Scheme 4. Proposed reaction mechanism.Scheme 5. Derivatization of 2 a and 3 a. mCBPA = meta-chloroperbenzoic acid, PMHS = poly(methylhydroxysilane).Scheme 6. The mode of oxidative addition.

A metal-free stereoselective reductive coupling reaction between isatins and aldehydes is reported. The reaction relies on commercial diethyl phosphite (∼€70 kg−1) as the stoichiometric reductant. Base-catalyzed Pudovik addition and phosphonate/phosphate rearrangement achieved polarity inversion on the isatin, and the derived carbanions were trapped by aldehydes with subsequent dialkoxyphosphinyl migration. Chiral iminophosphoranes were used as basic catalysts to achieve high diastereo- and enantioselectivities with excellent yields.

An acid-promoted novel cascade cyclization is described. Using 8 equiv of trifluoroacetic acid or a catalytic amount of Lewis acid as the promoter, structurally diverse polycyclic cyclopenta[b]indoles were obtained in moderate to excellent yield. This cascade process was extremely effective for the synthesis of 8-membered ring-fused cyclopenta[b]indole derivatives.