Ring enantioselective

Eq. (10.31)]. Particularly noteworthy here is the abihty to form six-, seven-, and eight-membered rings enantioselectively ... 

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

Relationships between stereocenters vary between two extremes. On the one hand, stereocenters may interact strongly in a spatial sense if they are directly joined, proximate to one another, or part of a compact rigid-ring structure. On the other hand, two stereocenters which are remote from one another and/or flexibly connected may be so independent that one cannot be used to provide substrate spatial control for the other. Nonetheless, this latter type of stereorelationship may still be clearable if the target molecule can be disconnected to divide the two stereocenters between two precursors or if an appropriate enantioselective transform is available. 

In general, 2-substituted allylic alcohols are epoxidized in good enantioselectivity. Like glycidol, however, the product epoxides are susceptible to ring opening via nucleophilic attack at the C-3 position. Results of the AE reaction on 2-methyl-2-propene-l-ol followed by derivatization of the resulting epoxy alcohol are shown in Table 1.6.1. Other examples are shown below. 

As with i -substituted allyl alcohols, 2,i -substituted allyl alcohols are epoxidized in excellent enantioselectivity. Examples of AE reactions of this class of substrate are shown below. Epoxide 23 was utilized to prepare chiral allene oxides, which were ring opened with TBAF to provide chiral a-fluoroketones. Epoxide 24 was used to prepare 5,8-disubstituted indolizidines and epoxide 25 was utilized in the formal synthesis of macrosphelide A. Epoxide 26 represents an AE reaction on the very electron deficient 2-cyanoallylic alcohols and epoxide 27 was an intermediate in the total synthesis of (+)-varantmycin. 

The catalytic enantioselective cycloaddition reaction of carbonyl compounds with conjugated dienes has been in intensive development in recent years with the main focus on synthetic aspects the number of mechanistic studies has been limited. This chapter will focus on the development and understanding of cycloaddition reactions of carbonyl compounds with chiral Lewis acid catalysts for the preparation of optically active six-membered ring systems. 

We are the first group to succeed with the highly enantioselective 1,3-dipolar cycloadditions of nitronates [75]. Thus, the reaction of 5,6-dihydro-4H-l,2-oxazine N-oxide as a cyclic nitronate to 3-acryloyl-2-oxazilidinone, at -40 °C in dichloro-methane in the presence of MS 4 A and l ,J -DBFOX/Ph-Ni(II) complexes, gave a diastereomeric mixture of perhydroisoxazolo[2,3-fe][l,2]oxazines as the ring-fused isoxazolidines in high yields. The J ,J -DBFOX/Ph aqua complex prepared from... 

Tlie constrLiction of carbocydic cotnpoutidi by ring-annulation procedures frequently plays a prominent role in total syntliesis. Tlie tolerance of various functional groups in tlie zinc reagents employed in copper-catalyzed asymmetric 1,4-additions fornis tlie basis for tliree novd catalytic enantioselective annulation metliods discussed bete. 

Asymmetric ring-opening of saturated epoxides by organoctiprates has been studied, hut only low enantioselectivities f -c 1596 ee) have so far been obtained [49, 50]. Muller et al., for example, have reported that tlie reaction between cyclohexene oxide and MeMgBr, catalyzed by 1096 of a chiral Schiffhase copper complex, gave froiis-2-metliylcyclohexanol in 5096 yield and with 1096 ee [50]. 

The fourth factor becomes an issue when anti betaine formation is reversible or partially reversible. This can occur with more hindered or more stable ylides. In these cases the enantiodifferentiating step becomes either the bond rotation or the ring-closure step (Scheme 1.12), and as a result the observed enantioselectivities are generally lower (Entry 5, Table 1.5 the electron-deficient aromatic ylide gives lower enantioselectivity). However the use of protic solvents (Entry 6, Table 1.5) or lithium salts has been shown to reduce reversibility in betaine formation and can result in increased enantioselectivities in these cases [13]. Although protic solvents give low yields and so are not practically useful, lithium salts do not suffer this drawback. [18]... 

Although the enantioselective intermolecular addition of aliphatic alcohols to meso-epoxides with (salen)metal systems has not been reported, intramolecular asymmetric ring-opening of meso-epoxy alcohols has been demonstrated. By use of monomeric cobalt acetate catalyst 8, several complex cyclic and bicydic products can be accessed in highly enantioenriched form from the readily available meso-epoxy alcohols (Scheme 7.17) [32]. 

The first example of asymmetric catalytic ring-opening of epoxides with sp2-hybridized carbon-centered nucleophiles was reported by Oguni, who demonstrated that phenyllithium and a chiral Schiff base ligand undergo reaction to form a stable system that can be used to catalyze the enantioselective addition of phenyllithium to meso-epoxides (Scheme 7.24) [48]. Oguni proposed that phenyllithium... 

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