Oligomers, ribbon-type

If the oxo (or methano) bridges are not exclusively syn to one another in either the bis-dienophiles or bis-dienes, then the pressure-induced repetitive Diels-Alder reactions (proceeding again highly stereoselectively) produce rigid ribbon-type oligomers on a nanometer scale (Scheme 10 entry 1). Bis-diene 45 reacts less stereoselectively than bis-diene 44 and forms with bis-dienophiles such as 46 the ribbon-type oligomers 47... 

Stoddard and co-workers [45-47] describe the synthesis of cyclic and linear ribbon-type oligomers starting from the monomers 21 [39] and 23 [42]. The double stranded macrocycles, e.g. 27, generated are intermediates in the preparation of cyclic oligoarenes (cycloacenes) - attractive compounds with two dimensional cyclic J7-systems of the Huckel-type. Synthetic approaches to remove the oxo-bridges reductively and to generate the final fully unsaturated hydrocarbons, lead only to intermediates, e.g. 28, which are partially hydrogenated [46]. 

SCHEME 10. Ribbon-type oligomers and polymers via repetitive Diels-Alder reactions... 

Scheme 2.15. Synthesis of ribbon-type oligomers by means of pressure-induced repetitive Diels-Alder reactions.

The syntheses of the first sulfonio-bridged, p-phenylene ladder oligomers (44) that have a planar ribbon-type structure have also been reported (60). 

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