Cyclobutene, fused ring derivatives

ROMP of fused-ring cyclobutene derivatives followed by a reverse Diels-Alder reaction to eliminate a benzene compound, leaving polyacetylene (see Section 13.2). This method can also be adapted to make the centre block of a triblock copolymer with norbomene, having morphologies ranging from spherical to cylindrical to lamellar (see Section 14.4). 

The benzo-fused derivative 8 of l,4-dimethyTl,4-dihydro-l,4-diazocine can be prepared by gas phase thermolysis of the cyclobutene derivative 7,31 a reaction that can be reversed photo-chemically. In contrast to the parent system, the heterocyclic ring of the benzannulated compound displays no aromatic character. 

Finally, it is important to mention that there are other related publications in which porphyrin macrocycles are not directly used as dipolarophiles but are transformed into new derivatives that can react with carbonyl ylides via ACE (alkene cyclobutene epoxide) reactions. This idea arose in 1997, when Russell and co-workers found that fused ester-activated cyclobutene epoxides 86 can be ring-opened to give carbonyl ylides 87, and that these can be trapped stereospecifically by ring-strained alicyclic dipolarophiles, such as 2,5-norbomadiene, to form hetero-bridged norbomanes 88 in good yields, through ACE transformations (Scheme 31) <97CC1023>. 

As an application of this procedure, Snapper succeeded in the formal total synthesis of (+)-astericanolide. Treatment of iron complex 62, which is prepared from cyclopentenol derivative 60 and iron complex 61, with MeaNO gives fused cyclobutene derivative 63. ROM of cyclobutene of 63 using Ig under ethylene gas smoothly proceeds to produce an eight-membered ring of 64 via Cope rearrangement. The resulting product 64 is converted into Wender s intermediate for the synthesis of (+)-astericanolide ... 

In refluxing xylene (160 °C), fused Dewar s thiophene 40 mainly desulfurized to give cyclobutene 41 but under flash vacuum pyrolysis conditions, a mixture of isomeric dihydrothiophenes 42, 43, and 5H-benzocycloheptatriene 44 was formed (Scheme 9). Evidently the latter product derived from a formal electrocyclic ring opening of the cyclobutene intermediate 41 <1990H(31)1769>. 

Dewar thiophenes are thiiranes fused to cyclobutene rings. While the reversion of the tetrakis(trifluoromethyl) derivative to the normal thiophene structure requires a high temperature (r 1/2 160 , C Hg] 5.1 hr), the transformation can be achieved at room temperature by phosphines (86) such as Ph3P. A 1 1 adduct can be isolated when PhjPCl is added to the Dewar thiophene. However, the harder PhPCl2 and PCI3 and pentacovalent phosphorus compounds are totally ineffective for the isomerization. 

Dimethyl acetylenedicarboxylate has been added to norbornene and several of its derivatives using H2Ru(PPh3) as catalyst. The [2+ 2] adducts are formed in moderate yields all the adducts had the exo-fused cyclobutene ring. 

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