Benzopyrylium triflates

Benzopyrylium triflates formed in situ from 3-formylchromones react in a sequential fashion with silyl enol ethers to give chromones such as 111 (Scheme 7) <2003TL7921, 2006T7674, 2002CC168, 2006CEJ1221, 2006T9694, 2006EJO3638>. 

Benzopyrylium triflates (e.g., 122) can be reacted with diazoesters to give cyclopropanes that are readily converted to 2,3-benzoxepins such as 123 and related heterocycles (Scheme 10) <2005TL4057>. 

Ring enlargement of chromone to benzoxepins has been reported (see also Section 7.07.5). The in situ generation of benzopyrylium triflate 335 by reaction of 334 with Me3SiOTf followed by addition of ethyldiazoacetate and catalytic copper(ll) triflate yielded cyclopropane 336 as a single diastereomer. Treatment of compound 336 with trifluoroacetic acid resulted in the formation of 2,3-benzoxepin 337 (Scheme 47). 

An account of domino reactions of benzopyrylium triflates with l,3-bis(silyl enol ethers) has been published <07SL1016>. 

As shown in Scheme 60, treatment of chromones 299b-c with f-butyldi-methylsilyl triflate affords the 1-benzopyrylium triflates 45b-c. Reaction of 45b-c with 3-(trimethylsilyl)-l-butene gives the cyclopentane annulation products 49a (87%) and 49b (37%) along with a small amount of the allyla-tion product 48 from 299c (90CL1725 91JOC2058). 

Silylation of Chromones The preparation of 4-r-butyldimethylsilyloxy-l-benzopyrylium triflate is carried out by heating chromone and TBDMS triflate at 80 °C for 1 h (without solvent) under nitrogen (eq 9). The silylated chromones undergo addition reaction with enol silyl ethers and... 

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