Metathesis reactions in total synthesis

Allenylidene-to-Indenylidene Rearrangement in Arene−Ruthenium Complexes: A Key Step to Highly Active Catalysts for Olefin Metathesis Reactions

The allenylidene-ruthenium complexes [(eta6-arene)RuCl(=C=C=CR2)(PR'3)]OTf (R2 = Ph; fluorene, Ph, Me; PR'3 = PCy3, P(i)Pr3, PPh3) (OTf = CF3SO3) on protonation with HOTf at -40 C are completely transformed into alkenylcarbyne complexes [(eta6-p-cymene)RuCl([triple bond]CCH=CR2)(PR3)](OTf)2. At -20 degrees C the latter undergo intramolecular rearrangement of the allenylidene ligand, with release of HOTf, into the indenylidene group in derivatives [(eta6-arene)RuCl(indenylidene)(PR3)]OTf. The in situ-prepared indenylidene-ruthenium complexes are efficient catalyst precursors for ring-opening metathesis polymerization of cyclooctene and cyclopentene, reaching turnover frequencies of nearly 300 s(-1) at room temperature. Isolation of these derivatives improves catalytic activity for the ring-closing metathesis of a variety of dienes and enynes. A mechanism based on the initial release of arene ligand and the in situ generation of the active catalytic species RuCl(OTf)(=CH2)(PR3) is proposed.

Total Synthesis and Structural Elucidation of Azaspiracid-1. Construction of Key Building Blocks for Originally Proposed Structure

Syntheses of the three key building blocks (65, 98, and 100) required for the total synthesis of the proposed structure of azaspiracid-1 (1a) are described. Key steps include a TMSOTf-induced ring-closing cascade to form the ABC rings of tetracycle 65, a neodymium-catalyzed internal aminal formation for the construction of intermediate 98, and a Nozaki-Hiyama-Kishi coupling to assemble the required carbon chain of fragment 100. The synthesized fragments, obtained stereoselectively in both their enantiomeric forms, were expected to allow for the construction of all four stereoisomers proposed as possible structures of azaspiracid-1 (1a-d), thus allowing the determination of both the relative and absolute stereochemistry of the natural product.

Ring-Opening/Ring-Closing Metathesis (RORCM) Reactions of 7-Azanorbornene Derivatives. An Entry into Perhydroindolines

[reaction: see text]. 7-Azanorbornenes undergo ring-opening/ring-closing metathesis upon treatment with the second-generation Grubbs catalyst to give hexahydroindoline derivatives.

Total synthesis of floresolide B and Δ6,7-Z-floresolide B

The total syntheses of the cytotoxin marine natural product floresolide B (1) and its Delta(6,7)-Z isomer (2) have been achieved through an olefin metathesis-based strategy.

A practical synthetic route to functionalized THBCs and oxygenated analogues via intramolecular Friedel–Crafts reactions

A practical catalytic approach to the synthesis of 4-substituted 1,2,3,4-tetrahydro-beta-carbolines (THBCs, 1) and 1,2,3,9-tetrahydropyrano[3,4-b]indoles (2) via InBr3-catalyzed intramolecular Friedel-Crafts (F-C) cyclization is described. The use of cross-metathesis reaction represents a direct route to the cyclization precursors and the use of InBr3 (5 mol%) allowed polycyclic indole compounds to be isolated in high yields under mild reaction conditions (rt, DCM, minutes). Finally, efforts toward the development of a stereocontrolled version of the present cyclization are presented, highlighting [salenAlCl] and bimetallic [(salenAlCl)2-InBr3] system as promising chiral Lewis acids (ee up to 60%).

A short enantioselective total synthesis of the phytotoxic lactone herbarumin III

The phytotoxic lactone herbarumin III has been synthesized in 11% overall yield. The approach applied uses Keck's asymmetric allylation and Sharpless epoxidation to build the key fragment. Esterification with 5-hexenoic acid and a ring closing metathesis was used to arrive at the target.

P-Heterocyclic carbenes as potential ligands in the design of new metathesis catalysts. A computational study

Density functional calculations are reported concerning the olefin metathesis characteristics of a variety of P-heterocyclic carbene (PHC) complexes. The calculations employ model catalysts of the type (PMe3)(PHC)Cl2Ru=CH2, the PHC ligands being 1,3-dihydro-1,3-diphosphol-2-ylidene PH, 1,3-diphenyl-1,3-diphosphol-2-ylidene PPH, and 1,4-dihydro-1,4-diphosphol-2-azol-5-ylidene PNH. Complexes with N-heterocyclic carbenes (NHC) are included for comparison. Associative and dissociative reaction pathways are considered, the latter ones representing the favored reaction mechanisms. Calculations show that the rate determining step is ring opening of a ruthena-cyclobutane intermediate. In comparison with NHC model catalysts, the PHC compounds have lower phosphine dissociation energies, and also form weaker pi-complexes with an olefinic substrate. Compared to the initially formed pi-complexes, the ruthena-cyclobutane is more stable for PHC- than for NHC-catalysts. The catalytic activity of model PHC-compounds in comparison with NHC-compounds is discussed on the basis of the calculated reaction profiles. In this context, different models for enhanced reactivity of NHC-based catalysts that have been proposed in the literature are considered as well. It is demonstrated that the nature of the substituent of the carbene phosphorus not only exhibits a steric influence on the course of the reaction, but a significant stereoelectronic effect as well. Further, agostic interactions in ruthena-cyclobutane intermediates are investigated.

Synthesis of iso-epoxy-amphidinolide N and des-epoxy-caribenolide I structures. Revised strategy and final stages

A general and highly convergent synthetic route to the macrocyclic core structures of the antitumour agents amphidinolide N (1) and caribenolide I (2) has been developed, and the total synthesis of iso-epoxy-amphidinolide N and des-epoxy-caribenolide I structures is described. Central to the revised strategy was the use of a Horner-Wadsworth-Emmons olefination between beta-ketophosphonate 51 and aldehyde 14 to construct the C1-C13 sector common to both 1 and 2. Stereoselective alkylation of hydrazone 11 with iodide 65 and then with bromide 56 allowed for the rapid assembly of the complete caribenolide I carbon skeleton. Key steps in the completion of the synthesis of des-epoxy-caribenolide I structure 78 included hydrolysis of a sensitive methyl ester using Me(3)SnOH, followed by regioselective macrolactonisation of the resulting diol seco-acid and global deprotection. Coupling of hydrazone 11, bromide 56 and iodide 64 was followed by an analogous sequence of late-stage manoeuvres to arrive at the fully deprotected des-epoxy-amphidinolide N framework, obtained as a mixture of hemiacetal and bicyclic acetal 84. Regio- and diastereo-selective epoxidation of the C6 methylene group in bicyclic acetal 84 provided access to iso-epoxy-amphidinolide N stereoisomer 89.

Surprisingly Efficient Catalytic Cr-Mediated Coupling Reactions

[reaction: see text] With use of 1 mol % of Cr catalyst 5, surprisingly efficient Cr-mediated couplings of aldehydes with various types of nucleophiles have been realized. The catalyst set of Cr catalyst 5 and Ni catalyst 4 is used for alkenylation, alkynylation, and arylation, whereas the catalyst set of Cr catalyst 5 and CoPc (cobalt phthalocyanine) is used for 2-haloallylation, alkylation, and propargylation. Only the Cr catalyst 5 is required for allylation. The reaction rates in DME and THF have been found significantly faster than that in MeCN.

Joys of Molecules. 1. Campaigns in Total Synthesis

Our bodies are made of molecules, and it is from molecules that we derive our strength and joys. The joys of molecules manifest themselves in many ways. These include beautiful colors, exquisite aromas, distinct tastes, psychological ups and downs, and intellectual inspirations, among other forms of stimulation, material or spiritual. In this Perspective, written on the occasion of the 2005 American Chemical Society Arthur C. Cope Award address, I recount some of the joys I have experienced and shared with my students during campaigns to synthesize some of Nature's most intriguing and complex molecules.