Acetals conformational analysis

This chapter is divided into two paits. The first, and major, portion is devoted to caibohydrate structure. You will see how the principles of stereochemistry and conformational analysis combine to aid our understanding of this complex subject. The remainder of the chapter describes chemical reactions of caibohydrates. Most of these reactions aie simply extensions of what you have aheady leained concerning alcohols, aldehydes, ketones, and acetals. 

Stereochemistry. - There has been a far infra-red spectroscopic study of ethyl phosphine,130 and in combination with X-ray fluorescence, further work completed on the conformational analysis of dialkylphenyl phosphines.131 The influence of water on the conformational equilibria of trimethyl phosphate has received attention.132 There has been a low temperature solid state and matrix isolation study of methyl phosphorodichi oridate,133 and a conformational study of polymorphic modifications of diphenyl-phosphinyl acetic acid hydrazide. 34... 

The C,C-coupling reactions of six-membered cyclic nitronates were studied in most detail (274, 478). Here silyl ketene acetal was also used as the test nucleophile. The configurations of most of the starting nitronates and the resulting nitroso acetals were determined by NMR spectroscopy and X-ray diffraction, and also a conformational analysis was performed (see Tables 3.24 and 3.25). 

One example of the use of 2D-NMR experiments in conformational analysis is the study of molecular interactions between cinchonidine and acetic acid [26]. These alkaloids are used as chiral auxiliaries in enantioselective hydrogenations, and the enantiomeric excess is dependent on solvent polarity, acetic acid being a good solvent This suggests that protonation and a preferred conformation play a role in achieving high enantioselectivities. With a combination of COSY-experiments, 3J coupling constants and NOESY experiments, it was shown that one conformer is preferred in acidic solutions. 

Like the cWo-derivative, acetals may be formed using a similarly structured exo-annulated MBF-OH43. Conformational analysis can be carried out to determine the absolute configuration of compounds of a similar structure as with the endo-annulated reagent. Differences in chemical shift of the same hydrogens and carbons are similar and reveal that other acetals may be equally useful for enantioselectivity studies. 

Several reviews have already been published on the subject, for example, the acetala-tion of alditols [4], of aldoses and aldosides [5,6], and of ketoses [7]. Some aspects of the stereochemistry of cyclic acetals have been discussed in a review dealing with cyclic derivatives of carbohydrates [8], also in a general article [9] and, more recently, in a chapter of a monograph devoted to the stereochemistry and the conformational analysis of sugars [10], Aspects on predicting reactions patterns of alditol-aldehyde reactions are reviewed within a general series of books on carbohydrates [11]. The formation and migration of cyclic acetals of carbohydrates have also been reviewed [12,13],... 

Conformational analysis of 4-oxo-6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidine-3-acetates and -3-carboxylates 30 (R = H) and their mono-methylated (R = Me, R1 = H) and 6,9-, 7,9-, and 8,9-dimethylated derivatives were carried out by H and l3C NMR spectroscopy (86JOC394). At ambient temperature the 6-methyl derivatives predominantly adopt the energetically most favorable half-chair conformation with a pseudoaxial methyl group. In the other half-chair conformation a serious 1-3 allylic strain exists between the pseudoequatorial methyl group and the adjacent carbonyl group. At the 7- and 8-methyl derivatives the half-chair conformations with equatorial methyl group occur almost exclusively, but the 9-... 

Conformational analysis explains why the relatively strained 2,6-dioxa-bicyclo[3.2.1]octane skeleton 98 of 3,6-anhydro-D-gluco- and -D-manno-pyranose tends to recyclize rapidly in the presence of methanolic hydrogen chloride into the less-strained 2,6-dioxabicyclo[3.3.0]octane skeleton 99 of the 3,6-anhydrohexofuranose. Obviously, such rearrangement cannot take place with the galacto or talo configurations, and consequently, acyclic acetals are formed.390,391... 

Conformation of CH, 1 CHCfjHg HN CH(CH2)2CeHg ch3 Stereoisomer Analysis of Crude Dilevalol Acetate Stereoisomer Analysis of Dilevalol DBTA Salt Yield of Dilevalol DBTA Salt Based on Starting RR Secondary Amine... 

A tertiary carbonium ion, generated at C(2o> hy heating the tertiary alcohol 19) with iodine or an acid in acetic acid, rearranged to give the i7,i7a-dimethyl-A -olefin(2o) [22 /]. A conformational analysis of the possible transition states suggests that migration of the C(i3)-"C(i7> bond is likely to... 

Epoxycyclohexane, conformational analysis, 11 Epoxy-derivatives of enol acetates, rearrangement, 367-388, 445... 

A conformational analysis of various substituted A-homo-steroids is reported. The preferred conformation of the amide group in some acetamido-substituted steroids is of the type (5), with anti-periplanar orientation of C—H and N—H bonds the eclipsing of the C—H and C=0 bonds is similar to that accepted for acetates of secondary alcohols. 

The special electronic structure of the anomeric center in pyranoid derivatives results in experimentally significant differences in molecular geometry (e.g., as reflected by bond lengths and valence angles) about the anomeric carbon atom between a- and 0-pyranoses and a- and /3-pyranosides. In the article by Jeffrey, a discussion of the structural properties of the anomeric center in pyranoses and pyranosides is given, and a comparison is made of crystallographic data of some carbohydrates with the results of theoretical calculations performed on model compounds. Paulsen and co-workers also present x-ray crystallographic data in their discussion of some aspects of the conformational analysis of pentopyranosyl acetates, benzoates, and halides, in comparison with extensive conformational data compiled by Durette and Horton for these compounds in solution. 

Aspects of Conformational Analysis of Pentopyranosyl Acetates, Benzoates, and Halides... 

N.m.r. spectroscopy has become an indispensable tool in the conformational analysis of acetals. This development is largely attributable to the fact that the spectra can be assigned, and analyzed, on a first-order basis. A well resolved spectrum is a necessary prerequisite for any conformational analysis. In addition to the established aids for analyzing complex spectra, the use of lanthanide shift-reagents now provides an often simpler approach to this problem. 

Ahyd H was measured n acetic acid solution. No experimental uncertainty was given. Results were applied to a conformational analysis of the 1,1,4,4-tetramethylcyclodecane system. 

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