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New Strategies for the Synthesis of Biologically Important Tetrapyrroles. The \"B,C + D + A\" Approach to Linear Tetrapyrroles J. Org. Chem. 2000, 65, 205.

Linear tetrapyrroles related to phytochrome were prepd. in enantiospecific fashion by a new strategy beginning with ring-B,C synthons of type (bis-iododipyrrins)(I). Rings A and D were elaborated by Pd(0)-mediated coupling of I with the appropriate alkyne acid or amide derivs. (II) [R = Et, Me; R1 = Me, Et, CH=CH2, (S)-CH(OH)Me; R2 = OH, NH2], followed by intramol. cyclization (method C: BC + D + A .fwdarw. ABCD).

Total Syntheses of (.+-.)- and (-)-Stemoamide J. Am. Chem. Soc. 2000, 122, 4295.

(.+-.)-Stemoamide (I) was prepd. in seven steps beginning with .gamma.-chlorobutryl chloride and succinimide, which were efficiently converted to the key alkyne oxazole (II) on a multigram scale. Intramol. (Diels-Alder)-(retro-Diels-Alder) reaction of II then gave butenolide (III) directly upon aq. workup. The remaining two stereocenters in I were established in a single step by a highly selective redn. of III (NaBH4/NiCl2), followed by equilibration to the thermodynamically favored natural configuration. In analogous fashion (-)-stemoamide was prepd. beginning with L-pyroglutamic acid.

On the Mechanism of Pd0-Initiated Coupling-Cyclization of .gamma.-Aminoalkynes J. Org. Chem. 2000, 65, 7676.

The mechanism of the title reaction, to give dipyrrins vs. amidines, is discussed.

New Syntheses of the C,D-Ring Pyrromethenones of Phytochrome and Phycocyanin J. Org. Chem. 2000, 65, 8478.

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).