Organic Functional Group Interconversion

2017 ◽  
Author(s):  
Douglass F. Taber
Keyword(s):  
Reduced Form ◽  
Ni Catalyst ◽  
Max Planck ◽  
University Of Maryland ◽  
Ru Catalyst ◽  
Maryland School ◽  
National University ◽  
Rh Catalyst ◽  
Seoul National University ◽  
The University

Alois Fürstner of the Max-Planck-Institut Mülheim devised (Angew. Chem. Int. Ed. 2013, 52, 14050) a Ru catalyst for the trans- selective hydroboration of an alkyne 1 to 2. Qingbin Liu of Hebei Normal University and Chanjuan Xi of Tsinghua University coupled (Org. Lett. 2013, 15, 5174) the alkenyl zirconocene derived from 3 with an acyl azide to give the amide 4. Chulbom Lee of Seoul National University used (Angew. Chem. Int. Ed. 2013, 52, 10023) a Rh catalyst to convert a terminal alkyne 5 to the ester 6. Laura L. Anderson of the University of Illinois, Chicago devised (Org. Lett. 2013, 15, 4830) a protocol for the conversion of a ter­minal alkyne 7 to the α-amino aldehyde 9. Dewen Dong of the Changchun Institute of Applied Chemistry developed (J. Org. Chem. 2013, 78, 11956) conditions for the monohydrolysis of a bis nitrile 10 to the monoamide 11. Aiwen Lei of Wuhan University optimized (Chem. Commun. 2013, 49, 7923) a Ni catalyst for the conversion of the alkene 12 to the enamide 13. Kazushi Mashima of Osaka University optimized (Adv. Synth. Catal. 2013, 355, 3391) a boronic ester catalyst for the conversion of an amide 14 to the ester 15. Jean- François Paquin of the Université Laval prepared (Eur. J. Org. Chem. 2013, 4325) the amide 17 by coupling an amine with the activated intermediate from reaction of an acid 16 with Xtal- Fluor E. Steven Fletcher of the University of Maryland School of Pharmacy designed (Tetrahedron Lett. 2013, 54, 4624) the azodicarbonyl dimorpholide 18 as a reagent for the Mitsunobu coupling of 19 with 20. The reduced form of 18 was readily separated by extraction into water and reoxidized. Jens Deutsch of the Universität Rostock found (Chem. Eur. J. 2013, 19, 17702) simple ligands for the Ru-mediated borrowed hydro­gen conversion of an alcohol 22 to the amine 23. Ronald T. Raines of the University of Wisconsin devised (J. Am. Chem. Soc. 2013, 135, 14936) a phosphinoester for the efficient conversion in water of an azide 24 to the diazo 25.

Transition Metal-Mediated Construction of Carbocycles: Dimethyl Gloiosiphone A (Takahashi), Pasteurestin A (Mulzer), and Pentalenene (Fox)

2011 ◽  
Author(s):  
Douglass Taber
Keyword(s):  
Transition Metal ◽  
South Florida ◽  
Health Science ◽  
Pt Catalyst ◽  
Max Planck ◽  
Au Catalyst ◽  
Ru Catalyst ◽  
National University ◽  
Rh Catalyst ◽  
The University

There continue to be new developments in transition metal- and lanthanide-mediated construction of carbocycles. Although a great deal has been published on the asymmetric cyclopropanation of styrene, relatively little had been reported for other classes of alkenes. Tae-Jeong Kim of Kyungpook National University has devised (Tetrahedron Lett. 2007, 48, 8014) a Ru catalyst for the cyclopropanation of simple α-olefins such as 1. X. Peter Zhang of the University of South Florida has developed (J. Am.Chem. Soc. 2007, 129, 12074) a Co catalyst for the cyclopropanation of alkenes such as 5 having electron-withdrawing groups. Alexandre Alexakis of the Université de Genève has reported(Angew. Chem. Int. Ed. 2007, 46, 7462) simple monophosphine ligands that enabled enantioselective conjugate addition to prochiral enones, even difficult substrates such as 8. Seunghoon Shin of Hanyang University has found (Organic Lett. 2007, 9, 3539) an Au catalyst that effected the diastereoselective cyclization of 10 to the cyclohexene 11, and Radomir N. Saicic of the University of Belgrade has carried out (Organic Lett. 2007, 9, 5063), via transient enamine formation, the diastereoselective cyclization of 12 to the cyclohexane 13. Alois Fürstner of the Max-Planck- Institut, Mülheim has devised (J. Am. Chem. Soc. 2007, 129, 14836) a Rh catalyst that cyclized the aldehyde 14 to the cycloheptenone 15. Some of the most exciting investigations reported in recent months have been directed toward the direct diastereo- and enantioselective preparation of polycarbocyclic products. Rai-Shung Liu of National Tsing-Hua University has extended (J. Org. Chem. 2007, 72, 567) the intramolecular Pauson-Khand cyclization to the epoxy enyne 16, leading to the 5-5 product 17. Michel R. Gagné of the University of North Carolina has devised (J. Am. Chem. Soc. 2007, 129, 11880) a Pt catalyst that smoothly cyclized the polyene 18 to the 6-6 product 19. Yoshihiro Sato of Hokkaido University and Miwako Mori of the Health Science University of Hokkaido have described (J. Am. Chem. Soc. 2007, 129, 7730) a Ru catalyst for the cyclization of 20 to the 5-6-5 product 21. Each of these processes proceeded with high diastereocontrol.

C–O Ring Construction: The Oger/Lee/Galano Synthesis of 7(RS)-ST-Δ8-11-dihomo-Isofuran

2017 ◽  
Author(s):  
Douglass F. Taber
Keyword(s):  
Ring Expansion ◽  
Radical Mechanism ◽  
Columbia University ◽  
University Of Michigan ◽  
Max Planck ◽  
University Of British Columbia ◽  
University Of Technology ◽  
National University ◽  
Seoul National University ◽  
The University

Shaorong Yang and Huanfeng Jiang of the South China University of Technology assembled (Angew. Chem. Int. Ed. 2014, 53, 7219) the β-lactone 3 by the Pd-catalyzed addition of 2 to the alkyne 1. Jack R. Norton of Columbia University observed (J. Am. Chem. Soc. 2015, 137, 1036) that the vanadium-mediated reduc­tive cyclization of 4 proceeded by a free radical mechanism, leading to the cis 3,4-disubstituted tetrahydrofuran 5. The cyclization of 6 to 7 developed (J. Org. Chem. 2015, 80, 965) by Glenn M. Sammis of the University of British Columbia also involved H atom transfer. Amy R. Howell of the University of Connecticut devised (J. Org. Chem. 2015, 80, 5196) the ring expansion of the β-lactone 8 to the tet­rahydrofuran 9. Dmitri V. Filippov and Jeroen D. C. Codée of Leiden University showed (J. Org. Chem. 2015, 80, 4553) that the net reductive alkylation of the lac­tone 10 led to 11 with high diastereocontrol. A. Stephen K. Hashmi of the Ruprecht-Karls-Universität Heidelberg optimized (Chem. Eur. J. 2015, 21, 427) the gold-mediated rearrangement of the ester 12 to the lactone 13. This reaction apparently proceeded by the coupling of the metalated lac­tone with a propargylic carbocationic species. Benjamin List of the Max-Planck-Institut für Kohlenforschung developed (Angew. Chem. Int. Ed. 2015, 54, 7703) an organocatalyst that mediated the addition of 15 to 14, leading to 16 in high ee. Scott E. Denmark of the University of Illinois published (Nature Chem. 2015, 6, 1056) a detailed study of the enantioselective cyclization of 17 to 18. Shunichi Hashimoto of Hokkaido University established (Tetrahedron Lett. 2015, 56, 1397) that his catalyst was effective for the cycli­zation of 19 to 20. Debendra K. Mohapatra of the Indian Institute of Chemical Technology showed (J. Org. Chem. 2015, 80, 1365) that allyl trimethylsilane could trap the intermediate from the cyclization of 21, leading to 22 with high diastereocontrol. Young-Ger Suh of Seoul National University used (Chem. Commun. 2015, 51, 9026) a Pd catalyst to cyclize 23 to (−)-deguelin 24. John Montgomery of the University of Michigan showed (Org. Lett. 2015, 17, 1493) that the Ni-catalyzed reduc­tive cyclization of 25 to 26 proceeded with high diastereoselectivity.

Heteroaromatic Synthesis: The Tokuyama Synthesis of (−)-Rhazinilam

2017 ◽  
Author(s):  
Douglass F. Taber
Keyword(s):  
Ni Catalyst ◽  
State University ◽  
Colorado State University ◽  
Ru Catalyst ◽  
Opioid Ligands ◽  
University Of Wisconsin ◽  
Rh Catalyst ◽  
Indian Institute Of Science ◽  
The University ◽  
École Polytechnique

Mei-Huey Lin of the National Changhua University of Education rearranged (J. Org. Chem. 2014, 79, 2751) the initial allene derived from 1 to the γ-chloroenone. Displacement with acetate followed by hydrolysis led to the furan 2. A. Stephen K. Hashmi of Ruprecht-Karls-Universität Heidelberg showed (Angew. Chem. Int. Ed. 2014, 53, 3715) that the Au-catalyzed conversion of the bis alkyne 3, mediated by 4, proceeded selectively to give 5. Tehshik P. Yoon of the University of Wisconsin used (Angew. Chem. Int. Ed. 2014, 53, 793) visible light with a Ru catalyst to rearrange the azide 6 to the pyrrole 7. Cheol-Min Park, now at UNIST, found (Chem. Sci. 2014, 5, 2347) that a Ni catalyst reorganized the methoxime 8 to the pyrrole 9. A Rh catalyst converted 8 to the corresponding pyridine (not illustrated). In the course of a synthesis of opioid ligands, Kenner C. Rice of the National Institute on Drug Abuse optimized (J. Org. Chem. 2014, 79, 5007) the preparation of the pyridine 11 from the alcohol 10. Vincent Tognetti and Cyrille Sabot of the University of Rouen heated (J. Org. Chem. 2014, 79, 1303) 12 and 13 under micro­wave irradiation to give the 3-hydroxy pyridine 14. Tomislav Rovis of Colorado State University prepared (J. Am. Chem. Soc. 2014, 136, 2735) the pyridine 17 by the Rh-catalyzed combination of 15 with 16. Fabien Gagosz of the Ecole Polytechnique rearranged (Angew. Chem. Int. Ed. 2014, 53, 4959) the azirine 18, readily available from the oxime of the β-keto ester, to the pyridine 19. Matthias Beller of the Universität Rostock used (Chem. Eur. J. 2014, 20, 1818) a Zn catalyst to mediate the opening of the epoxide 21 with the aniline 20. A Rh cata­lyst effected the oxidation and cyclization of the product amino alcohol to the indole 22. Sreenivas Katukojvala of the Indian Institute of Science Education & Research showed (Angew. Chem. Int. Ed. 2014, 53, 4076) that the diazo ketone 23 could be used to anneal a benzene ring onto the pyrrole 24, leading to the 2,7-disubstituted indole 25.

Enantioselective Construction of Alkylated Stereogenic Centers

2011 ◽  
Author(s):  
Douglass Taber
Keyword(s):  
Ni Catalyst ◽  
Max Planck ◽  
Enantiomerically Pure ◽  
University Of Oxford ◽  
Princeton University ◽  
Oxidation Reduction ◽  
Single Diastereomer ◽  
Rh Catalyst ◽  
The University ◽  
Diastereomeric Excess

Carsten Bolm of RWTH Aachen developed (Angew. Chem. Int. Ed. 2008, 47, 8920) an Ir catalyst that effected hydrogenation of trisubstituted enones such as 1 with high ee. Benjamin List of the Max-Planck-Institut Mülheim devised (J. Am. Chem. Soc. 2008, 130, 13862) an organocatalyst for the enantioselective reduction of nitro acrylates such as 3 with the Hantzsch ester 4. Gregory C. Fu of MIT optimized (J. Am. Chem. Soc. 2008, 130, 12645) a Ni catalyst for the enantioselective arylation of propargylic halides such as 6. Both enantiomers of 6 were converted to the single enantiomer of 8. Michael C. Willis of the University of Oxford established (J. Am. Chem. Soc. 2008, 130, 17232) that hydroacylation with a Rh catalyst was selective for one enantiomer of the allene 9, delivering 11 in high ee. Similarly, José Luis García Ruano of the Universidad Autónoma de Madrid found (Angew. Chem. Int. Ed. 2008, 47, 6836) that one enantiomer of racemic 13 reacted selectively with the enantiomerically- pure anion 12, to give 14 in high diastereomeric excess. Ei-chi Negishi of Purdue University described (Organic Lett. 2008, 10, 4311) the Zr-catalyzed asymmetric carboalumination (ZACA reaction) of the alkene 15 to give the useful chiron 16. David W. C. MacMillan of Princeton University developed (Science 2008, 322, 77) an intriguing visible light-powered oxidation-reduction approach to enantioselective aldehyde alkylation. The catalytic chiral secondary amine adds to the aldehyde to form an enamine, that then couples with the radical produced by reduction of the haloester. Two other alkylations were based on readily-available chiral auxiliaries. Philippe Karoyan of the Université Pierre et Marie Curie observed (Tetrahedron Lett . 2008, 49, 4704) that the acylated Oppolzer camphor sultam 20 condensed with the Mannich reagent 21 to give 22 as a single diastereomer. Andrew G. Myers of Harvard University developed the pseudoephedrine chiral auxiliary of 23 to direct the construction of ternary alkylated centers. He has now established (J. Am. Chem. Soc. 2008, 130, 13231) that further alkylation gave 24, having a quaternary alkylated center, in high diastereomeric excess.

Metal-Mediated Ring Construction: The Hoveyda Synthesis of (–)-Nakadomarin A

2015 ◽  
Author(s):  
Douglass F. Taber
Keyword(s):  
Preliminary Investigation ◽  
State University ◽  
Colorado State University ◽  
Nakadomarin A ◽  
Sterically Demanding ◽  
National University ◽  
Rh Catalyst ◽  
Seoul National University ◽  
The University ◽  
Technological University

John F. Hartwig of the University of California, Berkeley effected (J. Am. Chem. Soc. 2013, 135, 3375) selective borylation of the cyclopropane 1 to give 2. It would be particularly useful if this borylation could be made enantioselective. Eric M. Ferreira of Colorado State University showed (Org. Lett. 2013, 15, 1772) that the enantomeric excess of 3 was transferred to the highly substituted cyclopropane 4. Antonio M. Echavarren of ICIQ Tarragona demonstrated (Org. Lett. 2013, 15, 1576) that Au-mediated cyclobutene construction could be used to form the medium ring of 6. Joseph M. Fox of the University of Delaware developed (J. Am. Chem. Soc. 2013, 135, 9283) what promises to be a general enantioselective route to cyclobutanes such as 8 by way of the intermediate bicyclobutane (not illustrated). Huw M.L. Davies of Emory University reported (Org. Lett. 2013, 15, 310) a preliminary investigation in this same direction. Masahisa Nakada of Waseda University prepared (Org. Lett. 2013, 15, 1004) the cyclopentane 10 by enantioselective cyclization of 9 followed by reductive opening. Young-Ger Suh of Seoul National University cyclized (Org. Lett. 2013, 15, 531) the lactone 11 to the cyclopentane 12. Xavier Ariza and Jaume Farràs of the Universitat de Barcelona optimized (J. Org. Chem. 2013, 78, 5482) the Ti-mediated reductive cyclization of 13 to 14. The hydrogenation catalyst reduced the intermediate Ti–C bond without affecting the alkene. Erick M. Carreira of ETH Zürich observed (Angew. Chem. Int. Ed. 2013, 52, 5382) that a sterically demanding Rh catalyst mediated the highly diastereoselective cyclization of 15 to 16. The ketone 16 was the key intermediate in a synthesis of the epoxyisoprostanes. Jianrong (Steve) Zhou of Nanyang Technological University used (Angew. Chem. Int. Ed. 2013, 52, 4906) a Pd catalyst to effect the coupling of 17 with the prochiral 18. Geum-Sook Hwang and Do Hyun Ryu of Sungkyunkwan University devised (J. Am. Chem. Soc. 2013, 135, 7126) a boron catalyst to effect the addition of the diazo ester 21 to 20. They showed that the sidechain stereocenter was effective in directing the subsequent hydrogenation of 22.

Enantioselective Construction of Alkylated Centers: The Shishido Synthesis of (+)-Helianane

2015 ◽  
Author(s):  
Douglass F. Taber
Keyword(s):  
Allylic Alcohol ◽  
National University ◽  
Cu Catalyst ◽  
Rh Catalyst ◽  
Enantioselective Acylation ◽  
Enantioselective Addition ◽  
Institute Of Technology ◽  
Enantioselective Alkylation ◽  
Seoul National University ◽  
The University

Teck-Peng Loh of Nanyang Technological University developed (Org. Lett. 2011, 13, 876) a catalyst for the enantioselective addition of an aldehyde to the versatile acceptor 2 to give 3. Kirsten Zeitler of the Universität Regensburg employed (Angew. Chem. Int. Ed. 2011, 50, 951) a complementary strategy for the enantioselective coupling of 4 with 5. Clark R. Landis of the University of Wisconsin devised (Org. Lett. 2011, 13, 164) an Rh catalyst for the enantioselective formylation of the diene 7. Don M. Coltart of Duke University alkylated (J. Am. Chem. Soc. 2011, 133, 8714) the chiral hydrazone of acetone to give 9, then alkylated again to give, after hydrolysis, the ketone 11 in high ee. Youming Wang and Zhenghong Zhou of Nankai University effected (J. Org. Chem. 2011, 76, 3872) the enantioselective addition of acetone to the nitroalkene 12. Takeshi Ohkuma of Hokkaido University achieved (Angew. Chem. Int. Ed. 2011, 50, 5541) high ee in the Ru-catalyzed hydrocyanation of 15. Gregory C. Fu, now at the California Institute of Technology, coupled (J. Am. Chem. Soc. 2011, 133, 8154) the 9-BBN borane 18 with the racemic chloride 17 to give 19 in high ee. Scott McN. Sieburth of Temple University optimized (Org. Lett. 2011, 13, 1787) an Rh catalyst for the enantioselective intramolecular hydrosilylation of 20 to 21. Several general methods have been devised for the enantioselective assembly of quaternary alkylated centers. Sung Ho Kang of KAIST Daejon developed (J. Am. Chem. Soc. 2011, 133, 1772) a Cu catalyst for the enantioselective acylation of the prochiral diol 22. Hyeung-geun Park of Seoul National University established (J. Am. Chem. Soc. 2011, 133, 4924) a phase transfer catalyst for the enantioselective alkylation of 24. Peter R. Schreiner of Justus-Liebig University Giessen found (J. Am. Chem. Soc. 2011, 133, 7624) a silicon catalyst that efficiently rearranged the Shi-derived epoxide of 26 to the aldehyde 27. Amir H. Hoveyda of Boston College coupled (J. Am. Chem. Soc. 2011, 133, 4778) 28 with the alkynyl Al reagent 29 to give 30 in high ee. Kozo Shishido of the University of Tokushima prepared (Synlett 2011, 1171) 31 by the Mitsunobu coupling of m-cresol with the enantiomerically pure allylic alcohol.

Metal-Mediated Carbocyclic Construction: The Simpkins Synthesis of Ialibinones A and B

2013 ◽  
Author(s):  
Douglass F. Taber
Keyword(s):  
Gold Catalyst ◽  
Conjugate Addition ◽  
South Florida ◽  
Pt Catalyst ◽  
General Strategy ◽  
Ru Catalyst ◽  
University Of Technology ◽  
Rh Catalyst ◽  
Institute Of Technology ◽  
The University

Adriaan J. Minnaard and Ben L. Feringa of the University of Groningen devised (J. Am. Chem. Soc. 2010, 132, 14349) what promises to be a general strategy for the construction of enantiomerically pure cyclopropanes, based on conjugate addition to acceptors such as 1 . X. Peter Zhang of the University of South Florida developed (J. Am. Chem. Soc. 2010, 132, 12796) a Co catalyst for the enantioselective cyclopropanation of α-olefins such as 3. Seiji Iwasa of Toyohashi University of Technology designed (Angew. Chem. Int. Ed. 2010, 49, 8439) a resin-bound Ru catalyst that could be used repeatedly for the enantioselective cyclization of the ester 6. Rai-Shung Lin of National Tsing-Hua University showed (Angew. Chem. Int. Ed. 2010, 49, 9891) that a gold catalyst could expand the alkyne 8 to the cyclobutene 9. Takao Ikariya of the Tokyo Institute of Technology reported (J. Am. Chem. Soc. 2010, 132, 16637) a detailed study of the enantioselective conjugate addition of malonate 11 to cyclopentenone 10. Vladimir A. D’yakonov of the Russian Academy of Sciences, Ufa, showed (Tetrahedron Lett. 2010, 51, 5886) that a cyclic alkyne 13 could be annulated to the cyclopentenone 14. Shunichi Hashimoto of Hokkaido University also designed (Angew. Chem. Int. Ed. 2010, 49, 6979) a resin-bound Rh catalyst that could also be used repeatedly for the enantioselective cyclization of the ester 15. Tushar Kanti Chakraborty of the Central Drug Research Institute used (Tetrahedron Lett. 2010, 51, 4425) Ti(III) to mediate the diastereoselective cyclization of 17 to 18. Alexandre Alexakis of the University of Geneva extended (Synlett 2010, 1694) enantioselective conjugate addition of isopropenyl to the more difficult enone 19. Joseph P. A. Harrity of the University of Sheffield showed (Org. Lett. 2010, 12, 4832) that Pd could catalyze the rearrangement of 21 to 22. Strategies for the controlled construction of polycyclic ring systems are also important. Günter Helmchen of the Universität Heidelberg showed (J. Org. Chem. 2010, 75, 7917) that 23 was efficiently cyclized to the diene with Pt catalyst. The reaction could be carried out in the presence of the dienophile 24 to give 25 directly.

Establishing Arrays of Stereogenic Centers: The Sato/Chida Synthesis of (-)-Kainic Acid

2013 ◽  
Author(s):  
Douglass F. Taber
Keyword(s):  
Catalyst System ◽  
Allylic Alcohol ◽  
University Of Pennsylvania ◽  
Ni Catalyst ◽  
Max Planck ◽  
Stereogenic Centers ◽  
University College London ◽  
Enantioselective Addition ◽  
La Jolla ◽  
The University

Benjamin List of the Max-Planck-Institut, Mülheim, devised (J. Am. Chem. Soc. 2010, 132, 10227) a catalyst system for the stereocontrolled epoxidation of a trisubstituted alkenyl aldehyde 1. Takashi Ooi of Nagoya University effected (Angew. Chem. Int. Ed. 2010, 49, 7562; see also Org. Lett. 2010, 12, 4070) enantioselective Henry addition to an alkynyl aldehyde 3. Madeleine M. Joullié of the University of Pennsylvania showed (Org. Lett. 2010, 12, 4244) that an amine 7 added selectively to an alkynyl aziridine 6. Yutaka Ukaji and Katsuhiko Inomata of Kanazawa University developed (Chem. Lett. 2010, 39, 1036) the enantioselective dipolar cycloaddition of 9 with 10. K. C. Nicolaou of Scripps/La Jolla observed (Angew. Chem. Int. Ed. 2010, 49, 5875; see also J. Org. Chem. 2010, 75, 8658) that the allylic alcohol from enantioselective reduction of 12 could be hydrogenated with high diastereocontrol. Masamichi Ogasawara and Tamotsu Takahashi of Hokkaido University added (Org. Lett. 2010, 12, 5736) the allene 14 to the acetal 15 with substantial stereocontrol. Helen C. Hailes of University College London investigated (Chem. Comm. 2010, 46, 7608) the enzyme-mediated addition of 18 to racemic 17. Dawei Ma of the Shanghai Institute of Organic Chemistry, in the course of a synthesis of oseltamivir (Tamiflu), accomplished (Angew. Chem. Int. Ed. 2010, 49, 4656) the enantioselective addition of 21 to 20. Shigeki Matsunaga of the University of Tokyo and Masakatsu Shibasaki of the Institute of Microbial Chemistry developed (Org. Lett. 2010, 12, 3246) a Ni catalyst for the enantioselective addition of 23 to 24. Juthanat Kaeobamrung and Jeffrey W. Bode of ETH-Zurich and Marisa C. Kozlowski of the University of Pennsylvania devised (Proc. Natl. Acad. Sci. 2010, 107, 20661) an organocatalyst for the enantioselective addition of 27 to 26. Yihua Zhang of China Pharmaceutical University and Professor Ma effected (Tetrahedron Lett. 2010, 51, 3827) the related addition of 27 to 29. There have been scattered reports on the stereochemical course of the coupling of cyclic secondary organometallics. In a detailed study, Paul Knochel of Ludwig-Maximilians- Universität München showed (Nat. Chem. 2020, 2, 125) that equatorial bond formation dominated, exemplified by the conversion of 31 to 33.

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