Diels-Alder fragments

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. 

Also the mirror image of the strueture I, eorreetly denoted as exo-3,10-dihydroxy-3,5,8,10-tetra-methyltrieyelo[6.2.2.0 ]dodeea-5,l 1-diene-4,9-dione, would be possible sinee enantiomers are not differentiated by NMR. A retro-Diels-Alder fragmentation of I to CsH/oO explains why the moleeular ion eorresponding to the moleeular formula C16//20O4 is not deteeted in the mass spee-trum. The metabolite I eould be formed by Diels-Alder dimerisation of 1,5-dimethyleyelohexa-l,3-dien-5-ol-6-one J as the primary metabolite which acts as diene and dienophile as well... 

Mandelbaum A. Stereochemical Effects in the Retro-Diels-Alder Fragmentation... 

Keywords retro-Diels-Alder fragmentation occurring in gas-phase, fragmentation in mass spectrometry... 

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>. 

In 2007, Padwa reported the first synthesis of valparicine using his Diels-Alder/fragmentation methodology. See Boonsombat J, Zhang H, Chughtai MJ, Hartung J, Padwa A (2007) J Org Chem 73 3539-3550... 

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. 

For a tandem Diels-Alder/fragmentation approach to the eleutherobin aglycone, Winkler et al. used Marshall s protocol in an early step of the synthesis. The building block 95 was prepared in that way (Scheme 15.24) [56]. 

Budzikiewicz, H. Brauman, J.I. Djerassi, C. Mass Spectrometry and Its Application to Structural and Stereochemical Problems. LXVII. Retro-Diels-Alder Fragmentation of Organic Molecules Under Electron Impact. Tetrahedron 1965, 27, 1855-1879. 

Karpati, A. Rave, A. Deutsch, J. Man-delbaum, A. Stereospecificity of Retro-Diels-Alder Fragmentation Under Electron Impact. J. Am. Chem. Soc. 1973,90,4244. 

Dixon, J.S. Midgley, I. Djerassi, C. Mass Spectrometry in Stractural and Stereochemical Problems. 248. Stereochemical Effects in Electron Impact Induced Retro-Diels-Alder Fragmentations. J. Am. Chem. Soc. 1977,99,3432-3441. 

Retro Diels-Alder fragmentation is another type of fragmentation which occurs in compounds with ring systems. 

El mass spectrum of limonene produced by retro Diels-Alder fragmentation. 

The spectroscopic distinction between thiocoumarins and thiochromones is best made by observing the low-field 5-proton in the NMR spectrum of the latter and also the characteristic retro-Diels-Alder fragmentation always observed in the mass spectrum of thiochromones. Thiochromones also show distinctive, strong ultraviolet absorptions at 250-270 nm. ... 

The Diels—Alder reaction results in a [47c + 27r] cycloaddition to give a six-membered ring. The reverse process, the rctro-Diels—Alder fragmentation. results in ring opening, and is also common in the MS of sixmembered rings containing a double bond (Scheme 5.17). 

Scheme 5.18 The retro-Diels-Alder fragmentation of benzofc]-pyran...

For example, the ion of [M-69]+, which is observed in the tandem mass spectra of lycopene, neurosporene, and y-carotene but not a-carotene, p-carotene, lutein, or zeaxanthin, indicates the presence of a terminal acyclic isoprene unit. Elimination of a hydroxyl group or a molecule of water, [M-17]+ or [MH-18]+, from carotenoids such as astaxanthin or zeaxanthin is characteristic of the presence of a hydroxyl group. Also, tandem mass spectrometry can be used to distinguish between isomeric carotenoids such as a-carotene and p-carotene, or lutein and zeaxanthin. For example, the ring of a-carotene containing the double bond that i s not conj ugated to the rest of the polyene chain shows unique retro-Diels-Alder fragmentation to form the ion of [M-56]+. In a similar manner, isomeric lutein and zeaxanthin differ by the... 

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). 

---