Acetal ization

Variant 2 of the coupled acetalizations/orthoester hydrolyses succeeds completely without alcohol. Here the carbonyl compound is acetalized only with the orthoester. This method is not simply an option, but sheer necessity if the carbonyl compound that is to be acetalized is an a,/l-unsaturated carbonyl compound as in the example of Figure 9.14. Otherwise, such substrates bear the risk of methanol being added to the Ca=C/3 double bond before the acetal-ization of the C=0 group takes place. 

It is possible that a trace of alcohol contaminating the orthoester would make the acetal-ization of a,/)-unsaturated carbonyl compounds via the mechanism of the preceding paragraph conceivable. But most probably the alternative acetalization mechanism of Figure 9.14 comes into play. The orthoester A would then first be cleaved so that MeOH would be eliminated in an equilibrium reaction This would provide the carboxonium ion E, which would... 

N-cyclohexyl-1-chlorophthalimlde (250 g) was dissolved in glacial acetic acid (2.5 8),concentrated hydrochloric acid (555 ml) and tin (27B g) were added and the suspension was heated on a steam bath for 16 hours. The cooled solution was filtered and concentrated to dryness in vacuo to give a white solid. This solid was dissolved in water and the precipitated oil extracted with chloroform. The chloroform solution was dried and concentrated in vacuo to give a solid which, after recrystal I izat ion, yielded 5-chloro-2-cyclohexylisoindolin-1-one (43%), MP 140°Cto 142°C. 

Although the C-3 stereocenter in 6 may be susceptible to epimer-ization in the presence of a basic organolithium reagent, enal 6 condenses smoothly in the desired and expected way with lithio sul-fone 5 at -78 °C to give, after quenching with acetic anhydride, a stereoisomeric mixture of acetoxy sulfones (see 35, Scheme 7). ( ,E,7f)-Triene 36 is then unveiled on reduction of the stereoisomeric acetoxy sulfones with 5 % sodium amalgam (77 % overall yield from 6).3... 

Mitulovic et al. [Ill] presented a procedure for the preparation of a-carbon deuterium-labelled a-amino acids from native amino acids via a Schiff-base racem-ization protocol in deuterated acetic acid involving a preparative chromatography step of the obtained Z-protected deuterium-labeled amino acid derivatives on the tBuCQN-CSP [ill]. The analytical control of the enantiomeric products after DNP, Z, or DNZ derivatization showed a high enantiomeric excess (97-98%) and also a high isotopic purity (99%) by MS. 

N-eth yl-N/ e thoxyphenylurea. Decompose ti on occurs on attempted recry stall1 ization from acet,benz,chif,or petr eth Refs 1) Beil 13, [295] 2) H.F.J. 

Preparation.l Dialkyl acetals of DMF are generally prepared by transacetal-ization of the dimethyl or diethyl acetal, but this reaction is not useful in the case of the di-r-butyl acetal. Replacement of one methoxy group of DMF dimethyl acetal occurs on refluxing in f-butyl alcohol conversion to the dw-butyl acetal is effected in the presence of 2,4,6-tri-f-butylphenol (equation I). 

Pyridineacetic and thiazoleacetic acid can also be decarboxylated by way of the zwitterions or analogous cyclic intermediates.20 In these cases the formation of the zwitterion does not lead to electron attraction by the -carbon atom but, instead, produces a cationoid centre at the -position. 2-Ethyl-2-methyl-2-(2 -pyridyl)acetic acid, for instance, is stable in boiling concentrated hydrochloric acid or boiling sodium carbonate solution but is readily decarboxylated in neutral solution. 4-Pyridineacetic acid behaves similarly. Decarboxylation of optically active pyridineacetic acid derivatives having an asymmetric a-carbon atom leads to completely racemic products, and as the racem-ization is simultaneous with, and not subsequent to, the decarboxylation it is assumed that an enamine is formed as intermediate, e.g. ... 

A solution of 1.50 g 3-methoxy-4,5-ethylenedioxybenzaldehyde in 6 ml acetic acid was treated with 1 ml nitroethane and 0.50 g anhydrous ammonium acetate, and held on the steam bath for 1.5 h. To the cooled mixture H20 was cautiously added until the first permanent turbidity was observed, and once crystal-1 ization had set in, more H20 was added at a rate that would allow the generation of additional crystals. When there was a residual turbidity from additional H20, the addition was stopped, and the beaker held at ice temperature for several h. The product was removed by filtration and washed with a little 50% acetic acid, providing 0.93 g 1-(3-methoxy-4,5-ethylenedioxyphenyl)-2-nitropropene as dull yellow crystals with a mp of 116-119 °C. Recrystallization of an analytical sample from MeOH gave a mp of 119-121 °C. 

There is no commercial use for the individual substances. The D. have supporting effects in the parasit-ization of insects by Metarrhizium anisopliae. For biosynthesis and incorporation experiments of, e.g., C labelled acetate, see Lit., for detection, see Ut.. D. B reversibly suppresses the expression of hepatitis B viral surface antigen (HBsAg) gene in human hepatoma cells and may have potential for development as a specific anti-HBV drug. ... 

Biocatalytic Dynamic Kinetic Resolution of (R,S)-1- 2,3-Dihydrobenzo[b]Furan-4-yl -Ethane-1,2-Diol. Most commonly used biocatalytic kinetic resolution of racemates often provide compounds with high e.e., although the maximum theoretical yield of product is only 50%. In many cases, the reaction mixture contains a roughly 50 50 mixture of reactant and product which have only slight differences in physical properties (e.g., a hydrophobic alcohol and its acetate), and thus separation may be very difficult. These issues with kinetic resolutions can be addressed by employing a Dynamic Kinetic Resolution process involving a biocatalyst or biocatalyst with metal-catalyzed in situ racem-ization (26,27). 

Another example of polymer-supported A1 based Lewis acid is cross-linked polystyrene-supported aluminum triflate (79). Cross-linked polystyrene-supported AICI3 (72) was easily converted into (79) by treatment with triflic acid. This catalyst was applied to dithioacetalization of carbonyl compounds and transdithioacetal-ization of acetals (Scheme 19.19) [44]. From benzaldehyde the corresponding dithioacetal (84) was obtained in the presence of the polymeric catalyst (79) in 98% yield in 30 minutes. The same product was also obtained from the dimethyl acetal (85) in 94% yield. Chemoselectivity of the polymeric catalyst was also demonstrated in Scheme 19.19. Aldehydes reacted faster in the presence of ketone (34) to give the dithioacetals with (79). Aliphatic ketone (89) exclusively reacted with dithiol (81) in the presence of aromatic ketone (34). These chemoselectivities were higher than those obtained from the reactions using nonsupported Al(OTf)3. 

---