Intramolecular cycloadditions thiocarbonyl ylides

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

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 attractive approach toward the preparation of polycyclic systems containing a thiophene ring involves the intramolecular [3 - - 2] cycloaddition of thiocarbonyl ylides. A number of representative examples were reported using mesoionic compounds. Gotthardt et al. (151) used l,3-dithiolium-4-olates such as 89 bearing an olefinic side chain. Upon heating to 120 °C in xylene, the polycyclic tetrahy-drothiophene 90 was formed (Scheme 5.33). 

It is also worthy to note that the intramolecular [3 + 2] cycloaddition of thiocarbonyl ylides occurs easily with nonactivated C=C bonds, whereas the corresponding intermolecular process does not occur. 

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. 

Intramolecular [3- -2]-cycloadditions of thiocarbonyl ylides with nonactivated acetylenes have also been described. Most representative examples involved the use of mesoionic substrates. The initially formed polycyclic adducts of type 110 undergo spontaneous elimination of phenyl isocyanate (24,62,151). A typical example leading to compound 111 is shown in Scheme 5.40. 

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). 

There are two mesoionic sulfur heterocycles which have been shown to undergo intramolecular cycloaddition as thiocarbonyl ylides 1,3-dithiolones130 and 1,3-thiazolones.131 Thus, the alkenyl 1,3-di-thiolone (235) gave a 90% yield of cyclization product (Scheme 70). The analogous alkynyl... 

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. 

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