Dienone precursor

Several syntheses of 2//-pyrans are based on the preparation of the acyclic precursors (157b) in the hope that the dienone zZ 2//-pyran equilibrium will favor the heterocycle (157a). Often the product will contain both valence isomers. Such dienone precursors can be obtained by Knoevenagel condensation of 1,3-dicarbonyl compounds with a,(3-unsaturated aldehydes (Scheme 57). Simple 1,3-diketones yield the 2//-pyran directly (88IZV1815) and cyclohexan-l,3-diones afford fused pyrans (82S683, 84JHC913, 87JOC1972>. 

Stork and Ponaras have developed a method, involving the intermediacy of P-hydroxyhydrazones, for the introduction of alkyl and aryl groups on the a-carbon of an aP-unsaturated ketone, based on principles discussed previously. The method is particularly useful in situations where the thermodynamic enolate ion (y-carbon) cannot, or must not for the sake of other sensitive groups, be formed. Acid-catalysed dehydration of substituted cyclohex-3-ene-l,2-diols, readily formed from dienone precursors, yielded mixtures of cyclohex-2-enones and substituted benzenes. The product ratio was strongly dependent on reaction conditions and on the substituent group at C-1, but independent of the initial diol configuration, thus indicating a common cationic intermediate. Conditions were found which allow the synthetic use of the transformation sequence shown (Scheme 2) in up to 90 % yield. 

While the mechanistic scheme as outlined so far accounts for the majority of structural changes in ring A-dienone isomerizations, a few cases require modifications of this general pathway. The B-nor dienone (215) is transformed exclusively to the linear dienone (217) in dioxane solution. The preferential fission of the 5,10-bond in the hypothetical precursor (216) has... 

Dienones, such as 4-[4-(trimethylsilyl)-2-butenyl]-3-vinyl-2-cyclohexenone, are useful precursors for these particular transformations the allylsilane side chain is too short for effective 1,4-addition, but just right for 1,6-addition, resulting in six-ring annulation. Three different Lewis acids can be used titanium(IV) chloride, boron trifluoride diethyl ether complex, and ethylaluminum dichloride. The best chemical yields and complete asymmetric inductions were obtained with ethylaluminum dichloride. 

Different directions in rearrangement of the same precursor were shown by using the bicyclic dienones 337 (equation 121)177. The formation of two different products, 338 by methyl migration and 339 by a stepwise cyclohexane ring migration, is considered confirmation of a multistage mechanism. 

In the latter compounds, the cross-conjugated dienone is replaced by a fu-ran ring conjugated to an E double bond. Biosynthetically, it is not known if these compounds arise from 1,4-dione precursors such as 138 by a Paal-Knorr type cyclization (Scheme 2) or from the a-angelica lactones 134 by reduction of the lactone carbonyl followed by loss of water. 

Cyclohexadienones 61 and 64 are readily available from monoprotected hydro-quinones or para-substituted phenols, respectively. Conjugate additions to these symmetrical dienones result in desymmetrization of the prochiral dienone moieties, providing access to multifunctional chiral synthons in two steps from the aromatic precursors (Scheme 7.17) [72]. 

The formation of the pyrrolidone group involves condensation of an acetate with L-serine. Since a significant proportion of L-[l,2,3-l3C, 15N]serine was incorporated into pramanicin, the carbon skeleton of serine is incorporated intact. Acylation with the 14-carbon moiety then ensues leading to the conjugated dieneone tetramic acid intermediate 122. In fact, this compound co-occurs with 121 and, interestingly, is almost exclusively produced when 123 and 124 are used as precursors. The remaining steps involve formation of the trans-diol at C-3 and C-4 and ep-oxidation of the terminal alkene in the dienone chain. 

The p,0 -dihydroxystilbene III does not give any quinone on oxidation with H202/Cu+2, but the stilbene is destroyed. Neither do simple catechols give any significant color under the oxidation conditions used. Recently (75, 28) structures of type XI, or precursors of dienone type, have been demonstrated in Bjorkman lignin. Kraft cooking of XI presumably... 

Similarly, many A-norsteroids 244 have been subjected to phenol-dienone rearrangement in HF—SbF5 medium854 [Eq. (5.312)]. 1H NMR spectroscopic studies at low temperature confirms the formation of 0-protonated intermediates (Scheme 5.87), which subsequently rearrange to diprotonated precursors of the dienones 245. 

Cycloalkenones.3 Cycloalkenones can be prepared by a retro Diels-Alder reaction of norbornenes of type 1, conducted at 25-70° in the presence of CH3A1C12 (1 equiv.) and a reactive dienophile, usually maleic anhydride or fumaronitrile. The [4 + 2]cycloreversion was used to prepare 12-oxophytodienoic acid (4), which epi-merizes at Cu to the trans-isomer on brief exposure to acid. The precursor nor-bornene 3 was prepared from the known dienone 2 as shown. Treatment of 3 at room... 

Dienones 12A-12D were also detected as trace components in quince fruit volatiles after SDE sample preparation. However, as shown in Figure 3, except for the low amount of hydrocarbon 5, the distribution of thermal degradation products from 8 did not correspond to the composition of the major norisoprenoids 5-7 obtained after SDE of quince fruit juice. Consequently, diol 8 had to be excluded as their precursor. 

The importance of conformation in selective attack of the 1 l/ -alkoxy radical on C18 and C19 was nicely illustrated by the photolysis of the nitrite 18 derived from the dienone 19. This afforded only C18 attack.7 Treatment of the derived oxime with nitrous acid gave 1-dehydro-aldosterone acetate 20. This is a convenient precursor for labelled aldosterone acetate since selective hydrogenation with tritium gives 1,2-ditritiated aldosterone acetate. 

The Schmidt reaction with the dienone 11/102 (Scheme 11/15) yields the 1,4-thiazepine, 11/103. Treatment of its dihydroderivate, II/104, gives exclusively the enlargement product 11/105, in which the methylene group migrated [82]. A similar reaction can be observed if the synthetic ergot alkaloid precursors of type 11/106 are treated with in situ generated hydrazoic acid. Again no trace of the isomeric lactam can be observed [83]. 

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