Xiaoxiang Liu

Synthesis and structure–activity relationship studies on tryprostatin A, an inhibitor of breast cancer resistance protein

Tryprostatin A is an inhibitor of breast cancer resistance protein, consequently a series of structure-activity studies on the cell cycle inhibitory effects of tryprostatin A analogues as potential antitumor antimitotic agents have been carried out. These analogues were assayed for their growth inhibition properties and their ability to perturb the cell cycle in tsFT210 cells. SAR studies resulted in the identification of the essential structural features required for cytotoxic activity. The absolute configuration L-Tyr-L-pro in the diketopiperazine ring along with the presence of the 6-methoxy substituent on the indole moiety of 1 was shown to be essential for dual inhibition of topoisomerase II and tubulin polymerization. Biological evaluation also indicated the presence of the 2-isoprenyl moiety on the indole scaffold of 1 was essential for potent inhibition of cell proliferation. Substitution of the indole N(a)-H in 1 with various alkyl or aryl groups, incorporation of various L-amino acids into the diketopiperazine ring in place of L-proline, and substitution of the 6-methoxy group in 1 with other functionality provided active analogues. The nature of the substituents present on the indole N(a)-H or the indole C-2 position influenced the mechanism of action of these analogues. Analogues 68 (IC(50)=10 microM) and 67 (IC(50)=19 microM) were 7-fold and 3.5-fold more potent, respectively, than 1 (IC(50)=68 microM) in the inhibition of the growth of tsFT210 cells. Diastereomer-2 of tryprostatin B 8 was a potent inhibitor of the growth of three human carcinoma cell lines: H520 (IC(50)=11.9 microM), MCF-7 (IC(50)=17.0 microM) and PC-3 (IC(50)=11.1 microM) and was equipotent with etoposide, a clinically used anticancer agent. Isothiocyanate analogue 71 and 6-azido analogue 72 were as potent as 1 in the tsFT210 cell proliferation and may be useful tools in labeling BCRP.

Enantiospecific total synthesis of the enantiomer of the indole alkaloid affinisine

Abstract The enantiomer of affinisine has been synthesized (from l -tryptophan) with 100% diastereoselectivity via the asymmetric Pictet–Spengler reaction coupled with a Pd 0 (enolate mediated) coupling process. This enantiomer has also been converted into a key intermediate which provides a route to the enantiomers of both macroline and alstonerine following the previous work of LeQuesne.

Concise synthesis of optically active ring-A substituted tryptophans

A concise synthesis of optically active trytophan derivatives was developed via diastereoselective alkylation of the Schollkopf chiral auxiliary 4 to provide alkyne 2 which underwent palladium-catalyzed heteroannulation with iodoanilines 1 to furnish protected tryptophans 3. Hydrolysis and subsequent saponification of 3 provided the desired tryptophans 12 in good yields.

Efficient asymmetric synthesis of important tryptophan analogs for biological research via the Schöllkopf chiral auxiliary

An efficient method has been developed via the Schollkopf chiral auxiliary for the asymmetric synthesis of the important tryptophan analogs: l-isotryptophan, l-benzo[f]tryptophan and l-homotryptophan.

EFFECT OF THE LEAVING GROUP ON THE ALKYLATION DIASTEREOSELECTIVITY OF THE SCHOLLKOPF CHIRAL AUXILIARY

Tosylates, diphenylphosphates and bromides were employed to study the effect of the leaving group on the alkylation diastereoselectivity of the Schollkopf chiral auxiliary 1 at aliphatic, benzylic, propargylic and allylic electrophilic centers. The diastereoselectivity generally follows the pattern: diphenylphosphate > tosylate > bromide. In terms of both diastereoselectivity and yield, each leaving group possesses a different advantage.

Enantiospecific total synthesis of the enantiomer of the indole alkaloid intermediate macroline

Abstract The total synthesis of the enantiomer 3En of macroline 3 was completed (from l -tryptophan methyl ester 6 ) in an overall yield of 12.3% (11 isolated intermediates). The corresponding macroline equivalent 18 was prepared in 14.3% yield. This work provides, for the first time, an opportunity to synthesize mismatched bisindole alkaloids for studies on the mechanism of action of Alstonia antimalarial alkaloids at the receptor level.