Diastereomeric protons

The detection of chiral recognition with a mass spectrometer was reported first in 1977 by Fales and Wright. Their study showed that the chirality of dialkytartrates (T) strongly influences the stability of their diastereomeric proton-bound dimers,... 

Table 7 Relative abundances of diastereomeric proton-bound complexes from Cl, FAB, and API mass spectrometry... 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

Fluoride ion catalyzed reaction of the cyclic nitronate 1 with benzaldehyde provides a cyclic hcmiaeetal 2 in 95% diastereoselectivity18 with tra ,v,tr u.v-relationship of the protons in positions 2, 3 and 4. The relative configuration of the hemiacetal carbon position was not assigned. Upon reaction of the diastereomeric nitronate 3 at 25 °C the 4a-stereocenter is epimerized by fluoride ion such that hemiacetal 2 is once again obtained. In contrast, reaction of 3 at 0°C furnishes the diastereomeric product 4 in 23% yield 8. 

The addition of the anions of racemic cyclic allylic sulfoxides to various substituted 2-cyclopentenones gives y-l,4-adducts as single diastereomeric products22. The modest yields were due to competing proton-transfer reactions between the anion and enone. The stereochemical sense of these reactions is identical to that for the 1,4-addition reaction of (Z)-l-(/erf-butylsulfinyl)-2-methyl-2-butene to 2-cyclopentenone described earlier. 

Thus, the enantiomeric contents in a pair of sulphoxides can be determined by the NMR chemical shifts in the methine or methylene protons in the two diastereomeric complexes which are stabilized by the hydrogen bond between the hydroxyl and the sulphinyl groups147-151 (Scheme 13). Similarly, the enantiomeric purity and absolute configurations of chiral sulphinate ester can be determined by measuring the H NMR shifts in the presence of the optically active alcohols152. 

The diastereomeric ratio was determined by the NMR integral ratio of the methine protons adjacent to the hydroxyl group (3.33 ppm for the (//-isomer 3.44 ppm for the waw-isomer). 

The key step in achieving catalytic turnover is protonation of the titanium—oxygen and titanium—carbon bonds, which is readily achieved by employing collidine hydrochloride as a protic acid. An interesting feature of the cyclization shown above is its diastereocon-vergent nature. From a diastereomeric mixture of the epoxides, the cyclization product is obtained as essentially a single isomer. Unfortunately, this is not always the case, as shown in Table 12.1 [36],... 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

A more recent application of this chemistry was reported by Oestreich and Hoppe [74] and involved the enantioselective deprotonation of the enyne carbamate ester 125 with sec-butyllithium in the presence of (-)-sparteine (Scheme 2.41). Removal of the pro-S hydrogen atom led to the corresponding organolithium intermediate, which then underwent a highly enantioselective intramolecular 1,4-addition to the enyne. Protonation of the resulting allenyllithium species 126 provided a 70 30 mixture of the two diastereomeric allenes 127. 

In complexes of crown ethers [303] to [305] (types IV and V) both large groups are located in the larger of the two non-equivalent chiral cavities (Kyba et al., 1978). This was concluded from H nmr spectra of the diastereomeric complexes of [303] with a-phenylethylammonium salts. In the spectra the position of the methyl protons in both diastereomeric complexes is the same, in contrast to the methyl protons in complexes of [284] or [285] with the same salt. In the latter the upheld shift of the methyl protons in the more stable SS-(R) enantiomeric complex differs from that in the less stable SS-(S) enantiomer. Taking into account the available data summarized above, the tentative conclusion seems to be that the simple steric model (see structure... 

Protonated imines are effective dienophiles. Thus in the reaction of methyl glyoxylate with the hydrochloride 127 of alanine methyl ester in the presence of cyclopentadiene, a mixture of hydrochlorides of the exo- and ewrfo-adducts 128-131 was formed (equation 69). The diastereomeric ratio of the exo-compounds was 83 1761. 

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