Academic literature on the topic 'Gruber, Karl'

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As alkene metathesis is extended to more and more challenging substrates, improved catalysts and solvents are required. Robert H. Grubbs of Caltech developed (Organic Lett. 2008, 10, 441) the diisopropyl complex 1, that efficiently formed the trisubstituted alkene 6 by cross metathesis of 4 with 5. Hervé Clavier and Stephen P. Nolan of ICIQ, Tarragona, and Marc Mauduit of ENSC Rennes found (J. Org. Chem. 2008, 73, 4225) that after cyclization of 7 with the complex 2b, simple filtration of the reaction mixture through silica gel delivered the product 8 containing only 5.5 ppm Ru. The merit of CH2Cl2 as a solvent for alkene metathesis is that the catalysts (e.g. 1 - 3) are very stable. Claire S. Adjiman of Imperial College and Paul C. Taylor of the University of Warwick established (Chem. Commun. 2008, 2806) that although the second generation Grubbs catalyst 3 is not as stable in acetic acid, for the cyclization of 9 to 10 it is a much more active catalyst in acetic acid than in CH2Cl2 . Bruce H. Lipshutz of the University of California, Santa Barbara observed (Adv. Synth. Cat . 2008, 350, 953) that even water could serve as the reaction solvent for the challenging cyclization of 11 to 12, so long as the solubility- enhancing amphiphile PTS was included. Ernesto G. Mata of the Universidad Nacional de Rosario explored (J. Org. Chem. 2008, 73, 2024) resin isolation to optimize cross-metathesis, finding that the acrylate 13 worked particularly well. Karol Grela of the Polish Academy of Sciences, Warsaw optimized (Chem. Commun. 2008, 2468) cross-metathesis with a halogenated alkene 16. Jean-Marc Campagne of ENSC Montpellier extended (J. Am. Chem. Soc. 2008, 130, 1562) ring-closing metathesis to enynes such as 19. The product diene 20 was a reactive Diels-Alder dienophile. István E. Markó of the Université Catholique de Louvain applied (Tetrahedron Lett. 2008, 49, 1523) the known (OHL 20070122) ring-closing metathesis of enol ethers to the cyclization of the Tebbe product from 23. The ether 24 was oxidized directly to the lactone 25.

To assemble the framework of the cytotoxic macrolide Amphidinolide X 3, Fèlix Urpí and Jaume Vilarrasa of the Universitat de Barcelona devised (Organic Lett. 2008, 10, 5191) the ring-closing metathesis of the alkenyl silane 1. No Ru catalyst was effective, but the Schrock Mo catalyst worked well. In the course of a synthesis of (-)-Dactylolide 6, Michael P. Jennings of the University of Alabama offered (J. Org. Chem. 2008, 73, 5965) a timely reminder of the particular reactivity of allylic alcohols in ring-closing metathesis. The cyclization of 4 to 5 proceeded smoothly, but attempted ring closing of the corresponding bis silyl ether failed. Polyenes such as ( + )-Cytotrienin A 8 are notoriously unstable. It is remarkable that Yujiro Hayashi of the Tokyo University of Science could (Angew. Chem. Int. Ed. 2008, 47, 6657) assemble the triene of 8 by the ring-closing metathesis of the highly functionalized precursor 7. Bicyclo [2.2.2] structures such as 9 are readily available by the addition of, in this case, methyl acrylate to an enantiomerically-pure 2-methylated dihydropyridine. André B. Charette of the Université de Montréal found (J. Am. Chem. Soc. 2008, 130, 13873) that 9 responded well to ring-opening/ring-closing metathesis, to give the octahydroquinoline 10. Functional group manipulation converted 10 into the Clavelina alkaloid ( + )-Lepadin B 11. The construction of trisubstituted alkenes by ring-closing metathesis can be difficult, and medium rings with their transannular strain are notoriously challenging to form. Nevertheless, Karl-Heinz Altmann of the ETH Zürich was able (Angew. Chem. Int. Ed. 2008, 47, 10081), using the H2 catalyst, to cyclize 12 to cyclononene 13, the precursor to the Xenia lactone ( + )-Blumiolide C 14. It is noteworthy that these fi ve syntheses used four different metathesis catalysts in the key alkene forming step. For the cyclization of 7, the use of the Grubbs first generation catalyst G1, that couples terminal alkenes but tends not to interact with internal alkenes, was probably critical to success.

Jon D. Rainier of the University of Utah has put forward (J. Am. Chem. Soc. 2007 , 129 , 12604) an elegant alternative to Ru-catalyzed alkene metathesis, demonstrating that an ω-alkenyl ester such as 1 will cyclize to the enol ether 2 under Tebbe conditions. The particular reactivity of free alcohols in Ru-catalyzed alkene metathesis is underscored by the observation (Tetrahedron Lett. 2007, 48, 6905) by Javed Iqbal of Dr. Reddy’s Laboratories, Ltd., Miyapur that attempted metathesis of the ether 4a failed, but metathesis of the diol 4b proceeded efficiently. Kazunori Koide of the University of Pittsburgh has demonstrated (Organic Lett. 2007, 9, 5235) that the yields of cross-metathesis with an alkenyl alcohol could be enhanced by binding it to a trityl resin. He observed that the Grela catalyst 8 was particularly effective in this application. Residual Ru species do not interfere with some subsequent transformations. Rodrigo B. Andrade of Temple University has demonstrated (Tetrahedron Lett. 2007, 48, 5367) that metathesis with an α, β-unsaturated aldehyde such as 11 can be followed directly by phosphonate condensation to give the doubly-homologated product 12. Philip J. Parsons of the University of Sussex has found (Organic Lett. 2007, 9, 2613) that the nitro functional group is compatible with the Ru catalyst. The product nitro alkene 15 could be cyclized (intramolecular Michael addition) to the cyclopentane 16, or (intramolecular dipolar cycloaddition) to the cyclopentane 17. There has been much interest in carrying out the several alkene metathesis transformations (cross metathesis, ring closing metathesis, ring-opening metathesis polymerization) in water. Robert H. Grubbs of the California Institute of Technology has designed (Angew. Chem. Int. Ed. 2007, 46, 5152) the ammonium salt 18 for this purpose, and Karol Grela of the Polish Academy of Sciences in Warsaw and Marc Mauduit of ENSC Rennes have jointly(Chem. Commun. 2007, 3771) put forward the pyridinium salt 19. Remarkably, Ronald T. Raines of the University of Wisconsin has shown (Organic Lett. 2007, 9, 4885) that the Hoveyda catalyst 20 is sufficiently stable in aqueous acetone and aqueous DME to function efficiently.