Conformational memory

Rychnovsky et al. considered the formation of achiral conformers from chiral molecules and trapping the prochiral radical with a hydrogen atom donor based on memory of chirality (Scheme 12) [41], The photo-decarboxylation of optically active tetrahydropyran 40 leads to an intermediate 43, which now does not contain a stereocenter. If the intermediate 43 can be trapped by some hydrogen atom source before ring inversion takes place, then an optically active product 41 will be formed. This is an example of conformational memory effect in a radical reaction. It was reported that the radical inversion barrier is low (< 0.5 kcal/mol) while the energy for chair flip 43 44 is higher (5 to... 

Guamieri, F. and Weinstein, H. (1996). Conformational memories and the exploration of biologically relevant peptide conformations An illustration for... 

If the conformational memories can be used to understand the differences in the conformations and bioactivity of known analogs, perhaps it could also be used to predict the activity of unknown analogs. To test this hypothesis, we ran Anneal-Hex on a new compound vitamin D-23-oxy ketone 5. The conformational memories of this compound are... 

Barnett-Norris J, Guarnieri F, Hurst DP, Reggio PH (1998) Exploration of biologically relevant conformations of anandamide, 2-arachidonylglycerol, and their analogues using conformational memories. J Med Chem 41 4861-4872... 

Barnett-Norris J, Hurst DP, Lynch DL, Guarnieri F, Makriyannis A, Reggio PH (2002) Conformational memories and the endocannabinoid binding site at the cannabinoid CBI receptor. J Med Chem 45 3649-3659... 

Griesbeck, A. G., Mauder, H., Stadtmuller, S., Intersystem Crossing in Triplet 1,4 Biradicals Conformational Memory Effects on the Stereoselectivity of Photocycloaddition Reactions, Acc. Chem. Res. 1994, 27, 70 75. 

Decarboxylation and Decarbonylation. - A study has demonstrated that conformational memory plays a major part in the photochemical dissociation of formic acid. The results of a photophysical study of the photochemical decomposition of formic acid in the vacuum-UV have been reported as has the photochemical decomposition of formic acid using 212.8 nm irradiation. The equilibrium geometries of iV,iV-dimethylformamide in the singlet and triplet excited states have been calculated. ... 

Conformational memory in the reductive decyanation of optically active cyanohydrins [31] is shown in Scheme 21. In order to obtain optically active products by this process, reduction of the initially-formed radical intermediate must be faster than any other racemizing process such as ring inversion. Typically in these reductions, the products possess only modest enantiomeric excesses (entries 1 and 2), but if high concentrations of lithium in ammonia are used, then the reduced products can be obtained in up to 90% ee (entry 3). 

Conformational memory is also observed in Barton radical decarboxylations of optically active tetrahydropyrans [31]. Photolysis of thiohydroxamate esters derived from optically pure tetrahydropyrans in the presence of various hydrogen atom... 

Scheme 23. Conformational memory in intramolecular cyclizations to make spiro compounds...

Conformational memory has also been demonstrated in intramolecular cyclizations to form spiro compounds [32]. In the event, treatment of an optically active cyanohydrin with LiDBB in THF at -78 °C for 10 min alfords a high yield of a single diastereomeric c -spiroether in 42% ee (Scheme 23). The cyclization proceeds most likely via an equatorial a-alkoxylithium, which arises from ring inversion of the kinetic axial alkyllithium intermediate [34]. 

Additionally, the torsional minima for the rotatable bonds in DHA are very broad. Results of the biased sampling phase from conformational memories (CM) calculations of AA are consistent with all of these results (Bamett-Norris et al., 1998), as they reveal a relatively broad distribution of populated torsional space about the classic skew angles of -119°(s ) and 119°(s) for the... 

FIGURE 2.4 Conformational memories (CM) results for a series of fatty acid ethanolamides [22 4,... 

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