S. R. Ghantwal, S. D. Samant
Nov 1, 1999
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0
Influential Citations
4
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Journal
ChemInform
Abstract
7-Hydroxy-4-methylcoumarin 1 has been brominated under different conditions to obtain the corre~poniling 3-bromo2; 3,6-dibromo- 4; 3,8-dibromo- 3; and 3,6,8-tribromo- 8 derivatives. The 2-4 are allylated and subjected to Claiseh -rear ran ge ment 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 migra tion 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 re mains int ac t. The Claisen rearrangement of 7-allyloxycoumarins is intere sting as it provides an entry into naturally oc curring 6,7- and 7,8-furocoumarins I . The rearrange·· ment is sensitive with respect to the substituents pres ent on the ring. There is a high tendency for the mi gration to the 8-position 2 . The 4-methyV phenyl-7allyloxycoumarin undergoes rearrangement to the 8position 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 bro mine 4 .5 or iodine 6 . 7 at the 8-position, it is reported that the group is lost, but th e migration takes place to the 6-position . The rearrangement of 7-allyloxy-3,4dihydrocoumarin takes pl ace at the 6-position 8. In the present work we have studied the Claisen rearran gement 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 vative 9 and with excess of Br2 3 and 4 10 . 11 are ob tai ned . When 1 was reacted with Br2 in acetic acid, dioxane, chloroform, and carbon tetrachlorid~ at R.T.,