Anti configuration

For the stable conformers 13a-c of a substituted ethane the vicinal HH coupling constants 3Hz for syn-protons and 15 Hz for anti-protons can be derived from Fig. 2.18. If there is rotation around the C-C single bond, the coupling protons pass through the syn configuration twice and the anti configuration once. 

Analogous addition to benzaldehyde afforded a 80 20 mixture of diastereomers 37 A. However, both diastereomers have an anti configuration between C-2 and C-3. The stereochemical difference between the two diastereomers is located at C-l. X-ray crystallography shows that the dia-stereomer bearing a / -configurated hydroxy group is predominant. Therefore, the diastereose-lectivity with respect to C-2 and C-3 in 37 A and C-l and C-2 in 36 is virtually complete in both cases. 

Provided that the silanolate elimination proceeds with anti selectivity, it must be concluded, that the intermediate homoallylic alcohol has an anti configuration, and thus the reagent has an ( -configuration. Acidic hydrolysis of the enol ether leads to enones the overall sequence consists of a nucleophilic acroylation. This has also been applied in the total synthesis of the marine diterpene ( )-aplysin-2067. 

The kinetic stereoselection of this process is rationalized in terms of the formation of Schiff bases with anti configuration. 

Consecutive Michael additions and alkylations can also be used for the diastereoselective synthesis of 5- and 6-membered ring systems. For instance when 6-iodo-2-hexenoates or 7-iodo-2-heptenoates are employed the enolate of the Michael adduct is stereoselectively quenched in situ to provide the cyclic compound with trans stereochemistry (>94 6 diastereomeric ratio). As the enolate geometry of the Michael donor can be controlled, high stereoselectivity can also be reached towards either the syn or anti configuration at the exocyclic... 

All the asymmetric thiirane oxides which have been obtained through this procedure are mixtures of the two possible cis- and trans- syn- and anti-) configurations, but the antiisomer predominates. 

This crisscross or von Halban-White-type cyclization product is formed from the (E)-configured intermediate 87, which cannot undergo the 67r-electrocy-clization like the (Z)-configured isomer 88, to yield the benzannelation product 86 [78,79]. While the diastereoselectivity of the alkyne insertion must have been controlled by the electronic and not the steric factors of the substituents on the alkyne, the anti-configuration of the tricyclic system 85 was confirmed by an X-ray structure analysis [77]. 

Fig. 3.16 Catalysts 43 and 44 with syn and anti configurations of the iV-heterocyclic backbone...

Thiirane oxides (3 x = 1) were rather rare and not well characterized until about 20 years ago °. Since 1965 synthetic methods for their preparation have been consistently and systematically explored . They are rather thermodynamically stable compounds— compared to their closely-related thiirane dioxides—provided they have an anti-configuration with respect to the substituents and the sulfinyl oxygen. Also they are more resistant than the corresponding sulfones toward ring opening by either nucleophiles or electrophiles. 

The silyl ketene acetal rearrangement can also be carried out by reaction of the ester with a silyl triflate and tertiary amine, without formation of the ester enolate. Optimum results are obtained with bulky silyl triflates and amines, e.g., f-butyldimethylsilyl triflate and (V-methyl-Af, /V-dicyclohcxylaminc. Under these conditions the reaction is stereoselective for the Z-silyl ketene acetal and the stereochemistry of the allylic double bond determines the syn or anti configuration of the product.243... 

This synthesis is shown in Scheme 13.59. Two enantiomerically pure starting materials were brought together by a Wittig reaction in Step C. The aldol addition in Step D was diastereoselective for the anti configuration, but gave a 1 1 mixture with the 6S, 1R-diastereomer. The stereoisomers were separated after Step E-2. The macrolactonization (Step E-4) was accomplished by a mixed anhydride (see Section 3.4.1). The final epoxidation was done using 3-methyl-3-trifluoromethyl dioxirane. 

With two y,8 double bonds, two a,/3 double bonds, and the possibilities of cis and trans ring fusions with syn and anti configurations, 20 isomeric dimers are possible. Surprisingly, only one product is formed in a head-to-tail fashion. The sole product of the irradiation of the 3,5-diene-7-ketosteroid (76), however, is the head-to-head dimer. The specificity and mode of addition arise presumably through the effect of the specific environment of the chromaphore. The dimerization of (75) is believed to involve the addition of the a,fi double bond of a photoexcited molecule to the less hindered y,8 double bond of a ground state molecule. The photocondensation of (76) with cyclopentene, in which steric hindrance should not be a controlling factor, was found to yield a cyclobutane product involving the a,/ bond of the steroid in contrast to dimerization across the y,8 bond. 

Before complexes can be used in nuclear medicine, especially for brain function studies, in vivo reactivity has to be imposed upon them. Retention in the brain is necessary for perfusion imaging. This can be achieved by substituents on the ligand that bind them to binding sites in the brain. N-alkylation with methyl and other alkyl groups led to the class of complexes shown in Fig. 19. Upon complexation to technetium, the IV-alkyl substituent can assume a syn or anti configuration with respect to the oxo ligand, as proved by X-ray crystal... 

Recently, Bafquiren and Eddine have studied hindered Schiff base derivatives of fra/ s-l,2-diaminocyclohexane23 The differences in chemical shifts found for imino carbons have indicated the presence of anti- and syn-geometric configurations. The intense NOESY effects between protons of naphthyl and alkyl units have proved the anti-configuration of the major stereoisomer. 

The Diels-Alder reaction of phospholes was not studied extensively. It was known, however, that the cycloadditionoftheMatheyphosphole(46)withA-phenyhnaleimide afforded 7-phosphanorbomene 47 (Scheme 11) [50], Similar reactions with fuma-ronitrile or with another unit of phosphole led to products with the same anti configuration of the bridging P-moiety [51, 52],... 

The interconversion between octadienediyl-Ni11 stereoisomers may involve two different processes namely the syn-anti isomerization and also the enantioface conversion of one or both terminal allylic groups (Fig. 4). The isomerization of the allylic group is connected with two aspects firstly the interconversion of its syn and anti configuration and secondly the inversion of its enantioface.36 On the other hand, the process of enantioface conversion is not accompanied by alternation of the allylic configuration. 

In entries 10-13 (Table 21.8) of trisubstituted alkenes, very high diastereo-selectivity is realized by the use of a cationic rhodium catalyst under high hydrogen pressure, and the 1,3-syn- or 1,3-anti-configuration naturally corresponds to the ( )- or (Z)-geometry of the trisubstituted olefin unit [48, 49]. The facial selectivity is rationalized to be controlled by the A(l,3)-allylic strain at the intermediary complex stage (Scheme 21.2) [48]. 

Assignment of an anti configuration to a [2](2,5)furano[2](l,4)naph-thalenophane (42) synthesized by Wasserman and Keehn 65> followed from a comparison of its 1H—NMR spectrum with that of [2]paracyclo-[2](2,5)furanophane (41 a) 66>. The absorption band assigned to the /3-furanoid proton Ha in the spectrum of 42 (r=4.38) appears in the same region as the corresponding band for 41 a. In the case of syn-42, a chemical shift would be expected due to the transannular shielding effect of the naphthalene nucleus. 

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