Mercuration electrophilic, cyclization

The nucleophiles that are used for synthetic purposes include water, alcohols, carboxylate ions, hydroperoxides, amines, and nitriles. After the addition step is complete, the mercury is usually reductively removed by sodium borohydride. The net result is the addition of hydrogen and the nucleophile to the alkene. The regioselectivity is excellent and is in the same sense as is observed for proton-initiated additions.16 Scheme 4.1 includes examples of these reactions. Electrophilic attack by mercuric ion can affect cyclization by intramolecular capture of a nucleophilic functional group, as illustrated by entries 9-11. Inclusion of triethylboron in the reduction has been found to improve yields (entry 9).17... 

Different electrophiles, such as I2, Br2, NBS, NIS, PhSeCl or metachloperoxy-benzoic acid have been used in the cyclization of glycoenitols [34], but the best stereochemical results have been obtained with mercuric salts and iodine. Iodine however can give rise to a debenzylation with formation of the more favored 5-member cyclization product. This is the case for 72 which, when treated with iodine, affords the C-fructosidic structure 83 [35] (Scheme 23). 

The second example27 belongs to the category of addition via onium-type intermediates . It exemplifies how the choice of the electrophile may affect the stereochemical course of the cyclization. When the hydroxy cnones rac-4a, b were treated with mercury(II) trifluoroacetate, both (E)- and (Z)-enone afforded basically the same yield and distribution of the diastereomer-ic cyclization products rac-5a, b. However, when phenylselenenyl chloride was used as electrophile, the (Z/Zj-isomers rac-4a, b produced quite contrasting ratios of the products rac-5 a, b. This result was interpreted in terms of EjZ equilibration in a single-bonded cationic adduct of the mercuric ions vs. nucleophilic attack on a rigid epi-selenonium ion27. 

Several cases have been reported in which the reactivity of the C-C douhle hond towards nucleophilic attack is further enhanced by an external electrophile. An example of an exo-trig cyclization induced by mercuration is shown in the case of the unsaturated acid rac-1 which was converted to the cyclic acetals (otR,2S)-2 and (y.S.2S)-2 employing mercury(ii) acetate in methanol followed by subsequent demercuration40. In the latter step, sodium trithiocarbonate afforded the highest selectivity in terms of retention of configuration (cf. Section 4.7.1.1.1.1.2.). 

The intramolecular cyclization of unsaturated carboxylic acids is one of the most straightforward routes to the synthesis of lactones, which leads directly to the desired products. In general, electrophilic reagents such as iodine, phenylselenium chloride, and mercuric or palladium salts promote this cyclization [26]. However, a second step is necessary to remove the reagent. Although the cyclization of unsaturated carboxylic acids promoted by a Brpnsted acid is a well-known reaction, it occurs mostly with more than stoichiometric quantities of a Brpnsted acid [27], On the other hand, it was reported that a variety of substituted y- and 5-lactones could be easily prepared from unsaturated carboxylic acids in excellent yields using trifluoromethane-sulfonic acid (TfOH) as a catalyst (Scheme 9.17) [28]. However, applications of this method, such as an asymmetric... 

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