Mass spectrometry in structural and stereochemical problems. 248. Stereochemical effects in electron impact induced retro-Diels-Alder fragmentations

PRODUCTION OF FREE STEROLS BY INFECTED TUBERS OF OPHRYSLUTEA CAV.: IDENTIFICATION BY GAS CHROMATOGRAPHY‐MASS SPECTROMETRY

“…Peak 5 in Figure l Wyllie & Djerassi, 1968). The lower intensity of ions at m/z 394 (M+-90) and 355 (M+-129), when compared with the corresponding ions of campesterol and sitosterol (Knights, 1967) allied to a Retro-Diels-Alder fragmentation oftheB-ring (m/z 159, 161, 188, 215, 257) from m/z 253, 255, 282, 309 and 351, respectively (Dixon, Midgley & Djerassi, 1977) are indicative of the presence of a A^ double bound. The identification of the reported sterols was confirmed by comparison with the spectra obtained from authentic samples.…”

Energy-resolved electrospray mass spectrometry of diexo- norbornane and norbornene derivatives containing a fused, phenyl-substituted, 1,3-oxazine ring: competitive retro-Diels–Alder fragmentation processes

“…[1][2][3][4] Strong relationships between the stereospecificity of this fragmentation pathway and its concerted nature were established such that retention of the precursor ion configuration in the structure originating from the RDA reaction, in the absence of any interfering isomerisation event, was considered to be a test of a concerted mechanism. It should be noted, however, that the stereospecificity or non-stereospecificity of the RDA reaction can also be related to charge-site location 5 or to the degree of substitution of the covalent bonds involved in the dissociation mechanism as well as to associated energetic factors. 4h Whereas many studies of M + ions generated by electron ionisation have been published, only a few articles deal with the behaviour of protonated molecules towards RDA fragmentation.…”

Steroidal Saponins from the Rhizomes of Smilacina henryi

“…The aequatorial orientation of OHÀC (17) was deduced from the NOESY cross-peak ( Fig. 2) between d(H) 0.87 (Me (18)) and d(H) 2.19 (HÀC(20)) [2]. The resonances at d(C) 44.0 and d(H) 0.87 were attributed to C (13) and Me (18), respectively, on the basis of the HMBCs from d(H) 0.87 to d(C) 44.0 and C (17).…”

“…2) between d(H) 0.87 (Me (18)) and d(H) 2.19 (HÀC(20)) [2]. The resonances at d(C) 44.0 and d(H) 0.87 were attributed to C (13) and Me (18), respectively, on the basis of the HMBCs from d(H) 0.87 to d(C) 44.0 and C (17). Thus the tertiary Me group at d(H) 0.80 was ascribable to Me (19).…”

“…In particular, henryiosides A(1), B(2), and D(4) are based on steroidal aglycones never reported before. Although the aglycones of henryioside C(3) and E (5) have been synthesized before [18] [19], they are reported in natural products for the first time.…”