Hydroxy acetal product

Enol ethers are interesting substrates for epoxidations since a-hydroxy ketones or the corresponding acetals are isolated, depending on the choice of solvent. Kat-suki has used enol ethers as substrates, including the cyclic enol ether (4.67), which affords the hydroxy acetal product (4.68). ° Adam has used silyl enol ethers and silyl ketene acetals as substrates. A typical example is provided by the asymmetric oxidation of silyl enol ether (4.69), generating the oi-hydroxy ketone (4.70) after a suitable work up. ... 

Katsuki et al. have reported that high enantioselectivity can be obtained in the oxidation of nonconjugated cyclic enol ethers by using Mn(salen) (34) as the catalyst.138 The reactions were performed in an alcoholic solvent to obtain a-hydroxy acetals as the products, because a-hydroxy acetals are tolerant to a weak Lewis acid like Mn(salen) and do not racemize during the reaction and the isolation procedure (Scheme 29). 

Lieb, F., Oediger, H., and Streible, G., Phosphono-Hydroxy-Acetic Acid and Its Salts, Their Production and Their Medicinal Use, U.S. Patent 4,340,599, 1982. 

Tartaric acid has been used as a chiral auxiliary in a patented route to (S)-naproxen (20) (Scheme 23.3).45" 9 In initial studies, the acetal 21 was used to allow a stereoselective bromination that resulted in a 91 9 ratio of the (RRS)- and (RRR)-bromo derivatives 22 and 23. The bromo acetal diesters could be completely separated. Debromination of 22, followed by acid hydrolysis, led to formation of (S)-naproxen (20) in 80% yield, >99% ee, and recovery of the auxiliary. Conversely, debromination and hydrolysis of 23 gave only 12% yield of (R)-naproxen and 86% ee. In this case, the hydroxy acetal 24 was the major product (68%). However, the auxiliary was recovered in enantiomerically pure form. 

When treated with sodium hydroxide dissolved in THE the product of Step 1 was converted into 2-propyl-cr-hydroxy-acetic acid as is illustrated in Eq. 2 ... 

In the aliphatic series there has been little work performed with fluoroalkylamine reagents (FAR). They do not seem to work well, since, for example, methyl 2-hydroxy-2-melhyl-propanoate and the Yarovenko reagent, 2-chloro-/V,/V-diethyl-l,1.2-trinuoroethylamine (1), give the desired methyl 2-fluoro-2-mclhylpropanoate in only 10% yield elimination is the main reaction. Good yields of fluorinated products are obtained from 2-aryl-2-hydroxy-acetates, where elimination cannot occur (sec Table 9). When alkyl substituents arc present in the a-position, elimination products arc again found. 

Conversely, the same catalyst (2) can be used for the protection of hydroxy benzal-dehydes, substrates that usually need protection of the phenol function prior to acetal formation. Azeotropic distillation in benzene give good yields of the acetal product with both 1,2-ethanediol and 1,3-propanediol (Scheme 10.3) [4]. 

The role of the trialkylsilyl group is unclear. Changing the trimethylsilyl group not only increases the selectivity but also affects the product of the reaction—/3-hydroxy acetals are obtained instead of (5)-hydroxy esters. They investigated the course of the reaction with different terr-butyldimethylsilyl ketene acetals and aldehydes with catalyst 3f(Eq. 48). 

The next higher hydroxy acid is hydroxy acetic acid CH2(OH)— COOH, known also as glycolic acid. It may be prepared (a) from chlor acetic acid, (b) from the cyan-hydrine obtained from formic aldehyde, or (c) hy the oxidation of ethylene glycol, by reactions which have been already discussed. Its relation to ethylene glycol gives it the name of glycolic acid. It may be considered as a direct oxidation product of ethane. 

The formation of high boiling products, not previously considered in mechanistic studies of this reaction, presents several problems. Simple addition of the elements of water or acetic acid to the vinylation product to form the hydroxy acetates and the diacetates is unsatisfactory for several reasons. First, there is no analogy in the literature for this type of reaction second, this type of addition cannot explain the increased amounts of high boiling products found as reaction time is shortened, and finally, simple addition of the elements of acetic acid to the vinyl ester would not explain the results obtained in some of the deuterium studies (36). 

Drimane-8a,l 1-diol (41) and its 11-monoacetate (42) are suitable starting compounds for the synthesis of a series of drimanes and not only of them. Only the diol (41) was found in natural sources and was isolated from tobacco [51] and from a special gland of African elephant [52], Data about the synthesis of these compounds from the ambreinolide [39] and of the hydroxy acetate (42) from the sclareol (3) have been already reported [42]. Barrero et al. [42] showed also that if the reduction of the ozonolysis product of the mixture of esters (54) is done with LiAlFE instead of NaBH4, the diol (41) is obtained in a 95% yield (Scheme 11). 

Ohloff and Giersch [53] accomplished the synthesis of the drimanediol (41) from the norambreinolide (69). The latter was reduced into the semiacetal (74), whose acetate (75) on pyrolysis gave the dihydrofuran (76). Its ozonolysis and subsequent reduction of the ozonolysis products with NaBFU afforded the diol (41). Unfortunately, the yields of the products in [53] are not given. As it was described above, the hydroxy acetate (42) was transformed into the drimenol (2) [39]. It should be mentioned that the reverse conversion of the drimenol (2) into the diol (41) [54] also takes place. For this purpose, the drimenyl acetate (12) was... 

They obtained the enol acetate (170) in 92% yield, probably, as a mixture of cis- and trans- isomers. Compound (170) was ozonised, and the resulting product was reduced with NaBfLi to the hydroxy acetate (171) which, on the Swem oxidation, afforded the acetoxy aldehyde (172). The... 

Disperse Accosoft 806 into water. Slowly add the hydroxy-acetic acid while mixing until product is homogeneous. Properties ... 

Both at 70 and 80 °C, selectivity starts to decrease above 60% conversion, but more rapidly at 80 °C (product yield at 95% conversion <60% at 80 C >75% yield at 70 °C). Selectivity loss at higher conversions can be explained by competing addition of coproduced water instead of methanol to the 3-methoxyacrylate intermediate. In contrast to the stable 3,3-dimethoxy methyl propionate acetal product formed by methanol addition, the hemiacetal 3-methoxy-3-hydroxy methyl propionate generated by addition of water is prone to overoxidation, especially at higher reaction temperatures. Consequently, 3,3-dimethoxy methyl propionate selectivity benefits from increasing the methanol/methyl acrylate ratio. The results of variation of methanol/methyl acrylate ratio on activity and selectivity are depicted in Chart 11.2c,d, respectively (Pd/Cu/Fe/methyl acrylate 1/500/500/25000 oxygen pressure 0.2 MPa). Whereas conversion rates are equal at different methanol/methyl acrylate ratios, 3,3-dimethoxy methyl propionate selectivity erodes to 50% above 90% conversion at a low methanol/methyl acrylate... 

From a comparison study of asymmetric reduction of a selected a-keto acetal, 2,2-di-ethoxy-l-phenylethanone (28), using various OAB catalysts, the use of 2a or 2b and 8 or 9 as catalyst and borane reagent, respectively, provided the best result to give product a-hydroxy acetal 29 in 96-99% yield and >90% ee (Scheme 11.8) [57], Such reduction was highly effective for aromatic ketones but less so for aliphatic ones. The 2d-catalyzed reductions were highly enantioselective for both aliphatic and aromatic a-keto thioketals 30 [58]. 

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