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Molecular sieves catalyse pyridinium chlorochromate and pyridinium dichromate oxidation and so give rise to an efficient synthesis of ketonucleosides and ketosugars a t room temperature.PYRIDINIUM CHLOROCHROMATE (PCC) and pyridinium dichromate (PDC)2 allow mild and large scale oxidation of a wide range of alcohols to carbonyl compounds in methylene chloride at room temperature. Unfortunately, in the case of nucleosides these conditions lead to a very long and incomplete reaction.In order to enhance the reactivity of PCC and P P C towards nucleosides at room temperature we have tried to promote the reactions by using insoluble inorganic compounds which are active in the case of chromic3t4 or permanganate oxidations.6 Since attempts to catalyse the reaction by celite, alumina, and silica gel were unsuccessful we performed the oxidation using molecular sieves and found that the reaction rate was considerably accelerated (Table 1).

Previous studies have shown that ligand or immunoaffinity chromatography can be used to purify the human platelet thromboxane A 2 (TXA 2 ) receptor-G␣ q complex. The same principle of co-elution was used to identify another G-protein associated with platelet TXA 2 receptors. It was found that in addition to G␣ q , purification of TXA 2 receptors by ligand (SQ31,491)-affinity chromatography resulted in the co-purification of a member of the G 12 family. Using an antipeptide antibody specific for the human G 13 ␣-subunit, this G-protein was identified as G␣ 13 . In separate experiments, it was found that the TXA 2 receptor agonist U46619 stimulated [35 S]guanosine 5-O-(3-thiotriphosphate) incorporation into G 13 ␣-subunit. Further evidence for functional coupling of G 13 to TXA 2 receptors was provided in studies where solubilized platelet membranes were subjected to immunoaffinity chromatography using an antibody raised against native TXA 2 receptor protein. It was found that U46619 induced a significant decrease in G␣ q and G␣ 13 association with the receptor protein.These results indicate that both G␣ q and G␣ 13 are functionally coupled to TXA 2 receptors and dissociate upon agonist activation. Furthermore, this agonist effect was specifically blocked by pretreatment with the TXA 2 receptor antagonist, BM13.505. Taken collectively, these data provide direct evidence that endogenous G␣ 13 is a TXA 2 receptor-coupled G-protein, as: 1) its ␣-subunit can be co-purified with the receptor protein using both ligand and immunoaffinity chromatography, 2) TXA 2 receptor activation stimulates GTP␥S binding to G␣ 13 , and 3) G␣ 13 affinity for the TXA 2 receptor can be modulated by agonist-receptor activation.

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