ORGANIC MASS SPECTROMETRY, VOL. 27, zyxwvutsr 621-624 (1992) Negative Ion Mass Spectra of Some Unsaturated C-Glycosides Following keV Ion Bombardment Mohamed Brakta and Denis Sinou* Laboratoire de Synthbse Asymttrique, associt au CNRS, ESCIL, Universitt Claude Bernard Lyon I, 43 Bd du 11 Novembre 1918,69622 Villeurbanne, France Michel k c h i SCACNRS, BP 22,69390 Vernaison, France Joseph Banoub Department of Biochemistry, Memorial University of Newfoundland, St John’s, Newfoundland, AlB 3x9, Canada A series of anomeric unsaturated C-glycosides were studied by negative ion mass spectrometry following keV ion bombardment. The compounds bearing an acidic hydrogen show a prominent [M zyxw - HI- zyxw peak; in the other cases, the [ M - HI - zyxwvu peak is of very low intensity. A retro-Diels-Alder fragmentation and the abstraction of a hydrogen atom from the aglycon residue are observed in all cases. INTRODUCTION The use of mass spectrometry for the structural analysis of carbohydrates has increased considerably in the last few years. Much effort has been devoted to positive ion mass spectrometric investigations by means of electron impact (EI),1-9 chemical ionization (CI),9-1 field ionization/field desorption (FI/FD)18-2Zand fast atom bombardment (FAB)23-36 mass spectrometry. The negative ion spectra of carbohydrates have only been studied to a small extent ;28*29,35-43 for example, nega- tive ion FAB proved to be of great value in the differen- tiation of isomeric D-aldohexoses and substituted glucoses and in the determination of the linkage posi- tion in disaccharides and small oligosaccharides. During our studies of unsaturated C-glycosides by positive ion FAB mass ~pectrometry,4~-~’ we have noticed that some anomeric unsaturated C-glycosides showed a distinct and discriminating ion pattern, the daughter ion mass spectra of the two anomers having an ion intensity ‘fingerprint’which was characteristic of each anomer. As a continuation of our interest in the study of these compounds, we now report on the nega- tive ion mass spectrometric behaviour of these unsatu- rated C-glycosides in relation to the configuration at the anomeric centre and the nature of the anomeric substit- uent. EXPERIMENTAL The synthesis of compounds 1-4 (Scheme 1) was described previo~sly.~~ The FAB mass spectra were recorded on a VG ZAB 2-SEQ (Fisons, Manchester, UK) mass spectrometer fitted with a caesium ion gun delivering about 2 pA of caesium ion current with about 35 kV energy. All the samples were analysed by dis- 0030-493X/92/050621-04 $05.00 zyxwvut 0 1992 by John Wiley & Sons, Ltd. solving them in dichloromethane and mixing the solu- tion (1-2 pl) into 1-2 p1 of matrix. 3-Nitrobenzyl alcohol can be used as a matrix, but the best results were obtained with a mixture of 2-nitrophenyl octyl ether and tetraglyme. FAB collisionally activated disso- ciation mass-analysed ion kinetic energy (CAD-MIKE) analyses were performed using helium as the collision gas at a pressure such that the intensity of the mass- selected beam was reduced by -80% of its original value. The laboratory collision energy was 8 kV. RESULTS AND DISCUSSION The negative ion FAB mass spectra of only compounds z 2a and 28 (Fig. 1 and Table 1) show a prominent zy m/z 440 [M - HI- peak which is the base peak for the two anomers. The other compounds show a peak of low intensity for [M - HI- in the case of the a-anomer and no peak at [M - HI- for the fl-anomer. The differences are due to the easy deprotonation of compounds 2 con- taining a very acidic hydrogen. The observed differences between the zyxw a- and fl-anomers for the other compounds are more diffcult to explain, owing to the possible elimination of a hydrogen atom from various positions, which allows the formation of mixtures of ions having pha20a: PhCH20 zyxwvut la R’ = H, R~ = CH,CO,C,H, zyxw Za R’ = H, R~ = CH(NO,)CO,C,H, lp R’ = CH~COZC~H,, R2 = H Zp R’ = CH(N02)C02C2H, R2 = H 3a R’ = H, R2 = C(NOz)(CO,C2H,), 3p RI = C(NOZ)(M~C~H&, R2 = H 4p R’ = C(COCH3)2(COOCH,), R2 = H 4a R’ = H, R2 = C(COCH3)2(COOCH3) Scheme 1 Received 30 November 1991 Revised manuscript received 29 January 1992 Accepted 2 February 1992