Cyclic derivatives formation

The presence of neighbouring hydroxy group in most carbohydrate samples makes them ideal candidates for cyclic derivative formation. Examples suitable for GC-MS include butylboronate acetates [180] and several boronate-TMS derivatives [181]. Many other early examples of cyclic derivatives, such as acetals and iso-propylidene derivatives, for these compounds have been reviewed by Knapp [182] and by Darbre [183]. 

Aldoses incorporate two functional groups C=0 and OH which are capable of react mg with each other We saw m Section 17 8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal When the hydroxyl and carbonyl groups are part of the same molecule a cyclic hemiacetal results as illustrated m Figure 25 3 Cyclic hemiacetal formation is most common when the ring that results is five or SIX membered Five membered cyclic hemiacetals of carbohydrates are called furanose forms SIX membered ones are called pyranose forms The nng carbon that is derived... 

Cyclic herniacetal formation is most common when the ring that results is five- or six-rnernbered. Five-rnernbered cyclic herniacetals of carbohydrates are called furanose forms six-rnernbered ones are called pyranose forms. The ring carbon that is derived... 

The activation of an anomeric hydroxyl group from partially protected or unprotected monosaccharides can be achieved via 1,2-cyclic sulfite formation. A subsequent trans-ring opening with azide N3 affords one anomeric derivative exclusively 133... 

The reaction of dicarbonyl compounds, such as glyoxal or phenylglyoxal, with a guanidinyl group, such as that of an arginine residue, proceeds to yield a more stable linkage due to the formation of a cyclic derivative (Reaction 46). 

Ha and Choun [51] confirmed these findings from the investigation of cyclic oligomer formation at 270 °C. They derived a rate equation for cyclic oligomer formation taking thermal degradation of the polymer into account. 

In contrast, the HRP-catalyzed kinetic resolution of racemic secondary hydroperoxides in the presence of guaiacol afford the hydroperoxides and their alcohols in high enantiomeric excesses (Eq. 3) [69]. In the case of the aryl alkyl-substituted hydroperoxides and cyclic derivatives (Table 4, entries 1 -3,6-10), HRP preferentially accepts the (R)-enantiomers as substrates with concurrent formation of the (R)-alcohols the (S)-hydroperoxides are left behind, further-... 

Ring-chain equilibria involving cyclic tautomer formation by intramolecular reversible amidrazone NH-group addition to the C=0 bond were observed (85KGS849 86ZOR500) in solutions of the products formed in the reactions of pentane-2,4-dione and its 3-methyl derivatives with benzamidrazinium iodides. The ring-chain equilibrium was detected only in solutions of iodides or picrates. The free bases exist as the open-chain tautomers, like the hydrazones or enhydrazines. In contrast with the acyl-... 

The palladium catalysed formation of indole derivatives has been extended by Grigg, who used carbon monoxide and unsaturated amines to trap the palladium complex formed in the insertion step. Reaction 3.10. provides an example of such a transformation. The amides were converted to the cyclic derivatives using ring closing metathesis.13... 

The mechanism of the photochemical degradation of catenated silicon derivatives has received considerable attention (25). Substituted cyclic derivatives photochemically extrude a silylene fragment which can be intercepted by appropriate trapping reagents (e.g., trialkylsilanes or 2,3-dimethyl butadiene). This extrusion results in the formation of the corresponding ring contracted cyclopolysilane. The process continues upon additional irradiation until a cyclotetrasilane results which then undergoes... 

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],... 

Several novel modified salen derivatives of cobalt(III) have provided convincing evidence for the importance of the propagating copolymer chain staying in the vicinity of the metal center, so as to avoid the formation of cyclic carbonates this procedure is especially relevant to processes involving the PO monomer. Both, computational and experimental studies have shown that cyclic carbonate formation is enhanced relative to monomer enchainment under conditions where the growing polymer chain is outside the influence of the metal catalyst [50, 51]. To circumvent this issue, Nozaki and coworkers prepared a salen complex containing a piperidinium end-capping arm (Scheme 8.4) [52]. 

Much attention was given in CHEC-II(1996) <1996CHEC-II(4)409> to the synthesis of 1,2,3-dithiazoles. Cyclic and acyclic oximes were found to be important precursors to their preparation. The discovery of 4,5-dichloro-1,2,3-dithiazolium chloride (Appel salt 20) from the reaction of commercial and cheap acetonitrile and disulfur dichloride gave strong impulse to the synthesis of various 1,2,3-dithiazole derivatives. Formation of the 1,2,3-oxathiazole ring involved almost exclusively the conversion of m -aminohydroxyl compounds to the A-oxide derivatives. 

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