Bidirectional chain elongation

Such aldolase-catalyzed bidirectional chain elongation ( tandem aldolization) of simple, readily available dialdehydes has been developed into an efficient method for the generation of higher carbon sugars (e.g. (87)/(89)) by simple one-pot operations (Figure 10.32) [126,156]. The choice offuranoid (87) or pyranoid (89) nature of the products can be determined by a suitable hydroxyl substitution pattern in a corresponding cycloolefinic precursor (86) versus (88)). The overall specific substitution... 

Long-chain dialdehydes can be also used as substrates and can react independently at both electrophilic sites, undergoing a bidirectional chain elongation by twofold enzymic aldolization, without the need for isolation of intermediates [110], If hydroxylation is suitably placed in the substrate, this furnishes C-glycosidically linked saccharide units that can be considered as disaccharide mimics (Scheme 10.5) [111]. 

Bidirectional chain elongation by two fold tandem enzymic aldol addition of DHAP to a meso-dihydroxydialdehyde catalyzed by FniA from rabbit muscle (RAMA). 

Synthetic strategies differ by binding either the glycosyl donor [105] or the glycosyl acceptor to the solid phase. A bidirectional approach [106], which allows for chain elongation in both directions, has also been reported. 

A symmetric oligomer chain elongation approach was introduced by Tour et al. [168-170]. This group developed a bidirectional approach to the solid phase synthesis of oligo phenylene ethynylenes (Scheme 7.8). 

Solution phosphotriester syntheses have usually been performed by blockwise chain-elongation. The strategy of using recyclable, bidirectionally applicable (47) blocks may help to reduce the cost and facilitate the separations. Eventually, solution and support synthesis may join efforts, the latter providing for a rapid assemblage of solution-synthesized blocks—the problems, which still remain to be solved, have been discussed in Section 2. The reverse option of joining support-synthesized blocks in solution (224) seems realizable in view of recent developments (225,226) however, this alternative will probably be limited to special applications. 

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