Development of Useful Reactions Involving Tandem Cyclizations Based on the Novel Reactivities of Allenic Compounds

[Elucidation of Biological Mechanisms Using Synthetic Natural Products and Their Derivatives]

Natural products are useful sources in the search for biochemical probes and drug leads because of their unique biological activities. However, synthetic studies or functional analyses of polycyclic complex natural products or conjugated lipids (e.g., glycolipids) are often hampered because of their synthesis and handling are challenging. On the basis of rational designs, synthetic studies, and chemical modifications, natural products need to be optimized to more potent compounds with improved activities, selectivities and/or physical properties. We have been synthesizing natural products and their derivatives for the elucidation of their biological mechanisms and discovery of drug leads. This review describes three topics for developing functional compounds derived from natural products for prospective involvement in pharmaceutical research: 1) direct construction of the ergot alkaloid scaffold by palladium catalyzed domino cyclization of amino allenes; 2) identification of novel sphingosine kinase inhibitors through a structure-activity relationship study of jaspine B; and 3) design, synthesis and biological evaluation of novel CD1d glycolipid ligands containing modified lipid moieties.

Ring-construction/stereoselective functionalization cascade: total synthesis of pachastrissamine (jaspine B) through palladium-catalyzed bis-cyclization of propargyl chlorides and carbonates

Palladium(0)-catalyzed cyclization of bromoallenes, propargyl chlorides, and carbonates bearing hydroxy and benzamide groups as internal nucleophiles stereoselectively provides functionalized tetrahydrofuran. Cyclization reactivity is dependent on the relative configuration of the benzamide and leaving groups, and on the nature of the leaving groups. This bis-cyclization was used as the key step in a short total synthesis of pachastrissamine, which is a biologically active marine natural product.

Quantitative Determination of the Regioselectivity of Nucleophilic Addition to η-Propargyl Rhenium Complexes and Direct Observation of an Equilibrium Between η-Propargyl Rhenium Complexes and Rhenacyclobutenes

PMe(3) adds selectively to the central carbon of the η(3)- propargyl complex [C(5)Me(5)(CO)(2)Re(η(3)-CH(2)C≡CCMe(3))][BF(4)] (1-t-Bu) to form the metallacyclobutene [C(5)Me(5)(CO)(2)Re(CH(2)C(PMe(3))=CCMe(3))][BF(4)] (7). The rate of rearrangement of the metallacyclobutene 7 to η(2)-alkyne complex [C(5)Me(5)(CO)(2)Re(η(2)-Me(3)PCH(2)C≡CCMe(3))][BF(4)] (8) is is independent of phosphine concentration, consistent with a dissociative mechanism proceeding via η(3)-propargyl complex 1-t-Bu. The rate of this rearrangement is 480 times slower than the rate of exchange of PMe(3) with the labeled metallacyclobutene 7-d(9). This rate ratio provides an indirect measurement of the regioselectivity for addition of PMe(3) to the central carbon of η(3)-propargyl complex 1-t-Bu to give 7 compared to addition to a terminal carbon to give 8. The addition of PPh(3) to 1-t-Bu gives the metallacyclobutene [C(5)Me(5)(CO)(2)Re(CH(2)C(PPh(3))=CCMe(3))][BF(4)] (11). Low temperature (1)H NMR spectra provide evidence for an equilibrium between metallacyclobutene 11 and η(3)-propargyl complex 1-t-Bu (K(eq) ≈ 44 M(-1) at -46 °C and ΔG° (0 °C) = -1.2 ± 0.2 kcal mol(-1)).

Total synthesis of (+/-)-lysergic acid, lysergol, and isolysergol by palladium-catalyzed domino cyclization of amino allenes bearing a bromoindolyl group

Ergot alkaloids and their synthetic analogs have been reported to exhibit broad biological activity. We investigated direct construction of the C/D ring system of ergot alkaloids based on palladium-catalyzed domino cyclization of amino allenes. With this biscyclization as the key step, total synthesis of (+/-)-lysergic acid, (+/-)-lysergol, and (+/-)-isolysergol was achieved.

Palladium-catalyzed diastereo- and enantioselective Wagner-Meerwein shift: control of absolute stereochemistry in the C-C bond migration event

Inducing absolute stereochemistry in Wagner-Meerwein shifts was examined in a ring expansion protocol. Initiated by generation of a pi-allylpalladium intermediate by hydropalladation of allenes, the ring expansion of allenylcyclobutanol substrates proceeded with excellent diastereo- and enantioselectivities. The results demonstrate that, during the C-C bond migration process, our chiral catalysts can control the stereochemistry of both the pi-allylpalladium intermediate and the corresponding migration bond. Moreover, the stereochemical outcome of the reaction can be rationalized very well with the working model of the chiral catalyst. The method provides an efficient way to synthesize highly substituted cyclopentanones with an alpha-chiral O-tertiary center which has various synthetic applications.

Computational investigations on the general reaction profile and diastereoselectivity in sulfur ylide promoted aziridination

Mechanism and diastereoselectivity of sulfur ylide promoted aziridination reactions were studied by density functional theory with inclusion of solvent effects through the continuum solvation model. The general reaction pathway was modeled for the addition of substituted sulfur ylides (Me(2)S(+)CH(-)R) to an aldimine ((E)-methyl ethylidenecarbamate, MeHC=NCO(2)Me). The nature of the substituents on the ylidic carbon atom substantially affects the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode leading to trans aziridines via anti betaine intermediates. The simplest model ylide (unstabilized, R=H) underwent cisoid addition in a similar fashion. In the case of stabilized ylides product diastereoselectivity is controlled by the barriers of the elimination step leading to the 2,3-trans aziridine, whereas it is decided in the addition step in the case of semistabilized ylides. The importance of steric and electronic factors in diastereoselective addition (2 and 5) and elimination (5) transition states was established. Comparison of results obtained with the gas-phase optimized geometries and with the fully optimized solvent-phase geometries reveals that the inclusion of solvent effects does not bring about any dramatic changes in the reaction profiles for all three kinds of ylides. In particular, diastereoselectivity for both kinds of ylides was found to be nearly the same in both these approaches.