6jr-Electrocyclizations

A related 1,7-cyclization has been invoked to account for the formation of diazepine 292 from the electrophilic diazoalkane substitution reaction of ethyl diazoacetate and dimethyl diazomethylphosphonate, with the 2,4,6-trimethylpyr-ylium salt 289 (337) (Scheme 8.70). While the expected 4-(diazomethyl)-4/f-pyran 290 could be isolated (20-22%), the 2-substituted isomer 291 was not. It was proposed that this latter species underwent 6jr-electrocyclic ring opening followed... 

The reaction of 2-(phenylamino)- and 2-(dimethylamino)-thiazoles (38) with DMAD produced dimethyl 6-(phenylamino)- and 6-(dimethylamino)-3,4-pyridinedicarboxyl-ates exclusively. The intermediate 2 + 2-cycloadduct (39) undergoes ring opening, 6jr-electrocyclization with the extrusion of sulfur to produce the final pyridine derivatives (40) (Scheme ll).38... 

Scheme 26 Dibenzo[2.2]paracyclophanedienes by fourfold Heck reactions followed by two independent 6jr-electrocyclizations [94,95]...

Scheme 30 Preparation of ring-annelated arenes [108] and cyclohexadienes [110] by intra-intermolecular cross couplings with subsequent 6jr-electrocyclizations...

Scheme 31 Intra-intermolecular cross-coupling-6jr-electrocyclization sequences incorporating bicyclopropylidene (37) [111]...

Scheme 33 A domino cross coupling-6jr-electrocyclization reaction featuring a 4-exo-dig cyclization [112,113]...

The strategy of a vicinal twofold Heck reaction was applied on tetrabromo[2.2]para-cyclophanediene 164. After its fourfold coupling with styrene (165-H) or substituted styrenes 165-R, the products 166 can be 6jr-electrocyclized with subsequent aromatization (cf. Scheme 3-21) to the benzannelated [2.2]paracyclophanedienes 167, molecules with eight orthogonal biphenyl moieties (Scheme 3-43) [196a]. 

For the construction of highly condensed polycyclic skeletons, the cascade consisting of two intramolecular Heck-type reactions and a 6jr-electrocyclization as performed with the... 

Scheme 10 Tandem 4jr-6jr electrocyclic concerted process with a dihydropyridine ring [83]...

Howard and Woerpel have also reported silylene transfer to a-ketoesters using dimethylsilirane 21 derived from f-Bu2SiCl2 and isobutene. This results in direct formation of silalactones via 6jr-electrocyclization and subsequent Ireland-Claisen rearrangement. The silalactones underwent hydrolysis upon treatment with HF to provide the desired a-hydroxy acids (eq 13). ... 

An intramolecular [4+2] cycloaddition reaction is also observed when the naphthyl derivative 172 is generated in situ. The cycloadduct 173 is obtained in 40 % yield. As a byproduct the acridine derivative 174 is obtained in 37 % yield. The formation of 174 is the result of a [1,5]H shift followed by a 6jr electrocyclization reaction... 

Isomerism along the backbone of cw-poly(arylacetylene)s can arise from the different dihedral angles that are possible about the C-C bonds of the main chain. The transoid (s-trans) conformation is more extended, whereas the cisoid (s-cis) conformation is more compact and both are helical [126-129]. Degradation of the main chain structure occurs through cyclization reactions of the polyene backbone [126-129, 137-142], which require the polymer to adopt a cis-cisoidal conformation. Scheme 3 illustrates the stmctural transformations that degrade the polymer backbone. 6jr-Electrocyclization of triene segments in the backbone occurs under... 

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