Acetate formation

If you re not sure of the mechanism of acetal formation or just want to know a bit more about acetals, read frames 1-21 and 62-64 of the Carbonyl Programme. 

Since (A) does not contain any other functional group in addition to the formyl group, one may predict that suitable reaction conditions could be found for all conversions into (A). Many other alternative target molecules can, of course, be formulated. The reduction of (H), for example, may require introduction of a protecting group, e.g. acetal formation. The industrial synthesis of (A) is based upon the oxidation of (E) since 3-methylbutanol (isoamyl alcohol) is a cheap distillation product from alcoholic fermentation ( fusel oils ). The second step of our simple antithetic analysis — systematic disconnection — will now be exemplified with all target molecules of the scheme above. For the sake of brevity we shall omit the syn-thons and indicate only the reagents and reaction conditions. 

The syntheses of brevicomin (67)[109,110] and frontalin[l 11] have been achieved as an elegant application of the intramolecular acetal formation with the diol 66 in dry DME. Optically active frontalin (68) has been synthesized by this cyclization in triglymejl 12]. 

FIGURE 17 9 The mechanism of acetal formation from benzaldehyde and ethanol... 

The position of equilibrium is favorable for acetal formation from most aldehydes especially when excess alcohol is present as the reaction solvent For most ketones the position of equilibrium is unfavorable and other methods must be used for the prepara tion of acetals from ketones... 

This reaction is simply the reverse of the reaction by which acetals are formed—acetal formation is favored by excess alcohol acetal hydrolysis by excess water Acetal for matron and acetal hydrolysis share the same mechanistic pathway but travel along that pathway m opposite directions In the following section you 11 see a clever way m which acetal formation and hydrolysis have been applied to synthetic organic chemistry... 

The subject has been reviewed (37,38). Water may be added to the feed to suppress methyl acetate formation, but is probably not when operating on an industrial scale. Water increase methanol conversion, but it is involved in the unavoidable loss of carbon monoxide. A typical methanol carbonylation flow sheet is given in Figure 2. 

Vanillin, being an aldehyde, is able to form acetals and hemiacetals. Therefore, in flavor formulations using high concentrations of vanillin in conjunction with carriers such as propylene glycol, a glc analysis often shows a reduced vanillin peak after storage of the compounded flavor, and the presence of new peaks indicating acetal formation. Addition of about 0.5% of water to the formula reverses the reaction, ie, there is a reduction of acetal, and the reappearance of vanillin peaks. 

The rates of both formation and hydrolysis of dimethyl acetals of -substituted benzaldehydes are substituent-dependent. Do you expect to increase or decrease with increasing electron-attracting capacity of the pam substituent Do you expect the Ahydroi to increase or decrease with the electron-attracting power of the substituent How do you expect K, the equilibrium constant for acetal formation, to vary with the nature of the substituent ... 

Melamine can also be added to a UF-resin in the form of melamine salts like acetates, formates or oxalates [31-34]. These decompose in the aqueous resin mix at higher temperatures and enable considerable savings of melamine for the same degree of water resistance compared to traditionally prepared MUF-resins. These salts can act additionally as hardeners. 

One of the most widely used methods for the introduction of a 17a-hydroxy group into a 20-ketopregnane is via A ° -enol acetate formation. The overall reaction is ... 

In a situation where severe steric hindrance e.g., 16,16-dimethyl-20-keto-pregnanes) prevents enol acetate formation, an alternate scheme has been devised. Condensation of ethyl oxalate at C-21 produces, after hydrolysis, the 21-glyoxylic acid this on treatment with acetic anhydride and a strong acid catalyst such as perchloric acid gives both lactone acetates. 

With 11/3-hydroxy steroids, 11/3, 19-ether formation competes with hemi-acetal formation.This is a consequence of steric hindrance of the 11/5-oxygen by the C-18 and C-19 iodomethyl groups which reduces the rate of hypoiodite formation [(3) (5)] even though the conformation of the... 

Structurally, 0-glycosides are mixed acetals that involve the anomeric position of furanose and pyranose forms of carbohydrates. Recall the sequence of intermediates in acetal formation (Section 17.8) ... 

Hydrogenation of enones in MeOH with Pd/C resulted in acetal formation. When ethylene glycol/THF is used as solvent, the related dioxolane is formed in 86% yield. 

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