Total synthesis of calonyctin A2, a macrolidic glycolipid with plant growth-promoting activity

Article abstract of DOI:10.1016/S0040-4039(00)00427-5

Calonyctin A2, a tetrasaccharidic glycolipid having a 22-membered macrolidic structure, has been synthesized by the assembly of three 6- deoxygenated thioglycoside intermediates. The short-step synthesis was achieved by preparation of the most complicated

Full text of DOI:10.1016/S0040-4039(00)00427-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3453–3457
Total synthesis of calonyctin A2, a macrolidic glycolipid with
plant growth-promoting activity
Jun-ichi Furukawa, Shigeru Kobayashi, Motoyoshi Nomizu, Norio Nishi and Nobuo Sakairi ∗
Division of Bio-Science, Graduate School of Environmental Earth Science, Hokkaido University, Kita-ku, Sapporo 060-0810,
Japan
Received 31 January 2000; accepted 10 March 2000
Abstract
Calonyctin A2, a tetrasaccharidic glycolipid having a 22-membered macrolidic structure, has been synthesized
by the assembly of three 6-deoxygenated thioglycoside intermediates. The short-step synthesis was achieved by
preparation of the most complicated b–c disaccharide unit from phenyl 2,20:4,6:40,60-tri-O-benzylidene-1-thio-β-
D-laminaribioside without any glycosidation reaction and by regioselective macrolactonization. © 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: carbohydrates; glycolipids; calonyctin A; glycosides; macrolides; thioglycosides.
Calonyctin extracted from the leaves of Yue-Guang-Hua (Calonyction aculeatum L. House) is a plant
growth regulator, which promotes the tuber production of sweet potato and increases the crop yields
of beans and wheat.1 It is a mixture of homologous glycolipids consisting of a common deoxygenated
tetrasaccharide residue and 11-hydroxy fatty acids,2 which are named calonyctin A1 (1) and A2 (2),
respectively (Fig. 1).3 Furthermore, one of the sugar hydroxyl groups is acylated with (2R,3R)-3-hydroxy-
2-methylbutyric acid. The most remarkable feature of their structure is that they have a 22-membered
macloride ring. The absolute configuration of the aglycon moiety was determined as S by Schmidt’s
synthesis of 1 from a racemic fatty acid.4 In this communication, we describe an expeditious synthesis of
calonyctin A2 (2) using laminaribiose as the starting material. On the basis of our studies on the chemical
modification of laminaribiose,5 we envisioned that the readily accessible tri-O-benzylidene derivative 3
could be used as a synthon for the most complicated disaccharide unit (Qui-b and Qui-c) of 2. Compound
3 was converted into a crucial disaccharide donor, phenyl 30-O-allyl-2,20-di-O-benzoyl-4,40-di-O-benzyl-
6,60-dideoxy-1-thio-β-D-laminaribioside 8, which had to be coupled with a monosaccharide intermediate
17 and a L-rhamnosyl donor 21. Furthermore, an optically active 11(S)-hydroxymyristic acid derivative
14 could be prepared from the known (S)-γ-tosyloxymethyl-γ-butyrolactone in an enantioselective
manner.
∗
Corresponding author. Tel/fax: +81 11 706 2257; e-mail: nsaka@ees.hokudai.ac.jp (N. Sakairi)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00427-5
tetl 16707

3454
Fig. 1.
The synthetic route of the disaccharide donor 8 is illustrated in Scheme 1. Thus, 3 was allylated
at the unprotected hydroxyl group to give the fully protected disaccharide6 4. Upon treatment of 4
with PPTS in CHCl3–MeOH at room temperature, the most labile O-benzylidene group with an eight-
membered ring underwent selective cleavage, to give the 2,20-diol6 5 in 79% yield based on consumed
4. The following reductive cleavage of the benzylidene group in 5 was one of the most difficult steps
in the present synthesis, and this was overcome by applying Kusumoto’s reagent system.7 Treatment
of 5 with BH3·Me2NH–BF3·OEt2 in CH2Cl2 gave the desired 2,6,20,60-tetraol 6 in 77% yield without
any affection on the other protecting groups.8 The conventional reduction of 5, or its 2,20-di-O-acetyl
derivative with BH3·Me3N–AlCl39 or DAIBAL-H,10 Super-Hydride®, failed due to poor regioselectivity
and undesirable cleavage of the protecting group. Subsequently, 6 underwent selective esterification in
a one-pot manner with TsCl and then with excess BzCl, giving the labile 2,20-di-O-benzoyl-6,60-di-O-
tosyl derivative 7. Without purification, 7 was reduced with NaBH4 in DMF at 70°C into the dideoxy
derivative6 8 in an overall yield of 47% from 6.
Scheme 1. Reagents and conditions: (a) AllylBr, NaH, DMF; (b) PPTS, MeOH/CHCl3; (c) BF3·OEt2, Me2NH·BH3, CH2Cl2,
under N2; (d) TsCl, pyridine, then BzCl; (e) NaBH4, DMF, 70°C
For the synthesis of the aglycon moiety of 2, the optically active pentanetriol11 9 derived from L-
glutamic acid was subjected to selective silylation and subsequent basic treatment with tert-BuOK in
THF to give the epoxide 10 (Scheme 2). Treatment of 10 in THF with 9-(benzyloxy)nonyl magnesium
bromide in the presence of CuI and subsequent benzoylation gave 116 in an overall yield of 72%. A
three-step reaction involving de-O-silylation with Bu4NF, bromination with CBr4–PPh3,12 and reduction
with Bu3SnH–AIBN was carried out to give 12.6 Treatment of 12 with methanolic sodium methoxide
followed by Birch reduction gave the tetradecan-1,11-diol6 13. Finally, a primary hydroxyl group of
13 was subjected to two-step oxidation13 with TEMPO and subsequently with sodium chlorite to give
11(S)-hydroxymyristic acid, which was isolated and characterized as the methyl ester 14.6 In order

3455
to construct the target compound 2, two other intermediates, phenyl 2-O-acetyl-3,4-di-O-benzyl-6-O-
mesyl-1-thio-β-D-glucopyranoside14 15 and phenyl 2,3,4-tri-O-benzyl-1-thio-α-L-rhamnopyranoside15
21, were prepared.
Scheme 2. Reagents and conditions: (a) TBDPSCl, pyridine/CH2Cl2, then tert-BuOK, THF; (b) 9-(benzyloxy)nonyl magnesium
bromide, CuI, THF, 0°C: BzCl, pyridine; (c) n-Bu4NF, THF: CBr4, Ph3P, DMF, then Bu3SnH, AIBN, toluene, 80°C; (d) NaOMe,
MeOH, 70°C, then liq. NH3, Na, −80°C; (e) TEMPO, TBACl, NCS, CH2Cl2/H2O, pH 8.6: NaClO2, NaH2PO4, tert-BuOH,
2-methyl-2-butene/H2O, then CH2N2, Et2O; (f) thioglycoside14 15, NIS–TfOH, CH2Cl2, MS 4 Å, −20°C, then NaOMe, MeOH;
(g) NaBH4, DMF, 70°C; (h) 8, NIS–TfOH, CH2Cl2, MS 4 Å, −20°C; (i) 2 M KOH, MeOH, 40°C; (j) Cl3C6H2COCl, Et3N,
DMAP, toluene; (k) thioglycoside15 21, NIS–TfOH, CH2Cl2, MS 4 Å, −20°C; (l) RhCl(PPh)3, diazabicyclo[2,2,2]octane,
EtOH, reflux: 2 M HCl, 45°C, then (2R,3R)-3-benzyloxy-2-methylbutyric acid, DMAP, WSC, CH2Cl2; (m) Pd/C, H2, MeOH
Assembly of the four intermediates (8, 14, 15 and 21) were performed by iodonium activation of
the thioglycosides. At first, 14 and the glucosyl donor 15 were treated with NIS–TfOH16 in CH2Cl2 at
−20°C, and the β-glucoside 16 was isolated after de-O-acetylation. Reduction of the mesyloxy group
in 16 with NaBH4 in DMF gave the glycosyl acceptor6 17, which was coupled with 8 in a similar way
to afford the trisaccharide6 18 in 71% yield. After saponification with aqueous NaOH, the resulting
carboxylic acid 19 was subjected to macrolactonization by a mixed anhydride procedure17 under high-
dilution conditions in toluene (1.71×10−4 M). Although two possible hydroxyl groups are present at the
2b- and 2c-positions, the desired 22-membered macrolide6 20 was obtained as a single product in 60%
yield together with recovered starting material 19. The structure of 20 was unambiguously corroborated
by two-dimensional COSY and NOESY NMR spectroscopy. Similar glycosylation of 20 with the L-
rhamnosyl donor 21 gave the tetrasaccharide6 22 in 15% yield, which was not improved by use of
another thiophilic reagent, MeOTf. The low yield was probably due to steric hindrance of the hydroxyl

3456
group at the 2a-position. To synthesize the target compound, the allyl group in 22 was isomerized with
Wilkinson’s catalyst and the propenyl group was removed by acid hydrolysis. The resulting alcohol was
esterified with (2R,3R)-3-benzyloxy-2-methylbutyric acid4 in the presence of water-soluble carbodiimide
in CH2Cl2, giving the fully protected derivative 23 in 62% yield. Finally, removal of all benzyl groups
in 23 by catalytic hydrogenolysis gave calonyctin A2 (2), of which the NMR spectra in pyridine-d5 were
consistent with those already reported.2
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry
of Education, Science, Sports and Culture of Japan (No. 0924010).
References
1. Ding, J.-L. Acta Nong Ye 1952, 3, 17–24; Gou, Q.; Wang, Z.; Fang, Y.; Bian, Z. J. Xiamen Univ. 1980, 83–91.
2. Fang, Y.-W.; Chai, W.-A.; Chen, S.-M.; He, Y.-Z.; Zhao, L.; Peng, J.-H.; Huang, H.-W.; Xin, B. Carbohydr. Res. 1993, 245,
259–270.
3. Hu, Y.; Guo, O.; Pastor, R.; Serratrice, G.; Cambon, A.; Bosso, C. Youji Huaxue 1989, 9, 146–150.
4. Jiang, Z.-H.; Geyer, A.; Schmidt, R. R.; Angew. Chem., Int. Ed. Engl. 1995, 34, 2520–2524.
5. Sakairi, N.; Okazaki, Y.; Furukawa, J.; Kuzuhara, H.; Nishi, N.; Tokura, S. Bull. Chem. Soc. Jpn. 1998, 71, 679–683.
6. All new compounds had satisfactory elemental analysis data or high-resolution mass spectra. Optical rotations were
1
measured in CHCl3 at 25°C unless otherwise noted. Selected data for 4: H NMR (300 MHz, CDCl3) δH 5.72, 5.59,
5.56 (each s, PhCH); [α]D −59.2 (c 2.0). Compound 5: δH 5.58, 5.53 (each s, PhCH), 4.68 (d, J=9.6, H-1), 4.65 (d, J=7.0,
H-10); [α]D −43.4 (c 0.47 in CHCl3); HRMS (FAB+) m/z for C35H39O10S (M+H)+, calcd: 651.2263; found: 651.2236.
Compound 6: δH 5.72, 5.59, 5.56 (each s, PhCH); [α]D −3.8 (c 5.74). Compound 8: δH 4.75 (d, J=8.0, H-10), 4.70 (d,
J=10.1, H-1). Compound 9: δH 3.85 (m, CH), 3.65 (t, J=5.6, CH2). Compound 11: δH 3.69 (t, J=6.0, 14-CH2), 3.46 (t,
J=6.7, 1-CH2), 2.00 (bs, OH); [α]D +2.1 (c 0.40 in CHCl3); HRMS (FAB+) m/z for C44H59O4Si (M+H)+, calcd: 679.4155;
found: 679.4183. Compound 12: δH 0.93 (t, J=7.3, 14-CH3), 4.49 (s, PhCH2), 5.15 (m, 11-CH); [α]D +3.7 (c 0.29); HRMS
(FAB+) m/z for C28H41O3 (M+H)+, calcd: 425.3056; found: 425.3060. Compound 13: δD 3.64 (t, J=6.6, 1-CH2), 3.61 (m,
11-CH); [α]D +1.8 (c 0.80). Compound 14: m.p. 41.4–42°C; δD 3.67 (s, CH3O), 3.60 (m, 11-CH); [α]D 0.7 (c 0.25).
Compound 17: δD 4.25 (d, J=7.3, H-1), 3.66 (s, COOCH3), 0.90 (t, J=7.1, CH3); [α]D −17.5 (c 5.44). Compound 18: δD
4.99 (d, J=7.5, H-1b), 4.73 (d, J=7.7, H-1c), 4.22 (d, J=7.5, H-1a); [α]D +10.1 (c 0.43). Compound 20: δD 4.99 (d, J=7.5,
H-1b), 4.73 (d, J=7.7, H-1c), 4.22 (d, J=7.5, H-1a); [α]D −18.7 (c 0.69); HRMS (FAB+) m/z for C63H84O14Na (M+Na)+,
calcd: 1087.5759; found: 1087.5740. Compound 22: δH ((CD3)2CO): 5.44 (d, J=0.9, H-1d), 4.32 (d, J=7.6, H-1a), 4.27
(bs, 1H, H-2d); [α]D −6.3 (c 0.20); HRMS (FAB+) m/z for C90H112O18Na (M+Na)+, calcd: 1503.7746; found: 1503.7730.
Compound 23: δH ((CD3)2CO): 5.51 (t, J=9.4, H-3c), 5.41 (s, H-1d), 5.15 (d, J=7.9, H-1c), 5.08 (t, J=8.8, H-2c), 4.85 (d,
J=8.8, H-1b), 4.38 (bs, H-2d), 4.31 (d, J=7.6, H-1a), 4.12 (bd, J=9.3, H-3d), 4.01 (t, J=9.4, H-3b), 3.61 (t, J=8.6, H-2a),
3.45 (t, J=8.4, H-4c), 3.37–3.27 (m, H-5a), 3.18 (t, J=8.8, H-4a), 3.21–3.11 (m, H-5a), 2.92 (t, J=9.1, H-4b), 2.33–2.22
(m, CH2COO), 0.86 (t, J=7.0, CH3); δC ((CD3)2CO): 174.0, 172.8, 139.9, 139.4, 139.3, 139.2, 129.2, 128.7, 128.6, 128.5,
128.3, 128.2, 128.1, 127.9, 127.8, 127.7, 127.6, 127.5, 103.0, 100.7, 99.6, 86.3, 84.4, 82.0, 81.9, 81.4, 78.8, 77.5, 75.8, 75.4,
74.9, 74.8, 74.7, 73.9, 72.8, 71.9, 71.6, 71.1, 45.3, 38.7, 35.6, 33.7, 31.0, 30.5, 30.3, 30.1, 29.8, 29.6, 29.3, 29.1, 28.8, 28.6,
25.9, 25.8, 25.4, 25.2, 22.8, 18.5, 18.2, 18.0, 17.9, 17.8, 15.9, 14.1, 13.9, 12.9, 12.1; [α]D −8.9 (c 0.23); HRMS (FAB+) m/z
for C99H123O20Na (M+H+Na)+, calcd: 1654.8505; found: 1654.8634. Compound 2: [α]D −51.7 (c 0.12, EtOH); HRMS
(FAB+) m/z for C43H74O20Na (M+Na)+, calcd: 933.4671; found: 933.4670.
7. Oikawa, M.; Liu, W.-C.; Nakai, Y.; Koshida, S.; Fukase, K.; Kusumoto, S. Synlett 1996, 1179–1180.
8. Similar reduction of the 2,20-dibenzoate of 5 resulted in the formation of an intractable mixture of O-benzoyl and O-benzyl
derivatives.
9. Ek, M.; Garegg, P. J.; Hultberg, H.; Oscarson, S. J. Carbohydr. Chem. 1983, 2, 305–311.
10. Mikami, T.; Asano, H.; Mitsunobu, O. Chem. Lett. 1987, 2033–2034.
11. Ackermann, J.; Waespe-Sarcevic, N.; Tamm, C. Helv. Chim. Acta 1984, 67, 254–260.

3457
12. Hooz, J.; Gilani, S. S. H. Can. J. Chem. 1968, 46, 86–87.
13. Einhorn, J.; Einhorn, C.; Ratajczak, F.; Pierre, J.-L. J. Org. Chem. 1996, 61, 7454–7454.
14. New compound 15 [1H NMR: 3.21 (s, Ms); [α]D +12.6 (c 0.83)] was prepared from phenyl 4,6-O-benzylidene-3-O-benzyl-
1-thio-β-glucopyranoside followed by reductive cleavage of the benzylidene group and O-mesylation at the 6-position.
15. New compound 21 [1H NMR: 5.49 (bs, H-1); [α]D +0.65 (c 0.70)] was prepared by the Lewis acid-promoted
thioglycosidation of the fully acetylated rhamnose, followed by de-O-acetylation and O-benzylation.
16. Veeneman, G. H.; van Leeuwen, S. H.; van Boom, J. H. Tetrahedron Lett. 1990, 31, 1331–1334.
17. Hikota, M.; Sakurai, Y.; Horita, K.; Yonemitsu, O. Tetrahedron Lett. 1990, 31, 6367–6370; Larson, D. E. P.; Heathcook, C.
H. J. Org. Chem. 1996, 61, 5208–5209.