Mass spectral fragmentation retro Diels-Alder

JHC297>. In a continuation of the above work, l,3,4-thiadiazolo[3,2-a][l,3,5]-triazines were also analyzed by electron impact mass spectrometry. A retro Diels-Alder process gave two fragments (Equation (1)) which in turn were further fragmented into smaller molecules. The mass spectral data for the 11 compounds analyzed has been tabulated in great detail <890MS(24)379>. 

Mass spectra of the cis- and /ram-isomers of the pyrimido[2,TA [l,3]thiazin-6-ones 294 and 295 were studied. Retro-Diels-Alder fragmentation of the hydrocarbon ring was of medium to low stereospecificity. A number of highly selective processes were discovered allowing differentiation between stereoisomers <1996RCM721>. The mass spectral fragmentation pattern of 296 was studied in detail <1996PS(113)67>. 

Mass-spectral fragmentation of 5 -steroidal 1-enes, including a quasi-thermal retro-Diels-Alder cleavage... 

Mass spectral data have been reported for an oxetane derivative of (1) formed by the reaction of benzophenone with l-acetyl-pyrrolo[2,3-6]pyridine (Scheme 1). The major fragments upon electron impact result from a retro-Diels-Alder process to give the original component ions. These ions then undergo further fragmentation <76JHC585>. 

Similarities between mass spectral and thermal fragmentations are particularly common in certain reaction types. Electrocyclic reactions, for example, are frequently similar in the two processes. The thermal process has in general a higher stereoselectivity (because of the higher aromaticity in even-electron systems). Retro-Diels-Alder reactions are typical examples for the similarity of the two processes. Internal displacement reactions may also be similar in the two processes, mainly in the case of internal radical displacements. The relationship between mass spectra and thermal fragmentation is complex, and it is useful to discuss it for separate classes of compounds. 

A new and very neat synthesis of evodiamine and rutaecarpine is described by the authors as a retro mass spectral synthesis , since the original conception was derived from the mode of fragmentation of these alkaloids in the mass spectrometer this involves a familiar retro Diels-Alder fission of ring c. Evodiamine (16a) was thus constructed by a 27r + 47r cycloaddition of 3,4-dihydro-/8-carboline with the keteneimine (17), prepared in situ by elimination of sulphur dioxide from the sulphinamide anhydride (18) (Scheme 7). When the anthranilic acid derivative (19) was used the product was rutaecarpine (20) itself, the initially formed dihydro-... 

The mass spectral fragmentation can be explained by a "retro" or reverse Diels-Alder fragmentation ... 

The spectral features of 1,4-disubstituted quinolizidines are quite similar to those of the 5,8-disubstituted indolizidine analogues. Their mass spectra are characterised by a major fragment arising from a-cleavage of the alkyl side-chain at C-4 and by an additional peak at m/z 110 (C H 2N+, ca. 30-50%) in ion-trap spectra, arising from a subsequent retro Diels-Alder process, Fig. (5). Since, as seen for the indolizidine analogues, the alkyl substituent at C-l is commonly a methyl group the base peak of... 

In many cases, fragmentations corresponding to the retroreaction of a powerful reaction (sec. lO.S.B.vi) can be identified and used retrosynthetically. Retro-mass spectral analysis has been applied to total synthesis by Kametani. The mass spectra of tetrahydroisoquinoline alkaloids show fragmentations by the three major pathways shown in Scheme 10.24.1 Kametani observed that path c (a retro Diels-Alder reaction) was the major fragmentation pathway (sec. ll.S.B). Xylopinene (274) gave this specific fragmentation (to 275 and... 

The mass spectral fragmentation of tetrahydroprotoberberine bases is usually easy to interpret, since the major cleavage results from retro-Diels-Alder fragmentation of ring C, to produce ions 57 (base) and 58. 

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