Pyrrole ring-closure sequence

Analogously, when the above sequence was followed with pyrrole, ring closure of 141 did occur (when n = 2 or 3) but the cycloadduct 142 underwent rearomatization to an oxime which added to phenylisocyanate providing carbamate 143 in moderate yield [40]. 

Alder/retrograde Diels-Alder reaction sequence of a diaryl alkyne with a 3,6-dicarbomethoxy tetrazine. The resulting diazine (14) is then reduced, cleaved and cyclized with Zn/acetic acid to the 2,3,4,5-tetrasubstituted pyrrole (15), which is then N-alkylated with a-bromo-4-methoxyacetophenone to give a pentasubstituted pyrrole (16). The synthesis of lukianol A is completed by ester hydrolysis, decarboxylation, ring closure and deprotection. 

Reductive ring closure of l-(2-nitrobenzyl)-2-pyrrole carbaldehyde 200 results in pyrrolo[2,l-c][l,4]benzodiazepine 201 (Scheme 42 (1999BMCL1737)). On the other hand, oxo derivative 203 can be synthesized starting from aldehyde 200 through a nitrile formation/cyclizations multistep sequence. The alternate synthetic strategy included reduction of the intermediate acid (R = H) or ester (R = Et) 205 followed by CDI or thermal cyclization (1992JHC1005). 

The ring closure of aminoalkynes bearing a leaving group in the appropriate position might lead to the formation of pyrroles in an addition-elimination sequence. 2-Phcnylcthynyl-1 -amino-sec-butanol. for example, gave 4-ethyl-2-phenylpyrrole on treatment with palladium dichloride in acetonitrile in excellent yield (3.46.),57... 

Not only acetylene derivatives do undergo palladium catalysed intarmolecular carbon-nitrogen bond formation with amines. The similar reaction of olefins in a Wacker-type process also leads to ring closure. (0-Aminopentenes bearing a suitable leaving group in the 4-position were converted to pyrroles in a cyclization-isomerisation-elimination sequence (3.65.),82... 

Moreover, 2,4-disubstituted pyrroles can be synthesized from N,0-acetal 23b, which in turn can be obtained by three-component couphng of sulfonamide 21a, methoxyallene and phenyl iodide (08T809). The aryl group thus inserted into the aUene is part of the coupling process and does not add any extra steps to the sequence. Subsequent RCM and aroma-tization furnished pyrrole 13c in 54% isolated yield the reduced yield is probably due to the steric hindrance of N,0-acetal 23b resulting in less efficient ring closure (Scheme 7). 

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