Chemoselectivity of Intramolecular Aza-Wittig Cyclizations

The chemoselectivity of bifunctional systems is important for the selective ring construction by intramolecular aza-Wittig reactions. The selective cycli-zation of methyl 1-(2-azidobenzoyl)-5-oxo-L-prolinate 76a with TBP and TPP involves the ester carbonyl group rather than the imidoyl carbonyl group affording 77a selectively; but both 77a and 78a are formed with triethylphos-phite based on the product ratios 77/78 as determined by 1H NMR spectra of the reaction mixture (Scheme 17, Table 1) [219,220]. The isolated 77a is sensitive to moisture and is converted quantitatively into 78a by treatment with a catalytic amount of conc. HCl in THF at rt for 3 h. The imi-docarbonyl group of amide 76b is more reactive yielding 78b exclusively. The product ratio of 7-membered rings 77a,b versus 6-membered rings

co2h co2h

79 linaric acid

Scheme 17 Chemoselectivity of intramolecular aza-Wittig cyclization [219,220]

Table 1 Chemoselectivity of the Staudinger/aza-Wittig tandem reaction of 76a,b [219, 220]


76: X

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