SYNTHESIS OF O-QUINODIMETHANES AND BENZOCYCLOBUTENES FROM DIMETHYL SQUARATE

Abstract Selected 3-alkylidene(and benzylidene)-4-allenylcyclobutenes were shown to undergo an unusual thermal ring expansion to o-quinodimethanes and thus to benzocyclobutenes upon electrocyclic ring closure. The mechanism of this rearrangement is envisaged to involve ring opening of the starting cyclobutenes to the corresponding octa-1,2,4,6,7-pentaenes which lead to the quinodimethanes upon ring closure.

The starting allenyl-substituted alkylidenecyclobutenes 10,12a, b, 14 stem from dimethyl squarate 7 which was readily converted to 2-methoxy-3-phenylcyclobutenedione 8 by standard methods (Scheme 2).4 Wittig olifination of 8 with benzylidenetriphenylphosphorane in anhydrous ethyl ether at ambient temperature gave 9a (6085% isolated yield) as a 1.7:1 mixture of (E)-and (Z)-isomers. The stereochemistry of these isomers was readily assigned from their TH NMR spectra which showed the expected relatively greater deshielding of the vinyl proton absorption of the E-isomer (5, 6.48) vs. the Z-isomer (6, 6.38).5 Both isomers were observed to be stable, having an indefinite shelf life when stored at -2o'C. The (E)-and (Z)-alkylidenecyclobutenones 9b were prepared by the Peterson olefination using the lithium salt of trimethylhexylsilane.
Addition of this reagent to 8 proceeded smoothly to give the diastereomeric alcohols in 59% isolated yield. Treatment of these with 8F3-etherate in CH2Cl2 at 0 'C then gave 9b as a mixture of (E)-and (Z)-isomers in 62% isolated yield. Unlike the benzylidene analogs (i.e., 9a), the alkylidenecyclobutenones 9b are very unstable and must be used immediately.6 Thecydobutencnes 9a and 9b were converted directly to the corresponding isomeric mixtures of the cyclobutenes 10, 12r and 14 in 47-5896 yield upon treatment with the respective liihiiallene reagent.7 The two possible diastereomers of 10 were formed in a ratio of approximately 3:2 and independently subjected to thermolysis in refluxing pxylene for 5-7.5 h. As expected, based upon the mechanism outlined in Scheme-l, they resulted in the benzocyclobutene 11, mp, 58-59.5 'C (49-82%), the structure of which was unambiguously established by a single crystal X-ray analysis. In a similar manner 12a and 12b gave the the respective benzocyclobutenes 13a (49-64%) and 13b (35%), the structures of which are based upon characteristic spectral data.8 A varient of the rearrangement was observed for the thermolysis of the isomerlc mixture of 14. Here, the benzocyclobutene 15 was isolated in 30% yield along with the phenol 16 in 5% yield. The former appears to be the primary precursor to the latter as evidenced by the fact it decreases in yield as 16 increases with prolonged thermdysis time. This is envisaged to involve an equilibrium between the benzocyclobutene 15 ring opened oquinodimethane which then leads to the phenol 16 by a 1 ,5-hydrogen shift.
In conclusion, the ring expansion of 3-alkylidene+allenylcyclobutenones reported here provides a potentially general route to highly substituted benzocyclobutenenes.
In this regard, the synthesis starts with dimethyl squarate which can be conveniently converted to a variety substituted cyclobutenediones and these in turn to the synthetically useful qulnodimethane intermediates.

6.
The instability of the alkylidenecyclobutenones Sb oxygen in an ene reaction at the alkylidene site.