A short synthesis of (+)-harzialactone A and (R)-(+)-4-hexanolide via proline-catalyzed sequential α-aminooxylation and Horner-Wadsworth-Emmons olefination of aldehydes

Article abstract of DOI:10.1016/j.tetasy.2007.07.031

An efficient and short enantioselective synthesis of the antitumor marine metabolite, (+)-harzialactone A 1 and pheromone, (R)-(+)-4-hexanolide 2 using l-proline-catalyzed sequential α-aminooxylation and Horner-Wadsworth-Emmons olefination of the respecti

Full text of DOI:10.1016/j.tetasy.2007.07.031

Tetrahedron: Asymmetry 18 (2007) 1795–1798
A short synthesis of (+)-harzialactone A and (R)-(+)-4-hexanolide
via proline-catalyzed sequential a-aminooxylation and
Horner–Wadsworth–Emmons olefination of aldehydes
Shriram P. Kotkar, Gurunath S. Suryavanshi and Arumugam Sudalai*
Chemical Engineering and Process Development Division, National Chemical Laboratory, Pashan Road, Pune 411 008, India
Received 29 May 2007; accepted 28 July 2007
Abstract—An efficient and short enantioselective synthesis of the antitumor marine metabolite, (+)-harzialactone A 1 and pheromone,
(R)-(+)-4-hexanolide 2 using L-proline-catalyzed sequential a-aminooxylation and Horner–Wadsworth–Emmons olefination of the
respective aldehydes is described.
ꢀ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
In recent years, proline has emerged as a ‘universal cata-
lyst’ because of its high utility, especially in enantioselective
aldol,6 Diels–Alder,7 Michael addition,8 and a-functionali-
zation of aldehydes9 among many others.10 Proline-cata-
lyzed sequential transformations in particular,11 are an
emerging research field in organic synthesis. Herein we
report a highly efficient and short synthesis of (+)-harzi-
alactone A 1 and (R)-(+)-4-hexanolide 2 via L-proline-
catalyzed sequential aminooxylation–olefination11a,12 of
the corresponding aldehydes (Schemes 2 and 3).
Marine microorganisms have been a rich source of bio-
active metabolites, especially those with unique structural
features that might represent possible leads in drug discov-
ery processes.1 (+)-Harzialactone A 1, a marine metabolite
isolated from the culture broth of a strain of Trichoderma
harianium OUPS-N115, has exhibited significant antitumor
and cytotoxic activities against cultured P388 cells.2
Mereyala et al. have synthesized harzialactone A 1 starting
from D-glucose3a and D-xylose,3b each in seven steps with
an overall yield of 15% and 24%, respectively. Recently,
Wu et al. have synthesized the antipode of harzialactone
A from L-malic acid in 40% overall yield.3c (R)-(+)-4-Hex-
anolide 2 is a component of a pheromone secreted by the
female of dermestid beetles Trogoderma glabrum.4 The bee-
tle has been reported to respond only to the (R)-isomer of
hexanolide.4 Several asymmetric syntheses of 2 have been
reported, which include (i) the stereospecific synthesis from
chiral building blocks;5a,b (ii) the asymmetric reduction of
prochiral ketones with Baker’s yeast or chiral reducing
agents;5c–e (iii) the resolution of alcohols5f and other asym-
metric catalytic methods.5g,h Some of these methods are
not amenable to scale up, due to large number of steps cou-
pled with low yields.
2. Result and discussion
Scheme 1 shows the retrosynthetic analysis of (+)-harzia-
lactone A, wherein we envisaged the L-proline-catalyzed
sequential aminooxylation–olefination strategy for intro-
ducing chirality into the molecule. Thus, 3-phenylpropanal
was subjected to a-aminooxylation13 with nitrosobenzene
and L-proline at ꢀ20 ꢁC, followed by in situ Horner–
Wadsworth–Emmons (HWE) olefination with LiCl and
DBU (Masamune–Roush protocol)14 to furnish aminooxy
olefinic ester 4 in 77% yield. Simultaneous reduction of both
the C@C bond and the anilinoxy group in ester 4 was
achieved with 10% Pd/C, H2 (1 atm) to produce
c-hydroxy ester 5 in 92% yield. Intramolecular cyclization
of hydroxyl ester 5 gave lactone 3 in 88% yield and 97%
ee (determined by chiral HPLC analysis, Chiracel OD-H).
Finally, diastereoselective a-hydroxylation15 of lactone 3
*
Corresponding author. Tel.: +91 20 25902174; fax: +91 20 25902676;
e-mail: a.sudalai@ncl.res.in
0957-4166/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.07.031

1796
S. P. Kotkar et al. / Tetrahedron: Asymmetry 18 (2007) 1795–1798
HO
O
O
O
Ph
O
(+)-Harzialactone A, 1
(R)-(+)-4-Hexanolide, 2
ONHPh
O
HO
O
Ph
Ph
CHO
O
Ph
O
O
1
Ph
OEt
4
3
Scheme 1. Retrosynthetic analysis of (+)-harzialactone A 1.
was achieved with KHMDS and 2-[(4-methylphenyl)sulfo-
nyl]-3-phenyloxaziridine (Davis oxaziridine) to afford 1 as
a separable mixture of diastereomers (dr 2:1 trans/cis, as
determined by 1H NMR analysis of the crude mixture) with
fination of their respective aldehydes. Excellent yields, sim-
ple and environmentally friendly procedures, and the easy
availability of the starting materials are some of the salient
features of this approach.
trans-lactone 1 [(+)-harzialactone A] as a major product
25
(42% yield for trans-lactone) (>99% ee); ½aꢁD ¼ þ40 (c
25
0.3, CHCl3) {lit.3c ½aꢁD ¼ ꢀ39:6 (c 0.32, CHCl3) for
4. Experimental
antipode of (+)-harzialactone A} (Scheme 2).
4.1. General information
Similarly, the synthesis of (R)-(+)-4-hexanolide started
with the same sequential protocol used for harzialctone A
with n-butyraldehyde to produce 6 in 75% yield. Reduction
of both the C@C bond and the anilinoxy group in ester 6
with 10% Pd/C, H2 (1 atm) produced c-hydroxy ester 7
in 84% yield, which on subsequent cyclization furnished
Solvents were purified and dried by standard procedures
before use. Melting points are uncorrected. Optical rota-
tions were measured using the sodium D line on a JAS-
CO-P-1020 digital polarimeter. H and 13C NMR spectra
1
were recorded on Brucker AC-200 spectrometer. Enantio-
meric excesses were measured using either chiral HPLC
or by comparison with the specific rotation. Elemental
analysis was carried out on a Carlo Erba CHNS-O
analyzer.
(R)-4-hexanolide
2 in 95% yield and in 97% ee;
25
25
½aꢁD ¼ þ51:7 (c 1, MeOH) {lit.5c ½aꢁD ¼ þ53:1 (c 1,
MeOH)} (Scheme 3).
3. Conclusion
4.2. (R)-Ethyl 4-anilinoxy-5-phenylpent-2-enoate 4
In conclusion we have achieved a short and efficient synthe-
sis of (+)-harzialactone A (overall yield 26.1%) and
(R)-(+)-4-hexanolide (overall yield 59.8%) by employing
proline-catalyzed sequential a-aminooxylation–HWE ole-
To a solution of nitrosobenzene (3 g, 28 mmol) and L-pro-
line (483 mg, 15 mol %) in CH3CN (60 mL) was added 3-
phenylpropanal (4.5 g, 33.6 mmol) at ꢀ20 ꢁC. The reaction
mixture was stirred at the same temperature for 24 h fol-
OH
ONHPh
c
b
a
Ph
Ph
Ph
CHO
CO2Et
CO2Et
5
4
HO
O
d
O
Ph
O
Ph
O
3
(+)-Harzialactone A
97% ee (HPLC)
1
Scheme 2. Reagents and conditions: (a) PhNO, L-proline (15 mol %), CH3CN, ꢀ20 ꢁC, 24 h then triethyl phosphonoacetate, LiCl, DBU, 1 h, 77%; (b) H2
(1 atm), Pd/C (10%), MeOH, 12 h, 92%; (c) EtOH, reflux, 5 h, 88%; (d) 2-[(4-Methylphenyl)sulfonyl]-3-phenyloxaziridine, KHMDS, THF, ꢀ78 ꢁC, 1 h,
63%.
ONHPh
OH
c
a
b
CHO
O
CO2Et
CO2Et
O
6
7
(R)-(+)-4-Hexanolide
2
Scheme 3. Reagents and conditions: (a) PhNO, L-proline (15 mol %), ꢀ20 ꢁC, 24 h then triethyl phosphonoacetate, LiCl, DBU, 1 h, 75%; (b) H2 (1 atm),
Pd/C (10%), MeOH, 12 h, 84%; (c) EtOH, reflux, 5 h, 95%.

S. P. Kotkar et al. / Tetrahedron: Asymmetry 18 (2007) 1795–1798
1797
lowed by the addition of LiCl (1.7 g, 1.5 equiv), triethyl
4.5. (+)-Harzialactone A 1
phosphonoacetate (9.4 g, 1.5 equiv), and 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU) (4.2 g, 1 equiv). After stirring
for 1 h, the reaction mixture was quenched with half satu-
rated NH4Cl and extracted with ethyl acetate (3 · 60 mL).
Combined organic phases were concentrated and dried
over anhydrous Na2SO4. Purification by flash column
chromatography (pet. ether–EtOAc = 88:12) afforded
To a solution of KHMDS in dry THF (8 mL) at ꢀ78 ꢁC
was added lactone 3 (0.5 g, 2.8 mmol) in THF and the reac-
tion mixture was stirred for 10 min followed by the addi-
tion of [(4-methylphenyl)sulfonyl]-3-phenyloxaziridine
(Davis oxaziridine) (814 mg, 2.8 mmol). After stirring at
the same temperature for 1 h, the reaction mixture was
quenched with aq NH4Cl and extracted with ethyl acetate
(3 · 15 mL). Purification by flash column chromatography
gave 1 as a colorless solid in 63% yield with dr = 2:1. The
diastereomers were separated by flash column chromato-
graphy (pet. ether–EtOAc = 70:30). Yield: 229 mg, 42%
aminooxy olefinic ester 4. Yield: 6.7 g, 77% yield;
25
½aꢁD ¼ þ47 (c 1, CHCl3); IR (CHCl3) mmax 3028, 2979,
2937, 2358, 1714, 1600, 1494, 1454, 1369, 1271, 1178,
1
1029, 910, 754 cmꢀ1; H NMR (200 MHz, CDCl3): d 1.29
(t, J = 7.1 Hz, 3H), 2.87–2.97 (dd, J = 5.9, 13.8 Hz, 1H),
3.04–3.15 (dd, J = 7.7, 13.8 Hz, 1H), 4.19 (q, J = 7.2,
2H), 4.58 (m, 1H), 5.97 (d, J = 15.8, 1H), 6.64 (d,
J = 7.9 Hz, 1H), 6.97 (m, 3H), 7.16–7.30 (m, 8H); 13C
NMR (50 MHz, CDCl3): d 14.1, 40.0, 60.4, 83.9, 114.1,
121.9, 122.9, 126.6, 128.4, 128.8, 129.5, 136.8, 146.2,
148.1, 165.9; Elemental Anal. Calcd for C19H21NO3: C,
73.29; H, 6.8; N, 4.5. Found: C, 73.33; H, 6.73; N, 4.67.
yield (after diastereomeric purification and recrystallization
25
from n-hexane); mp = 80 ꢁC (lit.11 82–84 ꢁC); ½aꢁD ¼ þ40
25
(c 0.3, CHCl3) {lit.14 ½aꢁD ¼ ꢀ39:6 (c 0.32, CHCl3) for anti-
pode of (+)-harzialactone A}; IR (CHCl3) mmax 3373, 2921,
1
2360, 2343, 1772, 1456, 1188, 1031, 700 cmꢀ1; H NMR
(200 MHz, CDCl3): d 2.27–2.43 (m, 2H), 2.43 (br s, 1H),
2.96 (d, J = 5.7 Hz, 2H), 3.96 (t, J = 8.1 Hz, 1H), 4.91
(m, 1H), 7.19–7.34 (m, 5H); 13C NMR (50 MHz, CDCl3):
d 34.3, 41.1, 67.0, 78.1, 127.2, 128.4, 129.5, 135.2, 177.2;
Elemental Anal. Calcd for C11H12O3: C, 68.74; H, 6.29.
Found: C, 68.55; H, 6.58.
4.3. (S)-Ethyl 4-hydroxy-5-phenylpentanoate 5
To a solution of 4 (5.0 g, 16 mmol) in MeOH was added
10% Pd/C (300 mg) carefully. The reaction mixture was
then stirred in a hydrogen atmosphere (1 atm of H2) for
12 h. After completion of the reaction (monitored by
TLC), the reaction mixture was filtered through Celite
pad and concentrated to near dryness. The crude product
was then purified by flash column chromatography (pet.
4.6. (R)-Ethyl 4-anilinoxyhex-2-enoate 6
To a solution of nitrosobenzene (3 g, 28 mmol) and L-pro-
line (476 mg, 15 mol %) in CH3CN (60 mL) was added n-
butyraldehyde (2.94 mL, 33.6 mmol) at ꢀ20 ꢁC. The reac-
tion mixture was stirred at the same temperature for 24 h,
followed by the addition of LiCl (1.7 g, 1.5 equiv), triethyl
phosphonoacetate (9.4 g, 1.5 equiv) and DBU (4.2 g,
1 equiv). After stirring for 1 h, the reaction mixture was
quenched with half saturated NH4Cl and extracted with
ethyl acetate (3 · 60 mL). The combined organic phases
were concentrated and dried over anhydrous Na2SO4. Puri-
fication by flash column chromatography (pet. ether–
ether–EtOAc = 75:25). Yield: 3.28 g, 92% yield;
25
½aꢁD ¼ þ14:5 (c 1, CHCl3); IR (CHCl3) mmax 3488, 2927,
2360, 2331, 1770, 1731, 1602, 1494, 1178, 1022, 923,
750 cmꢀ1
;
1H NMR (200 MHz, CDCl3): d 1.25 (t,
J = 7.2 Hz, 3H), 1.77–1.90 (m, 3H), 2.48 (dt, J = 7.5,
1.3 Hz, 2H), 2.65–2.75 (dd, J = 7.9, 13.5 Hz, 1H), 2.78–
2.87 (dd, J = 4.7, 13.5 Hz, 1H), 3.85 (m, 1H), 4.11 (q,
J = 7.1, 2H), 7.28 (m, 5H); 13C NMR (50 MHz, CDCl3):
d 14.1, 30.7, 31.4, 44.0, 60.4, 71.8, 126.4, 128.4, 129.3,
138.1, 174.0; Elemental Anal. Calcd for C13H18O3: C,
70.24; H, 8.16. Found: C, 70.14; H, 8.22.
EtOAc = 90:10) afforded aminooxy olefinic ester 6 as brown
25
liquid. Yield: 5.2 g, 75% yield; ½aꢁD ¼ þ88 (c 2, CHCl3); IR
(CHCl3) mmax 3433, 2975, 2937, 2877, 2360, 1703, 1654,
1
1454, 1274, 1178, 1039, 777 cmꢀ1; H NMR (200 MHz,
CDCl3): d 1.02 (t, J = 7.4 Hz, 3H), 1.31 (t, J = 7.1 Hz,
3H), 1.62 (s, 1H), 1.66–1.99 (m, 2H), 4.19 (q, J = 7.1 Hz,
2H), 4.30 (m, 1H), 5.77 (d, J = 15.9 Hz, 1H), 6.93 (m, 4H),
7.24 (m, 2H); 13C NMR (50 MHz, CDCl3): d 9.5, 14.1,
26.2, 60.3, 84.0, 114.2, 121.9, 122.8, 128.8, 146.9, 148.4,
165.9 ppm. Elemental Anal. Calcd for C14H19NO3: C,
67.45; H, 7.68; N, 5.62. Found: C, 67.15; H, 7.28; N, 5.80.
4.4. (S)-5-Benzyl-dihydrofuran-2(3H)-one 3
A solution of hydroxyl ester 5 (2 g, 9 mmol) in dry ethanol
(20 mL) was refluxed for 5 h. Removal of the solvent under
reduced pressure and flash chromatographic purification
gave lactone 3 as a brown colored liquid. Yield: 1.39 g,
25
88% yield; ½aꢁD ¼ þ24:7 (c 1, CHCl3); IR (CHCl3) mmax
2925, 2360, 1771, 1602, 1456, 1180, 1022, 912, 734 cmꢀ1
;
4.7. (R)-Ethyl 4-hydroxyhexanoate 7
1H NMR (200 MHz, CDCl3): d 1.86–2.05 (m, 1H), 2.17–
2.53 (m, 3H), 2.87–2.98 (dd, J = 6.29, 14.0 Hz, 1H), 3.03–
3.13 (dd, J = 6.0, 14.0 Hz, 1H), 4.67–4.80 (m, 1H), 7.21–
7.36 (m, 5H); 13C NMR (50 MHz, CDCl3): d 26.9, 28.4,
41.0, 80.5, 126.7, 128.4, 129.2, 135.7, 176; Elemental Anal.
Calcd for C11H12O2: C, 74.98; H, 6.86. Found: C, 75.11; H,
6.77. Chiracel OD-H column; 97% ee, n-hexane–i-PrOH
(90:10 v/v); flow rate 1.0 mL/min; UV 214 nm; column
temperature 25 ꢁC; retention time: 14.95 min major (S-
isomer) and 17.53 min minor (R-isomer) (compared with
reported conditions16).
To a solution of 6 (3.0 g, 12 mmol) in MeOH was added
10% Pd/C (200 mg) carefully. The reaction mixture was
then stirred under a hydrogen atmosphere (1 atm of H2)
for 12 h. After completion of the reaction (monitored by
TLC), the reaction mixture was filtered through a Celite
pad and concentrated to near dryness. The crude product
was then purified by flash column chromatography
(pet. ether–EtOAc = 80:20). Yield: 1.6 g, 84% yield;
25
½aꢁD ¼ þ27 (c 2, CHCl3); IR (CHCl3) mmax 3523, 2970,
2939, 2881, 2358, 2331, 1770, 1602, 1461, 1353, 1182,

1798
S. P. Kotkar et al. / Tetrahedron: Asymmetry 18 (2007) 1795–1798
1H NMR (200 MHz, CDCl3): d 0.95 (t,
5. (a) Ravid, U.; Silverstein, R. M.; Smith, L. R. Tetrahedron
960, 850 cmꢀ1
;
1978, 34, 1449; (b) Dhotare, B.; Hassarajni, S. A.; Chatto-
padhyay, A. Enantiomer 1999, 4, 431; (c) Mori, K.; Mori, H.;
Sugai, T. Tetrahedron 1985, 41, 919; (d) Ghosh, S. K.;
Chattopadhyay, S.; Mamdapur, V. R. Tetrahedron 1991, 47,
3089; (e) Ramachandran, P. V.; Brown, H. C.; Pitre, S. Org.
Lett. 2001, 3, 17; (f) Jacobs, H. K.; Mueller, B. H.; Gopalan,
A. S. Tetrahedron 1992, 48, 8891; (g) Nunez, M. T.; Martin,
V. S. J. Org. Chem. 1990, 55, 1928; (h) Arceo, E.; Odriozola,
J. M.; Garcia, J. M.; Gonzalez, A.; Gil, P. Tetrahedron:
Asymmetry 2003, 14, 1617.
J = 7.4 Hz, 3H), 1.26 (t, J = 7.1 Hz, 3H), 1.48 (m, 2H),
1.76 (m, 2H), 2.44 (m, 3H), 3.53 (m, 1H), 4.11 (q,
J = 7.2 Hz, 2H); 13C NMR (50 MHz, CDCl3): d 9.64,
13.8, 29.8, 30.4, 31.4, 59.9, 71.8, 173.9 ppm. Elemental
Anal. Calcd for C8H16O3: C, 59.97; H, 10.07. Found: C,
59.69; H, 10.3.
4.8. (R)-(+)-4-Hexanolide 2
6. List, B.; Lerner, R. A.; Barbas, C. F., III. J. Am. Chem. Soc.
2000, 122, 2395.
A solution of hydroxyl ester 7 (1 g, 6.2 mmol) in ethanol
was refluxed for 5 h. Removal of the solvent under reduced
pressure and flash chromatographic purification gave
hexanolide 2 in 95% yield. Yield: 676 mg, 95% yield;
7. (a) Sabitha, G.; Fatima, N.; Reddy, E. V.; Yadav, J. S. Adv.
Synth. Catal. 2005, 347, 1353; (b) Ramachary, D. B.;
Chowdari, N. S.; Barbas, C. F., III. Synlett 2003, 1910.
8. Hechavarria Fonseca, M. T.; List, B. Angew. Chem., Int. Ed.
2004, 43, 3958.
9. For a-functionalization reviews: (a) Franzen, J.; Marigo, M.;
Fielenbach, D.; Wabnitz, T. C.; Kjarsgaard, A.;
Jørgensen, K. A. J. Am. Chem. Soc. 2005, 127, 18296; (b)
Guillena, G.; Ramon, D. J. Tetrahedron: Asymmetry 2006,
17, 1465.
25
25
½aꢁD ¼ þ51:7 (c 1, MeOH) {lit.6 ½aꢁD ¼ þ53:1 (c 1,
MeOH)}; IR (CHCl3) mmax 754, 970, 1176, 1353, 1458,
1602, 1770, 2360, 2941, 2970, 3020 cmꢀ1
;
1H NMR
(200 MHz, CDCl3): d 1.01 (t, J = 7.4 Hz, 3H), 1.58–1.96
(m, 3H), 2.31 (m, 1H), 2.50–2.59 (m, 2H), 4.44 (m, 1H);
13C NMR (50 MHz, CDCl3): d 9.35, 27.4, 28.4, 28.7,
82.13, 177.2 ppm. Elemental Anal. Calcd for C6H10O2: C,
63.14; H, 8.83. Found: C, 63.0; H, 8.54.
10. For a review of proline-catalyzed asymmetric reactions, see:
List, B. Tetrahedron 2002, 58, 5573.
11. (a) Zhong, G.; Yu, Y. Org. Lett. 2004, 6, 1637; (b) Kotkar, S.
P.; Chavan, V. B.; Sudalai, A. Org. Lett. 2007, 9, 1001; (c)
Zhao, G.-L.; Liao, W.-W.; Cordova, A. Tetrahedron Lett.
2006, 47, 4929; (d) Liao, W.-W.; Ibrahem, I.; Cordova, A.
Chem. Commun. 2006, 674; (e) Kumarn, S.; Shaw, D. M.;
Longbottom, D. A.; Ley, S. V. Org. Lett. 2005, 7, 4189.
12. Mangion, I. K.; MacMillan, D. W. C. J. Am. Chem. Soc.
2005, 127, 3696.
Acknowledgements
S.P.K. thanks CSIR, New Delhi, for the award of senior
research fellowship. The authors are thankful to Dr. B.D.
Kulkarni, Head, Chemical Engineering and Process Devel-
opment Division, for his support and encouragement.
13. (a) Hayashi, Y.; Yamaguchi, J.; Hibino, K.; Shoji, M.
Tetrahedron Lett. 2003, 44, 8293; (b) Zhong, G. Angew.
Chem., Int. Ed. 2003, 42, 4247; (c) Hayashi, Y.; Yamaguchi,
J.; Sumiya, T.; Shoji, M. Angew. Chem., Int. Ed. 2004, 43,
1112; (d) Bøgevig, A.; Sunden, H.; Cordova, A. Angew.
Chem., Int. Ed. 2004, 43, 1109; (e) Brown, S. P.; Brochu, M.
P.; Sinz, C. J.; MacMillan, D. W. C. J. Am. Chem. Soc. 2003,
125, 10808.
14. Blanchette, M. A.; Choy, W.; Davis, J. T.; Essenfeld, A. P.;
Masamune, S.; Roush, W. R.; Sakai, T. Tetrahedron Lett.
1984, 25, 2183.
15. (a) Curtis, N. R.; Holmes, A. B.; Looney, M. G. Tetrahedron
Lett. 1992, 33, 671; (b) Caputo, R.; Cecere, G.; Guaragna, A.;
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