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-(omega'-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-2-one (8) or-more favorably-from the corresponding alpha-(omega'-alkenyl)-substituted delta-valerolactams 9-11 by a selenylation/elimination sequence (56-62 % overall yield). 3-(omega'-Alkenyloxy)-substituted 5,6-dihydro-1H-pyridin-2-ones 5 and 6 were accessible in 43 and 37 % overall yield from 3-diazopiperidin-2-one (15) by an alpha,alpha-chloroselenylation reaction at the 3-position followed by nucleophilic displacement of a chloride ion with an omega-alkenolate and oxidative elimination of selenoxide. Upon irradiation at lambda=254 nm, the precursor compounds underwent a clean intramolecular [2+2] photocycloaddition reaction. Substrates 2 and 5, tethered by a two-atom 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 degrees 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-2-ones and quinolones.