Diastereo- and enantioselective intramolecular [2+2] Photocycloaddition reactions of 3-(ω'-alkenyl)- and 3-(ω'-alkenyloxy)-Substituted 5,6-Dihydro-1H-pyridin-2-Ones

Abstract: 3-(ω'-Alkenyl)-substituted 5,6-dihydro-1H-pyridin-2-ones 2-4 were prepared as photocycloaddition precursors either by cross-coupling from 3-iodo-5,6-dihydro-1H-pyridin-2one (8) or-more favorably-from the corresponding α-(ωalkenyl)-substituted δ-valerolactams 9-11 by a selenylation/elimination sequence (56-62% overall yield). 3-(ω'- Alkenyloxy)-substituted 5,6-dihydro-1H-pyridin-2-ones 5 and 6 were accessible in 43 and 37% overall yield from 3-diazopiperidin-2one (15) by an ααa-chloroselenylation reaction at the 3-position followed by nucleophilic displacement of a chloride ion with an ω-alkenolate and oxidative elimination of selenoxide. Upon irradiation at λ = 254 nm, the precursor compounds underwent a clean intramolecular [2+2] photocycloaddition reaction. Substrates 2 and 5, tethered by a twoatom chain, exclusively delivered the respective crossed products 19 and 20, and substrates 3, 5, and 6, tethered by longer chains, gave the straight products 21-23. The completely regio- and diastereoselective photocycloaddition reactions proceeded in 63-83% yield. Irradiation in the presence of the chiral templates (-)-1 and (+)-31 at -75°C in toluene rendered the reactions enantioselective with selectivities varying between 40 and 85% ee. Truncated template rac-31 was prepared as a noranalogue of the well-established template 1 in eight steps and 56% yield from the Kemp triacid (24). Subsequent resolution delivered the enantiomerically pure templates (-)-31 and (+)-31. The outcome of the reactions is compared to the results achieved with 4-substituted 5,6-dihydro-1H-pyridin-2ones and quinolones.