Ascorbic acid labelled, synthesis

The possibility that the biosynthesis of ascorbic acid occurred through sorbose and 2-keto-L-gulonic acid, as in the chemical synthesis, was eliminated when the latter was not converted to ascorbic acid, and L-sorbose-6-C gave rise to ascorbic acid labeled in both carbons 1 and 6. Sorbose was evidently degraded and converted to glucose before being transformed into ascorbic acid. 

F. Yamamoto, S. Sasaki, M. Maeda, Positron labeled antioxidants Synthesis and tissue biodistribution of 6-deoxy-6-[ F]fluoro-L-ascorbic acid, Appl. Radiat. Isot. 43 (1992) 633-639. 

A separate Section on the synthesis of variously labelled L-ascorbic acid is included here, because of the important role that these molecules have played in helping to elucidate the metabolic fate of 1 in plants and animals. Tritium, deuterium, and carbon-14 have each been incorporated into 1. 

Uniformly labelled 1 was prepared by the Reichstein-Griissner synthesis (see Scheme 4), starting with unifonnly labelled D-glu-cose.597-599 L-[2,3,4,5,6-14C]Ascorbic acid was prepared 800 from L-[U-14C]xylose by way of L-threo-pentos-2-ulose (9) (see Scheme 2). The L-[U-14C]xylose was prepared from D-[U-,4C]glucitol by using the route shown in Scheme 21. 

L-[4-14C]-,-[3-14C]-,-[6-,4C -, and-[U-14C]-Ascorbic acid were prepared802 from the corresponding, labelled D-glucose by the Bakke-Theander synthesis (see Scheme 8). 

The 5,6-O-isopropylidene acetal (152) of L-ascorbic acid has been prepared,340 and von Schuching and Frye341 prepared the corresponding cyclohexylidene acetal. These compounds were found to be more resistant than L-ascorbic acid toward oxidation, and the parent acid can be readily regenerated by acid hydrolysis. The derivative was used in the synthesis of 14C-labeled vitamin C. The C-2 and C-3 enols of L-ascorbic acid or its acetone derivative (152) can be readily methylated with diazomethane, yielding the corresponding dimethoxy analogues. 

Calcium oxalate results from the reaction of oxalic acid and calcium ions (McNair, 1932), and oxalic acid itself is an effective deterrent to herbivores in its own right (Amott and Webb., 1983). At least four different biochemical pathways have been shown to result in the synthesis of oxalate in plants (Raven et al., 1982). Utilizing radiocarbon-labeled ascorbic acid administered to roots of Yucca torreyi L. (Torrey s yucca) the incorporation into vacuole crystal bundles demonstrated that ascorbate is an important precursor to oxalate in this plant (Horner et ai, 2000). 

The Reichstein-Gnissner synthesis has been used to prepare a number of labeled derivatives of L-ascorbic acid, starting with labeled D-glu-cose (36-40). This synthesis is not amenable to the preparation of analogues. 

An alternative method of preparing L-ascorbic acid was reported by Bakke and Theander (Scheme 14) (43), In this synthesis D-glucose was first oxidized at C6, then at C5, and then reduced at Cl. This contrasts with the Reichstein-Griissner synthesis in which glucose was first reduced at Cl, then oxidized at C5, and then at C6 to achieve the requisite inverted carbon chain. The key intermediate in the Bakke-Theander synthesis, ketolactone (25) was prepared earlier (44,45) but was not converted to 1. Hydrolysis of 25 afforded 6-aldehydo-L-ascorbic acid (26, aldehydo-L-threo-hex-4-enurono-6,3-lactone) as an unisolated intermediate. Compound 26 was not previously synthesized. The reduction of 26 afforded 1. This synthesis of 1 is used effectively in the preparation of labeled derivatives of 1 (46). It is not useful for the preparation of analogues. 

Either this synthesis or that shown in Scheme 16 is suitable for the preparation of C6 deuterated or tritated derivatives of L-ascorbic acid reducing 27 with deuterium or tritium enriched gas or with labeled sodium borohydride (51). 

Fig. 5. Determination of protein synthesis rates in H9 ceils in the presence and absence of ascorbate (AA). Protein synthesis was assayed as described by Somasundaran and Robinson (17). Each point is the mean of S-labeled amino acid incorporation per 10 cells.

The most convincing evidence that ascorbic acid can be synthesized from a hexose sugar in animals comes from the work of Jackel and his collaborators (1950). When D-glucose labeled uniformly in all positions with was supplied to rats which had been fed chloretone to stimulate the synthesis of the vitamin, L-ascorbic acid was excreted which was also labeled uniformly. This clearly indicates that n-glucose can serve as an ultimate precursor but does not exclude the possibility that the glucose may have to be converted to other sugars before being transformed. 

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