reaction of Oxime Ether 13
Yield
(%)a)
Entry
RI
EtnM
Product
1
2
None
None
i-PrI
c-Hexyl I
c-Pentyl I
s-Bu I
Et3B
Et2Zn
Et3B
Et3B
Et3B
Et3B
Et3B
19a
19a
19b
19c
19d
19e
72
12
69
54
59
55
ND
3b)
4b)
5b)
6b)
7b)
t-Bu I
Chart 4
a) Yields are for the isolated products after purification by preparative TLC. b)
Reactions were carried out with RI (30 eq) and Et3B (3 eqꢀ3) in toluene at 100 °C.
bonds on a solid support by using Wang resin-bound oxime
ether 13 (Chart 6).
The reaction of 13 with an ethyl radical proceeded effec-
tively by the treatment of simple triethylborane (Table 2,
entry 1). To a flask containing aldoxime ether 13 in toluene
was added three times a commercially available 1.0 M solu-
tion of triethylborane in hexane (3 eq) at 100 °C. After the re-
action mixture was stirred at 100 °C totally for 2 h, the resin
was then filtered and successively washed with CH2Cl2,
AcOEt, and MeOH. The cleavage of the resin by treatment
with NaOMe gave the desired azacyclic product 19a in 72%
isolated yield. However, the tandem radical reaction of oxime
ether 13 did not proceed at 25 °C. The reaction using di-
ethylzinc as a radical initiator gave the desired product 19a in
12% isolated yield, after cleavage of the resin (entry 2). The
treatment of 13 with i-PrI (30 eq) and triethylborane (3 eqꢀ
3) in toluene at 100 °C gave the desired azacyclic product
19b in 69% isolated yield. A favorable experimental feature
of this method is that the reaction proceeds smoothly even in
the absence of toxic tin hydride or heavy metals via a route
Chart 5
carbon–carbon double bond by using triethylborane as a radi- involving an iodine atom-transfer process (Chart 7).51) More-
cal initiator. Treatment of 13 with triethylborane in the pres- over, it is noteworthy that this reaction provided the new
ence of Bu3SnH gave the cyclic product 17 in 64% yield, method for the construction of two carbon–carbon bonds on
after cleavage of the resin.
solid support under mild reaction conditions without strictly
The development of tandem carbon–carbon bonds-form- anhydrous solvents and reagents. Good chemical yields were
ing radical reactions is a new subject of considerable interest; also observed in the radical reaction using different radical
thus, a number of extensive investigations were reported in re- precursors such as cyclohexyl, cyclopentyl, and sec-butyl io-
cent years.21) We also reported a new Mannich-type reaction dides under the iodine atom-transfer reaction conditions,
based on tandem carbon–carbon bonds-forming radical reac- except for a bulky tert-butyl radical (entries 3—7). In this re-
tion, which has been successfully applied to the asymmetric action, triethylborane acted as an effective reagent for trap-
synthesis of g-butyrolactones and b-amino acids.37—39) We ping the intermediate aminyl radicals E to regenerate an ethyl
newly investigated the construction of two carbon–carbon radical; therefore, more than a stoichiometric amount of tri-

July 2004
845
CH2Cl2, the organic phase was washed with diluted aqueous NaHCO3, and
water, dried over MgSO4, and concentrated at reduced pressure. Purification
of the residue by preparative TLC (CHCl3/MeOH 20 : 1) afforded product 6a
(57.4 mg, 45%).
Radical Reaction of Oxime Ether 4 Using Et3B (Table 1, Entries 2 and
8) To a suspension of 4 (250 mg, 0.208 mmol) and Bu3SnH or (TMS)3SiH
(0.675 mmol) in toluene (10 ml) was added portionwise a 1.0 M solution of
Et3B in hexane (2.7 ml, 2.70 mmol) under a nitrogen atmosphere at 80 °C.
After being stirred at 80 °C for 3 h, Bu3SnH or (TMS)3SiH (0.675 mmol)
and Et3B in hexane (2.7 ml, 2.70 mmol) were added to the reaction mixture.
After being stirred at 80 °C for 3 h, Bu3SnH or (TMS)3SiH (0.675 mmol)
and Et3B in hexane (2.7 ml, 2.70 mmol) were added to the reaction mixture.
After being stirred at 80 °C for 2 h, the resin was filtered, washed well with
CH2Cl2, AcOEt, and MeOH, and then dried in vacuo. To a flask with the re-
sulting resin was added TFA/CH2Cl2 (1 : 5, v/v, 4.0 ml) under a nitrogen at-
mosphere at 20 °C. After the reaction mixture was stirred at the same tem-
perature for 30 min, the reaction mixture was filtered and washed with
CH2Cl2 (50 ml), and the filtrate was concentrated at reduced pressure. After
the resulting residue was dissolved in CH2Cl2, the organic phase was washed
with diluted aqueous NaHCO3, and water, dried over MgSO4, and concen-
trated at reduced pressure. Purification of the residue by preparative TLC
(CHCl3/MeOH 20 : 1) afforded products 6a and 6b.
5-[3-(Benzyloxyamino)-4-((tributylstannyl)methyl)pyrrolidin-1-yl]-5-
oxopentanoic Acid (6a) The presence of rotamers and isomers precluded
a comprehensive assignment of all proton resonances. As a colorless oil and
a 1 : 1 trans/cis mixture: IR (CHCl3) cmꢁ1: 2927, 1713, 1626. 1H-NMR
(CDCl3) d: 7.40—7.30 (5H, m), 6.21 (2H, br s), 4.68 (1H, s), 4.67 (1H, br s),
3.90—2.10 (11H, m), 1.97 (2H, m), 1.62—0.67 (28H, m). HR-MS m/z:
610.2794 (Calcd for C29H50N2O4Sn120 (Mꢃ): 610.2790) and 608.2778 (Calcd
for C29H50N2O4Sn118 (Mꢃ): 608.2784).
Chart 7
ethylborane would be required.
In conclusion, we have demonstrated the solid-phase tan-
dem radical reaction of oxime ethers connected with olefin
moieties. The reaction of oxime ethers with stannyl radical
proceeded smoothly by the use of triethylborane as a radical
initiator to provide the functionalized pyrrolidines. The tan-
dem carbon–carbon bonds-forming reaction of oxime ether
with alkyl radicals proceeded smoothly via a route involving
an iodine atom-transfer process without strictly anhydrous
solvents and reagents.
Experimental
General 1H- and 13C-NMR spectra were recorded at 200, 300, or 500
MHz and at 50 or 125 MHz, respectively. IR spectra were recorded using
FTIR apparatus. Mass spectra were obtained by EI or CI methods. Prepara-
tive TLC separations were carried out on precoated silica gel plates (E.
Merck 60F254). Flash column chromatography was performed using E.
Merck Kieselgel 60 (230—400 mesh).
5-[3-(Benzyloxyamino)-4-((tristrimethylsilyl)methyl)pyrrolidin-1-yl]-
5-oxopentanoic Acid (6b) The presence of rotamers and isomers pre-
cluded a comprehensive assignment of all proton resonances. As a a color-
less oil and a 1 : 1 trans/cis mixture: IR (CHCl3) cmꢁ1: 3691, 1730, 1602.
1H-NMR (CDCl3) d: 7.39—7.30 (5H, m), 7.15 (2H, br s), 4.67 (2H, br s),
4.00—2.90 (5H, m), 2.50—2.20 (4H, m), 1.95 (2H, m) 1.10—0.60 (3H, m),
0.19 (27H, s). HR-MS m/z: 566.2850 (Calcd for C26H50N2O4Si4 (Mꢃ):
566.2845).
N-(2-Propenyl)aminoethanal O-Benzyloxime (2) Preparation of 2 was
reported in our previous article.40)
4-[N-(2-(Benzyloxyimino)ethyl)-N-(2-propenyl)carbamoyl]butanoic
Acid (3) To a solution of 2 (100 mg, 0.526 mmol) in pyridine (2.0 ml) were
added glutaric anhydride (72.1 mg, 0.632 mmol) and DMAP (32.16 mg,
0.263 mmol) at room temperature. After being stirred at room temperature
for 4 h, the reaction mixture was diluted with AcOEt, washed with 5% HCl,
water, and brine, dried over Na2SO4 and concentrated at reduced pressure.
Purification of the residue by flash columun chromatography (hexane/AcOEt
1 : 2) afforded 3 (136 mg, 81%) as a colorless oil and a 3 : 2 mixture of E/Z-
oxime. The presence of rotamers and E/Z-isomers precluded a comprehen-
sive assignment of all proton resonances. IR (CHCl3) cmꢁ1: 3682, 2938,
1711, 1649, 1455. 1H-NMR (CDCl3) d: 9.13 (1H, br s), 7.37 (3/5H, t, Jꢂ5.7
Hz), 7.40—7.26 (5H, m), 6.69 (2/5H, t, Jꢂ4.4 Hz), 5.82—5.60 (1H, m),
5.24—5.02 (4H, m), 4.27—3.80 (4H, m), 2.48—2.24 (4H, m), 2.04—1.84
(2H, m). HR-MS m/z: 318.1559 (Calcd for C17H22N2O4 (Mꢃ): 318.1578).
Attachment of Oxime Ether 3 to Wang Resin To a suspension of
Wang resin (0.83 mmol/g, 1.0 g, 0.83 mmol) in CH2Cl2 (50 ml) were added
the acid derivative 3 (792 mg, 2.49 mmol), DCC (856 mg, 4.15 mmol) and
DMAP (50.7 mg, 0.415 mmol) under a nitrogen atmosphere at room temper-
ature. After the reaction mixture was stirred at the same temperature for 1 h
and then staid for 11 h, the resin was filtered, washed well with CH2Cl2,
AcOEt followed by MeOH and then dried in vacuo to give the Wang resin-
bound oxime ether 4.
Radical Reaction of Oxime Ether 4 Using AIBN (Table 1, Entry 1)
To a suspension of 4 (250 mg, 0.208 mmol) and Bu3SnH (0.182 ml, 0.675
mmol) in toluene (10 ml) was added portionwise a solution of AIBN (34 mg,
0.208 mmol) in toluene (1.5 ml) under a nitrogen atmosphere at 80 °C. After
being stirred at 80 °C for 5 h, Bu3SnH (0.182 ml, 0.675 mmol) and a solution
of AIBN (34 mg, 0.208 mmol) in toluene (1.5 ml) were added to the reaction
mixture. After being stirred at 80 °C for 5 h, Bu3SnH (0.182 ml, 0.675 mmol)
and a solution of AIBN (34 mg, 0.208 mmol) in toluene (1.5 ml) were added
to the reaction mixture. After being stirred at 80 °C for 10 h, the resin was
filtered, washed well with CH2Cl2, AcOEt, and MeOH, and then dried in
vacuo. To a flask with the resulting resin was added TFA/CH2Cl2 (1 : 5, v/v,
4.0 ml) under a nitrogen atmosphere at room temperature. After the reaction
mixture was stirred at the same temperature for 30 min, the reaction mixture
was filtered and washed with CH2Cl2 (50 ml), and the filtrate was concen-
trated at reduced pressure. After the resulting residue was dissolved in
N-(2-Propynyl)aminoethanal O-Benzyloxime (7) Preparation of 7 was
reported in our previous article.40)
4-[N-(2-(Benzyloxyimino)ethyl)-N-(2-propynyl)carbamoyl]butanoic
Acid (8) To a solution of 7 (5.85 g, 31.1 mmol) in pyridine (15.0 ml) were
added glutaric anhydride (4.26 g, 37.4 mmol) and DMAP (761 mg, 6.23
mmol) at room temperature. After being stirred at room temperature for 3 h,
the reaction mixture was diluted with AcOEt, washed with 5% HCl, water,
and brine, dried over Na2SO4 and concentrated at reduced pressure. Purifica-
tion of the residue by flash columun chromatography (hexane/AcOEt 1 : 2)
afforded 8 (7.31 g, 74%) as a colorless oil and a 3 : 2 mixture of E/Z-oxime.
The presence of rotamers and E/Z-isomers precluded a comprehensive as-
signment of all proton resonances. IR (CHCl3) cmꢁ1: 3694, 1711, 1655. 1H-
NMR (CDCl3) d: 7.80 (1H, br s), 7.42 (3/10H, t, Jꢂ4.9 Hz), 7.36 (3/10H, t,
Jꢂ5.8 Hz), 7.34—7.27 (5H, m), 6.80 (2/10H, t, Jꢂ4.1 Hz), 6.74 (2/10H, t,
Jꢂ4.3 Hz), 5.13 (2/5H, s), 5.11 (2/5H, s), 5.06 (6/5H, br s), 4.34—3.94 (4H,
m), 2.56—2.20 (5H, m), 2.06—1.84 (2H, m). HR-MS m/z: 316.1444 (Calcd
for C17H20N2O4 (Mꢃ): 316.1421).
Attachment of Oxime Ether 8 to Wang Resin To a suspension of
Wang resin (0.83 mmol/g, 12.2 g, 10.2 mmol) in CH2Cl2 (200 ml) were
added the acid derivative 8 (6.42 g, 20.3 mmol), DCC (10.5 g, 50.8 mmol)
and DMAP (621 mg, 5.08 mmol) under a nitrogen atmosphere at room tem-
perature. After the reaction mixture was stirred at the same temperature for
1 h and then staid for 11 h, the resin was filtered, washed well with CH2Cl2,
AcOEt followed by MeOH and then dried in vacuo to give the Wang resin-
bound oxime ether 9.
N-[2-(Benzyloxyimino)ethyl]-N-(2-hydroxyethyl)-2-propenamide (11)
Preparation of 11 was reported in our previous article.40)
4-[(2-(N-(2-(Benzyloxyimino)ethyl)-N-(2-propenoyl)amino)ethoxy)car-
bonyl]butanoic Acid (12) To a solution of 11 (100 mg, 0.382 mmol) in
pyridine (1.0 ml) was added glutaric anhydride (65.3 mg, 0.573 mmol) at
room temperature. After being stirred at 100 °C for 2 h, glutaric anhydride
(43.6 mg, 0.382 mmol) was added to the reaction mixture. After being
stirred at 100 °C for 2 h, the reaction mixture was diluted with AcOEt,
washed with 5% HCl, water, and brine, dried over Na2SO4 and concentrated
at reduced pressure. Purification of the residue by flash columun chromatog-
raphy (CHCl3/MeOH 30 : 1) afforded 12 (134 mg, 93%) as a colorless oil

846
Vol. 52, No. 7
C26H46N2O3Sn120 (Mꢃ): 554.2528).
and a 1 : 1 mixture of E/Z-oxime. The presence of rotamers and E/Z-isomers
precluded a comprehensive assignment of all proton resonances. IR (CHCl3)
cmꢁ1: 3619, 1965, 1735, 1651. 1H-NMR (CDCl3) d: 8.25 (1H, br s), 7.45—
7.25 (11/2H, m), 6.34 (1/2H, t, Jꢂ4.5 Hz), 6.64—6.28 (2H, m), 5.78—5.66
(1H, m), 5.14 (1/2H, s), 5.13 (1/2H, s), 5.06 (1H, br s), 4.33—4.14 (4H, m),
3.69—3.55 (2H, m), 2.43—2.28 (4H, m), 1.99—1.82 (2H, m). HR-MS m/z:
376.1649 (Calcd for C19H24N2O6 (Mꢃ): 376.1632).
Attachment of Oxime Ether 12 to Wang Resin To a suspension of
Wang resin (0.83 mmol/g, 4.27 g, 3.55 mmol) in CH2Cl2 (100 ml) were
added the acid derivative 12 (2.00 g, 5.32 mmol), DCC (2.74 g, 13.3 mmol)
and DMAP (162 mg, 1.33 mmol) under a nitrogen atmosphere at room tem-
perature. After the reaction mixture was stirred at the same temperature for
1 h and then staid for 11 h, the resin was filtered, washed well with CH2Cl2,
AcOEt followed by MeOH and then dried in vacuo to give the Wang resin-
bound oxime ether 13.
General Procedure for Reaction of Oxime Ether 13 with Alkyl Radi-
cal To a suspension of 13 (200 mg, 0.166 mmol) and RI (4.98 mmol) in
toluene (5 ml) was added portionwise a 1.0 M solution of Et3B in hexane
(0.498 ml, 0.498 mmol) under a nitrogen atmosphere at 100 °C. After being
stirred at 100 °C for 30 min, Et3B in hexane (0.498 ml, 0.498 mmol) was
added to the reaction mixture. After being stirred at 100 °C for 30 min, Et3B
in hexane (0.498 ml, 0.498 mmol) was added to the reaction mixture. After
being stirred at 100 °C for 1 h, the resin was filtered, washed well with
CH2Cl2, AcOEt, and MeOH, and then dried in vacuo. To a flask with the re-
sulting resin were added THF (6 ml), MeOH (3 ml), H2O (0.1 ml), and
NaOMe (5.2 M solution of in MeOH, 0.064 ml, 0.332 mmol) under a nitro-
gen atmosphere at room temperature. After the reaction mixture was stirred
at the same temperature for 1 h, the reaction mixture was filtered and washed
with CH2Cl2 (50 ml), and the filtrate was concentrated at reduced pressure.
After the resulting residue was dissolved in CH2Cl2, the organic phase was
washed with diluted aqueous NaHCO3, and water, dried over MgSO4, and
concentrated at reduced pressure. Purification of the residue by preparative
TLC (AcOEt) afforded product 19a—e.
5-[3-(Benzyloxyamino)-4-((tributylstannyl)methylene)pyrrolidin-1-yl]-
5-oxopentanoic Acid (15) To a suspension of 9 (250 mg, 0.208 mmol) and
Bu3SnH (0.208 ml, 0.675 mmol) in toluene (10 ml) was added portionwise a
1.0 M solution of Et3B in hexane (2.08 ml, 2.08 mmol) under a nitrogen at-
mosphere at 80 °C. After being stirred at 80 °C for 2 h, Bu3SnH (0.208 ml,
0.675 mmol) and Et3B in hexane (2.08 ml, 2.08 mmol) were added to the re-
action mixture. After being stirred at 80 °C for 2 h, Bu3SnH (0.208 ml,
0.675 mmol) and Et3B in hexane (2.08 ml, 2.08 mmol) were added to the re-
action mixture. After being stirred at 80 °C for 2 h, the resin was filtered,
washed well with CH2Cl2, AcOEt, and MeOH, and then dried in vacuo. To a
flask with the resulting resin was added TFA/CH2Cl2 (1 : 5, v/v, 4.0 ml) under
a nitrogen atmosphere at room temperature. After the reaction mixture was
stirred at the same temperature for 30 min, the reaction mixture was filtered
and washed with CH2Cl2 (50 ml), and the filtrate was concentrated at re-
duced pressure. After the resulting residue was dissolved in CH2Cl2, the or-
ganic phase was washed with diluted aqueous NaHCO3, and water, dried
over MgSO4, and concentrated at reduced pressure. Purification of the
residue by preparative TLC (CHCl3/MeOH 20 : 1) afforded product 15 (91
mg, 77%). The presence of rotamers and E/Z-isomers precluded a compre-
hensive assignment of all proton resonances. As a colorless oil and a 1 : 1
4-(Benzyloxyamino)-1-(2-hydroxyethyl)-3-propylpyrrolidin-2-one
(19a)38) trans-Isomer: As a colorless oil: IR (CHCl3) cmꢁ1: 3401, 1670.
1H-NMR (CDCl3) d: 7.38—7.30 (5H, m), 5.66—5.40 (1H, br m), 4.70 (2H,
s), 3.72 (2H, t, Jꢂ5.0 Hz), 3.57—3.50 (2H, m), 3.43 (1H, m), 3.36—3.30
(2H, m), 2.30 (1H, dt, Jꢂ8.5, 4.5 Hz), 2.08—1.90 (1H, br m), 1.71 (1H, m),
1.51—1.36 (3H, m), 0.92 (3H, t, Jꢂ7.0 Hz). 13C-NMR (CDCl3) d: 176.6,
137.3, 128.6, 128.5, 128.1, 76.7, 60.6, 59.2, 51.5, 46.1, 45.8, 32.0, 20.2,
14.0. HR-MS m/z: 292.1803 (Calcd for C16H24N2O3 (Mꢃ): 292.1785). cis-
Isomer: As a colorless oil: IR (CHCl3) cmꢁ1: 3371, 1673, 1454. H-NMR
1
(CDCl3) d: 7.37—7.30 (5H, m), 5.69—5.43 (1H, br m), 4.69 (1H, d,
Jꢂ11.5 Hz), 4.66 (1H, d, Jꢂ11.5 Hz), 3.71 (2H, t, Jꢂ5.0 Hz), 3.49—3.29
(4H, m), 2.51—2.47 (1H, m), 1.75—1.71 (1H, m), 1.47—1.33 (3H, m), 0.94
(3H, t, Jꢂ7.0 Hz). 13C-NMR (CDCl3) d: 176.2, 137.2, 128.7, 128.5, 128.1,
76.5, 60.5, 56.1, 51.3, 46.1, 44.8, 26.1, 21.1, 14.1. HR-MS m/z: 292.1781
(Calcd for C16H24N2O3 (Mꢃ): 292.1785).
4-(Benzyloxyamino)-1-(2-hydroxyethyl)-3-isobutylpyrrolidin-2-one
(19b)38) trans-Isomer: As a colorless oil: IR (CHCl3) cmꢁ1: 2959, 1670,
1488, 1455. 1H-NMR (CDCl3) d: 7.40—7.25 (5H, m), 5.69—5.43 (1H,
br m), 4.70 (2H, s), 3.78—3.63 (2H, m), 3.60—3.23 (6H, m), 2.34 (1H, m),
1.82—1.50 (2H, m), 1.40—1.21 (1H, m), 0.928 (3H, d, Jꢂ6.2 Hz), 0.908
(3H, d, Jꢂ6.2 Hz). 13C-NMR (CDCl3) d: 176.7, 137.3, 128.6, 128.4, 128.1,
76.7, 60.4, 59.6, 51.2, 45.9, 44.1, 39.1, 25.8, 23.1, 21.8. HR-MS m/z:
306.1955 (Calcd for C17H26N2O3 (Mꢃ): 306.1941). cis-Isomer: As a color-
1
E/Z mixture: IR (CHCl3) cmꢁ1: 2960, 1636. H-NMR (CDCl3) d: 7.40—
7.23 (5H, m), 5.28—5.06 (2H, m), 4.74—4.64 (2H, m), 4.24—3.96 (3H, m),
3.80—3.50 (2H, m), 2.42—2.22 (4H, m), 1.94 (2H, m), 1.59 (6H, m), 1.34
(6H, m), 1.25 (6H, m), 0.91 (9H, t, Jꢂ7.4 Hz). HR-MS m/z: 608.2646
(Calcd for C29H48N2O4Sn120 (Mꢃ): 608.2633) and 606.2617 (Calcd for
C29H48N2O4Sn118 (Mꢃ): 606.2627).
4-(Benzyloxyamino)-3-[(tributylstannyl)methyl]-1-(2-hydroxyethyl)-
pyrrolidin-2-one (17)40) To a suspension of 13 (200 mg, 0.166 mmol) and
Bu3SnH (0.179 ml, 0.664 mmol) in toluene (5 ml) was added portionwise a
1.0 M solution of Et3B in hexane (0.664 ml, 0.664 mmol) under a nitrogen at-
mosphere at 80 °C. After being stirred at 80 °C for 30 min, Bu3SnH
(0.179 ml, 0.664 mmol) and Et3B in hexane (0.664 ml, 0.664 mmol) were
added to the reaction mixture. After being stirred at 80 °C for 30 min,
Bu3SnH (0.179 ml, 0.664 mmol) and Et3B in hexane (0.664 ml, 0.664 mmol)
were added to the reaction mixture. After being stirred at 80 °C for 1 h, the
resin was filtered, washed well with CH2Cl2, AcOEt, and MeOH, and then
dried in vacuo. To a flask with the resulting resin were added THF (6 ml),
MeOH (3 ml), H2O (0.1 ml), and NaOMe (a 5.2 M solution of in MeOH,
0.08 ml, 0.415 mmol) under a nitrogen atmosphere at room temperature.
After the reaction mixture was stirred at the same temperature for 1 h, the re-
action mixture was filtered and washed with CH2Cl2 (50 ml), and the filtrate
was concentrated at reduced pressure. After the resulting residue was dis-
solved in CH2Cl2, the organic phase was washed with diluted aqueous
NaHCO3, and water, dried over MgSO4, and concentrated at reduced pres-
sure. Purification of the residue by preparative TLC (CHCl3/MeOH 20 : 1)
afforded product 17 (58.4 mg, 64%). Major isomer: as a colorless oil: IR
less oil: IR (CHCl3) cmꢁ1: 2959, 1674, 1486, 1467. H-NMR (CDCl3) d:
1
7.40—7.25 (5H, m), 5.62—5.39 (1H, br m), 4.69 (1H, d, Jꢂ11.8 Hz), 4.66
(1H, d, Jꢂ11.8 Hz), 3.79—3.62 (3H, m), 3.58—3.23 (4H, m), 3.18—2.97
(1H, br m), 2.57 (1H, m), 1.80—1.21 (3H, m), 0.926 (3/3H, d, Jꢂ6.2 Hz),
0.911 (3/3H, d, Jꢂ6.2 Hz). 13C-NMR (CDCl3) d: 176.3, 137.0, 128.6, 128.3,
128.0, 76.4, 60.5, 56.2, 51.2, 46.0, 42.8, 32.5, 26.0, 23.1, 21.7. HR-MS m/z:
306.1941 (Calcd for C17H26N2O3 (Mꢃ): 306.1941).
4-(Benzyloxyamino)-3-(cyclohexylmethyl)-1-(2-hydroxyethyl)pyrro-
lidin-2-one (19c)38) trans-Isomer: As a colorless oil: IR (CHCl3) cmꢁ1
:
2926, 1671, 1488, 1450. 1H-NMR (CDCl3) d: 7.40—7.25 (5H, m), 4.70
(2H, s), 3.70 (2H, t, Jꢂ5.2 Hz), 3.61—3.26 (6H, m), 2.37 (1H, m), 1.77—
0.83 (14H, m). 13C-NMR (CDCl3) d: 177.1, 137.3, 128.6, 128.5, 128.1,
76.7, 60.4, 59.7, 51.2, 45.9, 43.4, 37.6, 35.2, 33.9, 32.5, 26.5, 26.2, 26.1.
HR-MS m/z: 346.2249 (Calcd for C20H30N2O3 (Mꢃ): 346.2254). cis-Isomer:
As a colorless oil: IR (CHCl3) cmꢁ1: 2926, 1674, 1486, 1449. 1H-NMR
(CDCl3) d: 7.40—7.25 (5H, m), 5.61—5.33 (1H, br m), 4.71 (1H, d,
Jꢂ11.6 Hz), 4.65 (1H, d, Jꢂ11.6 Hz), 3.72 (2H, t, Jꢂ5.0 Hz), 3.66 (1H, m),
3.52—3.24 (4H, m), 3.15—2.92 (1H, br m), 2.59 (1H, m), 1.80—0.45 (13H,
m). 13C-NMR (CDCl3) d: 176.5, 137.2, 128.8, 128.5, 128.2, 76.5, 60.7, 56.5,
51.3, 46.2, 42.3, 35.6, 33.9, 32.7, 31.2, 26.5, 26.3, 26.2. HR-MS m/z:
346.2239 (Calcd for C20H30N2O3 (Mꢃ): 346.2254).
1
(CHCl3) cmꢁ1: 3368, 2925, 1674, 1488. H-NMR (CDCl3) d: 7.40—7.25
(5H, m), 4.70 (2H, s), 3.72 (2H, t, Jꢂ5.1 Hz), 3.55 (1H, m), 3.46—3.26 (4H,
m), 2.43 (1H, dt, Jꢂ5.7, 8.1 Hz), 1.52—1.41 (4H, m), 1.36—1.22 (6H, m),
0.92—0.81 (21H, m). 13C-NMR (CDCl3) d: 177.4, 137.2, 128.4, 128.3,
128.0, 76.5, 63.5, 60.6, 50.9, 46.1, 43.9, 29.0, 27.3, 13.6, 9.7. One carbon
peak was missing due to overlapping. HR-MS m/z: 554.2548 (Calcd for
C26H46N2O3Sn120 (Mꢃ): 554.2528). Minor isomer: as a colorless oil: IR
4-(Benzyloxyamino)-3-(cyclopentylmethyl)-1-(2-hydroxyethyl)pyrro-
lidin-2-one (19d)38) trans-Isomer: As a colorless oil: IR (CHCl3) cmꢁ1
:
1
2952, 2468, 1670, 1489, 1454. H-NMR (CDCl3) d: 7.40—7.25 (5H, m),
4.70 (2H, s), 3.71 (2H, t, Jꢂ5.6 Hz), 3.62—3.26 (6H, m), 2.30 (1H, m),
2.02—0.90 (11H, m). 13C-NMR (CDCl3) d: 176.7, 137.2, 128.5, 128.3,
127.9, 76.5, 60.3, 59.2, 51.2, 45.9, 45.2, 37.6, 36.0, 32.9, 32.1, 25.1, 24.8.
HR-MS m/z: 332.2112 (Calcd for C19H28N2O3 (Mꢃ): 332.2099). cis-Isomer:
1
(CHCl3) cmꢁ1: 3368, 2925, 1675, 1486. H-NMR (CDCl3) d: 7.40—7.25
(5H, m), 4.70 (2H, s), 3.72 (2H, t, Jꢂ5.4 Hz), 3.64 (1H, m), 3.51—3.26 (4H,
m), 2.69 (1H, dt, Jꢂ7.5, 9.3 Hz), 1.55—1.40 (4H, m), 1.38—1.21 (6H, m),
0.92—0.81 (21H, m). 13C-NMR (CDCl3) d: 177.1, 137.1, 128.5, 128.4,
128.0, 76.4, 60.6, 58.0, 51.0, 46.2, 43.2, 29.1, 27.3, 13.6, 9.9. One carbon
peak was missing due to overlapping. HR-MS m/z: 554.2540 (Calcd for
As a colorless oil: IR (CHCl3) cmꢁ1: 3401, 2951, 2464, 1678, 1485. H-
1
NMR (CDCl3) d: 7.40—7.25 (5H, m), 5.72—5.28 (1H, br m), 4.71 (1H, d,
Jꢂ11.6 Hz), 4.65 (1H, d, Jꢂ11.6 Hz), 3.76—3.67 (3H, m), 3.52—3.24 (4H,
m), 3.18—2.81 (1H, br m), 2.51 (1H, m), 2.00—1.00 (11H, m). 13C-NMR

July 2004
847
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(CDCl3) d: 176.2, 137.1, 128.6, 128.3, 128.0, 76.4, 60.5, 56.3, 51.2, 46.1,
44.1, 38.0, 33.0, 32.0, 29.7, 25.0, 24.9. HR-MS m/z: 332.2127 (Calcd for
C19H28N2O3 (Mꢃ): 332.2099).
4-(Benzyloxyamino)-1-(2-hydroxyethyl)-3-(2-methylbutyl)pyrrolidin-
2-one (19e)38) trans-Isomer: As a colorless oil and a 1 : 1 mixture of di-
astereomers concerning sec-butyl group: IR (CHCl3) cmꢁ1: 3396, 2963,
1
1664, 1488, 1455. H-NMR (CDCl3) d: 7.40—7.25 (5H, m), 4.70 (2H, s),
3.71 (2H, t, Jꢂ5.4 Hz), 3.61—3.24 (5H, m), 2.42—2.30 (1H, m), 1.78—
0.80 (11H, m). 13C-NMR (CDCl3) d: 176.4, 137.2, 128.5, 128.4, 128.3,
127.9, 76.5, 60.3, 59.7, 59.4, 51.2, 51.0, 45.8, 43.8, 43.7, 37.1, 36.7, 31.94,
31.85, 29.9, 28.4, 19.2, 18.3, 11.1, 10.9. HR-MS m/z: 320.2088 (Calcd for
C18H28N2O3 (Mꢃ) 320.2098). cis-Isomer: As a colorless oil and a 1 : 1 mix-
ture of diastereomers concerning sec-butyl group: IR (CHCl3) cmꢁ1: 3401,
1
2962, 1674, 1455. H-NMR (CDCl3) d: 7.40—7.25 (5H, m), 4.71 (1H, d,
Jꢂ11.6 Hz), 4.65 (1H, d, Jꢂ11.6 Hz), 3.75—3.65 (3H, m), 3.52—3.24 (4H,
m), 2.64—2.50 (1H, m), 1.80—0.80 (11H, m). 13C-NMR (CDCl3) d: 176.3,
137.0, 128.6, 128.4, 128.0, 76.4, 60.5, 56.6, 56.0, 51.22, 51.18, 46.1, 42.7,
42.6, 32.3, 32.2, 30.6, 30.14, 30.08, 28.5, 19.4, 18.5, 11.3, 11.0. HR-MS
m/z: 320.2088 (Calcd for C18H28N2O3 (Mꢃ) 320.2098).
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Acknowledgements We thank Grant-in-Aids for Scientific Research
(B) (T.N.) and for Young Scientists (B) (H.M.) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology of Japan, the Science Re-
search Promotion Fund of the Japan Private School Promotion Foundation
for research grants, and Mitsubishi Chemical Corporation Fund (H.M.).
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