Standard type donors

Initial smdies to construct the a-glycoside of Al-acetyl neuraminic acid (NeuSAc) employed the 2-chloride donor derivative [5]. It is relatively easy to access most 

---

Sialic acid donors that have been reported to date can be classified into two major types based on the mode of chemical modification. These have been termed standard type and appended type donors (Figure 9.2). All of the donors have a suitable leaving group such as bromide, chloride, alkyl- or arylthio group (SMe, SEt, SPh, etc.) or phosphite group [OP(OEt)2 and OP(OBn)2] at the anomeric C-2 carbon. In the appended type donors an assisting nucleophilic functionality (auxiliary) is mounted at C-3 or C-1 to restrain the nucleophile from attacking the (3-face. In contrast, the standard type donors maintain the 3-deoxy structure. 

Thus Meyrick and Thompson [1] reported strong P=0 and P—H absorptions in a number of compounds of this type, and the same effects have been noted by Bellamy and Beecher [6] and by Nyquist [39]. The absence of P—OH absorptions and the fact that the P—0 absorption frequency corresponds to an unbonded phos-phoryl group confirm that not more than a very small proportion of the whole can exist as hydrogen phosphite. The basicity of the P=0 link has been measured by hydrogen-bonding methods in which the frequency shift of some standard XH donor such as CDCI3 is measured. Gramstad [40, 41] has shown that Si> can then be related to the Taft a values of the phosphorus substituents. These therefore determine both the frequency and the polarity. The intensity of the P=0 band is also related to the 6v values of proton donors and is therefore similarly determined by the polarity [41]. 

A subclass of lyases, involved in amino acid metabolism, utilizes pyridoxal 5-phosphate (PLP, 3-hydroxy-2-methyl-5-[(phosphonooxy)methyl]-4-pyridinecarbaldehyde) as a cofactor for imine/ enamine-type activation. These enzymes are not only an alternative to standard fermentation technology, but also offer a potential entry to nonnatural amino acids. Serine hydroxymethyl-tansferase (SHMT EC 2.1.2.1.) combines glycine as the donor with (tetrahydrofolate activated) formaldehyde to L-serine in an economic yield40, but will also accept a range of other aldehydes to provide /i-hydroxy-a-amino acids with a high degree of both absolute and relative stereochemical control in favor of the L-erythro isomers41. 

Metastable solutions of GaX (X = C1, Br) as well as conventionally prepared Gal react with lithium or sodium organometallics in standard salt metathesis reactions with formation of M2R4 (Section 3.07.4.1) as well as neutral and anionic clusters of the type MnRm]x m > n) (chapter 4.1). Moreover, metalloid clusters [MnRm]x (m < n), which feature different types of metallic core structures, have been obtained. Their formation strongly depends on the reaction conditions, in particular the reaction temperature, and the (donor) solvent. 

---