A conjugate addition/dipolar-cycloaddition cascade sequence for the synthesis of (±)-cylindricine C

Synthesis of the Marine Alkaloid Cylindricine C and Serendipitous Synthesis of Its 2,13-Di-epi Stereoisomer

A new approach to the marine alkaloid cylindricine C afforded its previously unreported (±)-2,13-di-epi stereoisomer as the major product along with a minor amount of the racemic parent alkaloid. Key steps included a stereoselective dianion alkylation of a monoester of 1,2-cyclohexanedicarboxylic acid and an annulation based on the tandem conjugate addition of a primary amine to an acetylenic sulfone, followed by intramolecular acylation of the resulting sulfone-stabilized carbanion. The cis-azadecalin moiety thus formed, comprising the cyclohexane A-ring and enaminone B-ring of the products, was further elaborated by the selenenyl chloride-induced cyclofunctionalization of a pendant butenyl substituent with the enaminone moiety, followed by a seleno-Pummerer reaction. Desulfonylation and enaminone reduction afforded the final products. Molecular modeling and X-ray crystallography provided further insight into these processes.

Enantioselective total syntheses of marine natural products (+)-cylindricines C, D, E and their diastereomers

On the basis of the amide strategy, we have achieved the concise enantioselective total synthesis of (+)-cylindricines D, C, and E, and their 2-epimers in seven, eight, and nine steps, respectively.

Total Synthesis of Fawcettimine-Type Alkaloid, Lycojaponicumin A

The efficient total synthesis of lycojaponicumin A (1) has been accomplished for the first time. The remarkable features of this novel strategy include the following: (1) rapid construction of tricyclic intermediate 4 through a regio- and stereoselective semipinacol ring expansion, which simplified the construction of rings A and B of 1; (2) the subsequent regio- and stereoselective formation of the highly strained rings C-E of 1 through a tandem oxa-hetero [3 + 2] cycloaddition/N-cycloalkylation.

Approach to Fully Substituted Cyclic Nitrones from N-Hydroxylactam Derivatives: Development and Application to the Total Synthesis of Cylindricine C

An approach to cyclic nitrones from N-hydroxylactam derivatives is documented. The nucleophilic addition of an organolithium reagent to an N-OSEM [SEM=2-(trimethylsilyl)ethoxymethyl] lactam forms a five-membered chelated intermediate, which undergoes both elimination and deprotection to give a fully substituted nitrone in a one-pot process. When combined with the N-oxidation of easily available chiral lactams, this method becomes especially useful for the quick synthesis of chiral nitrones in enantio-pure form, enabling the concise total synthesis of cylindricine C.

Enantiodivergency and enantioconvergency in the synthesis of the dendrobate alkaloid 241D

A diastereodivergent preparation of two N-alkenylnitrones (9 and 11) from easily available (R)-2,3-O-cyclohexylideneglyceraldehyde (5) led to an enantiodivergent synthesis of both enantiomers of the dendrobate alkaloid 241D in a sequential two-directional approach involving intramolecular nitrone cycloaddition as the key step. Either of these two nitrones could, in principle, be utilized for the preparation of the title compounds in an enantioconvergent fashion as well. The methodology was extended to prepare an analogue (33) of (-)-241D.

Use of sulfur fragments for the synthesis of nitrogen heterocycles

This contribution contains a representative sampling from the Padwa laboratory of the conjugate addition of oximes with 2,3-bis(phenylsulfonyl)-1,3-butadiene followed by a subsequent dipolar cycloaddition cascade to produce a variety of alkaloids. The resulting cycloadducts are cleaved reductively to provide azapolycyclic scaffolds with strategically placed functionality for further manipulation of the target compounds.

Radical azidation reactions and their application in the synthesis of alkaloids

Recent advances in radical azidation using sulfonyl azides are presented. For instance, radical carboazidation using α-iodoketones, desulfitative carboazidation, and anti-Markovnikov hydroazidation of alkenes are described. These novel methods tolerate a large number of functional groups and allow the synthesis of organic azides that would be difficult to synthesize otherwise. The transformation of the azides using reductive processes as well as a Schmidt reaction under nonacidic conditions were used to synthesize alkaloids including indolizidine 167B, monomorine I, cylindricine C, and lepadiformine C.

Synthesis of fused tricyclic amines unsubstituted at the ring-junction positions by a cascade condensation, cyclization, cycloaddition then decarbonylation strategy

Heating aldehydes that contain a protected hydroxymethyl group, a tethered alkyl chloride and a tethered alkenyl group at the α-position of the aldehyde with an amine sets up a cascade (tandem) reaction sequence involving condensation to an intermediate imine, then cyclization and formation of an intermediate azomethine ylide and then intramolecular dipolar cycloaddition. The fused tricyclic products are formed with complete or very high stereochemical control. The hydroxymethyl group was converted into an aldehyde – which could be removed to give the tricyclic amine products that are unsubstituted at the ring junction positions – or was converted into an alkene, which allowed the formation of the core ring system of the alkaloids scandine and meloscine.