Uncatalyzed addition of N-(tert-butoxycarbonyl)-2-tert-butyldimethylsilyloxypyrrole to activated quinones

The uncatalyzed addition of N-(tert-butoxycarbonyl)-2-tert-butyldimethylsilyloxypyrrole 3 to 1,4-quinones bearing an electron withdrawing group at C-2 is reported. Use of 2-methoxycarbonyl-1,4-benzoquinone 4 and 2-methoxycarbonyl-1,4-naphthoquinone 5 provided an efficient synthesis of the pyrrolidinobenzofuran adduct 9 and the corresponding pyrrolidinonaphthofuran adduct 10, respectively, whereas use of 2-acetyl-1,4-benzoquinone 6, 2-acetyl-1,4-naphthoquinone 7 and 2-acetyl-8-methoxy-1,4-naphthoquinone 8 formed silyloxypyrroles 11, 12, and 13 resulting from direct electrophilic substitution of the silyloxypyrrole by the electrophilic quinone. Addition of PPTs to the reaction of 2-acetyl-8-methoxy-1,4-naphthoquinone 8 with N-(tert-butoxycarbonyl)-2-tert-butyldimethylsilyloxypyrrole 3 afforded pyrrolidinonaphthofuran adduct 14 which then underwent smooth oxidative rearrangement to pyrrolidino pyranonaphthoquinone 15, thereby providing a novel approach for the synthesis of aza-analogues of the pyranonaphthoquinone antibiotics such as kalafungin 16.