Cyclic sulfates ring opening reactions

The use of cyclic sulfates in synthetic applications has been limited in the past because, although cyclic sulfites are easily prepared from diols, a convenient method for oxidation of the cyclic sulfites to cyclic sulfates had not been developed. The experiments of Denmark [70] and of Lowe and co-workers [71 ] with stoichiometric ruthenium tetroxide oxidations and of Brandes and Katzenellenbogen [72a] and Gao and Sharpless [68] with catalytic ruthenium tetroxide and sodium periodate as cooxidant have led to an efficient method for this oxidation step. Examples of the conversion of several diols (67) to cyclic sulfites (68) followed by oxidation to cyclic sulfates (69) are listed in Table 6D.7. The cyclic sulfite/cyclic sulfate sequence has been applied to 1,2-, 1,3-, and 1,4-diols with equal success. Cyclic sulfates, like epoxides, are excellent electrophiles and, as a consequence of their stereoelectronic makeup, are less susceptible to the elimination reactions that usually accompany attack by nucleophiles at a secondary carbon. With the development of convenient methods for their syntheses, the reactions of cyclic sulfates have been explored, Most of the reactions have been nucleophilic displacements with opening of the cyclic sulfate ring. The variety of nucleophiles used in this way is already extensive and includes H [68], [68,73-76], F" [68,72,74], PhCOCT [68,73,74], NOJ [68], SCN [68],... 

Ring-opening reactions of functionalized 1,2-cyclic sulfates and oxetanes with carbanions derived from diisopropyldifluoromethylphosphonate (115) have been reported (Scheme 40). This approach allowed an easy access to y-hydroxy-a,a -difluorophosphonates (116) as building blocks in synthesis of acyclic nucleosides. 

Organic azides can be obtained from cyclic sulfates and from cyclic sulfiles (Schane 3.4). Avenoza et al. studied the nucleophilic ring-opening reactions of gm-disubstituted cyclic sulfates with sodium azide, which lead to the synthesis of azido alcohols 37 and 38. They observed that the legioselectivity depends on the substiment present on the cyclic sulfate. Amide substituents lead preferentially to products arising from nucleophilic attack at the least substituted Cp position, whereas reverse regioselectivity is obtained... 

The cyclic sulfates undergo ring opening with a wide variety of nucleophiles, such as hydride, azide, fluoride, benzoate, amines, and Grignard reagents. The reaction of an amidine with a cyclic sulfate provides an expeditious entry to chiral imidazolines 21 and 1,2-diamines (Scheme 9.27).169... 

IZV118) and the formation of (31) is analogous to the reaction (197)->(98) via a four-membered 1,2-oxathietane 2,2-dioxide intermediate. Subsequent products derived from (31) by electrophilic addition reactions at the alkenic double bond have been described in Section 4.33.3.2.2 and the synthesis of 4,5-dichloro-l,3,2-dioxathiolane 2,2-dioxide (154) by chlorination of ethylene sulfate (18) is discussed in Section 4.33.3.5. Cyclic sulfites, on the other hand, cannot be halogenated without ring opening (cfSection 4.33.3.2.4). 

For cyclic sulfites and sulfates, reaction is favored adjacent to the aryl group while for alkyl enoic acid derivatives, nucleophilic substitution is favored next to the ester group [347]. This ring-opening selectivity was exploited in a synthesis of P-lactams (Scheme 3.32) [348]. 

Sharpless and Kim reported a one-pot synthesis of cyclic sulfates 96 from 1,2-diols via catalytic oxidation with ruthenium chloride51. The cyclic sulfates 96 thus formed on treatment with nucleophiles give /2-sulfates 97, which in turn are hydrolyzed to the / -hydroxy compounds 98 (equation 54). Hence the cyclic sulfates 96 are synthetically equivalent to epoxides. The results of ring opening of cyclic sulfates 96 are shown in Table 4. When the reaction of 99 with malonate anion is carried out in DME, the /2-sulfate moiety serves as a leaving group to give cyclopropane 100 (equation 55)51. 

Likewise, cyclic sulfate 134 underwent ring opening with tbe anion derived from a-pbenyltbio-substituted pyran derivatives 135, leading to tbe formation of a spiro ketal 137 (Scheme 33). A similar reaction with cyclic sulfate 138 led to spiro ketal 140. In contrast, it has been reported that the reaction carried out with the corresponding epoxides did not succeed. 

We explained in Chapter 15 that Sn2 reactions adjacent to carbonyl groups are very fast. The regioselectivity of the ring opening of a cyclic sulfate, like that of an epoxide, is directed by the competition between relative rates of two nucleophilic substitution reactions. Benzylic and carbonyl-substituted positions usually open faster. There is more discussion of the regioselectivity of epoxide opening on p. 351. 

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