Dimethoxy acetal

A second, less well-known example is the anodic conversion of toluenes in methanol as solvent to benzaldehyde-dimethoxy acetals (180, 7S7) ... 

The Mukaiyama reaction with the dimethoxy acetal of azidoacetaldehyde gives 2,3-disubstituted pyrroles (Scheme 50) (90AG(E)777). 

A synthesis in solution of a protected K2K dendron and its functionalization by a dimeth-oxyacetyl derivative as a masked aldehyde185 is illustrated in Scheme 6. Synthesis of the tetravalent K2K dendron 29 was accomplished in three steps. The acetal derivative 30 was obtained from saponification of the commercially available methyl dimethoxyacetate ester and directly coupled to the tetravalent lysinyl dendron. Cleavage of the dimethoxy acetal requires an acid stronger than TFA and is removed successfully with cold concentrated HC1 because of the stabilization effect of the adjacent amide bond. Pallin and Tam 88 have found that a higher homologue with an extra methylene unit between the dimethoxy and acetyl moiety is susceptible to TFA conversion into the aldehyde unit for the same purpose. 

The comparable process for alkyl enol ethers involves participation of solvent,residual peiacid or water ° in cleaving the initially formed epoxide.Thus vinyl ether (74) produces the a-hydroxy derivative directly, while (75) provides the dimethoxy acetal. ... 

In the case of alkyl enol ethers the normal oie process competes with solvent incorporated and 1,2-di-oxetane products. Here however the ene process seems to be less inevitaUe when allylic protons are available and the product distribution may be effectively contndled by manipulation of solvent and temperature combinations. Best results are nonetheless achieved where the competitive processes are restricted. Thus enol ether (79) produces hydroxylated dimethoxy acetal (W) via direct incorporation of methanol or through reduction of the 1,2-dioxetane (81). 

Of the possible acetals, benzylidene and isopropylidene have been utilized most frequently in carbohydrate chemistry for simultaneous protection of two hydroxyls. They are introduced by standard acetalization conditions that is, either the aldehyde or the dimethoxy acetal is employed as reagent together with some acid catalyst (Scheme 3.4) [9]. With isopropylidene acetals, the methoxy propenyl ether has been utilized to provide the kinetic product, and formation of benzylidene acetals has been achieved under basic conditions using a,a-dibromotoluene in refluxing pyridine [10]. 

Isopropylidene, Benzylidene, and Related Acetals Isopropylidene or benzylidene acetals are formed either by direct condensation of the diol with the appropriate carbonyl compound (acetone or benzaldehyde, respectively) or by transacetalation with the corresponding dimethoxy acetal. Both processes are carried out in acidic conditions [ 123]. 

To demonstrate the versatility of this process and to provide a relevant model study for the synthesis of 1, we investigated an extension to medium-sized heterocycles. Thus, the diastereomeric silyl ethers 8a-b were selected to test this application by generation of the corresponding 9-membered oxacyclic dienes (Scheme 3). The preparation of 8a-b began with the reduction of pyruvic aldehyde dimethoxy acetal with NaBH4 in MeOH/THF to afford hydroxy acetal 2 (84%). Alkylation of the sodium salt of 2 with propargylic bromide afforded 3 (85%). Conversion of 3 to 5 was achieved by alkyne iodination followed by a c/s-reduction of... 

Stereoselective reactions with acetals. Noyori et al. (10,438) have used this Lewis acid to promote an aldol-type reaction between enol silyl ethers and acetals and have noted high. syn-selectivity in this process. Molander and Haar report that reaction of acetals with cyanotrimethylsilane promoted by TMSOTf results in a-alkoxy cyanides and that this reaction can be diastereoselective when the acetal is substituted at the 4-position by an alkoxy group. The diastereoselectivity depends on the nature of the acetal and the 4-alkoxy group. Dimethoxy acetals show slight diastereoselectivity, but diisopropoxy and dibenzyl acetals can show diastereoselectivity of 5-10 1. The diastereoselectivity also depends on the type of 4-substituent. Acetoxy and t-butyldimethylsilyloxy groups have no effect on the diastereoselectivity, but methoxy, benzyloxy, and allyloxy groups promote anri-selectivity. Since a metal template is not involved, the diastereoselectivity... 

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