Acetal derivatives regioselectivity

Cleavage of acetals. In the presence of a Pd(II) catalyst, /-butyl hydroperoxide oxidatively cleaves a five- or six-membered acetal to an ester of a diol. Pd(OAc), and PdCl2 can catalyze this reaction, but CFjCOjPdtJOCfCHj), (10, 299) is most effective. The cleavage of acetals derived from unsymmeti ical diols is not regioselective. Example ... 

Table XIX shows the results of reactions of silyl ketene acetals derived from propionates with crotonate, cinnamate, sorbate, and fumarate in the presence of aluminum ion-exchanged montmorillonite (Al-Mont) (62). The reactions proceeded at low temperatures. The Michael products could be isolated in the unstable form of a trimethylsilyl ketene acetal in good yield owing to an easy work-up procedure (removal of the solid catalyst). It is noteworthy that the montmorillonite-induced Michael addition to a polyenoate occurred regioselectively in a 1,4-fashion in the case of methyl sorbate (Table XIX, Entry 4), the preference for 1,4-addition (98%) over 1,6-addition (2%) is notable because the addition of a lithium enolate (a conventional...

H 2]-Cycloaddition occurs between 3-methoxycarbonyl-2-pyrone 309 and a trimethylenemethane Pd[(/-PrO)3P]4 complex to give the cycloadduct 310 in 71% yield (88JA1602). (See Scheme 59.) Reactions of 309 and 3-acetylcoumarin 236a with cyclopropenone acetal derivative 312 afford regioselectivity the cyclopentenone acetal compound 313 and 314, respectively (92JA5523). 

Arylations. Copper(II) acetate catalyzes the reaction of arylboronic acids with thiols and of amines with hypervalent arylsiloxanes. 4-Substituted imidazoles react with aryllead(IV) reagents (in dichloromethane at room temperature) to afford N-1 arylatcd derivatives regioselectively under catalysis of Cu(OAc)2. ... 

The unstable cycloadducts (24) and (25) are obtained from the photolysis of the enone ethers (26).A similar approach has been reported by Barker and Pattenden in their study of the photocyclization of enol acetates. Thus the intramolecular photocycloaddition affords the adducts (27) from the mixture of enol acetates derived from (28). An analogous regioselective cycloaddition is encountered in the irradiation of the enol acetates derived from (29) to afford the adduct (30). 

Various versions of the acyl anion equivalent (32) are described by Stork etal. (use of Et2NCH2CN), Krief and co-workers (1,3-dithianes in regioselective Michael addition), Gokel et al. (use of sulphur heterocycles with thermal deblocking), Cohen and Weisenfeld" (conversion of acids into vinyl sulphides), Meyers and Campbell " (acetals derived from aryl aldehydes), and Hunig and co-workers (cyanohydrin derivatives of aryl aldehydes). 

The reaction of both open-chain and cyclic 2,3-epoxy alcohols with molecular Bromine or Iodine in the presence of Ti(0-i-Pr)4 at 0 °C leads to the regioselective formation of halo diols (eq 9). Interestingly, if these reactions are conducted at 25 °C a 1 1 mixture of the C-2 and C-3 cleavage products is obtained, and the same outcome is observed, even at 0 °C, when the acetate derivative of the 2,3-epoxy alcohol is involved as substrate. Dialkylamine hydrochlorides can be used as sources of halide nucleophiles in these types of epoxide ring-cleavage reactions. ... 

Bu2SnO, benzene BnBr, DMF, heat, 80% yield. This method has also been used to protect selectively the anomeric hydroxyl in a carbohydrate derivative. The replacement of Bu2SnO with Bu2Sn(OMe)2 improves the process procedurally. The use of stannylene acetals for the regioselective manipulation of hydroxyl groups has been reviewed. ... 

Since the Fries rearrangement is a equilibrium reaction, the reverse reaction may be used preparatively under appropriate experimental conditions. An instructive example, which shows how the regioselectivity depends on the reaction temperature, is the rearrangement of m-cresyl acetate 8. At high temperatures the ortho-product 9 is formed, while below 100°C the para-derivative 10 is formed ... 

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