Stereospecificity quantitation

Keywords phenylboronic acid, phenylboronic esters, mannitol, inositol, regio-specificity, stereospecificity, quantitative, solid-solid reaction, ball mill... 

Bordwell and coworkers reported the stereospecific quantitative desulfurization of episulfides with organolithiums.39) The mechanism of this reaction, however, remained undefined. In addition to a disrotatory concerted sulfurane decomposition [i.e. Eq. 9), p elimination from a 2-alkylthioalkyl-lithium may be envisioned (see Eq. 10). However, independent stereo-... 

Decarboxylation. Decarboxylation of linear and aromatic carboxyUc acids and of amino acids is common and of practical interest. L-Lysine [56-87-1] (48) can be synthesized by stereospecific decarboxylation of meso- (but not DL-) aa -diaminopimehc acid [2577-62-0] (49). The reaction is catalyzed by Bacillus sphaericus and proceeds in quantitative yields (92). 

This reaction, now termed hydroboration, has opened up the quantitative preparation of organoboranes and these, in turn, have proved to be of outstanding synthetic utility. It was for his development of this field that H. C. Brown (Purdue) was awarded the 1979 Nobel Prize in Chemistry . Hydroboration is regiospecific, the boron showing preferential attachment to the least substituted C atom (anti-Markovnikov). This finds ready interpretation in terms of electronic factors and relative bond polarities (p. 144) steric factors also work in the same direction. The addition is stereospecific cis (syn). Recent extensions of the methodology have encompassed the significant development of generalized chiral syntheses. 

It may be of interest to note that the stereospecific transformation shown in equation 15 has been cited as the first reported observation of an 1 - 3 chirality transfer. It is evident that on rearrangement of optically active 6d to 7d, the chiral center at C-a is eliminated and a new one created at C-y. The term self-immolative asymmetric synthesis has also been used to describe syntheses of this kind. As pointed out by Hoffmann , quantitative 1 - 3 chirality transfer will follow from the suprafacial - course of rearrangement, provided the reactant has a uniform configuration at the j8, y-double bond. This stereochemical prediction has also been confirmed by the results obtained in several other [2,3]sigmatropic rearrangements, subsequently reported " . 

Sulphoximines may also be obtained from sulphoxides by reaction with benzene- and toluenesulphonyl azides . Yields are often nearly quantitative and the reaction is usually carried out in the presence of copper. This reaction shows a high degree of stereospecificity for example, if (R)-methyl p-tolyl sulphoxide is the reactant then (S)-methyl-S-p-tolyl-iV-p-toluenesulphonylsulphoximine is the product (equation 66). It has been suggested that this reaction involves the intermediacy of a nitrene species . ... 

Kanegafuchi Chemical Industries produce D-p-hydroxyphenyl glycine, which is a key raw material for the semisynthetic penicillins ampicillin and amoxycillin. Here, an enantioselective hydantoinase is applied to convert the hydantoin to the D-p-hydroxyphenyl glycine. The quantitative conversion of the amide hydrolysis is achieved because of the in situ racemization of the unreacted hydantoins. Under the conditions of enzymatic hydrolysis, the starting material readily racemizes. Therefore, this process enables the stereospecific preparation of various amino acids at a conversion of 100% [38]. 

Reduction of either the exo or endo isomer of 2-phenyl-2-norbornanol with trifluoroacetic acid and triethylsilane, triphenylsilane, or phenylsilane in dichloro-methane gives endo-2-phenylnorbomane quantitatively (Eq. 24).164 The stereospecific formation of only the endo-hydrocarbon can be understood on the basis that only exo approach by organosilicon hydride toward the 2-phenylnorbornyl cation intermediate is kinetically competitive for product formation.164... 

Selenosulfonylation of olefins in the presence of boron trifluoride etherate produces chiefly or exclusively M products arising from a stereospecific anti addition, from which vinyl sulfones can be obtained by stereospecific oxidation-elimination with m-chloroper-benzoic acid134. When the reaction is carried out on conjugated dienes, with the exception of isoprene, M 1,2-addition products are generally formed selectively from which, through the above-reported oxidation-elimination procedure, 2-(phenylsulfonyl)-l,3-dienes may be prepared (equation 123)135. Interestingly, the selenosulfonylation of butadiene gives quantitatively the 1,4-adduct at room temperature, but selectively 1,2-adducts at 0°C. Furthermore, while the addition to cyclic 1,3-dienes, such as cyclohexadiene and cycloheptadiene, is completely anti stereospecific, the addition to 2,4-hexadienes is nonstereospecific and affords mixtures of erythro and threo isomers. For both (E,E)- and ( ,Z)-2,4-hexadienes, the threo isomer prevails if the reaction is carried out at room temperature. 

In ethyl 3-keto-2-oximino-3-phenylpropanoate catalytic hydrogenation over palladium on carbon reduced both the keto and oximino group, giving a 74% yield of ethyl ester of -phenylserine (ethyl 2-amino-3-hydroxy-3-phenylpropionate). The reduction is stereospecific and only the erythro dia-stereomer was obtained, probably via a cyclic intermediate 11097]. Similarly, hydrogenation over Raney nickel at 25-30° and 1-3 atm converted ethyl a-oximimoacetoacetate quantitatively to ethyl 2-amino-3-hydroxybutanoate [45]. 

Various unsubstituted solid alkenes are able to quantitatively add gaseous halogenohydrides. Prominent examples are the cholesterol esters 128 that give stereospecifically the bromides 129 at -30 °C [75,75al and camphene (130) that gives stereospecifically the rearranged bromide 131 or the elusive camphene hydrochloride (132) with 100% yield [11] (Scheme 15). The quantitative solid-... 

The marked stereoselectivities and clean solid-state reactions of oxiranes were used for synthetic purposes in the steroid field. The stereospecifically obtained trans-chlorohydrins 147 ensue quantitatively from the crystalline 5a,6a-epoxides 146 with gaseous HCl [77]. Similarly, the crystalline 16a,17a-epoxide 148 reacts with gaseous HCl to yield exclusively the traws-chlorohydrin 149 which easily loses HCl to re-form the starting epoxide 148. Therefore, an equilibrium situation is reached in that case [77] (Scheme 16). 

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