Scope and limitations of the double [4+3]-cycloadditions of 2-oxyallyl cations to 2,2 '-methylenedifuran and derivatives

The reactivity of various 2-oxyallyl cations toward 2,2'-methylenedifuran (1b), 2,2'-(hydroxymethyl)difuran (1c), 2,2'-(trimethylsilylmethylene)difuran (1d), and di(2-furyl)methanone (1e) has been explored. Difuryl derivatives 1c, 1d, and le refused to undergo formal double [4+3]-cycloadditions. Conditions have been found to convert 1b into meso-1,1'-methylenedi[(1R,1'S,5S,5'R)- (3) and (+/-)-1,1'- methylenedi[(1RS, 1'SR,5SR,5'RS)-8-oxabicyclo[3.2.1]oct-6-en-3-one] (4) that do not require CF3CH(OH)CF3 as solvent. High yields of meso-1,1'-methylenedi[(1R,1'S,2S,2'R,4R,4'S,5S,5'R)- (5) and (+/-)-1,1'-methylenedi[(1RS,1'RS,2SR,2'SR,4RS,4'RS,5SR,5'SR)-2,4-dimethy l-8-oxabicyclo[3.2.1]oct-6- en-3-one] (6) have been obtained when 1b was reacted with 2,4-dibromopentan-3-one (7h) and NaI/Cu.