The photolysis of N‐alkylpyridinium halides 9a–e in alkaline H2O gave 6‐azabicyclo[3.1.0]hexenol derivatives 10a–e. N–Substituents bearing ether, acetal, and alcohol functions were found to do not adversely influence the photochemical reaction course. The free OH groups of the N–(3‐hydroxypropyl) derivative 10d were protected by benzoylation. The ensuing dibenzoate 14 underwent stereocontrolled opening of the aziridine ring on reaction with MeSH/BF3 to give a thioether 15. With benzoic acid in CHCI3, 10d gave the 4‐hydroxy‐5‐aminocyclopent‐2‐enyl benzoate 11. The meso‐2‐aminocyclopent‐4‐ene‐1,3‐diol 12 was obtained by hydrolysis of 11. On reaction with Boc2O and NaI, the aziridine ring of 14 was converted to a bicyclic compound 17. Hydrolysis of 17 provided the trans‐1,3‐diol 18, the epimer of 12. Face‐selective dihydroxylation of Boc‐protected 12 gave a meso‐aminocyclopentanetetrol 23 which was characterized upon peracetylation. Dihydroxylation of 15 provided a racemic analogue of epi‐mannostatin A (26).

The title compounds 4 are obtained by photolysis of simple N‐alkylpyridinium salts in H2O or alcohol. On reaction with [Fe2(CO)9] in THF, 4 gives bicyclic tricarbonyliron complexes 13a–d, which on oxidative decomplexation with ceric ammonium nitrate afford cis‐fused cyclopenteno‐β‐lactams 15a–d.

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Bicyclic aziridines formed by the irradiation of pyridinium salts in basic solution have recently been recognized to have great synthetic potential. We have undertaken a joint computational and experimental investigation of the mechanism of this photoreaction. We have computationally determined the structures and relative energies of the relevant stationary points on the lowest potential energy surface (PES) of the pyridinium and methylpyridinium ions. Two important intermediates are shown to be bound minima on the ground-state PES: azoniabenzvalene and a 6-aza[3.1.0]bicyclic ion with an exo-oriented substituent (analogous to prefulvene). We advance a mechanism which involves initial formation of this exo-bicyclic ion, followed by nitrogen migration around the ring via the azoniabenzvalene intermediate. Thus, the barrier separating the two intermediates is the factor that determines the degree of scrambling observed in the photoproducts when the carbon atoms are labeled with deuterium or substituted with additional methyl groups. For N-methylpyridinium, the exo-methyl bicyclic ion was computed to be approximately 1 kcalmol(-1) lower in energy than N-methyl-azoniabenzvalene. The transition state was computed to lie several kcal mol(-1) above the exo-methyl bicyclic ion (+8.4kcalmol(-1), 6-31G* RHF; +3.7kcalmol(-1), 6-31G* B3LYP), but still well below the energy available from the 254 nm excitation of the N-methylpyridinium ion. The computed relative energies correspond splendidly with several experimental findings which include the preference for exo products, the results of deuterium labeling, and the impact of additional substituent methyl groups on the product distribution.

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