Radical cyclization route to the stereoselective synthesis of ( )-trans-cognac lactone and ( )-trans-aerangis lactone

Article abstract of DOI:10.1080/00397910601055057

Stereoselective total synthesis of two chiral lactones, (+)-trans-cognac lactone (1b) and (+)-trans-aerangis lactone (2c), has been achieved from the same intermediate using a radical-based cyclization route. Copyright Taylor & Francis Group, LLC.

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Radical Cyclization Route to the
Stereoselective Synthesis of (+)‐trans‐Cognac
Lactone and (+)‐trans‐Aerangis Lactone
Gowravaram Sabitha a , M. Bhikshapathi a & J. S. Yadav a
a Organic Division I , Indian Institute of Chemical Technology , Hyderabad,
India
Published online: 06 Mar 2007.
To cite this article: Gowravaram Sabitha , M. Bhikshapathi & J. S. Yadav (2007) Radical Cyclization Route
to the Stereoselective Synthesis of (+)‐trans‐Cognac Lactone and (+)‐trans‐Aerangis Lactone, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:4,
559-567, DOI: 10.1080/00397910601055057
To link to this article: http://dx.doi.org/10.1080/00397910601055057
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Synthetic Communicationsw, 37: 559–567, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910601055057
Radical Cyclization Route
to the Stereoselective Synthesis
of (1)-trans-Cognac Lactone and
(1)-trans-Aerangis Lactone
Gowravaram Sabitha, M. Bhikshapathi, and J. S. Yadav
Organic Division I, Indian Institute of Chemical Technology,
Hyderabad, India
Abstract: Stereoselective total synthesis of two chiral lactones, (þ)-trans-cognac
lactone (1b) and (þ)-trans-aerangis lactone (2c), has been achieved from the same
intermediate using a radical-based cyclization route.
Keywords: aerangis lactone, chiral lactones, cognac lactone, natural products
INTRODUCTION
The g-lactones (five-membered) and d-lactones (six-membered) functiona-
lized at the ring carbons are useful as building blocks in natural product
syntheses and are used in the perfume industry. Examples in this family are
(2)-cis and (þ)-trans cognac lactones 1a and 1b, and cis and trans
aerangis lactones 2a, 2b, 2c, and 2d. The cis and trans cognac lactones
were isolated from different types of wood, and they were identified as key
flavors of aged alcoholic beverages such as whisky, brandy, wine, and
cognac.[1–3] These two diastereomeric lactones have the same (S)-configu-
ration at C-(4) and different configurations at C-(5). Aerangis lactones 2a–
2d[4] are natural fragrant molecules identified from the Aerangis species
Received July 7, 2006
IICT Communication No. 060310.
Address correspondence to Gowravaram Sabitha, Organic Division I, Indian
Institute of Chemical Technology, Hyderabad 500 007, India. E-mail:
gowravaramsr@yahoo.com
559

560
G. Sabitha, M. Bhikshapathi, and J. S. Yadav
(native to Kenya and Tanzania). In 1995, Bartschat et al.[4b] obtained samples
of four stereo isomers of aerangis lactone by chiral high-performance liquid
chromatography.
A variety of syntheses of racemic and enantiomerically pure cognac
lactones in both diastereomeric forms is reported in the literature.[5] (+)-cis-
Aerangis lactone was first obtained as a 1:1 mixture with the trans-diastereomer
2c by hydrogenation of dihydrojasmone and subsequent Baeyer–Villiger
oxidation.[6] The cis and trans isomers were found to show a very pleasant
odor, reminiscent both of the smell of tuberose and gardenia and of the
fragrance of caramel, condensed milk, and coconut.[6] As a consequence, the
synthesis of disubstituted g- and d-lactones 1 and 2 has drawn much
attention. As a part of our program on the synthesis of naturally occurring
bio-active lactones,[7] we herein report a radical cyclization route for the
synthesis of (þ)-trans-cognac lactone (1b) and (þ)-trans-aerangis lactone (2c).
RESULTS AND DISCUSSION
Even though the radical-based five-membered lactone synthesis was reported
from our group,[8] this is the first report of a synthesis of a six-membered
lactone using the same strategy. While targeting synthesis of the six-
membered lactone, (þ)-trans-aerangis lactone 2c, we could get a five-
membered lactone, (þ)-trans-cognac lactone 1b, from the key intermediate

Stereoselective Synthesis of Chiral Lactones
561
3. The intermediate 3 can be prepared from a chiral epoxy alcohol 4 as given in
Scheme 1.
The synthesis of the two lactones 1b and 2c began with the readily
available E-allyl alcohol 5 (Scheme 2). The alcohol 5 was subjected to
Sharpless epoxidation conditions using (2) diethyl-D(-)-tartarate (DET),
Ti(OiPr4), and cumenehydroperoxide in dichloromethane (DCM) at 2248C
to afford 2,3-epoxy alcohol 4 in 78% yield. The epoxy alcohol 4 was
converted to the corresponding 2,3-epoxy-1-chloride 6 by treatment with tri-
phenylphosphine (TPP) in refluxing CCl4 for 4 h. Reductive opening of 6 was
achieved with Na in ether to furnish the desired terminal alkenic alcohol 7 in
70% yield. Treatment of 7 with N-bromosuccinamide and ethyl vinyl ether in
DCM afforded the bromoacetal 8.[9] The key step of the synthesis was the
radical cyclization, which could be achieved by treatment of bromoacetal 8
with tri-n-butylstannane (Bu3SnH), N,N0-azoisobutyronitrile (AIBN)
catalyst (as a radical initiator), in refluxing benzene[10] for 4 h to give the
cyclic ethyl acetal 9. The cyclic acetal 9 was hydrolyzed using 60% AcOH
under refluxing conditions to give a key intermediate lactol 3, which on
oxidation with pyridiniumchlorochromate (PCC) in DCM afforded the
target molecule 1b in 92% yield. The stereochemistry of the lactone 1b was
established to be trans by NMR spectral analysis, which showed a methyl
resonance at 1.01 (d, J ¼ 5.9 Hz) and H-5 at 3.82–3.94 (m, 1H), and also
comparing the natural product spectral and optical rotation data.[5i]
The synthesis of trans-aerangis lactone was achieved from the key
intermediate lactol 3 by manipulating the following reactions (Scheme 2).
Scheme 1. Retro synthetic analysis.

562
G. Sabitha, M. Bhikshapathi, and J. S. Yadav
Scheme 2. Reagents and conditions: (a) (2)-DET, Ti(OiPr)4, cumenehydroperoxide,
DCM, 2248C, 78% (b) TPP, CCl4, reflux, 70%, (c) Na, ether, 08C–r.t., 70%, (d) NBS,
ethyl vinyl ether, DCM, 08C–r.t., (e) (n-Bu)3SnH, benzene, reflux, (f ) 60% AcOH,
reflux, 68% (for three steps), (g) PCC, DCM, 92%, (h) t-BuOK, CH3PPh3I, 08C–r.t.,
.
53%, (i) MOMCl, iPr2NEt, DCM, 08C–r.t., 81%, ( j) (i) BH3 THF, (ii) NaOH,
H2O2, 08C–r.t., 88%, (k) (i) IBX, DCM–DMSO, 08C–r.t., (ii) NaClO2, NaH2PO4,
DMSO, (iii) PTSA, MeOH 68% (for three steps).
Thus, chain elongation by one carbon atom was achieved via one-carbon
Wittig olefination reaction with methyltriphenylphosphonium iodide in the
presence of t-BuOK to afford the homologated compound 10 in 53% yield.
The secondary hydroxyl group in compound 10 was protected as its methoxy-
methyl (MOM) ether by treatment with MOMCl, in the presence of Hunig’s

Stereoselective Synthesis of Chiral Lactones
563
.
base in DCM. Hydroboration of 11 using a BH3 THF complex (1 M solution)
afforded a primary hydroxy compound 12 in 88% yield. The alcohol 12 was
subjected to oxidation using iodoxybenzoic (IBX) in DCM–dimethylsulphox-
ide (DMSO) to afford the aldehyde; without isolation it was further oxidized to
the corresponding acid using NaClO2 and NaH2PO4 in DMSO. Acid was also
used directly for the next step without further purification. Thus, acid on
treatment with para-toluenesulphonic acid (PTSA) in MeOH produced the
target molecule 2c by deprotection of MOM group followed by in situ cycliza-
tion. Its spectral data and specific rotation were in complete agreement with
those reported for the natural product.[5i]
In summary, we have described an elegant radical-based strategy for the
synthesis of disubstituted five-and six-membered lactones, (þ)-trans-cognac
lactone (1b) and (þ)-trans-aerangis lactone (2c), from the same key
intermediate.
EXPERIMENTAL
All solvents were distilled before use. Dry solvents were prepared according to
the standard procedures. All reactions were carried out under an N2 atmos-
phere and monitored by thin-layer chromatography (TLC) on silica gel
(60–120 mesh, Merck). NMR spectra were recorded on Bruker (300-MHz
1H; 75-MHz 13C) and Varian (200-MHz 1H; 50-MHz 13C) NMR spec-
trometers using CDCl3 as solvent. Electrospray ionisation technique (ESI)-
mass spectra were recorded with LC-MSD-Trap-SL (Agilent technologies).
IR spectra were recorded with Fourier transform infrared spectroscopic
technique (FTIR) (Thermo Nicolet Nexus 670 spectrometer). Optical
rotations were measured with Jasco DIP-370 polarimeter at 208C.
((2S,3S)-3-Pentyloxiran-2-yl) Methanol (4)
˚
DCM (25 ml) was added to a powdered and activated molecular sieve (4 A,
1.5 g), and the suspension was cooled to 2248C. D-(-)-DET (3.8 g,
18 mmol) and Ti(OiPr)4 (5.3 g, 18 mmol) were added subsequently while
stirring, and the resultant mixture was stirred for 30 min. To this stirred
solution, allyl alcohol 5 (12 g, 93 mmol) was added and stirred for 30 min
more. Then Cumene hydroperoxide (16.6 ml, 112 mmol) was added, and
the reaction mixture was stirred for 3 h. After monitoring by TLC, the
reaction mixture warmed to 08C and quenched with water and sat.
NaOH þ NaCl solution, stirred for 30 min, and filtered through celite, and
the filtrate was concentrated and eluted on a silica-gel column using
hexane–ethyl acetate (70:30) to afford epoxy alcohol 4 (10.5 g, 78%) as a
viscous liquid; IR (neat) ¼ 3428, 2960, 2852, 1040 cm21
;
1H NMR

564
G. Sabitha, M. Bhikshapathi, and J. S. Yadav
(CDCl3, 300 MHz) d ¼ 0.91 (t, J ¼ 6.7 Hz, 3H), 1.29–1.60 (m, 8H),
2.83–2.94 (m, 2H), 3.53–3.62 (m, 1H), 3.82–3.91 (m, 1H).
(2R,3S)-2-(Chloromethyl)-3-pentyloxirane (6)
Epoxy alcohol 4 (6 g, 41.6 mmol) was taken in a round-bottomed flask, and
TPP (16.8 g, 62 mmol), NaHCO3 (3.5 g, 41.6 mmol), and dry CCl4 (10 ml)
were added and refluxed for 4 h. After monitoring by TLC, the solvent
was removed under vacuum, and the crude compound was purified using
silica-gel column chromatography to afford the corresponding chloro
compound
6
(4.7 g, 70%); 1H NMR (CDCl3, 300 MHz) d ¼ 0.94
(t, J ¼ 6.7 Hz, 3H), 1.29–1.63 (m, 8H), 2.79–2.85 (dt, J ¼ 6.0, 2.2 Hz,
1H), 2.92–2.98 (dt, J ¼ 5.2, 2.2 Hz, 1H), 3.37–3.44 (dd, J ¼ 6.0, 5.2 Hz,
1H) 3.59–3.66 (dd, J ¼ 6.0, 5.2 Hz, 1H).
(3R)-1-Octen-3-ol (7)
Small shiny pieces of sodium metal (2.2 g, 96 mmol) were suspended in ether
(10 ml), and a solution of chloro compound 6 (3.9 g, 24 mmol) in ether
(10 ml) was added at 08C slowly dropwise. After stirring for 2–3 h, the
turbid liquid was decanted carefully into a conical flask (Na pieces should
not come), washed with water, and extracted into ether. The organic layer
was separated, dried over sodium sulphate, concentrated, and eluted on a
silica-gel column to afford the neat olefin 7 (2.2 g, 70%) as a colorless oil;
[a]2D5 24.5 (c ¼ 0.5, CHCl3); IR (neat) ¼ 3445, 2927, 2859, 1709 cm21; H
1
NMR (CDCl3, 300 MHz) d ¼ 0.90 (t, J ¼ 6.7 Hz, 3H), 1.24–1.54 (m, 8H),
4.01–4.12 (m, 1H), 5.04–5.09 (dt, J ¼ 9.8, 1.5 Hz, 1H), 5.16–5.21 (dt,
J ¼ 17.3, 1.5 Hz, 1H), 5.77–5.90 (m, 1H); EIMS: 127 (Mþ 2 1).
(4S,5R)-4-Methyl-5-pentyltetrahydro-2-furanol (3)
A mixture of enol 7 (2.2 g, 17.8 mmol) and N-bromosuccinimide (NBS)
(3.05 g, 17.8 mmol) in DCM was stirred at 08C, ethyl vinyl ether (2.8 ml,
35.6 mmol) was added dropwise, and the reaction mixture was stirred for
4 h at rt. After monitoring by TLC, the solvent was removed under vacuum
(the product formed was found to be unstable, so it was used directly). To
the bromo acetal, tri-n-butyl tin hydride (5 ml, 18.75 mmol) was added over
30 min followed by AIBN (catalytic), and the mixture was refluxed in dry
benzene for 2 h. Then the benzene was removed under vacuum. To the
residue, 60% acetic acid was added, and the reaction mixture was again
refluxed for 2–4 h. Then the mixture was quenched with sodium bicarbonate
solution and extracted into ethyl acetate, concentrated, and purified with

Stereoselective Synthesis of Chiral Lactones
565
silica-gel column chromatography to afford an inseparable anomeric mixture
of two lactols 3 (2 g, 68%); 1H NMR (CDCl3, 200 MHz) d ¼ 0.90
(t, J ¼ 6.7 Hz, 6H), 1.05 (dd, J ¼ 6.6, 6.4 Hz, 6H), 1.25–1.65 (m, 18H),
1.70–1.81 (m, 1H), 1.99–2.16 (m, 2H), 2.27–2.37 (m, 1H), 3.22 (br s, 1H),
3.39–3.47 (m, 2H), 3.62–3.69 (m, 1H), 5.37 (dd, J ¼ 2.2, 2.4 Hz, 1H), 5.45
(m, 1H).
(4S,5R)-4-Methyl-5-pentyltetrahydro-2-furanone
(Cognac Lactone) (1)
PCC (0.67 g, 3.48 mmol) and celite were added to the lactol 3 (0.5 g,
2.9 mmol) in DCM (1 ml) and stirred at rt for 2–3 h. The solvent was
removed under vacuum, and the mixture was directly eluted on silica gel to
afford the target molecule cognac lactone (1) (0.45 g, 91.8%);
[a]2D5 ¼ þ71.5 (c ¼ 0.5, CHCl3); IR (neat) ¼ 2932, 2865, 1777 cm21
;
1H NMR (CDCl3, 200 MHz) d ¼ 0.90 (t, J ¼ 6.7 Hz, 3H), 1.01
(d, J ¼ 5.9 Hz, 3H), 1.17–1.62 (m, 8H), 2.02–2.20 (m, 2H), 2.46–2.60
(m, 1H), 3.82–3.94 (m, 1H); 13C NMR (CDCl3, 75 MHz) d ¼ 177.68, 85.07,
36.00, 32.10, 30.96, 28.60, 24.21, 20.08,19.4, 14.12; EIMS: 169 (Mþ 2 1).
(4S,5R)-4-Methyl-1-decen-5-ol (10)
Wittig salt (1.4 g, 3.5 mmol) was taken in a round-bottomed flask, and THF
(2 ml) was added, followed by t-BuOK (0.325 g, 3 mmol), at 08C. After
20 min, a solution of compound 3 (0.2 g, 1.2 mmol) in THF (1 ml) was
added. The mixture was stirred at rt for 2 h, and after monitoring by TLC,
was quenched with water and extracted into ethyl acetate. The organic layer
was dried over sodium sulphate and concentrated to get the crude
compound, which was eluted on a silica-gel column to afford the compound
10 (0.1 g, 52.6%); IR (neat) ¼ 3444, 2956, 2925, 2856, 1611 cm21
;
1H NMR (CDCl3, 200 MHz) d ¼ 0.86–0.97 (m, 6H), 1.19–1.67 (m, 10H),
1.84–2.04 (m, 1H), 2.12–2.36 (m, 1H), 3.34–3.49 (m, 1H), 4.95–5.11
(m, 2H), 5.66–5.91 (m, 1H); EIMS: 169 (Mþ 2 1).
(4S,5R)-5-(Methoxymethoxy)-4-methyl-1-decan (11)
To a solution of compound 10 (0.5 g, 2.94 mmol) in DCM (2 ml), Hunig’s
base (2.0 ml, 11.7 mmol) was added at 08C and stirred for 10 min. Then
MOMCl (0.35 ml, 4.4 mmol) was added and stirred at rt. After 1 h by
checking the TLC, the reaction mixture was quenched with sodium bicarbo-
nate and taken up into ethyl acetate. The organic layer was dried over
sodium sulphate, and the solvent was removed under vacuum. Residue was
subjected to a silica-gel column to get the corresponding MOM ether 11

566
G. Sabitha, M. Bhikshapathi, and J. S. Yadav
(0.5 g, 80.6%); IR (neat) ¼ 3074, 2929, 1640, 1041 cm21; 1H NMR (CDCl3,
200 MHz) d ¼ 0.86–0.97 (m, 6H), 1.22–1.45 (m, 8H), 1.71–1.91 (m, 2H),
2.10–2.28 (m, 1H), 3.35 (s, 3H), 3.36–3.39 (m, 1H), 4.59 (s, 2H), 4.94–
5.03 (m, 2H), 5.66–5.81 (m, 1H); LCMS: 215 (Mþ þ 1).
(4S,5R)-5-(Methoxymethoxy)-4-methyl-1-decan-1-ol (12)
To the borane tetrahydrofuran complex (1 M, 1.8 ml, 1.8 mmol) in THF
(1 ml), the MOM ether 11 (0.4 g, 1.8 mmol) in THF (1 ml) was slowly
added at 08C dropwise. The reaction mixture was left overnight, basified
with sodium hydroxide (3 N, 1 ml), and oxidized with 30% hydrogen
peroxide (1 ml) in cold conditions. The mixture was taken up into ethyl
acetate; the organic layer was dried over sodium sulphate and concentrated
under vacuum. The residue was eluted on a silica-gel column to afford the
hydroxy compound 12 (0.38 g, 88%); [a]2D5 ¼ þ11.8 (c ¼ 0.5, CHCl3); IR
1
(neat) ¼ 3427, 2931, 1041 cm21; H NMR (CDCl3, 200 MHz) d ¼ 0.86–
0.97 (m, 6H), 1.10–1.17 (m, 13H), 3.36–3.40 (m, 4H), 3.60 (t, J ¼ 6.0 Hz,
2H), 4.59 (s, 2H); EIMS: 231 (Mþ 2 1).
(5S,6R)-5-Methyl-6-pentyltetahydro-2H-2-pyranone
(Aerangis Lactone) (2)
The alcohol 12 (0.3 g, 1.28 mmol) in DCM (1 ml), DMSO (0.3 ml), and IBX
(0.45 g, 1.54 mmol) were added at 08C and stirred for 3 h. The reaction
mixture was quenched with sodium bicarbonate and taken up into DCM,
and the solvent was removed under the vacuum. The resultant crude
compound was further oxidized as its corresponding acid by treating with
.
NaClO2 (0.11 g, 1.28 mmol) and NaH2PO4 2H2O (0.2 g, 1.28 mmol) in
DMSO (1 ml). The acid, upon treatment with PTSA in methanol, resulted in
deprotection of MOM group and in situ cyclization, resulting in the target
molecule aerangis lactone 2 (0.16 g, 68%); [a]2D5 ¼ þ41.05 (c ¼ 0.5,
CHCl3); IR (neat) ¼ 2922, 2857, 1737, 1080 cm21
;
1H NMR (CDCl3,
200 MHz) d ¼ 0.86–0.97 (m, 6H), 1.19–1.47 (m, 8H), 1.55–1.76 (m, 2H),
1.92–2.07 (m, 1H), 2.23–2.34 (m, 2H), 4.04–4.14 (m, 1H); 13C NMR
(CDCl3, 75 MHz) d ¼ 171.92, 76.21, 34.50, 32.63, 31.04, 29.98, 28.76,
26.04, 20.01, 18.63, 14.02; LCMS: 182 (Mþ 2 2).
ACKNOWLEDGMENT
M. B. thanks the Council for Scientific and Industrial Research (CSIR), New
Delhi, for the award of a fellowship.

Stereoselective Synthesis of Chiral Lactones
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