Oxymercuration alcohols from

Alcohols from Alkenes through Oxymercuration-Demercuration... 

OXYMERCURATION-REDUCTION ALCOHOLS FROM OLEFINS 1-METHYLCYCLOHEXANOL, 53, 94... 

Problem 13.43 Write the structural formulas for the alcohols formed by oxymercuration-demercuration from... 

Problem 13.37 Write the structural formulas for the alcohols formed by oxymercuration-demercuration from (a) l-heptene, (b) 1-methylcyclohexene, (c) 3,3-dimethyl-1-butene. M... 

ALCOHOLS FROM ALKENES THROUGH OXYMERCURATION-DEMERCURATION... 

We have already studied the acid-catalyzed hydration of alkenes, oxymercuration-demer-curation, and hydroboralion-oxidation as methods for the synthesis of alcohols from alkenes (see Sections 8.5,8.6, and 8.7, respectively). Below, we briefly summarize these methods. 

Because borane additions to double bonds and subsequent oxidation are so selective, this sequence allows the stereospeeific and regioselective synthesis of alcohols from alkenes. The anti-Markovnikov legioselectivity of the hydroboration-oxidation sequence complements that of acid-catalyzed hydration and oxymercuration-demercuration. In addition, hydroboration, like oxymercuration, occurs without the participation of carbocations therefore, rearrangements are not observed. 

Three of the six reactions are well suited for the synthesis of alcohols acid-catalyzed hydration, oxymercuration (after reduction of the C-Hg bond by NaBHi), and hydroboration (by oxidation of the C-B bond by H2O2). Compare the alcohol from hydration (shown earlier) with that from demer-curation of the oxymercuration product ... 

The approach to polyketide synthesis described in Scheme 5.2 requires the relatively nontrivial synthesis of acid-sensitive enol acetals 1. An alternative can be envisioned wherein hemiacetals derived from homoallylic alcohols and aldehydes undergo dia-stereoselective oxymercuration. Transmetallation to rhodium could then intercept the hydroformylation pathway and lead to formylation to produce aldehydes 2. This proposal has been reduced to practice as shown in Scheme 5.6. For example, Yb(OTf)3-cata-lyzed oxymercuration of the illustrated homoallyhc alcohol provided organomercurial 14 [6]. Rhodium(l)-catalyzed hydroformylation of 14 proved successful, giving aldehyde 15, but was highly dependent on the use of exactly 0.5 equiv of DABCO as an additive [7]. Several other amines and diamines were examined with variation of the stoichiometry and none proved nearly as effective in promoting the reaction. This remarkable effect has been ascribed to the facilitation of transmetallation by formation of a 2 1 R-HgCl DABCO complex and the unique properties of DABCO when both amines are complexed/protonated. 

Both hydroboration/oxidation and oxymercuration yield the same alcohol product from the symmetrical alkene starting material. 

Conversion of synthetic picrotoxinin (1) to picrotin (2) was achieved by Corey and Pearce in four steps and 30% overall yield 124). To prevent intramolecular oxymercuration, the tertiary alcohol of picrotoxinin (1) was protected as trifluor-oacetate 346 prior to addition of mercury trifluoroacetate in a benzene/THF mixture as solvent. Demercuration with sodium borohydride failed, thus the covalent C-Hg bond was cleaved by tributylstannane in ethanol. Mild hydrolysis of the bistrifluor-oacetate 347 afforded picrotin (2) in 30% overall yield (75% corrected for recovered 1) from picrotoxinin (1). 

In the area of pheromone synthesis, oxymercuration-oxidative demercuration has also proven valuable. For example, all four stereoisomers (rf tetrahydro-2,2,6-trimethyi-2//-pyran-3-ol, from the eim bark beetle Pteleobius vittatus have been acquired by a sequence from (/ )- and (S)-sulcatol, which incorporates this mercury chemistry. The epimeric alcohols (47) and (48 Scheme 37) were separable (MPLC)... 

Alcohols can be conveniently made from compounds containing carbon-carbon double bonds in two ways by oxymercuration-demercuradon and by hydroboradon-oxidadon. Both amount to addition of water to the double bond, but with opposite orientation—Markovnikov and anti-Markovnikov—and hence the two methods neady complement each other. 

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