Enones 3-substituted

A series of different cyclic enones has been subjected to photochemical [2-1-2] cycloaddition [337, 340]. Some examples are shown in Figure 4.14. As expected, enones substituted with bulky groups hinder addition for example, the tert-butyl substituted enone 295 is unreactive toward Cr,... 

Electrophilic additions with activated alkynes also occur readily with ri2-pyrrole complexes.12bl3b In methanol solution, the pyrrole (20) and 1-methylpyrrole (21) complexes undergo conjugate addition cleanly at C-3 with 3-butyn-2-one to give the p-enone-substituted pyrrole complexes 56 and 58 in high yield (Figure 13). In contrast to what is observed... 

Photocyclization of enones substituted by an a-silylamino group. Irradiation (>320 nm) of these substrates with DCA as sensitizer results in N-heterocycles. 

Related chemistry of a cationic rf-a- yrm Mo(ll) complex is shown in Scheme 28. Hydride addition to the cationic reactant complex gives a neutral allyl-Mo that is oxidized by pyridinum dichromate (PDC) to a cation. Nucleophilic addition of water and oxidative decomplexation of the Mo fragment gives an enone. Substituted a-pyran hgands follow one of two paths depending on the electronic and steric effects of the substituent, as shown. 

Intra-molecular examples provide an annulation process which is also feasible in cases of less reactive /3,/3 -dialkyl enone substitution, as shown by the transformation of 178 to the cw-fusedketone 179 (Scheme 5.2.39).55... 

Diels-Alder reactions. Enones substituted at the a -position with an acetal group form cyclic vinyloxocarbenium ions. The enhanced dienophilic reactivity of such species is synthetically useful, and diasteroselective processes have been developed in which the ionization is mediated by fluoroboric acid. Scandium(III) triflate provides lower diastereoselectivity, and BFj OEtj is useless, as extensive decomposition of the substrates occurs. 

Danishefsky s interest in the development of synthetically useful Diels-Alder reaction methodology has recently extended to two different dienophiles. First, he studied the reactivity of enyne dienophiles to show that ynones react preferentially to enones. Substituted styrene 78 is the exclusive product on intermolecular Diels-Alder reaction of diene 76 with dienophile 77 followed by a retro Diels-Alder reaction (to generate 2-methylpropene) and two desilylations. ... 

Suggest a mechanism for this reaction, which is another synthetic route to enones substituted at the /3-carbon. (Hint See Exercise 18-24.)... 

Cydopentane reagents used in synthesis are usually derived from cyclopentanone (R.A. Ellison, 1973). Classically they are made by base-catalyzed intramolecular aldol or ester condensations (see also p. 55). An important example is 2-methylcydopentane-l,3-dione. It is synthesized by intramolecular acylation of diethyl propionylsucdnate dianion followed by saponification and decarboxylation. This cyclization only worked with potassium t-butoxide in boiling xylene (R. Bucourt, 1965). Faster routes to this diketone start with succinic acid or its anhydride. A Friedel-Crafts acylation with 2-acetoxy-2-butene in nitrobenzene or with pro-pionyl chloride in nitromethane leads to acylated adducts, which are deacylated in aqueous acids (V.J. Grenda, 1967 L.E. Schick, 1969). A new promising route to substituted cyclopent-2-enones makes use of intermediate 5-nitro-l,3-diones (D. Seebach, 1977). 

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

Wlien tlie diiral molybdenum -K-allyl-substituted enone 147 was treated witli litliium dimetliylciiptate, formation of adduct 148 witli fait selectivity was observed tSdieme 6.29) [69], Interestingly, bigber selectivities were obtained in tlie presetice of boron ttlbuotlde etlierate. It is assumed tliat Lewis acid coordination induces tlie s-trans reactive conformation 149 [64], Consequently, nudeopb de attack anti to tlie molybdetiLim ftagmetit sbould afford tlie major diastereomer 148. 

In this synthesis, we have witnessed the dramatic productivity of the intramolecular enone-olefin [2+2] photocycloaddition reaction. This single reaction creates three contiguous and fully substituted stereocenters and a strained four-membered ring that eventually provides the driving force for a skeletal rearrangement to give isocomene. 

The reversal of the stereoselectivity is attributed to the ability of chlorotrimethylsilane to trap the initially formed cuprate-enone complex, thereby suppressing equilibration of the diastereomeric complexes. The copper-catalyzed 1,4-addition of Grignard reagents to 5-substituted 2-cyclo-hexenone also proceeded with very high trans diastereoselectivity22. 

Combination of nickel bromide (or nickel acetylacetonate) and A. A -dibutylnorephcdrinc catalyzed the enantioselective conjugate addition of dialkylzincs to a./Tunsaturated ketones to afford optically active //-substituted ketones in up to ca. 50% ee53. Use of the nickel(II) bipyridyl-chiral ligand complex in acetonitrile/toluenc as an in situ prepared catalyst system afforded the //-substituted ketones 2, from aryl-substituted enones 1, in up to 90% ee54. 

Another chiral cuprate that shows high diastereoselectivity on addition to cycloalkcnones is the dihydropyrazine derivative 12. The adducts 13 were obtained in good chemical yield with diastereoselectivities exceeding 90% A Lower diastereoselectivities resulted on addition of 12 to /(-substituted cycloalkenones or to acyclic ( )-enones. 

The addition of the anions of racemic cyclic allylic sulfoxides to various substituted 2-cyclopentenones gives y-l,4-adducts as single diastereomeric products22. The modest yields were due to competing proton-transfer reactions between the anion and enone. The stereochemical sense of these reactions is identical to that for the 1,4-addition reaction of (Z)-l-(/erf-butylsulfinyl)-2-methyl-2-butene to 2-cyclopentenone described earlier. 

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