Diels-Alder molecular fragmentation

Their electron impact mass spectra indicated that the molecular ions of diexo norbornane-fused 1,3-oxazines underwent retro-Diels-Alder (RDA) fragmentations, leading to cleavage of the heterocyclic ring with the loss of norbornene, whereas... 

The retro-Diels-Alder (RDA) fragmentation is highly stereospecific in a variety of bi-, tri-, tetra- and pentacyclic systems. Two examples are shown in Scheme 13. The diene radical cations are the most abundant species observed in the El mass spectra of the cis isomers 14c and 15c, and practically absent in the trans counterparts 14t and 15t. The very high efficiency of this process in the cis isomers and the high stability of the molecular ions of the trans analogues suggest that the RDA dissociation of gas-phase ions takes place by a concerted mechanism, which exhibits symmetry conservation characteristics similar to those of the ground-state RDA process in neutral species. 

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

Woodward and Hoffmann pointed out that the Diels-Alder reaction involved bonding overlap of the highest-occupied molecular orbital (HOMO) on the diene and the lowest-unoccupied molecular orbital (LUMO) on the dienophile. Display the HOMO for 2-methoxybutadiene. Where is it localized Display the LUMO for acrylonitrile. Where is it localized Orient the two fragments such that the HOMO and LUMO best overlap (A clearer picture is provided by examining-the HOMO map for 2-methoxybutadiene and the LUMO map for acrylonitrile.) Which product should result ... 

Two new pseudotabersonine-related alkaloids were isolated by Husson et al. from the stem bark of T. albiflora from French Guiana (29) and characterized as (20/ )-18,19-dihydroxypseudovincadifformine (166, C2iH26N204, MP 190°C, [a]D +264°) and 19-hydroxy-20-epipandoline (167, C21H26N204, MP 204°C, [a]D +511°. Both compounds exhibited similar UV maxima attributed to a (3-anilinoacrylic chromophore, and their mass spectra showed, in addition to a molecular ion at m/z 370, a base peak at m/z 156 formed by retro-Diels-Alder fragmentation in ring C, followed by C-6—C-5 bond cleavage. 

With one exception, naphthalen-l,4-imines with a double bond between C-2 and C-3 are not known to dissociate thermally by either possible retro-Diels-Alder pathway (the reverse of reactions described in Section III, A, 1 and 2), and the enthalpy requirements for the formation of a benzyne or an acylic acetylene are doubtless unfavorable. However, the mass spectra of compounds 93-99 reveal one important fragmentation of the molecular ions to be loss of dimethyl acetylene-dicarboxylate, and another fragmentation pathway involves the formation of nitrilium ions MeC=NR and PhC=NR from 93-95 and 96-99, respectively. ... 

The fragmentation pattern of l,5-dimethyl-7-methoxy-l,2,3,4,5,10-hexahydro-4fl//-pyridazino[l,6-h]isoquinoline (22) has been studied (73JHC999). The base peak at m/e 148 might be derived through the retro Diels-Alder fission of the molecular ion. The 7-hydroxy derivative gives the same fragmentation pattern. 

In all the foregoing examples, the prominent fragmentation of pyran-4-ones involves a retro-Diels-Alder (RDA) cleavage of the molecular ion. However, atypical behaviour is sometimes displayed, particularly when C-3 bears an oxygen function, which can cause other more complex pathways to supervene. Some examples are given below. 

An independent determination of the isotope effect could be derived from the mass spectrum of the 3-trimethylsiloxycyclohexene-retro Diels-Alder fragmentation (m/e = 142). 

An earlier off-odor problem that surfaced was the presence of 4-phenyl-cyclohexene in styrene-butadiene coated paper (Koszinowski et al. 1980). This compound was created by a Diels-Alder condensation reaction involving a molecule of styrene and butadiene and is differentiated by its odor from the isomeric 3 and 1-phenyl-cyclohex-ene compounds which cannot be formed by such a condensation reaction. The recognition threshold of this compound in the headspace over an aqueous solution lies around a concentration of 10 pg/kg (10 ppb). The typical odor of this compound at concentrations of 4-phenyl-cyclohexene in paper over 4 mg/kg (4 ppm) is easily identified. AGC determination in this concentration range is also possible without difficulty and its identification with MS using the relative molecular mass of 158 and one of the retro Diels-Alder decomposition product fragments at m/e = 104 (styrene) and mJe = 54 (butadiene) is definitely possible. 

Also the mirror image of the structure I, correctly denoted as cyro-3,10-dihydroxy-3,5,8,10-tetra-methyltricyclo[6.2.2.02 7]dodeca-5,ll-diene-4,9-dione, would be possible since enantiomers are not differentiated by NMR. A retro-Diels-Alder fragmentation of I to Cg//yo04 explains why the molecular ion corresponding to the molecular formula is not detected in the mass spec-... 

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