Appended type donors

The appended type donors incorporate neighbouring functionahties at C-3 or C-1 in order to direct the a-selective assembly of sialosides. Therefore, a-selective sialylation using appended type donors is independent of solvent. 

---

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. 

Notes and discussion. The 2-chloride derivative can be prepared via several protocols [8, 9, 10, 11]. This protocol is the simplest and safest method, and allows facile large-scale preparation. The 2-chloro derivative is often used to prepare the 2,3-dehydro derivative [8b], which is pivotal for access to C-3 appended type donors. It is also central to strategies involving Koenigs-Knorr reactions with simple alcohols. Immediate use is recommended after preparation, despite the moderate shelf life of the title compound in the freezer. 

A real rotaxane has recently been described which consists of a porphyrin-appended ring threaded by a string whose two ends are attached to a porphyrin. In this way, if the porphyrins are of different types (donor D and acceptor A), both are maintained in the same molecule by mechanical (or topological, for a catenane) bonds only. If... 

This approach, in which an arene appended to a coordinated donor atom replaces a labile ligand, is by far the most generally useful procedure and has been applied to the preparation of tethered arene complexes of Ti(IV), Cr(0), Mo(0), Mo(II), W(II), Ru(II), Rh(I), and Ir(I). The labile ligand itself is often an arene, as shown in Scheme lb, in which case the replacement is an intramolecular version of the intermolecular exchange between free and coordinated arenes, which has been studied for complexes of the type [M(CO)3(ri6-arene)] (M = Cr, Mo),26 31 [Ir(ri6-arene)(ri4-l,5-COD)]BF4,28 32 and [Ru(ri6-arene)(ri4-l,5-COD)]28 (COD = cyclooctadiene). 

Type I ligands have a pyridine ring as the central unit, which carries two branched aliphatic sidearms, resulting in a 2,6-disubstitution pattern. The sidearms are C3 units whose central carbon atom -i.e., C-2 - is connected to the pyridine ring. The donor atoms are appended in the 1 and 3 positions, respectively. Suitable donors are N, O, S, or P (1-4). 

Many types of aromatic substrate are known to undergo a cyclometallation reaction when exposed to alkylpentacar-bonylmanganese complexes under thermal conditions. It is well established that the treatment of ligand appended arenes with alkylmanganesepentacarbonyl complexes can lead to the formation of [C,Y] heterochelates of Mn(CO)4 (Y being a two-electron donor ligand) (Equation 6). For instance, aromatic compounds such as W,W-dimethylbenzyl-amine, alkyl benzyl thioethers, 2-phenylpyridine, acetophenone, benzaldehyde, and diazobenzene can be readily... 

The first ester functionalized cobaltocenium receptor (74) developed by us in 1989, was based solely on electrostatic interactions (as discussed earlier) (9, 186). It was therefore decided to append the cobaltocenium moiety with secondary amide functionalized arms . In this manner, the receptors became more resistant to hydrolysis (a recurrent problem with ester-based systems), and also incorporated neutral hydrogen-bond donors capable of coordinating anions. [An interaction between a secondary amide (acetamide) and the bromide anion was physically investigated (by IR spectroscopy) as long ago as 1961 (211)]. We made the first report of this novel type of receptor (91 and 92) in 1992 (212), which was, in fact, the first class of inorganic anion receptor that incorporated hydrogen-bonding functionalities. 

Gandelman and coworkers have recently reported a series of pincer click ligands, 88a-b, that incorporate two phosphine donor groups appended to a triazole (Scheme 13) [88-90,192]. They have shown that the triazole binds to a metal ion as a triazolidene and then, after a postmodification of the coordinated ligand in which the triazole backbone is alkylated, as a carbene. Square planar Pd(II) and Pt(II) complexes of this type have been reported and characterized (89-90a,b). X-ray structures confirm the N-alkylation, and the expected changes in metal-donor bond lengths are observed. Palladium(II) complexes of this type have been shown to be excellent catalysts for the Heck reaction. 

---