Cyclization of monosaccharides

Understand the cyclization of monosaccharides to form pyranose and furanose rings. 

In addition to their work on tricyclic systems, Chmielewski and co-workers have used monosaccharides to obtain polycyclic derivatives of oxacepham. For instance, cyclization of 479 (R2 = Tos or TIBS) in a two-phase system including a phase-transfer catalyst and potassium carbonate gave 480 in 65-90% yields. The protecting group R1 was lost if it was a silyl group, for example, TMS, TBDMS <1998T14065>. 

Preparation of highly functionalized medium-sized carbocycles (210) and (212), derived from carbohydrates, can also be carried out with the same procedure via 8-endo-trig and 1-exo-trig manners, as shown below (eqs. 3.81, 3.82). Moreover, preparation of (3-C-disaccharide (214) proceeds selectively via 8-endo-trig radical cyclization of two temporarily tethered monosaccharide (213), by means of the addition of a carbohydrate-... 

This strategy was mostly exploited by Sinay and his coworkers, enabling them to prepare various C-linked disaccharides in an expeditious fashion, via radical cyclization of temporarily tethered monosaccharidic precursors [111]. Medium-sized ring intermediates are obtained in reasonable yields, leading to the desired compounds after removal of the tether. The prototype of this approach is described below [112a] (O Scheme 52). 

Cyclization of a monosaccharide results in the formation of an additional asymmetrical center, known as... 

Oxidative cyclization of (5,6-diphenyl-l,2,4-triazin-3-yl)hydrazones of aldose monosaccharides (820) and concurrent acetylation of the alditolyl chain hydroxyls provided the corresponding O-acetylated 3-(alditol-l-yl)-... 

Some physicochemical measurements on mono-l-triethyl silatranyl ethers prepared from a range of monosaccharide derivatives have been reported. Treatment of D-glucose, D-galactose, and D-xylose with (EtO)3SiH followed by tris[HOCH2CH2)3N] gave the cyclized glycosylsilatranes isolated as their peracetates (16)-(18). ... 

The representation of monosaccharide molecules as open-chain structures is only partially correct. In solution, the molecules with five- and six carbon atoms can also appear in cyclic forms which are in equilibrium with the open-chain structures. The cyclization process represents the intramolecular nucleophilic attack of the hydroxyl group on the aldehyde or keto group. 

Chiral thiopyran derivatives have been obtained in Diels-Alder reactions from monosaccharide O-thioformates and dithiooxalates. Intramolecular Diels-Alder cyclization of 1,6,8-nonatrienes and 1,7,9-decatrienes prepared from pentoses or D-glucose afforded multichiral hexahydroindene and octahydronaphthalene derivatives with very high diastereoselectivity. 

The monosaccharide D-glucose, whose chemistry is representative of all aldoses containing four or more carbon atoms, exists predominantly in the two pyranosc forms 4 and 5. These are six-membered hemiacetals formed by the reversible cyclization of the acyclic polyhydroxy aldehyde 3 (Eq. 23.1). In the cyclic forms 4 and 5, the ring carbon that is derived from the carbonyl group is referred to as the anomeric carbon atom. The specific rotation, [a] (Sec. 7.5), of a-D-(+)-glucose (4) is +112 whereas that of the -anomer 5 is +19°. When crystals of either pure 4 or pure 5 are dissolved in water, the [a]p changes to an equilibrium value of +52.7°. This process is termed mutarotation. At equilibrium in water, the a- and p-forms are present in the ratio of 36 64 only about 0.03% of D-glucose is in the acyclic form 3. 

The carbon in position 1 is called the anomeric carbon. The location (equatorial or axial) of the oxygen linked to this carlxMi is determined upon cyclization of the monosaccharides and all monosaccharides can exist in two anomeric forms. In the D-pyranoside ring, the a anomer is formed if 01 is axial, and if 01 is equatorial the P anomer is formed both are illustrated in Fig. 2. When an OH group is bound to Cl, the anomers may co-exist and intercOTivert (mutarotation), in relative... 

Typical applications of the DHAP aldolases indude the synthesis of monosaccharides and derivatives of sugars from suitable functionalized aldehyde precursors. High conversion rates and yields are usually achieved with 2- or 3-hydroxyaldehydes, because for these compounds reaction equilibria benefit from the cyclization of the products in aqueous solution to give more stable fiiranose or pyranose isomers (Figure 5.32). For example, enantiomers of glyceraldehyde are good substrates, and stereoselective addition of dihydroxyacetone phosphate produces enantiomerically pure ketohexose... 

The hetero-Diels-Alder cyclization reaction of tra s-l-methoxy-3-trimethylsilyloxy-1,3-butadiene (Si) (= Danishefsky s diene) with benzaldehyde (S ) (Scheme 12.23) [217-221] is a promising reaction for evaluating the catalytic properties of Lewis acidic lanthanide centers, and has enormous potential for asymmetric synthesis of natural products (e.g., monosaccharides) [222-225]. 

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