Total syntheses of dl-mesembrine, dl-dihydromaritidine and dl-epi-dihydromaritidine via regioselective NaBH4/H+ reduction of imides

Construction of the 5,10b‐Phenanthridine Skeleton Using [3+2]‐Cycloaddition of a Non‐Stabilized Azomethine Ylide: Total Synthesis of (±)‐Maritidine and (±)‐Crinine Alkaloids

“…In the former approach, spiro-fused dienone precursor 7 is obtained by the para-para coupling of substituted norbelladine derivatives employing various oxidizing agents, [24,25,27] photochemical cyclization, [26] intramolecular Heck reaction, [39] and cyclization of an intermediate iron carbonyl complex. [27,28] Substituted 3a-arylhydroindoles 6, which are used for the Pictet-Spengler reaction, are synthesized through key reactions such as regioselective reduction of 1-methyl-3,3-disubstituted pyrrolidine-2,5-dione, [30] intramolecular ene cyclization [31] of an appropriately constructed acylnitroso olefin, or condensation of 3-arylated Δ 1 -pyrrolinium salts with tert-butyl 3-oxopent-4-enoate. [32] A few other approaches reported for the synthesis of 1 have involved intramolecular cycloamination reactions from an appropriate spiro precursor for the carbon-nitrogen bond formation in the construction of the substituted angular phenanthridine skeleton ( Figure 2).…”

Synthesis of Alkaloid Natural Products Using Solid-Supported Reagents and Scavengers

“…The dihydro analogues (16) and (17) of these natural products [18] were also obtained by further reduction using hydrogenation with palladium on carbon (Scheme 5). Once again all products were produced with high purity without recourse to conventional chromatographic work-ups.…”

“…Acid catalysed hydrolysis of the acetal protecting group using trifluoroacetic acid (TFA) furnished the aldehyde which underwent spontaneous cyclisation to the imine. This material was further transformed via an in-situ reduction with palladium on (18) as its TFA salt. Evaporation of the solution followed by dissolution of the residue in ethyl acetate and addition of a polymer-supported carbonate base liberated the free amine base (18), which in turn was captured on to a sulfonic acid resin (Amberlyst 15).…”

“…This material was further transformed via an in-situ reduction with palladium on (18) as its TFA salt. Evaporation of the solution followed by dissolution of the residue in ethyl acetate and addition of a polymer-supported carbonate base liberated the free amine base (18), which in turn was captured on to a sulfonic acid resin (Amberlyst 15). This allowed easy purification of the bound material by filtration of the resin and washing with CH2Cl2 to remove impurities.…”

“…The natural products nornicotine (18) and nicotine (19) are of considerable interest to medicinal chemists owing to their interactions with nicotinic acetylcholine receptors [20]. They have been shown to improve cognitive function and give neuroprotection [21].…”