2.4- Disubstituted-benzo pyrans

Various 1,3-disubstituted benzo[c]pyran-5,8-diones are accessible by rearrangement of the transient Diels-Alder adducts from styrene and 2-substituted 1,4-benzoquinones <01T8653>. 

Yamashita, M., Inaha, T., Nagahama, M., Shimizu, T., Kosaka, S., Kawasaki, I., and Ohta, S. (2005) Novel stereoconvergent transformation of l,2a-disubstituted l,2,2a,8b-tetrahydro-3H-benzo[b] cydobuta[d]pyran-3-ones to 1,3-disubstituted l,2,4a,9b-tetrahydro-dibenzofuran-4-ols and its application to the second-generation synthesis of ( )-linderol A. Organic el Biomolecular Chemistry, 3, 2296-2304. 

Cleavage of the hetero ring of dibenzofuran by Li followed by the addition of ketones offers a useful route to 6,6-disubstituted dibenzo[/>,<7]pyrans 13 <06JOC8291> and a photochemical rearrangement of naphtho[2,3-b]benzofuranones was used to prepare intensely fluorescent indeno[ 1,2-/>]benzo[4,5-e]pyranones 14 <060BC3406>. 

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