Hemiketal structure

Note. The term potential aldehydic carbonyl group refers to the hemiacetal group arising from ring closure. Likewise, the term potential ketonic caibonyl group refers to the hemiketal structure (see 2-Carb-5). 

The aldehyde or ketone group of monosaccharides can undergo an intramolecular reaction with one of its own hydroxyl groups to form a cyclic, hemiacetal, or hemiketal structure, respectively (Figure 1.26). In aqueous solutions, this cyclic structure actually predominates. The open-chain aldehyde or ketone form of monosaccharides is in equilibrium with the cyclic form, but the open structure exists less than 0.5 percent of the time in aqueous environments. It is the... 

Discussion of Structural Studies. Collected data from the NMR spectra are shown in Table II. l-DHA and D-isoDHA have similar chemical shifts, suggesting a similar structure. They both show an assigned shift for C6 that is further downfield than for the C6 of the respective starting materials. One would expect the C6 to be shifted downfield if the compound is in the hemiketal form. This fact, with the information from the preceding proton experiments, supports a proposed hemiketal structure. 

Problems associated with construction of C(8), C(ll) bridged hemiketal structural array, which is common to numerous naturally occurring quassinoids. To follow is described the synthesis of pentacyclic alcohol (243) which features a protocol for elaboration of ring C functionality found in chaparrinone and related quassinoids A facile five-step sequence commencing with picrasane derivative (237) has been developed for elaboration of the sensitive ring C hemiketal unit chaparrinone (226) (c.f. pentacyclic alcohol 243) [118],... 

The synthesis of glaucarubolone described above commences with the tetracyclic alcohol (319), which we had prepared in conjunction with a synthesis of chaparrinone (226). The transformation of (319) into glaucarubolone requires (a) incorporation of a 2-oxo-A3,4 olefin unit into ring A, (b) elaboration of the C(8), C(ll) bridged hemiketal structural array in ring C, and (c) introduction of a C(15) p-hydroxyl group into the eventual ring D 8-lactone. 

Problem 18.15. What is the distinguishing feature of the hemiketal structure in fructose ... 

Ans. The cyclic hemiacetal and hemiketal structures have no carbonyl function but are in equilibrium with the open-chain carbonyl structure. As the small amount of the open-chain carbonyl structure reacts, some of the a- and /3-anomers undergo ring-opening to replenish the equilibrium concentration of the open-chain structure. The newly formed open-chain structure undergoes reaction. More a- and -anomers open up. There is a continuous shifting of a- and /3-anomers to the open-chain carbonyl structure until all of the monosaccharide initially present has reacted. 

The OH unit derived from the carbonyl group, formed by cycliza-tion to a furanose or a pyranose, is attached to the so-called anomeric carbon. An aldose will cyclize to form a stable hemi-acetal structure a furanose or a pyranose. A ketose will cyclize to form a stable hemiketal structure a furanose or a pyranose. The Haworth projection is an older representation for carbohydrates that is based on a planar pyranose or furanose ring. Cyclization of an aldose or a ketose to a furanose or pyranose is accompanied by mutarotation at the anomeric carbon. Mutarotation is the change in optical rotation of a pure furanose or pyranose derivative to that resulting from a mixture of isomers at the anomeric carbon. 

For germine derivatives having a group other than hydroxyl at C-7, ample evidence supported a 4,9-hemiketal structure. However, an unequivocal choice between the 4,9-hemiketal and 4,7-hemiketal alternative structures was impossible for those compounds which possess... 

---