Strained Alkenes in Natural Product Synthesis
Strained Alkenes in Natural Product Synthesis 15 October 2013 Denmark Group MeeAng Hyung Min Chi
Overview • Types of strains in alkene system • PreparaAon of strained alkenes • Strained alkenes used in natural product synthesis • Strained Alkenes in [4+2] CycloaddiAons –
Rubrolone aglycon, Platensimycin, CorAstaAn A, Cycloclavine, Asteriscanolide, Pleocarpenone & Pleocarpenene, Vinigrol
• Sigmatropic Strain-‐Driven Siloxy-‐Cope Rearrangement –
Phomoidride B
• Metal-‐Catalyzed ReacAons –
Spirofungin A, RouAennocin, Pentalenene, Isoquinoline alkaloids
• Nucleophilic AddiAon –
Hyacinthacine A2
Types of Strains in alkene system • Angle compression
• TwisAng
• PyramidalizaAon Distor'on energy: The deformaAon of the reactants required for an opAmal transiAon-‐state geometry Interac'on energy: Stabilizing electrostaAc, charge-‐transfer & repulsion interacAons between approaching reactants Houk and Bickelhaupt suggests that the lower acAvaAon energy correlates most accurately with distorAon/interacAon energy Schoenebeck, F.; Ess, D. H.; Jones, G. O.; Houk, K. N., J. Am. Chem. Soc. 2009, 131, 8121 van Zeist, W. J.; Bickelhaupt, F. M., Org. Biomol. Chem. 2010, 8, 3118
AcAvaAon, DistorAon and InteracAon energies • DistorAon/InteracAon model (acAvaAon/strain model) – – – –
Diene distorAon energy: green Dienophile distorAon energy: blue InteracAon energy: red AcAvaAon energy: black
Paton, R. S.; Kim, S.; Ross, A. G.; Danishefsky, S. J.; Houk, K. N. Angew. Chem. Int. Ed. 2011, 50, 10366
PreparaAon of the strained alkenes • Barton-‐Kellogg’s olefinaAon
• Tying-‐back methods
• Carbene coupling methods
Lenoir, D.; Wahenbach, C.; Liebman, J. F., Struct. Chem. 2006, 17, 419.
Strained alkenes • First synthesis of cyclopropene (1922) – Dem’yanov & Doyarenko
• Strained alkenes used in synthesis
D. L. Boger, C. E. Brotherton, Tetrahedron 1986, 42, 2777
Cyclopropene Ketals • Boger, 1986 – Accelerate both normal and inverse electron demand Diels-‐Alder reacAon – Cyclopropene ketals show exclusive exo selecAvity
D. L. Boger, C. E. Brotherton, Tetrahedron 1986, 42, 2777
Cyclopropene Ketals • Boger, 2000 – Synthesis of rubrolone aglycon – UAlizing cyclopropene ketal to access tropone moiety
D. L. Boger, S. Ichikawa, H. Jiang, J. Am. Chem. Soc. 2000, 122, 12169.
Cyclopropene Ketals • Boger, 2000 – [4+2] cycloaddiAon with cyclopropene ketals show high exo selecAvity – nearly quanAtaAve yield of cycloadduct as a single exo diastereomer.
D. L. Boger, S. Ichikawa, H. Jiang, J. Am. Chem. Soc. 2000, 122, 12169.
Tetrabromocyclopropene
• Oblak & Wright, 2011 – near instantaneous exo-‐selecAve [4+2] cycloaddiAon – Synthesis of a Key Oxabicyclo[3.2.1]octadiene Intermediate
E. Z. Oblak, D. L. Wright, Org. LeD. 2011, 13, 2263.
Intramolecular [4+2]: Cyclopropene
• Magnus and Lilch, 2009 – corAstaAn A – tandem nucleophilic addiAon/intramolecular cyclopropene Diels–Alder reacAon
P. Magnus, R. Lilch, Org. LeD. 2009, 11, 3938.
Intramolecular [4+2]: Cyclopropene
• De Clercq, 1979/1982 – Failed with unstrained alkene system (De Clercq) – Intramolecular Diels-‐Alder furan approach
P. Magnus, R. Lilch, Org. LeD. 2009, 11, 3938. P. J. De Clercq, L. A. Van Royen, Synth. Commun. 1979, 9, 771. L. A. Van Royen, R. Mijngheer, P. J. De Clercq, Tetrahedron LeD. 1982, 23, 3283.
Intramolecular [4+2]: Methylenecyclopropane
• Petronijevic & Wipf, 2011 – ergot alkaloid cycloclavine – simultaneous installaAon of the quaternary cyclopropane stereocenter and the trans-‐hydroindole ring system – The observed rate acceleraAon results from methylenecyclopropane ring strain (40 kcal/mol) – single trans diastereomer
F. R. Petronijevic, P. Wipf, J. Am. Chem. Soc. 2011, 133, 7704.
Intramolecular [4+2]: Methylenecyclopropane • Parker and Iqbal, 1987 – Diels–Alder reacAons with unstrained dienophile – 200 oC for 10 hours gave cycloadducts less than 20% yield and with poor cis/ trans selecAvity
F. R. Petronijevic, P. Wipf, J. Am. Chem. Soc. 2011, 133, 7704. K. A. Parker, T. Iqbal, J. Org. Chem. 1987, 52, 4369.
Intramolecular [4+2]: Cyclobutadiene
• Snapper, 2000 – (+)-‐asteriscanolide – [4+2] cycloaddiAon upon oxidaAve decomplexaAon – Cross-‐metathesis followed by in situ Cope rearrangement
J. Limanto, M. L. Snapper, J. Am. Chem. Soc. 2000, 122, 8071.
Intramolecular [4+2]: Cyclobutadiene • Snapper, 2007 – Pleocarpenone and pleocarpenene
M. J. Williams, H. L. Deak, M. L. Snapper, J. Am. Chem. Soc. 2007, 129, 486.
Intramolecular [4+2]: Bicyclo[2.2.2]octene
• Baran, 2009 – Vinigrol – intramolecular Diels–Alder reacAon/Grob fragmentaAon – Diels–Alder cycloaddiAon with acyclic alkadiene results in only 46% aper 90 hours at 162oC.
T. J. Maimone, A. F. Voica, P. S. Baran, Angew. Chem. Int. Ed. 2008, 47, 3054 T. J. Maimone, J. Shi, S. Ashida, P. S. Baran, J. Am. Chem. Soc. 2009, 131, 17066. Y. T. Lin, K. N. Houk, Tetrahedron LeD. 1985, 26, 2269.
Sigmatropic Strain-‐driven Siloxy-‐Cope Rearrangement
• Clive, 1997/1997 – Phomoidride B
• Leighton, 2003 – one-‐pot Pd-‐catalyzed carbonylaAon/siloxy-‐Cope rearrangement with enol triflate
P.W. M. Sgarbi, D. L. J. Clive, Chem. Commun. 1997, 2157; D. L. J. Clive, S. Sun, X. He, J. Zhang, V. Gagliardini, Tetrahedron LeD. 1999, 40, 4605. M. M. Bio, J. L. Leighton, J. Am. Chem. Soc. 1999, 121, 890; M. M. Bio, J. L. Leighton, Org. LeD. 2000, 2, 2905. M. M. Bio, J. L. Leighton, J. Org. Chem. 2003, 68, 1693.
Cross-‐Metathesis: Cyclopropenone Ketal • Marjanovic and Kozmin, 2007 – spirofungin A
J. Marjanovic, S. A. Kozmin, Angew. Chem. Int. Ed. 2007, 46, 8854.
Cross-‐Metathesis: Cyclopropenone Ketal • Mazumoto and Kozmin, 2008 – rouAennocin
K. Matsumoto, S. A. Kozmin, Adv. Synth. Catal. 2008, 350, 557.
Intramolecular Pauson-‐Khand ReacAon • Pallerla and Fox, 2007 – mulAcomponent formal [2+2+1] cycloaddiAon – (-‐)-‐Pentalenene – higher reacAvity of strained alkenes in Pauson–Khand reacAons results from greater back donaAon to the lower-‐lying LUMO of a strained alkene
M. K. Pallerla, J. M. Fox, Org. LeD. 2007, 9, 5625.
AddiAon to Strained Azabicyclic Alkenes
• Lautens, 2008 – Isoquinoline alkaloids – [4+2] cycloaddiAon of benzyne intermediate, generated in situ from dibromobenzene
H. A. McManus, M. J. Fleming, M. Lautens, Angew. Chem. Int. Ed. 2007, 46, 433; M. J. Fleming, H. A. McManus, A. Rudolph, W. H. Chan, J. Ruiz, C. Dockendorff, M. Lautens, Chem. Eur. J. 2008, 14, 2112.
Nucleophilic AddiAon: Intramolecular trans-‐ Cyclooctene HydroaminaAon • Fox, 2011 – Hyacinthacine A2 – The increased reacAvity likely results from significant twisAng of the alkene p-‐system – HOMO of the alkene becomes significantly higher in energy, allowing the destabilizing NH lone pair to encounter the electron-‐rich olefin p-‐system
M. Royzen, M. T. Taylor, A. DeAngelis, J. M. Fox, Chem. Sci. 2011, 2, 2162.
Conclusion • Use of strained alkenes allow access to reacAons than typically require significant acAvaAon. • Strategic use of strained alkenes can promote complex transformaAons in natural product syntheses. – CycloaddiAons, rearrangements, metal-‐catalyzed reacAons, and nucleophilic addiAons
• Development of syntheAc methods has facilitated access to highly strained alkenes
References • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
Wilson, R.; Taylor, R. E., Angew. Chem. Int. Ed. 2013, 52, 4078. Meijere, A. de.; Blechert, S., Strain and its implicaKons in organic chemistry : organic stress and reacKvity. Dordrecht ; Kluwer Academic Publishers, 1989. Liebman, J. F.; Greenberg, A., Chem. Rev. 1976, 76, 311. Shea, K. J., Tetrahedron, 1980, 36, 1683 Paquehe, L. A et al, J. Am. Chem. Soc. 1971, 93, 4516. Vazquez, S.; Camps, P. Tetrahedron 2005, 61, 5147. Schoenebeck, F.; Ess, D. H.; Jones, G. O.; Houk, K. N., J. Am. Chem. Soc. 2009, 131, 8121 van Zeist, W. J.; Bickelhaupt, F. M., Org. Biomol. Chem. 2010, 8, 3118 Paton, R. S.; Kim, S.; Ross, A. G.; Danishefsky, S. J.; Houk, K. N. Angew. Chem. Int. Ed. 2011, 50, 10366 Lenoir, D.; Wahenbach, C.; Liebman, J. F., Struct. Chem. 2006, 17, 419. D. L. Boger, S. Ichikawa, H. Jiang, J. Am. Chem. Soc. 2000, 122, 12169. E. Z. Oblak, D. L. Wright, Org. LeD. 2011, 13, 2263. P. Magnus, R. Lilch, Org. LeD. 2009, 11, 3938. P. J. De Clercq, L. A. Van Royen, Synth. Commun. 1979, 9, 771. L. A. Van Royen, R. Mijngheer, P. J. De Clercq, Tetrahedron LeD. 1982, 23, 3283. F. R. Petronijevic, P. Wipf, J. Am. Chem. Soc. 2011, 133, 7704. K. A. Parker, T. Iqbal, J. Org. Chem. 1987, 52, 4369. J. Limanto, M. L. Snapper, J. Am. Chem. Soc. 2000, 122, 8071. M. J. Williams, H. L. Deak, M. L. Snapper, J. Am. Chem. Soc. 2007, 129, 486. T. J. Maimone, A. F. Voica, P. S. Baran, Angew. Chem. Int. Ed. 2008, 47, 3054 T. J. Maimone, J. Shi, S. Ashida, P. S. Baran, J. Am. Chem. Soc. 2009, 131, 17066. Y. T. Lin, K. N. Houk, Tetrahedron LeD. 1985, 26, 2269. P.W. M. Sgarbi, D. L. J. Clive, Chem. Commun. 1997, 2157; D. L. J. Clive, S. Sun, X. He, J. Zhang, V. Gagliardini, Tetrahedron LeD. 1999, 40, 4605. M. M. Bio, J. L. Leighton, J. Org. Chem. 2003, 68, 1693. J. Marjanovic, S. A. Kozmin, Angew. Chem. Int. Ed. 2007, 46, 8854. K. Matsumoto, S. A. Kozmin, Adv. Synth. Catal. 2008, 350, 557. M. K. Pallerla, J. M. Fox, Org. LeD. 2007, 9, 5625. H. A. McManus, M. J. Fleming, M. Lautens, Angew. Chem. Int. Ed. 2007, 46, 433 M. J. Fleming, H. A. McManus, A. Rudolph, W. H. Chan, J. Ruiz, C. Dockendorff, M. Lautens, Chem. Eur. J. 2008, 14, 2112. M. Royzen, M. T. Taylor, A. DeAngelis, J. M. Fox, Chem. Sci. 2011, 2, 2162.
Overview • Types of strains in alkene system • PreparaAon of strained alkenes • Strained alkenes used in natural product synthesis • Strained Alkenes in [4+2] CycloaddiAons –
Rubrolone aglycon, Platensimycin, CorAstaAn A, Cycloclavine, Asteriscanolide, Pleocarpenone & Pleocarpenene, Vinigrol
• Sigmatropic Strain-‐Driven Siloxy-‐Cope Rearrangement –
Phomoidride B
• Metal-‐Catalyzed ReacAons –
Spirofungin A, RouAennocin, Pentalenene, Isoquinoline alkaloids
• Nucleophilic AddiAon –
Hyacinthacine A2
Types of Strains in alkene system • Angle compression
• TwisAng
• PyramidalizaAon Distor'on energy: The deformaAon of the reactants required for an opAmal transiAon-‐state geometry Interac'on energy: Stabilizing electrostaAc, charge-‐transfer & repulsion interacAons between approaching reactants Houk and Bickelhaupt suggests that the lower acAvaAon energy correlates most accurately with distorAon/interacAon energy Schoenebeck, F.; Ess, D. H.; Jones, G. O.; Houk, K. N., J. Am. Chem. Soc. 2009, 131, 8121 van Zeist, W. J.; Bickelhaupt, F. M., Org. Biomol. Chem. 2010, 8, 3118
AcAvaAon, DistorAon and InteracAon energies • DistorAon/InteracAon model (acAvaAon/strain model) – – – –
Diene distorAon energy: green Dienophile distorAon energy: blue InteracAon energy: red AcAvaAon energy: black
Paton, R. S.; Kim, S.; Ross, A. G.; Danishefsky, S. J.; Houk, K. N. Angew. Chem. Int. Ed. 2011, 50, 10366
PreparaAon of the strained alkenes • Barton-‐Kellogg’s olefinaAon
• Tying-‐back methods
• Carbene coupling methods
Lenoir, D.; Wahenbach, C.; Liebman, J. F., Struct. Chem. 2006, 17, 419.
Strained alkenes • First synthesis of cyclopropene (1922) – Dem’yanov & Doyarenko
• Strained alkenes used in synthesis
D. L. Boger, C. E. Brotherton, Tetrahedron 1986, 42, 2777
Cyclopropene Ketals • Boger, 1986 – Accelerate both normal and inverse electron demand Diels-‐Alder reacAon – Cyclopropene ketals show exclusive exo selecAvity
D. L. Boger, C. E. Brotherton, Tetrahedron 1986, 42, 2777
Cyclopropene Ketals • Boger, 2000 – Synthesis of rubrolone aglycon – UAlizing cyclopropene ketal to access tropone moiety
D. L. Boger, S. Ichikawa, H. Jiang, J. Am. Chem. Soc. 2000, 122, 12169.
Cyclopropene Ketals • Boger, 2000 – [4+2] cycloaddiAon with cyclopropene ketals show high exo selecAvity – nearly quanAtaAve yield of cycloadduct as a single exo diastereomer.
D. L. Boger, S. Ichikawa, H. Jiang, J. Am. Chem. Soc. 2000, 122, 12169.
Tetrabromocyclopropene
• Oblak & Wright, 2011 – near instantaneous exo-‐selecAve [4+2] cycloaddiAon – Synthesis of a Key Oxabicyclo[3.2.1]octadiene Intermediate
E. Z. Oblak, D. L. Wright, Org. LeD. 2011, 13, 2263.
Intramolecular [4+2]: Cyclopropene
• Magnus and Lilch, 2009 – corAstaAn A – tandem nucleophilic addiAon/intramolecular cyclopropene Diels–Alder reacAon
P. Magnus, R. Lilch, Org. LeD. 2009, 11, 3938.
Intramolecular [4+2]: Cyclopropene
• De Clercq, 1979/1982 – Failed with unstrained alkene system (De Clercq) – Intramolecular Diels-‐Alder furan approach
P. Magnus, R. Lilch, Org. LeD. 2009, 11, 3938. P. J. De Clercq, L. A. Van Royen, Synth. Commun. 1979, 9, 771. L. A. Van Royen, R. Mijngheer, P. J. De Clercq, Tetrahedron LeD. 1982, 23, 3283.
Intramolecular [4+2]: Methylenecyclopropane
• Petronijevic & Wipf, 2011 – ergot alkaloid cycloclavine – simultaneous installaAon of the quaternary cyclopropane stereocenter and the trans-‐hydroindole ring system – The observed rate acceleraAon results from methylenecyclopropane ring strain (40 kcal/mol) – single trans diastereomer
F. R. Petronijevic, P. Wipf, J. Am. Chem. Soc. 2011, 133, 7704.
Intramolecular [4+2]: Methylenecyclopropane • Parker and Iqbal, 1987 – Diels–Alder reacAons with unstrained dienophile – 200 oC for 10 hours gave cycloadducts less than 20% yield and with poor cis/ trans selecAvity
F. R. Petronijevic, P. Wipf, J. Am. Chem. Soc. 2011, 133, 7704. K. A. Parker, T. Iqbal, J. Org. Chem. 1987, 52, 4369.
Intramolecular [4+2]: Cyclobutadiene
• Snapper, 2000 – (+)-‐asteriscanolide – [4+2] cycloaddiAon upon oxidaAve decomplexaAon – Cross-‐metathesis followed by in situ Cope rearrangement
J. Limanto, M. L. Snapper, J. Am. Chem. Soc. 2000, 122, 8071.
Intramolecular [4+2]: Cyclobutadiene • Snapper, 2007 – Pleocarpenone and pleocarpenene
M. J. Williams, H. L. Deak, M. L. Snapper, J. Am. Chem. Soc. 2007, 129, 486.
Intramolecular [4+2]: Bicyclo[2.2.2]octene
• Baran, 2009 – Vinigrol – intramolecular Diels–Alder reacAon/Grob fragmentaAon – Diels–Alder cycloaddiAon with acyclic alkadiene results in only 46% aper 90 hours at 162oC.
T. J. Maimone, A. F. Voica, P. S. Baran, Angew. Chem. Int. Ed. 2008, 47, 3054 T. J. Maimone, J. Shi, S. Ashida, P. S. Baran, J. Am. Chem. Soc. 2009, 131, 17066. Y. T. Lin, K. N. Houk, Tetrahedron LeD. 1985, 26, 2269.
Sigmatropic Strain-‐driven Siloxy-‐Cope Rearrangement
• Clive, 1997/1997 – Phomoidride B
• Leighton, 2003 – one-‐pot Pd-‐catalyzed carbonylaAon/siloxy-‐Cope rearrangement with enol triflate
P.W. M. Sgarbi, D. L. J. Clive, Chem. Commun. 1997, 2157; D. L. J. Clive, S. Sun, X. He, J. Zhang, V. Gagliardini, Tetrahedron LeD. 1999, 40, 4605. M. M. Bio, J. L. Leighton, J. Am. Chem. Soc. 1999, 121, 890; M. M. Bio, J. L. Leighton, Org. LeD. 2000, 2, 2905. M. M. Bio, J. L. Leighton, J. Org. Chem. 2003, 68, 1693.
Cross-‐Metathesis: Cyclopropenone Ketal • Marjanovic and Kozmin, 2007 – spirofungin A
J. Marjanovic, S. A. Kozmin, Angew. Chem. Int. Ed. 2007, 46, 8854.
Cross-‐Metathesis: Cyclopropenone Ketal • Mazumoto and Kozmin, 2008 – rouAennocin
K. Matsumoto, S. A. Kozmin, Adv. Synth. Catal. 2008, 350, 557.
Intramolecular Pauson-‐Khand ReacAon • Pallerla and Fox, 2007 – mulAcomponent formal [2+2+1] cycloaddiAon – (-‐)-‐Pentalenene – higher reacAvity of strained alkenes in Pauson–Khand reacAons results from greater back donaAon to the lower-‐lying LUMO of a strained alkene
M. K. Pallerla, J. M. Fox, Org. LeD. 2007, 9, 5625.
AddiAon to Strained Azabicyclic Alkenes
• Lautens, 2008 – Isoquinoline alkaloids – [4+2] cycloaddiAon of benzyne intermediate, generated in situ from dibromobenzene
H. A. McManus, M. J. Fleming, M. Lautens, Angew. Chem. Int. Ed. 2007, 46, 433; M. J. Fleming, H. A. McManus, A. Rudolph, W. H. Chan, J. Ruiz, C. Dockendorff, M. Lautens, Chem. Eur. J. 2008, 14, 2112.
Nucleophilic AddiAon: Intramolecular trans-‐ Cyclooctene HydroaminaAon • Fox, 2011 – Hyacinthacine A2 – The increased reacAvity likely results from significant twisAng of the alkene p-‐system – HOMO of the alkene becomes significantly higher in energy, allowing the destabilizing NH lone pair to encounter the electron-‐rich olefin p-‐system
M. Royzen, M. T. Taylor, A. DeAngelis, J. M. Fox, Chem. Sci. 2011, 2, 2162.
Conclusion • Use of strained alkenes allow access to reacAons than typically require significant acAvaAon. • Strategic use of strained alkenes can promote complex transformaAons in natural product syntheses. – CycloaddiAons, rearrangements, metal-‐catalyzed reacAons, and nucleophilic addiAons
• Development of syntheAc methods has facilitated access to highly strained alkenes
References • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
Wilson, R.; Taylor, R. E., Angew. Chem. Int. Ed. 2013, 52, 4078. Meijere, A. de.; Blechert, S., Strain and its implicaKons in organic chemistry : organic stress and reacKvity. Dordrecht ; Kluwer Academic Publishers, 1989. Liebman, J. F.; Greenberg, A., Chem. Rev. 1976, 76, 311. Shea, K. J., Tetrahedron, 1980, 36, 1683 Paquehe, L. A et al, J. Am. Chem. Soc. 1971, 93, 4516. Vazquez, S.; Camps, P. Tetrahedron 2005, 61, 5147. Schoenebeck, F.; Ess, D. H.; Jones, G. O.; Houk, K. N., J. Am. Chem. Soc. 2009, 131, 8121 van Zeist, W. J.; Bickelhaupt, F. M., Org. Biomol. Chem. 2010, 8, 3118 Paton, R. S.; Kim, S.; Ross, A. G.; Danishefsky, S. J.; Houk, K. N. Angew. Chem. Int. Ed. 2011, 50, 10366 Lenoir, D.; Wahenbach, C.; Liebman, J. F., Struct. Chem. 2006, 17, 419. D. L. Boger, S. Ichikawa, H. Jiang, J. Am. Chem. Soc. 2000, 122, 12169. E. Z. Oblak, D. L. Wright, Org. LeD. 2011, 13, 2263. P. Magnus, R. Lilch, Org. LeD. 2009, 11, 3938. P. J. De Clercq, L. A. Van Royen, Synth. Commun. 1979, 9, 771. L. A. Van Royen, R. Mijngheer, P. J. De Clercq, Tetrahedron LeD. 1982, 23, 3283. F. R. Petronijevic, P. Wipf, J. Am. Chem. Soc. 2011, 133, 7704. K. A. Parker, T. Iqbal, J. Org. Chem. 1987, 52, 4369. J. Limanto, M. L. Snapper, J. Am. Chem. Soc. 2000, 122, 8071. M. J. Williams, H. L. Deak, M. L. Snapper, J. Am. Chem. Soc. 2007, 129, 486. T. J. Maimone, A. F. Voica, P. S. Baran, Angew. Chem. Int. Ed. 2008, 47, 3054 T. J. Maimone, J. Shi, S. Ashida, P. S. Baran, J. Am. Chem. Soc. 2009, 131, 17066. Y. T. Lin, K. N. Houk, Tetrahedron LeD. 1985, 26, 2269. P.W. M. Sgarbi, D. L. J. Clive, Chem. Commun. 1997, 2157; D. L. J. Clive, S. Sun, X. He, J. Zhang, V. Gagliardini, Tetrahedron LeD. 1999, 40, 4605. M. M. Bio, J. L. Leighton, J. Org. Chem. 2003, 68, 1693. J. Marjanovic, S. A. Kozmin, Angew. Chem. Int. Ed. 2007, 46, 8854. K. Matsumoto, S. A. Kozmin, Adv. Synth. Catal. 2008, 350, 557. M. K. Pallerla, J. M. Fox, Org. LeD. 2007, 9, 5625. H. A. McManus, M. J. Fleming, M. Lautens, Angew. Chem. Int. Ed. 2007, 46, 433 M. J. Fleming, H. A. McManus, A. Rudolph, W. H. Chan, J. Ruiz, C. Dockendorff, M. Lautens, Chem. Eur. J. 2008, 14, 2112. M. Royzen, M. T. Taylor, A. DeAngelis, J. M. Fox, Chem. Sci. 2011, 2, 2162.
