Stereogenic centers chiral alkenes

Steam distillation, TEARS assay, 667 l-Stearoyl-2-arachidonoyl-GPC, 737, 738 Stereogenic centers chiral alkenes, 884-5, 1144 dihydrogen tiioxide, 131 Stereoisomerism... 

When the diphosphine is chiral, binding of a prochiral alkene creates diastereomeric catalyst-alkene adducts. (Diastereomers result because binding of a prochiral alkene to a metal center generates a stereogenic center at the site of unsaturation.) Through a powerful combination of3lP and l3C NMR methods, Brown and Chaloner first demonstrated the presence of two diastereomeric catalyst-enamide adducts with bidentate coordination of the substrate to the metal (Figure 1) [19]. 

The introduction of umpoled synthons 177 into aldehydes or prochiral ketones leads to the formation of a new stereogenic center. In contrast to the pendant of a-bromo-a-lithio alkenes, an efficient chiral a-lithiated vinyl ether has not been developed so far. Nevertheless, substantial diastereoselectivity is observed in the addition of lithiated vinyl ethers to several chiral carbonyl compounds, in particular cyclic ketones. In these cases, stereocontrol is exhibited by the chirality of the aldehyde or ketone in the sense of substrate-induced stereoselectivity. This is illustrated by the reaction of 1-methoxy-l-lithio ethene 56 with estrone methyl ether, which is attacked by the nucleophilic carbenoid exclusively from the a-face —the typical stereochemical outcome of the nucleophilic addition to H-ketosteroids . Representative examples of various acyclic and cyclic a-lithiated vinyl ethers, generated by deprotonation, and their reactions with electrophiles are given in Table 6. 

The results of the dioxirane epoxidation of some 3-alkyl-substituted cyclohexenes and of 2-menthene indicate that the diastereoselectivity control is subject to the steric interactions of the dioxirane with the substituents of the substrate, while the size of the dioxirane substituents has only a minimal effect . In the favored transition structure, the alkyl groups of the dioxirane cannot interact effectively with the substituents at the stereogenic center of the chiral alkene . ... 

In many addition reactions of alkenes generating two chiral units, the configuration at the C —C double bond is translated into the relative configuration at the newly created stereogenic centers. Thus, knowledge of the stereochemical course of the addition reaction (s> n or anti)... 

All the above examples share high stereofacial selectivity defined by the configuration of the stereogenic center that connects the enone chromophore with the alkenyl side chain. However, chiral induction at the enone and/or the alkenyl tethered must be introduced to achieve stereofacial selectivity in the more general systems in which the alkene is connected at the a-carbon or /1-carbon of the enone. One of the successful early examples is found in Pirrung s120 synthesis of ( )-isocomene 263. Irradiation of 261 afforded the single product 262, which was transformed to isocomene in a two-step sequence. 

The fadal diastereoselectivity of intermolecular cyclopentenone [2 + 2]-photocy-cloaddition reactions is predictable if the cyclopentenone or a cyclic alkene reaction partner is chiral. Addition occurs from the more accessible side, and good stereocontrol can be expected if the stereogenic center is located at the a-position to the double bond. In their total synthesis of ( )-kelsoene (11), Piers et al. [22] utilized cyclopentenone 9 in the [2 + 2]-photocycloaddition to ethylene (Scheme 6.5). The cyclobutane 10 was obtained as a single diastereoisomer. In a similar fashion, Mehta et al. have frequently employed the fact that an approach to diquinane-type cis-bicydo [3.3.0]octenones occurs from the more accessible convex face. Applications can be found in the syntheses of (+)-kelsoene [23], (—)-sulcatine G [24], and ( )-merri-lactone A [25]. 

The [2 + 2]-photocycloaddition chemistry of a,(3-unsaturated lactones has been widely explored. The factors governing regio- and simple diastereoselectivity are similar to what has been discussed in enone photochemistry (substrate class Al, Section 6.2). The HT product is the predominant product in the reaction with electron-rich alkenes [84]. A stereogenic center in the y-position of ot,P-unsaturated y-lactones (butenolides) can serve as a valuable control element to achieve facial diastereoselectivity [85, 86]. The selectivity is most pronounced if the lactone is substituted in the a- and/or P-position. The readily available chiral 2(5H)-furanones 79 and 82 have been successfully employed in natural product total syntheses (Scheme 6.30). In both cases, the intermediate photocycloaddition product with 1,2-dichloroethylene was reductively converted into a cyclobutene. In the first reaction sequence, the two-step procedure resulted diastereoselectively (d.r. = 88/12) in product 80, which was separated from the minor diastereoisomer (9%). Direct excitation (Hg lamp, quartz) in acetonitrile solution was superior to sensitized irradiation (Hg lamp, Pyrex) in acetone, the former providing the photocycloaddition products in 89% yield, the latter in only 45%. Cyclobutene 80 was further converted into the monoterpenoid pheromone (+)-lineatin (81) [87]. In the second reaction... 

The induced diastereoselectivity in a Paterno-Buchi reaction resulting from a stereogenic center in the alkene part was recently described by Bach and coworkers in the photocycloaddition of chiral silylenol ethers 153 with benzaldehyde (Sch. 53) [151]. The substituents R at the stereogenic center... 

The necessary starting material is an alkene. Addition of H2 to the double bond forms an A -acetyl amino acid with a new stereogenic center on the a carbon to the carboxy group. With proper choice of a chiral catalyst, the naturally occurring S configuration can be obtained as product. 

With suitable substrates, addition of two OH groups creates one new stereo-genic center from a terminal alkene and two new stereogenic centers from internal alkenes. Addition to alkenes of the form RCH=CH2 has been made enantioselective, and addition to RCH=CHR both diastereoselective and enan-tioselective, by using chiral additives or chiral catalysts, such as 173, 174 (derivatives of the... 

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