2-Lithio-2-trimethylsilyl-l,3-dithiane

When 2-lithio-2-(trimethylsilyl)-l,3-dithiane,9 formed by deprotonation of 9 with an alkyllithium base, is combined with iodide 8, the desired carbon-carbon bond forming reaction takes place smoothly and gives intermediate 7 in 70-80% yield (Scheme 2). Treatment of 7 with lithium diisopropylamide (LDA) results in the formation of a lactam enolate which is subsequently employed in an intermolecular aldol condensation with acetaldehyde (6). The union of intermediates 6 and 7 in this manner provides a 1 1 mixture of diastereomeric trans aldol adducts 16 and 17, epimeric at C-8, in 97 % total yield. Although stereochemical assignments could be made for both aldol isomers, the development of an alternative, more stereoselective route for the synthesis of the desired aldol adduct (16) was pursued. Thus, enolization of /Mactam 7 with LDA, as before, followed by acylation of the lactam enolate carbon atom with A-acetylimidazole, provides intermediate 18 in 82% yield. Alternatively, intermediate 18 could be prepared in 88% yield, through oxidation of the 1 1 mixture of diastereomeric aldol adducts 16 and 17 with trifluoroacetic anhydride (TFAA) in... 

Thermolysis of 2-diazo-l,3-dithiane, prepared in situ from the reaction of 2-lithio-2-trimethylsilyl-l,3-dithiane and tosyl azide, occurs already below 0°C. The resulting carbene dimerizes efficiently even in the presence of alkenes and alkynes to give bis(l,3-dithianylidene) in 78% yield (Scheme 41) <1997T9269>. 

Chiral /J-amino acyl silanes have been prepared through the addition of 2-lithio-2-trimethylsilyl-l,3-dithiane to enantiomerically pure A-tosylaziridines followed by mercury-mediated thioacetal hydrolysis113. 

The ketenethioacetal (277), prepared from the aldehyde (275) and 2-lithio-2-trimethylsilyl-l,3-dithiane (276), upon treatment with tributyltin hydride gave, via a radical tandem cyclization, the 2-(3-thiol-l-propyl)thieno[3,2-b]thiophene (278) (Scheme 19) <93TL5653>. 

Vinylketene thioaeetais. 2-Lithio-2-trimethylsilyl-l,3-dithiane (1) also reacts with 3,/8-unsalurated aldehydes and ketones exclusively with the carbonyl group to give vinylketene thioaeetais (2). These undergo Dicls-Alder cycloaddition, which Carey and... 

CYCLIZATION n-Butyllithium. Hydrobromic acid. Hydrogen fluoride. Ion-exchange resins. 2-Lithio-2-trimethylsilyl-l,3-dithiane. Lithium diisopropylamide. Polyphos-phoric acid. Tetra-n-butylammonium iodide. Stannic chloride. Trifluoroacetic... 

Ketene thioacetals also are readily availabte > by a Wittig-type reaction of 2-lithio-2-trimethylsilyl-l,3-dithiane (61) with aldehydes or... 

Acyl Anion Conjugate Additions. The lithio reagent readily undergoes 1,4-addition to unsaturated substrates (eq 6), in direct contrast to the corresponding 2-lithio-2-trimethylsilyl-l,3-dithiane, which is a poor Michael donor. The initial Michael adducts can also be alkylated to provide highly functionalized products. Very good levels of diastereoselectivity have been observed in the 1,4-addition and enolate alkylations of cyclic enoates (eq 7)2 and acyclic enones (eq 8). ... 

Lithio-2-trimethylsilyl-l,3-dithiane is the most widely utilized reagent for the conversion of ketones and aldehydes to the corresponding ketene dithioacetals (Scheme 2.49) [126-128]. It is used for the synthesis of functionalized 2-alkylidene-1,3-dithianes 79 [129-135]. The 2-alkylidene-l,3-dithianes 79 thus synthesized are useful synthetic intermediates, which are conveniently accessible by means of Peterson reactions, and they can be transformed into various compounds [136, 137]. For example, compounds 79 are converted to the corresponding carboxylic acids, aldehydes, and enones by hydrolysis, hydrogenation followed by hydrolysis, and deprotonation followed by alkylation and hydrolysis, respectively (Scheme 2.49) [138-140]. 

On the other hand, the reactions of 2-lithio-2-trimethylsilyl-l,3-dithiane with esters and thioesters give not the ketene dithioacetals, but the 2-acyl-l,3-dithianes 81 (Scheme 2.50). The possibly formed silyl enol ethers 80 undergo subsequent hydrolysis to give 81 [141],... 

Scheme 2.50. Reaction of 2-lithio-2-trimethylsilyl-l, 3-dithiane with esters and thioesters.

The reaction of 2-lithio-2-trimethylsilyl-l,3-dithian (94) with aldehydes and ketones is apparently the method of choice for the synthesis of ketone... 

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