Stereocontrol, anionic

The enantioselective construction of a key tricyclic intermediate of spinosyn A utilizing a highly stereocontrolled anionic oxy-Cope rearrangement was accomplished in the laboratory of L.A. Paquette.The precursor tertiary alcohol was treated with potassium hydride in THF and the oxy-Cope rearrangement was complete within 3 hours at room temperature. Interestingly, the yield varied between 77 and 91% depending on the source of KH. 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

Cations which are covalently attached to the allyl anion part by a cr-bond and have sufficient Lewis acid properties offer the broadest versatility and highest levels of stereocontrol, since the C—C bond-forming step can occur in a pericyclic process9 accompanied by allylic inversion. It is reasonable to assume the prior assembly of both reaction partners in an open-chain complex, in which usually the (F )-oxonium ion, avoiding allylic 1,3-strain10, is predominant. 

Double asymmetric induction operates when the azomethine compound is derived from a chiral a-amino aldehyde and a chiral amine, e.g., the sulfin-imine 144 [70]. In this case, the R configuration at the sulfur of the chiral auxihary, N-tert-butanesulfinamide, matched with the S configuration of the starting a-amino aldehyde, allowing complete stereocontrol to be achieved in the preparation of the diamine derivatives 145 by the addition of trifluo-romethyl anion, which was formed from trifluoromethyltrimethylsilane in the presence of tetramethylammonium fluoride (Scheme 23). The substituents at both nitrogen atoms were easily removed by routine procedures see, for example, the preparation of the free diamine 146. On the other hand, a lower diastereoselectivity (dr 80 20) was observed in one reaction carried out on the imine derived from (it)-aldehyde and (it)-sulfinamide. 

In a similar fashion to those results obtained for the oxidation process, on switching from poly-(L)-leucine to poly-(D)-leucine the opposite configuration of the polyether was observed (absolute configuration of products unknown). Unexpected, however, was the observation that poly-(L)-phenylalanine furnished the opposite enantiomer to that observed employing poly-(L)-leucine. Thus, it has been shown that addition of nucleophiles other than peroxide anion can be catalysed by polyamino acids with significant stereocontrol [22]. 

Stereocontrolled addition of 2-furyl anion to 2,3-0-isopropylidene-D-glyceraldehyde can be achieved by the addition of some metal salts. 

Steroid side chain.1 The key step in a method for stereocontrolled addition of the side chain to 17-kelo steroids is hydroboration of a 17(20)-(Z)-ethylidene steroid (I), which proceeds selectively to give 2, with the desired natural configuration at C,- and C2ft. The product reacts with most alkylating reagents in rather low yield, possibly because of stcric factors however alkylation with the anion of chloroacetonitrile (potassium 2,6-di-r-butyl-4-methylphenoxide) in T1IF gives the nitrile 3 in 60 70% yield. One added attraction of this route is that 9-BBN reacts preferentially with a 17(20)-double bond in the presence of a 5(6 )-double bond. 

To broaden our overall knowledge of process kinetics the first chapter of this volume deals with elementary reactions in radical and anionic polymerization it was written by G. V. Schulz, the first recipient of the H. Staudinger Award. It is followed by a discussion on monomer constitution and stereocontrol in radical polymerization by H. G. Elias et al. 

Stereopure epoxide 1 was prepared and treated with 3.6 equivalents of t-butyllithium in THF/HMPA at -78°C. The intention was that formation of the anion at the benzylic carbon would lead to a 4-exo-epoxide ring opening reaction a subsequent [l,2]-silyl shift (Brook rearrangement) would generate the oxetane 2 with stereocontrol at all three stereocentres. Anion formation proceeded smoothly at -78°C, then 1 ml of 1 M hydrochloric acid was added and the product isolated. Obtained pure in 40% yield, this was shown to be the aldehyde 3. No oxetane 2 was obtained. 

A highly Z-selective olefination of a-oxy and a-amino ketones via ynolate anions has been reported (Scheme 9).43 The stereocontrol mechanism has been explained by (g) orbital interactions between the s orbital of the breaking C-O bond or n orbital of the enolate and the s orbital of the C-O or C-N bonds of the substituent in the ring opening of the /I-lactone enolate intermediates, and/or the chelation to lithium. 

Sunay, U. Fraser-Reid, B. Synthetic studies relating to the C1-C9 eastern" half of rosara-micin. Tetrahedron Lett. 1986, 27, 5335-5338. Smith, A. B. Pitram, S. M. Boldi, A. M. Gaunt, M. J. Sfouggatakis, C. Moser, W. H. Multicomponent linchpin couplings. Reaction of dithiane anions with terminal epoxides, epichlorohydrin, and vinyl epoxides efficient, rapid, and stereocontrolled assembly of advanced fragments for complex molecule synthesis./. Am. Chem. Soc. 2003, 125, 14435— 14445. 

Acetals such as 34 have also been used as substrates in anionic-radical cascades treatment of 34 with SmI2-HMPA gives bicyclic acetal 35 with excellent stereocontrol (Scheme 6.14).19... 

A simple, stereocontrolled, and economical synthesis of m-2,4,5-triarylimidazolines 276 starts from the reaction of aromatic aldehydes 274 with ammonia to form 2,4-diazapentadienes 275. Deprotonation of 2,4-diazapentadienes 275 with a strong base or under thermal conditions results in formation of the transient 2,4-diazapentadienyl anion that cyclizes in a disrotatory fashion to furnish m-imidazolines 276 (Scheme 134) microwave irradiation facilitates the ring closure <1997TL8631, 2003S1236, 2005H(65)353, CHEC-III(4.02.9.1.3)295>. 

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