Structural elucidation of Compound i: 1,2,3-Trimethoxyacridone (i)

Extractives from Toddaliopsis bremekampii leaves and branches 6.1. Introduction

6.2. Results and Discussion

6.3.1. Structural elucidation of Compound i: 1,2,3-Trimethoxyacridone (i)

H

°

_OMe

OMe

The positive Cl mass spectrum [242] of compound (i) showed a [M+Ht peak at mlz 286.1081 corresponding to the molecular formula Ct6HtsN04 for the compound. The IR spectrum [240] of compound (i) showed an N-H stretching band at 3275 cm-t and a C=O stretching band at 1633 cm-t.

Inspection of the tH NMR spectrum [243] of compound (i) showed the presence of five aromatic proton resonances (OH 8.41,7.53 (2H), 7.17, 6.75), three methoxy group proton resonances (OH 4.00,3.84, 3.74) and a NH group proton resonance(OH 10.39).

Inspection of the t3C NMR [244] and ADEPT [245] spectra of compound (i) showed the presence of sixteen carbon resonances, four fUlly substituted carbon resonances (oc 140.39, 140.10, 121.88, 110.31), three oxygenated fUlly substituted carbon resonances (oc 158.30, 153.55, 137.81), a carbonyl carbon resonance (oc 176.99), five methine carbon resonances (oc 132.73, 126.56, 121.38, 116.65, 94.27) and three oxygenated methyl carbon resonances (oc 61.97, 61.54, 55.84).

The COSY spectrum [249] showed correlations between the four aromatic proton resonances H-5 (OH 7.53), H-6 (OH 7.53), H-7 (OH 7.17) and H-8 (OH 8.41), which indicated the presence of an ortha- disubstituted aromatic ring A in compound(i).

Chapter 6: Extractive. fromToddllliopa. bremelUJmpii

o

OMe

OMe

. . . COSY correlation . . . NOESY correlation

COSY and NOESY NMR correlations

The HMBC [247-248] correlations between the C-9 carbonyl carbon resonance at oc 176.99 and the H- 5 and H-8 proton resonances confirmed that compound (i) is an acridone alkaloid. The H-8 proton resonance atOH8.41 is characteristic of acridone alkaloids as a result of the deshielding effects of the NH and C=O groups. The methoxy groups were attached to ring C.

The H-6, H-7 and H-8 proton resonances showed HMBC [247-248] correlations to the fully substituted carbon resonance at Oc 140.10, which was assigned to C-5a. The H-7 proton resonance showed a HMBC [247-248] correlation to the fully substituted carbon resonance at Oc 121.88, which was assigned to C-8a.

OMe

OMe

HMBC NMR correlations (C~H)

When three oxygenated carbons are adjacent to each other, the middle carbon resonance is doubly shielded and resonates at > 130 ppm. The other two carbon resonances are relatively deshielded and occurs around 159 - 151 ppm. Therefore the resonance at Oc 137.81 was assigned to C-2.

The NH group proton resonance(OH 10.39) showed correlations in the NOESY NMR spectrum [250] to the H-5 proton resonance of ring A and to the proton resonance (OH 6.75) of ring C, which indicated that this ring C proton was attached at C-4. The H-4 proton resonance showed a correlation in the NOESY NMR spectrum [250] to the methoxy group proton resonance atOH 3.74, which lead to the assignment of this methoxy group to C-3. The oxygenated fully substituted carbon resonance at Oc 158.30 was therefore assigned to C-3 in the HMBC NMR spectra [247-248]. The H-4 proton resonance showed HMBC [247-248] correlations (4 J coupling across the zig-zag bond) to the C-9 carbonyl carbon resonance (oc 176.99), the three oxygenated fully substituted carbon resonances at Oc 137.81 (C-2), 153.55 (C-1) and 158.30 (C-3), and the two fUlly substituted carbon resonances at Oc 110.31 and 140.39, which were assigned to C-1a and C-4a respectively.

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Chapter 6: Extractlvea fromToddBllopsfs bremekampll

Compound (i)is the novel 1,2,3-trimethoxyacridone(i). The NMR data for 1,2,3-trimethoxyacridone(i) is shown in table 6.1 .

Table6.1. :NMRdata for1,2,3-trimethoxyacridone (i)

C li'~CIppm (CDCI₃₎ li HIppm (CDCI3) HMBC(~H) COSY NOESY

1 .153.55 (C)

-

^1-0CH3,4

2 137.81(C)

-

^2-0CH3,4

3 158.30 (C)

-

^3·0CH3,4

4 94.27 (CH) 6.75 (5) 3-0CH3,NH

5 116.65 (CH) 7'.53· ·7 6 .6,NH

6 132.73 (CH) 7.53· 8 5,7 5, 7

7 121.38 (CH) 7.17 (m) 5 6,8 6,8

8 126.56 (CH) 8.41 (d,8.06) 6 7 7

9 176.99 (C)

-

^4,5,8

10 NH

-

^{10.39 (bs) 4,5}

4a 140.39 (C)

.

4

1a 110.31 (C)

-

⁴

8a 121.88 (C)

-

⁷

5a 140.10 (C)

-

^6,7,8

1-0CH3 61.97 (CH₃₎ 4.00(5)

2-0CH₃ 61.54 (CH3) 3.84 (5)

3-0CH₃ ·55.84 (CH₃₎ 3.74(5) 4

• The H-5 and H-6 resonances are superimposed.

6.3.2.

Structural elucidation of Compound

ii:

1,2,3-

Trimethoxy-1 O-acetoxymethylacridone

(ii)

H

o

OMe

OMe

OMe

(H)

The positive Cl mass spectrum [254] of compound (ii) showed a [M+Ht peak at m/z 358.1291 corresponding to the molecular formula C19H19N06 for the compound. The IR spectrum [252] of compound (ii) showed C=O stretching bands at 1733 and 1644 cm-1 for the acetoxy and pyridone respectively,? and no stretching bands were observed for the N-H group.

Inspection of the 1H NMR spectrum [255] of compound (ii) showed the presence of five aromatic proton resonances (OH8.43, 7.65, 7.50, 7.30, 6.88), three methoxy group proton resonances(OH4.008, 4.006, 3.99), a two proton methylene group proton resonance (OH 6.29) and an acetoxy methyl group proton resonance(OH2.20).

Chapter 6: Extractives fromToddMiopaia bremeampii

Inspection of the 13C NMR [256] and ADEPT [257] spectra showed the presence of nineteen carbon resonances, comprising two carbonyl carbon resonances (oc 176.82, 170.56), four fully substituted carbon resonances (oc 141.30, 140.85, 123.99, 112.14), three oxygenated fully substituted carbon resonances (oc 158.16, 154.67, 138.80), five methine carbon resonances (oc 133.27, 127.58, 122.62, 114.34,93.22), one methylene carbon resonance (oc 71.48), one methyl group carbon resonance (oc 21.01) and three oxygenated methyl carbon resonances (oc 61.87, 61.43, 56.20). The NMR correlations in compound (ii) were similar to those of compound (i), and suggested that the structure of compound(ii)was similar to compound(i).

The aromatic ring proton resonances atOH7.50 (d, 8.61 Hz), 7.65 (dd, 6.96, 8.61 Hz), 7.30 (dd, 6.96, 8.06 Hz) and 8.43 (d, 8.06 Hz) showed correlations to each other in the COSY NMR spectrum [261], which indicated the presence of an ortho-disubstituted aromatic ring A in compound (ii), thereby leaving the three methoxy groups on ring C, as observed for compound(i).

o

OMe

OMe

(ii)

COSY and NOESY NMR correlations

The H-5(OH7.50) and H-7(OH7.30) proton resonances of ring A showed HMBC [259-260] correlations to the fully substituted carbon resonance atOc123.99, which was assigned to C-8a. The H-6(OH7.65) and H-8 (OH 8.43) proton resonances of ring A showed HMBC [259-260] correlations to the fully substituted carbon resonance atOc 141.30, which was assigned to C-5a.

(ii)

OMe

OMe

HMBC NMR correlations (C4H)

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Chapter 6: Extractlves from Toe/dal/opsls bremekampl/

The proton resonance atOH6.88 showed a correlation in the NOESY NMR spectrum [262-263] to the methoxy group proton resonance atOH4.006, and also to the methylene group two proton resonance at OH 6.29, which, in turn, showed a correlation in the NOESY NMR spectrum [262-263] to the H-5 resonance (OH 7.50). This indicated that the proton (OH 6.88) was attached at C-4 and the methylene group was attached to the nitrogen atom. The acetoxy group was placed on the methylene group as a result of the methylene group carbon resonance occurring at Oc 71.48. This is expected for methylene group carbons that are attached to both a nitrogen atom and an oxygen atom.a

The H-4 proton resonance showed HMBC [259-260] correlations to the C-9 carbonyl carbon resonance (oc 176.82), the two oxygenated fully substituted carbon resonances at Oc 138.80 (C-2) and 158.16 (C-3), and the two fully substituted carbon resonances at Oc 112.14 and 140.85, which were assigned to C-1 a and C- 4a respectively. A characteristic feature of acridone alkaloids that can be seen here is the deshielded C-4a and C-5a carbon resonances which occur at - OH140.

Compound (ii) is the novel 1,2,3-trimethoxy-1 O-acetoxymethylacridone (ii). The NMR data for 1,2,3- trimethoxy-10-acetoxymethylacridone (ii) is shown in table 6.2.

Table 6.2. : NMR data for 1,2,3-trimethoxy-1 O-acetoxymethylacridone (ii)

C

aUc

I ppm(CDCb) 8 HI ppm(CDCb) HMBC(~H) COSY NOESY

1 154.67 (C)

-

^1-0CHa

2 138.80(C)

-

^2-0CHa,4

3 158.16(C)

-

^3-0CHa,4

4 93.22 (CH) 6.88 (s) 3-0CHa 3-0CHa, N-CH2

5 114.34 (CH) 7.50 (d;8.61) 7 6 6, N-CH2

6 133.27 (CH) 7.65 (dd, 6.96, 8.61) 8 5,7 5, 7

7 122.62 (CH) 7.30 (dd, 6.96, 8.06) 5 6,8 6,8

8 127.58 (CH) 8.43 (d, 8.06) 6 7 7

9 1,76.82 (C)

-

^4,8

.

4a 140.85 (C)

-

⁴

1a 112.14(C)

-

⁴

8a 123.99(C)

-

^5,7

5a 141.30 (C)

-

^6,8

N-CH2 71.48 (CH2) 6.29 (bs) 4,5

1-0CHa 61.87 (CHa) 4.008 (s)

2-0CHa 61.43 (CHa) 3.99 (s)

3-QCHa 56.20 (CHa) 4.006 (s) 4 4

O~OCHa 170.56 (C)

.

_QCOC!:!.a

OCO~Ha 21.01 (CHa) 2.20 (s)

Chapter 6: Extractlves fromTodd8liopa.bremeampll