Sulfur enantiomerically pure

Under inversion of the configuration at sulfur, enantiomerically pure 4,5-dihydro-2-[(7 )-sulfinylmethyl)]oxazoles (e.g 2) are obtained from metalaled 4,5-dihydrooxazoles and (-)-menthyl (5,)-4-methylbenzenesulfinate31. 

The enantiomerically pure isobomeol allyl sulfoxide derivatives (17 ,2Y,3/ ,4S )-1,7,7-tri-methyl-3-[(S)- or -(/ )-2-propenylsulfmyl]bicyclo[2.2.1]heptan-2-ol are thermally more stable inversion of configuration at sulfur, S -> / , occurs at 135-145 °C. Their lithio derivatives give exclusively y-1,4-adducts with 2-cyclopentenone19. 

An unprecedented stereoselective procedure to obtain enantiomerically pure transition cluster M3Q4 complexes consists of the direct excision of the M3Q7X4 n polymers using chiral diphosphanes, namely (+)-l,2-bis[(2J ,5R)-2,5-(dimethylphospholano)]ethane [(R,R)-Me-BPE] and its respective enantiomer [(S,S)-Me-BPE] to afford the trinuclear complexes (P)-[Mo3S4Cl3(J ,J -Me-BPE)3] and (Af)-[Mo3S4Cl3(S,S-Me-BPE)3] , respectively [30]. The structures of both enantiomers are shown in Fig. 7.3. The symbols (P) and (M) refer to the rotation of the chlorine atoms around the C3 axis, with the capping sulfur pointing towards the viewer. 

This chapter, however, does not deal with above-mentioned reactions of sulfoxides. Rather it is limited to asymmetric synthesis using a-sulfinyl carbanions and -unsaturated sulfoxides, specifically in which the stereogenic sulfoxide sulfur atom is enantiomerically pure. Therefore reactions of racemic sulfoxides are for the most part excluded from this review. For more general discussions, the reader is referred to other chapters in this volume and to other reviews on the chemistry of sulfoxides. Especially useful are the reviews by Johnson and Sharp and by Mislow in the late 1960s and by Oae and by Nudelman as well as a book by Block . A review by Cinquini, Cozzi and Montanari" through mid-1983 summarizes the chemistry and stereochemistry of optically active sulfoxides. This chapter emphasizes results reported from 1984 through mid-1986. 

In an earlier study the authors proposed a [3.2.0] bicyclic sulfonium salt 8 as the reactive intermediate in the trimethylsilyl iodide mediated ring contraction of 4-methoxythiephane <1996T5989>. Enantiomerically pure thio-lane derivatives were synthesized via a ring contraction of a seven-membered sulfur heterocycle by nucleophilic transannular substitution <2000TA1389>. The thiepane derivative 15, derived from d-sorbitol, was converted into the dimesyl derivative 16 following deprotection under acidic conditions. Treatment of 16 with sodium azide in DMSO at 120°C yielded the corresponding thiolane as a mixture of two diastereoisomers, 17a and 17b, in a 5 1 ratio (see Scheme 1). 

Theoretical studies on thiepine-thianorcaradiene (benzene sulfide) valence isomerization have been of continuous interest. In addition, hypervalency of sulfur in thiepines has been theoretically studied. The interest in fused systems, such as dibenzo[/)/]thiepines, toward pharmacologically active compounds is growing. In the last decade, many reports on the synthesis, characterization, and utilization of enantiomerically pure dihydro- and tetrahydrothiepines and thiepanes have appeared. 

Cz Symmetrical and enantiomerically pure thiepines were prepared <1999TL813, 20060BC2218>. Diaryl compounds with two chiral centers 176 were lithiated with sec-butyl lithium in THF at — 78 °C to give the bis-ortho-lithiated 177. Reaction of sulfur diimidazole led to thiepines 26 and 178 in 37—48% yield (Scheme 20). The use of elemental sulfur, thionyl chloride, sulfuryl chloride, SC12, S02(imidazolyl)2, SO(imidazolyl)2, sulfur ditriazole, and sulfur bisbenzotriazole, as a sulfur electrophile, gave cyclized products in poor results. 

Ferrocene is best deprotonated by r-BuLi/r-BuOK in THF at 0 °C,360 since BuLi alone will not lithiate ferrocene in the absence of TMEDA and leads to multiple lithiation in the presence of TMEDA. In the example below,216 a sulfur electrophile and a Kagan-Sharpless epoxidation lead to the enantiomerically pure sulfinyl ferrocene 398. The sulfinyl group directs stereoselective ortholithiation (see section 2.3.2.2), allowing the formation of products such as 399. Nucleophilic attack at sulfur is avoided by using triisopropylphenyllithium for this lithiation. 

Most sulfoxides will retain their configuration at sulfur up to temperatures of about 200 °C—indeed, it is estimated that the half-life for racemization of an enantiomerically pure sulfoxide is about 5000 years at room temperature. However, sulfoxides carrying allyl groups are much less stable—they racemize rapidly at about 50-70 °C. A clue to why this should be is provided by the reaction of an allylic sulfoxide ith trimethyl phosphite, P(OMe)3-... 

Hydrogenation of a sulfur-containing substrate. Usually sulfur-compounds are not satisfactory substrates for catalytic hydrogenations. However, the diacetyl derivative (2) of dehydrobiotin (1) can be hydrogenated successfully with Pd/C as catalyst to enantiomerically pure N,N-diacetylbiotin methyl ester (3). The yield can be made quantitative by using hydrogen pressures of about 3000 psi and increased amounts of catalyst. ... 

The examples shown in Scheme 16, which are representative for all other sulfinates studied, demonstrate a high level of stereoselectivity (around 80-90% of the original enantiomeric excess is retained after switching the stereogenic atom from sulfur to carbon). Despite these encouraging results, the somewhat tedious and stereochemically unreliable preparation of the starting sulfinates [47] may be the reason for the reluctance of the chemical community to use this chemistry as an entry to enantiomerically pure 2-alkenyl sulfones. 

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