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VIII. Summary

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    Synthesis of O-thiocarbonyl compounds [(xanthates, (thio­carbonyl)­imid­azolides, aryl thi­ono­carbonates, cyclic thionocarbonates, thionoesters)] is the first step in using them to generate carbon-centered radicals.

    Xanthates usually are prepared by deprotonating a partially protected carbohydrate and then reacting the resulting alkoxide ion with carbon disulfide and methyl iodide. The primary limitation of this approach is that it involves conditions in which base-sensitive compounds are unstable. Xanthate synthesis by phase-transfer reaction or by reaction with phenyl chlorodithioformate avoids this difficulty.

    (Thiocarbonyl)imidazolides are formed by reacting a partially protected carbohydrate with N,N-thiocarbonyl­di­imid­azole. These conditions for synthesis are much less basic that those used for preparing xanthates.

    Aryl thionocarbonates typically come from reaction of a partially protected carbohydrate with phenoxythiocarbonyl chloride in the presence of DMAP (4-dimethyl­amino­pyridine). Side reactions are rare and tend to arise when DMAP promotes base-catalyzed reactions that compete with thiono­carbonate formation. Phenyl thionocarbonates also can be prepared in reactions cata­lyzed by N‑hydroxy­succinimide (NHS). This alternative procedure normally is implemented to im­prove product yields or avoid side reactions caused by DMAP. An additional option for phenyl thionocarbonate preparation consists of reacting a partially protected sugar with thiophosgene and treating the product with a phenol. This procedure is useful in preparing phenyl thiono­carbonates with groups, usually electron-withdrawing ones, in the aromatic ring.

    If a partially protected carbohydrate has vicinal, cis-related hydroxyl groups, reaction with N,N-thiocarbonyldiimidazole will form a cyclic thiono­carbonate. A second procedure for syn­the­sis of these compounds consists of formation of a stannylene complex of a carbohydrate, and then reaction of this complex with thiophosgene or phenoxythiocarbonyl chloride.

    Thionoesters are less frequently used in deoxygenation reactions than other O-thiocarbonyl compounds, in part, due to the difficulty in their prep­aration. The only thionoesters used to a sig­nif­icant extent in deoxygenation are thionobenzoates.

    When O-thiocarbonyl compounds are unable to form under the standard reaction conditions, sometimes they can be synthesized by converting the partially protected carbohydrate reactant into its corresponding alkoxide ion. Forming an alkoxide ion also increases the possibility that group migration will compete with formation of an O-thiocarbonyl, carbohydrate derivative.

    This page titled VIII. Summary is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.