Sulfones unsaturated, reduction

Reduction of iodates by ene-diols. . 100 Reduction of unsaturated acetates. 153 Replacement of a sulfonate ester... 

In contrast cyclic y-(rey/-butyldimethylsilyloxy)-a,/ -unsaturated sulfones undergo addition of organometallic reagents to give mainly trans-adducts after reductive removal of the sulfonyl group. 

Skrydstrup, Beau and co-workers122 have adapted Stork s method to the SmI2-reduction of glycosyl pyridyl sulfones bearing a silicon-tethered unsaturated group at HO-C(2). An example is shown with the synthesis of methyl a-C-zso-maltoside 172 from alkyne 170 via the 5-exo-dig radical cyclization of 171 (Scheme 56).144... 

In a water/chlorobenzene biphasic system, reduction of aromatic aldehydes by hydrogen transfer from aqueous sodium formate catalyzed by [ RuCl2(TPPMS)2 2] provided unsaturated alcohols exclusively (Scheme 10.7). Addition of 3-CD shghtly inhibited the reaction [13]. It was speculated that this inhibition was probably due to complexation of the catalyst by inclusion of one of the non-sulfonated phenyl rings of the TPPMS ligand, however, no evidence was offered. 

The sulfone moiety was reductively removed and the TBS ether was cleaved chemoselectively in the presence of a TPS ether to afford a primary alcohol (Scheme 13). The alcohol was transformed into the corresponding bromide that served as alkylating agent for the deprotonated ethyl 2-(di-ethylphosphono)propionate. Bromination and phosphonate alkylation were performed in a one-pot procedure [33]. The TPS protecting group was removed and the alcohol was then oxidized to afford the aldehyde 68 [42]. An intramolecular HWE reaction under Masamune-Roush conditions provided a macrocycle as a mixture of double bond isomers [43]. The ElZ isomers were separated after the reduction of the a, -unsaturated ester to the allylic alcohol 84. Deprotection of the tertiary alcohol and protection of the prima-... 

Reduction of a,P-unsaturated sulfones.1 Sodium amalgam (7, 326-327) is not satisfactory for reductive cleavage of alkenyl sulfones, R CH=CR2S02C6H5. These substrates are best reduced to alkcncs wth potassium-graphite (65-85% yields). 

Review. Use of C8K as a heterogeneous reagent in organic synthesis has been reviewed, particularly as a reducing agent for C=C and C—N bonds, and for reductive cleavage of the C—S bond of a,p-unsaturated sulfones. C8K has been used to obtain active metals dispersed on graphite by reduction of metal halides. 

The reagent effects selective reduction in the presence of nitro, halogen, alkoxy, and sulfone groups, but carbonyl groups undergo competitive reduction and unsaturated substrates undergo hydroboration. 

In the previous subsection, it was shown that the Ferrier reaction offers an opportunity to convert glycal derivatives into unsaturated sugar derivatives, which have an isolated double bond between C(2) and C(3). The Tipson-Cohcn reaction is another important reaction for the introduction of isolated double bonds.29 In this procedure, a cis or tram diols are converted into disulfonates (mesylates or tosylates) which are reductively eliminated with sodium iodide and zinc in refluxing DMF (Scheme 3.6a). In this reaction, the C(3) sulfonate is substituted by an iodide, which then is reductively removed by zinc with concomitant elimination of the second sulfonate moiety, introducing a double bond. Stereoelectronic effects make nucleophilic substitutions at C(3) more favourable than similar reactions at C(2) (see Section 3.2.3). Probably, the elimination proceeds through a boat conformation. In this case, the iodide and tosylate are in a syn relation. In most cases, E2 elimination proceeds via a transition state involving an anti orientation. Nevertheless, syn elimination becomes the dominant mode of reaction when structural features prohibit an anti orientation. 

Mulzer (Scheme 8 upper left) obtained the a,/(-unsaturated ester 33 with Z configuration from aldehyde 26a via a Still-Gennari olefination with phosphonate ester 34. Reduction of the ester with DIBAH and application of L-imidazole-PPhj gives allylic iodide 35. This acts as electrophile on the -anion of sulfone 36. After reductive removal of the phenylsulfone, group 28b is obtained [23]. 

This modification resulted in a yield improvement for the pentacyclization process from 47 % to 66 %. Treatment of the amino ether 192 with diisobutylaluminum hydride in refluxing toluene accomplished Eschenmoser-Grob fragmentation and reduction of the initially formed immonium ion, to give the unsaturated amino alcohol 193 in 86% yield. It was gratifying to find that 193 was the only product formed in this reaction. In the tetrahydropyran derivative, reduction of 192 to 193 is accompanied by about 15 % simple elimination. Displacement of the tosyl group in 196 gives sulfide 197, which is oxidized to sulfone 198. This material is metallated and coupled with enantiomerically pure aldehyde to secure the codaphniphylline skelton [74]. 

The lower reactivity of Raney Ni towards sulfones allows the chemoselective desulfurization of (4) with Raney W-2, but an appreciable amount of reduction of keto groups was observed. Raney Ni W-2 also tolerates sulfonamides in this work an ionic mechanism was suggested in order to explain the substitution reaction observed. Lactones,keto lactones," and unsaturated lactoneswere tolerated during the desulfurization of sulfide or dithioketal" groups, but deactivated (e.g. by acetone) Raney Ni was generally used. In contrast, carbon-halide bonds are cleaved where the desulfurization is stereoselective only because the product is the thermodynamically more stable d5-lactone (equation 2)." e-" f... 

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