Demercuration stereochemistry

On the basis of the mechanistic pattern for oxymercuration-demercuration, predict the structure and stereochemistry of the alcohol(s) to be expected by application of the reaction to each of the following substituted cyclohexenes. 

The threo stereoisomer was the major product obtained by the synthesis in Scheme 13.14. This stereochemistry was established by the conjugate addition in Step A, where a significant (4-6 1) diastereoselectivity was observed. The C(4)-C(7) stereochemical relationship was retained through the remainder of the synthesis. The other special features of this synthesis are in Steps B and C. The mercuric acetate-mediated cyclopropane ring opening was facilitated by the alkoxy substituent.19 The reduction by NaBH4 accomplished both demercuration and reduction of the aldehyde group. 

Fig. 7). The ring closure also proved to be remarkably stereoselective. Thus intramolecular oxymercuration, followed by reductive demercuration, provided the tetrahydropyran (22, ring A of X-206) with the desired 2,6-cis-relative stereochemistry in excellent overall yield as a single diastereomer. 

No tetracyclic products were isolated. The importance of the vinyl group stereochemistry in cyclisation is shown " by the oxymercuration-demercuration of methyl pimarate and methyl sandaracopimarate. The former affords an... 

The challenge to the synthetic chemist that was posed by these structures was the control of C6 stereochemistry. An early solution to this problem involved the intramolecular oxymercuration-demercuration approach, as illustrated in Scheme 9 (46,58). Thus, the stereoselective cyclization of 38 to the tricycUc cannabinoid skeleton by means of the intramolecular oxymercuration-demercuration reaction (58,61) gave the 6P-isomer predominantly (6P 6a = 85 15). The enantiomers were separated by chiral high-performance liquid chromatography (HPLC) using Chiralpak OD columns. The probable... 

Different stereochemistry of Grignard and oxymercuration-demercuration alcohol syntheses. 

Oxymercutation-demercutation occurs with Markovnikov regiochemistry and results in hydration of alkenes without complication from carbocation rearrangement. It is often the preferred choice over acid-catalyzed hydration for Markovnikov addition. The overall stereochemistry of addition in acid-catalyzed hydration and oxymercuration-demercuration is not controlled—they both result in a mixture of cis and trans addition products. 

The well known valerolactone (115), commonly described as the Prelog-Djerassi lactone, has attracted the attention of many synthetic chemists over the years and another synthesis of this important macrolide antibiotic intermediate has appeared (Scheme 13). The synthesis involves the Hg -induced cycliz-ation of the aldehyde (114) to control the stereochemistry at C(2) and C(3). This was followed by demercuration with sodium trithiocarbonate in methanol at -60 C, hydrolysis, and oxidation to give the lactone (115) and its epimer in a 3.5 1 ratio. Demercuration with sodium borohydride followed by hydrolysis and oxidation gave almost complete inversion. 

One of the features that makes the hydroboration reaction so useful is the regiochemistry that results when an unsymmetrical alkene is hydroborated. For example, hydroboration-oxidation of 1-methylcyclopentene yields trans-2-methylcyclopentanol. Boron and hydrogen add to the alkene from the same face of the double bond—that is, with syn stereochemistry, the opposite of anti— with boron attaching to the less highly substituted carbon. During the oxidation step, the boron is replaced by an OH with the same stereochemistry, resulting in an overall syn non-Markovnikov addition of water. This stereochemical result is particularly useful because it is complementary to the Markovnikov regiochemistry observed for oxymercuration-demercuration. 

When predicting the product of a reaction, you have to recall what you know about the kind of reaction being carried out and then apply that knowledge to the specific case you re dealing with. In the present instance, recall that the two methods of hydration— hydroboratlon-oxidation and oxymercuration-demercuration—give complementary products. Hydroboration-oxidation occurs with syn stereochemistry and gives the non-Markovnikov addition product oxymercuration-demercuration gives the Markovnikov product. 

Sfli Stereochemistry. Treatment of the 3-mercurated cyclic peroxides with sodium "Borohydride leads to mixtures of demercure ted cyclic peroxides and epoxy-alcohols. Thus, reacts to give the cyclic peroxides and the epoxy-alcohol in a 3 1 ratio. 

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