Enol carbonate

Oxidation of silylenol ethers and enol carbonates to enones... 

The decarboxylation-allylation of allyl enol carbonates proceeds smoothly[450]. The isomeric enol carbonates 699 and 701 of the enone 698 undergo regiospecific allylation, giving the regioisomers 700 and 702 selectively. 

Ailyl enol carbonates derived from ketones and aldehydes undergo Pd-cat-alyzed decarboxylation-elimination, and are used for the preparation of a, /3-unsaturated ketones and aldehydes. The reaction is regiospecific. The regio-isomenc enol carbonates 724 and 726, prepared from 723, are converted into two isomeric enones, 725 and 727. selectively. The saturated aldehyde 728 can be converted into the a,/3-unsaturated aldehyde 730 via the enol carbonate 729[459]. 

When 2-lithio-2-(trimethylsilyl)-l,3-dithiane,9 formed by deprotonation of 9 with an alkyllithium base, is combined with iodide 8, the desired carbon-carbon bond forming reaction takes place smoothly and gives intermediate 7 in 70-80% yield (Scheme 2). Treatment of 7 with lithium diisopropylamide (LDA) results in the formation of a lactam enolate which is subsequently employed in an intermolecular aldol condensation with acetaldehyde (6). The union of intermediates 6 and 7 in this manner provides a 1 1 mixture of diastereomeric trans aldol adducts 16 and 17, epimeric at C-8, in 97 % total yield. Although stereochemical assignments could be made for both aldol isomers, the development of an alternative, more stereoselective route for the synthesis of the desired aldol adduct (16) was pursued. Thus, enolization of /Mactam 7 with LDA, as before, followed by acylation of the lactam enolate carbon atom with A-acetylimidazole, provides intermediate 18 in 82% yield. Alternatively, intermediate 18 could be prepared in 88% yield, through oxidation of the 1 1 mixture of diastereomeric aldol adducts 16 and 17 with trifluoroacetic anhydride (TFAA) in... 

Carboxylic acids can be directly alkylated by conversion to dianions with two equivalents of LDA. The dianions are alkylated at the a-carbon, as would be expected, because the enolate carbon is a more strongly nucleophilic than the carboxylate anion.76... 

Substituted benzo[b]furan can be oxidatively converted to the lactone 57 in 81% yield, which is allowed to react with chloroformate containing a chiral trans-( R,2R)-2-phenylcyclohexyl group (R ) to give the benzo[ft]furan-based enol carbonate. When the carbonate is treated with DMAP, nearly complete conversion to the new benzo-fused y-lactone 58 is observed in dichloromethane or in THF with a diastereomeric ratio of 3 1 <00OL1031>. 

HYDROGENATION OF ENOL CARBONATES AND 4-MKTIIYI. KNK-N-ACYI OYA/.OI IPINONK USING [Rh((R)-BiNAP)l COMPLEXES... 

The reaction of silyl enol ethers with fluoroformates and fluoroformamides, catalysed by the addition of tetra-n-butylammonium fluoride, produces enol carbonates and carbamates in acceptable yields [60],... 

C-alkylation is predicted to occur via attack on the enolate carbon perpendicular to the C — C — O plane for maximum overlap of the re-system. Such a transition state 5 can be easily achieved when the ring to be formed is six-membered. In the five-membered ring closure there is less strain in a nucleophilic substitution involving the lone-pair orbital on oxygen. 

Quenching this reaction with deuteriomethanol gave 2-methylcycloheptanone having deuterium at the 2-position (199 E = D) in 75% yield with 95% deuterium incorporation. Aldehydes and benzoyl chloride gave the desired products in 60-70% yields. Alkylation of the enolate intermediate (198) was successfully carried out with alkyl halides in the presence of HMPA in good yields. The reaction with ethyl chloroformate and chlorotri-ethylsilane gave enol carbonate (200) and sUyl enol ether (201) in 74 and 75% yield, respectively. 

Enol carbonates and carbamates. Olofson and Cuomo have extended the regio-ipccilic alkylation of ketones (6, 44) to a regiospeeific preparation of enol carbonates and carbamates. The enol carbonates are prepared by reaction of silyl... 

Tributyltin hydride, 316 Tributyltinlithium, 319 Trichloroacetonitrile, 321 Other carbohydrates (Diethylamino)sulfur trifluoride, 110 Triethyloxonium tetrafluoroborate, 44 Carbonates (see also Enol carbonates) Carbon dioxide, 65 Di-/-butyl dicarbonate, 94 Carboxylic acids (see also Dicarbonyl compounds, Unsaturated carbonyl compounds)... 

Enol carbonates Di-f-butyl dicarbonate, 94 Potassium hexamethyldisilazide, 257 Enol esters Enol acetates Acetic anhydride, 93 Bis(7i5-cyclooctadienyl)ruthenium(II), 35... 

Recently, a general synthesis of a-formyloxycarbonyl compounds was reported. Yields ranging from 35-90% were achieved via electrochemical generation of enol carbonate cation radicals in DMF156. The cation radicals are trapped by the solvent, and the resulting formiminium ion is hydrolyzed during workup. The mechanism is shown in Scheme 62. 

Enol carbonates. Site-specific metal enolates, such as those generated by conjugate addition of cuprates to a./i-enones, can be trapped as the enol carbonates by a quench with methyl chloroformate. Unlike silyl enolates, enol carbonates are only weakly nucleophilic. Thus they are stable to ozone, peracids, and Wittig-like... 

Mercuric triflate has been shown to be a powerful catalyst for the cyclization of alkynyl /-butylcarbonates (36) and (37) giving rise to cyclic enol carbonates under mild conditions. Internal alkynyl carbonate (36) afforded the endo-dig cyclization (g) product (38) selectively, whereas terminal alkynyl carbonate (37) provided only the exo-dig product (39).48... 

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