Cyclobutenylideneammonium salts

The cyclobutenylideneammonium salts (2) and (4) are reactive dienophiles. Thus (4, X = BF4 ) reacts with cyclopentadiene at 20° to give the adduct (5) in 84% yield. The adduct can be hydrolyzed to the corresponding cyclobutanone. 

Cyclobutenylideneammonium salts, and hence cyclobutenones, are formed by the [2 + 2] addition of ketenimmonium salts to acetylenes. The reaction of dimethylketenimmonium tetrafluoroborate with the acetylenes gives the salts (205) in good yield, but the reaction is not highly regioselective. The salts are, themselves, excellent dienophiles, and may be used directly in reactions with dienes. The salt (206) gives the adducts (207) and (208) with butadiene or cyclopentadiene. Dimethyl-cyclobutenone (209) has also been prepared by the adcfition of dimethylketen to methyl vinyl sulphide, followed by methylation at sulphur and elimination of dimethyl sulphide. The ketone itself is a useful dienophile, but is less reactive than its imminium salt. The cyclobutenone (209) is functionally equivalent to terelactone which is an extremely reluctant dienophile. 

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