Wai-Lun Lam

C-10 Ester and Ether Derivatives of Dihydroartemisinin − 10-α Artesunate, Preparation of Authentic 10-β Artesunate, and of Other Ester and Ether Derivatives Bearing Potential Aromatic Intercalating Groups at C-10

Preparative and stereochemical aspects of reactions providing new C-10 ester and ether derivatives of the antimalarial drug dihydroartemisinin (DHA, 2) have been examined. β-Artesunate has been prepared for the first time, and has been differentiated from the antimalarial α-artesunate; the latter has been incorrectly designated as the β-epimer in Chemical Abstracts and some primary literature. New ester and ether derivatives bearing potential intercalating groups have been synthesised by means of the Schmidt, Mitsunobu and DCC coupling procedures, by acylation in the presence of DMAP, or by hydroxy activation with BF3 as catalyst. When the hydroxy group of DHA acts as a nucleophile towards activated carboxy groups in acylating agents or the DCC intermediate, α-esters are obtained exclusively. When the hydroxy group is activated for displacement by nucleophiles, as in the Schmidt or Mitsunobu procedures, β-esters and β-ethers are obtained either exclusively or predominantly. An exception is represented by the Mitsunobu procedure involving DHA and 1- and 2-naphthols, in which mixtures of epimers are obtained; however, exclusive formation of β-aryl ethers takes place when the Schmidt procedure is used, with activation of the intermediate trichloracetimidate by SnCl2. The latter method is therefore superior to patented procedures for the preparation of β-aryl ethers from nonbasic aryl alcohols without detectable rearrangement to C-aryl compounds. However, the Mitsunobu procedure is better when basic aromatic alcohols are used as nucleophiles. The formation of α-products in which the hydroxy group of DHA acts as a nucleophile is of biological significance in relation to enzyme-mediated Phase II glucuronidation of DHA, in which only the α-DHA glucuronide is formed.

Reaction of Metallated tert‐Butyl(phenyl)phosphane Oxide with Electrophiles as a Route to Functionalized Tertiary Phosphane Oxides: Alkylation Reactions

P-Chiral tertiary phosphane oxides have been prepared from each of the secondary phosphane oxides racemic 1, (SP)-(−)-4 and (RP)-(+)-tert-butylphenylphosphane oxide (5) by lithiation with LDA or nBuLi, or sodiation with sodium hydride, in THF, and then by treatment with a series of primary alkyl halides. Doubly P-chiral ditertiary bis(phosphane oxides) are also obtained from these metallated secondary phosphane oxides by treatment with electrophiles based on straight-chain, tartrate-derived, and bishalomethylarene dihalides. In general, the bis-phosphane oxides are obtained in good yields. However, when the α,ω-dihalide bears an embedded heteroatom (O or Si), yields are diminished. The enantiomeric purity of each of the products was assessed through admixture with (RP)- and (SP)-tert-butyl(phenyl)phosphanylthioic acids and measurement of the tert-butyl resonances in the 1H-NMR spectra. In all cases, the act of metallation of the enantiomerically pure secondary phosphane oxide followed by its alkylation is not accompanied by detectable racemization. This method for preparing P-chiral tertiary phosphane oxides is therefore more straightforward than those described previously.

Relaxant actions of nonprostanoid prostacyclin mimetics on human pulmonary artery.

The specific prostacyclin (IP) receptor agonist cicaprost relaxed human pulmonary artery preparations precontracted with phenylephrine [50% inhibitory concentration (IC50) approximately 0.6 nM], U-46619 (IC50 approximately 0.9 nM), and K+ (approximately 40% maximal relaxation); endothelium removal had little effect on relaxant activity. Ranking of relaxant potencies for prostacyclin and five of its analogs was 17 alpha, 20-dimethyl-delta 6,6a-6a-carba PGI1 (TEI-9063) > or = cicaprost > iloprost > prostacyclin > taprostene > benzodioxane prostacyclin > 15-deoxy-16 alpha-hydroxy-16 beta,20-dimethyl-delta 6,6a-6a-carba PGI1 (TEI-3356). The potency of the isocarbacyclin TEI-3356 may have been under-estimated because of its contractile (EP3 receptor agonist) activity. The potency ranking of four nonprostanoid prostacyclin mimetics was 3-[4-(4,5-diphenyl-2-oxazolyl)-5-oxazolyl]phenoxy] acetic acid (BMY 45778; IC50 approximately 2.5 nM) > > 2-[3-[2-(4, 5-diphenyl-2-oxazolyl)ethyl]phenoxy]acetic acid (BMY 42393) > octimibate > CU 23 (a novel diphenylindole). From IP receptor binding affinities obtained on human platelet membranes, it is suggested that the slightly shallower log concentration-response curves for BMY 45778, BMY 42393, and CU 23 may reflect the near-maximal receptor occupancy required for complete relaxation. A fifth nonprostanoid, CU 602, had much shallower log concentration-response curves than cicaprost against phenylephrine tone but not against U-46619 tone; this may indicate IP receptor partial agonism coupled with TP receptor antagonism. The relaxant actions of the nonprostanoid mimetics were more persistent than those of the prostacyclin analogs on washout of the organ bath; by the inhalation route, this type of compound may be retained within pulmonary tissue and thus afford greater pulmonary/systemic selectivity than currently used pulmonary vasodilators.