Cyclizations SrnI reactions

Formation of cyclized products, resulting from intramolecular attack of the intermediate aryl radical in the SrnI reaction on a second aromatic ring (i.e. intramolecular homolytic arylation) are rare and not synthetically useful.64-158-161... 

Two further quite different cyclization processes each have an initial intermolecular Srn 1 reaction followed by a second in which the ring closure is made. The reactions of o-dihalobenzenes (24) and (25) with the dithiolate (26) to give (27) have already been mentioned (see equation 28), but the yield of analogous product formed between (26) and 1 -brotno-2-iodonaphthalene is only 24% and use of 1,2-eth-anedithiolate with o-diiodobenzene gives only 13% of benzo-l,4-dithiane.98 A rather convenient one-step synthesis of [m.m]-mera-cyclophanediones (m = 3-8) in most acceptable yields is obtained on tandem SrnI reactions of the ketones (45 equation 50).165... 

Carbanions derived from side chain tertiary amides have also been cyclized to provide isoquinolones and isoindoles (equation 36).125 126 While benzyne intermediacy in the formation of the former is likely, the latter seems to arise through a SrnI reaction pathway. Synthesis of indole from the meta bromo compound (87), on the other hand, clearly involves an aryne cyclization. 27 A more versatile route to indoles is based on intramolecular addition of aminyl anions to arynes (equation 38).128 A somewhat similar dihydroindole preparation constitutes the first step in a synthesis of lycoranes (equation 39).129 The synthesis of (88) also falls in the same category of reactions, but it is noteworthy because only a few examples of ring closure of heteroarynes are mentioned in literature.27 28... 

Although the intramolecular SrnI reaction has been studied to a lesser extent as compared to the intermolecular process, it has been shown to represent a highly useful route for the synthesis of various heterocyclic compounds. For example, the SrnI cyclization of N-alkyl-N-acyl-o-haloaniUnes and N-acyl-N-methyl-o-chlorobenzylamines (Equation 13.5) gives N-aUcylindol-2-one and 1,4-dihydro-2H-isoquinolin-3-ones in moderate to good yields [21]. lodoketone 1 reacts under SrnI conditions to provide the complex heterocyclic compound 2 with excellent yield (Equation 13.6) [22]. 

An intramolecular SrnI reaction was used as key step in the synthesis of the alkaloid 0-demethyleupoularamine 3. The intermediately formed SrnI product is readily transformed by photochemical oxidative biaryl formation and subsequent methylation to the target compound 3 in good yield (Equation 13.7) [23]. The SrnI cyclization of l-(2-bromobenzyl)-l,2,3,4-tetrahydroisoquinolin-7-ol derivatives 4 was recently applied to the synthesis of aporphine alkaloids 5 (Equation 13.8) [24] and, by using the same approach, homoaporphine alkaloids can also be synthesized. 

Intramolecular photochemical dehydrohalogenation reactions have featured prominently in heterocyclic and carbocyclic synthesis. The novel photocyclization of the enamine (110) was the key reaction in the synthesis of the antileukaemic lignan steganacin (111) (Scheme 36). This cyclization could have occurred by the SrnI mechanism. ... 

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