Transform enantioselective

Other physical methods are also useful for asymmetric transformation. Enantioselective photochemical sensitization has been noted in stereo-... 

Sulfamate indan-2-yl ester 145 is oxidized by iodobenzene diacetate to give condensed 1,2,3-oxathiazole di-A-oxides 146 (Equation 35). Various rhodium <2001JA6935, 2004HCA1607>, manganese(m) Schiff base <2005TL5403>, and ruthenium porphyrin <2002AGE3465> catalysts can be used for this transformation. Enantioselective intramolecular amidation is achieved with good yields. 

However, the results should be taken as an exception, since in our understanding of enantioface differentiating transformations enantioselectivity is obtained by discrimination of one of the two enantiofaces of a molecule usually due to steric hindrance. Since under high pressure the sterically more crowded transition structure is preferred, a decrease in enantioselectivity should be observed under high pressure. This indeed was found for the intermolecular all-carbon Diels-Alder reaction of 2-methyl-1,3-butadiene to a crotonic acid derivative [552], A divergent result may be obtained if the differentiation is caused by electrostatic reasons. 

In seeking ways to capitalize on this particular advantage that photochemistry enjoys over ground-state chemistry, it would be desirable to be able to carry out the A — B transformation enantioselectively. Aside from the intellectual challenge posed by such a problem, the preparation of theoretically interesting, highly strained compounds in optically pure form could be of considerable interest in subsequent mechanistic studies of the chemical behavior of such species as well as in their use as synthons in total syntheses. The present volume, as well as a number of recent review articles and symposia [1], attest to the growing interest in the field of photochemical asymmetric synthesis. 

Takemoto and colleagues demonstrated that bifunctional ammonium-thiourea catalyst 110, prepared in situ by treatment of a chiral thiourea precatalyst with a strong Br0nsted acid, is also able to effect the same sequential transformation enantioselectively (Scheme 42.40). With ee-values of up to 69%, however, the enantiodiscrimination remained comparatively modest [90]. 

On the other hand, this lack of synthetic application clearly points out that further improvements are required to achieve enantioselective proton transfer with a broad substrate scope and high enantiomeric excess. The development of such kinds of catalytic system remains as a holy grail, and the synthetic chemist will probably continue to pay attention and focus his or her creativity. Indeed, in flie future, to address these main issues, organic chemists are expected to develop new creative and innovative processes to transform enantioselective protonation from an emerging tool into a major method toward the formation of tertiary chiral carbon centers. 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

To the best of our knowledge the data in Table 3.2 constitute the first example of enantio selectivity in a chiral Lewis-acid catalysed organic transformation in aqueous solution. Note that for the majority of enantioselective Lewis-acid catalysed reactions, all traces of water have to be removed from the... 

Of all the work described in this thesis, this discovery is probably the most significant. Given the fact that the arene - arene interactions underlying the observed enantioselectivity of ftie Diels-Alder reactions described in Chapter 3 are also encountered in other organic reactions, we infer that, in the near future, the beneficial influence of water on enantioselectivity can also be extended to these transformations. Moreover, the fact that water can now be used as a solvent for enantioselective Lewis-add catalysed reactions facilitates mechanistic studies of these processes, because the number of equilibria that need to be considered is reduced Furthermore, knowledge and techniques from aqueous coordination chemistry can now be used directly in enantioselective catalysis. 

The wM-diacetate 363 can be transformed into either enantiomer of the 4-substituted 2-cyclohexen-l-ol 364 via the enzymatic hydrolysis. By changing the relative reactivity of the allylic leaving groups (acetate and the more reactive carbonate), either enantiomer of 4-substituted cyclohexenyl acetate is accessible by choice. Then the enantioselective synthesis of (7 )- and (S)-5-substituted 1,3-cyclohexadienes 365 and 367 can be achieved. The Pd(II)-cat-alyzed acetoxylactonization of the diene acids affords the lactones 366 and 368 of different stereochemistry[310]. The tropane alkaloid skeletons 370 and 371 have been constructed based on this chemoselective Pd-catalyzed reactions of 6-benzyloxy-l,3-cycloheptadiene (369)[311]. 

Perhaps the biggest impact on the practical utilization of enzymes has been the development of nonaqueous enzymology (11,16,33,35). The use of enzymes in nonaqueous media gready expands the scope of suitable transformations, simplifies thek use, and enhances stabiUty. It also provides an easy means of regulation of the substrate specificity and regio- and enantioselectivity of enzymes by changing the reaction medium. 

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. 

Enantioselective processes involving chiral catalysts or reagents can provide sufficient spatial bias and transition state organization to obviate the need for control by substrate stereochemistry. Since such reactions do not require substrate spatial control, the corresponding transforms are easier to apply antithetically. The stereochemical information in the retron is used to determine which of the enantiomeric catalysts or reagents are appropriate and the transform is finally evaluated for chemical feasibility. Of course, such transforms are powerful because of their predictability and effectiveness in removing stereocenters from a target. 

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. 

Reduce stereochemical complexity and molecular size concurrently by applying diastereoselective or enantioselective transforms which are also disconnective. 

Diisocyanoadociane, a novel marine-derived diterpenoid, was analyzed retrosynthetically using the intramolecular Diels-Alder transform as T-goal concurrently with topological and stereochemical guidance. The enantioselective synthesis outlined below allowed assignment of absolute configuration. 

Interestingly, phase-transfer catalysts including crown ethers have been used to promote enantioselective variations of Darzens condensation. Toke and coworkers showed that the novel 15-crown-5 catalyst derived from d-glucose 33 could promote the condensation between acetyl chloride 31 and benzaldehyde to give the epoxide in 49% yield and 71% A modified cinchoninium bromide was shown to act as an effective phase transfer catalyst for the transformation as well. ... 

Enantioselective synthesis and transformations of oxirane and aziridine derivatives 99PAC423. 

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