4-t-Butyldimethylsiloxy-l-benzopyrylium salt (2,) was prepared in situ from chromone (1) and t-butvldimethvlsilvl triflate and the salt reacted with enol silyl ethe?s, ketene silyl acetals and active methylene compounds to afford 2-substituted 4-t-butyldimethylsiloxy-2H-l-benzopyrans (33, in high yield. These products reacted with acid chlorides and an iminium salt to give 2,3-disubstituted 2,3-dihydro-4H-1-benzopyran-4-ones.
A variety of substituted 4H-l-benzopyran-4-ones (chromone) have been reported to have biological activity 1 and in addition have been used as synthon for more complex heterocycles. 192 Recently alkyl copper boron trifluoride complex was reported to effect the conjugate addition to chromone.
In this case, however, the substituents introduced into the 2-position on chromone were limited to alkyl groups.
3 We now report that 2-oxoalkyl groups are introduced in high yield by the reaction of 4-siloxy-1-benzopyrylium salt with enol silyl ethers or active methylene compounds. Furthermore the product