D. A. Evans Introduction to Photochemistry Chem 206 Worthwhile General reviews http:/www.courses.fasharvardedu/-chem206/ New insights into an old mechanism: [2+ 2] photocycloaddition of enones to alkenes Schuster, D L; Lem, G; Kaprinidis, N. A. Chem. Rev. 1993, 93, 3 Chemistry 206 Stereoselective intermolecular[2+2-photocycloaddition reactions and their application in synthesis. Bach, T. Synthesis 1998, 683-703 Advanced Organic Chemistry 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- Lecture number 2020. i "Synthetic Applications of Intramolecular Enone-Olefin Photocycloadditions Crimmins, M T.chem.Rev.1988.88.1453 Introduction to Photochemistry The meta photocycloaddition of arenes to alkenes. Cormelisse, Chem. Rev. 1993, 93, Introduction to electronic Excitation 615 a Franck-Condon Principle, Jabolonski Diagram a Photochemistry of Olefins and Dienes a Photochemistry of Carbonyl Compounds a Norrish Type-I& II Processes Paterno Buchi Reaction Chem 206, 1999 Final Exam Question. Provide a mechanism for the followin transfomation that explains the observed stereochemistry ( Marshall, JoC, 1971, 214). I[2+ 2] Photocycloaddition of Olefins Reading Assignment for this Lecture fourth Edition, Chepter 13, "Photochemistry",pp743-789 y Carey, and Sundberg, Advanced Organic Chemistry, Part Stereoselective intermolecular[2+2 -photocycloaddition reactions and their application in synthesis. Bach, T. Synthesis 1998, 683-703. Mechanism?? [2+2 photocycloaddition/fragmentation strategies for the synthesis of natural and unnatural prod Minkler. J. D: Bowen. C. M . Liotta. F. Chem. Rev. 1995. 95, 2003-2020.(handout Matthew d. shair Friday December 20. 2002 The above reaction forms the basis of a photolabile protecting troup strategy for amines and alcohols
D. A. Evans Chem 206 Matthew D. Shair Friday, December 20, 2002 http://www.courses.fas.harvard.edu/~chem206/ Reading Assignment for this Lecture: Introduction to Photochemistry Chemistry 206 Advanced Organic Chemistry Lecture Number 36 Introduction to Photochemistry ■ Introduction to Electronic Excitation ■ Franck-Condon Principle, Jabolonski Diagram ■ Photochemistry of Olefins and Dienes ■ Photochemistry of Carbonyl Compounds ■ Norrish Type-I & II Processes ■ Paterno-Büchi Reaction ■ [2 + 2] Photocycloaddition of Olefins Carey, and Sundberg, Advanced Organic Chemistry, Part A fourth Edition, Chepter 13, "Photochemistry", pp 743-789 "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. "Stereoselective intermolecular [2+2]-photocycloaddition reactions and their application in synthesis.", Bach, T. Synthesis 1998, 683-703. "[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. "The meta photocycloaddition of arenes to alkenes.", Cornelisse,Chem. Rev. 1993, 93, 615. Worthwhile General Reviews Chem 206, 1999 Final Exam Question. Provide a mechanism for the following transformation that explains the observed stereochemistry (Marshall, JOC, 1971, 214). HO Me hn PhH O Me "Stereoselective intermolecular [2+2]-photocycloaddition reactions and their application in synthesis.", Bach, T. Synthesis 1998, 683-703. (handout) "[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. (handout) O Me hn Me O Mechanism?? NO2 O H N O R hn R –NH2 + CO2 NO CHO The above reaction forms the basis of a photo-labile protecting gtroup strategy for amines and alcohols
B Breit D.A. Evans Introduction to Photochemistry Chem 206 Background Reading Knowing the absorption wavelength in nm, you can calculate the the energ Carey, and Sundberg Advanced Organic Chemistry, Parts A Third Edition, Chepter 13, "Photochemistry, pp 729-765 E( kcal/mol)=286×10 for 1= 200 nm:E=143 kcal Wo a( in nm) forλ New insights into an old mechanism: [2+2 photocycloaddition of enones to alkenes ,I Schuster, D L; Lem, G. Kaprinidis, N. A Chem. Rev. 1993, 93, 3. Stereoselective intermolecular[2+2H-photocycloaddition reactions and their application in synthesis., Bach, T. Synthesis 1998, 683-703 Consider a simple diatomic molecule A-B 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- ·A-B A-B Synthetic Applications of Intramolecular Enone-olefin Photocycloadditions Crimmins, M T.chem.Rev198888,1453 The meta photocycloaddition of arenes to alkenes Cornelisse, J. Chem. Rev. 1993, 93, internuclear distance B A electronic ■ Morse curve a Important Regions of the Electromagnetic Spectrum gA-B Basic Types Information Energy Range excited state Ultraviolet-Visible Electronic States 40-140 kcal/mol Infrared Functional Groups 2-12 kcal/mol NMR H&c Connectivity 1010a Light-induced electron excitation ground state Electrons are excited to higher energy levels when a molecule absorbs 二 E vibration Time scale: electron excitation: 10-15s a photon of energy equal to the energy difference between the ground- state electronic level and the excited state electronic level Time scale: nuclei movement: 10-12s E= h for 2= 200 nm E= 143 kcal a Franck-Condon Principle: Upon light-induced electronic excitation, only the electrons are reorganized; the heavier nuclei stay in their ground-state for 1=700 nm: E =40.9 kcal geometry. (Vertical Transitions)
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Background Reading Carey, and Sundberg, Advanced Organic Chemistry, Parts A Third Edition, Chepter 13, "Photochemistry", pp 729-765 "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. "Stereoselective intermolecular [2+2]-photocycloaddition reactions and their application in synthesis.", Bach, T. Synthesis 1998, 683-703. "[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. "The meta photocycloaddition of arenes to alkenes.", Cornelisse, J. Chem. Rev. 1993, 93, 615. Worthwhile General Reviews 10-5–10-6 kcal/mol 2-12 kcal/mol Electronic States 40-140 kcal/mol Functional Groups NMR H & C Connectivity Infrared Ultraviolet-Visible Basic Types Information Energy Range ■ Important Regions of the Electromagnetic Spectrum Electrons are excited to higher energy levels when a molecule absorbs a photon of energy equal to the energy difference between the groundstate electronic level and the excited state electronic level. E = hn = h c l ■ Light-induced electron excitation E (kcal/mol) = 2.86 x 10+4 Knowing the absorption wavelength in nm, you can calculate the the energy l( in nm) for l = 200 nm: E = 143 kcal for l = 700 nm: E = 40.9 kcal for l = 200 nm: E = 143 kcal for l = 700 nm: E = 40.9 kcal DE electronic s * A–B s A–B r 0 internuclear distance Consider a simple diatomic molecule A–B A B ■ Morse Curve Potential Energy r 0 (internuclear distance) E electronic DE vibration excited state ground state ■ Franck-Condon Principle: Upon light-induced electronic excitation, only the electrons are reorganized; the heavier nuclei stay in their ground-state geometry. (Vertical Transitions) Time scale: electron excitation: 10-15 s Time scale: nuclei movement: 10-12 s B A
B. Breit. D. A. Evans Introduction to Photochemistry Chem 206 Summary of Photochemical Processes Selection rules Not all excitations/transitions are allowed(have high probability) (a)Spin-forbidden: Transitions between states of different multiplicity M nonradiative decay M=2s+1 s=∑s ersystem S: total spin Spins Paired: Singlet State: S= ES=(+1/2-12)=0 2 Excitation FLuorescence Spins Unpaired: Triplet State: S= ES=(+12+12)=1 S, or S2 Phosphorescence forbidden (hv3 (b )Space forbidden: transitions between orbitals which do not overlap vacuum UV only(△ E large) probability for that ro(internuclear distance) transition is low.) Absorption max(nm) Simple alkenes 190200 I Sensitizer: e.g. acetophenone, benzophenone 220-250 Small AE between So& S, Facile excitation into S, followed by IsC intoT1 Cyclic dienes 250-270 Interaction with substrate 270-300 Saturated ketones 270-280 Substrate(So)+ Sens. T1) Substrate(1)+ Sens(So) Unsaturated ketones 310-330 Aromatic ketones (aldehydes) 280-300 Aromatic compounds 250-280 Substrate reacts via T, excited state transmits absorbs Pyrex Glass 400nm 3Triplet-Quencher. e.g.O2, piperylene Me transmits Quartz Glass Reacts immediately with molecules in T, excited state, depopulate T1 400nm Quencher(So)+ Substrate (1)- Quencher(T)+ Substrate(So) a ubstrate reacts via S, excited state
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Summary of Photochemical Processes Energy r 0 (internuclear distance) S0 S1 T1 Excitation (+ hn1 ) Fluorescence (– hn2 ) Intersystem crossing Phosphorescence (– hn3 ) nonradiative decay (–D) Substrates Absorption max (nm) 190–200 220–250 250–270 270–300 270–280 310–330 280–300 250–280 Simple alkenes Acyclic dienes Cyclioc dienes Styrenes Saturated ketones Unsaturated ketones Aromatic ketones (aldehydes) Aromatic compounds Pyrex Glass 300 nm absorbs Quartz Glass 200 nm absorbs transmits transmits Hg lamp 254 nm, 313 nm, 366 nm M = 2S + 1 S = S s M: multiplicity S: total spin s: spin of a single electron (±1/2) S0 S1 or S2 T1 allowed S0 forbidden s s * p p * n p * vacuum UV only (DE large) UV(VIS) allowed forbidden (Does not mean impossible. Implies only that probability for that particular transition is low.) Selection Rules Not all excitations/transitions are allowed (have high probability): (a) Spin-forbidden: Transitions between states of different multiplicity M (b) Space-forbidden: transitions between orbitals which do not overlap. ■ Sensitizer: e.g. acetophenone, benzophenone Small DE between S0 & S1 . Facile excitation into S1 followed by ISC into T1 . Interaction with substrate: Substrate (S0 ) + Sens. (T1 ) Substrate (T1 ) + Sens. (S0 ) ■ Triplet-Quencher: e.g. O2 , piperylene Me Reacts immediately with molecules in T1 excited state, depopulate T1 Quencher (S0 ) + Substrate (T1 ) Quencher (T1 ) + Substrate (S0 ) 400 nm 400 nm Spins Paired: Singlet State: S = S s = (+1/2 –1/2) = 0 Spins Unpaired: Triplet State: S = S s = (+1/2 +1/2) = 1 Substrate reacts via S1 excited state. Substrate reacts via T1 excited state. Substrate reacts via S1 excited state
B Breit. D. A. Evans Introduction to Photochemistry Chem 206 Jablonski Diagram conversio ≥10 conversion 106-102sec crossing 10-10se ≥10 Figure 12.2 Jablonski diagram Energy levels of excited states of a polyatomic molecule. The west vibrational energy levels of a state are indicated by thick horizontal lin other horizontal lines represent associated vibrational levels. Vertical straight lines represent radiative transitions, wavy lines nonradiative transitions. The orders of magnitude of the first-order rate constants for the various processes are indicated From Cundall, R. B; Gilbert, A. Photochemistry. "Thomas Nelson: London 1970. Reproduced by permission of Thomas Nelson and Sons Limited
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Jablonski Diagram
D A. Evans Introduction to Photochemistry Chem 206 Photochemical reactions Hexatriene. Frontier MO Description derotation Fleming I. Frontier Orbitals and Organic Chemical Reactions", Chapter 6. " Photochemical reactio Reactions of olefins Consider [2+ 2] cycloaddition: Photochemical activation MO HOMO 中1e 中3 199 The rotatory motion of ning closure may be reversed by photo-activation One of the first cases where heat and light induced electrocyclization t concerted followed different pathways energy Havinga, Tetrahedron, 1961, 16, 1 The(2+ 2 cycloaddition of two olefins exhibits the option of being concerted if one of the olefins reacts out of its photochemically excited state Excited State Geometry disputation connotation 84 xcited state('s and T
D. A. Evans Introduction to Photochemistry Chem 206 Photochemical Reactions Frontier MO Description Fleming, I. "Frontier Orbitals and Organic Chemical Reactions", Chapter 6, "Photochemical Reactions" Reactions of Olefins p* p concerted The [2 + 2] cycloaddition of two olefins exhibits the option of being concerted if one of the olefins reacts out of its photochemically excited state. ✻ bonding bonding HOMO + energy ✻ + light ✻ p p* new HOMO light Consider [2 + 2] cycloaddition: Photochemical activation LUMO C C C C C C C C C C C C C C C C C C The rotatory motion of ring closure may be reversed by photo-activation controtation 1eY4 (hexatriene HOMO) 2e- 2e- 1e- 2e- 2e- 2elight Hexatriene: disrotation Y3 (hexatriene HOMO) H Me H Me H Me H Me Me H H Me Me H Me H Y4 Excited State Geometry p* p p p* new HOMO light C R H R H Excited state (1S and 3T) Rotation Me Me RO R H Me Me RO R heat disrotation light controtation Me Me RO R H One of the first cases where heat and light induced electrocyclizations followed different pathways. Havinga, Tetrahedron, 1961, 16, 146. Me Me RO R H Me Me RO R H Y4 Y1 Y3 Y3 Y1
B. Breit D. A. Evans Introduction to Photochemistry Chem 206 Cis-Trans Isomerization Cis trans olefins may be interconverted by either direct or sensitized irradiation sensitizer: benzene △Hmm2=-33 (T)-benzene ('S)-benzene trans-cyclohexene cis-cyclohexene Sensitized Photohydration t-BUOH/H2O sensitizer Review: Marshall Accounts Chem. Res 1969. 2. 33 ROH Ground state trans cyclohexene is now sufficiently basic to deprotonate alcohols HOH tBuOH Kropp, JACS 1966, 88 Marshall, JAcs 1966. 8 Marshall, JOC, 1971, 214)
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Hmm2 = – 33 kcal/mol trans-cyclohexene cis-cyclohexene Me Me Me Me O O hn benzene t-BuOH Me CH2 Me Me O O 85% yield Evans et. al. unpublished results, 1972 HO Me hn PhH O Me (Marshall, JOC, 1971, 214). Cis–Trans Isomerization Cis & trans olefins may be interconverted by either direct or sensitized irradiation C R H R H C R H C H R hn sensitizer 3 C H R C H R sensitizer: benzene ( 0S)benzene hn (1S)-benzene ( 3T)-benzene ( 0S)-olefin (0S)-benzene (3T)-olefin Me hn sensitizer Me (trans-cyclohexene) Me H Me H OR Sensitized Photohydration CH2 H Review: Marshall, Accounts Chem. Res 1969, 2, 33 Ground state trans cyclohexene is now sufficiently basic to deprotonate alcohols Me H Me H OR CH2 H Product Partitioning Me Me Me Me Me Me Me Me Me hn sensitizer HOH tBuOH 28% 72% 70% 30% Kropp, JACS 1966, 88, 4091 Marshall, JACS 1966, 88, 4092 Me Me Me hn xylene t-BuOH/H2O Me CH2 Me 50% Marshall Me Me Me Me Me Me –H+ ROH ROH RO
B. Breit. D. A. Evans Introduction to Photochemistry Chem 206 Excited State Geometry of Dienes Photochemistry of carbonyl compounds Photochemical excitation of carbonyl group:n→π Allylmethylene diradical TI 75-80 kcal/mol n→ pyramidal trans-trans not observed dipole: 2.34D dipole: 1.56D Explain why trans-trans isomer is not observed in this photosiomerization 十++ +++ ++++ ground n-I n-丌π-丌丌-π aglet trip 人(mb(A Note the diradical character of the excited states photochemically excited carbonyl compounds undergo many of the reactions typical of Rationalize the course of the sensitized transformation
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Excited State Geometry of Dienes C C H C H R C R H H light C H R C C H H R H Allylmethylene diradical C C H C R H C H H R light cis-cis C C H C H R C H H R cis-trans C C H C H R C R H H trans-trans not observed Explain why trans-trans isomer is not observed in this photosiomerization C H Me C C H H H H C C H C H Me C H H H light Me Me H Me H direct H2C Me Ph Ph hu Ph Ph sensitized hu direct Ph Ph Rationalize the course of the sensitized transformation O H H hn Photochemistry of Carbonyl Compounds Photochemical excitation of carbonyl group: n ® p* * planar pyramidal dipole: 2.34D dipole: 1.56D p * p n ground state n - p * singlet n - p * triplet p - p * singlet p - p * triplet Note the diradical character of the excited states. Photochemically excited carbonyl compounds undergo many of the reactions typical of radical species. n ® p* S 1 T 1 C O H H 80–85 kcal/mol 75–80 kcal/mol C O C O
B. Breit. D.A. Evans Introduction to Photochemistry Chem 206 Norrish-type I reactions (a-cleavage C/N bond cleavage: Aza-Fries rearrangement Barton,JAcS1985,107,3607. occurs from both recombination Chem. ber. 1969. 342 Regioselectivity of bond cleavage: Depends on relative stability of the two radicals formed Norrish-type lI reactions Acc. Chem. Res 1971, 4, 168 here: benzyl vs phenyl radical H Intermolecular (Reaction with solvent Reaction occurs readily if stabilized radicals are formed 是sH ket71963,1863. OHOH CIO and CIN bond cleavage: Photo-Fries rearrangement Pinacoh-type products Intramolecular(a)Photoenolizati ChaR H H-Abstr
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Norrish-type I reactions ( -cleavage) O R R' hn n ® p* O R R' * occurs from both S 1 and T1 O R + R' Regioselectivity of bond cleavage: Depends on relative stability of the two radicals formed. O Barton, JACS 1985, 107, 3607. Ph hn O Ph here: benzyl vs. phenyl radical H O Ph Ph hn Ph Ph - CO Quinkert, TL 1963, 1863. C/O and C/N bond cleavage: Photo–Fries rearrangement O O R hn OH O R OH R O + 1 : 1 O O R phenoxy radical (resonance stabilized) solvent cage O O R O O R Reaction occurs readily if stabilized radicals are formed. Ph H O S products (decarbonylation, recombination, disproportionation) N Ph O hn N O H N H O N O Chem. Ber. 1969, 342. Norrish-type II reactions Photo-excitation of carbonyl is followed by hydrogen atom abstraction. Acc. Chem. Res. 1971, 4, 168. Intermolecular (Reaction with solvent) Ar O R hn O Ar R * S-H OH Ar R * + S Dim. OHOH Ar Ar R R OHOH Ar R R Ar Pinacol-type products Intramolecular (a) Photoenolization CH2R O Ph C O Ph H CHR OH Ph CHR OD Ph CHR O Ph hn * H-Abstr. D C/N bond cleavage: Aza–Fries rearrangement R H
B. Breit. D. A. Evans Introduction to Photochemistry Norrish-type ll reactions continued.gamma H Abstraction terno-Buchi Reaction Femptosecond Dynamics of Norrish Type-l Reaction: Nonconcerted Hydrogen Transfer and Diradical Intermediacy, Zewail et al., Angew. Chemie, Int. Ed 2000, 39, 260 Reviews: Scharf. ACIEE 1991 30. 477: Ynoue. Chem. Rev. 1992. 92. 741 ∵cH2OH Photochemical 2+2 cycloaddition of ketones/aldehydes with olefins lifetime: 900 ns e general reaction 76 + With some aliphatic aldehydes stereospecific reactions have been observed 991,93,1277T 63;,JAcs1972,94 人 Me Me I RXn may take place out of either singlet or triplet state I Fragmentation to enol is favored entrop I The decomposition of the diradical is essentially barrierless I Fragmentation to enol is favored entropically I Fagmentation similar to McLafferty Rearrangement in mass spectroscopy Remote hydrogen abstraction Aromatic aldehydes/ ketones show stereo-convergent behavi (a simple diastereoselectivity T. Bach, Liebigs Ann. 997.162 H(D) R TI 0 OTMS Breslow. Acc. Chem. Res. 1980. 13. 170 dr>9.1 ond rotation therme oduct also for r2
B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Norrish-type II reactions continued...gamma H Abstraction "Femptosecond Dynamics of Norrish Type-II Reaction: Nonconcerted Hydrogen Transfer and Diradical Intermediiacy", Zewail et. al., Angew. Chemie, Int. Ed. 2000, 39, 260 Me O Me OH +76 +37 Me OH +26 OH Me lifetime: 900 ns ■ Rxn may take place out of either singlet or triplet state ■ Fragmentation to enol is favored entropically ■ The decomposition of the diradical is essentially barrierless ■ Fragmentation to enol is favored entropically ■ Fagmentation similar to McLafferty Rearrangement in mass spectroscopy Me Me O barrier +88 Remote hydrogen abstraction O Me H Me H R O C H2 O hn O Me H Me R O C H2 OH H(D) O Me H Me R O C H2 OH H(D) Breslow, Acc. Chem. Res. 1980, 13, 170. H H H hn Paterno-Büchi Reaction Photochemical [2+2] cycloaddition of ketones/aldehydes with olefins: The general reaction: O R + hn O R + * aliphatic via S1 aromatic via T1 O R triplet or singlet diradical general: via most stable diradical. O R Oxetane With some aliphatic aldehydes stereospecific reactions have been observed: O Me * S 1 O O Me Me Me Me H Me H Me O Me via singletdiradical bond formation faster than rotation! Me Me JACS 1991, 93, 1277; TL 1964, 1425; JACS 1968, 90, 6863; JACS 1972, 94, 8761. Aromatic aldehydes/ketones show stereo-convergent behavior: T. Bach, Liebigs Ann. 1997, 1627. (a) simple diastereoselectivity R 2 R 1 OTMS R 1 OTMS R 2 O Ph * T 1 + O Ph * T 1 + Ar O R 1 OTMS R 2 life time long enough to undergo bond rotation O R 2 OTMS R 1 Ph T 1 ISC to S1 recomb. thermodyn. most stable Ar product formed H H R 2 R 1 O OTMS (Main product also for R2 = H) dr > 9:1 Reviews: Scharf, ACIEE 1991, 30, 477; Ynoue, Chem. Rev. 1992, 92, 741. H H H H CH2
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, 485
B. 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
