A alkoxyacetals

Alkylcarboxonium ions may be synthesized easily by the dehalogenation of a-haloethers. In Magic Acid, first the oxygen is protonated to oxonium ion 319, which, upon increasing temperature, decomposes to yield alkylcarboxonium ion 320 [Eq. (3.87)].624 In a similar manner, protonation of a-alkoxyacetic chlorides625 or acids626 also leads to carboxonium ions. The latter approach was used to generate cyclic alkylcarboxonium ion 321 [Eq. (3.88)]. 

Copper(I) and (II) Lewis acids affect the alkoxyselenation of olefins (Sch. 18). Thus, cyclohexene affords trans adducts 71 in the presence of PhSeCN and CUCI2 [44]. The Lewis acid enhances the electrophilic nature of the selenium by coordination to the nitrile. The reaction is regioselective as terminal olefins afford primary selenides (alcohol addition to the internal carbon) and vinyl acetates yield (8-seleno-a-alkoxyacetates. 

Lanthanide Lewis acids catalyze many of the reactions catalyzed by other Lewis acids, for example, the Mukaiyama-aldol reaction [14], Diels-Alder reactions [15], epoxide opening by TMSCN and thiols [14,10], and the cyanosilylation of aldehydes and ketones [17]. For most of these reactions, however, lanthanide Lewis acids have no advantages over other Lewis acids. The enantioselective hetero Diels-Alder reactions reported by Danishefsky et al. exploited one of the characteristic properties of lanthanides—mild Lewis acidity. This mildness enables the use of substrates unstable to common Lewis acids, for example Danishefsky s diene. It was recently reported by Shull and Koreeda that Eu(fod)3 catalyzed the allylic 1,3-transposition of methoxyace-tates (Table 7) [18]. This rearrangement did not proceed with acetates or benzoates, and seemed selective to a-alkoxyacetates. This suggested that the methoxy group could act as an additional coordination site for the Eu catalyst, and that this stabilized the complex of the Eu catalyst and the ester. The reaction proceeded even when the substrate contained an alkynyl group (entry 7), or when proximal alkenyl carbons of the allylic acetate were fully substituted (entries 10, 11 and 13). In these cases, the Pd(II) catalyzed allylic 1,3-transposition of allylic acetates was not efficient. 

Exquisite diastereochemical control is attained by tuning the relative bulk of the two alkoxy groups in ketene silyl acetals derived from a-alkoxyacetic esters, during aldol reaction with aldehydes. A chiral version is promoted SiCU in the presence of the phos-photriamide 5. The same set of reaction conditions is also applicable to create asymmetric quaternary carbon centers, for example, in the reaction of Al-silyl ketenimines with ArCHO. °... 

McGill, J. M. (1993). Generation of stable synthetic equivalents of unstable a-a alkoxyacetal-dehydes An improved preparation of dirithromycin. Synthesis 1089-1091. 

Benzeneselenyl chloridejsilver nitrate a-Alkoxyacetals from a,p-ethylenehalides... 

Diastereo- and Enantioselective Aldol Reactions. Optically active 1,2-diol units are widely distributed in natural products such as macrolides, polyethers, and carbohydrates, etc. The aldol reactions of the enolates derived from a-alkoxyacetic acid ester derivatives with aldehydes provide a useful way to construct 1,2-diols, and several as)unmetric reactions have been developed. 

With Lewis acids as catalysts, compounds containing more than one alkoxy group on a carbon atom add across vinyl ether double bonds. Acetals give 3-alkoxyacetals since the products are also acetals, they can react further with excess vinyl ether to give oligomers (228—230). Orthoformic esters give diacetals of malonaldehyde (231). With Lewis acids and mercuric salts as catalysts, vinyl ethers add in similar fashion to give acetals of 3-butenal (232,233). 

The Lewis acid-catalyzed 1,3-migration of divinyl esters allows the formation of 1,3-butadienes, which can undergo cycloaddition. In this respect, Dai and coworkers described a rearrangement of the divinyl alkoxyacetate 1-203 followed by a Diels-Alder reaction with a dienophile such as maleic anhydride 1-204 in the presence of catalytic amounts of Ln(fod)3 to produce 1-205 in up to 61 % yield (Scheme 1.47) [53],... 

In addition to cyclopropane 145 and the expected [2,3] rearrangement product 143 of an intermediary oxonium ylide, a formal [1,2] rearrangement product 144 and small amounts of ethyl alkoxyacetate 146 are obtained in certain cases. Comparable results were obtained when starting with dimethyl diazomalonate. Rh2(CF3COO)4 displayed an efficiency similar to Rh2(OAc)4, whereas reduced yields did not recommend the use of Rh6(CO)16 and several copper catalysts. Raising the reaction temperature had a deleterious effect on total product yield, as had... 

A 2-alkoxyethanol with a fluorine-containing alkoxy group can be selectively oxidized by nitric acid to the 2-alkoxyacetic acid without cleavage of the ether bond, e.g. oxidation of 9 to 10.148... 

Chloroacetic Acid (ClCHiCOOHf. [CAS 79-11-8 J. Chloroacelic acid can be synthesized by the radical chlorination of acetic acid, treatment of trichloroethylene with concentrated H S04. oxidation of 1.2-dichloro ethane or chloroaceialdehyde. amine displacement from glycine, or chlorination of ketene. It behaves as a very strong monobasic acid and is used as a strong acid catalyst for diverse reactions. The Cl function can be displaced in base-catalyzed reactions. For example, it condenses with alkoxides to yield alkoxyacetic acids CICH COOH... 

Furthermore, it is found that enol silyl ethers derived from phenyl alkoxyacetates react with aldehydes to afford the corresponding awti-1,2-diol derivatives with high diastereo- and enantioselectivities through use of a tin(II) Lewis acid in the presence of chiral diamine 1 (eqs 26 and 27). ... 

The photodecarboxylation of alkoxyacetic acids in the presence of HgO and iodine has been studied.Karauchi et al. have studied the photodecarboxylation of the Schilf base (117). Low-temperature matrix isolation techniques have been used to trap and study the pyranone (118) obtained by the decarboxylation of the dilactone (119). Photodecarboxylation of the carbamates (120) in acetonitrile provides a simple route to the azabicyclobutanes (121). ... 

It was previously noted that enolates of a-allyloxy ketones were capable of either 2,3- or 3,3-rear-rangement, depending upon counterion, reaction conditions and substituents. Ester enolates show a greater propensity for the 2,3-pathway, as illustrated by the geraniol-derived alkoxyacetate (198) which afforded hydroxy ester (200) as the sole product upon treatment with LDA in THF at -78 to 0 C. The 3,3-product (201) could be obtained by addition of TBDMS-Cl and HMPA to the enolate at -78 °C and thermal rearrangement of the TBS enol ether (199b Scheme 14). ... 

Allylsilanes attack cyclohexanone ketals axially (93 7) and attack 2-phenylpropionaldehyde acetal with a degree of Cram selectivity that is influenced by the choice of Lewis acid, tin(IV) chloride giving the highest ratio (3.5 1). ° With -alkoxyacetals, chelation control is not possible as it is with the corresponding aldehydes stereoselectivity is not high, it is in the opposite sense (Scheme 41, compare Scheme 16), and it depends upon the choice of Lewis acid. °... 

Since carotenoid synthesis began, the enol ether condensation has frequently been used for the formation of carbon-carbon double bonds and this reaction was also applied for large-scale industrial production of carotenoids. The reaction is an addition of an enol ether 12 to an acetal 13, promoted by a Lewis acid, especially BFa-etherate or ZnCl2, and involves a chain lengthening of two or more carbon atoms to yield an intermediary 3-alkoxyacetal 14 which subsequently is converted into an unsaturated aldehyde 15 (Scheme 3). 

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