Stereoselective cyclo-condensation of a-sulfanylcarboxylic acids (or esters) 6 with N-methylarylimines 7 afforded the title compounds, 2-arylthiazolidin-4-ones, some of which exhibit highly potent anti-PAF activity. The reaction without added catalyst gave predominantly cis products, however, when titanium (iv) isopropoxide was added as catalyst trans products were predominantly formed. Allylation of 3-methyl-2-(3-pyridyl)thiazolidin-4-one 22 with ally1 bromide using lithium amides gave the trans-5-allyl-2-(3-pyridyl)thiazolidin-4-one 23 with good selectivity. To study the stereostructure-activity relationship, the four optically active isomers of 3,5-dimethyl-2-(3pyridyl) t hiazolid i n -4one 4 and 5 -(4 -c hlorop henyl) -3-methyl -2-(3-pyridyl) thiazol id i n -4-one 5 were synthesized. The absolute configurations of compounds 4 and 5 were determined by X-ray analyses and 'H NMR measurements. Epimerization of the 5-position of compounds 4 and 5 was found to be effected by bases with high regioselectivity (>99%), which was checked by a cross-over experiment using several optically active compounds.Since the discovery2 of the platelet activating factor (PAF), a number of investigations have been devoted to developing it for the medical applications. ' In addition, increasing interest has centred on anti-PAF active agents (PAF antagonist), and research in these areas is currently one of the most challenging problems in pharmacology. As part of our continuing programme in search of biologically active sulfur and nitrogen containing heterocycle^,^^^"^^^ we have attempted to synthesize several thiazolidin-4-ones 1 and perhydro-l,3,4-thiadiazin-5-ones 2 4c and evaluate their various biological activities.(cis: trans)? in the construction of thiazolidin-4-ones 1 (R2 = Ar) and 3 through a cyclo-condensation reaction; (ii) relative stereoselectivity in the allylation of the 5-position of the thiazolidin-4-one 22; (iii) syntheses of four optically active isomers of the two representative compounds, 4 and 5, to clarify the stereostructure-activity relationship; and (iv) regioselective epimerization of the 5-position of the thiazolidin-4-ones 4 and 5 promoted by bases.

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The cyclo-condensation of silyl 2-mercaptoalkanoates and arylmethyleneamines proceeded with high stereoselectivity to give alternatively both the cis- and trans-2,5-disubstituted 4-thiazolidinones, some of which are known as anti-platelet activating factor-active drugs. The use of the piperidine catalyst and no catalyst showed very high cis-stereoselectivity (cis/trans = 10/1—50/1) during the reaction. On the other hand, the trans-selective reaction was promoted by Ti(O-i-Bu)4 and Al(O-s-Bu)3 catalysts (cis/trans = 1/8—1/25). Both reactions were conducted with higher cis- and trans-selectivities as compared with those of the alkyl 2-mercaptoalkanoates under mild conditions. The cyclo-condensation using trimethylsilyl 2-(trimethylsilylthio)propionate and methyl 2-(trialkylsilylthio)propionates proceeded by the action of a catalytic amount (0.02 molar amount) of tetrabutylammonium fluoride with moderate cis-selectivity.

SUMMARYFluorination with 18F-F2 and 18F-AcOF were compared for the synthesis of 18F-fluorophenylalanines. L-phenylalanine in CF,COOH trapped 18F-AcOF more effectively than 18F-F2. The main product was ortho-18F-fluorophenylalanine when 18F-AcOF was used as a reagent. Lower radiochemical yield of 18F-fluorophenylalanines and si nificant formation of by-product were observed in the case o f feF-F,.