Enantioselectivity rearrangements

The scope of this methodology was extended to the enantioselective rearrangement of difluorovinyl allyl ethers by these authors, furnishing a novel powerful tool for the synthesis of chiral p-substituted a,a -difluorocarbonyl compounds. As shown in Scheme 10.36, moderate to good enantioselectivities... 

The origin of the enantiodiscrimination appears to be strongly dependent on the structure of the HCLA employed. For HCLA bases of type A (53 to 56), stereoselectivity has been empirically deduced to arise [in the transition state (TS)] from the difference of energy between the two diastereoisomeric 1/1 HCLA/oxirane complexes TSl and TS2 (Scheme 27). Indeed, the steric repulsions between cyclohexene oxide and the pyrrolidinyl substituents in TS 1 favor TS 2, as proposed by Asami in 1990 for enantioselective rearrangement of cyclohexene oxide by HCLA 53 (Scheme 26) . ... 

It is interesting to note that such model, which is in good agreement with the enantio-and diastereoselectivity observed, is consistent with the model proposed for the enantioselective rearrangement of meso oxiranes mediated by this base (see Section in.B.l.a). 

Several examples are of enantioselective rearrangement reactions of oxonium, sulfonium, or selenonium ylides, generated by the catalytic decomposition of diazo compounds (199-203), are known. The first reports of enantioselective dipolar... 

A. Fiirstner, Olefin Metathesis and Beyond, Angew. Chem. Int. Ed. Engl. 2000, 39, 3012. D. M. Hodgson, F. Y. T. M. Pierand, and P. A. Stupple, Catal3dic Enantioselective Rearrangements and Cycloadditions Involving Ylides from Diazo Compounds, Chem. Soc. Rev. 2001, 30, 50. 

Scheme 2.1.4.14 Pd(0)/BPA-catalyzed enantioselective rearrangement of allylic sulfinates.

The Mo-catalyzed transformations shown in Scheme 14 may also be viewed as AROM/RCM processes [24], Furthermore, it is possible that initiation occurs at the terminal olefin, followed by an ARCM involving the cyclic alkene. Regardless of these mechanistic possibilities, the enantioselective rearrangements... 

Scheme 14. Mo-catalyzed enantioselective rearrangement of meso-cyclopentenes to chiral unsaturated pyrans...

The method involves a highly enantioselective rearrangement of an epoxide and a subsequent Ireland-Claisen rearrangement (see Scheme 31). The enolate Claisen rearrangements of [4-7- /4-4-(l-acyloxy-2,4,6-octatrienyl)]tricarbonyl iron complexes... 

The MacMillan group has also shown that cycloaddition reactions (see also Chapter 8) can be performed highly diastereo- and enantioselectively. The [3+2]-cycloaddition of nitrones and a,/i-un saturated carbonyl compounds in the presence of 20 mol% of a phenylalanine-derived imidazolidinone acid salt led to products with 99% ee [32]. An example of an enantioselective rearrangement reaction (see also Section 13.6) with 99% ee has been reported by the Fu group [33], who used 2 mol% of a planar chiral DMAP derivative as catalyst. 

Isomerization of epoxides to allylic alcoholsThis rearrangement has been effected with strong bases and various Lewis acids. Enantioselective rearrangement to optically active allylic alcohols can be effected with catalytic amounts of vitamin B, at 25°. Thus cyclopentene oxide rearranges to (R)-2-cyclopentene-l-ol in 65% ee. The rearrangement of the as-2-butene oxide to (R)-3-butene-2-ol in 26% ee is more typical. 

Enantioselective rearrangement of epoxides to allylic alcohols 91TA1. Photochemical reactions of glycidyl esters 92MI11. 

Lithium amide deprotonation of epoxides is a convenient method for the preparation of allylic alcohols. Since the first deprotonation of an epoxide by a lithium amide performed by Cope and coworkers in 19585, this area has received much attention. The first asymmetric deprotonation was demonstrated by Whitesell and Felman in 19806. They enantioselectively rearranged me.vo-cpoxidcs to allylic alcohols for example, cyclohexene oxide 1 was reacted with chiral bases, e.g. (S,S) 3, in refluxing TFIF to yield optically active (/ )-2-cyclohexenol ((/ )-2) in 36% ee (Scheme 1). 

Table 10 The effect of adding THF to enantioselective rearrangement reactions catalyzed by (97i)...

Rearrangements. By applying Corey s diazaborolidine catalyst to aromatic Claisen rearrangement, a chiral side chain is introduced into the ortho position of catechols. Highly enantioselective rearrangement of 0-allylimidates 111 containing a methoxybinaphthyl substituent has been witnessed. ... 

D. M. Hodgson, R. Wisedale, Enantioselective rearrangement of exo-norbornene oxide to nortricyclanol, Tetrahedron Asymm. 7 (1996) 1275. 

Catalytic enantioselective rearrangements and cycloadditions involving ylides from diazo compounds 01CSR50. 

Asami, M., Ishizuka, T. and Inoue, S. (1994) Catal3ftic enantioselective deprotonation of mejo-epoxides by the use of chiral lithium amide. Tetrahedron.Asymmetry, 5, 793-796 Seki, A. and Asami, M. (2002) Catalytic enantioselective rearrangement of mejo-epoxides mediated by chiral lithium amides in the presence of excess cross-linked polymer-bound hthium amides. Tetrahedron, 58, 4655 663. 

On this topic, several outstanding contributions were reported by Fu and coworkers,in which new asymmetric nucleophilic catalysts based on chiral ferrocene-type heterocycles were designed. To this end the planar-chiral PPY ferrocene complex (PPY = 4-(pyrrolidino)-pyridine (3.61) was prepared and resolved. Complex 3.61 catalysed the enantioselective rearrangement of A-acylated Azlactones to give C-acylated isomers with high yields and ee of 82-90%. The powerful effect of the chiral ferrocene scaffold was clearly evident if compared to the same reaction with the organic catalyst DMAP reported in 1970 by Steglich and Hofle where only racemic compounds were formed (Scheme 3.26). 

In general, such an enantioselective rearrangement involves two possible enantio-determining steps, namely the lithiation step and the bond-rearrangement step. Also, a dynamic equilibrium may or may not exist between the two... 

---