Diesters, symmetrical unsymmetrical

Although electroreduction of a mixture of esters with an Mg electrode gives a mixture of symmetrical and unsymmet-rical 1,2-diketones (Scheme 19), the reduction of diesters affords unsymmetrical 1,2-diketones as the single product (Scheme 20) [38, 39]. The selectivity of the reaction shown in Scheme 20 is highly dependent on the value of n and the best result is obtained when n = 2. 

Carbonates are indexed in Chemicaly hstracts under carbonic acid, esters. Symmetrical diesters have the prefix di or bis. Unsymmetrical diesters are listed with the two radicals following each other. For example, ethyl phenyl carbonic diester is C2H OCOOC H. Table 6 Hsts commonly used carbonates, their Chemicaly hstracts Service Registry Number, and formulas. 

A further approach for the synthesis of nonsymmetrically protected lanthionines is the conversion of thiosulfinates of symmetrically protected cystine derivatives into nonsymmetrically protected cystines via a reaction with a cysteine derivative and subsequently the conversion of the resulting unsymmetrically protected cystine into the nonsymmetrically protected lanthionines with a tris(dialkylamino)phosphineJ26l The oxidation of the symmetrically protected cystine, e.g. A,AT-bis(benzyloxycarbonyl)-L-cystine diethyl ester, of one stereochemical configuration to the thiosulfinate with m-chloroperoxybenzoic acid is essentially quantitative. The nonsymmetrical cystine is then formed in a subsequent step by the addition of the /V-/er/-butoxycarbonyl-L-cysteine tert-butyl ester derivative to give N-Z-N -Boc-L-cystine ethyl ferf-butyl diester. The desired 2f ,6f -lanthionine is then formed in the presence of P(NEt2)3 in yields of >50%. 

In addition to cyclic allylic substrates, malonate nucleophiles have been used in reactions with both symmetrical" and unsymmetrical" acyclic systems, and with geminal dicarboxylates. Malonates and Meldrum s acid have also been used as nucleophiles in the desymmetrization of meso diesters. 

Monoderivatives of symmetrical diols.2 One possible route to these derivatives is treatment of the diol with di-n-butyltin oxide to form a stannoxane, which is then allowed to react with 1 equivalent of an acid chloride or sulfonyl chloride (equation I). Unsymmetrical diesters can be obtained by consecutive reaction of the stannoxane with two different reagents. 

Full details of the synthesis of unsymmetrical diesters of pyrophosphoric acid by the reaction of the disilver salt of a monoester of phosphorothioic acid and a phosphate monoester have been published. In view of the good yields, the mild conditions, and the absence of symmetrical pyrophosphate byproducts it appears to be superior to previous routes to the nucleotide pyrophosphate coenzymes. Further, the findings that the procedure is insensitive to... 

Perhaps the most important new development in the ac-biladiene route has been the synthesis of discrete unsymmetrical tripyrrene intermediates. Russian workers had shown that a,a -disubstituted pyrromethanes could be condensed successively with one and two formyl pyrrole units to form first a tripyrrene and then a symmetrical ac-biladiene, which could be cyclized to porphyrin. The Liverpool group, however, showed that by use of unsymmetrical differentially protected pyrromethane diesters, condensation could be effected with two different formyl pyrroles to yield first tripyrrenes and then unsymmetrical ac-bi-ladienes. Cyclization of the latter afforded unsymmetrically substituted porphyrins. This procedure represents the first truly stepwise rational porphyrin synthesis, as exemplified by the synthesis of isocoproporphyin (26) (cf. Scheme... 

Lewis acid-mediated addition of silyl ketene acetals to a chiral sulfimine gives precursors of /1-amino acids in fair to excellent deP Mixed diesters of both symmetrical and unsymmetrical diols have been prepared by reaction of carboxylic acids with cyclic ketene acetals of the diols, with the less substituted carbon of the cyclic dioxonium ion intermediate being attacked in most cases. Hydration of trifluoroacetylketene is discussed later under Enolization. 

If the first and second steps in Scheme 1 had been adequately separable, oligomer synthesis might have been off to a rapid start. True, the reactivity of the intermediate phosphorochloridate (2) is reduced by the increased electron accession from oxygen to the phosphoryl center, but not adequately thus symmetrical diesters (4) are formed concomitantly with the required unsymmetrical diester (3). The alternative, to isolate the diester intermediate (or generate it in an alternative way)... 

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