1.3- Dithiane dioxide anions

Aggarwal has discovered that the anion derived from 1,3-dithiane dioxide undergoes rapid reaction with a range of aldehydes leading to the formation of diastereoisomeric mixtures in good yield (Scheme 20).76b... 

Hexafluorophosphate derivatives, such as [M(Me2SO)n][PF6]3 (M = La, Lu, Y), have been synthesized (365), and infrared data show O-bonding of the sulfoxide with ionic hexafluorophosphate groups. Analytical data are incomplete for this series, as decomposition, postulated to be to lanthanide fluorides, occurs. The (CH2)4SO complexes [M( CH2 4SO)t.5][PF6]3 are reported (145), which are 3 1 electrolytes with uncoordinated anions, implying a possible semibridged structure, as previously mentioned. Complexes of other cyclic sulfoxides, including thioxane oxide (146) and trans-1,4-dithiane-1,4-dioxide (147) derivatives of hexafluorophosphate salts have also been prepared. 

The l,3-dithiane-l,3-dioxides can also be prepared by sodium periodate oxidation of the 1,3-dithiane, and the irons isomer was thus obtained in a 58% recrystallized yield [81]. Chiral molecules of this type (C2 symmetry ) are particularly suitable for use as chiral acyl anion equivalents [11,82]. 

Halo-1,3-dithiane trans-l,3-dioxides (80 X = Cl, Br) act as diastereoselective carbonyl anion equivalents in reactions with aldehydes.117 The scope of the reaction has been explored by varying the temperature, the aldehyde, and the metal used as counterion. Similarly, metal 1,3-dithianides (81 M = Li, Cu1) can be added diastereoselec-tively to chiral aldehydes subsequent hydrolysis yields an a-hydroxyaldehydc.118... 

The scope and limitations of the metal anions of 2-halo-l,3-dithiane trans-1,3-dioxide as diastereoselective carbonyl anion equivalents has been explored with regard to reaction with aldehydes.79 Reactions of metallated trans-, 3-dithiolanc 1,3-dioxide (five-membered ring) with aldehydes under kinetic and thermodynamic control have also been studied and contrasted with those of the metallated monooxide, parent sulfide, and 1,3-dithiane 1,3-dioxide (six-membered ring).80... 

Dropwise addition of butyllithium to a solution of ( )-l,3-dithiane 5,5 -dioxide (110) in pyridine-THF (1.5 1) generates an anion (111), which reacts with an aldehyde to give an adduct (112) as a 1 1 diastereomeric mixture. The reaction is extremely rapid at -78 °C, but the kinetic selectivity is moderate. In the reaction with benzaldehyde or pivalaldehyde, equilibration is attained at 0 C to give predominantly a single diastereomer in good yield (Scheme 31). ... 

Anions of type (4), for example (7), react with carbon dioxide at —70° to give 1,3-dithianes of a-keto carboxylic acids (11) in 70-75% yield. The reaction of carboxylic acid derivatives is more complex, but the reaction with nitriles to give 1,3-dithianes of 1,2-diketones such as (12) usually proceeds without complication.4... 

Scheme 8.27. (a) An example of the Andersen synthesis of chiral sulfoxides [119]. (b) Catalytic oxidation of an aromatic sulfide using a chiral titanium complex [118]. fc) Synthesis of a C2-symmetrical fra s-l,3-dithiane-l,3-dioxide and its use as an asymmetric acyl anion equivalent [120,121]. 

Although chlorohydrins are obtained in good yield and with high diastereoselectivity by the reaction of the anion derived from 2-chloro-1,3-dithiane 1,3-dioxide with aromatic aldehydes, subsequent ring closure to the epoxide is prevented by a competing retro-aldol reaction <97JCS(P1)11>. 

Anionic Additions to Aldehydes. The /dCa of trans-, >-dithiolane 1,3-dioxide has been determined by Bordwell and disclosed by Aggarwal to be 19.1, a surprisingly low value compared to Trans-1,3-dithiane 1,3-dioxide (24.9). While the deprotonation of 1,3-dithiolane and 1,3-dithiolane 1-oxide leads to unstable carbanions that cleave, the anion of Trans-1,3-dithiolane 1,3-dioxide has shown sufBcient stability to undergo addition reactions with aldehydes. Moreover, because of the Cj-symmetry incorporated into a five-membered ring, its potential to serve as a chiral acyl anion equivalent has been tested. 

Related Reagents. Several other chiral acyl anion equivalents have been published. Conqtarison with 1,3-disulfur derived compounds is reported below. Benzaldehyde has been taken as the reference electrophile. The equivalents are of two kinds mono-and dioxides. Generally, monoxides are slightly less efficient (85 15 ds) or much more difficult to prepare and thus of scarce synthetic interest (100 0 ds). Furthermore, they lead to additional problems because they lack C -sjunmetry. Dioxides are either cyclic or acyclic. Acyclic bis(sulfoxides) work also slightly less efficiently with benzaldehyde (90 10 to 95 5 ds), but slightly better with aliphatic aldehydes. The two reported cyclic dioxides (trans-l,3-dithiane 1,3-dioxide and trans-1,3-dithiolane 1,3-dioxide) are comparable and work equally well (>98 2 ds). Yet, there are some differences trans-1,3-dithiolane 1,3-dioxide proceeds under kinetic control, while trans-1,3-dithiane 1,3-dioxide is best under thermodjuiamic conditions. The latter also offers the advantage to be available in enantioenriched form. 

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