An unusual dichotomy in the regioselectivity of a metal catalyzed versus thermal ene reaction

A Pd(+2) catalyzed cyclization of a 1,6-enyne complements a thermal Alder ene reaction; a rationale invoking a remote binding site is proposed.     

Cyclizations via Palladium-Catalyzed Allylic Alkylations [New Synthetic Methods (79)]

The history of ring systems in organic chemistry parallels their synthetic accessibility. Transition-metal-catalyzed cyclizations offer a new opportunity to create carbo- and heterocyclic compounds with great facility. Among these methods, allylic alkylations catalyzed by palladium have proven unusually productive because of the extraordinary chemo-, regio-, and diastereoselectivity and the continuing possibility for the development of enantioselectivity. The rules for ring closure differ from those for non-transition-metal-catalyzed reactions. A major benefit is the ability to generate medium (eight-, nine-, ten-, and eleven-membered) and large rings in preference to normal (five-, six- and seven-membered) rings. With the appropriate substrate, efficient macrocyclizations are possible under conditions of normal concentrations. A second major benefit derives from the complementary stereochemistry of the metalcatalyzed substitution (net retention of configuration) compared to non-metal-catalyzed reactions (inversion of configuration). Further, the requirement for the substrate to conform to the transition-metal template may impose a stereochemical preference in the intermediate that ultimately translates into the thermodynamically less stable organic product regardless of the stereochemistry of the starting material. While more work has focused on carbocyclic synthesis, the possibilities for heterocyclic synthesis are just beginning to be tapped. In addition to forming heterocycles by C? C bond formation, use of a heteroatom as a nucleophile has already proven effective for oxygen and nitrogen, with other nucleophiles awaiting investigation. New dimensions for cyclization via allylic alkylation arise by generating the requisite π-allylpalladium intermediates by methods other than palladium(0)-initiated allylic ionizations. In addition, metals other than palladium will clearly expand the possibilities, but as yet remain untapped.     

Total synthesis of (+)-amphidinolide A. Structure elucidation and completion of the synthesis

The structure elucidation of (+)-amphidinolide A, a cytotoxic macrolide, has been accomplished by employing a combination of NMR chemical shift analysis and total synthesis. The 20-membered ring of amphidinolide A was formed by a ruthenium-catalyzed alkene-alkyne coupling to forge the C15-C16 bond. Using the reported structure 1 as a starting point, a number of diastereomers of amphidinolide A were prepared. Deviations of the chemical shift of key protons in each isomer relative to the natural material were used as a guide to determine the locations of the errors in the relative stereochemistry. The spectroscopic data for the synthetic and natural material are in excellent agreement.     

Syntheses of seven-membered rings: ruthenium-catalyzed intramolecular [5+2] cycloadditions

The Ru-catalyzed intramolecular [5+2] cycloaddition of cyclopropylenynes is investigated with respect to the regio- and diastereoselectivity as well as the functional group compatibility of the reaction. Evidence for the mechanism as occurring through a ruthenacyclopentene intermediate is elucidated from 1) the study of the diastereoselectivity of the cycloaddition; 2) the effect of variation of substituents on the regioselectivity of cyclopropyl bond cleavage in 1,2-trans- and 1,2-cis-disubstituted cyclopropanes and 3) examples that clearly do not involve ruthenacyclohexene as intermediates as products still incorporate the cyclopropyl moiety. The scope and limitations of the Ru-catalyzed cycloaddition are discussed and compared with the Rh-catalyzed reaction. The potential power of this methodology towards natural product total synthesis is demonstrated by the formation of several polycyclic systems with the chosen reaction conditions and readily available cyclopropylenyne substrates.     

A chemoselective reduction of alkynes to (E)-alkenes

The trans reduction of all types of alkynes to give (E)-olefins is achieved through a two-stage trans hydrosilylation and protodesilylation. Reaction of an alkyne and a silane with the ruthenium catalyst [Cp*Ru(MeCN)3]PF6 results in clean hydrosilylation to give only the (Z)-trans addition product at ambient temperature with catalyst loadings of 1-5 mol %. The crude vinylsilane products are then protodesilylated by the action of cuprous iodide and TBAF at rt-35 degrees C. The reaction is compatible with many sensitive functional groups and provides a general trans-alkyne reduction not possible by other means.