Ketones sulfinylation

The photochemistry of dibenzyl sulfoxide 10 was briefly reported in the mid 1960s [21,22]. It was said to decompose mainly to benzyl mercaptan (isolated as the disulfide 17) and benzaidehyde 16. Though no mechanism was suggested at the time, it is now clear that these products arise from a standard a-cleavage mechanism, followed by secondary photolysis of the sulfenic ester 13. The careful reader will note that the yield of bibenzyl (19) is very low in comparison to photolysis of dibenzyl ketone. Sulfinyl radicals rarely lose SO, though some net extrusions are discussed later. 

Metalation ofa-sulfinyl dimethylhydrazones with terf-butylmagnesium bromide, butyllithium or lithium diisopropylamide, and reaction of the generated azaenolates with aldehydes, provides aldol adducts (e.g., 6) as mixtures of diastereomers. Reductive desulfurization leads to fi-hydroxy dimethylhydrazones (e.g., 7) which are cleaved to the desired /(-hydroxy ketones in 25% overall yield10 u. The enantiomeric excesses are about 50%, except for (- )-3-hydroxy-4-methyl-1-phenyl-1-pentanone (8) which was obtained in 88% ee. 

The a-sulfinyl carbanion attacks a-halocarboxylic acid esters at the carboxylic carbon atom, not at the a-carbon atom, and the corresponding ketones are obtained slereoselectively in high yield62. 

Formation of a-Sulfinyl Anions with Lithium Diisopropylamide and Subsequent Addition to a,/ -Unsaturatcd Ketones General Procedure1 ... 

In this chapter the addition of carbon nucleophiles to simple a,j8-unsaturated sulfoxides, a-sulfinyl-a,/ -unsaturated ketones and a-sulfmyl-a,/ -unsaturated lactones will be discussed separately, in most cases the asymmetric induction arises from the chirality at sulfur. 

Enolates derived from various imino compounds have been sulfinylated in reactions analogous to those shown by equations (14) and (15). Some representative examples are shown in equations 16-18. Here again, these compounds have been utilized in asymmetric syntheses. Addition of sulfinate ester 19 to a THF suspension of a-lithio-N,N-dimethylhydrazones, derived from readily available hydrazones of aldehydes and ketones, leads to a-sulfinylhydrazones in good yield, e.g. 53 and 54 (equations 16 and 17)85,86. Compounds 53 and 54 were obtained in a 95/5 and 75/25 E/Z ratio, respectively. The epimer ratio of compound 53 was 55/45. Five other examples were reported with various E/Z and epimeric ratios. 

Introduction of an oxygen substituent at C-6 of a Hantzsch-type dihydropyridine having a sulfinyl group at C-5 affects the reduction of ketones with respect to both reactivity and stereoselectivity <96CC2535>. 

No stereoselectivity was observed in the formation of a 1 1 diastereomeric mixture of 2-hydroxy-2-phenylethyl p-tolyl sulfoxide 145 from treatment of (R)-methyl p-tolyl sulfoxide 144 with lithium diethylamide . However, a considerable stereoselectivity was observed in the reaction of this carbanion with unsymmetrical, especially aromatic, ketones The carbanion derived from (R)-144 was found to add to N-benzylideneaniline stereoselectivity, affording only one diastereomer, i.e. (Rs,SJ-( + )-iV-phenyl-2-amino-2-phenyl p-tolyl sulfoxide, which upon treatment with Raney Ni afforded the corresponding optically pure amine . The reaction of the lithio-derivative of (-t-)-(S)-p-tolyl p-tolylthiomethyl sulfoxide 146 with benzaldehyde gave a mixture of 3 out of 4 possible isomers, i.e. (IS, 2S, 3R)-, (IS, 2R, 3R)- and (IS, 2S, 3S)-147 in a ratio of 55 30 15. Methylation of the diastereomeric mixture, reduction of the sulfinyl group and further hydrolysis gave (—)-(R)-2-methoxy-2-phenylacetaldehyde 148 in 70% e.e. This addition is considered to proceed through a six-membered cyclic transition state, formed by chelation with lithium, as shown below . ... 

In contrast to a, -ethylenic ketones or even a, -ethylenic sulfones, a, ) -ethylenic sulfoxides generally are not sufficiently electrophilic to undergo successful nucleophilic j8-addition . a-Carbonyl-a, j8-ethylenic sulfoxides, however, are potent, doubly activated alkenes which undergo rapid and complete -addition of various types of nucleophiles even at — 78 °C. A brief account summarizing this area is available . The stereochemical outcome of such asymmetric conjugate additions to enantiomerically pure 2-sulfmyl 2-cycloalkenones and 2-sulfinyl-2-alkenolides has been rationalized in terms of a metal-chelated intermediate in which a metal ion locks the -carbonyl sulfoxide into a rigid conformation (36 cf. 33). In this fixed conformation, one diastereoface of the cyclic n... 

In the reaction of ketones with thionyl chloride the intermediate sulfinyl (or sulfenyl) chloride can cyclize on to an aromatic ring located two carbon atoms away giving benzo[b]thiophenes (48) (Scheme 7).54 Additional... 

The same methodology was successfnUy applied for the preparation of alkynes 152 and alkenes 153 from a-sulfinyl ketones 150 and S-mesyloxy snlfoxides 151, respectively (equation 52). 

It should be noted that this desulfinylation is totally regioselective and gives the trisub-stituted enolate. In the cases of a-halo a-sulfinyl ketones, reactions are carried out at —78 °C in Et20 with 1.1 equivalent of ethylmagnesium bromide. Interesting results have been obtained with a-fluoro-, a-bromo- and a-chloro a-sulfinyl ketones. Because of the... 

Chiral sulfinimines 236 are very useful intermediates for the preparation of enantiomer-ically pure primary amines 237 (equation 158) . This reaction has been applied to the synthesis of a-amino acids . For sulfinimines obtained from simple ketones, lithium reagents are preferable for the addition , while for cyclic ketones organomagnesium compounds gave the best results. Addition of alkyl and aryl Grignard compounds to sulfinimines, derived from 3- and 4-substituted cyclohexanones, proceeds with excellent diastereoselectivity, depending on the stereochemistry of the ring substituents rather than the sulfinyl group . 

Application of the method described above to unsymmetrical cyclic ketones such as 2-substituted cyclohexanones gave 2,7-disubstituted and 2,2,7-trisubstituted cycloheptanones (Scheme 8). Treatment of a-sulfinyl lithium carbanion of 1-chloroethyl p-tolyl sulfoxide with 2-substituted cyclohexanones (210a and 210b) afforded adducts as a mixture of two diastereomers. The main adducts were first treated with f-BuMgCl followed by i-PrMgCl (4 equiv) at 0°C to room temperature to give the magnesium /3-oxido carbenoid 211. The... 

Chiral 2,2-disubstituted cyclobutanones have been obtained by asymmetric rearrangement of chiral sulfinyl- 177,178 and sulfanylcyclopropanes.179 Using readily available cyclopropyl 4-tolyl (/ )-sulfoxide (l),180 the requisite sulfinylcyclopropanes 3 and 3 were obtained by a sequence of lithiation, reaction with carboxylic acid esters and stereoselective addition of Grignard reagents to the ketones 2 thus formed.178 The corresponding sulfanylcyclopropanes 4 and 4 resulted from a sequence of protection, reduction and deprotection.179... 

The submitters report that the sulfinyl ketone may be stored in vials in a desiccator at 0°C for more than 1 year without evidence of decomposition. Although storage under an inert atmosphere is not necessary, the checkers found that product exposed to the atmosphere at room temperature became discolored after several weeks. 

Addition to aldehydes or ketones of a-sulfinyl carbanions or of a-lithio-silanes (Peterson)... 

Asymmetric synthesis of fi-hydroxy carboxylic estersEsters of 1 in the presence of f-butylmagnesium bromide as base react with aldehydes and ketones to form optically active a-sulfinyl-jS-hydroxy carboxylic esters, which are desulfurized by aluminum amalgam in aqueous THF (equation I). Chemical yields are 75% of... 

The oxidation step can be avoided if sulfinylation of the carbonyl compound can be conveniently realized. Methyl 2-pyridinesulfinate (2) has been prepared and used for sulfinylation-dehydrosulfinylation of ketones (Table 3.1) and esters [216]. 

A new catalytic cycle for the enantioselective protonation of cyclic ketone enolates with sulfinyl alcohols has been developed (Scheme 2)25 In this method, the achiral alcohol plays two roles it is involved in the turnover of the chiral proton source and also in the generation of a transient enolate through the reaction of its corresponding alkoxide with the enol trifluoroacetate precursor. Stereoselectivity was found highly dependent on the structure of the achiral alcohol. 

---