Acetals alkoxyacetals

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). 

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... 

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. 

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. °... 

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. °... 

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). 

Stannous triflatejtriphenylcarbonium chloride p-Alkoxyacetals from enolethers and acetals s. 44, 625... 

Synthesis of a,y -ethylenealdehydes via y -alkoxyacetals Acetal-enolether condensation... 

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