Diastereoselective synthesis of spiro[carbazole-3,5'-pyrimidines] and spiro[carbazole-3,1'-cyclohexanes] via four-component reaction

Functionalized spiro[carbazole-3,5'-pyrimidines] and spiro[carbazole-3,1'-cyclohexanes] were efficiently synthesized in satisfactory yields with high diastereoselectivity by CuSO4 catalyzed multicomponent reaction of indole-2-acetate, aromatic aldehyde and 1,3-dimethylbarbituric acid or dimedone. The reaction was finished with sequential Diels-Alder reaction of both in situ generated indole-2,3-quinodimethane and a dienophile. Additionally, the initially formed spiro[carbazole-3,5'-pyrimidines] were converted to dehydrogenated spiro[carbazole-3,5'-pyrimidines] by DDQ oxidation. The initially formed spiro[carbazole-3,1'-cyclohexanes] were converted to δ-valerolactone-substituted carbazoles by a DDQ promoted Baeyer-Villiger oxidation process.

Recent approaches to the synthesis of tetrahydrocarbazoles

The tetrahydrocarbazole (THC) motif is ubiquitous in natural products and biologically active compounds.

Total synthesis of (-)-aspidospermine via diastereoselective ring-closing olefin metathesis

An enantiocontrolled total synthesis of (-)-aspidospermine has been achieved. The key element of the strategy is the diastereoselective construction of the quaternary stereogenic center employing 1,4-asymmetric induction during the ring-closing olefin metathesis.

Total synthesis of (+)-aspidospermidine: a new strategy for the enantiospecific synthesis of aspidosperma alkaloids

A new strategy was developed for the enantiospecific synthesis of aspidosperma alkaloids. The key steps involve a novel ketene-lactonization reaction of a chiral vinyl sulfoxide to efficiently set up the quaternary carbon center, and a tandem Michael addition-alkylation reaction sequence to form the polycyclic core structure. This new strategy was employed in the total synthesis of natural product (+)-aspidospermidine.

An efficient approach to Aspidosperma alkaloids via [4 + 2] cycloadditions of aminosiloxydienes: stereocontrolled total synthesis of (+/-)-tabersonine. Gram-scale catalytic asymmetric syntheses of (+)-tabersonine and (+)-16-methoxytabersonine. Asymmetric syntheses of (+)-aspidospermidine and (-)-quebrachamine

Described is a concise, highly stereocontrolled strategy to the Aspidosperma family of indole alkaloids, one that is readily adapted to the asymmetric synthesis of these compounds. The strategy is demonstrated by the total synthesis of (+/-)-tabersonine (rac-1), proceeding through a 12-step sequence. The basis for this approach was provided by a highly regio- and stereoselective [4 + 2] cycloaddition of 2-ethylacrolein with 1-amino-3-siloxydiene developed in our laboratory. Subsequent elaboration of the initial adduct into the hexahydroquinoline DE ring system was accomplished efficiently by a ring-closing olefin metathesis reaction. A novel ortho nitrophenylation of an enol silyl ether with (o-nitrophenyl)phenyliodonium fluoride was developed to achieve an efficient, regioselective introduction of the requisite indole moiety. The final high-yielding conversion of the ABDE tetracycle into pentacyclic target rac-1 relied on intramolecular indole alkylation and regioselective C-carbomethoxylation. Our approach differs strategically from previous routes and contains built-in flexibility necessary to access many other members of the Aspidosperma family of indole alkaloids. The versatility of the synthetic strategy was illustrated through the asymmetric syntheses of the following Aspidosperma alkaloids: (+)-aspidospermidine, (-)-quebrachamine, (-)-dehydroquebrachamine, (+)-tabersonine, and (+)-16-methoxytabersonine. Of these, (+)-tabersonine and (+)-16-methoxytabersonine were synthesized in greater than 1-g quantities and in enantiomerically enriched form ( approximately 95% ee). The pivotal asymmetry-introducing step was a catalyzed enantioselective Diels-Alder reaction, which proceeded to afford the cycloadducts in up to 95% ee. Significantly, the synthetic sequence was easy to execute and required only four purifications over the 12-step synthetic route.

Regiocontrol in an intramolecular Schmidt reaction: total synthesis of (+)-aspidospermidine

[reaction--see text] A total synthesis of (+)-aspidospermidine (1) is described, featuring an intramolecular Schmidt reaction as the key step. The effects of stereochemistry and protecting group status on the regio- and chemoselectivity of this reaction were examined.