Regiocontrol in Pd(0)-Mediated Couplings of a Bis(enol triflate) with Alkynes — Stereoselective Syntheses of Monocyclic Dienediynes Related to the Neocarzinostatin Chromophore

Cascade cyclization: an easy access to highly unsaturated polycyclic ring systems through a tandem stille/[4 + 2] reaction under mild conditions

The synthesis of several polycyclic compounds 1a-c, 2, and 3 has been performed through a tandem Stille/[4 + 2] cascade reaction from cyclic bis(enoltrifluomethanesulfonate) 4a-c, 5, and 6, respectively. The reaction proceeds very efficiently in a one-pot operation at roomtemperature in DMF in the presence of a catalytic amount of Pd(CH(3)CN)(2)Cl(2) and LiCl.[reaction: see text]

Unexpected hydrodeiodo Sonogashira-Heck-Casser coupling reaction of 2,2'-diiodobiphenyls with acetylenes

2,2'-Diiodobiphenyl-4,4'-dicarboxylic acid dimethyl ester (3) undergoes either a ring-closure reaction with phenylacetylene to give 4 or hydrodeiodo phenylethynylation to give 5 under the catalytic conditions of Pd(OAc)(2)/CuI/phosphine in amines. In these reactions, the amine and the phosphine ligands play important roles in controlling the reactivity. Among the ligands we used, tris(o-tolyl)phosphine is the best ligand for hydrodeiodo phenylethynylation, while the bidentate phosphine ligand retards both of the reactions. On the basis of our results, we propose that 5 is formed through a fast hydrodeiodination, followed by a Sonogashira phenylethynylation. The results of the deuterium labeling experiments show that proton exchange between the acetylenic proton and the alkyl protons of amine occurs effectively under the reaction conditions. In addition, the hydrogen that replaces the iodide in the hydrodeiodination process arises mainly from the acetylenic proton.

New syntheses of the C,D-ring pyrromethenones of phytochrome and phycocyanin

Pyrromethenone 7, the C,D-ring segment of phytochrome (Pr, 4), has been prepared in an efficient fashion employing three new strategies. Each of these has potential advantages for the synthesis of labeled material. Our first approach is related to the Gossauer synthesis, with the difference that strong alkali is avoided in the condensation of the C- and D-ring components 8 and 17. The key silyloxypyrrole 17 was readily prepared on multigram scales beginning with inexpensive butyrolactone (10). A second synthesis began with 2-acetylbutyrolactone (41). The key steps involved conversion of 41 to the Z-enoltriflate 42, followed by Pd(0)-catalyzed coupling with trimethylsilylacetylene, p-chlorophenylselenide ring opening, and finally, amidation to afford the ring-D synthon 45 having the proper geometry and oxidation state for conversion to 7. Sonogashira coupling of 45 with the iodopyrrole 22, followed by oxidative elimination, and F(-)-induced 5-exo-dig cyclization of the resultant pyrroloalkyne 47, then completed the synthesis. In similar fashion, we have also prepared pyrromethenone 6, the C,D-ring segment of phycocyanin (2).