Highly enantioselective organocatalytic [4+2]-cycloaddition of in situ generated trienamines with 4-nitro-5-styrylisoxazoles as α,β-unsaturated ester surrogates is presented. The synthetic utility of this strategy is demonstrated by transforming the formed cycloadducts into optically active carboxylates.

The enantio- and diastereoselective formation of indenes spirofused to highly substituted cyclopropanes is described. The application of a new cinchona alkaloid derived ammonium salt in a phase transfer catalysis setup facilitates high selectivity and excellent yields at low catalyst loadings (0.1-1.0 mol%).

The asymmetric formation of spiroindenes containing up to four contiguous stereocenters from the reaction of benzofulvenes with 2,4-dienals through trienamine catalysis is described. The benzofulvene core was found to be an excellent starting point for the synthesis of interesting spiroindenes through a formal cycloaddition pathway. The reaction was mediated by a diphenylprolinol silyl ether catalyst, and a diverse array of spiroindenes were obtained in high yields with excellent stereoselectivity. An attractive feature of the developed system is the possibility to diversify the product scaffold significantly by further manipulation of the chiral spiroindenes. Thus, three intramolecular ring-closing reactions following the organocatalytic step resulted in highly complex polycyclic systems.

The first enantioselective formal [4+ +2] cycloadditions of 3-nitroindoles are presented. By using 3-nitroindoles in combination with an organocatalyst, chiral dihydrocarbazole scaffolds are formed in moderate to good yields (up to 87 %) and enantioselectivities (up to 97 %e e). The reaction was extended to include enantioselective [4+ +2] cycloadditions of 3-nitrobenzothiophene.T he reaction proceeds through a[ 4 + +2] cycloaddition/elimination cascade under mild reaction conditions.F urthermore,adiastereoselective reduction of an enantioenriched cycloadduct is presented. The mechanism of the reaction is discussed based on experimental and computational studies.

Scheme 4. Intramolecular cyclization reactions and crystal structures. Bn = benzyl, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene;D MP = Dess-Martin periodinane. See the Supporting Information for further details.Angewandte Chemie Communications