Synthesis of novel [3,2-b] furan-fused pentacyclic triterpenoids via gold - Catalyzed intramolecular heterocyclization of 2-alkynyl-3-oxotriterpene acids

Effective Synthesis of a Novel Tetrahydrofuran Containing Triterpenoid: 5′(Z)-Benzylidene-tetrahydrofurano[3,2-b]lup-20(29)-en-28-oate

“…In the synthesis of the target triterpenoid 6, C-2 propargyl derivative of betulinic acid 5 was used as the starting compound, which was obtained in several stages from betulin by the method previously developed by our research group [14] (Scheme 1). The key stage of the scheme was α alkylation with propargyl bromide of potassium enoxytriethylborate generated from methylbetulonate 2 under the action of KN(SiMe 3 ) 2 -Et 3 B. Stereoselective reduction of the keto group in the propynyl derivative of betulonic acid 3 using NaBH 4 modified with CeCl 3, produced methylbetulinate 4 in good yield [14] (Scheme 1).…”

“…The key stage of the scheme was α alkylation with propargyl bromide of potassium enoxytriethylborate generated from methylbetulonate 2 under the action of KN(SiMe3)2-Et3B. Stereoselective reduction of the keto group in the propynyl derivative of betulonic acid 3 using NaBH4 modified with CeCl3, produced methylbetulinate 4 in good yield [14] (Scheme 1).…”

“…During the purification of triterpenoid 6 by the method of column chromatography on SiO 2 , an analytically pure sample of the compound was isolated in 32% yield along with its hydrolysis product, that is triterpenoid 7 in 51% yield. In the synthesis of the target triterpenoid 6, C-2 propargyl derivative of betulinic acid 5 was used as the starting compound, which was obtained in several stages from betulin by the method previously developed by our research group [14] (Scheme 1). The key stage of the scheme was α alkylation with propargyl bromide of potassium enoxytriethylborate generated from methylbetulonate 2 under the action of KN(SiMe3)2-Et3B.…”

“…At the same time, polysubstituted furans, tetrahydrofurans and their precursors, 2-alkylidenetetrahydrofurans, are present in numerous natural products or used as important synthetic building blocks in the synthesis of promising biologically active substances [10][11][12][13]. Recently, we developed an efficient method for the synthesis of 2-propargyl 3-oxo-triterpene acid derivatives [14]. The resulting triterpene compounds containing a 4-pentyn-1-one structural unit in ring A have been successfully used in the anionic 5-exo-dig cycloisomerization induced by a strong base, KN(SiMe 3 ) 2 -DME [15].…”

Progress in Cyclizations of 4‐Acetylenic Ketones: Synthesis of Furans and Pyrroles

“…[24] [3,2-b] Furan-fused pentacyclic triterpenoids 136 were smoothly prepared by Spivak and co-workers via intramolecular heterocyclization of 2-alkynyl-3-oxotriterpene acids derivatives 135 in the presence of the PPh 3 AuCl/AgOTf catalyst system (Scheme 26c). [50]…”