Mass Spectrometric Fragmentation of 1-(Benzyloxycarbonyl)amino-2-alkyl/cycloalkyl Thioacetates: a Th

Chinese Chemical Letters Vol. 17, No.8, pp 1069-1072, 20061069http://www.imm.ac.cn/journal/ccl.htmlMass Spectrometric Fragmentation of 1-(Benzy loxycarbony)amino-2-alky/cycloalkyl Thioacetates: a Thioester Pyrolysis-typeRearrangement under Electron Impact IonizationShu XU, Jia Xi XU*Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education,College of Chemistry and Molecular Engineering, Peking University, Beijing 100871Abstract: The mass spectrometric fragmentation of 1-(benzyloxycatbonyl)amino-2- alkylcyclo-alkyl thioacetates has been studied with the aid of mass. analysed ion kinetic energy spectrometryunder electron impact ionization. All compounds show a tendency to eliminate a ketene,thioacetic acid, and benzyl carbamate molecule, or an acetyl and benzyloxy radicals. A thioesterpyrolysis-type rearrangement under electron impact ionizations was observed.Keywords: Electron impact ionization, mass spectrometry, rearrangement, thioacetate.Many four- and six-membered ring rearrangements have been observed under massspectrometric ionization conditions to datel"7.1-(Benzyloxycarbonyl)amino-2-alkyl/cycloalkyl thioacetates are key intermediates for the synthesis of 1-substituted and cyclictaurines derivatives', which are sulfur analogues of naturally occurring amino acids andpossess many biological activities- "1Herein, we report a thioester pyrolysis-typesix-membered ring rearrangement of 1-(benzyloxycarbonyl)amino-2 alkyl/cycloalkylthioacetates under electron impact (EI) ionization conditions.The characteristic EI fragment ions of compounds 1-5 are listed in Table 1.Compound 2 was taken as an example to conduct the MIKES analysis to proposefragmentation mechanism of the title compounds. The EI spectrum of compound 2 isshown in Figure 1.Scheme 11-(Benzyloxycarbonyl)amino-2-lkyl/cycloalkyl thioacetates 1-51.R=Me，, NHCbz2.R=n -Bu4.n=!NHCbz3.R= Ph5.n=2R中國煤化工YHCNMH GE- mail: jxxu@ pku.edu.cn1070Shu XU et al.Figure 1 EI spectrum of 1-(benzyloxycarbonyl)amino-2-hexyl thioacetate 291100 78C6C404320-11523317626730950o1505030035m/zThe title compounds showed very low intensity of M+ ions with 0.5-1.2% relativeabundances. The fragmentation pathways of the title compounds may be proposed asshown in Scheme 2, as suggested by observations of the metastable ions (Table 2). Themolecular ions M+ could undergo four membered ring rearrangements to produce benzylN-(2-mercaptoalkyl/cycloalkyl)carbamate ions (a) by loss of a ketene molecule, and1-(cyclo)alken-2-yl thioacetate ions (h) by loss of benzyl carbamate (detailed ringrearrangement mechanisms were shown in Scheme 3). The ions (a) could furtherundergo an a-cleavage to give rise to benzyl ion (k) and N-(2-mercaptoalkyI/ cycloalkyl)carbamate ions (d), or N-(2-mercaptoalkyl/cycloalkylamino)formoyl ions (e), respect-tively, or undergo a four-membered ring rearrangement to form benzyl alcohol ion (j).The ions M+ could also undergo a six-membered ring rearrangement to yield benzylN-(1-alkenyl/cycloalkenyl)carbamate ions (C) by loss of a thioacetic acid molecule (alsosee Scheme 3). The ions M+ could eliminate an acetyl radical to give 1-benzyloxy-carbonylamino-2-(cyclo)alkylsulfurium ions (b)， which could further eliminate1-benzyloxycarbonylamino radical or benzyl carbamate molcule to form odd electronTable 1 EI-MS characteristic ions of compounds 1-5Comd.Ion [m/z (relative abundance, %)] [Refer to Scheme 2 for proposed ion structures]M+defgij225224191134118176116747:10891 43(1.1)(1.9) (1.2) (9.3) (3.3) (6.9)(0.9) (2.3) (2.0) (1.0) (14)(100) (31)2267266 233 176 160218158116 115 1080.8)(0.9) (0.8)(10) (5.1) (5.6) (0.5) (3.1) (1.8) (12) (8.0)(100) (33)3329287286253196180238178136 135 1081.2) (3.6) (15) (8.6) (5.6) (4.9) (20) (55) (7.1) (6.5) (5.9) (100) (494293251250 217 160 144 202中國煤化工(100) (39307265264231174 158 2MYHCNMHG(0.9) (0.8) (0.9) (10)(1.9) (7.1) (0.9) (10) (7.3) (8.1) (6.1)(100) (18)Mass Spectrometric Fragmentation of 1-(Benzyloxycarbonyl)amino- 10712-alkyl/eycloalkyl ThioacetatesScheme 2 Fragmentation pathways proposed for title compounds_CbzNHSRH2*R-. NHCbzBnOH思khj_CbzNH2d-Ac-Bn-AcSH .-CH2=C=OSH-BnOw NHCbz是_o+c, NHCbzaeM:-CbzNH2Ac+Bn+k20+Only 1-(benzyloxycarbonyl)amino 2-alkyl thioacetates were shown.Scheme 3 Ring rearrangement mechanisms for the formation of ions (a), (C), (g), and ()三.三(二HNHCbz .g-AcSHions (h) and 1-(cyclo)alken-2-ylsulfurium ions i), respectively. The ions M+ couldalso undergo an cx-cleavage to produce N-(2-acetothioalkyl/cycloalkylamino)formoylions (f) by loss of benzyloxy radical.It is . interesting to note that a characteristic fragmentation mechanism of thethioacetates under EI ionizations involved the elimination of a neutral thioacetic acidmolecule to produce alkene ions. The elimination is very similar to that under thermalconditions either in the gas phase or in中國煤化工rolysis-typerearrangement under electron impact ionizations.TYHCNMH G1072Shu XU et al.AcknowledgmentThis work was supported by NNSFC (No. 20472005).References1. L. Ceraulo, P. Agozzino, M. 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