Acetals a,p-unsaturated

TABLE 6.5. RATIOS OF 4- AND 5-REGIOISOMERS 19 AND 20 IN CYCLOADDITIONS TO a,P-UNSATURATED ACETALS AND THIOACETALS"... 

The reaction of a,P-unsaturated acetals predominantly affords regioisomer 19 with the acetal group attached to the C(4) position of the isoxazoline (127). With the analogous thioacetal, the regioselectivity was reversed (117) (Table 6.5). 

Regioselectivity and stereoselectivity of additions to a,P-unsaturated acetals and ketals... 

By using POQ3 in pyridine in the reductive elimination of an iodohydrin intermediate (Scheme 12), the acid sensitive a,p-unsaturated acetal (28) was prepared. ... 

The relative reactivities of acetals and orthoesters in Lewis acid catalyzed reactions with enol ethers have been investigated. For BF3-OEt2-catalyzed reactions with methyl vinyl ether, the following order has been found saturated acetals < methyl orthoformate < benzaldehyde acetals < a,p-unsaturated acetals. ... 

Reaction with a,p-unsaturated acetals. Under catalysis with cuprous bromide, Grignard reagents condense with a ,j3-unsaturated acetals to give enol ethers (Z-isomers predominating). The products are hydrolyzed to aldehydes by acid. The reaction can be carried out with a- or j3-substituted a, unsaturated acetals. 

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. 

The acetals of a,p-unsaturated aliphatic aldehydes show higher reactivity [6] than the aromatic and saturated species and react, at low temperatures, uniformly and with a high yield, to give the 1 1 product [7], because of the lower reactivity of the resulting saturated carbocation 7 towards the enol ether 7. It is possible, in this manner, to extend the carbon chain on the a,p-unsaturated acetal end specifically in the presence of a saturated acetal (Table 1 example 8 and Table 2 example 4). The outcome of the reaction with aromatic and saturated aliphatic acetals can be controlled by using an excess (up to 5 1) of the starting acetal [6]. The quantitative relative reactivities of acetals have been investigated in detail [8]. 

The reaction made in solution with acetonitrile N-oxide (from nitroethane and phenyl isocyanate) and trichloroacetonitrile as the dipolarophile gave after 24 h sonication (no experimental details given), a 53% yield of adduct, unfortunately without comparison to a conventional method. Nitrile oxides formed by dehydrochlorination of chlorooximes (Fig. 5) added to a,p-unsaturated acetals under pulsed irradiation. ... 

Alkenylboronic acids can be synthesized from reactive alkenylmetal species in a way similar to that described above for arylboronic acids (Section 1.3.1.1.1) [224]. Typically, alkenyl bromides or iodides are treated sequentially with n-Buli and a trialkylbo-rate (entry 1, Table 1.4). A nonpolar trienylboronic acid was synthesized using this approach [226]. As described in Section 1.2.2.2, small boronic adds tend to be highly soluble in water and may be difficult to isolate when made using the traditional approach involving an aqueous workup. In these cases, exemplified with the polymerization-prone ethyleneboronic acid synthesized from vinyknagnesium bromide, it has proved more convenient to isolate the product as a dibutyl ester by extraction of the addic aqueous phase with butanol [227]. Recently, aUcoxy-functionaHzed butadi-enyl- and styrenyl boronic esters were synthesized from a,P-unsaturated acetals by treatment with Schlosser s base and subsequent trapping with triisopropylborate (entry 2) [228]. 

The TiCU-promoted Michael reaetion proceeds under very mild conditions (—78 °C) this suppresses side-reactions and 1,5-dicarbonyl compounds are formed in good yields. For TiCU-sensitive compounds, a mixture of TiCU and Ti(0-/-Pr)4 is used. From silyl enol ethers and a,p-unsaturated ketones, 1,5-dicarbonyl compounds are formed (eq 32). The reaction also proceeds for a,p-unsaturated acetals. Silylketene acetals react with a,p-unsaturated ketones or their acetals to form 8-0x0 esters (eq 33). ... 

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