Vinyl enantiotopic

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

It should be noted that the vinyl and methyl proton resonances of the Z-isomer [i.e., of the achiral tetracarbonylirondimethyl (Z)-butenedioate complex] are also discriminated in the presence of Eutbfc), because the internally enantiotopic nuclei are rendered diastereotopic in the presence of the nonracemic LSR81. The vinyl protons of the. E-isomer show two lines (external diastereotopism) while the vinyl protons of the Z-isomer exhibit an AB-system (internal diastereotopism)5. 

Tetronates derived from l,3-divinyl-2-cyclopentanol were employed to study the possibility of a differentiation of enantiotopic or diastereotopic double bonds in their [2 + 2]-photocydoaddition [140]. It was found that tetronate 148 underwent a selective [2 + 2]-photocydoaddition (r.r. = 75/25) at one of the two possible double bonds to deliver product 149 in 67% yield (Scheme 6.52). The reaction was analyzed regarding the preferred conformations of the cyclopentanol, with the notion that the tetronate resides in a pseudoequatorial position, and the vinyl group in a pseudoaxial position of the envelope conformation. Intermediate 149 served as starting material for the first total synthesis of the tetracydic sesquiterpene (T)-punctaporonin C (150) [141]. 

The famous ligand BINAP controls an intramolecular Heck reaction to give decalin derivatives with good enantiomeric excess. BINAP is the optically pure phosphine built into the palladium catalyst. The presence of silver ions accelerates the reaction as well as preventing double bond isomerization in the original substrate. This time the chiral ligand selects which double bond is to take part in the reaction. The vinyl palladium species is tethered to the alkene and can reach only the same face. The faces of the alkenes are diastereotopic but the two alkenes are enantiotopic and you must know your right from your left to choose one rather than the other. 

The starting material is not chiral and chirality first arises in the Heck reaction when the vinyl Pd complex can attack one of two enantiotopic alkenes in the diene. In the presence of catalytic BINAP, good asymmetric induction 237 can be achieved. 

Enantiotopic polymerization of a prochiral vinyl monomer leads to either isotactic or syndiotactic polymer. Stereocontrol of the polymerization is a main issue of the polymer chemistry. Contrary to the situation for the vinyl monomers, a masked disilene such as 1 is chiral. Monomers are obtained as a racemic mixture, which can be separated into each enatiomer by using a chiral column on liquid chromatography. Herein we report the first highly enatioselective polymerization of the racemic masked monomer. The stereochemical course of the propagation step in the anionic polymerization of 1 should be extremely interesting. 

Another reaction that has been successfully employed as an enantioselective desymmetrization processes is the Heck reaction. The Heck reaction, which is described in more detail in Chapter 19, couples an aryl or vinyl electrophile with an olefin. As shown in Equation 14.19, this reaction can be run as a desymmetrization in which the catalyst reacts preferentially at one of the enantiotopic olefins over the other to generate the intermediate enol that undergoes isomerization to the ketone product. This ketone as formed in 76% yield and 86% enantioselectivity and was an intermediate in the synthesis of vemolepin. ... 

The enantioselectivity of [Cu(S,S)-tBu-box](OTf)2 (13b)Claisen rearrangement is explained as follows (Fig. 2.4). The alkoxycarbonyl and ether oxygens coordinate in a bidentate fashion to the Cu (box) complexes. The square planer geometry around the copper(II) cation has been proposed and a chair-hke transition-state model is suggested. The aUyUc ether moiety should approach the vinyl ether moiety from the opposite direction of the t-Bu substituents on the box-hgand. The Cu"(box) catalyst differentiates between two enantiomeric chair-hke transition state by selective coordination of enantiotopic lone pairs on oxygen to form (S,S,pro-S)-14a. 

The Claisen rearrangement is a suprafacial, concerted, nonsynchronous pericyclic process that is considered occasionally as an intramolecular 8, 2 alkylation (Eq. 3.1.18) [2]. When the Claisen rearrangement can produce enantiotopic faces at both terminals of allyl vinyl ether, the rearrangement can proceed through two pairs of chiral transition states to prepare two racemic diastereomers bearing newly created stereocenters of the products 15 and 16 (Eq. 3.1.19). Achiral allyl vinyl ether 14 can provide two enantiomeric chair-like transition states chair 1 and 2, both of which lead to the racemic, diastereomeric aldehyde 15. Similarly, the enantiomeric boat-like transition states boat 1 and 2 provide racemic diastereomer 16. The two transition states are essentially different in energy and the ratio 15/16 reflects the transition state geometry. 

The high enantioselectivity is ascribed to preferential ion-pairing on one enantiotopic face of the enolate of (77) with the chiral quaternary ammonium ion of the catalyst. The remaining steps consist of acid hydrolysis of the vinyl chloride and cyclisation to (81) (the Wichterle reaction) and installation of the acetic acid side-chain. It is likely that this asymmetric synthesis is actually used for large-scale production of the product. 

According to the (4,4) CASSCF/6-31G calculations, the nitrene 29a in the A" state can close to the azirine without any barrier and this state was found to be the transition state for interchange of the enantiotopic pair of hydrogens in 2H-azirine (27a). ° Therefore, if a barrier does exist, it is probably very small. This conclusion, based on the results of calculations, is wholly consistent with the fact noted above, that the triplet and singlet vinyl nitrenes have escaped detection. However, further experimental studies, using very fast laser flash photolysis techniques, along with higher level ab initio calculations are certainly warranted. 

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