The electrochemical reduction of benzyl thiocyanate and p-nitrobenzyl thiocyanate was investigated in acetonitrile at an inert electrode. These two compounds reveal a change in the reductive cleavage mechanism, and more interestingly, they show a clear-cut example of a regioselective bond dissociation. Both phenomena may be understood on the basis of the dissociative ET theory and its extension to the formation/dissociation reactions of radical ions. While the effect of the standard oxidation potential of the leaving group seems to be predominant in understanding the change in the ET mechanism by changing the driving force, the regioselective cleavage is dictated by changes in the intrinsic barrier related to the nature of the substituent on the aryl moiety.
We present an experimental and computational study of the reaction of aryl substituted benzyl 1-alkynyl sulfides with potassium alkoxide in acetonitrile, which produces 2-aryl 2,3-dihydrothiophenes in poor to good yields. The cyclization is most efficient with electron withdrawing groups on the aromatic ring. Evidence indicates there is rapid exchange of protons and tautomerism of the alkynyl unit prior to cyclization. Theoretical calculations were also conducted to help rationalize the base induced 5-endo cyclization of benzyl 1-propynyl sulfide (1a). The potential energy surface was calculated for the formation of 2,3-dihydrothiophene in a reaction of benzyl 1-propynyl sulfide (1a) with potassium methoxide. Geometries were optimized with CAM-B3LYP/6-311+G(d,p) in acetonitrile with the CPCM solvent model. It is significant that the benzyl propa-1,2-dien-1-yl sulfane (6) possessed a lower benzylic proton affinity than the benzyl prop-2-yn-1-yl sulfane (8) thus favoring the base induced reaction of the former. From benzyl(propa-1,2-dien-1-yl sulfane (6), 2,3-dihydrothiophene can be formed via a conjugate base that undergoes 5-endo-trig cyclization followed by a protonation step.
SummaryA convenient synthesis of leflunomide and its primary pharmacologically active metabolite A77 1726, each labeled with four 13 C atoms has been achieved. Starting from 13 C 4 -ethyl acetoacetate, each step of the synthesis proceeds in 60-82% yield. Analysis of the mass spectra of the labeled and unlabeled materials demonstrates that the compounds prepared herein will be suitable as analytical standards for bioavailability studies.
Enantiomerically enriched α,β-unsaturated sulfinate esters of ()-cholesterol undergo stereospecific substitutions at sulfur when treated in benzene at 6°C with Grignard reagents. Sulfoxides with ees of 8599.5% are obtained when enantiopure sulfinates are used. The substitution reactions proceed with inversion of sulfur configuration. Enantiomerically pure cholesteryl (E)-2-carbomethoxyethenesulfinate is not a suitable reactant under the Grignard reaction conditions. It is suggested that the ester group induces unwanted reactions significantly lowering both the yield and sulfur stereogenicity.Key words: sulfinate, sulfoxide, Grignard reagents, stereospecific, unsaturated.
Using established and new 1 H NMR data for sulfinate esters and sulfinyl chlorides, the parameters required for predicting position-dependent alkene 1 H NMR chemical shifts of vinylic sulfinate esters and vinylic sulfinyl chlorides have been obtained. Standard deviations of the new Z parameters lie in the range 0.08 to 0.15 ppm. Sulfinyl chloride and cyclohexyl sulfinate derivatives of (E) and (Z )-2-cyanoethenesulfinic acids have been prepared for the first time.The alkene additivity chemical shift rule Eq. (1) has long been used by organic chemists to interpret 1 H NMR spectra [1][2][3][4]. When a double bond configuration is uncertain, application of the rule, using tabulated chemical shift influences of substituents (Z values) usually allows a rapid and tentative, if not unequivocal, assignment of double bond geometry.Since its introduction [1], the table of substituent constants for common alkene substituents has found its way into several organic spectroscopy reference books. However, such entries are usually a reproduction of the original work [2-4], despite the emergence of new Z parameters for less common functional groups [5][6][7][8].Sulfinyl chloride and sulfinate ester functional groups are part of modern investigations [9, 10] and, as such, double bonded compounds bearing these substituents are of interest to us [11][12][13][14][15][16] and to others [17][18][19][20][21][22][23][24][25][26][27]. Unfortunately the alkene additivity rule is unavailable for investigators in this area due to the lack of appropriate substituent constants.
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