Total syntheses of spiroviolene and spirograterpene A: a structural reassignment with biosynthetic implications†

The recent natural product isolates spiroviolene and spirograterpene A are two relatively non-functionalized linear triquinane terpenes with a large number of structural homologies. Nevertheless, three significant areas of structural disparity exist based on their original assignments, one of which implies a key stereochemical divergence early in their respective biosyntheses. Herein, using two known bicyclic ketone intermediates, a core Pd-catalyzed Heck cyclization sequence, and several chemoselective transformations, we describe concise total syntheses of both natural product targets and propose that the structure of spiroviolene should be reassigned. As a result, these natural products possess greater homology than previously anticipated.


Total Synthesis of Spiroviolene (4)
Bicyclic Ketone 20. To a flask containing 18 2 (0.166 g, 1.00 mmol, 1.0 equiv) was added N,N-dimethyl hydrazine (0.380 mL, 5.00 mmol, 5.0 equiv) and the resultant mixture stirred at 23 °C for 12 h. Upon completion the reaction was quenched with NH 4 Cl (2 mL) and extracted with Et 2 O (3 × 2 mL). The combined organic layers were then washed with H 2 O (1 mL), brine (1 mL), dried (Na 2 SO 4 ), and concentrated to give the desired hydrazone 19 (0.200 g, 96% yield) as a yellow oil. To a flame-dried 30-mL Schlenk flask was added solid LDA (0.070 g, 0.65 mmol, 1.3 equiv, used to exclude any incorporation of hexanes) in a glove box at 23 °C. The flask was S5 capped with a septum and transferred from the glove box to a Schlenk manifold and place under positive pressure of N 2 . The flask was cooled to −78 °C and THF (2.5 mL) was added. Once all solids had dissolved, a solution of a portion of hydrazone 19 (0.104 g, 0.50 mmol, 1.0 equiv) in THF (2.5 mL) was added dropwise over the course of 5 min, and the resulting pale yellow solution was stirred at 0 °C for 2 h. The reaction mixture was then recooled to −78 °C and HMPA (0.113 mL, 0.65 mmol, 1.3 equiv) was added dropwise. After stirring for 10 min, a solution of alkyl iodide 15 (0.150 g, 0.60 mmol, 1.2 equiv) in THF (0.6 mL) was added at −78 °C slowly over the course of 5 min. The yellow solution was stirred at -78 °C for 15 h, where temperature gradually warmed to room temperature. The reaction mixture was quenched with saturated aqueous NH 4 Cl (5 mL) and extracted with Et 2 O (3 × 5 mL). The combined organic layers were dried (Na 2 SO 4 ), filtered, and concentrated to afford the crude product as a pale yellow oil. The crude oil was then further purified by flash column chromatography (silica gel, hexanes/EtOAc, 4/1) to give alkylated hydrazone S3 (0.118 g, 71% yield) as a pale yellow oil. S3: R f = 0.36 (hexanes/EtOAc, 4/1). [Note: First, solid LDA is necessary for the success of this alkylation. Hexanes from standard commercial n-BuLi solutions used for the preparation of LDA was found to be detrimental to the overall conversion and yield. Such efforts included using commercial 1.6 M or 2.5 M n-BuLi in THF and gave low conversion (< 30%); by contrast, using 11 M n-BuLi in THF gave a similar yield as using solid LDA. For safety concerns, we suggest using solid LDA. Second, hydrazone 19 needs to be used immediately after preparation; much lower conversion was observed when using material that had been stored in a refrigerator at 0 °C for 3 days. Third, both the reaction and the acidic workup are suggested to be performed under a N 2 or Ar atmosphere]. Pressing forward, to a 25-mL round bottomed flask was added alkylated hydrazone S3 (0.118 g, 0.36 mmol, 1.0 equiv), THF (5 mL), and 1 M aqueous HCl (5 mL) at 23 °C. The reaction mixture was then vigorously stirred at 23 °C for 12 h. Upon completion, the resulting mixture was extracted with Et 2 O (5 × 10 mL), and the combined organic layers were dried (Na 2 SO 4 ), filtered, and concentrated to afford a yellow oil. The crude oil was purified by filtration through a short silica gel plug (hexanes/EtOAc, 4/1) to afford alkylated ketone 20 (0.094 g, 92% yield) as a pale yellow oil. 20: R f = 0.69 (silica gel, hexanes/EtOAc, 4 Diene 22. To a flame-dried 4-mL scintillation vial was added triflate 21 (30.0 mg, 0.071 mmol), Ph 3 P (3.7 mg, 0.014 mmol, 0.2 equiv), Pd(OAc) 2 (1.6 mg, 0.0071 mmol, 0.1 equiv), and toluene (1.2 mL). After sparging the reaction mixture with argon for 30 min at 23 °C, Et 3 N (0.0195 mL, 0.14 mmol, 2.0 equiv) was added via syringe, and the vial was sealed with a Teflonlined cap. The reaction mixture was heated at 90 °C for 20 h. The yellowish-black mixture was cooled to 23 °C and diluted with hexanes (1 mL). The reaction mixture was subjected directly to purification by flash column chromatography (silica gel, hexanes/Et 3 N, 99/1) to give diene 22 (16.9 mg, 88% yield) as a colorless oil. (-)-Spiroviolene (4). 3 To a 4-mL scintillation vial at 23 °C was added diene 22 (9.0 mg, 0.033 mmol, 1.0 equiv), Pd/C (20.0 mg, 10 wt %, reduced dry powder), and EtOH (0.33 mL). The vial was sealed with a septum and the reaction mixture was sparged with hydrogen for 5 min at 23 °C. The reaction mixture was then stirred for 30 min at 23 °C. Upon completion, the black mixture was filtered through a short plug of Celite and silica gel, rinsing with hexanes (3 × 2 mL). The filtrate was evaporated and purified via flash column chromatography (silica gel, hexanes) to afford (-)-spiroviolene (4, 9.

Total Synthesis of Spirograterpene A (5)
Bicyclic Ketone 24. To a flask containing 23 2 (1.37 g, 6.05 mmol, 1.0 equiv) was added N,N-dimethyl hydrazine (1.38 mL, 18.15 mmol, 3.0 equiv) and the resultant mixture stirred at 23 °C for 12 h. Upon completion, the reaction contents were quenched with NH 4 Cl (5 mL) and diluted with Et 2 O (5 mL). After stirring for 30 min, the resultant mixture was transferred to a separatory funnel and further diluted with NH 4 Cl (30 mL) and Et 2 O (50 mL). The organic layer was separated and further washed with NH 4 Cl (2 × 30 mL), H 2 O (30 mL), brine (30 mL), dried (Na 2 SO 4 ) and concentrated to give hydrazone S4 (1.56 g, 96% yield) as a yellow oil. Pressing forward without any further purification, to a flame-dried, 250 mL Schlenk flask was added solid LDA in a glovebox at 23 °C. The flask was then transferred from the glovebox, attached to a Schlenk manifold, and placed under positive pressure of N 2 . THF (35 mL) was then added and the flask was cooled to -78 °C. After all solids had dissolved, freshly prepared S4 was added as a solution in THF (20 mL) slowly over the course of 5 min. Once that addition was complete, the flask was then warmed to 0 °C and the contents were stirred for 2 h. The reaction contents were then recooled to -78 °C and HMPA (1.51 mL, 8.70 mmol, 1.5 equiv) was added dropwise. After stirring at -78 °C for 30 min, a solution of alkyl iodide 15 (1.74 g, 7.00 mmol, 1.2 equiv) in THF (5 mL) was added over the course of 5 min. The reaction contents were then allowed to slowly warm to 23 °C over the course of 5 h and were stirred at 23 °C for an additional 10 h. Upon completion, the reaction contents were quenched by the addition of 1 N HCl (50 mL) and stirred under a N 2 atmosphere for an additional 12 h. Upon completion, the reaction mixture was diluted with Et 2 O (50 mL) and transferred to a separatory funnel. After separating the resultant layers, Diene 26. To a flame-dried, 50 mL flask was added ketone 24 (0.593 g, 1.70 mmol, 1.0 equiv) and THF (17 mL) at 23 °C. The flask was cooled to -78 °C and KHMDS (4.42 mL, 0.5 M in toluene, 2.2 mmol, 1.3 equiv) was added dropwise. The reaction contents were warmed to 0 °C and stirred for 2 h. The reaction mixture was recooled to -78 °C and a solution of Comins' reagent (0.734 g, 1.87 mmol, 1.1 equiv) in THF (3 mL) was added over the course of 10 min. The reaction was maintained at -78 °C for 20 min before being warmed to 23 °C and stirred for 1 h. Upon completion the reaction was quenched with saturated aqueous NaHCO 3 (5 mL) and diluted with Et 2 O (20 mL) and H 2 O (10 mL). The mixture was then transferred to a separatory funnel and the aqueous layer was extracted with Et 2 O (2 × 15 mL). The combined organic layers were washed with H 2 O (2 × 25 mL) and brine (25 mL), dried (Na 2 SO 4 ), and concentrated. The resultant yellow oil was further purified by flash column chromatography (silica gel, hexanes/EtOAc, 20/1) to give 25 (0.531 g, 65% yield) as a colorless oil. Next, to a flame-dried, 20-mL pressure vessel at 23 °C were added Pd(OAc) 2 (49.4 mg, 0.22 mmol, 0.2 equiv), Ph 3 P (0.115 g, 0.44 mmol, 0.4 equiv) and Et 3 N (0.460 mL, 3.3 mmol, 3.0 equiv). The contents of the flask were suspended in toluene (8 mL) and Ar was bubbled through the reaction mixture for 15 min. A solution of triflate 25 (0.528 g, 1.1 mmol, 1.0 equiv) in toluene (3 mL) was then added at 23 °C and the reaction mixture was heated to 90 °C for 16 h. Upon completion, the reaction contents were diluted with hexanes (5 mL) and directly purified via flash column chromatography (silica gel, hexanes/EtOAc, 20/1) to give the desired cyclization product 26 (0.200 g, 55% yield) as a pale yellow oil.

S13
contents were quenched with saturated aqueous NH 4 Cl (1 mL) and diluted with CH 2 Cl 2 (5 mL) and H 2 O (5 mL). The layers were separated and the aqueous layer was extracted with CH 2 Cl 2 (3 × 5 mL). The combined organic layers were dried (Na 2 SO 4 ) and concentrated to give an orange oil. Purification of this crude material by flash column chromatography (silica gel, hexanes/EtOAc, 10/1) gave the desired benzoate S5 (24.7 mg, 96% yield) as a pale yellow oil. Primary Alcohol 28. To a 4-mL vial at 23 °C containing a solution of MOM ether S5 (57.0 mg, 0.126 mmol, 1.0 equiv) in THF (2.5 mL) was added 6 M HCl (1.5 mL) dropwise. The reaction mixture was then heated to 50 °C for 4 h. Upon completion, the reaction contents were cooled to 23 °C and diluted with brine (5 mL). The resultant layers were separated and the aqueous layer was then extracted with Et 2 O (3 × 5 mL). The combined organic layers were then dried (Na 2 SO 4 ) and concentrated. Purification of the resultant residue by flash column chromatography (silica gel, hexanes/EtOAc, 5/1) gave alcohol 28 (47. 5

S15
Tetrasubstituted Alkene 30. To a flame-dried, 4-mL vial at 23 °C was added carboxylic acid 29 (19.9 mg, 0.047 mmol, 1.0 equiv) and HCl (1 mL, 4 M in dioxane). The reaction mixture was then heated to 80 °C for 48 h. Upon completion, the mixture was diluted with H 2 O (5 mL) and EtOAc (5 mL). The layers were then separated and the aqueous layer was extracted with EtOAc (3 × 5 mL). The combined organic layers were dried (Na 2 SO 4 ) and concentrated to give a brown oil. Purificaiton of that residue by flash column chromatography (silica gel, hexanes/EtOAc, 5/1) gave the desired isomerized product 30 (11.5 mg, 58% yield) as a pale yellow oil. Spirograterpene A (5). 5 To a solution of 30 (11.5 mg, 0.027 mmol, 1.0 equiv) in MeOH (0.25 mL) in a 4-mL vial was added K 2 CO 3 (11.2 mg, 0.081 mmol, 3.0 equiv). The vial was sealed and the reaction contents heated to 55 °C. After 15 h, the reaction was quenched by the addition of NH 4 Cl (1 mL). The aqueous layer was extracted with Et 2 O (3 × 2 mL), dried (Na 2 SO 4 ) and concentrated to give a yellow oil. This crude material was further purified by flash column chromatography (silica gel, hexanes/EtOAc, 2/1) to give 5 (4.7 mg, 52% yield

D. Computational General Information
DFT optimizations were performed through Gaussian 09 on the Midway2 Cluster at the University of Chicago's Research Computing Center. The ground state geometry optimizations and free energies (kcal/mol) of BH 3 and 26 were determined separately using the B3LYP/cc-pvDZ level of theory in the gas phase before being combined for subsequent transition state calculations using the B3LYP/cc-pvDZ level of theory in the gas phase. All ground state structures display no imaginary frequencies. Transition state structures display one negative frequency corresponding to the bond forming event. The complex and product were derived from IRC calculations using the B3LYP/cc-pvDZ level of theory in the gas phase. Further single point energy calculations were then performed at the B3LYP/aug-cc-pvDZ level of theory in the gas phase.