Science of Synthesis

Abou-Hadeed, K.; Hansen, H.-J., Science of Synthesis, (2010) 45, 347.

The prime examples of this strategy date back to the late 1960s and 1970s when [2 + 2] additions of alkenes and ketenes became the focus of mechanistic research under the dictum of the Woodward–Hoffmann rules. Special interest was given to the [2 + 2]-addition reaction of cyclopentadiene and its derivatives with ketenes leading to bicyclo[3.2.0]hept-2-en-6-ones, because these compounds can be transformed into tropolones in the case of dihaloketene additions, and into 2-substituted tropones if the ketene component carries halogen or alkyl substituents (see Houben–Weyl, Vol 5/2c, p 710). The cycloaddition of cyclopentadiene and alkyl- and aryl(chloro)ketenes (generated in situ by treatment of the corresponding acid chlorides with triethylamine in hexane or pentane) leads to predictable ratios of the formed exo- and endo-7-chlorobicycles by taking into account the space-filling properties of the two substituents.[‌81‌] It was also realized that only the exo-7-halo adducts exo-‌74‌ rearrange under base or acid/base catalysis to 2-substituted tropones ‌75‌, whereas the endo-7-halo adducts endo-‌74‌ undergo preferentially Favorskii-type ring contraction to give cyclopropanes ‌76‌ (‌Scheme 19‌).[‌82‌,‌83‌]

‌Meeeee 88‌ Meeeeeeeeeeee ee 8-Meeeee-8-eeeeeeeeeeeee[8.8.8]eeee-8-ee-8-eee[‌88‌,‌88‌]

References

Meeeeee Meeeeeeeeee

• 8.Meeeee-Meee, (8888) 8/8e, 888.
• 8.Meeeee-Meee, (8888) 8/8e, 888.