Indian JoUrnal of Chemistry Vol. 38B. November 1999. pp. 1242 - 1247

Claisen rearrangement of 3-bromo-, 3,6-dibromo-, 3,8-dibromo- and 8-iodo/aminomethyVacetyl-7 -allyloxy-4-methylcoumarins

S R Ghantwal & S D Samant * Organic Chemistry Research Laborat~ry. University Department of Chemical Technology. Matunga. Mumbai-400 019

Received 19 February 1999; accepted (revised) 10 September 1999

7-Hydroxy-4-methylcoumarin 1 has been brominated under different conditions to obtain the corre~poniling 3-bromo-2; 3,6-dibromo- 4; 3,8-dibromo- 3; and 3,6,8-tribromo- 8 derivatives. The 2-4 are allylated and subjected to Claiseh -rear­rangement in N,N-DMA when only 8-allyl-7-hydroxy-4-methylcoumarin' 13 is obtained. The Claisen rearrangement of 8-iodo and 8-aminomethyl coumarins also give 13. In all these rearrangements the 8-substituent is removed and the migra­tion takes place at the 8-position. The N,N-DMA serves as the scavenger fOf the substituent lost. The Claisen rearrangement of 8-acelyl-7-allyloxy-4-methylcoumarin gives 8-acetyl-6-allyl-7-hydroxy-4-methylcoumarin, in which the acetyl group re­mains intact.

The Claisen rearrangement of 7-allyloxycoumarins is interesting as it provides an entry into naturally oc­curring 6,7- and 7,8-furocoumarins I . The rearrange·· ment is sensitive with respect to the substituents pres­ent on the ring. There is a high tendency for the mi­gration to the 8-position2. The 4-methyV phenyl-7-allyloxycoumarin undergoes rearrangement to the 8-position in N,N-dimethylaniline (N,N-DMA) and to the 6-position in N,N-diethylaniline (N,N-DEA)2a. The rearrangement of 4-propyl-7-allyloxycoumarin gives a mjxture of the 6- and 8- allyl isomers3. When 7-allyloxycoumarin bears a labile group like bro­mine4

.5 or iodine6.7 at the 8-position, it is reported that

the group is lost, but the migration takes place to the 6-position . The rearrangement of 7-allyloxy-3,4-dihydrocoumarin takes place at the 6-position 8.

In the present work we have studied the Claisen rearrangement of 7-allyloxy-4-methylcoumarins bearing different substituents at the 8-position.

Bromination of 7-hydroxy-4-methylcoumarin 1: The bromination of 1 using Br2 gives the 3-bromo deri vative9 and with excess of Br2 3 and 410

.11 are ob­

tained. When 1 was reacted with Br2 in acetic acid, dioxane, chloroform, and carbon tetrachlorid~ at R.T., 2 was formed and its yield increased with inc'reasing the molar (Br2: I ) from I to 4. The best solvent was found to be acetic acid in which the reaction took place even at OaC. The yield of 2 decreased sharply with decreasing polarity of the solvent. When the re­action was carried out at the b.p. of acetic acid with higher concentration of Br2 , a mixture of 2, 3 and 4

was obtained; with increasing quantity of Br2 the yield of 4 increased, but that of 2 decreased. The bromination of 1 with (KBr+KBr03) also gave a mixture of 3 and 4 which could not be separated even by column chromatography.

The mixture of 3 and 4 was allylated using allyl bromide and K2C03 in acetone to obtain a mixture of allyl ethers 6 and 7 which could be separated from their acetone solution by fractional crystallization. The 2 was similarly converted into 5.

R· b . . f 1 312-14 d 61~16 •. 109 rommatlOn 0 at -an . -posItions is known. We found that when 1 was reacted with NBS in CHCl3 in lihe presence of light or BZ20 2, the only product obtained was 3,6,8-tribromo derivative 8. Surprisingly. when 7-0H group was blocked by methylation and the methyl ether 9 was reacted with NBS, 11 and 12 were obtained on using BZ20 2 and light as catalyst, respectively.

Claisen rearrangement of bromo-7-allyloxyc(]lumarins 5, 6 and 7: When 5, 6 and 7 were heated in N,N-DMA the only product obtained was 13 which is also obtained by the Claisen rearrangement of 7-allyloxy-4-methylcoumarin 17,18. In these rear­rangements the allyl group migrated to the 8-position and Br when present at the 3-position, is also elimi­nated through 1,S-hydrogen shift. The N,N-DMA pos­sibly served as the scavenger for Br, the electrophilic Bris transferred from the coumarin ring to N,N-DMA ring (Schemes I and II) . In the rearrangement of 6, side product 4-bromo-N,N-dimethylaniline 26a was isolated, which confirmed its role.

~ .

GHANTW AL et al.: CLAISEN REARRANGEMENT OF SUBSTITUTED COUMARINS 1243

I

~ m OIClpNBS bvOrBz~

CIs 0

12.

1-4, 8

R2

S-7

a-

a-

0

12b

&

H

o

1: R.t- R2- H

3: Rl- H R,2-Br

.. : Rl- Br R,2- H

8: RI-R,2-Br

s: RI-R2-H

6: Rl- H R2-Br

7: Rt-- Br R2-H

10: RI - Dr R2 -CH2Br

II: RI-Dr R2-CH3

:~(&)~1 j 0 0

13

Scheme [

Claisen rearrangement of 7-allyloxy-8-iodo-4-methy1coumarin 20: Compound 1 was iodinated at the 8-position using h-HI04-EtOH following the re-

d . d' 61920 b ' 19 h' h I porte proce ure " to 0 tam , w IC was a-Iylated to 20. Ahluwalia et at had reported the forma­tion of 23 in the Claisen rearrangement of a 7-allyloxy-8-iodocoumarin6 while Trivedi has re­ported the formation of an 8-allyl derivative with loss of 8-iodo atom in the Claisen rearrangement of a 7-allyloxy-8-iodoisoflavone 21. We found that when 20 was refluxed in N, N-DMA the only product ob­tained was 13,

Ahluwalia et at. had be'ueved that the reaction pro­ceeded through initial migration of the allyl group to the 6-position with concomitant 1,5-prototropic shift gi ving intermediate 21 , followed by the removal of I

by N, N-DMA.We feel that in the present case, due to the fixation of double bond between 7,8-position, the migration may sti ll take place to the 8-position to give 22, which may lose I and give 13 (Scheme III) . Care­ful work-lip of the mother liquor of this experiment gave equi valent amount of 26b, which confirmed the role of N,N-DMA.

Claisen rearrangement of 7-allyloxy-8-aminomethyl-4-methy1coumarins 24. The Mannich reaction on 1 takes place at the 8-position to give the corresponding 8-aminomethyl derivatives 22-25. 1 was reacted with secondary amines and formaldehyde to obtain a series of 8-aminomethyl-7-hydroxy-4-methylcoumarins 24 (Table I) which were allylated to 25 (Table I) . Whefl 25 was refluxed in N,N-DMA, 13 was obtained, Here the transfer of the amino

1244 INDIAN J CHEM, SEC B, NOVEMBER 1999

6

14

lJ ..

II

+NN-DMA -NN-D~(Br)

17

15

16

- ~ ~oyoAo

NN-DN.A -A Il

19 I

Scheme II

~ -- 10 @o " 0 I I H

11 11

1 1 ~ u cO::to

13

Scheme III

methyl residue from the 8-position to N,N-DMA ap­peared too favourable.

Claisen rearrangement of 8-acetyl-7-allyloxy-4-methylcoumarin 28. The Claisen rearrangement of o-acetylallyloxy arenes involving migration of allyl group to the carbon bearing acetyl group with loss of

2627 H . h the acetyl group are known . . owever, m t e case of coumarins it is reported that 7-allyloxy-8-acetylcoumarins undergo rearrangemt<nt to 6-position

. hI" . 2728 H Wit acety group remammg mtact ' . owever, proper proof has not been extended. 27 was prepared by the known procedure29. It was aHylated to obtained

28, which was refluxed in N,N-DMA when 'o'nly 29 was obtained.

Experimental Section General. The m.p.s were taken in open capillaries

on a Campbell precision melting point apparatus and

are uncorrected. For TLC silica gel (200-400 mesh) Acme grade was used. The IR spectra were recorded on a Perkin Elmer 397 spectrophotometer (vrnax in

I I ' cm-). The H-NMR spectra were recorded on a Varion T-60 spectrometer (60 MHz), with TMS as an internal standard and the values reported are in 5 units (ppm). The elemental analysis was found to be satisfactory. The 7-hydroxy-4-methylcoumarin was prepared from resorcinol and ethyl acetoacetate by the reported procedure30

.

3-Bromo-7-hydroxy-4-methylcoumarin 2. To 1 (3.52 g , 20 mmole) in acetic acid (30 mL), Br2 (1.1 mL, 20 ~ole) in acetic acid (20 mL) was added with stirring and the solution was stirred further at R.T. for 2hr. The solution was filtered and the solid was washed with water and crystallized from ethanol to "obtained 2. Yield 3.9 g (77%), m.p. 212-13°C (rep.1O 213-15°C); IR (KBr): 3300, 1710, 1620; 760;

Compd

24a

24b

24c

24d

24e

2Sa

2Sb

2Sc

2Sd

2Se

GHANTW AL et al. : CLAISEN REARRANGEMENT OF SUBSTITUTED COUMARINS

-NR2

4-morpholinyl

diethylamino

dimethylamino

I-piperidinyl

I-pyrrolidinyl

4-morpholinyl

diethylamino

dimethylamino

I-piperidin yl

I-pyrrolidinyl

I

Table I-8-Aminomethyl-7-hydroxy-/allyloxy-4-methylcoumarins 24 and 2S

Ri Mol formula

H C I5H17N04

H CI5HI9N03

H C 13HI5N03

H C 17HI9N03

H C I5H17N03

-CHr CH=CH2 CI RH21 N03

-CHr CH=CH2 C 1RH23N03

-CHr CH=CH2 CI6HI9N03

-C Hr CH=CH2 CI9H23N03

-CHr CH=CH2 CIRH21N03

24

NM~

~ 26.: X=& y 26b: X=I

X

9-J3

AC20"~1 AKl:3 :::::-.... o

COCl-I:3

27

29

o

Yield m.p. (%) (0C)

C

90 116-17 65 .33 (65.43

73 224-26 68.97 (68.88

61 210-11 66.88 (66.95

67 138-40 75.46 (75 .58

59 206-08 69.45 (69 .50

71 110-\2 68.49 (68 .50

67 118-15 71.71 (71.76

69 112-24 70.20 (70.33

6 1 138-39 72.86 (72.84

63 159-60 72.14 (72.24

25

28

Found (Calc.)%

H

6.24 6.22 6.48 6.50 6.41 6.40 6.62 6.67 6.46 6.50 6.65 6.67 7.62 7.64 6.92 6.96 7.55 7.53 6.98 7.00

J3

1245

N

5.16 5.19) 5.40 5.36) 5.98 6.00) 4.88 4.90) 5.40 5.41) 4.33 4.40) 4.58 4.60) 5.00 5.13) 4.46 4.47) 4.66 4.68)

INDIAN] CHEM, SEC B, NOVEMBER 1999

'H-NMR (CDCI): 2.63 (3H, s, CH), 6.93 (2H, m, C6H and CgH), 7.52 (lH, d, CsH).

7-Allyloxy-3-bromo-4-methylcoumarin S. 2 (2.54 g , 10 mmole) and anhyd K2CO) (6 g) were stirred in dry acetone (50 mL) for 20 min and allyl bromide (1.21 mL, 10 mmole) was added to it. The solution was refluxed for 7-8 hr and filtered to re­move K2C03. The filtrate was concentrated, diluted with water and the solid was filtered. The solid ob­tained was washed with water and cold dilute NaOH and crystallized from ethanol. Yield 2.3 g (78%); m.p.74- 76°C; IR (KBr): 1730, 1610, 1300, 1200, 750; 'H NMR (CDCl,): 2.6 (3H, s, CH3), 4.64 (2H, d, J= 6 Hz, OCH2), 5.2-5.6 (3H, m, CH2=CH), 6.85 (2H, m, C6H and CgH), 7.5 ( I H, d, J= 6 Hz , C5H) .

Bromination of 1 to obtain a mixture of 3,8 and 3,6-dibromocoumarins 3 and 4. To 1 (8.8 g , 50 mmole) in acetic acid (100 mL), Br2 (5.1 mL, 100 mmole) in acetic acid (50 mL) was added with stir­ring. The solution was reLuxed for 3 hr and then it was poured into crushed ice. The solid obtained was filtered and washed with cold water. The crude yield of a mixture of 3 and 4 was 11.6 g.

Allylation of the mixture of 3 and 4 to obtain 6 and 7. To the solution of the mixture of 3 and 4 (3.32 g) and anhyd K2CO, (6 g), in dry acetone (50 mL), allyl bromide ( 1.21 mL, 10 mmole) was added and the solution was refluxed for 6 hr. The solution was filtered, the filtrate concentrated and water added to it. The precipitate was filtered, washed with water, followed by cold dilute aq. NaOH. The yield was 2.6 g. The mixture of 6 and 7 was di ssolved in acetone (30 mL) and the solution chilled for 24 hr when 6 separated . The mother liquor was concentrated and ch illed further when 7 separated out. Both were crystallized from ethanol. 6: Yield 1.2 g (32 %); m.p. 175-76°C; IR (KBr): 1725, 1600, 1410, 1300, 1100, 1070; ' H-NMR (CDCI .. ): 2.35 (3H, s, CH), 4 .8 (2H, d, OCH2) ; 5 .2-6.4 (3H, m, -CH=CH2), 7.0 (tH, d, CoH), 7.6 ( I H, d, C5H) .

7: Yield 0.9 g (24 %), m.p. 163-64° C ; I R (KBr): 1740, 1600, 1370, 1200, 1080; IH NMR (CDCI3):

2.36 (3 H, s, CH .. ), 4.8 (2H, d, OCH2), 5.2-6.15(3H, m, CH=C H2) , 6.87 (I H, s, CgH). 26a : IR (nujol): 1450, 1370; ' H-NMR (CDCI,): 3.62 (6H, s, CH3), 7 .6-8.0 (4H, m, ArH)

3,6,8-Tribromo-7-hydroxy-4-methylcoumarin 8. To I (0.88 g, 5mmo le) in chloroform (50 mL), NBS (2.67 g , 15 mmole) and a pinch of Bz20 2 were added. The soluti on was refluxed for 3hr and the solution

was filtered . The filtrate was concentrated, cooled and the solid filtered . It was washed with water and crystalli zed from ethanol to give 8, yield 1.62 g (70 %), m.p. 242-44°C (repw 250-52°C) ; IR (KBr): 3320, 1730, 1590, 1320, 630;' H-NMR (DMSO-d6): 2.64 (3 H, s, CH1 ), 8 .1 ( I H, s, CsH) .

7-Methoxy-4-methylcoumarin 9. To 1 (4.4 g, 20 mmole) in aq . NaOH ( 10%, 20 mL) , dimethyl sul­phate (4 mL) was added and the solution stirred for I hr. The so lution was fi Itered and the residue washed with water and crystal · :zed from ethanol to obtained 9, yie ld 3.4 g (72 %), m.p. 156-57°C

3-Bromo-7-methoxy-4-methylcoumarin 11. To 9 (0.95 g, 5 mmole) in chloroform (50 mL), NBS (0.89 g, 5 mmole) and a pinch of Bz20 2 were added and then soluti on refluxed fo r 5 hr. The solution was fil­tered hot and the filtrate cooled when 11 separated out. Yield 1.3 g (75 %), m.p.128-29°C; IR (KBr): 171 0, 1610, 1200, 750; 'H NMR (DMSO-d6) : 2.64 (3 H, s, CH,), 3.8 (3 H, s, OCH,), 7.0 (2H, m, C6H, CgH), 7.8 (I H, d, C6H)

3-Bromo-4-bromomethyl-7-methoxycoumarin 10. To 9 (0.95 g, 5 mmole) in chloroform (50 mL), NBS ( 1.78 g, 10 mmole) was added and the solution was irradiated wi th tungsten lamp (60 W) for 4 hr, when the product separated. The solution was cooled and filtered . The residue was washed with water and crystallized from methanol. Yield 1.3 g (74 %), m.p. 83-84°C ; IR (KBr): 1740, 1590, 1120, 750, 620; 'H NMR (DMSO-d6): 2 .5 (2H, s, CH2), 3.8 (3H, s, OCH,), 6.9 (2H, m, C6H and CgH), 7 .7 ( IH, d, J = 9 Hz, C5H).

7-Hydroxy-8-iodo-4-methylcoumarin 19. To 1 (8.8 g, 50 mmole) in ethanol (min.), a mixture of h (4.5 g) and HI04 (2.5 g) in ethanol (50 mL) was added. The solution was stirred at R.T. for 4 hr and poured into ice cold water. The solid obtained was filtered and crystallized from ethanol. Yield 10.4 g (68 %), m.p.216-l7°C ; IR (KBr): 3300, 1720, 1620, 1420, 760; IH NMR (CDCh): 2.64 (3H, s, CH3), 6.18 (tH, s, C3H), 6.9 (tH, d, J = 8 Hz, C6H), 7 .63 (tH, d, J = 8 Hz, C5H), 11.8 (1 H, s, OH) .

7-Allyloxy-8-iodo-4-methylcoumarin 20. It was prepared by following the procedure adopted for 5.Yield 2.3 g (67%), m.p. I 62 .. 63°C; IR (nujol): 1730, 1470, 1400, 1100, 760; 'H NMR (DMSO-d6): 2.4 (3H, s, CH,), 4 .6-4.8 (2H , m, OCH2), 5.18-5.8 (3H, m, CH=CH2), 6.18 ( I H, S, C3H), 6.9 (lH, d, C6H), 7.6 (lH, d, CsH).

GHANTW AL et al.: CLAISEN REARRANGEMENT OF SUBSTITUTED COUMA'RlNS 1247

Claisen rearrangement of 5. Compound 5 (1 g) was refluxed in N,N-OMA (10 rnL) for 8 hr. The so­lution was concentrated and poured into ice-HCI. The residue was filtered, washed with dilute HCI followed by pet.ether (60-80°) and crystallized from ethanol to obtain 13. Yield 0.65 g (59 %), m.p.196-97°C; IR (KBr): 3300, 1700, 1600; IH NMR (COCl3) : 2.36 (3H, s, CH3), 6.1 (I H, S, CH=CH2 ), 6.6 (I H, d, CJI), 7.2 (lH, d, C5H). The Claisen rearrangement of 6,7 and 20 were carried out similarly to obtain 13 in re­spectively 52, 51 and 55% yields. 26b: IR (KBr): 1600, 1465, 930; IH NMR (DMSO-d6): 3.9 (6H, s, CH3), 7.5-8.2 (4H, m, ArH)

8-Aminomethyl-7 -hydroxy-4-methylcoumarins 24. To 1 (1.76 g, 10 mrnole) in ethanol (lOO rnL), amine (12 mrnole) and formalin (30 % ag., 1.0 rnL) were added and the solution was refluxed for 8-9 hr. The solvent was removed, the residue triturated with water and crystallized from methanol.

24a : IR (KBr) 1710, 1600, 1380, 1240, 1105, 1060; IH NMR (CDCh): 2.4 (3H, s, CH3), 2.6 (4H, t, N(CH2)2), 3.8 (4H, t, O(CH2)2), 4.2 (2H, s, 8-CH2), 6.18 (IH, s, C3H), 6.8 (lH, d, J = 9.6 Hz, C6H), 7.5 (H, d, J = 9.6 Hz, C5H), 8.3 (lH, bs, OH).

7 -A1lyloxy-8-aminomethyl-4-methylcoumarins 25 were prepared by the procedure adopted for the preparation of 5.

25a:IR(nujol) 1720, 1600, 1460, 1380, 1100; IH NMR (COCI): 2.5 (3H, s, CH3 ), 2.68 (4H, m, N(CH2)z), 3.75 (4H, m, O(CHzh), 4 .0 (2H, s, CH2-N), 4.75 (2H, d, OCH2), 5.3-6.1 (3H, m, CH=CH2), 6.2 (lH, s, C3H), 6.95 (I H, d, C6H), 7.6 (I H, d, C5H) .

Claisen rearrangement of 25 was carried out by the procedure adopted for 5. The product 13 was sepa­rated by column chromatography over silica gel using benzene as a eluent.

8-Acetyl-7-hydroxy-4-methylcoumarin 27. It was prepared by the reported procedure29

. It was al­lylated by the procedure given above for the prepara­tion of 5. The rearrangement of 28 to 29 was carried out by the procedure adopted for 5.

27: Yield 60%, m.p. 166-67°C 28: Yi eld 70%, m.p. 116-lrC (rep .28 1 19SC); IR

(KBr): 1740, 171 0, 161 0, 1500; IH NMR (CDCl): 2.4 (3 H, s. CH,), 2.6 (3H, s, COCH) ), ·1..05 (2H, d, OCH2). 5.15-6.15 (3 H, m, CH=CHz), 6.87 ( I H, d, C6H), 7.52 ( I H, d, C5H).

29: Yie ld 45%, m.p. 130-31 °C; IR (nujol) : 1745, 1710,620, 1470, 1400; 'H NMR (CDCl): 2.43 (3 H, s, CH) , 2.93 (3 H, s, COCH), 3.45 (2H, d, CH2) , 4 .9-

5.25 r3H, m, CH=CH2) . 6.1 (lH, s, C)H), 7.55 (lH, s, CsH), 14.03 ( I H, bs, OH)

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•

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