Methoxymercuration

Hg(OAc)2 easily reacts with allene to yield methoxymercuration products, i.e. vinylmercury compound 41, with the trans isomer being the major product. The chirality of allene was transferred into the final products, indicating the intermediacy of a cr-bridged mercurinium ion [23-26], The stereoselectivity of this reaction was determined by the relative stability of intermediates 40 and 42 and steric hindrance for the incoming methoxy group. 

The technique of methoxymercuration-demercuration was utilized to determine the position of double bonds in the side chains. Since this method is not successful with the free alkaloids (272), the secondary amino groups must be protected as the A -heptafluorobutyramide. These amides are treated with mercuric acetate and methanol followed by reduction with sodium borohydride to yield the methoxylated compounds (273). The mass spectra of these compounds show a fragment ion (274) at m/z 59 indicating terminal double bonds in every case (Scheme 22) 16,25,410,411). 

In contrast to another investigation,9 repetition of the methoxymercuration reaction of dimethyl tricyclo[4.2.2.02,5jdeca-3,9-diene-7,8-dicarboxylate gave a crystalline organomercurial, whose structure was reassigned as dimethyl 3-chloromercurio-4-methoxytrieyelo[4.2.2.02-5]dec-9-ene-7,8-dicarboxylate (4).1,5 Some representative electrophilic addition reactions of compounds la-d are summarized in Table l.1,3-7... 

Methoxythallation of allenes, such as 318, has been found to occur at the more substituted double bond, and to be far more regioselective than methoxymercuration, giving predominantly the product with (AcO)2Tl attached to the central carbon 319432. 

Another remarkable reaction is the methoxymercuration which leads, when applied to derivatives of lysergic acid, to the unique 10-methoxy-A8,9-ergolene derivatives (50) after treatment of the addition product with base and NaBH4 (51). In another recent paper (127) the synthesis of a new structural isomer of lysergic acid methyl ester (28) has been reported in which a novel modification of the Polonovski reaction has been used to introduce the 7,8 double bond. Thus, the A-oxide 51 on treatment with acetic anhydride and excess base, yielded the enamino ester (52) in about 50% yield. 

D. R. (1980). Application of methoxymercuration-demercuration followed by mass spectrometry as a convenient microanalytical technique for double-bond location in insect-derived alkenes. J. Chem. Ecol., 6, 257-269. 

Mori also succeeded in synthesizing the optically active ipsdienol 350 via carbonyl olefination of the acetonide 345 of (RX+)-glycerine aldehyde with isopropylidene-phosphorane to form 346. Methoxymercuration of346 followed by the splitting of the protective group, tosylation and treatment with alkali afford the epoxide 349 which is converted into the optically active (R)-350 via the same reaction sequence as in the synthesis of 344 according to Scheme 61 2101 (Scheme 62). 

An intermediate dimethyl acetal (172) was isolated83 3 in 63% yield by similar methoxymercuration of ether 151 followed by sodium borohydride reduction the product was then hydrolyzed to 2-deoxy-L-arabino-hexose. The same acetal (172) was formed83 3 by reaction... 

Deuterium labelling combined with deuterium decoupling is well known as a method in spectral analysis and in stereochemical and conformational studies. The deuterium-decoupled proton spectra of XCHD.CHDY compounds produced during mechanistic studies are useful for determining the stereochemical course of reactions such as the methoxymercuration of ethylene, (478) the conversion of tyramine into tyrosol, (479) the non-catalytic addition of deuterium molecules to cyclopentadiene, (480) and alkyl transfer and olefin elimination in 2-phenyl-l,2-dideuterioethyl transition metal compounds. (481) The proton spectra of [ H ]- and [ Hg]-t-butylcyclohexanes were... 

Even for symmetric alkenes such as CH2=CH2 and cis-MeCH=CHMe, nons5mi-metrically bridged species are proposed. The secondary deuterium isotope effect in the intramolecularly competing methoxymercuration of CH2==CD2 ... 

The solvent, HgX2, the nucleophile and even reagent concentrations influence the relative extents of cis trans addition °° ° ° . Both endo-cis and exo-cis (acetoxy-mercuration) products are found with homobarrelene and with bicyclo[2.2.2]octadiene with Hg(OAc)2 in THE, where also trans-(methoxymercuration) products are obtained in MeOH. 

AcOHgCH2CROMe in methoxymercurations using Hg(OAc)2 in MeOH however, with all other substituted a,)S-unsaturated carbonyls, the site of mercuration is the a-carbon. For a,j8-unsaturated aldehydes, acetals may be formed also ... 

In acetic acid, thallium(III) acetate reacts with alkylphenylacetylenes to give a mixture of the two possible regioisomers by trans acetoxythallation. In methanol, norbornene reacts with Hg(OAc)2 by cis-exo methoxymercuration, but under the same conditions, or in CHCI3, TlfOAcfs reacts by cis-exo acetoxythallation, and a concerted or near-concerted cyclic mechanism has been proposed . 

Although methanol undergoes addition to the C-C double bond of an (alk-l-enyl)cyclopropane in a proton-catalyzed reaction, " generally methanol addition is carried out in a two-step reaction involving methoxymercuration followed by sodium borohydride reduction. The pure product from the first step can be isolated, but this isolation is usually not performed ... 

---