Enolethers acetals

The complete ethanolysis of K CN in the presence of dry HCI gas and cone. H2SO4 furnishes triethyl [ C]orthoformate °. In normal synthetic organic chemistry triethyl orthoformate is used for the conversion of aldehydes and ketones into acetals, enolethers and ketals, respectively, as well as for the formylation of C- and Af-nucleophiles. In carbon-14 labeling the most frequent use has been for the preparation of purines and related compounds labeled at a methine position, through reaction of triethyl [ " C]orthoformate with appropriate substituted pyrimidine such as 131 (Figure 7.32), which reacts with the labeled reagent (in 10-25% excess) in the presence of catalytic amounts of a strong acid (p-TosOH, MsOH) under strictly... 

Synthesis of a,y -ethylenealdehydes via y -alkoxyacetals Acetal-enolether condensation... 

Anodic regioselective acetamidosulfeny-lation of alkenes is similarly achieved by oxidation of diphenyldisulfide in acetonitrile [81]. Cyclic enamines, which are intermediates in the oxidation of cyclic N-methoxycarbonyl amines, react in aqueous acetonitrile that contains chloride ions to a-hydroxy- 8-chloro compounds via intermediate chloronium ions [82]. Enolethers undergo a regioselective azidomethoxyla-tion to yield acetals of a-azido carbonyl compounds upon electrolysis in methanol containing sodium azide [83]. The reaction proceeds possibly via addition of an anodicaUy generated azide radical. 

Scheme 9 EGA-catalyzed diastereoselective aldol reaction of acetal with an enolether.

A solution of 1 g of the diacetate (III) in 100 cc of n-heptane containing 2.5 cc of cyclopentanol and 50 mg of p-toluenesulfonic acid is heated under reflux for 20 hours. After cooling, a few drops of pyridine are added and the solvent is eliminated by evaporation under vacuum. The residue is taken up with methanol to give 3-cyclopentyl enolether of 17a-ethynyl-19-nortestosterone acetate which, after recrystallization from methanol, melts at 182°C to 184°C. 

As an extension of this work, photoinduced [2+2]-cycloadditions of 1-acetylisatin (13) with cyclic enolethers (furan, benzofuran, 2-phenylfuran, 8-methoxypsoralen), and acyclic enolethers (//-butyl vinyl ether and vinyl acetate) were investigated which afforded the spiro-oxetanes in high yields (82-96%) and with high regio- and diastereoselectivity (Sch. 4) [19]. Treatment of the furan-derived oxetane 15 with acid resulted in oxetane ring opening and yielded the 3-(furan-3-yl)indole derivative 16. 

Even phenol, being less acidic than acetic acid, adds to (31) at 50 °C. However, the final product is not the enolether (60) but an o-substituted phenol (61), which is probably formed by a rearrangement of (60)204). 

Paternd-Biichi reactions [152] this competition has been investigated for electron-rich alkene substrates for several combinations of carbonyl compounds and electron-donors, e.g. a-diketones and ketene acetals [153], aromatic aldehydes and silyl ketene acetals, and enol ethers. In polar solvents, the assumption of a 1,4-zwitterion as decisive intermediate is reasonable. This situation then resembles the sequence observed for ET-induced thermal [2 -I- 2]-cycloaddition reactions [154]. Both regio- and diastereoselectivity are influenced by this mechanistic scenario. The regioselectivity is now a consequence of maximum charge stabilization and no longer a consequence of the primary interaction between excited carbonyl compound and alkene. Whereas 3-alkoxyoxetanes are preferentially formed from triplet excited aldehydes and enolethers, 2-alkoxyoxetanes result from the reaction of triplet excited ketones or aldehydes and highly electron-rich ketene silylacetals (Scheme 40) [155]. 

Ketocarboxylic esters condense with AT, as a rule, to give exclusively 7-oxo derivatives (see Scheme 1) an exception is that of a side reaction in a special case (93JHC1325). Esters having instead a 3-acetal or 3-enolether functionality, however, are known to tend to reverse reactivity (e.g., 99EUP947516, 04MI2). 2-Chlorocin-namic acid (15a, Scheme 6) or its ester remarkably also behave so (00CHE1329). 

In acyclic systems, reductions of the ketone group of 1.155 (Y = COR) give poor selectivities. Reactions of o-substituted aldehydes 1.155 (Y = CHO) with organomagnesium reagents, perfluoroalkyllithiums or nitromethane [540] or chloracetophenone [540, 544] anions are very selective. Such is also the case for their reactions with functionalized isonitriles [540], silyl enolethers or thioketene acetals in the presence of Lewis acids [545, 546], or in B aylis-Hillmann reactions [547],... 

The synthesis of the twistane-ketal 279 using endo-6-hydroxy-9-oxabicyclo[3.3.1]-nonan-2-one (274) or its acetate 227, resp., was nevertheless successful by applying the following route according to a procedure by Inhoffen etal. the keto-acetate 221 was treated with trimethyl orthoformate. From the reaction mixture which contained the dimethoxy-ketals 222 and 223 as well as the enolethers 224 and 225, the acetates 222 (63%) and 224 (5%) were isolated by chromatography. On base-hydrolysis they gave the alcohols 223 and 225. Pyrolysis of the ketal 222 and simultaneous distillation yielded 67% of the enolether 224. Finally by treating the dime-... 

Cyclopropanes are efficiently obtained from alkenes by Simmons-Smith reaction applying CH2l2/Zn-Cu couple or CH2l2/Et2Zn as reagents. A variety of compounds bearing a chiral moiety have been studied, as for example a,P-unsaturated acetals and oxazolidines, enolethers, allylic alcohols, alkenylboronic esters as well as a,p-unsaturated carbonyl compounds. 

By treatment of acetals, obtained from chiral 1,3-diols and cyclic a,p-unsaturated ketones with /BU3AI, enolethers are obtained bearing an alcohol functionality, which proved to be necessary to obtain high diastereoselectivity. ... 

Reactions of ketenes, e.g. dihaloketenes, with electron-rich alkenes bearing the chiral auxiliary, for example enolethers, carbohydrate-derived anomeric enolethers or ketene acetals, lead to cyclobutanones with moderate stereoselectivities. 

Without additional reagents Mixed O-heterocyclic acetals from enolethers and quinones... 

Stannous triflatejtriphenylcarbonium chloride p-Alkoxyacetals from enolethers and acetals s. 44, 625... 

Chiral (E)-enolethers. A degassed soln. of (5R)-5-cyclohexyl-2-ethenyl-l,3-dioxolan-4-one (2 1 cisjtrans) in THF added to ca. 1 eq. of a suspension of bis(l,5-cycloocta-diene)nickel(0) in the same solvent under N2, stirred until the complex dissolved (10 min), after 3h the resulting rust-coloured precipitate collected, suspended in methylene chloride, treated with MejSiCl, and stirred for 30 min intermediate 7c-allylnickel complex (Y 78%), in benzene treated with DMF and 5 eqs. 1-iodo-propane, irradiated with a sunlamp (GE 275 W Model RSW) for 2,5 h at 10°, stirred for a further 3 h, diluted with pentane to precipitate nickel halide, and stirred for a further 4 h product (Y 82% E/Z 9 1). Subsequent treatment with acetals afforded 2-p-alkoxy-l,3-dioxolan-4-ones with asym. induction, thereby providing an alternative to asym. aldol condensation. F.e. inch reaction with ar. and a,P-ethylene-bromides s. D.J. Krysan, P.B. Mackenzie, J. Am. Chem. Soc. 110, 6273 (1988). 

Palladous acetate j triethylamine 2-Aroylation of enolethers s. 39, lS5s43 details s. J. Org. Chem. 53, 4257-63 (1988). 

Grignard comics. 43,697 P-alkoxy-y-arylthio-a-methyleneal-dehydes from enolethers 44, 907 alkoxy-3-ethylenes from acetals 43, 795 44, 878... 

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