Indole ring electronic polarization

Another attractive method for E ring formation featured an intramolecular [2+3]cycloaddition of an azide moiety, emanating from the indole 3-position via a two-carbon linker, to, now, an electron-rich version of the C15-C16 double bond.19 The cycloaddition precursor 10 was made via 9, in turn assembled by regioselective cocylization of protected methoxyacetylene (Scheme 5). In a puzzling turn of events, thermolysis of the azide product in toluene at moderate temperature (to minimize nitrene formation) and in low concentration (to suppress intermolecular reactions) produced the two oxidized pentacyclic products 11 and 12 in a 2 1 ratio. Performing the reaction in a more polar solvent (DMF, 80 °C, 7 d) altered the ratio to 5 1.20... 

Based on NMR spectroscopic investigations, Corey proposed structure 254 for the activated aldehyde complex [128]. In this structure, one face of the aldehyde is effectively blocked from attack by the tryptophan ring [127]. The observation that the saturated system gave rise to products displaying considerably diminished selectivity is consistent with the unique electronic interaction between the electron-rich indole and the electron-deficient, polarized C=0 group coordinated to the chiral Lewis acid center. 

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