Synthesis of functionalized maoecrystal V core structures.

Two strategies toward the total synthesis of maoecrystal V (1) culminating in the construction of core structures 2 and 3 are described.

On the mechanism of the palladium catalyzed intramolecular Pauson-Khand-type reaction.

Density functional theory calculations and experimental studies have been carried out on the intramolecular Pauson-Khand-Type reaction mediated by a PdCl(2)-thiourea catalyst, which proceeds under mild reaction conditions and provides a useful alternative to traditional Pauson-Khand reactions. The classical mechanism of the Pauson-Khand reaction involving the alkyne/alkene C-C bond formation as the key step has been found to be energetically unfavorable and is not in line with the experimental observations. A novel reaction mechanism has been proposed for the reaction. The first step involves the cis-halometalation of the alkyne, followed by sequential alkene and carbonyl insertion. The rate-determining fourth step is an intramolecular C-Cl oxidative addition, leading to a Pd(IV) intermediate. A C-C bond formation by reductive elimination completes the reaction. The mechanism is in agreement with the key experimental observations including (1) the need of a chloride for catalytic activity and the absence of catalysis with Pd(OAc)(2) alone; (2) the rate acceleration by the addition of LiCl; both with PdCl(2) and Pd(OAc)(2) catalysts; and (3) the preferred formation of the trans diastereomer in substituted cases. The cis halometalation and the formation and stability of the Pd(IV) intermediate is studied in detail and provides general insights into these novel steps.

Formal total synthesis of N-methylmaysenine.

A novel synthetic approach for the formal total synthesis of N-methylmaysenine (1) has been developed. Key steps involve the Ti-mediated vinylogous Mukaiyama aldol reaction of chiral ketene silyl N,O-acetal with beta-dithiane-substituted aldehyde, an aldol condensation, and a ring-closing metathesis reaction.

Synthetic study of 1,3-butadiene-based IMDA approach to construct a [5-7-6] tricyclic core and its application to the total synthesis of C8-epi-guanacastepene O.

An efficient intramolecular Diels-Alder (IMDA) strategy for the construction of the [5-7-6] tricyclic core (18) of guanacastepenes has been developed from cis- and trans-1,3-butadiene-tethered 4-oxopent-2-ynoic acid ethyl esters 10 and 11. This method facilitates the synthesis of C8-epi-guanacastepene O (36) in a very efficient manner.

Thioureas as ligands in the pd-catalyzed intramolecular Pauson-Khand reaction.

[reaction: see text] The thiourea-Pd complex was established as a novel type of catalyst in the PKR of allylpropargylamine, and the demonstrated chemistry may prove to be valuable for developing thiuorea as a ligand for the Pd-catalyzed Pauson-Khand reaction.

Tetramethyl thiourea/Co2(CO)8-catalyzed Pauson-Khand reaction under balloon pressure of CO.

A Pauson-Khand type of conversion of enynes to bicyclic cyclopentenones employing the commercially available Co2(CO)8 and tetramethylthiourea (TMTU) as catalysts is described. This method allows a variety of enynes with diverse functional groups to be cyclized into cyclopentenones of interest. [reaction: see text]

A highly efficient synthesis of the FGH ring of micrandilactone A. Application of thioureas as ligands in the Co-catalyzed Pauson-Khand reaction and Pd-catalyzed carbonylative annulation.

The functionalized FGH ring system of micrandilactone A was successfully constructed in high selectivity and good yields. The key reactions in our strategy are the Co-thiourea-catalyzed stereoselective, intramolecular Pauson-Khand reaction and Pd-thiourea-catalyzed stereoselective, intramolecular annulation. [structure: see text]