Prismanes photochemical reactions

In addition to the information on specific lower prismanes, a few existing studies on higher prismanes and on the whole family should be mentioned. Photochemical reactions leading to [7]prismane analogues have been explored experimentally and by MM2 calculations by Mehta et al. (41b). A D4j structure has been proposed for [8]prismane (97) by Reddy and Jemmis (128) while Jemmis et al. (132) and Miller and Schulman (84) on the basis of MNDO and AMI calculations argued at the semiempirical level that this structure is not a local minimum. The calculation of geometries and vibrational frequencies of [n]prismanes (n = 3,...,9) has been carried out by Disch and Schulman (84) under symmetry constraint using STO-3G, 3-21G, and 6-31G basis... 

In this chapter, prototypes of the photochemical reactions useful for the synthesis of cage compounds are outhned first, and then photochemical approaches towards the synthesis of [n]prismanes as representative cage compounds are reviewed. 

Tetrafluoropyridazine is photochemically isomerized to tetrafluoropyrazine and thermally to tetrafluoropyrimidine. These reactions are proposed to proceed via the corresponding Dewar and the prismane analogs, and benzvalene analog, respectively (150) 162). 

The homolog of prismane, quadricyclane (29) was obtained through the homo-Diels-Alder reaction between diethyl azodicarboxylate and norborna-2,5-diene, ° although the deazetization has to be carried out photochemically. Thermal deazetization of the diazene 28 (and of closely related diazenes ) results in a reversion to the homodiene. 

In this context, 138 was converted into propella[34]prismane (179) via an intramolecular cycloaddition reaction, after the stabilizing CpCo-group had been removed photochemically (Fig. 46a) [112, 162],... 

Extrusion of Nitrogen.— Two important photochemical nitrogen extrusion reactions deserve mention. The first was in the synthesis of the highly strained prismane (188) in low yield by irradiation of the azo-precursor (187) in CD2CI2 at temperatures above 30 The second was in the conversion of the pyrazoline... 

As already described (Chapter 22, Scheme 34), Prinzbach et al. reported benzo/benzo [6 -I- 6]-photo-dimerization of benzene rings (70 to 71), and this is the second example of the photodimerization of benzene rings. Recently, they proposed a possible synthetic route of pagodane-condensed prismane derivatives 74 via a photochemical benzo/benzo [6-1-6] -photoreaction of 72 followed by a photochemical [4 -I- 4]-cycloaddition reaction of 73 (Scheme 9) °... 

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