The mass spectra of the TMS-ethers of the isomeric 2,3-dialkyl-1,4-cyclopentanediols are discussed. These compounds exhibit fragmentations, characteristic for the TMS-1,3-cyclopentanediol-system.The dimethyl substituted compounds display differences in the ion abundances for a number of ions, depending upon the stereochemistry of the trimethylsiloxy groups, while for longer alkyl groups only differences in the intensity of a rearrangement ion at m/e 141 are found. A considerable change in the mass spectral behaviour is found for the compounds with an 1',2'-unsaturated alkyl substituent.The observation of differences in the abundances of certain ions in the mass spectra of stereoisomers has already been used for configurational assignment of the parent compounds . This paper describes the mass spectral behaviour of the TMS-ethers of the six isomeric 2,3-dimethyl-l,4-cyclopentanediols.Characteristic fragmentations and relevant differences in the abundances of ions have been studied by deuterium labelling, experiments at low electron energy and high resolution measurements. Some of the fragmentation processes are of considerable diagnostic value for relative configurational assignments. This has also been confirmed by the study of TMS-ethers of some homologues with longer alkyl chains.An alkenyl (1',2')-substituent (R3) causes a change in fragmentation behaviour allowing differentiation between saturated and unsaturated side chains.The isomeric 2,3-dimethyl-1,4-cyclopentanediols were synthesised in our laboratory in order to obtain simple model compounds for study of more complex analogues2 : the prostaglandins from the F-serie and other 2,3-dialkyl-l , 4-cyclopentanedibls, which occur as intermediates during the total synthesis of the Fprostaglandins and result from a solved metal reduction in liquid ammonia of the corresponding 2, 3-dialkyl-4-hydroxy-2-cyclopentene-1-ones3 ' 4 .As this reaction can yield several stereoisomers, methods are required to elu-1

Abstract-The mass spectra of TMS ethers of 2,3-dialkyl-l ,4-cyclopentanediols having a 2'-carboxyl group or 2'-double bond in one of the sidechains, display a stereospecific rearrangement process. A mechanism is proposed to account for this stereospecificity. I N T R O D U C T I O NIN A RECENT study general breakdown patterns were established for the TMS derivatives of 2,3-dialkyl-1,4-~yclopentanediols.~ The present paper describes two stereospecific fragmentations, which are displayed upon introduction of a functional group (a 2'-double bond or 2'-carboxylic ester) in one of the sidechains. The compounds investigated are important intermediates in the total synthesis of prostaglandins and are ~b t a i n e d~'~ as shown in Scheme 1. The oxidative cleavage is performed on isolated isomers so that correlations between the two classes of compounds are possible. The configurational assignment has been solved by IH n.m.r.,3 chemical transformations3 and mined by mass spectrometry; orientation of both hydroxyl SCHEME 1 mass spectrometry. Partial structures can be detera rearrangement ion d (m/e 147) proves the relative fun~tionsl'~ while the hydrogen transfer reactions described in this paper yield information on the orientation of a functionalised sidechain in relation to the hydroxyl groups. For certain cases conclusive proof could only be obtained by mass spectrometry.The TMS ethers of 2-alkyl-3-(methoxycarbonyl)methyl-l,4-cyclopentanediols show the loss of methyl acetate. Our results fit in with a stereospecific process involving cis-y-hydrogen atoms (McLafferty rearrangement) or a cis-&hydrogen atom.The TMS derivatives of the 3-methyl-2-butenyl substituted compounds yield an intense ion at m/e 183 depending upon the relative configuration of the C-1 TMSO

thyl enol ethers of 1,3-cyclohexanediones and 1,3-cyclopentanediones is studied. Six membered rino; compounds behave quite straithforwardly and in accordance with the fragmentation suggested f o r 1,3-cyclohexanediones in the enol form. Some fragmentations are supported by deuterium labelling. tuted isomers display identical spectra. This isomerisation is studied by metastable transitions, low energy spectra and deuterium labelling; a possible mechanism is advanced.The influence of alkyl substituents on the mass spectral behaviour of meIn the case of enol ethers of 1,3-~yclopentanediones the 4-or 5-substi-The mass spectral behaviour of acyclic 1,3-diones1 to 4, 1,3-~yclohexane-diones5 to and 1,3-cycl~pentanediones~ has already been discussed. It could be proven that both tautomeric forms display different fragmentation patterns.It is therefore of interest to investigate corresponding enol ethers. The fragmentation behaviour of some enol derivatives of dirnedone'l and acyclic 1,3-diketones" has been published.ethers of alkyl substituted 1,3-cyclohexanediones and 1,3-cyclopentanediones.It appears, as well from the published data as from the present work that the most important ions are formed by initial cleavage 0 to the carbonyl function.The fragmentation of the cyclic enol ethers is discussed in function of the length and the position of the alkyl substituent; the mechanisms are in all cases, supported by high resolution measurements.We now present the fragmentation of methyl en01 I; R = H 11; R = Et 111; R = n.Pr IV; R = n.Bu).Holder of a bursary of V; R = Et VIII; R = Me VI; R = n.Pr IX; R = n.Pr VII; R = n.Bu X; R = n.Bu C. Van

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