Ketoses 1,3-dideoxy-, preparation

Mostly the baker s yeast transketolase has been used so far to prepare several valuable ketose sugars and derivatives 141 (cf. Sect. 7). Recently, the transketolase was utilized in the key stereogenic transformation of racemic 2-hydroxybutyraldehyde 142 into the homochiral synthon 5,6-dideoxy-D-t/ireo-hexulose 110 for the chemoenzymatic synthesis of (+ )-exo-brevicomin 107 [314], Transketolase has also been applied for the in-situ generation of Ery4P (35) from Fru6P (38) in a multi-enzymatic synthesis of 34 (Scheme 5). 

Xylose isomerase converts a wide range of D-glucose as well as L-idose derivatives modified at position 5 into the corresponding ketose. 5-Deoxy-5-fluoro-D-xylulose and a variety of 5,6-dimodified open-chain analogs of D-fructose, namely the 5,6-diazido-5,6-dideoxy, 6-azido-5,6-dideoxy, 6-azido-5,6-dideoxy-5-fluoro, 5,6-di-deoxy-5-fluoro, 5,6-dideoxy-6-fluoro and 5,6-dideoxy-5,6-difluoro derivatives were prepared with glucose isomerase (Fig. 17-24)[234, 235. ... 

Osmylation of the chirally masked 2-alkoxypent-3-enal 1 and subsequent functional group manipulations furnished enantiopure 5-deoxy-L-lyxose and -xylose derivatives. A new preparation of 5-deoxyribonolactone derivative 2 from ribonolactone by tributyltin hydride reduction of a 5-chloro-5-deoxy intermediate has been published. Conversion of compound 2 to a 5 -deoxy-nucleoside analogue is covered in Chapter 20. A multistep synthesis of methyl 5-0-benzoyl-2,3-dideoxy-P-D-g/ycero-pentofuranose (4) from D-xylose involving Raney nickel desulfurization of compound 3 (see Chapter 11) has been described. Several deoxy-ketoses have been prepared by Raney nickel desulfurization of 1-thio-ketose derivatives obtained by enzyme-catalysed aldol condensations (see Vol. 29, Chapter 2, Scheme 1). An example is given in Scheme 1. ... 

Transketolase has been used for the key steps in chemoenzymatic syntheses of (+)-exo-brevicomin 111 from racemic 2-hydroxybutyraldehyde [236], and of the azasugars l,4-dideoxy-l,4-imino-D-arabinitol [196] or N-hydroxypyrrolidine 124 [265] from 3-azido (95) and 3-O-benzyl (122) derivatives, respectively, of glyceraldehyde (Figure 5.55). Such syntheses were all conducted with intrinsic racemate resolution of 2-hydroxyaldehydes and profited from utilization of 119. Further preparative applications include the synthesis of valuable ketose sugars, particularly fructose analogs [258]. Transketolase has also been used for in-situ generation of erythrose 4-phosphate from fructose 6-phosphate in a multi-enzymatic synthesis of DAHP (26 Figure 5.17) [131]. 

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