Stereoselective conversion

Halogen atoms can be stereoselectively introduced by ring-opening of y-azir-idinyl-a,P-enoates (Scheme 2.39). Treatment of 149 with diethylaminosulfur tri-fluoride (DAST) results in stereospecific ring-opening to yield fluorinated derivative 150 [59]. A related stereoselective conversion of y-aziridinyl-a,P-enoates 151 into allyl halides 152 by use of lithium halide in the presence of Amberlyst 15 was also reported recently [60]. 

Stereoselective conversion of a thiane 57 to the corresponding tmns-thiane-l-oxide 58 was achieved by bromonium ion mediated electrooxidation while a preferential formation of the cis-sulphoxide 58 was observed under acidic electrolysis123 (equation 38). 

Suggest reaction conditions appropriate for stereoselective conversion of the octalone shown to each of the diastereomeric decalones. 

The direct, stereoselective conversion of alkynes to A-sulfonylazetidin-2-imines 16 by the initial reaction of copper(l) acetylides with sulfonyl azides, followed, in situ, by the formal [2+2] cycloaddition of a postulated A-sulfonylketenimine intermediate with a range of imines has been described <06AG(E)3157>. The synthesis of A-alkylated 2-substituted azetidin-3-ones 17 based on a tandem nucleophilic substitution followed by intramolecular Michael reaction of primary amines with alkyl 5-bromo-4-oxopent-2-enoates has been... 

Intramolecular cyclizations of silyl nitronates were also used in the preparation of aminosugars. In 2003 Kudoh et al. reported the stereoselective conversion of 2-nitroalkanols by silyl nitronate generation followed by an intramolecular nitronate-olefin [3 + 2] cycloaddition reaction (Scheme 51).88... 

Stereoselective conversion of an aldehyde to the corresponding E-vinyl iodide using CHI3 and CrCl2. 

A small number of examples is available for the synthesis of E and Z isomers of oximes. In many cases, E isomers were obtained either from the Z forms (by the hydrochloride method) or isolated by column chromatography. Often, the reagents that have been used for oximation of aldehydes and ketones also catalyze the interconversion of Z and E isomers. The rate of equilibration of a mixture of Z and E isomers and the position of the equilibrium is temperature-dependent ". In 2001, Sharghi and Sarvani reported a convenient method for controlling the stereochemistry of the reaction of hydroxylamine hydrochloride with aldehydes or ketones in the solid state. The highly stereoselective conversion of aldehydes and ketones to their corresponding oximes... 

Alternatively, the amino group was introduced subsequent to the reductive formation of the fluoroalkene [72]. The regio- and stereoselective conversion of the C5-hydroxyl group of the fluoroolefin to an amino group could be achieved through one-pot mesylation and azidation reaction (Scheme 25). 

Polyhydroxyalkanoate synthase (systematic name acyl-CoA 3-hydroxybutyrate 0-acyltransferase EC 2.3.1. class) is responsible for the polymerizations of PHAs in vivo because it catalyzes the stereoselective conversion of (/ )-3-hydroxyacyl-CoA substrates to PHAs with the concomitant release of CoA (see Fig. 2) [3]. 

Intramolecular cycloaddition of the nitrile oxide intermediate generated from the unsaturated oxime 221 was used for an evenmal synthesis of la,2p,25-trihydroxy-vitamin D3 (262) (Scheme 6.90). Oxime 221, prepared from tri-O-isopropyhdene-d-mannitol (220), was processed as usual to give isoxazoline 222 in good yield and with excellent stereoselectivity. Conversion of 222 to the aldol 223 proceeded in the normal manner and further elaboration gave the desired diene intermediate 224 (262). 

Yttrium-catalyzed cascade cyclization/hydrosilylation of 3-(3-butynyl)-l,5-hexadienes was stereospecific, and syn-19 (R =Gy, R = OGPh3) underwent cascade cyclization/hydrosilylation to form 80b (R = Gy, R = OGPh3) in 97% yield as a single diastereomer (Scheme 20). The regio- and stereoselective conversion of syn-19 to 80b was proposed to occur through an initial 5- x -intramolecular carbometallation via a chairlike transition state that resembles alkenyl olefin eomplex syn- m. followed by S-exo intramolecular carbometallation via a boatlike transition state that resembles alkyl olefin complex boat-llm. The second intramolecular carbometallation presumably occurs via a boatlike transition state to avoid the unfavorable 1,3-interaction present in the corresponding chairlike transition state (Scheme 20). 

M. J. Robins, J. S. Wilson, and F. Hansske, Nucleic acid related compounds. 42. A general procedure for efficient deoxygenation of secondary alcohols. Regiospeciflc and stereoselective conversion of ribotmdeoades to 2 deoxynodeosides, J. Am. Chem. Soc. 105 4059 (1983). 

In general, the method of enzymatic cyanohydrin synthesis promises to be of considerable value in asymmetric synthesis because of the synthetic potential offered by the rich chemistry of enantiomerically pure cyanohydrins, including their stereoselective conversion into other classes of compounds such as a-hydroxy carboxylic acids or respective esters, w c-diols, / -aminoalcohols, aziridins, a-azido(amino/fluoro)nitriles, and acyloins [501, 516]. 

In the total synthesis of mevinolin,122 a stereoselective conversion of (57) into (58) was called for. Several organocopper reagents were examined, but it was MeCu-BFa which exhibited die highest selectivity (Scheme 26). The reason for the selectivity is not obvious, but the results do indicate that there can be significant differences between the reactivity profile of the various organocopper reagents available. 

Pentadienyl cations complexed to Fe(CO)3 can be trapped intramolecularly by sulfides to yield tetrahydrothiopyrans with high stereoselectivity. Conversion to the sulfoxide is facile and occurs with good diastereoselectivity (95TL1849). 

Stereoselective conversion of 3,4-disubstituted-l,2-dithietane 1,1-dioxides 114 into symmetrical (Z)-alkenes 115 has been reported. When substituted 1,2-dithietane 1,1-dioxides 114 having a reactive thiosulfonate group in the four-membered ring were treated with lithium cyanide, the alkenes 115a-d were obtained with high (Z)-selectivity and high chemical yield (Equation 15) <1996BSF515>. 

It was noticed by Rickborn and Wood [4] and Field and Galagher [5] that cyclic tetrasubstituted alkenes undergo such a isomerization under much milder conditions. This approach enables highly stereoselective conversion of a variety of cyclic tetrasubstituted alkenes of type 34 into polyfunctional products of type 37 in one-pot reactions via the intermediate organoboranes 35 and 36 (Scheme 5) [6-8]. 

The same acidic or basic medium is also suitable for the stereoselective conversion of a-silylated tertiary alcohols into trisubstituted alkenes. Figure 4.9 showed an impressive series of examples of this. 

Fig. 16.15. Stereoselective preparations of trans-alkenyl-boronic acid esters (B) and trans-alkenylboronic acids (C) and their stereoselective conversion into c/s-bromoalkenes and trans-iodoalkenes, respectively.

In a general sense, [1,2]-Wittig rearrangements have only a limited application in synthesis because yields and selectivities are frequently moderate at best. For example, the valuable stereoselective conversion of 117 to 118 (which also works with higher homologues) proceeds in only 14% yield.85 86... 

A practical and efficient route for the stereoselective conversion of homoallylic alcohols to diastereomerically pure substituted cyclopropanes has been developed by Taylor et al. [73] (Eqs. 77,78). 

The Jacobsen-Katsuki-catalysts (Fig. 13) have recently received much attention as the most widely used alkene epoxidation catalysts. An example of Jacobsen s manganese-salen catalyst is shown in Fig. 13. They promote the stereoselective conversion of prochiral olefins to chiral epoxides with enantiomeric excesses regularly better than 90% and sometimes exceeding 98%.82,89,92,93,128 The oxidation state of the metal changes during the catalytic cycle as shown in Scheme 8. 

A series of N-protected substituted prolines have been prepared from /3-aminoaldehydes, as illustrated by the stereoselective conversion of 245 into the heterocycle 246 using benzyl diazoacetate as a source of the final required carbon atom (Equation 79) <2004JOC4361>. 

Lipoxygenases (LOs) are nonheme, mononuclear iron enzymes that catalyze the regio- and stereoselective conversion of polyunsaturated fatty acids with a di.di-1,4-diene functionality into products having a l-hydroperoxy-tra 5, cM-2,4-diene functionality. The mammalian LOs typically act on arachidonic acid and produce alkyl hydroperoxides that are converted into leukotrienes and lipoxins, which are involved as messengers in the inflammatory response. Plant enzymes act on linoleic acid, but the role of the product alkyl hydroperoxide is less well understood. 

Stereoselective conversion of 2-amino-3-hydroxy-4-pentenoic acid35 into the highly functionalized 2-furanones with the m-(4-hydroxy, 4-haloalkyl) relationship has been obtained through electrophile-promoted cyclization, due to the propensity of the hydroxy group to direct a cis substitution. Thus, treatment of (2.S, 3S)-2-tcrt-butoxycarbonylamino-3-hydroxy-4-pentenoic acid (32) with /V-bromosuccinimide in tetrahydrofuran gives the corresponding (35,4/ ,55 )-bromolactone 33 in 70% yield as the sole product35. 

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