Hydroboration-oxidation alcohols produced

Among these isomers, only two of them will imdergo hydroboration-oxidation to afford an alcohol with no chirality centers, shown below. The remaining fonr isomers can undergo hydroboration-oxidation to produce alcohols that do possess a chirality center. 

Hydroboration-oxidation of a pinene (page 235) like catalytic IS stereoselective Addition takes place at the less hindered face and a single alcohol is produced in high yield (89%) Sug... 

With ring G in place, the construction of key intermediate 105 requires only a few functional group manipulations. To this end, benzylation of the free secondary hydroxyl group in 136, followed sequentially by hydroboration/oxidation and benzylation reactions, affords compound 137 in 75% overall yield. Acid-induced solvolysis of the benzylidene acetal in 137 in methanol furnishes a diol (138) the hydroxy groups of which can be easily differentiated. Although the action of 2.5 equivalents of tert-butyldimethylsilyl chloride on compound 138 produces a bis(silyl ether), it was found that the primary TBS ether can be cleaved selectively on treatment with a catalytic amount of CSA in MeOH at 0 °C. Finally, oxidation of the resulting primary alcohol using the Swem procedure furnishes key intermediate 105 (81 % yield from 138). 

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

The dimer 111 was easily prepared in two steps from artemisinin. Hydroboration-oxidation produced alcohol 112, which was easily oxidized to give the carboxylic acid... 

Write the structure of the alcohol produced from the hydroboration-oxidation sequence when applied to a particular alkene. 

The starting material for the present synthesis was Wieland-Miescher ketone (24), which was converted to the known alcohol (25) by the published procedure [10], Tetrahydropyranylation of alcohol (25) followed by hydroboration-oxidation afforded the alcohol (26), which on oxidation produced ketone (27). Reduction of (27) with metal hydride gave the alcohol (28) (56%). This in cyclohexane solution on irradiation with lead tetraacetate and iodine produced the cyclic ether that was oxidized to obtain the keto-ether (29). Subjection of the keto-ether (29) to three sequential reactions (formylation, Michael addition with methyl vinyl ketone and intramolecular aldol condensation) provided tricyclic ether (30) whose NMR spectrum showed it to be a mixture of C-10 epimers. The completion of the synthesis of pisiferic acid (1) did not require the separation of epimers and thus the tricyclic ether (30) was used for the next step. The conversion of (30) to tricyclic phenol (31) was... 

The Diels-Alder reaction was utilized to construct bicyclo [2.2 1]heptane or bicyclo[2 2 l]heptene structures The reaction of isopropylidenecyclopentadiene with maleic anhydride produced the endo and exo configurational isomers of 8-isopropylidenebicyclo[2.2.1] hept-2-ene-5,6-dicarboxylic anhydride Similar reactions were applied to unsubstituted and l-(methoxycarbonyl)cyclopentadienes to give the corresponding anhydrides The anhydrides were reduced to alcohols, which were then allowed to react with thionyl chloride or tosyl chloride to give cyclic sulfites or tosylates Reaction of the tosylates with lithium chloride gave chlorinated compounds Hydration of the double bonds of the chlorinated compounds was accomplished by hydroboration-oxidation Diol 31 thus obtained was converted to 5,6-bis(chloromethyl)-7-isopropylidene-bicyclo[2 2 1] heptan-2-one [33] by chromium trioxide oxidation of the secondary hydroxyl group followed by dehydration at the C-7 substituent. 

The reduction of epoxides withborane is noteworthy as it gives rise to the less substituted alcohol as the major product (7.96), in contrast to reduction with complex hydrides (compare with Scheme 7.71). The reaction is catalysed by small amounts of sodium or lithium borohydride and high yields of the alcohol are obtained. With 1-alkylcycloalkene epoxides, the 2-alkylcycloalkanols produced are entirely cis, and this reaction thus complements the hydroboration-oxidation of cycloalkenes described in Section 5.1, which leads to trans products. Reaction with borane in the presence of boron trifluoride has also been used for the reduction of epoxides and for the conversion of lactones and some esters into ethers. 

The hydroboration-oxidation procedure is a valuable method to hydrate an alkene with anti-Markovnikov orientation and with syn addition of the H and OH groups. ° Addition of BH3 (which may be added to the reaction mixture as diborane, B2Hg) to an alkene occurs readily in diethyl ether, THF, or similar solvent. The hydroboration is strongly exothermic, with a AH of -33kcal/mol per B-H bond that reacts. If stoichiometry and the steric requirements of the alkyl substituents on the boron atom permit, the reaction proceeds until three alkyl groups are attached to each boron atom. The trialkylborane can then be oxidized with hydrogen peroxide in aqueous base to produce the alcohol. 

Hydroboration-Oxidation An alkene reacts with BHsTHF or diborane to produce an alkylborane. Oxidation and hydrolysis of the alkylborane with hydrogen peroxide and base yield an alcohol (Section 8.6). 

An important advantage of hydroboration-oxidation for hydration of alkenes is that rearrangements of the carbon skeleton do not occur, whereas they do in acid-catalyzed hydration. For example, acid-catalyzed hydration of (+)-a-pinene (65) proceeds with skeletal rearrangements to produce a number of isomeric alcohols. 

Further, it should be clear that if a chiral alkene such as the terpene a-pinene (Scheme 6.27) is used, and if steric effects are important, then the specificity of addition would result in a chiral borane reagent. Use of the chiral borane reagent, in, for example, oxidative hydroboration, would then produce a chiral alcohol. 

Compound A has molecular formula CsHio- Hydroboration-oxidation of compound A produces an alcohol with no chirality centers. Draw two possible structures for compound A. 

Addition reactions that will produce alcohols include acid-catalyzed hydration, oxymercuration-demercuration, and hydroboration-oxidation. 

Our previous method for alcohol synthesis, hydration of aikenes, necessarily produced the more substituted alcohol. So now we have complementary synthetic methods for producing hoth possible alcohols from a given alkene (Fig. 9.69). Direct hydration gives the more substituted product (Markovnikov addition) and the indirect hydroboration/oxidation method gives the less substituted alcohol (anti-Markovnikov addition). Be sure to add these reactions to your file-card collection of synthetically useful reactions. 

Addition of 9-BBN to alkynes is sufficiently slower than to similar alkenes to allow selective hydroboration-oxidation of skipped enynes (45) to produce 5,e-acetylenic alcohols (Scheme 22) without protection of the triple bond. ... 

Although this three-step synthesis is the most efficient answer, a related approach derives from our earlier consideration of ketone synthesis with the use of an alcohol. It, too, proceeds through an alkyne. Construction of bond a of the target molecule, shown earlier, requires addition of an organo-metallic reagent to a six-carbon aldehyde, which, in turn, may be produced through hydroboration-oxidation (Section 13-8) of the terminal alkyne shown in the preceding scheme. 

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