Sulfoxides elimination with base

In an interesting transformation, Nakai et al. (58) treated the sulfoxide (140) with base to generate the fluorovinylsulfoxide (141), then shoved that allylic alcohols (139) add, via an addition-elimination sequence, and the resulting enol (142) rearranges spontaneously to 143 (Scheme 42). 

Enantiomers (M)- and (P)-helicenebisquinones [32] 93 have been synthesized by high pressure Diels-Alder reaction of homochiral (+)-(2-p-tolylsulfo-nyl)-l,4-benzoquinone (94) in excess with dienes 95 and 96 prepared from the common precursor 97 (Scheme 5.9). The approach is based on the tandem [4 + 2] cycloaddition/pyrolitic sulfoxide elimination as a general one-pot strategy to enantiomerically enriched polycyclic dihydroquinones. Whereas the formation of (M)-helicene is explained by the endo approach of the arylethene toward the less encumbered face of the quinone, the formation of its enantiomeric (P)-form can be the result of an unfavourable interaction between the OMe group of approaching arylethene and the sulfinyl oxygen of 94. 

Rosell and Svennson (1975) described a procedure whereby a methylated polysaccharide that contained a limited number of free hydroxyls at specific positions could be selectively degraded. Using a dimethyl-sulfoxide-chlorine complex (Corey and Kim, 1973), the free hydroxyls are oxidized to keto (or aldehyde) groups. Treatment of the oxidized material with base results in the elimination of glycosyl or methoxyl substituents 3 to the activated group, and the resultant a,p-unsaturated... 

Thermolysis of 3-alkylsulflnyl-a-amino acid derivatives is an alternative reaction which has been applied to the synthesis of dehydroamino acid compounds 325, 326). Esters and peptides with dehydroalanine, dehydroaminobutyric acid, dehydrovaline and dehydrophenylalanine units have in many cases been obtained in excellent yields. The method is especially suitable for peptides with base sensitive groups. The elimination appears to take place with particular facility in compounds in which the amino acid unit bearing the sulfoxide group is present as a tertiary amide 326). Dehydrophenylalanine and dehydroaminobutyric acid derivatives are formed as mixtures of Z- and E-isomers on sulfoxide elimination. 

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

Besides simple alkyl-substituted sulfoxides, (a-chloroalkyl)sulfoxides have been used as reagents for diastereoselective addition reactions. Thus, a synthesis of enantiomerically pure 2-hydroxy carboxylates is based on the addition of (-)-l-[(l-chlorobutyl)sulfinyl]-4-methyl-benzene (10) to aldehydes433. The sulfoxide, optically pure with respect to the sulfoxide chirality but a mixture of diastereomers with respect to the a-sulfinyl carbon, can be readily deprotonated at — 55 °C. Subsequent addition to aldehydes afforded a mixture of the diastereomers 11A and 11B. Although the diastereoselectivity of the addition reaction is very low, the diastereomers are easily separated by flash chromatography. Thermal elimination of the sulfinyl group in refluxing xylene cleanly afforded the vinyl chlorides 12 A/12B in high chemical yield as a mixture of E- and Z-isomers. After ozonolysis in ethanol, followed by reductive workup, enantiomerically pure ethyl a-hydroxycarboxylates were obtained. 

Base-catalyzed ring expansion has heen observed vit i cyclo propyInictliy p-toluenesulfonate and bromide.205 Treatment with potassium rert-butoxide in dimethyl sulfoxide induced competing y- and -elimination yielding cyclobutene (14) and methylenecyclopropane (15), respectively. An initial a-elimination was ruled out by deuterium-labeling studies. 

Another example of based-catalyzed elimination in the 2-deoxy sugar series is that of l-[2-deoxy-3-0-(methylsulfonyl)-5-0-trityl-/3-D-eryMro-pentofuranosyl]uracil and 2, 3-anhydro-l-(2-deoxy-5- 0-trityl-/3-D-<. reo-pentofuranosyl) uracil, which, when treated with potassium ferf-butoxide in methyl sulfoxide, give a 70% yield of a 2,3-unsaturated nucleoside.28... 

Deoxygenation of sulfoxides.5 a-Cyano and carboalkoxy sulfoxides undergo eliminative deoxygenation with trimethylsilyl triflate in the presence of the hindered base hexamethyldisilazane to give vinyl sulfides. 

Readily available allylic and benzylic hichloromethyl sulfoxides undergo an unusual base-induced /(-elimination of chloroform, with formation of the corresponding a,/ -unsaturated sulfine, under mild conditions at room temperature die procedure has been applied to form vinylthioaldehyde 5-oxides and vinylthioketone 5-oxides. 89 N-Sulfinylamines (115) have likewise been prepared by /(-elimination of chloroform from trich Ioromcthanesulfinamidcs (114).90 The reaction is promoted rapidly at room temperature by pyrrolidine and Ht N and the sulfinylamines (115) can be trapped by o-phenylenediamme (116), to give benzothiadiazole, before desulfonative hydrolysis occurs. 

On treatment with weak base, 1,3-oxathiolane sulfoxide (80) was shown to undergo a reversible intramolecular ene reaction to sulfenic acid (81). Using stronger base, it was found that this pericyclic reaction cannot compete with elimination to enone (82), which then oxidizes or dimerizes to the observed products (83) and (84). The diastere-omeric sulfoxide (79) was found to undergo retro-ene elimination then dimerization to (85) or to (83) and (84) with even a weak base.85... 

The authors suggested a stepwise mechanism in which precomplexation of the sulfoxide to the lithium azaenolate would take place, thus allowing conjugate addition to follow. This notion was based on Posner s and Paquette s earlier work on conjugate additions of nucleophiles to 57. As before, thermal elimination of toluene sulfenic acid led to the conjugated products (not shown) in 92-93% yields for R2 = methyl.30 However, for R2 = H thermal elimination of arylsulfenic acid did not afford any dihydropyrrole product but rather led to the formation of pyrroles. Treating 58 or 59 with Raney nickel at low temperature led to the rapid formation of 60 and 61, respectively, in 82-86% yield. 

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