Five ingol esters and a 17-hydroxyingenol ester from the latex of Euphorbia Kamerunica . Assignment of esters using 13C.N.M.R. methods.

Tetrahedron Letters,Vo1.25,No.34,pp Printed in Great Britain

3773-3776,1984

FIVE INGOL ESTERS AND A 17-HYDROXYINGENOL EUPHORBIA

KAMERUNICA.

ASSIGNMENT

0040-4039/84 $3.00 + .oo 01984 Perqamon Press Ltd.

ESTER FROM THE LATEX OF

OF ESTERS USING

13C N.M.R. METHODS.

By Joseph D. Connolly*, Christopher 0. Fakunle+ and David S. Rycroft Department of Chemistry, University of Glasgow, Glasgow G12 8QQ. Scotland. Summary The structures of five ingol esters and a 17-hydroxyingenol ester from the latex of Euphorbia kamerunica have been determined. The 13C n.m.r. spectra of these compounds have been assigned using 2D 6C/6H correlations. The specific positions of attachment of esters have been assigned unambiguously using 13C n.m.r. methods including 2D long range 6c/6H correlations. The diterpenoids

of the Euphorbiaceae

have been a source of great interest

because of their biological

activity.'

nucleus

and it is difficult

to obtain pure compounds

various

positions

of the diterpenoid However,

solve the ester problem. cation

can occur during partial

These compounds

nucleus.

chemists

and to decide which ester is attached

In the past partial

while it has been reported'

hydrolysis

to organic

often occur as mixed esters of the same

this has not always

hydrolysis

to the

has been used to

that intramolecular

transesterifi-

been taken into account 3 in the

structural assignments. We have been able to circumvent the difficulties of transesterification 13 13 using C n.m.r. methods. Surprisingly there are few reports of C chemical shift data for 13 these compounds in the literature.' C chemWe now report the structural elucidation and the ical shifts of five ingol esters and a 17-hydroxyingenol Multiple

ester

preparative

(l), (2), (7), (lO),and

(16) [as its 3-deacetyl

(17) from the latex of Euphorbia

t.1.c. of the ether extract

third band in order of increasing

derivative

(15)1

kamerunica.

of the latex afforded

several

The

bands.

gave a crystalline mixture of compounds (1) and (2) in 13 C n.m.r. spectra (Tables 1 and 2) by comparison the ratio 3:2. It was evident from the 'H and 4 that with ingol tetra-acetate (31, whose relative stereochemistry has recently been reassigned, both compounds

contain

there are resonances cated

yielded

parative

t.1.c.

an ingol nucleus

for a tiglate

that the methoxyl

mixture

polarity

group

in (1) and a benzoate

is attached

the corresponding

(6).

to C-8 in both

3-deacetyl

derivatives

Thus the tiglate

Further

in (1) and

partial

and a methoxyl in (2). (1) and

group.

The chemical Partial

(2).

In addition shift of H-8 indihydrolysis

of the

(5) which were separated by pre13 C shifts, relative to (1) and the expected

In both cases C-2 and C-3 exhibited

(21, for removal of the 3-acetate. acetate

with two acetates

(4) and

hydrolysis

(4) must be attached

of

(4) gave 8-O-methyl-ingol-12-

to C-7.

The benzoate

group of

(5), and hence of (2) was shown to be attached to C-7 by selective decoupling experiments in the 13 C n.m.r. spectrum. Irradiation of H-7 (6 5.59) resulted in removal of a coupling 3 (3J = 3 Hz) to the benzoyl carbonyl group which then appeared as a triplet, J (H-ortho) = 3 Hz.

fully coupled

Similarly

for the acetate

(3J) and left a quartet

carbonyl

carbon

(2J = 7 Hz).

8-g-methyl-ingol-3,12-diacetate-7-tiglate

irradiation

of H-12

Thus the structures

(6 4.87) removed

of (1) and

a 4 Hz coupling

(2) are established

and 8-g-methyl-ingol-3,12-diacetate-7-benzoate

as respect-

ively. The third ingol ester, less polar +SeniOr

Commonwealth

Universities

than

(1) and

(2). was assigned

Fellow on leave from the University

3773

structure

(7), ingol-

of Ife, Nigeria.

3174

3,8,12-triacetate-7-tiglate, presence

of a 7-tiglate

on the basis of its spectroscopic

was established

based on long range couplings, carbonyl

group.

in particular

A more detailed

properties

by 2D &C/&H correlation

(Tables 1 and 2).

using polarisation

the three bond coupling

between

The

transfer

H-7 and the tiglate

of the use of the above methods for determining 5 will appear elsewhere. Partial hydrolysis of (7) yielded 6 inter alia ingol-12-acetate-8-tiglate (8) which, on acetylation, afforded (9). isomeric with the -sites of specific

natural

ester

hydroxyl

(7).

groups

structures recently

This result confirms

based on such evidence.

described3

these compounds

(9), 8-angelate

The least polar

The presence

the 3-deacetyl aration

of the tiglate

of a 7-angelate

(11) was obtained.

derivative

(see below) and also afforded

(13) and the 12-acetate The 3-deacetyl

(14).

derivative

by partial

hydrolysis

of

(11) enabled

(10) as a minor contaminant,

of (12) gave the natural product

method described

First

the sep-

as its 3-deacetyl

(12). the 12-acetate-8-angel-

on the basis of its spectroscopic

tion of the 2D long range hc/6H correlation

proper-

in two stages.

of the fifth ingol ester was readily assigned

8-g-methyl-ingol-12-acetate-7-angelate,

des(7).

with those of the tiglate

hydrolysis

the 12-acetate-7-angelate

Acetylation

to those of

(lo), has spectroscopic

resonances,

Further partial

of the fifth ingol ester, which accompanied

data for a compound

similarity

for

data3

at C-12.

apart from the angelate in (10) was proved

from E. kamerunica

of the published

The published

also bear remarkable

the C-7 and C-8 of assigning

of ingol-3,7,12-triacetate

Comparison

ingol-3,8,12-triacetate-7-angelate

identical,

derivative

(7).

this suggestion.

for the location

compound,

between

the hazards

and 8-benzoate

'I-esters, including

with our data supports

was presented

that ester transfer and illustrates

Thus it seems likely that the compounds

as the 8-tiglate

ties which are virtually

ate

hydrolysis

as ingol-3,7,8-triacetate-12-tiglate

No evidence

(7).

the report'

can occur during partial

are, in fact, the corresponding

cribed

description

ester attachment

(10).

structure

properties.

above confirmed

(15), Applica-

the presence

of the

7-angelate. Thus the natural ingol ester is 8-c-methyl-ingol-3,12-diacetate-7-angelate (16). 13 The C chemical shifts of these ingol esters were largely assigned using 2D 6c/6H correlations.

The problem

sidering

their long range proton couplings.

is a sharp quartet

of the assignment

show that the 6 73.4 resonance

the 6 71.1 signal is coupled

structure

data for ingol tetra-acetate

the torsion

expected.

6 73.4 is assigned

The most polar compound

has three acetates on C-15.

approximately function

and an angelate

The l3C resonances

138-9',

attached

of the geminal

is C-17

in (17) resonates (Table 3).

[rSH4.98

(H-3), 5.37

Using

the crystal C-15, C-4, C-5,

near zero are

to an ingenol methyls

(H-5), 4.07 and 4.16

(H-8), 0.89

nucleus

bearing

(H-13), 1.11

to cause an upfield

series appear

Introduction

(ABq,

(H-14)1,

an acyloxymethyl

in the ingol and ingenol

and 29 ppm for the a-methyl.

to either methyl would be expected

acyloxymethyl

to H-5, H-2 and H-18

to H-la.

for which =JCH values

(ABq, J 12.2 Hz, 2H-20), 4.32

16 ppm for the S-methyl

Since the only t-methyl

is coupled

to C-4 and 6 71.1 to C-15.

(171, m.p.

J 11.7 Hz. 2H-17), 4.11 and 4.55

of

angles C-4, C-15, C-l,H-laand

to be -105.2O and 111.6O respectively,

by con-

(7) the signal at 6 73.4 C of triplets (J 2, 7 Hz). The 2D

spectrum

to H-2, H-18 and more weakly

H-5 are inferred Hence

carbons , C-4 and C-15, was solved

In the coupled

(J 7 Hz) and that at 6C 71.1 a broad doublet

long range 6c/6H correlations whereas

of the epoxide

group at

of an acyloxy

shift of the remaining

methyl.

at 24.1 ppm it must be a, i.e. C-16, and hence the

377s

The position

of the angelate

of attachment

afforded

the 17-acetate-3-angelate

angelate

(19) [gH 4.39

(S, H-S),

of a 17-angelate of

(18) yielded

[6 4.95

C-3 are at lower

(s, H-S)]

in (17) since ester transfer

(H-3), 5.40

that the natural

3.65

compound

whereas

Formation

between

field than normal,

presumably

ester

(17).

Similarly

at 172.4 ppm.

hydrolysis

of

(18), followed

'H n.m.r.

(H-13), 1.15

16-hydroxyingenol

spectrum

tetra-acetate.

that the literature Table

(3)

(19) afforded

Acetylation

the isomer

(ZH-2011.

3.69

(21)

It follows,

therefore,

one acetate

[6.06 (H-l) , 4.93

1.

hydroxyl

is more deshielded

of the 20-angelate

derivatives

afforded (H-3), 5.37

(H-5), 6.21

for a compound

The 'H spectra of the isomers were predicted' 8. is identical to (22).

tetra-

(H-7), 4.37

(ABq, J 11.9 Hz, 2H-17), 4.57 and 4.14 to that published'

(19) and

hydrolysis.

17-hydroxyingenol

13C Chemical

Shifts of

Ingol

Derivatives

(ABq,

described to differ

as and

(CDC13)

(7)

(9)

(1)

(2)

(4)

(5)

1

31.3

31.5

31.7

31.5

31.4

31.5

31.5

31.8

31.7

2 3 4 5 6 7 8 9 10

29.3

29.5

31.9

29.5

29.3

29.4

29.4

31.9

31.9

76.2

76.5

76.1

76.6

76.3

76.8

76.9

76.1

75.9

73.3

73.4

75.7

73.4

73.4

73.5

74.4

75.7

75.7

117.0

117.0

117.5

116.8

116.8

116.7

117.1

117.3

117.6

139.5

139.8

139.5

139.9

139.9

140.0

139.7

139.5

139.2

76.8

76.6

73.6

76.7

77.0

74.0

74.6

73.7

74.3

71.2

71.5

78.8

71.7

71.2

78.8

78.8

78.9

78.9

24.7

25.1

27.2

25.0

25.0

27.2

27.2

27.1

27.1

19.3

19.3

19.2

19.3

19.3

19.3

19.4

19.2

19.2

11

30.6

30.7

30.5

30.7

30.8

30.5

30.5

30.4

30.5

12

70.6

70.6

71.0

70.6

70.8

71.0

71.0

70.8

71.0

13

42.9

43.1

42.9

43.1

43.0

43.0

43.1

43.0

43.0

14

207.4

207.6

207.8

207.6

207.6

207.6

207.6

207.8

207.8

15

71.0

71.1

72.8

71.1

71.1

71.1

71.2

72.8

72.8

16

16.9

16.9

16.1

16.9

17.0

16.9

16.9

16.1

16.1

17

17.3

17.5

17.8

17.5

17.4

17.7

17.8

17.8

17.8

18

29.0

29.1

29.2

29.2

29.2

29.3

29.3

29.3

29.3

19

16.0

16.1

16.5

16.1

16.1

16.5

16.5

16.5

16.5

20

13.3

13.4

13.2

13.3

13.4

13.3

13.3

13.2

13.2

Table

2. 9

1 H Chemical

Shifts of Ingol Derivatives

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

3

5.20

5.13

4.24 5.42 2.90

4.19 5.59 2.98

4.37 4.34 2.85

5.22 5.21 4.58

4.31 4.26 4.55

5.35 5.12 4.64

5.23 5.20 4.58

4.25 5.29 4.59

4.25 5.23 3.55

4.33

4.30

4.25

4.28 4.62

3.36

2.90

4.85

4.87

4.85

4.87

4.86

4.88

4.86

4.86

4.87

4.87

4.82

4.85

7

5.33

5.54

8

2.91

2.99

5.29 5.03 4.52

12

4.85

4.90

4.81

-4.13

to

than

compound

(15)

(10)

carbonyl

can occur during partial

by acetylation,

similarity

to the tertiary

is 167.3 ppm in (21) and moves downfield

The formation

(H-14), 4.22 and 4.11

J 12.7 Hz, 2H-20)] bears a striking

we suspect

of

as a result of H-bonding

carbonyl

(20) shows that, as in the ingol series , ester transfer

(22) whose

(s, H-3),

the possibility

The

usual in (21) and resonates

(H-8), 0.93

(20) [6D 4.41

(20) excludes

C-17 and C-20 is impossible.

acetylation

which

the 17-acetate-20-

(17), the most irritant of those isolated, is 17-acetoxyingenol-5,2013 C chemical shifts of the carbonyl group of the ester attached to

ester

168.8 ppm in the natural

acetate

of

hydrolysis

(s, H-5)1,

and the 20-angelate

(2H-17) , 4.31 and 4.55

(H-S), 4.13 and 4.24

Thus the l3C shift of the angelate

Complete

by partial

(s, 2H-20), 4.04

(ABq, J 11.7 Hz, 2~-17)].

the natural

diacetate-3-angelate.

group.

(s, H-3),

3.74 and 3.84

was determined

(18) [6H 4.13

5.47

3776

Table (17)

(22)

131.5 133.7 81.5 85.7 74.6

131.8 133.6

13 C Chemical

3.

82.0 85.7 74.6

Shifts of Ingenol Derivatives (17)

(22)

131.0

11 12

30.5 30.6

43.1 204.6 71.8

13 14 15

23.1 23.9 27.5

38.6 30.8 23.2 24.0 27.7

(17)

(22)

6

135.9

135.8

7

130.8

I3 9 10

43.1 204.6 71.9

(17) and

(22)

16 17 18 19 20

V7)

(22)

24.1 65.6 16.5 15.3 65.6

24.2 65.7 16.6 15.4 65.7

R4

16

19

OR” (1) 12) (3) (4) (5) (6! (7) (81 (9) (10) (11) (12) (131 (14) (15) (16)

R’

R2

R3

R4

AC AC AC

tig PhCO AC tiq PhCO H tig H AC ang ang ang H H

Me Me AC Me Me Me AC tig tiq AC AC H ang 9 Me Me

AC AC AC AC AC AC AC AC AC AC AC AC AC AC AC AC

H H H

AC H

AC AC H H H H H AC

ang ang

(17) (18) (19) (20) (21) (22)

R1

R2 R3

R4

ang w

AC H H H AC AC

AC AC AC H AC AC

H

H AC AC

AC H ang ang ang AC

References 1.

F.J. Evans and S.E. Taylor,

Progress

in the Chem. of Org. Nat. Prod.,'1983,

2.

K.A. Abo and F.J. Evans, J. Nat. Prod.,

1982, 45, 365.

3.

K.A. Abo and F.J. Evans, Planta Medica,

1981, 2,

J.D. Connolly,

C.O. Fakunle

and D.S. Rycroft,

J. Chem. Res., submitted.

5.

J.D. Connolly,

C.O. Fakunle

and D.S. Rycroft,

J. Chem. Res., submitted.

6.

H.J. Opferkuch

and E. Hecker,

7.

K.A. Abo and F.J. Evans, Phytochemistry,

8.

H.J. Opferkuch

9.

Only chemical

and E. Hecker,

Tetrahedron

shift data are given.

as reported

for the parent

(Received

in UK 4 June

1981, g, Letters,

Multiplicities

compounds. 1984)

Letters,

l-99.

392.

4.

Tetrahedron

44,

1973, 3611. 2535. 1974, 261. and coupling

constants

are essentially