Studies on the new useful coupling reagents

Abstract

I. Studies on the New Useful Coupling Reagents; S,S-Bis (4,6-dimethyl-2-pyrimidinyl) dithiocarbonate (DPDC) is prepared by the reaction of 4,6-dimethyl-2-pyrimidine thiol hydro chloride with phosgene. DPDC can be effectively utilized as a coupling reagent for the preparation of carboxylic esters including sterically hindered esters, thiol esters, and racemization-free peptides. It is found that bis(2-pyrimidinyl) dithiocarbonate derivatives are cleanly converted into the corresponding symmetrical sulfides via intramolecular rearrangement in toluene at reflux. 2-Pyridon-1-yl diphenyl phosphate (PDP) is easily prepared by the reaction of equimolar amounts of 1-hydroxy-2(1H)-pyridone and diphenyl chlorophosphate. PDP is found to be very effective coupling reagent for the preparation of carboxylic amides and racemization-free peptides. II. Studies on the Reductins with Thioborane Derivatives; The reagents, 1,3,2-dithiaborinane-dimethyl sulfide and 1,3,2-dithiaborinane-dimethyl sulfide, are readily prepared by treating borane-dimethyl sulfide complex with 1 equiv of 1,3-propanedithiol and 1,2-ethanedithiol, respectively. Reaction of carboxylic acids with these reagents in the presence of stannous chloride affords the corresponding 1,3-dithianes and 1,3-dithiolanes in high yields, respectively. Furthermore, the present method is sufficiently selective to distinguish between carboxylic acid and readily reducible substituents such as bromo, ester, tertiary amide, tosylate, and isolated olefinic double bond. Direct conversion of carboxylic acids into S,S``-diphenyl acetals is occurred by using phenyl thexylbromothioborinate/dibromoboranedimethyl sulfide. Additionally, in the presence of zinc iodide, this reagent can be successfully applied to the conversion of carboxylic esters and lactones into the corresponding phenyl sulfides and halogenatedalkyl phenyl sulfides, respectively.