1.3- Dipolar cycloaddition reactions tetrahydroisoquinolines

Likewise, benzyldihydroisoquinolinium derivatives can be used in a photochemical synthesis of tetrahydroisoquinolines. Thus, 2-(2-trimethyl-silylmethylphenylmethyl)-3,4-dihydroisoquinoliniun perchlorates have been successfully cyclized, as in the synthesis of the protoberbine alkaloids (+)xylopinine and (+)stylopine. The reaction proceeds via SET from the xylyl donor to the iminium moiety, fragmentation of the benzylsilane radical cation and carbon-carbon bond formation in the intermediate diradical. The synthesis is rather general and the yields compare favorably with those obtained from related substrates via a dipolar cycloaddition methodology [298] (Sch. 27). 

An interesting variation of the classical multicomponent reaction was recently described starting from a glyoxale and a tetrahydroisoquinoline generating in the first step an immonium species which underwent 1,3-dipolar cycloaddition with A-methyl maleimide as shown in Scheme 1.8.5.10.i ... 

Aleman, Fraile and coworkers described 1,3-dipolar cycloadditions of unsaturated aldehydes and cyclic azomethine imines (Scheme 8.36). The reaction proceeds either via iminium or dienamine activation of the corresponding aldehyde, giving chiral tetrahydroisoquinoline derivatives. Interestingly, it can be controlled either by the structure of the dipole or by additives such as tetrabutylammonium bromide. This transformation was investigated also by Du, Wang and coworkers. ... 

Benzoindole 114 has been synthesized by a remarkable sequence, in which the benzannulation precursor 113 is pre-assembled starting from two different chromium carbenes. It is formed in a [3+2] cycloaddition, in which the acylamino carbene complex 111 acts as the dipolar component and the alkynylcarbene complex 112 serves as the dipolarophile. The resulting 3-pyrrolylcarbene complex 113 undergoes a photoinduced intramolecular benzannu-lation to give the benzoindole 114 [84a]. This strategy complements an approach towards carbazoles [84b], Isoindolines and 1,2,3,4-tetrahydroisoquinolines are accessible from the reaction of pentacarbonyl (a-methoxyethylidene) chromium with 7r,co-dialkynes bearing a nitrogen atom in the carbon ether [84c]. 

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