Synthesis of racemic and each enantiomer of 3-methylnonacosanol, a new plant growth regulator from Lowsonia inermis

Article abstract of DOI:10.1016/S0957-4166(99)00150-0

An efficient synthesis of the title compound I in racemic and enantiomeric forms has been developed starting from 10-undecenoic acid. For the enantiomeric synthesis, a lipase catalyzed acylation strategy was employed to prepare the required methyl branched chiron which was subsequently derivatized to give the enantiomers of I. The plant growth regulatory (PGR) assay of I, carried out with hypocotyl cuttings of French beans (Phaseolus vulgaris L.) in Steinberg's nutrient medium revealed appreciable activity at a concentration of 2.5 ppm. The PGR activities of the compound both in racemic and enantiomeric forms were better than 1- triacontanol, the enantiomer, (S)-I being the best test candidate.

Full text of DOI:10.1016/S0957-4166(99)00150-0

Pergamon
Tetrahedron: Asymmetry 10 (1999) 1571–1577
Synthesis of racemic and each enantiomer of
3-methylnonacosanol, a new plant growth regulator from
Lowsonia inermis
Bheemashankar A. Kulkarni, Sivaraman Sankaranarayanan, Ayalur S. Subbaraman and
Subrata Chattopadhyay ∗
Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai 400 09\85, India
Received 26 March 1999; accepted 20 April 1999
Abstract
An efficient synthesis of the title compound I in racemic and enantiomeric forms has been developed starting
from 10-undecenoic acid. For the enantiomeric synthesis, a lipase catalyzed acylation strategy was employed to
prepare the required methyl branched chiron which was subsequently derivatized to give the enantiomers of I. The
plant growth regulatory (PGR) assay of I, carried out with hypocotyl cuttings of French beans (Phaseolus vulgaris
L.) in Steinberg’s nutrient medium revealed appreciable activity at a concentration of 2.5 ppm. The PGR activities
of the compound both in racemic and enantiomeric forms were better than 1-triacontanol, the enantiomer, (S)-I
being the best test candidate. © 1999 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction
The growing human demand for more food products caused by the population explosion and limited
availability of cultivable land has provided an arduous task of increasing agricultural productivity. In this
context, the importance of plant growth regulators (PGRs) has been well-recognized.1 Consequently, a
number of compounds such as 1-triacontanol2 have acquired prominence owing to their growth regulating
activities on different plants of agricultural and pharmacological interest. Earlier from our laboratory,
some of the olefinic analogues of 1-triacontanol were found3 to possess better plant growth regulating
activity compared to 1-triacontanol. In this context, 3-methylnonacosanol I, a branched analogue of 1-
triacontanol, seems an interesting candidate for PGR activity. Compound I has been isolated4 from the
Indian medicinal shrub Lowsonia inermis Linn., which is traditionally used for the treatment of jaundice,
enlargement of the liver and spleen, leprosy and other skin diseases.5 Thus, in addition to PGR assay,
∗
Corresponding author. E-mail: schatt@apsara.barc.ernet.in
0957-4166/99/$ - see front matter © 1999 Published by Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00150-0

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compound I warrants evaluation of other medicinal properties. For this, we have developed the first
synthesis of compound I in enantiomeric and racemic forms and used them for their PGR activity vis-
à-vis the stereochemistry. In this paper, the synthesis and the preliminary bioassay results are described.
Details of the latter will be discussed elsewhere.
2. Results and discussion
2.1. Synthesis of (+)-I
The known alcohol 16 was brominated with Ph3P/Br2/Py to give compound 2. Stearic acid was con-
verted to the bromide 37 via decarboxylative bromination by refluxing with HgO in the presence of Br2.
The Grignard reagent prepared from the bromide 2 was then coupled with 3 in the presence of Li2CuCl4
to afford the C28-alkene 4. For its conversion to the required ketone 5, an oxymercuration strategy was
envisaged. However, due to its poor solubility in the reaction medium, Hg2+-catalyzed hydration8 led
to poor conversion. Hence, the alkene 4 was subjected to a Wacker oxidation9 (PdCl2/CuCl2/DMF) to
give the ketone 5. Its Wittig–Horner reaction with triethyl phosphonoacetate led to the conjugated ester
6 which on catalytic hydrogenation over 10% Pd–C in ethanol furnished the ester 7. Its LAH reduction
provided the required alcohol (ꢀ)-I (Scheme 1) in good yield.
Scheme 1. (i) Ph3P/Br2/py/CH2Cl2; (ii) Mg/THF/Li2CuCl4/CH3(CH2)16Br (3); (iii) O2/PdCl2/CuCl2/aqueous DMF/70°C; (iv)
NaH/(EtO)2P(O)CH2CO2Et; (v) H2/10% Pd–C/EtOH; (vi) LAH/ether; (viii) lit.10; (viii) NaH/BnBr/Bu4NI/∆; (ix) MeOH/PTS;
(x) PCC/CH2Cl2; (xi) Dimsyl ion/DMSO/CH3(CH2)16PPh3Br (12)/THF
2.2. Enantiomeric synthesis of I
The target compound belong to the class of molecules which owe their chirality to the presence of
methyl branching. Realizing the importance of this structural feature in several bioactive compounds, we
have earlier designed10 a new methyl branched chiron viz. 12-tetrahydropyranyloxy-3-methyldodecanol
8 and used it for the synthesis of several insect pheromones of agricultural importance. Both the
enantiomers of compound 8 were synthesized in good enantiomeric purities via a chemoenzymatic
strategy starting from the alcohol 1. The chiron also seemed tailor-made for the present synthesis which

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1573
was executed as follows. For the synthesis, (S)-8 was benzylated11 with BnBr using NaH as the base in
the presence of Bu4NI to give (S)-9. Its depyranylation afforded the C13-hydroxy-derivative (S)-10 which
was oxidized with pyridinium chlorochromate (PCC) to give 11. Preparation of the target compound
from the above chiron required addition of a suitable C17-moiety to it. This was accomplished via a
Wittig olefination. Thus, the required C17-phosphonium salt 12 was prepared from the bromide 3. Its
reaction with (S)-11 in the presence of dimsyl anion as the base gave the C30-compound (S)-13. Its
catalytic hydrogenation over 10% Pd–C in EtOH led to simultaneous debenzylation providing (S)-I in
good yield.
The (R)-enantiomer of the target compound I was also synthesized using (R)-8 as the starting chiron.
The spectral data of all the synthetic samples were commensurate with those reported.4 For assaying
the enantiomeric purities of (S)-8 and its (R)-antipode, the individual compounds were oxidized with
pyridinium dichromate (PDC) in DMF and depyranylated to the corresponding acids. These were then
converted to the diastereomeric amides with (R)-phenylethylamine analyzed by capillary GLC under
the conditions specified in the experimental section. The ees of (S)- and (R)-8 were found to be 96 and
97.7%, respectively. Based on these result, the ees of the synthetic (S)-I and its enantiomer should be at
least 96%.
3. Experimental
All the boiling points are uncorrected. The IR spectra were scanned with a Perkin–Elmer spectropho-
1
tometer model 837. The H NMR spectra were recorded in CDCl3 with a Bruker AC-200 (200 MHz)
instrument. The optical rotations were measured with a Jasco DIP 360 polarimeter. The GLC analyses
were carried out with a Shimadzu GC-7A chromatograph fitted with a stainless steel column and flame
ionization detector using 3% OV-17 (2 m×0.5 mm) column and a N2 flow rate of 40 ml/min. Anhydrous
reactions were carried out under Ar using freshly dried solvents. The organic extracts were dried over
anhydrous Na2SO4.
3.1. 1-Bromoundecene 2
To a cooled (0°C) and stirred solution of PPh3 (29.63 g, 11.29 mmol) in CH2Cl2 (50 ml) was added
Br2 (18.72 ml, 5.5 M in CCl4, 10.35 mmol). After stirring for 0.5 h, the alcohol 1 (16.0 g, 9.41 mmol)
and pyridine (8.4 ml, 10.35 mmol) in CH2Cl2 (20 ml) were added to the mixture which was gradually
brought to room temperature and stirred for another 2 h. The mixture was concentrated in vacuo, the
residue dissolved in hexane and passed through a pad (2 in.) of silica gel. The eluent was concentrated in
vacuo to give the bromide 2. Yield: 17.3 g (79%); IR: 3100, 1650, 1010, 910 cm−1; 1H NMR: δ 1.3 (br.
s, 14H), 1.9–2.1 (m, 2H), 3.6 (t, J=6.5 Hz, 2H), 4.8–6.2 (m, 3H).
3.2. 1-Bromoheptadecane 3
To a refluxing mixture of stearic acid (36.0 g, 0.127 mol) and HgO (20.86 g, 0.096 mol) in CCl4 (150
ml) was dropwise added Br2 (20.27 g, 0.127 mol) so as to maintain a gentle reflux of the mixture. After
the addition was over, the mixture was further refluxed for 1 h, brought to room temperature, filtered and
the filtrate concentrated in vacuo. The residue was taken in hexane and washed with water and brine,
dried and concentrated. The residue was purified by column chromatography (silica gel, hexane) to give
the compound 3. Yield: 32.0 g (79%), bp: 140–143°C/1 mm; IR: 2980, 2860 cm−1; 1H NMR: δ 0.9 (dist.

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t, 3H), 1.25 (br. s, 30H), 3.4 (t, J=7.5 Hz, 2H). Anal. calcd for C17H35Br: C, 63.93; H, 11.05; Br, 25.02;
found: C, 64.21; H, 10.88; Br, 25.27.
3.3. 1-Octacosene 4
A Grignard reagent was prepared from 2 (8.0 g, 0.034 mol) and Mg (0.91 g, 0.038 mol) in THF (30
ml). To a stirred and cooled (−5°C) solution of the above reagent was added Li2CuCl4 (2.0 ml, 0.1 M
solution in THF) followed by 3 (9.57 g, 0.034 mol) in THF (40 ml). Stirring was continued for 3 h at
the same temperature and at room temperature overnight. The mixture was poured into ice cooled water,
the organic layer separated and the aqueous layer extracted with ether. The combined organic extract was
washed with water, brine and dried. Removal of solvent followed by column chromatography (silica gel,
1
hexane) of the residue yielded pure 4. Yield: 7.87 g (67%); IR: 3090, 1660, 1020, 990, 910 cm−1; H
NMR: δ 0.9 (dist. t, 3H), 1.25 (br. s, 48H), 1.9–2.2 (m, 2H), 4.8–6.2 (m, 3H). Anal. calcd for C28H56: C,
85.63; H, 14.37; found: C, 85.72; H, 14.49.
3.4. 2-Oxooctacosane 5
To a stirred mixture of PdCl2 (0.289 g, 1.63 mmol) and CuCl2 (0.28 g, 2.08 mmol) in aqueous DMF
(1:10, 10 ml) was gradually added the alkenol 4 (4.0 g, 1.02 mmol), while bubbling oxygen continuously.
After 1 h, the temperature was raised to 60–70°C and maintained for 1 h. It was poured into dilute HCl
(2 N) and extracted with ether. The extract was washed with water, brine, dried and concentrated. The
product was chromatographed (silica gel, 0–5% EtOAc–hexane) to furnish pure 5. Yield: 2.1 g (50%);
IR: 1720, 740 cm−1; 1H NMR: δ 0.9 (dist. t, 3H), 1.2–1.65 (m containing a br. s, 48H), 2.1 (s, 3H), 2.35
(t, J=6 Hz, 2H). Anal. calcd for C28H56O: C, 82.28; H, 13.81; found: C, 82.47; H, 14.01.
3.5. Ethyl 3-methyl-2-nonacosenoate 6
To a cooled (0°C) and stirred suspension of hexane-washed NaH (0.282 g, 50% dispersion in oil, 5.88
mmol) in THF (20 ml) was added triethyl phosphonoacetate (1.32 g, 5.89 mmol). After stirring for 1 h,
compound 5 (2.0 g, 4.90 mmol) in THF (20 ml) was added to it at 0°C. Stirring was continued for 3 h
at the same temperature and at room temperature overnight. The mixture was poured into ice cold water,
the organic layer separated and the aqueous layer extracted with ether. The combined organic extract was
washed with water, brine and dried. Removal of solvent followed by column chromatography (silica gel,
0–10% EtOAc–hexane) of the residue gave 6. Yield: 1.69 g (72%); IR: 3005, 1720, 1650 cm−1; 1H NMR:
δ 0.9 (dist. t, 3H), 1.18 (t, J=6 Hz, 3H), 1.3–1.6 (br. s, 48H), 2.1–2.3 (m, overlapped with a s at δ 2.12,
5H), 4.1 (q, J=5.5 Hz, 2H), 5.4 (s, 1H). Anal. calcd for C32H62O2: C, 80.27; H, 13.05; Found: C, 80.12;
H, 13.34.
3.6. Ethyl 3-methylnonacosanoate 7
A mixture of 6 (1.6 g, 3.35 mmol) and 10% Pd–C (0.05 g) in EtOH (20 ml) was shaken under a positive
pressure of hydrogen till the required uptake of the gas. The mixture was passed through a pad (200) of
silica gel which was thoroughly eluted with ether. The organic extract was concentrated in vacuo to give
the ester 7. Yield: 1.5 g (95%); IR: 1740, 730 cm−1; 1H NMR: δ 0.88–0.95 (merged d and t, 6H), 1.16
(t, J=6 Hz, 3H), 1.3 (br. s, 50H), 1.5–1.64 (m, 1H), 2.21 (d, J=7 Hz, 2H), 4.15 (q, J=5.4 Hz, 2H). Anal.
calcd for C32H64O2: C, 79.93; H, 13.42; found: C, 80.08; H, 13.62.

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3.7. (ꢀ)-3-Methylnonacosanol I
To a stirred solution of LiAlH4 (0.12 g, 3.16 mmol) in ether (50 ml) was added the ester 7 (1.0 g, 2.08
mmol) in ether (50 ml). After stirring for 3 h, the excess hydride was decomposed with aqueous saturated
Na2SO4, the supernatant organic layer decanted and the solid washed with ether. Concentration of the
1
organic extract furnished pure (ꢀ)-I. Yield: 0.76 g (83%); IR: 3400, 730 cm−1; H NMR: δ 0.85–0.95
(m, 6H), 1.25 (br. s, 52H), 1.5–1.8 (m, 1H), 2.1 (s, D2O exchangeable, 1H), 3.7 (t, J=6 Hz, 2H); MS: m/z
438 (M+). Anal. calcd for C30H62O: C, 82.11; H, 14.24; found: C, 82.24; H, 14.11.
3.8. (3S)-12-Tetrahydropyranyloxy-1-benzyloxy-3-methyldodecane 9
To a stirred suspension of NaH (0.25 g, 5.2 mmol, 50% suspension in oil) in THF (25 ml) was added
the alcohol (S)-8 (1.2 g, 4 mmol) and the mixture gently refluxed for 1 h. After the evolution of H2
ceased, Bu4NI (96 mg, 0.26 mmol) was added followed by benzyl bromide (0.89 g, 5.2 mmol) and the
reaction mixture refluxed for 2 h. It was cooled to room temperature, poured into ice-water and extracted
with diethyl ether. The ether layer was washed with water and brine and finally dried. Removal of solvent
followed by column chromatography of the residue (neutral alumina, grade II, 0–10% EtOAc–hexane)
afforded 9. Yield: 1.26 g (81%); [α]D25 +1.22 (c 1.18, CHCl3); IR: 3060, 3030, 1620, 870, 820 cm−1; 1H
NMR: δ 0.84 (d, J=6 Hz, 3H), 1.28 (br. s, 18H), 1.4–1.6 (m, 7H), 3.2–3.7 (m, 4H), 3.81 (t, J=6 Hz, 2H),
4.4 (s, 2H), 4.51 (br. s, 1H), 7.1–7.3 (m, 5H). Anal. calcd for C25H42O3: C, 76.87; H, 10.84; found: C,
77.02; H, 10.72.
3.9. (3S)-1-Benzyloxy-3-methyldodecan-12-ol 10
A solution of 9 (1.2 g, 3.1 mmol) in MeOH (30 ml) containing a catalytic amount of PTS was stirred
for 6 h at room temperature. Most of the solvent was removed in vacuo, the residue taken in ether
and the ether layer washed with aqueous 10% NaHCO3, water and brine. After drying, the extract was
concentrated in vacuo and the product obtained was chromatographed (silica gel, 0–15% ether–hexane)
to give 10. Yield: 0.85 g (90%); [α]D +3.44 (c 0.8, CHCl3); IR: 3400, 3060, 3030, 1620 cm−1; H
NMR: δ 0.88 (d, J=6 Hz, 3H), 1.31 (br. s, partially D2O exchangeable, 20H), 3.68 (t, J=6 Hz, 2H), 3.78
(t, J=6 Hz, 2H), 4.5 (s, 2H), 7.1–7.3 (m, 5H). Anal. calcd for C20H34O2: C, 78.38; H, 11.18; found: C,
78.52; H, 11.02.
25
1
3.10. (10S)-12-Benzyloxy-10-methyldodecanal 11
To a stirred suspension of PCC (0.84 g, 3.89 mmol) in CH2Cl2 (20 ml) was added 10 (0.8 g, 2.61 mmol)
in one lot. After stirring for 3 h, when the reaction was complete (cf. TLC), the mixture was diluted with
ether (30 ml) and the supernatant passed through a pad of silica gel. The eluent was concentrated to give
25
11 which was sufficiently pure (cf. TLC) and used as such for the next step. Yield: 0.56 g (70%); [α]D
−1.18 (c 0.78, CHCl3); IR: 3030, 2725, 1730, 700 cm−1; 1H NMR: δ 0.90 (d, J=6 Hz, 3H), 1.2–1.6 (m,
17H), 2.3–2.5 (m, 2H), 3.78 (t, J=6 Hz, 2H), 4.5 (s, 2H), 7.2–7.4 (m, 5H), 9.7 (t, J=1.5 Hz, 1H).
3.11. Heptadecyltriphenylphosphonium bromide 12
A mixture of 3 (6.38 g, 0.02 mol) and triphenyl phosphine (5.25 g, 0.02 mol) in CH3CN (50 ml)
was refluxed for 48 h. Most of the solvent was removed in vacuo, the residue thoroughly washed with

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anhydrous ether and dried in vacuum over P2O5 to get 12 as a pale brown gummy mass which was used
in the next step by making a 0.1 M solution in THF.
3.12. (12Z,3S)-1-Benzyloxy-3-methylnonacos-12-ene 13
To a stirred solution of dimsyl anion (2.13 mmol) in DMSO [prepared from NaH (0.102 g, 2.13 mmol,
50% suspension in oil) and DMSO (20 ml)] was added 12 (21.3 ml, 0.1 M in THF, 2.13 mmol) in
THF. After stirring for 1 h, compound 11 (0.5 g, 1.64 mmol) in THF (30 ml) was added to it and the
mixture stirred at room temperature for 48 h. It was poured into ice-water, the organic layer separated, the
aqueous portion extracted with ether, the combined organic extract washed with water and concentrated
after drying. The residue obtained was dissolved in a small amount of hexane, cooled to 0°C and filtered.
The solid residue was washed with cold hexane and the combined filtrate concentrated in vacuo. After
repeating the hexane dissolution process three times, the residue was chromatographed (silica gel, 0–5%
25
EtOAc–hexane) to furnish pure 13. Yield: 0.46 g (53%); [α]D +2.48 (c 1.2, CHCl3); IR: 3030, 1660,
720 cm−1; 1H NMR: δ 0.88–0.92 (m, 6H), 1.36 (br. s, 44H), 1.5–1.7 (m, 1H), 1.9–2.1 (m, 4H), 3.81 (t,
J=6 Hz, 2H), 4.5 (s, 2H), 5.3–5.4 (m, 2H), 7.2–7.4 (m, 5H).
3.13. (3S)-3-Methylnonacosanol I
Compound 13 (0.4 g, 0.8 mmol) was hydrogenated over 10% Pd–C (0.02 g) in EtOH (10 ml) and the
25
product chromatographed (silica gel, 0–20% EtOAc–hexane) to give (S)-I. Yield: 0.28 g (84%); [α]D
+1.61 (c 0.95, CHCl3).
3.14. (3R)-12-Tetrahydropyranyloxy-1-benzyloxy-3-methyldodecanol 9
[α]D25 −1.07 (c 2.35, CHCl3).
3.15. (3R)-1-Benzyloxy-3-methyldodecan-12-ol 10
[α]D25 −3.59 (c 1.12, CHCl3).
3.16. (10R)-12-Benzyloxy-10-methyldodecanal 11
[α]D25 +1.25 (c 1.6, CHCl3).
3.17. (12Z,3R)-1-Benzyloxy-3-methylnonacos-12-ene 13
[α]D25 −2.15 (c 2.6, CHCl3).
3.18. (3R)-3-Methylnonacosanol I
[α]D25 −1.43 (c 1.15, CHCl3).

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1577
4. PGR bioassay
Following the known procedure,12 the PGR assay of (ꢀ)-, (S)- and (R)-I were carried out with
hypocotyl cuttings of French beans, Phaseolus vulgaris L. in Steinberg’s nutrient medium.13 The
experiments were carried out with 8-day-old young, healthy and similar plants using nutrient medium
of 1/4th strength. The full strength medium was not suitable to realize any PGR effect of the samples and
standard. The assays were carried out using the medium as the control and 1-triacontanol as the standard.
The samples were applied in acetone solution and tested at four different concentrations viz. 1, 2.5, 5 and
7.5 ppm. Required amount of acetone was also added to the control. The results were compared with those
obtained from 1-triacontanol at the same concentration levels. The experiments were carried out with
three replicates and the mean differences in shoot and root weights were used for estimating the activity.
At a minimum concentration of 2.5 ppm, the samples and standard gave appreciable PGR activity. The
(S)-isomer of I showed better activity compared to that of its enantiomer which was slightly more active
than (ꢀ)-I. Most interestingly, all the test samples of I were more active than 1-triacontanol. Under the
optimized conditions, application of (S)-I led to increase in yield of both the shoots (24.1ꢀ2.3%) and the
roots (33.7ꢀ1.6%) as compared to those with the standard.
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