1.3- Dithiolanes, thiocarbonyl ylides

Historically, the first reactions involving thiocarbonyl ylides involve the preparation of thiiranes and 1,3-dithiolanes from diazomethane and thiocarbonyl compounds reported early in the last century by Staudinger and co-workers (12,13). Similar reactions have been smdied by Schonberg and co-workers (14—16) during the 1960s, but neither was the reaction mechanism understood nor have thiocarbonyl ylides been recognized as key intermediates. [For some remarks to this subject see (8) and (10) in (17).]... 

Based on a series of kinetic studies, Huisgen et al. (91-93) established that thiocarbonyl compounds, especially aromatic thioketones, function as very active dipolarophiles (superdipolarophiles) toward thiocarbonyl ylides. In fact, the trapping reaction of thiocarbonyl ylides with thiocarbonyl compounds represents an excellent method for the preparation of 1,3-dithiolanes. 

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-... 

When thiocarbonyl and ot-diazocarbonyl compounds are combined, acyl-substituted thiocarbonyl ylides 158 are generated from a nonisolable 3-acyl-1,2,4-thiadiazoline 157 (Scheme 8.36). In addition to giving acylthiiranes 159 and 1,3-dithiolanes 160, dipoles 158 can also 1,5-cyclize to produce 1,3-oxathioles 161. Acyl-thiocarbonyl ylides derived from diazoketones [e.g., HC(0)C(N2)R, R = Ph, f-Bu (219,220) 2-diazocyclohexanone (221)] produce 1,3-oxathioles [e.g., 162 (220), Scheme 8.36], while those derived from diazoesters (218,222,223) lead to thiiranes by 1,3-cyclization. Ylides derived from a-diazocarboxamides form 1,3-oxathioles (e.g., 163) and thiiranes (e.g., 159, R = f-Bu, R = NMePh, R = R" = Ph), depending on the nature of the substituents (220). A related 1,5-cyclization of an aminomethyl-thiocarbonyl ylide formed from dimethyl 3-anilino-2-diazobutanedioate was also reported (224). 

The 1,3-dipolar cycloaddition of the carbonyl ylide (31) to the aldimine (32) produces the adduct (33), which has been used to synthesize the taxol C(13) side-chain (34), which is known to be required for the antitumour activity of taxol (Scheme 9).35 The dirhodium tetraacetate-catalysed decomposition of l-diazo-5-phenylpentane-2,5-dione (35) yields the carbonyl ylide (36), which cycloadds to methylenecyclopropanes (37) to produce spirocyclopropanated 8-oxabicyclo[3.2.1]octan-2-ones [(38)-(40)] in 6-75% yields (Scheme 10).36 The 1,3-dipolar cycloadditions of aliphatic or alicyclic thiocarbonyl ylides with thiobenzophenone produce both regioisomeric 1,3-dithiolanes as expected. However, in the case of highly sterically hindered thiocarbonyl ylides, methylene transfer leads to the formation of 4,4,5,5-tetraphenyl-l,3-dithiolane.37,38... 

The dipolar 1,3-cycloaddition reaction of thiocarbonyl ylides to thiones can be a source of 1,3-dithiolanes. Its regioselectivity depends on the nature of substituents in the substrates. Thus, the. J-methylide 589, generated in situ by thermal decomposition of the corresponding l,5-dihydro-l,3,4-thiadiazoles, was reacted with the trithiocarbonate 588 to give a labile 4,4,5,5-tettasubstituted-l,3-dithiolane 590, which easily isomerized to an open-chain compound 591 in the presence of acids in solution (Scheme 84) <2000EJ01695>. 

The thiocarbonyl ylide 594 was intercepted by thiobenzophenone via a 1,3-dipolar cycloaddition reaction to afford two isomeric 1,3-dithiolanes 595 and 596 in 52% and 48% yields, respectively (Equation 79) <1999ACS360>. 

The exocyclic C=S bond of 5-thioxo-2-thiazolines (203) can act as a dipolarophile towards several compounds. For instance, cycloaddition reactions have been reported with nitrilium betaines (204) to give heterocyclic spiro compounds <84HCA534>, with thiocarbonyl ylides (205) to give spirocyclic 1,3-dithiolanes <9IHCA1386>, and with 2-diazopropane (206) to give thiadiazoles <92HCA1825>. On the other hand, the reaction of (203) with diazomethane leads to a mixture of five products (Scheme 49) <93hcai715>. 

The synthesis of the five-membered cyclic dithioate 65 is shown in Scheme 16 [52,53]. The parent y-dithiolactone (3,4,5-trihydrothiophene-2-thione) (146) is prepared by thionation of the corresponding thiolactone 147, as shown in Eq. 32 [74]. Preparations for the l,3-dithiolane-4-thione derivatives 148 have been reported [75,76] (Scheme 32). It was proposed that the adamantanespiro compound 148a was formed by a reaction of the thiocarbonyl ylide, generated from 149 by extrusion of N2, with carbon disulfide [75]. 

Thiocarbonyl Ylides. Thiocarbonyl ylides generated by thermolysis of 2,5-dihydro-1,3,4-thiadiazoles can be intercepted by 1 to give 1,3-dithiolane derivatives (so-called Schdnberg products)." Both aromatic and cycloaliphatic S-methylides undergo [2-i-3]-cycloadditions with 1 regioselectively. Whereas 39 yields the sterically crowded adduct 40, adamantane S-methylide (41) affords the less hindered 1,3-dithiolane (42) (eq However, the reaction of 8, the... 

Alkylidenethioketenes and thioketene 5-ylides (R2C=C=S=CH2) are also described in this chapter. The latter were trapped by an intramolecular cycloaddition reaction to give dithiolanes. Also, propadieneselon, CH2=C=C=Se, is described in this chapter. The unusual extended system, ArN=C=C=C=S, was recently generated by flash vacuum thermolysis ". Thiocarbonyl- -sulfides are treated as Thiosulfines in Section 4.2.2. 

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