Mercury-mediated

Chiral /J-amino acyl silanes have been prepared through the addition of 2-lithio-2-trimethylsilyl-l,3-dithiane to enantiomerically pure A-tosylaziridines followed by mercury-mediated thioacetal hydrolysis113. 

Figure 4-40. Mercury-mediated reactions of thioacid derivatives.

Figure 4-41. A mercury-mediated conversion of a thioamide to nitrile.

Barrett has reported a systematic study on the cleavage of multiple cyclopropanes (see e g. equation 20). Interestingly, it was found that while the mercury-mediated ring opening of monocyclopropanes usually proceeds via a concerted mechanism, in the case of multiple cyclopropanes, it occurs through a stabilized free carbocation. Furthermore, the presence of an adjacent cyclopropane increases the rate of the ring opening of the first cyclopropane. 

The complete cis stereoselectivity which is generally observed can be attributed to the adsorption of the radical species by its unhindered face on the electrode surface. The stereochemistry of 1 was assigned on the basis of comparison with authentic c/.v-isomers, prepared by hydrogenation of 2,5-disubstituted pyrroles, and mixtures of cis- and trans-isomers prepared by mercury-mediated cyclization of the same 4-alkenylamines, and also by X-ray analysis of a derived quaternary pyrrolidinium iodide24. 

High 1,3-asymmetric induction was obtained in the mercury-mediated cyclization of a carbamate containing the chloral moiety. The structure of the product 8 was determined by single crystal X-ray analysis157. 

Moreover, mercury-mediated cyclization of 4-pentenyl-substituted amidals gave the more stable 1, 4-c -diastereomer, cf. 16 and 17. This result was explained in terms of thermodynamic control. High 1,3-asymmetric induction was observed independent of the substituent, since the 1,4-c7j-isomer was always the major component of the reaction mixture165. 

Methyl 3- /7/-D-daunosaminide 154 has been derived from d-149 via a Wittig-type olefination using (2-thiazolylmethylene)triphenylphosphorane (Scheme 13.53). A 1 1 mixture of (E)- and (Z)-alkenes is obtained, which is isomerized in the presence of iodine into a 9 1 mixture of ( )-152 and (Z)-152. Methylation of the thiazole moiety increases the electrophilicity of the alkene, which then accepts nucleophiles such as benzylamine. The adduct is treated with NaBH4 to give a thiazolidine. Acetylation and mercury-mediated hydrolysis of the thiazolidine ring generates 153, which, on acidic treatment in methanol, yields the A-benzyl 3- /7/-D-daunosaminide 154 [99]. 

NaBH4 to give a thiazolidine. Acetylation and mercury-mediated hydrolysis of the thiazolidine ring and subsequent acidic treatment in methanol yields the A-benzyl 3-e/7j-D-daunosaminide 162 [292],... 

During the total synthesis of (+)-fredericamycin A, the spiro 1,3-dione center was introduced by R.D. Bach et al. utilizing a mild mercury-mediated semipinacol rearrangement that involved a [1,2]-acy shift. The indanone dithioacetal was reacted with 1,2-b/s[(trimethylsilyl)oxy]cyclobut-1-ene in the presence of mercuric trifluoroacetate and the rearrangement took place in situ. 

Mercury-mediated reactions of alkenes have largely been superseded by other, more efficient, less toxic procedures. 

The key step in the preparation of the inhibitor is shown in Scheme 1.4.10 and involves the elaboration of the glucose derivative, shown, to the olefin on reaction with methylenetriphenylphosphorane. Once the olefin was isolated, the C-glycoside was obtained via mercury mediated cyclization followed by oxidation of the mercury with iodine. Further elaboration of the resulting iodide gave the desired C-phosphate disaccharide shown. 

The bromo epoxide 402 used in these syntheses is prepared from 371 by initial conversion to (iS)-l,4-dibromo-2-butanol (401) [16] followed by cyclization with potassium hydroxide [18]. Alkylation of 1,3-dithiane first with EE-protected co-chloroalcohols to give 403 and then with the ( S)-epoxide 402 affords 404. Opening the oxirane with either Super-Hydride or methyl cuprate creates the requisite carbon skeletons 405 with the appropriate fimctionality patterns. Removal of the EE protecting group and mercury-mediated hydrolysis of the thioacetal directly furnishes the spiroacetals 407 as a 3 2 mixture of diastereomers [16]. 

Treatment of 925 with the carbanion derived from 926 yields the dialkylated dithiane 927. Mercury-mediated hydrolysis of the dithiane ring followed by acid-catalyzed cyclization furnishes the natural product 928. 

Acyclic alditols.-Studies of the vitreous transition in maltitol glasses and the thermal behaviour of four conunercially available hexitols during crystallization and vitrefication have appeared. Reports on the water sorption and solubility of polyols, and on the kinetics of mercury-mediated electroreduction of glucose, galactose and lactose have also been published. 

Intermediates similar to (170) are generated by conjugate addition, enolate trapping, and mercury-mediated cyclization by cleavage of a cyclopropane... 

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