Studies in organic mass spectrometry. XX—Stereospecificity in hydrogen transfer reactions in mass spectra of TMS derivatives of some 2,3‐dialkyl‐1, 4‐cyclopentanediols

Abstract

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