Fragmentation of Rings

Fragmentation of Rings.—A case that falls into this category has come from Just and Kim, who have synthesized the carba-analogues of the antiseptic nucleosides pyrazofurin A (33) and showdomycin. In a first step, they cleaved a norbornene derivative (31) to give the intermediate (32), having most of the carbon skeleton and also the correct relative stereochemistry of the desired products. 

A number of strategies are available, and are illustrated in the following sections. 

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The second major mechanism of VCD is the production of vibrationally generated electronic current density by the vibrational motion of a local oscillator. Usually, such currents are induced in molecular rings, or fragments of rings, and the local oscillator may either be attached to the ring or contained in it. The resulting VCD per oscillator is biased and monosignate. 

C-10, presumably via a path involving the intervention of a C-10 carbonium ion, elimination, and reprotonation. An unusual fragmentation of ring A has been observed47 in the treatment of 6a-hydroxy-7-oxoabieta-8,ll,13-triene (43) with toluene-p-sulphonyl chloride. The formation of the /3-naphthol (46) may proceed via the enol (44), followed by a methyl migration to (45) and then fragmentation. 

Similar fragmentation of rings carrying ketal substituents is observed in their mass spectra cf. Ref. 10 of Chapter i). 

The mass spectrum of tetrahydrothalifendine showed, besides the molecular ion at m/e 325, peaks at m/e 176 and 150 of ions resulting from the familiar retro-Diels-Alder fragmentation of ring C (Section III, C, 3). 

A similar structural unit to that contained in 281, that is, an iV-formyl group in an 11-membered ring, is known in some bisindole alkaloids, such as vinamidine. A possible mode of generation of this unit may thus be via the fragmentation of ring D in a precursor in which an appropriate leaving group is situated in the y-position to N. ... 

The mass spectrum of (6) contains,as expected, a major fragment at m/e 197. Deuteriation experiments unexpectedly show, however, that only ca. 20% of this ion originates via a retro-Diels-Alder fission of ring d instead, the ion appears to be mainly the result of a stepwise fragmentation of ring D (Scheme 4). 

In continuation of earlier work, a further series of mass spectra of eighteen erythrinan and erythrinan-8-one alkaloids and their homo -analogues have been analysed. All the compounds exhibit similar fragmentations of ring A, but an... 

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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