1.3- Dithiolanes, deprotonation

Although deprotonation of simple 1,3-dithiolanes at the 2 position is usually accompanied by cycloreversion to the alkene and dithiocarboxylate, this does not occur for the 2-ethoxycarbonyl compound 55. The anion of this is readily generated with LDA and undergoes conjugate addition to a,(3-unsaturated ketones, esters, and lactones to give, after deprotection, the a,8-diketoester products 56 (73TL2599). In this transformation 55 therefore acts as an equivalent of Et02C-C(0) . 

The (racemic) tmns disulfoxide of 1,3-dithiolane 59 is readily deprotonated at C2 by lithium hexamethyldisilazide, and the resulting anion reacts with aldehydes at -78°C with moderate to excellent diastereoselectivity to give mainly the products 60, although subsequent cleavage of these to give the a-hydroxyaldehydes was not described (97JOC1139). 

Ueno and Okawara (184) were the first to explicitly formulate a conjugated thiocarbonyl ylide as an intermediate in the reaction of l,3-dithiolane-2-thione with 4-bromophenacyl bromide. The initially formed thiocarbonyl ylide undergoes deprotonation with sodium hydride to give 2-(4-bromophenyl)-l-oxa-4,6,9-trithias-piro[4.4]non-2-ene. 1,3-Diacylated thiocarbonyl ylides of type 149 (Scheme 5.45) have also been proposed as intermediates in the reaction of 1,3-diphenylpropane-1,3-dione with thionyl chloride. This reaction leads to 2,2,4-tribenzoyl-5-phenyl-... 

A thioaldehyde complex (93) was formed, although in very poor yield (<1%), when the l,3-dithiolane-2-thione complex 92 was deprotonated with LDA at low temperature, treated with CS2, and finally alkylated with [Et30]BF4 [Eq. (19)]. No reaction took place in the absence of CS2. Studies using 14C-labeled CS2 confirmed that CS2 was incorporated into the complex.62... 

Dithiolane. the five-membered version of dithiane, cannot be used in this reaction because, although it is easy to deprotonate, once deprotonated it decomposes by the same mechanism as that used by lithiated THF. 

A route has been reported starting from l, 3-dithiolanes (21) that regio-specifically deprotonate in the presence of base to generate ylide (22), which spontaneously fragments to the thioaldehyde. Trapping reactions... 

Monosubstituted at C-2, 1,3-dithiolanes are easily deprotonated to give the corresponding carbanions which can easily react with various electrophiles. Thus, treatment of the 1,3-dithiolane 327 with EDA followed by... 

An interesting transformation was observed in the case of the 1,3-dithiolane derivative 332, which after deprotonation with LDA smoothly produced the thiolothionophthalic anhydride 335. The proposed mechanism assumed a loss of ethene and intermolecular attack of the resulting intermediate dithiocarboxylate 334 onto the ester function. Further heating of 335 afforded the 3,3 -bithiophthalide 336 in 83% yield (Scheme 43) <20000L3891>. 

Deprotonation of 1,3-dithiolanes leads to the formation of a cationic acetal carbon which easily reacts with nucleophiles such as hydride reagents <85CB3166>, Grignard reagents <81TLI821>,... 

Anionic Additions to Aldehydes. The /dCa of trans-, >-dithiolane 1,3-dioxide has been determined by Bordwell and disclosed by Aggarwal to be 19.1, a surprisingly low value compared to Trans-1,3-dithiane 1,3-dioxide (24.9). While the deprotonation of 1,3-dithiolane and 1,3-dithiolane 1-oxide leads to unstable carbanions that cleave, the anion of Trans-1,3-dithiolane 1,3-dioxide has shown sufBcient stability to undergo addition reactions with aldehydes. Moreover, because of the Cj-symmetry incorporated into a five-membered ring, its potential to serve as a chiral acyl anion equivalent has been tested. 

On treatment with LDA in THF-HMPA, 2-alkylidene-l,3-dithiolans [e.g. (36)] are deprotonated regioselectively and rearrange to give, following hydrolysis, 5-vinyldithioesters the use of HMPA is essential. ... 

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