B. Breit. D. A. Evans Introduction to Photochemistry Chem 206 Paterno-Buchi Reaction continued photocycloaddition of Enones New s into an old mechanism [2+ 2] photocyo Induced facial diastereoselectivity chuster, D L; Lem, G. Kaprinidis, N.A. controls rel. config photocycloaddition/fragmentation strategies for the synthesis of natural natural products. Winkler, J D; Bowen, C M; Liotta, F. Chem. Rei 95,2003 HcHO Synthetic Applications of Intramolecular Enone-Olefin Photocycloadditions. Crimmins. M. T. Chem. Rev. 19 1453. Bach et al up to 95:5 Reaction Regiochemistry (rel. config. atC1-/C3) A minimized Application in natural product synthesis OBn head-to-tail(HT) Favored: R=EWG Favored: R= Dono (±) ) Asteltoxin head to head. exo Schreiber, JAcs 1984. 106 4186 cience1985,857;JAcs,1983,105 R=CI Transannular reaction Ring Fusion Stereochemistry 0 互 Aco MeO OMe OM Stereoselective intermolecular[ 2+2H-photocycloaddition reactions and their lication in synthesis. Bach, T. Synthesis 1998, 683-703 i It is hard to rationalize the trans ring fusion 6%other products emistry through the intervention of a diradical Corey JACS 1964, 86, 485B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Induced facial diastereoselectivity R 1 OTMS Me RL * PhCHO hn O OTMS R 1 Ph RL Me H up to 95:5 (rel. config. atC1–/C3) controls rel. config. 1 3 H OTMS R 1 Me RL O Ar H A 1,3 minimized Bach et al. Application in natural product synthesis O Me Me O OBn + hn O Me O Me H OBn head to head, exo (±) Asteltoxin Schreiber, JACS 1984, 106, 4186; Science 1985, 857; JACS, 1983, 105, 660. Transannular reaction O AcO AcO O Me hn Paterno-Büchi Reaction conbtinued.... H "Stereoselective intermolecular [2+2]-photocycloaddition reactions and their application in synthesis.", Bach, T. Synthesis 1998, 683-703. Photocycloaddition of Enones "New insights into an old mechanism: [2 + 2] photocycloaddition of enones to alkenes.", Schuster, D. I.; Lem, G.; Kaprinidis, N. A. Chem. Rev. 1993, 93, 3. "[2+2] photocycloaddition/fragmentation strategies for the synthesis of natural and unnatural products.", Winkler, J. D.; Bowen, C. M.; Liotta, F. Chem. Rev. 1995, 95, 2003-2020. "Synthetic Applications of Intramolecular Enone-Olefin Photocycloadditions.", Crimmins, M. T. Chem. Rev. 1988, 88, 1453. Reaction Regiochemistry O R hn O R + + head-to-head (HH) Favored: R = EWG HN O Me Me R hn R = CN R = OEt HN O Me Me R HN O Me Me R 82 : 18 05 : 95 O R head-to-tail (HT) Favored: R = Donor Ring Fusion Stereochemistry O OMe hn MeO O OMe OMe H H 49% + O OMe OMe H H 21% + 6% other products Corey JACS 1964, 86, 485 It is hard to rationalize the trans ring fusion stereochemistry through the intervention of a 1,4-diradical