Dienones, protonated

When the reaction was followed by proton magnetic resonance, it was found that the yield reached a maximum after 18 hours longer refluxing resulted in decomposition and lower yields. At this time, also, the maximum proportion of the 1-methoxy isomer was produced this isomer is converted into the dienone complex. 

Metal-ammonia solutions reduce conjugated enones to saturated ketones and reductively cleave a-acetoxy ketones i.e. ketol acetates) to the unsubstituted ketones. In both cases the actual reduction product is the enolate salt of a saturated ketone this salt resists further reduction. If an alcohol is present in the reaction mixture, the enolate salt protonates and the resulting ketone is reduced further to a saturated alcohol. Linearly or cross-conjugated dienones are reduced to enones in the absence of a proton donor other than ammonia. The Birch reduction of unsaturated ketones to saturated alcohols was first reported by Wilds and Nelson using lithium as the reducing agent. This metal has been used almost exclusively by subsequent workers for the reduction of both unsaturated and saturated ketones. Calcium has been preferred for the reductive cleavage of ketol acetates. 

A variety of conjugated dienones are reduced by lithium-ammonia, presumably via dienyl carbanions analogous to the allyl carbanions encountered in enone reductions. Cross-conjugated l,4-dien-3-ones afford 4-en-3-ones as the major reduction products, indicating that the cyclohexadienyl carbanion (55) protonates largely at C-1. Some protonation at C-5 does occur as shown by examination of the NMR spectrum of the crude reduction product derived from the 17-ethylene ketal of androsta-l,4-diene-3,17-dione. The 17-ethylene ketal of androst-4-ene-3,17-dione is formed in 75%... 

Protonation of the dienyl carbanion formed by reduction of a linear dienone may occur at a carbanion center either p or S to the carbonyl group as is implied by the resonance structures shown below ... 

Most dienones that have been reduced have structures such that they cannot give epimeric products. However, reduction of 17 -hydroxy-7,17a-dimethyl-androsta-4,6-dien-3-one (63) affords 17 -hydroxy-7j9,17a-dimethylandrost-4-en-3-one (64), the thermodynamically most stable product, albeit in only 16% yield. The remainder of the reduction product was not identified. Presumably the same stereoelectronic factors that control protonation of the / -carbon of the allyl carbanion formed from an enone control the stereochemistry of the protonation of the (5-carbon of the dienyl carbanion formed from a linear dienone. The formation of the 7 -methyl compound from compound (63) would be expected on this basis. 

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

When dienones 39 and 40 are photolyzed in sulfuric acid they both rearrange to the same product, 2-methyl-5-hydroxybenzaldehyde (41) (Filipescu and Pavlik, 1970). The mechanism for this photorearrangement is consistent with that of the protonated cyclohexadienones already discussed, i.e., disrotatory closure to afford the intermediate bicyclic cations 42 and 43. In this case it is conceivable that the electron-withdrawing effect of the dichloromethyl group forces the subsequent thermal cyclopropyl migration entirely in the direction of the most stable cation 44 to yield the observed product. 

It is interesting to note that the parent dienone 67 and its 2-methyl derivative 68, when irradiated in strongly acidic media, gave no product which would con espond to the protonated bicyclo[4,l,0]heptenone observed in the photolysis of protonated eucarvone. In FHSO3 at — 66°... 

As in the photolysis of protonated eucarvone, an acyclic intermediate is proposed in the mechanistic pathway. The protonated dienones 73 and 74 should be thermally stable, since a symmetry-allowed ring closure in the conrotatory mode is precluded in the cyclic system (Woodward and Hoffmann, 1970). Upon irradiation it can undergo a conrotatory ring opening however, to produce the acyclic cations 79 and 80 which in... 

A related dienediol-phenol rearrangement which can occur by different pathways was reported as a new method for synthesis of the oxepine system180. Protonation of the starting diol 344 produces a cation 345 which can follow normal dienone-phenol rearrangement (path a) when the substituents R2 = Me, Ph and R1 = t-Bu are eliminated in the step 346 — 347. However, when R1 = t-Bu and R2 is a substituted phenyl which decreases the nucleophility, the cationoid intermediate 345 cyclizes to the oxonium ion 348 (path b) which then undergoes deprotonation to give the oxepine 349 (equation 124)180. 

As previously mentioned, allenes can only be obtained by 1,6-addition to acceptor-substituted enynes when the intermediate allenyl enolate reacts regioselectively with an electrophile at C-2 (or at the enolate oxygen atom to give an allenyl ketene acetal see Scheme 4.2). The regioselectivity of the simplest trapping reaction, the protonation, depends on the steric and electronic properties of the substrate, as well as the proton source. Whereas the allenyl enolates obtained from alkynyl enones 22 always provide conjugated dienones 23 by protonation at G-4 (possibly... 

Im ersten Schritt addiert sicb das Nitronium-Ion an das die Acyl-Gruppe tragcnde Ringatom. AnschlieBend lauft unter Proton-Abspaltung und Dienon-Bildung ein 1,3-Acyl-shift zum Carbonyl-O-Atom unter Bil-dung der Acctoxy-Gruppe ab ... 

The competition between Michael addition of a,(3-unsaturated ketones and Diels-Alder reactions involving furan and 2-methylfuran is affected by the catalyst used. Methyl vinyl ketone gives the alkylation product with furan and 2-methylfuran in the presence of silica gel (88TL175). Bis(alkylated) products have also been obtained in reactions of 2-methylene-1,3-dicarbonyl compounds (90H(31)1699). An intramolecular proton catalyzed alkylation reaction of an a,(3-unsaturated ketone provided a straightforward synthesis of norpinguisone (90TL4343) and in the example shown in Equation (4) the cyclization reaction involved an a,(3-y,8-dienone (94TL4887). 

In the presence of freeze-dried potassium fluoride, perfluoro-2-methylpent-2-ene reacts with activated methylene compounds to yield pyrans (81MI22400). The fluoride ion abstracts a proton from the methylene group and subsequent condensation of the carbanion with the perfluoroalkene affords a dienone (19) which ring closes to the pyran (20 Scheme 3). In the case of pentane-2,4-dione a divinyl ether (21) is also formed. This product is considered to arise from reaction of the alkene at the oxygen of the enolate ion. 

Suitable substitution at C-2 and C-4 thus causes a shift in the pyran-dienone equilibrium towards the heterocyclic system. Generally, whilst the adducts can be isolated, they tend to decompose during normal purification methods (80JOC5160). However, provided substituents are additionally present in the 3- or the 3- and 5-positions, the compounds are sufficiently stable to be isolated in a pure state (81ZC260). The pyran results when a suspension of the pyrylium salt is heated with an equimolar quantity of triethylamine, which acts as the proton acceptor, or simply on treatment with sodium alkoxide. 

The solvent effects observed in the photochemical rearrangements of cross-conjugated dienones suggest that the availability of a proton to an excited state of the ketone may be an important controlling factor. It is interesting to consider the possibility that protonation of an excited state may lie behind the apparent polar reactions. Triplet states are likely to be more polarizable and to be stronger bases than the corresponding... 

Electrocyclic closure of both pentadienyl cation and anion have been observed. Cations generated by protonation of dienones close in the predicted conrotatory manner as shown in Equation 12.55.99 The pentadienyl anion, a six-electron system, should close in the disrotatory sense a clear example is the rapid isomerization illustrated in Equation 12.56.100 Photochemical cyclization of pentadienyl cations has been observed Equation 12.57 shows an example in a cyclic system.101 The ready thermal reversion, which should be conrotatory and therefore difficult in the bicyclic system, may possibly occur by a stepwise path.102... 

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