Formate derivatives intramolecular trapping

A series of benzisothiazolone derivatives 238 has been prepared from methylthiosalicylate 235 O60L4811>. The key cyclization step features the formation of a TV-acylnitrenium ion 237, generated by the hypervalent iodine reagent, phenyliodine(III)bis(trifluoroacetate) (PIFA). This ion cyclizes to benzisothiazol-3-one 238 upon intramolecular trapping of the thiol moiety. 

Feldman has reported two interesting exeunples of epoxide ring opening. The opening reaction in the epoxides (34a) and (34b) affords an ylide-like species that is intramolecularly trapped to yield the two adducts (35a) and (35b). The 3-substituted derivative (34b) also affords another product identified as (36). The formation of this product is thought to involve a Norrish Type I process proceeding via (37) to the lactone (38) which then undergoes [2-1-2] cycloaddition. 

There are many examples of such reactivity and some of these have been reviewed by Roth and coworkers, a research group that is extremely active in this area. An example that is typical of the processes encountered involves the cyclization of the diene geraniol (1). In this case the sensitizer is 9,10-dicyanoanthracene (DCA) and the reactions are carried out in methylene chloride. The authors state that a contact radical-ion parr is involved, i.e. the radical cation of the diene is in close proximity to the radical anion of the DCA. Reaction within this yields the cyclopentane derivatives 2 and 3 in the yields shown. The ring formation is the result of a five centre CC cyclization within the radical cation of 1. When a more powerful oxidant such as p-dicyanobenzene is used as the sensitizer in acetonitrile as solvent, separated radical-ion pairs are involved. This leads to intramolecular trapping and the formation of the bicyclic ethers 4 and 5 . The bicyclic ether incorporates an aryl group by reaction of the radical cation of the diene with the radical anion of the sensitizer (DCB). This type of reactivity is referred to later. Other naturally occurring compounds such as (/fj-f-bj-a-terpineol (6) and (R)-(- -)-limonene (7)... 

Photoaddition of the ynone (47) to 1,1-diethoxyethene gave the oxetan (48) (50%), by a conventional (2+2)-cycloaddition route, as well as the furan derivative (49) (18%). The formation of this latter product arises by way of the carbene (50), itself a product of (3- -2)-addition of the ynone to the alkene. Intramolecular trapping of the carbene... 

A few reports have appeared on the thermal decomposition of sultams. The thermolysis of unsaturated -sultams leads to formation of the corresponding pyrrole and indole derivatives (equation 233 and 234)291 and resembles the decomposition of the related sultones. The high-temperature pyrolysis of naphthosultam gives a mixture of 2- and 3-cyanoindenes (equation 235)292. On the other hand, the AZ-phenyl derivative forms the benzacridine 144 formed by loss of S02 and intramolecular trapping of the intermediate which forms (equation 236). Flash vacuum pyrolysis of the bicyclic sultam 145 gave a... 

Photoreaction of the N-acylbenzoxazole-2-thiones (358) with alkenes yields iminothietanes (362) and 2-substituted benzoxazoles (363) by intramolecular trapping of the zwitterionic intermediates (361) and (360), respectively, derived from the spirocyclic aminothietanes (359) whose regiochemistry is in accord with formation of the more stable biradical intermediate in the [2+2] cycloaddition... 

Oxocanyl carbenium ions (42) are strongly indicated in the Lewis acid-catalyzed cyclization of 5-hexenylacetals. The evidence for formation of these species includes (i) the presence of aldehyde by-products (ii) the isolation of 2-alkoxyoxocanes (oxocanylacetals) (iii) intramolecular trapped products for some unsaturated acetal derivatives and (iv) the formation of 2-alkylated oxocanes when alkylaluminum complexes are used as the Lewis acid catalyst. 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

The intramolecular thermal cyclotrimerization of dodeca-1,6,11-triyne (110) at 450-600 °C afforded l,2,3,6,7,8-hexahydro[a5]indacene (112) and dehydro derivatives. An exothermic cycloaromatization mechanism has been proposed. An initial formation of a single bond gives diradical (111) which is then trapped by an alkyne. °... 

Intramolecular ylide formation with the lactone carbonyl oxygen (53) in 145 provided a carbonyl ylide 146 that was trapped with Al-phenyl maleimide to give cycloadduct 147. Likewise (54), carbonyl yhde 149, derived from ester 148, suffers intramolecular cycloaddition with the tethered alkene to deliver acetal 150 in 87% yield. An enantioselective version of this process has also been described (Scheme 4.33). 

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