addition in allylsilanes and silyl enol ethers. TiCl4 catalyzes 1,2-addition and ZnCl2 1,4-addition in silyl ketene acetals regardless of the substitution pattern. Only silyl enol ethers and silyl ketene acetals are reactive with diester 5 via 1,4-addition. In addition to the above carbonyl group activation, the ac-etal 10 is also a useful candidate for generating the electrophilic center under these conditions. Thus, besides typical organosilanes, azide functions can be introduced with BF3-catalysis (vide infra).

Similar electrophilic Friedel-Crafts-like reactions allow the most reactive dichloride 17 to furnish 1,4-diarylcyclobutenedione derivatives [44,45]; for example, 1,4-thieno[3,2-fr]pyrrole-substituted cyclobutenedione 23 was prepared by this method and an oxygen-inserted conjugation system 24 was attained as a photochromic devise [46] (Scheme 4).

Apart from these methods based on the squaric acid family, direct formation of cyclobutenedione rings by [4 + 2] and [2 + 2] cycloaddition reactions is a plausible approach to variably substituted 4-hydroxy-2-cyclobutenone systems [47-54].

squaric acid family

R1 R2

17 CI CI

18 CI OMe

19 CI NEt2 5 OEt OEt unsaturated organosilanes

TiCI4 1,2-addition

TiCI4 1,2-addition unsaturated organosilanes


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