Enantioselective Rearrangement

Allin, S. M., Baird, R. D., Lins, R. J. Synthetic applications of the amino-Cope rearrangement enantioselective synthesis of some tetrahydropyrans. Tetrahedron Lett. 2002, 43, 4195-4197. 

Cho GY, An KM, Ko SY (2001) Improving the Regioselectivity in the Cyclic Iminocarbonate Rearrangement Enantioselective Synthesis of the Paclitaxel Side Chain. Bull Korean Chem Soc 22 432... 

Up to now only a few attempts have been made to render Wittig [1,2]-rearrangements enantioselective. One noteworthy result has been accomplished with dibenzyl ether as the substrate. Metalation with ter/-butyllithium in DEE at -75 °C in the presence of catalytic or stoichiometric amounts of the homochiral bisoxazoline 364 gives 1,2-diphenylethanol in up to 94% yield and with an enantiomeric excess (ee) of up to 62% (Scheme 1-282). " ... 

R)-Pantolactone is also prepared in a sequence involving Claisen rearrangement of the chiral glycolate (24), although with poor enantioselectivity... 

There are a number of powerful synthetic reactions which join two trigonal carbons to form a CC single bond in a stereocontrolled way under proper reaction conditions. Included in this group are the aldol, Michael, Claisen rearrangement, ene and metalloallyl-carbonyl addition reactions. The corresponding transforms are powerfully stereosimplifying, especially when rendered enantioselective as well as diastereoselective by the use of chiral controller groups. Some examples are listed in Chart 20. 

By employing optically active enol borinates instead of silylketene acetals, the Ireland-Claisen rearrangement has been further developed to an enantioselective... 

Claisen rearrangements with 74 -, Diels-Alder reactions with 74 -, enantioselective reactions 74... 

Single isobacteriochlorin stereoisomers even in enantiomcrically pure form can be obtained230 when the Claisen rearrangement is performed with the pure hematoporphyrin stereoisomers23d which can be prepared by stereogenic enantioselective reduction from diacetyl deuteroporphyrin dimethyl ester. 

In a synthesis of (-i-)-asteltoxin, Cha applied the Suzuki-Tsuchihashi rearrangement to silyloxy epoxide 184 for the enantioselective construction of the unusual... 

The epoxidation of divinyl carbinol constitutes a special case of a dienol epoxida-tion, as the starting diene is not conjugated (Scheme 9.10). Desymmetrization by SAE, followed by a Payne rearrangement, furnishes the vinylepoxide in high yield and with excellent enantioselectivity (compare Table 9.2, Entry 1) [43]. 

The Rh2(DOSP)4 catalysts (6b) of Davies have proven to be remarkably effective for highly enantioselective cydopropanation reactions of aryl- and vinyl-diazoacetates [2]. The discovery that enantiocontrol could be enhanced when reactions were performed in pentane [35] added advantages that could be attributed to the solvent-directed orientation of chiral attachments of the ligand carboxylates [59]. In addition to the synthesis of (+)-sertraline (1) [6], the uses of this methodology have been extended to the construction of cyclopropane amino acids (Eq. 3) [35], the synthesis of tricyclic systems such as 22 (Eq. 4) [60], and, as an example of tandem cyclopropanation-Cope rearrangement, an efficient asymmetric synthesis of epi-tremulane 23 (Eq. 5) [61]. 

The premier example of this process in an ylide transformation designed for [2,3]-sigmatropic rearrangement is reported in Eq. 15 [107]. The threo product 47 is dominant with the use of the chiral Rh2(MEOX)4 catalysts but is the minor product with Rh2(OAc)4. That this process occurs through the metal-stabilized ylide rather than a chiral free ylide was shown from asymmetric induction using allyl iodide and ethyl diazoacetate [107]. Somewhat lower enantioselectivities have been observed in other systems [108]. 

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

The initial results of an early directed evolution study are all the more significant, because no X-ray data or homology models were available then to serve as a possible guide [89]. In a model study using whole E. coU cells containing the CHMO from Adnetohacter sp. NCIM B9871,4-hydroxy-cydohexanone (3 5) was used as the substrate. The WT leads to the preferential formation of the primary product (i )-36, which spontaneously rearranges to the thermodynamically more stable lactone (R)-37. The enantiomeric excess of this desymmetrization is only 9%, and the sense of enantioselectivity (R) is opposite to the usually observed (S)-preference displayed by simple 4-alkyl-substituted cydohexanone derivatives (see Scheme 2.10) [84—87]. 

Because of the nature of the transition state in the pericyclic mechanism, optically active substrates with a chiral carbon at C-3 or C-4 transfer the chirality to the product, making this an enantioselective synthesis (see p. 1451 for an example in the mechanistically similar Claisen rearrangement). ... 

In contrast to the synthesis of carbocyclic rings, the Cope rearrangement has been used sparsely for generating azepinones. Recently, the enantioselectivity of the conversion of 2-aza-divinylcyclopropane 286 has been investigated. The synthesis started from the optically active cyclopropanecarboxylic acid (90% ee), which had been converted into the isocyanate 286 by initial azidation to 285 and a consecutive Curtius rearrangement. Furthermore, the conditions of the iso-... 

An elegant enantioselective [2,3] sigmatropic rearrangement ofbisalkynyl ethers such as 75 was reported by Manabe in 1997 [20]. The deprotonation... 

---