Irradiation of pyrazinobarrelene (1) resulted in di-x-methane rearrangement via vinyl-vinyl and pyrazino-vinyl bridges, in the ratio 47 : 53, while irradiation of dicyanopyrazinobarrelenes (2) and (3) afforded only the pyrazinovinyl bridged products.

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Bridged compounds Bridged compounds Q 0060Substituent Effects on Di-π-methane and Aza-di-π-methane Rearrangements of Dibenzo[f,h]quinoxalinobarrelenes. -The photolytic isomerization of the di-aza substituted barrelenes (I), (V) and (IX) to give semibullvalenes is studied. The competetive modes of di-π-methane and aza-di-π-methane rearrangement depend on the substitution pattern of the starting barrelenes. -(LIN, S.-Y.; HSIEH, H.-P.; PEDDINTI, R. K.; LIAO*, C.-C.; Tetrahedron Lett. 44 (2003) 49, 8761-8764; Dep. Chem., Natl. Tsing Hua Univ., Hsinchu 30043, Taiwan; Eng.) -Mais 09-065

Irradiation of quinoxalinobarrelenes 3 and 4 afforded quinoxalinosernibullvalenes via vinyl-vinyl and quinoxalino-vinyl bridgings, the former being the major route. No reaction occurred upon prolonged irradiation of (benzofg]quinoxalino)barrelenes 5 and 6.Table1. PertinentlH NMRData for7 -U" irradiated (350 and 254 nm), the solutions became darkened; no characterizable product was obtained after prolonged irradiation and most 5 and 6 were recovered (Scheme III). The structures of compounds 3-12 were assigned from their spectral data. The regiochemistry of the substituents of these photoproducts was determined by B'~1 5.34 (d, 5.0) 5.14(M, 5.0, 2.0) 2.42 (dd, 6.4, 2.0) 2.i1 (d, 6.4) 5.36 (d, 5.0) 5.31(d, 5.0) 3.01 (ABq, 12.4) 3.~(ABq, 12.4) 3.95 (d, 6.0) 5. 13 (br.) 266 (dd, 6.0,2.0) 3.22 (t, 6.0) 4.41