Proximal Group Adducts

In addition of organometallic reagents to some arynes, prior counterion complexation with the substituent can direct the incoming group to the ortho position (kinetic control). Addition of alkyllithiums to oxazolinyl (OXZ) aryne (51) to give the ortho product (52) is explained in this manner. In contrast, lithium dialkylcuprates add to the aryne (51) exclusively at the meta position. This is ascribed to thermodynamic control of the reaction, which results in the formation of the more ligated and stable adduct (S3).i2 Control of nucleophilic addition to arynes by complex-induced proximity effects has not been explored with substituents other than OXZ,83 but has considerable synthetic potential if it can be achieved, say through solvent manipulation. 

Reaction of 2, 3 -ene-3 -phenylselenone 13 [R2 = C(4-CH3OC6H4)(C6H5)2] with 1,2-ethanedi-amine in tetrahydrofuran at about 20 °C gives a mixture of the 2, 3 -ribo-piperazine 20 (28%) and the 2, 3 -[A -(2-aminoethyI)]-ribo-aziridine 21 (54%)106. This shows that the second nucleophilic attack at C-3 by a more basic 2 -secondary amino group is favored over attack of the /(-amino group in the C-2 -substituted adduct. The proximity of the 2 -secondary amino group may also contribute to its facile attack at C-3. A similar reaction of 13 (R2 = H) gives direct access to the fully deprotected nucleosides 20 and 21 (R2 = H) in 34% and 38% yield, respectively. 

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