Thiocarbonyl ylides intramolecular reactions

Like other 1,3-dipolar species, thiocarbonyl ylides are able to enter intramolecular as well as intermolecular cycloaddition reactions. In this chapter, selected examples of both types will be illustrated. 

An intramolecular cycloaddition reaction was also used in the synthesis of the annelated tetrahydrothiophene (97), starting from l,3-oxathiolan-5-one (96) (131) (Scheme 5.36). Thiocarbonyl ylide formation occurred by thermal extrusion of CO2 at 250 °C, yielding 97 in 62% yield. 

Reactions of thiocarbonyl ylides with nitriles are scarce. Simple nitriles do not undergo bimolecular cycloaddition (171). There is, however, a single example of an intramolecular case that was reported by Potts and Dery (24c,62). By analogy to the intramolecular cycloaddition with acetylenic dipolarophiles (Scheme 5.40), the primary product derived from the reaction of a thiocarbonyl ylide with a nitrile group undergoes a subsequent elimination of phenylisocyanate to give the fused 1,3-thiazole (131). 

An example of an intramolecular cycloaddition reaction was offered by the mesoionic compound (129) which contains in the same molecule both a non-activated alkenic function and a cyclic thiocarbonyl ylide system. In an intramolecular [3 + 2] cycloaddition, (129) yields at 120 °C the tetracyclic primary adduct (130 90%) (81LA347). 

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. 

The thiocarbonyl-stabilized ylides 76, derived in a few steps from alanine and phenylalanine, undergo an intramolecular Wittig reaction involving one carbonyl group of the phthalimide function to give products 77, which are derivatives of the previously unknown pyrrolo[2,l-d]isoindol-5-one-2-thione ring system (Scheme 17 2014H(88)1135). 

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