An Abnormal Claisen Rearrangement

Suggest a mechanism for the conversion of 2d into 3 and account for the increase in yield which is observed when HMDS is added to the reaction mixture. 

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Photolysis of coumarin-3-carbonylazides gives nitrenes in the triplet state, and these have been trapped by hydrocarbons and by aniline. Prenyl ethers of 3-hydroxycoumarins undergo an abnormal Claisen rearrangement and give the 4-allyUc compound (193), but, in the presence of an 8-methoxy-group, the side-chain is further rearranged as in (192). ... 

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