Epimerization structure

Since the carbonyl group is planar and may be attacked from either side, two epimeric structures (anomers) are possible in each case, and in solution the two forms are frequently in equilibrium, because hemiacetal or hemiketal formation is reversible (see Section 7.2). The two anomers are designated a or P by comparison of the chiralities at the anomeric centre and at the highest numbered chiral centre. If these are the same (RS convention), the anomer is termed P, or a if they are different. In... 

A number of model esters related to lac resin have been prepared. These esters have the basic cedrane skeleton (149) or the C-2 epimeric structure. In an approach to the synthesis of cedrene (152), the intramolecular photocycloaddition of the dienone (150) was carried out at low temperature. A major tricyclic compound (151) was obtained and its structure was verified by X-ray analysis of its anisylidene derivative. Further work is required to bring about the rearrangement of a derivative of (151) to give cedrene. Further details on the hydroxylation (with O3) and chlorination (with PhICl2) of cedrol, patchoulol, and their derivatives have been presented. The results of this study are in keeping with those obtained previously. 

Because oxidation of D-erythrose gives an optically inactive aldaric acid, erythrose must correspond to structure I. Structure I would give /nc50-tartaric acid when it is oxidized to an aldaric acid. D-Arabinose must be one of the two epimeric structures that would degrade to this structure for D-erythrose. 

The El spectra of steroids provided an early example of the utility of mass spectrometry in structural elucidation. The CAD spectra (obtained by tandem MS/MS techniques) of various steroids have now been investigated and yield additional structural information (38—41). Items 1,2 and 4 in the positive features of structural identification listed above provided information which could be used to distinguish between epimeric structures and to identify constant structural features in different steroids. 

Most dienones that have been reduced have structures such that they cannot give epimeric products. However, reduction of 17 -hydroxy-7,17a-dimethyl-androsta-4,6-dien-3-one (63) affords 17 -hydroxy-7j9,17a-dimethylandrost-4-en-3-one (64), the thermodynamically most stable product, albeit in only 16% yield. The remainder of the reduction product was not identified. Presumably the same stereoelectronic factors that control protonation of the / -carbon of the allyl carbanion formed from an enone control the stereochemistry of the protonation of the (5-carbon of the dienyl carbanion formed from a linear dienone. The formation of the 7 -methyl compound from compound (63) would be expected on this basis. 

Steroids possessing an epoxide grouping in the side chain have likewise been converted to fluorohydrins. Thus, 20-cyano-17,20-epoxides of structure (19) furnish the 17a-fluoro-20-ketones (20) after treatment of the intermediate cyanohydrins with boiling collidine. The epimeric 5a,6a 20,21-oxides (21) afford the expected bis-fluorohydrins (22). The reaction of the allylic... 

Reaction of the cisoid methoxymethylene ketone (36) with decomposing sodium chlorodifluoroacetate yields two epimeric adducts in 42 and 28 % yield, respectively, to which structure (37) has been assigned on the basis of NMR... 

The alkynylation of estrone methyl ether with the lithium, sodium and potassium derivatives of propargyl alcohol, 3-butyn-l-ol, and propargyl aldehyde diethyl acetal in pyridine and dioxane has been studied by Miller. Every combination of alkali metal and alkyne tried, but one, gives the 17a-alkylated products (65a), (65c) and (65d). The exception is alkynylation with the potassium derivative of propargyl aldehyde diethyl acetal in pyridine at room temperature, which produces a mixture of epimeric 17-(3, 3 -diethoxy-T-propynyl) derivatives. The rate of alkynylation of estrone methyl ether depends on the structure of the alkyne and proceeds in the order propar-gylaldehyde diethyl acetal > 3-butyn-l-ol > propargyl alcohol. The reactivity of the alkali metal salts is in the order potassium > sodium > lithium. 

Kurath described the conversion of 3a,17y5-diacetoxy-5)9-androstan-12-one (partial structure 78) to diketone (79) by bromination of (78) and hydrolysis of the C-11 epimeric bromo ketones... 

It was noted (23) that the NMR spectrum of compound 32 was identical with that published (34) for the third product (assigned structure 29) isolated from the reaction of tetra-O-acetyl-2-hydroxy-D-glucal with acetic anhydride and zinc chloride. The identity of the compounds was fully established and a revised structure proposed for this third product. In the presence of zinc chloride, therefore, epimerization can occur at an allylic site and the quasi-equatorial C-4 acetoxy group in the erythro isomers 27 and 28 can assume the favored quasi-axial orientation (24). 

After 12 hours at 4 kbar. this reaction provided only 35% of a 63 27 mixture of 22 and a compound which was tentatively assigned structure 23. It is assumed that 23 derives from epimerization of 21 prior to reaction with (aS,S,S)-5l0b. Whether this stereochemical assignment is correct or not, this result shows that 5 may have problems with configurationally labile aldehydes in demanding cases of mismatched double diastereosclcction. For further examples of double asymmetric induction with 5 or related reagents, see refs 31, 34 and 47. 

Sodium borohydride reduction of aknadinine gave a pair of epimeric alcohols, one of which was found to be identical to natural epihernandolinol and the other identical to the known alkaloid hemandolinol (10) (28). As the structure of hemandolinol (10) had been proposed without resolution of the stereochemistry (28), the stereochemistry of epihernandolinol (9) was not definitely established (27). 

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