Cyclic optically enriched

Several examples exist in the literature in which cyclic ketone enolates are enantioselectively hydroxylated by chiral, nonracemic Davis oxaziridine reagents. In contrast to their acyclic counterpart, the enolate geometry is fixed in cyclic systems. During the preparation of enantiomerically pure (-)-blebbistatin, the enolate of the quinolone 61 was reacted with the Davis reagent 5 to afford the optically enriched 62 with 82% yield and 86% ee.54 The related reagent 6 was used in the synthesis of (-t-)-o-trimethylbrazilin, which was... 

The Diels-Alder cycloaddition of chiral cyclic 2-amidodienes, derived from chiral a-allyl allenamides, with enones yielded optically enriched [2.2.2]bicyclic adducts with dr up to >95 5. In the presence of HOMO-raised trienamines, e-deficient l-aza-l,3-butadienes containing 1,2-benzoisothiazole-1,1-dioxide or 1,2,3-benzoxathiazine-2,2-dioxide react as dienophiles in normal-electron-demand Diels-Alder reactions rather than the corresponding inverse-electron-demand versions. The Fe(III)-catalysed benzannulation of 2-(2-oxyethyl)-benzaldehydes with alkynes produced naphthalene derivatives in high yields (99%) under mild reaction conditions. ... 

Rogers LMA, Rouden J, Lecomte L, Lasne MC. Enantioselective decarboxylation-reprotonation of an a-amino malo-nate derivative as a route to optically enriched cyclic a-amino acid. Tetrahedron Lett. 2003 44 3047 3050. 

A subsequent study by Jprgensen et al. also demonstrated the enantioselective a-sulfenylation of (S-dicarbonyl compounds 420 using l-alkylsulfanyl[l,2,4]triazole derivatives 419 in the presence of a catalytic amount of cinchona alkaloid derivative 421. The use of cyclic (S-dicarbonyl compounds ensured the introduction of a quaternary sulfur center however, the observed enantioselectivity was modest in 51-89%. In 2009, Zhu and co-workers reported that a chiral a,a-diaryl prolinol 424 efficiently catalyzed the enantioselective sulfenylation of (S-ketoesters 420 using N-(phenylthio)phthalimide 423 as a sulfur electrophile. The absence of racemizable C—H bonds led to the optically enriched a-sulfenylated products 425 in excellent enantio-selectivities. In 2010, Fu developed a method for catalytic asymmetric 7-sulfenylation of carbonyl compounds using 2,3-allenoates 426 in the presence of a chiral bisphos-phine, TangPhos 427, and a bulky carboxylic acid 428. ... 

Kawakami et al. have prepared optically active bifunctional l,3-dimethyl-l,3-diphenyldisiloxanes.158,159 Strohmann et al. have prepared enantiomerically enriched Si-centered silyllithium compounds, which react stereo-specifically with triorganochlorosilanes.160-162 In solution, slow racemization of the silyllithium compounds takes place, which, however, can be circumvented by transmetallation with MgBr2. Oestreich et al. prepared new Si-centered cyclic silanes adopting the strategies developed by Corriu and Sommer.163 Bienz et al. have developed enantioselective routes for the preparation of C-centered chiral allenylsilanes.156,164-166... 

Another approach to chiral sulfinates was developed by Pirkle and Hoekstra (108) it is based on incomplete but stereoselective reaction of racemic sulfinates with chiral Grignard reagents. This kinetic resolution affords the unreacted sulfinates enriched in the (5 -enantiomers with optical purities in the 8-64% range. The chiral cyclic sulfinates 66 and 67 were first obtained by this method. 

As mentioned in Section 10.6.2, synthesis of 1-hydroxyethylene peptides can be initiated by adding a ferf-butoxycarbonyl N-protected a-amino aldehyde to an optically active Grignard reagent (Scheme 7)J11-13 This reaction affords a diastereomeric mixture of the C4 epimers of the hydroxy ether in good yields. In most cases the mixture is enriched in the 45-epimer and the epimers are readily separable. The yields and the ratios of the resulting 45- and 4R-epimers obtained from several examples of this reaction are summarized in Table 1. When this reaction was attempted with the aldehyde prepared from Aa,Ae-bis-tert-butoxycarbonyl-protected Lys, the desired product was not obtained. The anion of the Lys Ne-tert-butoxy-carbonylamino group probably reacts with the aldehyde to form a cyclic aminol that does not... 

Cyclic and acyclic enol derivatives 480 can be asymmetrically aziridinated with (A -tosylimino)iodobenzene 481 using a chiral copper catalyst prepared in situ from [Cu(MeCN)4]PF6 and the optically active ligand 479. Collapse of the aminal (i.e., 482) leads to the formation of enantiomerically enriched Q-amino carbonyl compounds 483, although ee s to date are modest <2000EJ0557>. Similarly, dienes can be selectively aziridinated using the chiral Mn-salen complex 484 to give vinyl aziridines 486 in scalemic form (Scheme 124) <2000TL7089>. 

Diels-Alder reactions of enantiomerically enriched 2H-azirine 3-phosphon-ates (281), a new class of chiral iminodienophiles, and dienes stereoselectively furnish optically pure, bicyclic aziridine adducts (282). Hydrogenation of (282) results in a ring opening that affords the first examples of optically pure quaternary piperidine phosphonates. Two step synthesis of an enantiomeric pure cyclic phosphite (283) and its application as a chiral phosphorus nucleophile in the asymmetric Michael addition to nitroalkenes (284) provides an efficient... 

It has been reported that histidine containing cyclic dipeptides catalyse the formation of optically active cyanohydrins. By this means a number of aromatic, aliphatic as well as heteroaromatic aldehydes and ketones could be converted into the corresponding enantiomerically enriched cyanohydrins [77 - 87]. Interestingly, cyanohydrin formation using this class of catalysts is suppressed by small amounts of benzoic acid. This finding suggests that the mechanism of the... 

The pilot scale plant was used to perform spray experiments with an excess of CO2. There is a continuous flow of CO2. The liquid which shall be atomized is added cyclic with a piston pump. Therefore, it is assumed that the flow regime in and upstream the nozzle is a two phase flow consisting of a compressed gas phase and a gas-enriched liquid phase. This flow regime equals to the flow regime of a flash atomization with internal flashing. The optically transparent capillary with an inner diameter of 1 x 10 m is assembled to the pilot scale plant to analyze the flow regime and to verify these assumptions. 

In the case of propylene sulfide the stereoelectivity could be highly enhanced when using monomers having an unbalanced R/S enantiomeric ratio. This is of interest because starting from a monomer of 50% optical purity one can isolate after 50% conversion an unreacted monomer almost optically pure. The stereoelective polymerization of cyclic monomers is therefore a potential method for their enantiomeric purification. Similarly using convenient conditions one can obtain optically pure polymers either by physical separation or by using enriched monomers. 

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