Photochemical cycloadditions sigmatropic shifts

In contrast to tetracyclo[4.3.0.0 .0 ]non-8-eneJ the corresponding 5-benzhydrylidene and 5-isopropylidene derivatives 15 (R = Ph, Me) do not undergo photochemical [27t + 2o-] cycloaddition but a formal [ 2 + 2 + 23] (retro-ene) reaction to furnish the substituted 5-methylenetricyclo[4.3.0.0 ]nona-3,7-dienes 16. The isopropylidene derivatives 17 also rearrange via a 1,3-sigmatropic shift and retro-Diels-Alder reaction to give the bridged cyclooc-tatrienes 18. 

Several rearrangements of 1, 5-dienes have already been discussed above they demonstrate that the reaction path depends on nature of the photochemically excited states [S or ]. The unsensitised (Sj) photochemical reaction lead to the two possible [1, 3] sigmatropic shifts to produce (1) and (3), while photolysis is presence of triplet sensitiser results in cross cycloaddition to give (2a) and (2b). 

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

Just as there are secondary interactions in cycloadditions, so too are there ancillary orbital-symmetry effects in sigmatropic reactions. In process (79), Jones and Jones (1967) find no products of [1,3] hydrogen or methyl shifts, e.g. 1,4,7-trimethylheptatriene, which ostensibly (Table 5) are photochemically allowed. They point out that in the first... 

Photochemical cyclization of l-(lV-methylanilino)cycloalkenes gives primarily trans-fused hexahydrocarbazole derivatives 170 (68JA5329). This cyclization may involve a conrotatory electrocyclization of divinylamines, followed by a suprafacial [l,4]-sigmatropic hydrogen shift (70CC531). The intermediates in the initial cyclization are believed to be cyclic azomethine ylide 1,3-dipoles 169, but no direct observation of the participation of such dipoles was made. All attempts to capture dipoles 169 by cycloaddition with maleic anhydride or furan were unsuccessful (71JA2918). 

Scheme 8. Enantioselective Photoreactions in TADDOL Inclusion Compounds with a Cou-marin, a Methacryl Anilide, and an Oxocyclohexenyl-carboxamide. In the first case, the packing of the coumarin molecules in the mixed crystal is such that the double bonds are predisposed for the (2+2) cycloaddition. In the second example, a photochemical electrocychc reaction is followed by a sigmatropic H shift. The third reaction is an intramolecular (2+2) cycloaddition with dia- and enantioselective formation of three new stereogenic centers. There are several more reactions of this type, described in the literature [54], and the Toda group has determined the crystal structures of a number of inclusion compounds to show the correlation between the crystal packing and the configuration of the photoproducts. EMastereoselective solid-phase reactions of chiral guests in TADDOL-host lattices have also been described by the...

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