Ketose, cyclization

Ketoses are characterized by the ending ulose m their name Most nat urally occurring ketoses have their carbonyl group located at C 2 Like aldoses ketoses cyclize to hemiacetals and exist as furanose or pyranose forms... 

OZTs from aldoses and ketoses. The second and the oldest-as well as the less studied—method is based on the condensation of O-unprotected sugars with thiocyanic acid, generated in situ from potassium thiocyanate and a protic acid. The reaction involves the free anomeric position and a y- or (5-hydroxyl group able to promote intramolecular cyclization of a transient open-chain isothiocyanate, to form the thermodynamically most stable OZT. The first results obtained by Zemplen in d-gluco and D-Fru series reported the formation of OZTs fused to pyran backbones (Scheme 20).42... 

The proportion of the acyclic form also increases with increasing temperature this is true for aldoses and ketoses,16,31 as well as for simple hydroxyketones.74 This would be expected from considerations of entropy, as the acyclic form has a greater degree of freedom, but studies on y- and d-hydroxyketones show that change in enthalpy contributes even more to the changing position of the equilibrium with increasing temperature. Evidently, cyclization of hydroxyketones is exothermic, and is favored by lower temperatures.74... 

In DERA reactions, where acetaldehyde is the donor, products are also themselves aldehydes. In certain cases a second aldol reaction will proceed until a product has been formed that can cyclize to a stable hemiacetal.71 For example, when a-substituted aldehydes were used, containing functionality that could not cyclize to a hemiacetal after the first aldol reaction, these products reacted with a second molecule of acetaldehyde to form 2,4-dideoxyhexoses, which then cyclized to a hemiacetal, preventing further reaction. Oxidation of these materials to the corresponding lactone, provided a rapid entry to the mevinic acids and compactins (Scheme 5.43). Similar sequential aldol reactions have been studied, where two enzyme systems have been employed72 (Scheme 5.44). The synthesis of 5-deoxy ketoses with three substitutents in the axial position was accomplished by the application of DERA and RAMA in one-pot (Scheme 5.44). The long reaction time required for the formation of these thermodynamically less stable products, results in some breakdown of the normally observed stereoselectivity of the DERA and FDP aldolases. In a two-pot procedure, DERA and NeuAc aldolase have... 

In addition, its proton magnetic resonance spectrum unambiguously supported the 5-ketose structure (69). Rearrangements of epoxides to ketones when dicobalt octacarbonyl is used as the catalyst at temperatures above 100 , or when cobalt hydrocarbonyl is used at lower temperatures, are well known. By applying the technique of double irradiation to a sample of (68), the main component was shown to possess structure (68). Presumably, the free aldehyde group of the hydroformylation product immediately cyclized with the free hydroxyl group on C-3 to give the tricyclic structure (68). A third component (68a) (isolated in less than 5 % yield) was undoubtedly formed by subsequent reduction of the dialdose derivative (68). 

Carbohydrates are either polyhydroxyaldehydes (aldoses, oses) or polyhydroxyke-tones (ketoses, uloses) there is an electron gap at their carbonyl carbon atom. Typically, aldehydes and ketones accept nucleophiles such as water to form hydrates or alcohols to form hemiketals (5.1 and 5.3) and hemiacetals (5.4 and 5.6), respectively. In pentoses, pentuloses, hexoses, hexuloses, and higher carbohydrates, one of the hydroxyl groups can play the role of internal nucleophile. Thus, open-chain structure (5.2 and 5.5) cyclizes into internal hemiacetals and ketals, all with either five- (5.1 and 5.3) or six- (5.4 and 5.6) membered cycles. 

Several 4-deoxy-4-nitro ketose derivatives have been synthesized. Three crystalline 2,7-anhydro-4-deoxy-4-nitro-/3-D-heptulopyranoses arose by nitromethane cyclization92 of the dialdehyde (74) produced by periodate oxidation of sedoheptulosan (73), and were assigned93 the D-alio (75), D-gulo (76), and D-altro (77) configurations by nuclear magnetic resonance spectroscopy of the corresponding, peracetylated amino sugars. When the cyclization of 74 was performed in methan-olic medium, a solid mixture of stereoisomeric nitronates was obtained in almost quantitative yield, and it could be partially separated into the individual nitronates, which furnished 75 and 76. The isomer 77... 

The basic monomeric unit of the polysaccharide polymer, the monosaccharide, exists in solution in equilibrium between the open chain and cyclized forms. As the open chain form (Figure 2), monosaccharides are represented by two main classes, the aldoses and ketoses. As the names imply, the aldoses contain a terminal aldehyde functionality while the... 

Fig. 13 Dehydration of tetroses (both ketose and aldose) via enolization and retro-Michael reactions (steps 1 and 2). An observed side reaction after the first dehydration is a cyclization (step 2 ), followed by isomerizations (= iso) to a-hydroxybutyrolactone HBL [100]...

In practice, these forms are present to less than 1% in glucose solutions, but they are important with other monosaccharides. The ketose D-fructose, for example, exists in solution mainly in two furanose forms. The carbonyl carbon at C-2 and hydroxyl group at C-5 cyclize to give the furanose ring. 

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. 

There are obviously many aldose monosaccharides and many ketose monosaccharides, but the discussion will focus only on the d diastereomers, as with the aldoses. The triose is l,3-dihydroxy-2-propanone (41 also called glycerone) and the tetrose is d-glycero-tetrulose (42). Two pentoses are named d-ribulose (43) and d-xylulose (44) and four hexoses are named d-psicose (45), d-fructose (46), d-sor-bose (47), and d-tagatose (48). All of these compounds are ketoses and are further classified according to the number of carbon atoms, as noted in Section 28.1. For example, 42 is a ketotetrose, 43 is a ketopentose, and 45 is a ketohexose. For 43-48, cyclization is possible to form a hemiketal. Just as aldehydes and alcohols react to form a hemiacetal, ketones and alcohols react to form a hemiketal (see Chapter 18, Section, 18.6). For 43, a five-membered ring (a furanose) is formed if the terminal CH2OH unit (in violet) reacts with the ketone carbonyl. The two anomers formed are 49 (a-d-ribulofuranose) and 50 (P-d-ribulofuranose). 

Methyl Ethers.—6-0-Methyl-L-sorbose, 5-O-methyl-D-fructose and -L-sorbose, and 5,6-di-O-methyl-L-sorbose have been synthesized. " U.v. spectroscopy was used to determine the proportion of the acyclic form of these and other ketoses present in methanol solution the extinction coefficient (e = 34.7) for the carbonyl group was determined using 5,6-di-O-methyl-L-sorbose, which cannot cyclize. Of the other ketoses examined, 6-O-methyl-L-sorbose was found to contain the highest proportion (7.4%) of the acyclic form at equilibrium. 

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