Novel Synthesis, Crystal Structure, and Plant-growth Regulation Activity of(1S,4R)-4,7,7-Trimethyl-6- oxabicyclo[3.2.1]octane-1,4-diol①

HUANG Do-Zhn XIE Zhi-Peng LAN Hong-Yun

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Novel Synthesis, Crystal Structure, and Plant-growth Regulation Activity of(1S,4R)-4,7,7-Trimethyl-6- oxabicyclo[3.2.1]octane-1,4-diol①

HUANG Dao-Zhana, b②XIE Zhi-PengaLAN Hong-Yuna, b

a(530008)b(530008)

The title compound, (1S,4R)-4,7,7-trimethyl-6-oxabicyclo [3.2.1] octane-1,4-diol (C10H18O3), has been synthesized from terpinoleneone-step catalytic synthetic method and structurally characterized by means of HRMS, IR,1H-NMR,13C-NMR and single-crystal X-ray diffraction. The compound crystallizes in trigonal, space group-3, with= 27.892(9),= 27.892(9),= 6.720(2) ?,= 120°,= 18,= 4527(3) ?3,D= 1.230 g/cm3,M= 186.24,(Mo) = 0.71073?,= 0.09 mm-1,(000) = 1836, the final= 0.051 and= 0.161. The title compound molecule contained a 6-oxabicyclo[3.2.1]octane skeleton and two hydroxyl groups, which were connected through intermolecular O–H···O hydrogen bonds to generate a two-dimensional network. Especially, the preliminary bioassay showed that the title compound can promote the root growth and shoot elongation of rape () at low concentration (0.625～70 mmol·L-1) and inhibit them at high concentration (> 70 mmol·L-1).

monoterpene oxybicyclodiol, one-step catalytic synthesis, crystal structure, plant-growth regulation activity, terpinolene;

1 INTRODUCTION

Compounds containing a 6-oxabicyclo [3.2.1] octane skeleton exist in a variety of natural and synthetic products[1-7]and some of them have been found to show diverse important bioactivities. The difference of bioactivities among those compounds is likely to be related to their molecule structures, in which the number and position of hydroxyl, methyl or other substituent groups connected with the 6-oxabicyclo [3.2.1] octane skeleton are different. Firstly, 8-isopropyl-6-oxabicyclo[3.2.1]octan-7-one[8]displays insecticidal activity. Secondly, Prostanoid analogues[9]with a 6-oxabicyclo [3.2.1] octane ring exhibit obvious biological activity on thromboxane receptors and prostaglandin I2receptors. Moreover, 5,7,7-trimethyl-6-oxabicyclo [3.2.1] octan-4-ols[10]and (3R,4R)-5,7,7-trimethyl-6-oxabicyclo [3.2.1] octan-3,4-diol[11, 12]show excellent herbicidal active- ties against many weeds.

The title compound (1S,4R)-4,7,7-trimethyl-6- oxabicyclo [3.2.1] octane-1,4-diol is an isomer of (3R,4R)-5, 7,7-trimethyl-6-oxabicyclo [3.2.1] octan-3,4-diol. It was traditionally synthesized from terpinolene by a two-step synthetic method[13], in which epoxidation of terpinolene with 3-chloro- peroxybenzoic acid (MCPBA) as oxidant and acid- catalyzed epoxidation-ring-opening rearrangement of terpinolene diepoxide with sulfuric acid as acid catalyst were carried out successively. However, the expensiveterpinolene diepoxide and complex synthetic process made it hard to be of practical use. So far, no report of the molecular crystal structure, bioactivity and application for the title compound including its derivatives has been found.

On the basis of our previous work[7], a new synthetic method for the title compound is discussed. Herein, the one-step catalytic method, spectroscopy and molecular crystal structure data are reported. Besides, the plant-growth regulation activity of the title compound on rape has also been investigated.

2 EXPERIMENTAL

2. 1 Materials and methods

All chemicals were of analytical grade and used without further purification. Rape () seeds were purchased from Wanhe County CSV seed industry co., Ltd. IR spectrum (KBr pellets) was recorded on a Magna-IR 550 (series II) Fourier transform spectrometer, Nicolet Co., U.S.A. Melting point was taken on a WRS-1B apparatus (Shanghai Precision and Scientific Instru- ments Co., Ltd., China) without correction for the thermometer. NMR spectra were recorded on a Bruker Avance AV 600 spectrometer (Bruker Co., Ltd., Switzerland) using DMSO-d6as the solvent,relative to TMS (0.00). A HRMS spectrum was performed on a Thermo Finnigan Trace GC/MAT 95XP instrument, Thermo Fisher Scientific Inc., U.S.A.

Catalyst [-C5H5N(CH2)15CH3]3PMo2W2O24was prepared from heteropoly acid and cetylpyridinium chloride according to the method of literatures[14].

2. 2 Synthesis

The synthesis procedure is shown in Scheme 1. To a 1000 mL CHCl3solution of 3.37 g catalyst was added 0.5 molterpinolene and 1.5 mol 30% aqueous H2O2by drops. The reaction mixture was stirred at 35 ℃ and the reaction was monitored by TLC or GC. The reaction mixture was cooled to room tempe- rature and placed on the separatory funnel after reaction. The aqueous layer was separated and then concentrated by a rotary evaporator to give yellow sticky liquid. The resulting sticky liquid was dissolved in hot ethyl ester for separating the insoluble substance by hot filtration and the filtrate was cooled to give a pale yellow solid crude product. The crude product was purified for four times by re-crystallization in ethyl acetate.

White solid, yield = 8.9%, m.p.: 159.0～159.7℃. IR (KBr, cm-1)max: 3367, 2968, 2928, 1462, 1367 and 1059.1H-NMRppm 3.731(1H, s, 4-OH), 3.721(1H, s, 1-OH), 2.203～2.245(1H, m, 5-H), 1.899～1.911(2H, m, 8-H), 1.755～1.770(2H, t, 3-H), 1.579～1.598(2H, t, 2-H), 1.242(3H, s, 9-H), 1.236(3H, s, 10-H), 1.202(3H, s, 11-H).13C-NMRppm 81.39(C-7), 80.81(C-1), 77.50(C-5), 71.64(C-4), 37.05(C-8), 34.19(C-3), 32.56(C-2), 27.96(C-11), 26.79(C-10), 21.25(C-9). HRMS (/) Calculated for C10H18O3, [M+] 186.1256, observed 186.1249.

Scheme 1. Synthetic route for the title compound

2. 3 X-ray crystal structure determination

A single crystal of the title compound for X-ray diffraction analyses was grown by slow solvent evaporation technique in ethyl acetate solvent at room temperature. A suitable single crystal posses- sing the dimensions of 0.28mm × 0.15mm × 0.11mm was selected and mounted on the top of a glass fiber. Diffraction data were collected on a BRULER SMART CCD area detector at 296(2) K using graphite-monochromated Moradiation (= 0.71073 ?). Absorption corrections were done by the multi-scan technique. The structure was solved by direct methods[15]and refined by full-matrix least-squares techniques using SHELXT program (Sheldrick, 2015) on2[16]. All non-hydrogen atoms of the molecule were refined with anisotropic para- meters. Molecular graphics were prepared using Diamond[17]. A total of 4332 reflections with 2220 independent ones (int= 0.0413) were measured in the range of 2.53≤≤27.75° by anscan mode, of which 1902 were observed with> 2(). The final= 0.051,= 0.161 (= 1/[2(F2) + (0.096)2+ 3.913], where= (F2+ 2F2)/3), (Δ/)max= 0.001,= 1.060, (Δ)max= 0.595 and (Δ)min= –0.548 e·?-3.

2. 4 Bioassay

Plant growth regulatory activity assay was carried out by using Petri dish method[18, 19]with seeds of rape. The title compound was made into different levels of aqueous solutions. Groups of 10 pre-ger- minated seeds of rape were placed in Petri dishes (i.d. = 9 cm) containing 10 mL of the corresponding solutions. The Petri dishes were sealed with parafilmand incubated at 28 °C in darkness. After 48 h, the lengths of root and shoot were measured. All experiments had three replications. The percentage of average inhibition against root growth and shoot elongation was calculated in relation to the blank control. The bioassay result is in comparison with that of Gibberellin A3acting as a control check (Ck).

3 RESULTS AND DISCUSSION

3. 1 Synthesis

The title compound was synthesized from ter- pinolene by one-step catalytic synthetic method, in which [-C5H5N(CH2)15CH3]3PMo2W2O24was used as the bifunctional redox-acid catalyst and aqueous 30% H2O2as the oxidant (Scheme 1). Compared to the two-step synthetic method[13], this method simplifies the process of operation and decreases the cost, as the material is cheap and abundant. Com- pared with 5,7,7-trimethyl-6-oxabicyclo [3.2.1] octan-3,4-diol[7]distributing in the aqueous and organic phase, the title compound mainly existed in the aqueous phase after synthetic reaction and was easily separated from the two-phase reaction mix- tures through phase separation, evaporation, con- centration and crystallization. The title compound was structurally identified by means of IR,1H-NMR,13C-NMR, HRMS and single-crystal X-ray diffraction.

3. 2 Structural description and analysis

Single-crystal X-ray diffraction was used to determine the crystal of the title compound. The molecular structure, packing arrangement and hydrogen bonds in the unit cell are shown in Figs. 1 and 2, respectively. The selected bond lengths and bond angles are given in Table 1. The hydrogen bond lengths and bond angles are presented in Table 2.

Single-crystal X-ray diffraction result reveals that the title compound crystallizes in trigonal, space group-3. The asymmetric unit contains two hydroxyl groups at C(3) and C(6), and an oxygen atom (O(1)) which bridges the adjacent carbon atoms (C(1) and C(10)), as shown in Fig. 1. Meanwhile, the1H-NMR statistics (O(3)–H(3),: 3.721, O(2)–H(2),: 3.731) and the medium intensity broad absorption band at 3390 cm-1nfrared spectra may be assignable to(O–H) in the title compound. The complete structure was finally confirmed by single-crystal crystallography. Furthermore, the title compound is interconnected into an intricate two-dimensional framework through the hydrogen-bonding interactionsbetween the hydroxyl groups (O(2), O(3)) and neighboring O(1) atom containing the hetero-ring (Fig. 2).

Fig. 1. Molecular crystal structure of the title compound

Fig. 2. Hydrogen bond viewing of packing arrangement of the title compound

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°)

Symmetry codes: (i) 2/3–x+y, 4/3–x, 1/3+z; (ii) 1/3+x–y, –1/3+x, 5/3–z

The O(3)–H(3)···O(1) (2.80 ?) and O(2)– H(2)···O(2) (2.93 ?) are both intermolecular hydro- gen bonds and play an important role in forming the 2network. Six title compound molecules associate through six unique O(3)–H(3)···O(1) interactions into a six-numbered ring. Two six-numbered rings are connected togetherO(2)–H(2)···O(2) interac- tion, which combined five adjacent title compound molecules (including two pairs of overlapping molecules and an isolated molecule) to form a triangular hydrogen bond combination when viewed along-axis.

The formation of intramolecular hydrogen bonds has a very pronounced effect on the molecular structure and properties. The title compound shows higher melting point and is more hydrophilic in HCCl3-H2O2two phase reaction system than its isomer (3R,4R)-5,7,7-trimethyl-6-oxabicyclo [3.2.1] octan-3,4-diol[7]. It extended into a simple 2networktwo different intermolecular hydrogen bonds and an intramolecular hydrogen bond, resulting in the HCCl3-H2O2two phase reaction mixtures, mainly existing in aqueous phase, which could be directly separated and purified from aqueous phase through evaporation, concentration and crystallization as discussed above.

3. 3 Plant-growth regulation activity

Plant growth regulator activity of the title compound is shown in Fig. 3. The title compound exhibits dual effects on both root growth and shoot elongation of rape, which could be promoted at low concentration (0.625～70 mmol·L-1) and inhibited at high concentration (> 70 mmol·L-1). For the growth promoting activity, with the increase of con- centration, the inhibition rate of the title compound against root growth and shoot elongation of rape first turns low and then becomes high; minimum inhibitions against root growth and shoot elongation were observed at the concentration of 40 and 2.5 mmol·L-1, respectively. For the growth inhibiting activity, the inhibition rate of the title compound against root growth and shoot elongation of rape increased with increasing the concentration, being up to 92.1% and 61.6%, respectively at the concentration of 160 mmol·L-1. In comparison with Gibberellin A3, which promoted the root growth and shoot elongation of rape at low concentration (0.28～0.46 mmol·L-1) and inhibited at high concentration (>1 mmol·L-1)[20], the title compound shows weaker inhibition against the root growth and shoot elongation of rape.

In general, the title compound shows dual plant growth regulator activities on root growth and shoots elongation of rape, indicating that it could be used as a botanical plant growth promoter or herbicide.

Fig. 3. Plant-growth regulator activity of the title compound on rape.

Note: Growth inhibited when the inhibition rate is positive, on the contrary promoted

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28 October 2017;

5 March 2018 (CCDC 1575491)

the National Natural Science Foundation of China (No. 31460174), the Science and Research Start-Up Project for the Recruit Talent of Guangxi University for Nationalities (No. 2014MDQD014) and Innovation Project of Guangxi Graduate Education(gxun-chxzs2016113)

. Huang Dao-Zhan, majoring in chemistry of forest products. E-mail: huangdaozhan@gxun.edu.cn

10.14102/j.cnki.0254-5861.2011-1876