J. Chem. Research (S), 1997, 34 J. Chem. Research (M), 1997, 0321–0335 Photocycloaddition of Cyanoethylenes onto 1,4-Dihydroand 1,4,5,6-Tetrahydro-pyridines Donato Donati, Stefania Fusi and Fabio Ponticelli* Istituto di Chimica Organica, Universit`a di Siena, pian dei Mantellini, 44, 53100 Siena, Italy The photochemical cycloaddition of cyanoethylenes onto the title compounds to give 2-azabicyclo[4.2.0]octanes shows a different degree of selectivity depending on the position of the chiral centre of the starting material, with a maximum effect in the case of the 4-position.Previously we reported that some enantiomeric induction is observed during the photoaddition of acrylonitrile onto 1,4-dihydropyridines carrying an easily removable tetraacetylglucoside substituent at the 1-position.1. In view of the continued interest in asymmetric synthesis, we now report a further insight into the above reaction, by considering the effect of the location of the chiral centre with respect to the C(2)–C(3) double bond.Photochemical addition of acrylonitrile onto dihydropyridine 2 followed by catalytic hydrogenation of the reaction mixture yielded two pairs of the diastereoisomeric amides, 3 and 4, with a low diastereoisomeric excess (d.e.) (5–15%). We then considered the 1,4-dibenzyl-1,4-dihydropyridine 5, but found it not suitable for our aims since upon irradiation it rearranges to the 1,6-dibenzyl-1,6-dihydropyridine 7. However, catalytic hydrogenation of 5 gave the tetrahydro derivative 8, which by acrylonitrile photoaddition gave nearly exclusively the 8-cyano-2-azabicyclo[4.2.0]octanes 11 and 12, showing a high degree of regio- and stereo-chemical control due to the 4-substituent.The structures of compounds 11 and 12 were assigned on the basis of 1H and 13C NMR chemical shift considerations and NOE experiments. In view of the synthetic potential of this reaction, a deeper insight into the mechanism was required.To this purpose we verified that the reaction requires a non-symmetrically substituted electron-deficient alkene. In fact, neither fumaronitrile nor vinyl ether underwent photoaddition onto 1,4-dihydropyridines. In addition, when we irradiated ethyl 1-benzyl- 1,4-dihydronicotinate in the presence of (Z)- or (E)-but- 2-enenitrile we obtained, after hydrogenation, the 2-azabicyclo[4.2.0]octanes 13, 14 or 15, 16, respectively. It is worth noting that the cross-over products, i.e. 13, 14 from the E-isomer or 15, 16 from the Z-isomer, were not formed. The structures of the above compounds were assigned on the basis of NMR, NOE and observed and calculated10 LIS data. In conclusion, alkene geometry is retained during the photoaddition onto the pyridine system, suggesting a concerted process and excluding long-lived radicals as intermediates. We thank the Ministero dell’Universit`a e della Ricerca Scientifica e Tecnologica, Rome (quota 60%) for financial support and Dr G.L. Giorgi, Centro di Analisi e Determinazioni Strutturali, University of Siena, Italy, for the recording of mass spectra. Techniques used: 1H and 13C NMR, IR, polarimetry, MS and HRMS References: 10 Schemes: 4 Figure 1: Stereoview of the conformation of 14 Table 1: 1H NMR data for 2-azabicyclo[4.2.0]octanes 3, 4, 11–18 Table 2: 13C NMR data for 2-azabicyclo[4.2.0]octanes 3, 4, 11–18 Table 3: Observed and calculated LIS data for 13–18 (17 and 18 are stereoisomers of 14/16). Received, 30th July 1996; Accepted, 29th November 1996 Paper E/6/05331I References 1 G. Adembri, D. Donati, S. Fusi and F. Ponticelli, J. Chem. Soc., Perkin Trans. 1, 1992, 2033. 10 J. Paasivirta, in Lanthanide Shift Reagents in Stereochemical Analysis, VCH, New York, 1986, pp. 119–126, 145–150. 34 J. CHEM. RESEARCH (S), 1997 *To receive any correspondence. Scheme 1 Reagents and conditions: i, CH2�CHCN, hv; ii, H2 Scheme 2 Reagents and conditions: i, H2/Pd; ii, hv; iii, CH2�CHCN, hv Scheme 3 Conditions: i, irradiation with (Z)-but-2-enenitrile, catalytic hydrogenation; ii, irradiation with (E)-but-2-enenitrile, catalytic hydrogena