Stereospecific conversion

Borane 2 adds to carbon-carbon double bonds without the need of catalytic activation. This reaction has been discovered and thoroughly investigated by H. C. Browm, and is called hydroboration It permits a regioselective and stereospecific conversion of alkenes to a variety of functionalized products. 

The stereospecific conversion of menthyl arenesulphinates into chiral aryl methyl sulphoxides may also be achieved by means of methyllithium . The reaction of methyllithium with diastereoisomerically or enantiomerically pure arenesulph-inamides 283 was found to give optically active aryl methyl sulphoxides 284 (equation 156). The preparation of optically active sulphoxides 285 and 286, which are chiral by virtue of isotopic substitution (H - D and - respectively), involves the reaction of the appropriate non-labelled menthyl sulphinates with fully deuteriated methyl magnesium iodide (equation 157) and with benzylmagnesium chloride prepared from benzyl chloride labelled with carbon (equation 158). 

Taking 1,2-disubstituted cyclopropane as an example, retro synthesis analysis shows that there are three possible ways to disconnect the three-membered ring—a, b, and c as shown in Figure 5-11. Route a involves the addition of methylene across a double bond, and this is often a stereospecific conversion or Simmons-Smith reaction.92 One can clearly see that route b or c will encounter the issue of cis/trans-product formation. 

In addition to the stereospecific sulfoxide syntheses mentioned above, optically active sulfoxides of high optical purity may be prepared from optically active sulfoximides 60, which are sufficiently basic to form salts with optically active sulfonic acids and can be easily resolved into enantiomers. The stereospecific conversion of sulfoximides 60 into the corresponding sulfoxides was effected by... 

The stereospecific conversion of sulfinates into sulfoxides of known chirality has been applied as a general method for determining the absolute configuration of a wide range of optically active sulfinic esters. For example, the absolute configurations of a series of alkyl alkanesulfinates obtained by asymmetric synthesis (107) or resolution via 3-cyclodextrin inclusion complexes (106) were determined by this method. 

The biotransformation of (/f,5)-Iinalool by fungi is a useful method for the preparation of natural linalool oxides. The stereospecific conversion of (J ,5)-linalool by Corynespora cassiicola DSM 62475 led to 5/f-configured furanoid linalool oxides and 55-configured pyranoid linalool oxides, both via bS -configured epoxylinalool as postulated intermediate (Figure 12.6). The biotransformation protocol affords an almost total conversion of the substrate with high enantioselectivities and a molar conversion yield close to 100% (Table 12.4). Pure linalool oxides are of interest for lavender notes in perfumery. ... 

LO catalyses the stereospecific conversion of arachidonic acid (1) to a 5-hydroperoxyeicosatetraenoic acid (5-HPETE) (2). The enzyme contains... 

Dihydroflavonol 4-reductase (DFR EC 1.1.1.219) is a member of the short-chain dehydrogenase/reductase family and catalyzes the stereospecific conversion of (+)-(2R,3R)-dihydroflavonols to the corresponding (2R,3S,4S) flavan-3,4-cw-diols (leucoanthocyanidins), with NADPH as a required cofactor. The enzyme activity was first identified in cell suspension cultures of Douglas fir (Pseudotsuga menziesii) and was shown to be related to the accumulation of flavan-3-ols and proanthocyanidins [96]. Leucoanthocyanidins and DFR were later shown to be required for anthocyanidin formation by complementation of Matthiola incana mutants blocked between dihydroflavonol and anthocyanidin biosynthesis [97, 98], DFR has been purified to apparent homogeneity and biochemically analyzed from flower buds of Dahlia variabilis [99]. DFR was shown to accept different substrates depending on the plant species from which it was isolated (reviewed in 100). 

Dihydroflavonol 4-reductase (DFR) catalyzes the stereospecific conversion of 2R,3R)-trans-DHFs to the respective (2R,35, 45)-flavan-2,3-traKi-3,4-cA-diols (leucoanthocyanidins) through a NADPH-dependent reduction at the 4-carbonyl. DNA sequences for DFR were first identified from A. majus and Z. mays, and the identity of the Z. mays sequence confirmed by in vitro transcription and translation of the cDNA and assay of the resultant protein. DNA sequences have now been cloned from many species, with the size of the predicted protein averaging about 38kDa. Stereospecificity to (2R,3R)-dihydroquercetin (DHQ) has been shown for some recombinant DFR proteins. ... 

Stereospecific conversion of vinylsilanes to silyl enol ethers This conversion can be effected by dihydroxylation with 0s04 in combination with (CH3)3NO and pyridine followed by anti p-elimination with NaH via an a-oxidosilane. The overall process converts vinylsilanes into silyl enol ethers with preservation of the geometry of the double bond. 

An interesting problem in stereoisomerism is found in the aldol reactions of the achiral aldehydes which are obtained by ozonolysis of the homoallylic alcohols 174. After stereospecific conversion by the FruA [230], the products can be readily induced to form an intramolecular glycoside 175 by acidic (R=OH) or alkaline treatment (R=C1), under which conditions the two equatorial ring hydroxyl groups completely direct the stereogenic acetal formation [234]. The corresponding RhuA catalyzed reactions deliver the enantiomeric... 

Guella G, Skropeta D, Mancini I, Petra F (2003) Calenzanane Sesquiterpenes from the Red Seaweed Laurencia microcladia from the Bay of Calenzana, Elba Island Acid-Catalyzed Stereospecific Conversion of Calenzanol into Indene- and Guaiazulene-Type Sesquiterpenes. Chem Eur J 9 5770... 

Numerous other stereospecific conversions have been reported, e.g. (137) (Zweifel and Steele, 1967). 

Huisgen, R., Scheer, W., and Huber, H. (1967) Stereospecific conversion of cis-trans isomeric aziridines to open-chain azomethine ylides. Journal of the American Chemical Society, 89, 1753-1755. 

These isomerizations can also occur with ring enlargement, as illustrated by the high-yielding and stereospecific conversion of epoxy alcohol 94 to hydroxy ketone 95 in the presence of tin(IV) chloride <02TL6637>. In another set of conditions, the macrocyclic epoxy alcohol 96... 

This procedure illustrates the stereospecific conversion of 1,2-diols... 

This enzyme catalyzes the stereospecific conversion of the (2R, JR)-dihydroflavonols, DHK and DHQ, to the corresponding flavonols, by introducing a double bond between C-2 and C-3. The enzyme was first reported in parsley cell cultures,58 and later shown to occur in several plant species.7 A cDNA clone encoding FLS was isolated from Petunia hybridal and its antisense expression strongly reduced flavonol synthesis in flower petals, thus allowing a higher flux of dihydroflavonols to be channeled towards anthocyanin synthesis.59... 

The stereospecific conversion of the synthetic tricyclic ketone (157)73 into ( )-hirsutic acid (159) has been accomplished74 by the short reaction sequence shown in Scheme 21. 

Additional evidence for the stereospecific conversion of fi-O-4 and arylglycerol lignin models to tetronic acids under conditions that selectively oxidize aryl rings also comes from acidic permanganate oxidation (Tsutsumi et al. 1990). This method gives essentially the same products as ozonation. 

The stereospecific conversion of c/s-cyclooctene epoxide to alcohol 51, first reported by Cope,87 has been examined in greater detail recently.88-90 Of importance is... 

The stereospecific conversion of cyclohexene into the corresponding amido selenide 54 is illustrated in Scheme 8. These amidoselenenylation reactions are commonly employed for the preparation of allylic and saturated amides by oxidative or reductive deselenenylation. Propionitrile, butyronitrile, benzonitrile and ethyl cyanoacetate may be used in place of acetonitrile. Styrene gave poor results and other electron-rich olefins such as 1-methylcyclohexene or 2,3-di-methylbut-2-ene did not give the amidoselenenylation products. The reaction can also be effected starting from the hydroxy- or methoxyselenenylation products of alkenes, in the presence of water and trifluoromethanesulfonic acid in this case the nitriles are used in stoichiometric amounts [48c]. This methodology was employed to prepare the amidoselenenylation products of styrene, 55, and of electron-rich olefins. It was necessary, however, to replace the phenyl-... 

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