Divergent Asymmetric Total Synthesis of (-)-Voacafricines A and B.

A divergent asymmetric total synthesis of voacafricines A and B, hexacyclic monoterpene indole alkaloids, has been accomplished featuring the following key steps: a) a catalyst-controlled asymmetric Pictet-Spengler reaction of 6-methoxytryptamine with a chiral α-ketoester affording a 1,1-disubstituted tetrahydro-ß-carboline in excellent yield and diastereoselectivity; b) oxidative cleavage of a 3,5-disubstituted cyclopentene furnishing a dialdehyde intermediate, which was effectively differentiated through spontaneous cyclization with the neighboring hydroxy and secondary amine functions; c) intramolecular nucleophilic addition of a tertiary amino nitrogen atom to the in situ generated oxonium species generating stereoselectively an unprecedented 8-alkyl octahydro-2H-5,8-methanofuro[2,3-b]azepin-8-ium motif bearing five contiguous stereocenters. The synthesis confirmed the absolute configuration of these two natural products.

Organocatalytic Enantioselective Pictet-Spengler Reaction of α-Ketoesters: Development and Application to the Total Synthesis of (+)-Alstratine A.

We report herein an asymmetric Pictet-Spengler reaction of α-ketoesters. In the presence of a catalytic amount of simple alanine-derived squaramide and p-nitrobenzoic acid, reaction of tryptamines with methyl 2-oxoalkanoates afforded the corresponding 1-alkyl-1-methoxycarbonyl tetrahydro-ß-carbolines (THBCs) in high yields and ee values. A primary kinetic isotope effect (KIE=4.5) using C2-deteurium-labelled tryptamine indicates that rearomatization through deprotonation of the pentahydro-ß-carbolinium ion could be the rate- and enantioselectivity-determining step. A concise enantioselective total synthesis of (+)-alstratine A, a hexacyclic cagelike monoterpene indole alkaloid, featuring this reaction as a key step, was subsequently accomplished. Remeasurement of the [a]D value of the natural product indicates that natural alstratine A is dextrorotatory rather than levorotatory as it was initially reported in the isolation paper.

Catalytic Enantioselective Pictet-Spengler Reaction of α-Ketoamides Catalyzed by a Single H-Bond Donor Organocatalyst.

The asymmetric Pictet-Spengler reaction (PSR) with aldehydes is well known. However, PSR involving ketones as electrophilic partners is far-less developed. We report herein the first examples of catalytic enantioselective PSR of tryptamines with α-ketoamides. A new class of easily accessible prolyl-urea organocatalysts bearing a single H-bond donor function catalyzes the title reaction to afford 1,1-disubstituted tetrahydro-ß-carbolines in excellent yields and enantioselectivities. The kinetic isotope effect using C2-deuterium-labelled tryptamine indicates that the rearomatization of the pentahydro-ß-carbolinium ion intermediate might be the rate- and the enantioselectivity-determining step.

Asymmetric Total Synthesis of (-)-Arborisidine and (-)-19-epi-Arborisidine Enabled by a Catalytic Enantioselective Pictet-Spengler Reaction.

A five-step total synthesis of arborisidine, a caged pentacyclic monoterpene indole alkaloid, has been accomplished in both racemic and enantioselective manners. The synthesis features the following three key steps: (a) a regioselective Pictet-Spengler reaction of tryptamine with 2,3-pentanedione; (b) a chemo- and stereoselective intramolecular oxidative cyclization; and (c) a complexity-generating aza-Cope/Mannich cascade followed by in situ oxidation and epimerization. A chiral pyrenylpyrrolidino-squaramide catalyzed enantioselective Pictet-Spengler reaction between tryptamine and 2,3-pentanedione is subsequently uncovered, allowing us to develop a five-step asymmetric synthesis of (-)-arborisidine, an enantiomer of the natural substance. Both (±)-19-epi-arborisidine and (-)-19-epi-arborisidine are also synthesized, which undergo epimerization at room temperature in the presence of aqueous 2 N KOH to (±)-arborisidine and (-)-arborisidine, respectively. The 19-epi-isomer is also partially epimerized to arborisidine upon preparative TLC purification (eluent: MeOH/CHCl3 saturated with NH3) and equilibrium studies indicate that arborisidine is thermodynamically more stable than its 19-epimer. In line with Kam's biosynthesis proposal and their purification protocol, we suspect that 19-epi-arborisidine could also be a natural product.

Catalytic Enantioselective α-Ketol Rearrangement.

A highly enantioselective α-ketol rearrangement has been developed. In the presence of a chiral Cu-bisoxazoline complex, achiral ß-hydroxy-α-dicarbonyls were isomerized to chiral α-hydroxy-ß-dicarbonyls and their bicyclic derivatives in excellent yields and enantioselectivities. Enantioenriched 2-acyl-2-hydroxy cyclohexan-1-ones, dihydroxyhexahydrobenzofuranones, and dihydroxyhexahydro-cycloheptafuranones, with up to three stereocenters, were readily prepared from achiral starting materials in one operation. The reaction is applicable to the desymmetrization of meso substrates and kinetic resolution of racemic alcohols.