Alcohol stereo specificity

It was also of interest to apply such lOOC reactions to formation of carbocyclic rings. Oxime olefins 230 a-e, formed in good yield via reaction of 229 with 0-silyl-a-bromoaldoximes 228 in the presence of F ions, cyclized in a sealed tube at 190 °C to provide 231 a-e (Eq. 24, Table 22) [63]. Reduction of 231a provided amino alcohol 232 a in 68% yield. Amino alcohol 232 e was converted stereo-specifically to the fused -lactam 233. 

The 10 OC route was followed for the synthesis of tetrahydrofurans possessing a y-amino alcohol moiety 247 (Eq. 29) 118]. Aldoximes 21a-f (see also Eq. 3 and Table 2), when heated in benzene in a sealed tube at 110 -120 °C for 6 h, underwent smooth intramolecular cycloaddition to the tetrahydrofuranoisoxazo-lidines 246a-f in 70-83% yield (Eq. 29). This ring closure proceeded stereo-specifically to generate three adjacent stereogenic centers. LAH reduction of 246 b resulted in isolation of stereospecifically functionalized tetrahydrofuran derivative 247b in 75% yield. 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

In the interim period, results have accumulated steadily, in endeavors to address and extend the chemistry beyond the initial perceived limitations. These limitations include the following (a) the effective catalytic syntheses are confined to the reactions utilizing catecholborane (b) the scope of alkenes for which efficient rate, regio- and enantio-selectivity can be achieved is limited, and (c) the standard transformation mandates the oxidation of the initially formed (secondary) boronate ester to a secondary alcohol, albeit with complete retention of configuration [8]. Nonetheless, for noncatalytic hydroboration reactions that lead to the formation of a trialkylborane, a wide range of stereo-specific transformations may be carried out directly from the initial product, and thereby facilitate direct C-N and C-C bond formation [9]. 

Cassipourine has been isolated from Cassipourea species (Rhizophoraceae).8 Wrobel and Glinski have devised a synthesis of racemic material (18).9 The starting material was the / -epoxide (13), which was converted into the three steps (see Scheme 4). Thiobenzyl alcohol reacted regio- and stereo-specifically with this material (14) to give the thiol (15), which oxidized in air to give dimeric products. The racemate (16) and the meso-ioxm were separated by column chromatography. Reduction of the racemate (16) with sodium in liquid ammonia yielded the dithiol (17), which afforded ( )-cassipourine (18) on aerial oxidation. 

The first step is a reaction you haven t yet met—it comes in Chapter 47. All you need to know now is that the reagent, a boron-containing compound called 9-borabicyclononane (9-BBN), hydrates one of the double bonds in the reverse fashion to what you would expect with acid or Hg2+ (Chapter 20) and stereo specifically (H and OH go in cis). The resulting alcohol is mesylaled (p. 486) in the usual way. This puts in H and OMs stereo specifically cis to each other. 

The ring-A/B moiety (286) of the naturally occurring C2g steroidal lactone withaferin A has been incorporated into cholestane as outlined in Scheme 14.156 Hydride reduction of the epoxide (280) gave the diol (281) which reacted stereo-specifically with peroxy-acid to yield the a-epoxide (282), and this was in turn converted into the epoxy-enone (283). The yield of (283) based upon the epoxy-dienone (261) is 70%. Ring-opening of the oxide (283) afforded the 5a-alcohol (284) which was dehydrated to the A2,5-diene (285). The A5-bond was then epox-idized stereoselectively and quantitatively to the 5/3-oxide (286). Ring A of this 5/3 -oxide was shown to be in the boat conformation. 

Bicyclobutanes are also cleaved, but small changes in conditions cause different bonds to break, as shown for Moore s bridged bicyclobutane 39 and its derivative 44 in Scheme 11. Thus water, alcohols, and HCN in chloroform or THF add stereo-specifically across the bridging bicyclobutane bond to form 40 [82], implying that... 

The aim of this section is to show how cyclic precursors can serve as an appropriate starting material for the construction of open-chained or monocyclic intermediates with defined regio- and/or stereo-specifity. The nucleofuge usually is halide or sulfonate and the electrofuge an alkoxide, generated from an alcohol by base or from a keto group by attack of a nucleophile. 

The synthesis of such eight-membered rings by the nickel-catalysed dimerisation of butadienes, is used here with 2-methylbutadiene. The regio-selectivity and stereo-specificity of the hydro-boration of symmetrical 124 are as expected for a trisubstituted alkene. Conversion of the alcohol 125 into a good leaving group 126 made it reactive enough to carry out electrophilic addition on the other alkene. In aqueous solution, the final nucleophile was water and the product a mixture of diastereoisomers of the tertiary alcohol 127. 

The QDH from C. t stosim i was further characterized It oxidizes stereo-specifically the fl ) enantiomer of secondary alcohols. Both, bsat/Km and M increased with the substrate chain length. J ritra, fetricyamde used as sacrificial electron acceptor. i idi O, the excess electrons are most probably transferred to molecular... 

The reaction of nitrone 101 with such olefins 5 as propylene, n-hexene, allyl alcohol, styrene, acrylic acid, methyl acrylate, acrylonitrile, and butadiene are all regiospecific to give S-substituted isoxazolidines of type l.77 The reaction with olefins 6 seems to be regio- and stereo-specific thus, nitrone 101 and methyl methacrylate give cycloadduct lilt as the sole product.77 Similarly, 101 and 1 -methyl-1-phenylethylene gives adduct Hit.77 In the reactions with olefins 7 or 8, the stereospecificity is more evident.77,78 The isoxazolidine obtained is always one of the two possible epimers. The reaction of nitrone 101 with trisubstituted olefins is highly stereoselective.64 Some results are summarized in Table III. 

Simple dialkyl ketones are not easily oxidized by less reactive peroxyacids and usually require the more powerful trifluoroperoxyacetic acid. Since most groups migrate in preference to methyl, "the Baeyer-Villiger reaction of methyl ketones serves as a stereo specific source of optically active alcohols by way of the acetate", 28 seen in the conversion of 310 to 311 in 85% yield. 29 example also illustrates that the stereochemical integrity of the migratory group is retained, which appears to be a general phenomenon. ... 

Cuprous chloride tends to form water-soluble complexes with lower olefins and acts as an IPTC catalyst, e.g., in the two-phase hydrolysis of alkyl chlorides to alcohols with sodium carboxylate solution [10,151] and in the Prins reactions between 1-alkenes and aqueous formaldehyde in the presence of HCl to form 1,3-glycols [10]. Similarly, water-soluble rhodium-based catalysts (4-diphenylphosphinobenzoic acid and tri-Cs-io-alkylmethylam-monium chlorides) were used as IPTC catalysts for the hydroformylation of hexene, dodecene, and hexadecene to produce aldehydes for the fine chemicals market [152]. Palladium diphenyl(potassium sulfonatobenzyl)phosphine and its oxide complexes catalyzed the IPTC dehalogenation reactions of allyl and benzyl halides [153]. Allylic substrates such as cinnamyl ethyl carbonate and nucleophiles such as ethyl acetoactate and acetyl acetone catalyzed by a water-soluble bis(dibenzylideneacetone)palladium or palladium complex of sulfonated triphenylphosphine gave regio- and stereo-specific alkylation products in quantitative yields [154]. Ito et al. used a self-assembled nanocage as an IPTC catalyst for the Wacker oxidation of styrene catalyzed by (en)Pd(N03) [155]. 

Reaction with allylic alcohols. French chemists have observed notable stereo-specificity in the reaction of the Simmons-Smith reagent with acyclic (Z)-allylic alcohols (equation I). In the reaction of the (E)-isomers, both erythro- and threo-cyclopropyl alcohols are formed, with a slight bias for the former products. 

Cu -Mediated additions of Grignard reagents to allenic alcohols occur regio- and stereo-specifically to give ( )-allylic alcohols as long as the y-allene position is unsubstituted [i.e. equation (5) = H]. 

Specificity for the alcohol substrate is very broad, ranging from small molecules (e.g. propanediol) to polysaccharides. Galactose oxidase is strictly stereo specific. It does not oxidise either o-glucose or l-galactose. 

AUylic alcohols, e.g. (269), can be converted into diols, e.g. (270), with a high regio- and stereo-specificity via acetal intermediates, e.g. (271). ... 

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