Academic literature on the topic 'Free radical homologation'

Masahito Ochiai developed (Org. Highlights, March 24, 2008) the iodosobenzene-mediated cleavage of alkenes to keto aldehydes. Thottumkara K. Vinod of Western Illinois University described (Org. Lett. 2010, 12, 5640) a modified protocol that delivered the keto acid 2. Chi-Ming Che of the University of Hong Kong established (J. Am. Chem. Soc. 2010, 132, 13229) a method for the preparative scale Fe-catalyzed cis dihydroxylation of an alkene 3. Ilhyong Ryu of Osaka Prefecture University devised (Synlett 2010, 2014) a practical procedure for the free radical addition of HBr to an alkene 5. Tetsuo Ohta of Doshisha University showed (Tetrahedron Lett. 2010, 51, 2806) that a Ru catalyst could add an aromatic acid to the internal carbon of a terminal alkene 7. Noriki Kutsumura and Takao Saito of the Tokyo University of Science found (Org. Lett. 2010, 12, 3316) conditions for bromination/dehydrobromination to convert 10 to 11. Tsuyoshi Taniguchi of Kanazawa University oxidized (J. Org. Chem. 2010, 75, 8126) the alkene 12 to the nitro alkene 13. Professor Taniguchi added (Angew. Chem. Int. Ed. 2010, 49, 10154) methyl carbazate to 14 to give the β-hydroxy ester 15. Philippe Renaud of the University of Bern effected (J. Am. Chem. Soc. 2010, 132, 17511) the free radical homologation of 16 to the azide 18. Daniel P. Becker of Loyola University described (Tetrahedron Lett. 2010, 51, 3514) the elegant diastereoselective Pd-catalyzed bis-methoxycarbonylation of 19 to the diester 20. Matthew S. Sigman of the University of Utah established (J. Am. Chem. Soc. 2010, 132, 13981) the oxidative Heck arylation of 21 to 23. F. Dean Toste of the University of California, Berkeley, found (Org. Lett. 2010, 12, 4728) that the intermediate in the gold-catalyzed alkoxylation of 24 could couple to an aryl silane 25 to give 26. Chun-Yu Ho of the Chinese University of Hong Kong used (Angew. Chem. Int. Ed. 2010, 49, 9182) a Ni catalyst to add styrene 27 to the alkene 24. Masahiro Miura of Osaka University effected (J. Org. Chem. 2010, 75, 5421) the oxidative coupling of 29 with styrene 27 to give the linear product 30.

Alkenes are usually reduced by catalytic hydrogenation. Diimide reduction is a mild and neutral alternative. Keith R. Buszek, now at the University of Missouri, Kansas City, has shown (J. Org. Chem. 2007, 72, 3125) that the reduction can conveniently be carried out on resin-bound alkenes, using 2-NBSH (o-nitrobenzenesulfonylhydrazide) with Et3 N for convenient room temperature diimide generation. Ozone can be difficult to dispense accurately on small scale. Masahito Ochiai of the University of Tokushima has uncovered (J. Am. Chem. Soc. 2007, 129, 2772) an alternative, using acid-promoted Ph-I=O. Isolated alkenes also work well. MCPBA is the reagent most commonly used for alkene epoxidation. Payne oxidation (H2O2 /CH3CN) is a convenient and inexpensive alternative. In the course of a study of the enantioselective enzymatic hydrolysis of 6, Takeshi Sugai of Keio University has described (Tetrahedron Lett. 2007, 48, 979) a practical procedure for multigram Payne epoxidation of 5. Several procedures have been put forward for functionalizing terminal alkenes, exemplified by 7. Stefan Grimme and Armido Studer of the Universität Münster have developed (J. Am. Chem. Soc. 2007, 129, 4498) a free radical alkene amination, represented by the conversion of 7 to 9. Tehshik P. Yoon of the University of Wisconsin has found (J. Am. Chem. Soc. 2007, 129, 1866) that Cu catalyzes the addition of oxaziridines such as 10 to alkenes, to make 11. Shinji Nakamura of the University of Tokyo and Masanobu Uchiyama of the University of Tokyo and RIKEN have established (J. Am. Chem. Soc. 2007, 129, 28) that the anion from Cu promoted addition of the silyl zinc reagent to alkenes is long-lived enough to be trapped by electrophiles, including H+ to give 12. Hideki Yorimitsu and Koichiro Oshima of Kyoto University have developed (J. Am. Chem. Soc. 2007, 129, 6094) a complementary transformation, Ni-catalyzed addition of 13 to give 14. The conversion of 7 to 15 reported (Organic Lett. 2007, 9, 53) by Li-Biao Han of the National Institute of Advanced Industrial Science and Technology, Tsukuba, is likely also a free-radical process.

Justin Du Bois of Stanford University developed (J. Am. Chem. Soc. 2010, 132, 10202) a Ru catalyst for the stereoretentive hydroxylation of 1 to 2. John T. Groves of Princeton University effected (J. Am. Chem. Soc. 2010, 132, 12847) equatorial chlorination of the test substrate 3. Kenneth M. Nicholas of the University of Oklahoma found (J. Org. Chem. 2010, 75, 7644) that I2 catalyzed the amination of 5. Thorsten Bach of the Technische Universität München established (Org. Lett. 2010, 12, 3690) that the amination of 7 proceeded with significant diastereoselectivity. Phil S. Baran of Scripps/La Jolla compiled (Synlett 2010, 1733) an overview of the development of C-H oxidation. Tethering can improve the selectivity of C-H functionalization. X. Peter Zhang of the University of South Florida devised (Angew. Chem. Int. Ed. 2010, 49, 10192) a Co catalyst for the cyclization of 9 to 10. Teck-Peng Loh of Nanyang Technological University established (Angew. Chem. Int. Ed. 2010, 49, 8417) conditions for the oxidation of 11 to 12. Jin-Quan Yu, also of Scripps/La Jolla, effected (J. Am. Chem. Soc. 2010, 132, 17378) carbonylation of methyl C-H of 13 to give 14. Sunggak Kim, now also at Nanyang Technological University, established (Synlett 2010, 1647) conditions for the free-radical homologation of 15 to 17. Gong Chen of Pennsylvania State University extended (Org. Lett. 2010, 12, 3414) his work on remote Pd-mediated activation by cyclizing 18 to 19. Many schemes have been developed in recent years for the oxidation of substrates to reactive electrophiles. Gonghua Song of the East China University of Science and Technology and Chao-Jun Li of McGill University reported (Synlett 2010, 2002) Fe nanoparticles for the oxidative coupling of 20 with 21. Zhi-Zhen Huang of Nanjing University found (Org. Lett. 2010, 12, 5214) that protonated pyrrolidine 25 was important for mediating the site-selective coupling of 24 with 23. Y. Venkateswarlu of the Indian Institute of Chemical Technology, Hyderabad, was even able (Tetrahedron Lett. 2010, 51, 4898) to effect coupling with a cyclic alkene 28. AB3217-A 32, isolated in 1992, was shown to have marked activity against two spotted spider mites. Christopher R. A. Godfrey of Syngenta Crop Protection, Münchwilen, prepared (Synlett 2010, 2721) 32 from commercial anisomycin 30a. The key step in the synthesis was the oxidative cyclization of 30b to 31.

Sunggak Kim of KAIST reported (Synlett 2009, 81) an improved protocol for the one-carbon free radical homologation of an iodide such as 1 to the nitrile. Primary, secondary, and tertiary iodides work well. We described (Tetrahedron Lett. 2009, 50, 2462) a procedure for the one-carbon homologation of a halide 4 directly to the benzyl ether 6. Bin Xu of Shanghai University showed (Chem. Commun. 2009, 3246) that conversion of a ketone 8 to the 1,1-dibromoalkene set the stage for the net one-carbon homologation to the amide 9. A. Fernández-Mateos of the Universidad de Salamanca uncovered (J. Org. Chem. 2009, 74, 3913) a powerful new branching reaction, condensing the more substituted center of an epoxide 10 with a nitrile 11 to deliver the adduct 12. Useful diastereocontrol was observed with cyclic epoxides. Uli Kazmaier of the Universität des Saarlandes optimized (Adv. Synthy. Cat. 2009, 351, 1395) a Mo catalyst for the hydrostannation of a terminal alkene 13 to the branched product 14. Dong-Mei Cui of the Zhejiang University of Technology and Chen Zhang of Zhejiang University (both in Hangzhou) developed (Chem. Commun. 2009, 1577) a complementary procedure, converting the terminal alkene 15 into the branched alkenyl tosylate 16. The Wittig reaction is notorious for racemizing sensitive aldehydes. Hélène Lebel of the Université de Montréal demonstrated (Organic Lett. 2009, 11, 41) a simple one-pot protocol for sequential oxidation and homologation of 17 that preserved the integrity of the adjacent stereogenic center. The stereocontrolled construction of trisubstituted alkenes is still a major issue in organic synthesis. Giancarlo Verardo of the University of Udine established (J. Phys. Org. Chem. 2009, 22, 24) that the α-diazo ester 19, readily prepared directly from the simple ester, was converted by I2 to the alkene 20 with high geometric control. Condensation with the Ohira reagent 22 is often the method of choice for converting an aldehyde into the homologated alkyne. Hubert Maehr and Milan Uskokovic of Bioxell and Carl P. Schaffner of the Waksman Institute described (Syn. Commun. 2009, 39, 299) an optimized, scalable procedure for the in situ preparation of 22 and the conversion of 21 to 23. Note, again, that the sensitive stereogenic center adjacent to the intermediate aldehyde was not epimerized under the reaction conditions.

Reaction with an enantiomerically-pure epoxide is an efficient way to construct a molecule incorporating an enantiomerically-pure oxygenated stereogenic center. The Jacobsen hydrolytic resolution has made such enantiomerically-pure epoxides readily available from the corresponding racemates. Christopher Jones and Marcus Weck of the Georgia Institute of Technology have now (J. Am. Chem. Soc. 2007, 129, 1105) developed an oligomeric salen complex that effects the enantioselective hydrolysis at remarkably low catalyst loading. Any such approach depends on monitoring the progress of the hydrolysis, usually by chiral GC or HPLC. In a complementary approach, we (J. Org. Chem. 2007, 72, 431) have found that on exposure to NBS and the inexpensive mandelic acid 2, a terminal alkene such as 1 was converted into the two bromomandelates 3 and 4. These were readily separated by column chromatography. Individually, 3 and 4 can each be carried on the same enantiomer of the epoxide 5. As 3 and 4 are directly enantiomerically pure, epoxide 5 of high ee can be prepared reliably without intermediate monitoring by chiral GC or HPLC. Another way to incorporate an enantiomerically-pure oxygenated stereogenic center into a molecule is the enantioface-selective addition of hydride to a ketone such as 6. Alain Burgos and his team at PPG-SIPSY in France have described (Tetrahedron Lett. 2007, 48, 2123) a NaBH4 -based protocol for taking the Itsuno-Corey reduction to industrial scale. In the past, aldehydes have been efficiently α-oxygenated using two-electron chemistry. Mukund P. Sibi of North Dakota State University has recently (J. Am. Chem. Soc. 2007, 129, 4124) described a novel one-electron alternative. The organocatalyst 10 formed an imine with the aldehyde. One-electron oxidation led to an α-radical, which was trapped by the stable free radical TEMPO to give, after hydrolysis, the α-oxygenated aldehyde 11. High ee oxygenated secondary centers can also be prepared by homologation of aldehydes. Optimization of the enantioselective addition of the inexpensive acetylene surrogate 13 was recently reported (Chem. Commun. 2007, 948) by Masakatsu Shibasaki of the University of Tokyo. Note that the free alcohol of 13 does not need to be protected.

Peter Legzdins of the University of British Columbia has described (J. Am. Chem. Soc. 2007, 129, 5372) a stoichiometric tungsten complex that specifically functionalized the primary H of an alkane 1 to give the organometallic 2. Neither the scope of the reactivity of 2 nor the functional group compatibility of this process have as yet been explored. Ruggero Curci of the Università di Bari has reported (Tetrahedron Lett. 2007, 48, 3575) the stereospecific hydroxylation of 1,3-dimethyl cyclohexane 4 to the diol 6. Yasuyuki Kita of Osaka University has developed (Organic Lett. 2007, 9, 3129) conditions for specific benzylic oxidation, converting 7 into 8 with high diastereocontrol. Larry E. Overman of the University of California, Irvine has established (Organic Lett. 2007, 9, 5267) that by using a slow H-atom donor, it was possible to effect intramolecular H abstraction, leading, by oxidation of the intermediate captodatively-stabilized radical, from 9 to the acetate 10. The target C-H of 9 is activated by being adjacent to the ring nitrogen. There are many other ways that nitrogen, easily oxidized, has been used to activate a C-H for bond formation. Takehiko Yoshimitsu and Tetsuaki Tanaka of Osaka University have established (Organic Lett. 2007, 9, 5115) a free-radical route for the homologation of a tertiary amine such as 11 with phenyl isocyanate 12. Chuan He of the University of Chicago has devised (Angew. Chem. Int. Ed. 2007, 46, 5184) an Ag catalyst for the oxidative cyclization of sulfamates such as 15. M. Christina White of the University of Illinois has developed (J. Am. Chem. Soc. 2007, 129, 7274) a ligand system that allows the diastereoselective Pd-mediated allylic oxidation of 16 to 17. The cyclization of 18 to 19 developed (J. Org. Chem. 2007, 72, 8994) by Renhua Fan of Fudan University is thought to be proceeding via H atom abstraction by an intermediate nitrogen radical. The oxidation of the amine 20 to the endocyclic enamine 21 reported (J. Am. Chem. Soc. 2007, 129, 14544) by Maurice Brookhart of the University of North Carolina depended on the ease of oxidative addition of an intermediate alkenyl Co complex into the C-H bond adjacent to the nitrogen. The multistep cyclization of 22 to 23 devised (Organic Lett. 2007, 9, 4375) by Philippe Renaud of the Universität Bern depended on the ease of H atom abstraction adjacent to nitrogen.

Fabio Doctorovich of the Universidad de Buenos Aires reported (J. Org. Chem. 2008, 73, 5379) that hydroxylamine in the presence of an Fe catalyst reduced alkenes such as 1, but not ketones or esters. Erick Carreira of ETH Zürich developed (Angew. Chem. Int. Ed. 2008, 47, 5758) mild conditions for the hydrochlorination of mono-, di- and trisubstituted alkenes. Ramgopal Bhattacharyya of Jadavpur University established (Tetrahedron Lett. 2008, 49, 6205) a simple Mo-catalyzed protocol for alkene epoxidation. Nitro alkenes are of increasing importance as acceptors for enantioselective organocatalyzed carbon-carbon bond formation. Matthias Beller of the Universität Rostock found (Adv. Synth. Cat. 2008, 350, 2493) that an alkene such as 7 was readily converted to the corresponding nitroalkene 8 by exposure to of NO gas. The reaction could also be effected with NaNO2/HOAC. Two complementary protocols for Rh-catalyzed alkene hydroformylation have been reported. Xumu Zhang of Rutgers University devised (Organic Lett. 2008, 10, 3469) a ligand system that cleanly migrated the alkene of 9, then terminally hydroformylated the resulting monosubstituted alkene, to give 10. Kian L. Tan of Boston College designed (J. Am. Chem. Soc. 2008, 130, 9210) a ligand such that the hydroformylation of the internal alkene of 11 was directed to the end of the alkene proximal to the directing OH, delivering 12. Several other methods for the functionalizing homologation of alkenes have been put forward. Chul-Ho Jun of Yonsei University assembled (J. Org. Chem. 2008, 73, 5598) a Rh catalyst that effected the oxidative acylation of a terminal alkene 13 with a primary benzylic alcohol, to give the ketone 14. For now, this approach is limited to less expensive alkenes, as the alkene, used in excess, is the reductant in the reaction. The other procedures outlined here require only stoichiometric alkene. Yasuhiro Shiraishi of Osaka University devised (Organic Lett. 2008, 10, 3117) a simple photoprocess for adding acetone to a terminal alkene 13 to give the methyl ketone 14, in what is presumably a free radical reaction.

The pentacyclic alkaloid manzamine A 4, isolated from a sponge collected in the Okinawa Sea, displays a range of antibacterial, anticancer, and antimalarial activity. The preparation of 4 reported (J. Am. Chem. Soc. 2012, 134, 17482) by Darren J. Dixon of the University of Oxford showcases the versatility of the nitro group in organic synthesis. The nitro alkene 2 was prepared from the commercial bromide 5. Displacement with acetate followed by Swern oxidation led to the aldehyde 6, which was condensed with nitromethane to give 2. Lactam 1 was an intermediate in Professor Dixon’s synthesis (Org. Highlights May 3, 2010) of (–)-nakadomarin A. Lactam 1 was prepared from the tosylate 7, which was derived from pyroglutamic acid. The addition of 1 to the nitroalkene 2 delivered 3 as the dominant diastereomer of the four that were possible. Mannich condensation with formaldehyde and the amine 12 gave 13. The nitro group of 13 was removed by free radical reduction. Exposure of the reduced product to trimethylsilyl iodide gave, via ionization of the ketal, the primary iodide, which was carried onto the nitro compound 14. Dibal selectively reduced the δ-lactam. Partial reduction of the γ-lactam then gave an intermediate that engaged in Mannich condensation with the nitro-activated methylene to give 15. Although there are many protocols for the conversion of a nitro compound to a ketone, most of those were not compatible with the functional groups of 15. Fortunately, Ti(III) was effective. Ce-mediated addition of the Grignard reagent 16 to the ketone followed by deprotection and protection then delivered the silyl ether 17. Remarkably, the ketone 17 could be deprotonated and carried on to the enol triflate 18 without eliminating the TMSO group. Coupling with the stannane 19 then completed the synthesis of manzamine A 4. One-carbon homologation of 18 led to ircinol A, ircinal A, and methyl ircinate (not illustrated).