Dienone system

Diels-Alder reactions of 2-bromo-2-cycloalkenones. A convenient approach to doubly cisoid fully conjugated dienone system [94]... 

In contrast with all the other species of Crematogaster studied till now, the venom of C. sp. 2 from Papua New Guinea did not contain mixtures of homologous compounds. Two derivatives, 136 and 137, characterized by the presence of a conjugated triene on one end of the chain, and by a 1,3-hydroxyketone at the other end, were isolated from this species (Fig. 23). These structures could constitute biosynthetic intermediates en route to the cross-conjugated dienone system. The venom of C. sp. 3 contained 4-oxo-2,5-dienyl acetates similar to... 

Accordingly, carbon-carbon bonds involving positions ortho or para to the original phenols, or ether linkages may be formed. The reactive dienone systems formed as intermediates may, in some cases, be attacked by other nucleophilic groupings (see Section 10.10), extending the range of structures ultimately derived from this basic reaction sequence. 

The demonstration that both double bonds of the cross-conjugated dienone system are not necessary for photoisomerization considerably extends the potential utility of these rearrangements because of the greater availability of a,/3-unsaturated ketones. It is safe to predict that photochemical rearrangements of a,/3-unsaturated ketones will be subject to intense investigation in the immediate future. 

The preparation of the acetates (61) and (62) from lapachol was the first example of the formation of a chromenol by base-catalysed cyclisation of an allylquinone. The reaction proceeds by normal acetylation, apparently giving both isomers (63) and (64), followed by proton abstraction from the activated methylene group and rearrangement of the dienone system, Fig. (12) [139],... 

Cobalt catalyzed double [2 + 2 + 1] cycloaddition reactions of branched triynes 169 have led to novel [5.5.5.6] tetracyclic dienone systems 172, instead of the expected [S.5.5.5] systems 173. These substrates underwent first... 

Attack of the trifluoromethyl radical on conjugated dienone systems occurs preferentially a to the carbonyl function. Thus, photocatalyzed reaction of trifluoroiodomethane with steroidal dienones leads to trifluoromethyl-substituted products, with the double bond regenerated by loss of hydrogen iodide, c.g. formation of I. ... 

Even 16-membered ring lactones, one with only two stereogenic methyl groups120, the other with six stereogenic centers, but in rather remote positions (in y,t>-position of the dienone system of a protected carbonolide117), have been epoxidized with peracid with at least 90% diastereoselectivity, the former directly to a triepoxide. 

This heteroannular dienone system causes rings A and B to assume a half-chair conformation. Carbon atoms 1, 3, 4, 5, 6, 8, 9, 10, 11 are all in one plane while carbon atom 2 projects above and carbon atom 7 below the plane. The rest of the molecule is below the plane. Addition of the second unsaturation to the A -3-ketone enone system provides an additional electron-rich unsaturated residue, i.e., it enhances the electron delocalization. The result will be an enlarged contribution of ionic resonance structures such as... 

In synthetic work in the Senecio alkaloid series, Adams and co-workers ° prepared a 1-hydroxypyrrolizidine of the desired stereochemistry (2) by Rh-AljOj hydrogenation of (1), a reaction involving complete saturation of the dienone system. 

Thus, irradiation of methyl 3-methoxy-l-methyl-4-oxocyclohexa-2,5-dienecarboxylate (11 a) in protic or aprotic solvents resulted in complex mixtures from which methyl 4-hydroxy-3-methoxy-2-methylbenzoate (12 a) was obtained in low yield ( 7%). The photoisomerization of 2-methylcyclohexa-2,5-dienone (11b) in methanol provided the corresponding phenol 12b in 41% yield. It has been shown that the formation of these phenols required the intermediacy of methyl 1 -methoxy-6-methyl- and 1,6-dimethyl-2-oxobicyclo[3.1.0]hex-3-enecarboxylates the latter has been isolated. Therefore, electron-releasing substituents at C2 of the cyclohexa-2,5-dienone systems facilitate formation of Cl-substituted bicyclo[3.1.0]hex-3-en-2-ones. ... 

The IR spectra of these alkaloids showed the characteristic absorptions of a cross-conjugated dienone system, and their UV spectra were similar to that of androcymbine (16). Their similarity to 16 was also evident from the mass spectra of CC-10 and collutine which established the molecular formula C21H25NO5. Alkaloid CC-10 is thus isomeric with androcymbine (16) and with collutine (13), but it was chromatographically different from both. Since methylation of 12 and 13 with diazomethane yielded 18, an... 

Oxo-1,5-di-tert.-butyl - cyclohexadien - (1,4) - (3-spiro-l)-cis- 2,3 -dime -thyl-cyelopropan isomerisiert sich durch einen intramolekularen Energie-Transfer vom Dienon-System iiber einen Dreiring zum 6-Oxo-l,5-di-tert.-butyl-cyclohexadien-(l,4)-(3-spiro-l) - trans -2,3- dimethyl-cyclopropan 2 ... 

The first compound, in approximately 4% yield, was regarded as demethoxydihydroschelhammeridine (44) which is presumably formed by hydrogenolysis of the allylic methoxy group at C-3 of 38 followed by 1,4 addition of hydrogen to the dienone system. The second product was obtained in 30% yield and has been shown to be l,2,6ce,7-tetra-hydroschelhammerine (45). The stereochemistry shown at C-6 of 45 cannot be deduced from spectral data but inspection of molecular models indicates that the attack from the side of the molecule is hindered by the bulky aromatic ring. The third compound, obtained in 30% yield, was postulated to be dihydroschelhammeridine (35), which was identical with schelhammericine, a natural product. The formation of 35 can be readily explained by 1,4 addition to the diene system. Further attempts to reduce it under the same conditions have been unsuccessful. The fourth minor product has been shown to be the... 

N-Methylcrotsparine (5) is a hypotensive agent and proaporphines incorporating a dienone system may possess both analgesic and hypotensive activity (no data). There is a claim that glaziovine (4) may be used as a tranquillizer " the alkaloid does possess anxiolytic properties similar to diazepam. ... 

Reductive cycttzation. Azide reduction with CrCla is followed by intramolecular addition of the resulting amine to a cross-conjugated dienone system. This efficient assemblage of fused-bridged tricycles has been incorporated as the final and key step in a synthesis of (—)-cylindricine. 

Transformation of santonin (1) into guaianolides has been carried out by two different methods, namely photochemical rearrangement of the dienone system present in the A ring of santonin, and solvolytic rearrangement of a proper 1-sulfonate derivative. 

An oxidative /pio-rearrangement mediated by a hypervalent iodine reagent that enables rapid generation of a functionalized dienone system 266 containing a quaternary carbon center has been developed (Scheme 3.110) [330]. The process occurs through transfer of an aryl group from a silyl segment present on the lateral chain of the phenol derivative 265. This transformation has been utilized in a total synthesis of an alkaloid sceletenone [330],... 

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