Expanding rings

Given stringent requirements for effective sensitizers and the desire to use wavelengths further to the red for therapeutic appHcations, definition of newer sensitizers has been a principal area of research since about 1987. Expanded theoretical and experimental understanding of photophysics has been a key element in identifying new classes of potential sensitizers (93—98). Research has focused on cationic derivatives of Nile Blue (93), metaHo-phthalocyanines (94), naphthalocyanines (95), chlorin-type compounds (96), expanded ring porphyrinoids (97), as well as porphyrins other than hematoporphyrin and its derivatives (98). This work has also been reviewed (10,91). Instmmentation for photodynamic therapy has been reviewed (99). 

The one-dimensional geometry of a radially expanding ring is perhaps the simplest for considering fundamental aspects of the fracture and fragmentation process. In a ductile metal ring, fracture proceeds through the multiple... 

Figure 8.20. Cumulative number distribution of fragments from four expanding ring experiments (10 fragments each) and comparison with one-dimensional theoretical distribution based on Poisson statistics.

In the numerical calculations, an elastic-perfectly-plastic ductile rod stretching at a uniform strain rate of e = lO s was treated. A flow stress of 100 MPa and a density of 2700 kg/m were assumed. A one-millimeter square cross section and a fracture energy of = 0.02 J were used. These properties are consistent with the measured behavior of soft aluminim in experimental expanding ring studies of Grady and Benson (1983). Incipient fractures were introduced into the rod randomly in both position and time. Fractures grow... 

Figure 8.22. Comparison of numerical cumulative fragment size data and aluminum expanding ring data.

Moreover, the frequent need for further elaboration of the ring expanded or ring expanded-ring contracted products required that they have functional groups suited for subsequent chemical transformations. 

Cesium fluonde in dimethylformamide catalyzes the isomerization offluori-nated cyclobutenes, perfluorobipyntmdines, and their oligomers to products with expanded rings [30, 31, 32] The product distribution in cobalt tnfluonde fluorina-tion depends strongly on the temperature of the reaction [33] Fluorinated 1-dimethylamino-5,6,7,8-tetrafluoro-l,4-dihydro-l,4-ethenonaphthalene rearranges in protic media to a biphenyl derivative [34] (equation 8)... 

By external stimulus at the wire electrode, a reaction is initiated which propagates from the center in all directions evenly and so forms an expanding ring [68]. 

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

Radicals more stable than 157 should be formed on thermolysis of l-vinyl[2.2]paracyclophane (164) and trans-/S-carbomethoxy-1 -vinyl-pi. 2]paracyclophane (165) 113>. When heated at 165 °C, crystalline 164 gave the expanded ring cis-[4.2]paracyclophan-l-ene (168) in 90% yield photolysis of 164 in methanol at room temperature also gave (13%) 168. When heated at 100 °C in benzene, trans-765 gave cis-3-carbo-methoxy[2.2]paracyclophan-l-ene (169). 

Since cis-trans isomerization of the dimethylcyclohutanes is observed, the same arguments about expanded ring versus biradical transition... 

As reported above, rotaxane 82 with the expanded ring 81 disassembles in solution. The reverse process, i.e. synthesis in the melt, turned out to be successful. 

Fig. 13 Types of intramolecular nucleophilic attack on P-lactams leading to expanded rings...

Several plants in the Liliaceae, notably the genus Veratrum (Liliaceae/Melanthiaceae), contain a remarkable group of steroidal alkaloids in which a fundamental change to the basic steroid nucleus has taken place. This change expands ring D by one carbon at the expense of ring C, which consequently becomes five-membered. 

In like manner, the oxygen atom of a purine 7-iV-oxide may be incorporated into an expanded ring, and theophylline 7-iV-oxides (501) give rise to pyrimido[5,4-c][l,2,5]oxadiazines (502) on heating (Equation (107)) <66LA(692)134,66LA(699)145,67FRP90508). 

Cyclopropenes and cyclopropenones can also be used as precursors for expanded ring systems. The intermediates of the ring enlargement reactions are generated by 1,3-dipolar addition or by a [2+2] addition across the cyclo-propenyl jr-bond [55]. The reaction principle is summarized in Scheme III/7. In the addition of a 1,3-dipole (III/42) to a cyclopropene, III/41, the bicyclic compound III/43 is formed. Depending on several factors the primary reaction product, III/43, may be stable. In cases where III/43 is formed from cyclo-propenone, it may eliminate carbon monoxide to yield the five-membered III/44. An alternative reaction possibility for III/43 is its spontaneous rearrangement to the monocyclic compound, III/45. Acylic decomposition products of compound III/43 are known, too. 

Treatment of cyclopropenones with isonitriles [56] [57] and of triafulvenes with isonitriles [58] gives cyclobutenes. Formations of expanded rings are observed by the reactions of enamines with diphenylcyclopropenones [59] [60]. 

The work on stannacycloalkanes and -cycloalkenes up to 1972 has been reviewed (9), and again, though in less detail, in 1982 (10). Most of the early studies concern the formation of five- and six-membered rings, and claims for smaller systems in particular should be treated with caution. More recent efforts have been directed to the synthesis of both strained and expanded rings. The synthetic methodology, however, remains dominated by the use of difunctional carbanion sources (Grignard and lithium reagents) or the hydrostannation reactions of tin dihydrides. 

In contrast, when a larger tetracationic cyclophane ring was employed in another set of experiments, the formation of new [3]-catenanes was found to be more facile. ° The expanded ring was obtained by replacing the phenylene linker units in the normal tetracationic cyclophane with biphenylene units. This larger ring was found to form readily around two threaded crowns when the latter are present in the acetonitrile reaction mixture in several-fold excess. Both bis-p-phenylene-34-crown-lO and l,5-dinaphtho-38-crown-10 were successfully employed in separate reactions to produce the corresponding [3]-catenanes in 20 and 31% yields, respectively. 

Under these conditions, the 1,2,4,5-tetraastannacyclohexanes do not give expanded rings by insertion of the alkyne into me Sn Sn2 and Sn4Sn5 bonds, but instead add me... 

Figure 17. Structure of the porphyrinoid [26] expanded ring system.

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