Asymmetric vinyl sulfides

In contrast to the limited success with vinyl sulfides as components of [2 + 2] cycloadditions, allenyl sulfides show wide applicability. As illustrated in Scheme 8.91, Lewis acid-catalyzed [2 + 2] cycloadditions of l-trimethylsilyl-l-methylthio-1,2-propadiene (333) with a variety of electron-deficient olefins 336 provide cycloadducts 337 with excellent regioselectivity but with moderate stereoselectivity [175c], Nara-saka and co-workers reported the first Lewis acid-catalyzed asymmetric [2 + 2] cycloaddition of C-l-substituted allenyl sulfides 319 with a,/3-unsaturated compounds 338 using a chiral TADDOL-titanium catalyst. The corresponding cycloadducts 339 were obtained with 88-98% ee, but a low level of trans/cis selectivity (Scheme 8.92) [169,175d[. 

Scheme 8C.4. Asymmetric carbonyl-ene reaction of vinylic sulfides and selenides.

CONTENTS Preface, Mark Lautens. Photocyclization and Photocycloaddition Reactions of 4- and 2-Pyrones, Frederick G. West. Intramolecular [4+3] Cycloaddition Reactions, Michael Harmata. Lewis Acid Catalyzed [2+2] Cycloaddition Reactions of Vinyl Sulfides and Their Analogues Catalytic Asymmetric [2+2] Cycloaddition Reactions, Koichi Narasaka and Yujiro Hayashi. Vinylboranes as Diels-Alder Dienophiles, Daniel A. Singleton. Preparation and Exo-Selective [4+2] Cycloaddition Reactions of Cobaloxime-Substituted 1,3-Dienes, P... 

The synthetic utility of the vinylic sulfide and selenide approach is exemplified by the synthesis of enantio-pure (i )-(-)-ipsdienol, an insect aggregation pheromone (Sch. 13) [54], Kabat and Uskokovic have demonstrated the asymmetric catalytic synthesis of la,25-dihydroxyvitamin D3 (la,25(OH)2D3) A-ring synthon by means of a glyoxylate-ene reaction catalyzed by BINOL-Ti complex (1) (Sch. 14) [55]. 

In many respects the Pummeter reaction can be regarded as the sulfur version of the Polonovski reaction (and vice versa), and by analogy to the Polonovski reaction the central intermediate is a sulfur-stabilized carbocation (thionium ion). Although the existence of this species is only transient, it reacts to give a number of different products, e.g. a-acetoxy sulfides, vinyl sulfides, cationic cyclization products, etc., depending upon the sulfoxide structure and reaction conditions. Other reaction pathways ate specific to the Pummerer reaction as a result of sulfur s ability to expand its valence shell (additive Pum-merer reactions). A moderate degree of asymmetric induction is also observed in certain Pummerer reactions, where optically pure sulfoxides are substrates. 

Scheme 3. Asymmetric carbonyl-ene reactions of vinyl sulfides with electron deficient aldehydes.

The Aggarwal group has used chiral sulfide 7, derived from camphorsulfonyl chloride, in asymmetric epoxidation [4]. Firstly, they prefonned the salt 8 from either the bromide or the alcohol, and then formed the ylide in the presence of a range of carbonyl compounds. This process proved effective for the synthesis of aryl-aryl, aryl-heteroaryl, aryl-alkyl, and aryl-vinyl epoxides (Table 1.2, Entries 1-5). 

The successful extension of this asymmetric reaction to the use of allyl halides (instead of benzyl halides) was also reported by the Metzner group [208]. The desired vinyl oxiranes were formed in a one-pot reaction starting from an allyl halide and an aromatic aldehyde in the presence of a sulfide, e.g. 215, and sodium hydroxide as base. A 9 1 mixture of tert-butanol and water was used as solvent. 

Sulfide has also been involved in a new two-step annulation method which featured stereospecific formation of three contiguous asymmetric centers. It was found that the lithium dienolate of the vinyl-ogous ester (6a) reacts rapidly with the a-thiophenyl butenolide (6b) to provide the corresponding adduct as a single diastereoisomer. Subsequent addition of vinyllithium, acidification and cyclization gave the tricyclic lactone (Scheme 30). 

Alkenyl sulfides are known to react with some labile electron-deficient olefins such as methyl vinyl ketone in the presence of AlCl3 to form cyclobutanes [25]. In the present chiral titanium-promoted asymmetric reaction, alkenyl sulfides can also be employed as electron-rich components. 2-Ethylthio-l-propene (7a) reacts with 2a in the presence of a catalytic amount of the chiral titanium reagent, giving the diastereomeric [2-1-2] cycloaddition products 8a and 9a in 51% (>98% ee) and 19% (79% ee) yields, respectively (Scheme 7 and Table 2). Although 2-ethylthio-l-propene (7a) is known as a good ene component in the reaction with carbonyl compounds, 3-(3-(methoxycarbonyl)-5-ethylthio-5-hex-enoyl)-l,3-oxazolidin-2-one, an ene product, is obtained only in 16% yield as a side product. 

Allenmark and coworkers also studied the asymmetric sulfoxidation catalyzed by vanadiumcarboxy)phenyl methyl sulfide and 2-(carboxy)vinyl methyl sulfides [74, 79]. 

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