Oxetans, carbohydrate

Carbohydrate oxetanes have been prepared from deoxyiodo sugars without die use of strongly alkaline reagents. Thus, treatment of 5-deoxy-5-iodo-l,2-0-isopropylidene-a-D-xylofuranose with silver fluoride in cold pyridine afforded 3,5-anhydro-l,2-0-isopropylidene-... 

K. Capek and T. Vydra, Oxirane-oxetane-l,4-dioxane anhydro-ring migration in sucrose derivatives, Carbohydr. Res., 168 (1987) C1-C4. 

Isatine derivatives gave the corresponding cycloadducts with high stereoselectivity when irradiated in the presence of furan and benzofuran [72]. The reaction of furan with acyl cyanides yields the corresponding oxetanes, but both diastereoisomeric endo- and exo-oxetanes are formed (Scheme 3.36). When chiral acyl cyanides are used, low asymmetric induction is observed [73]. Furan also reacts with chiral ketones. In this case, an ot-cleavage reaction before the 2+2 cycloaddition modifies the expected products (Scheme 3.37). When (—)-menthone was used as a substrate, a chiral product was obtained as a 2 1 diastereoisomeric mixture the most abundant product has the (1/ , 3R) configuration [74]. When the reaction was performed on carbohydrate 36, a complex reaction mixture was obtained (Scheme 3.38) [74, 75]. 

Carbohydrate templates have provided interesting applications of the Patemo-BUchi reaction. In a series of papers, Araki has demonstrated the feasibility of oxetane formation from glucals. - Photocy-... 

Another excellent example demonstrating the utility of organolithium compounds in the formation of C-glycosides from modified carbohydrates was reported by Koll, et al.38 This study, shown in Scheme 2.3.13, involves the addition of a lithium acetylide to a C-glycosyl oxetane. The result was the extension of a pre-existing C-glycosyl side chain. 

A new set of carbohydrate-based thermotropic liquid crystals (17) have been prepared via reaction of the oxetan (18)(derived from glucose) with lithium acetylide reagents followed by catalytic hydrogenation. A related series of 2,5-anhydro-hexitol derivatives (19) have been prepared by a similar route, reacting the oxetan with long-chain alcohols in presence of trifluoroacetic... 

Carbohydrates have been used as chiral auxiliaries in asymmetric Patemo-Buchi reactions described by Scharf and co-workers (52). The diastereomeric excess obtained in the oxetane formation was found to depend heavily on both the reaction temperature and the structure of the carbohydrate auxiliary. 

Cyclic ethers are components of many natural products as well as synthetic compounds. A simple one is erythritol anhydride (63), which contains the epoxide unit and is used to prevent microbial spoilage. The important anticancer drug taxol (64) contains an oxetane unit. The tetrahydrofiiran unit is a component of the important carbohydrate ribose (65), which is the sugar portion of RNA (see Chapter 28). A tetrahydrofuran unit is found in isosorbide dinitrate (Isordil , 66), which is a vasodilator used in the prevention and treatment of angina. The tetrahydropyran unit is found in common sugars such as glucose (67) and in desosamine (68), which is the sugar component of many macrolide antibiotics. Both the tetrahydrofuran and tetrahydropyran rmits are found in the antibiotic Lasalocid (69). 

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