Synthesis of Fused-Ring and Attached-Ring bis-Tetrahydrofurans via PdCatalyzed Carboetherification

Abstract

A five step-synthesis of fused bis-tetrahydrofurans and attached bis-tetrahydrofurans from butadiene diepoxide is described. Two sequential Pd-catalyzed carboetherification reactions between protected 1,2-diols and aryl/alkenyl bromides, each of which form both a C–O bond and a C–C bond, are used to generate the heterocyclic rings with >20:1 dr. Installation of different R¹ and R² groups is achieved in a straightforward fashion through use of different aryl or alkenyl bromide coupling partners.

A large number of interesting compounds contain attached-ring or fused-ring tetrahydrofuran scaffolds (Figure 1). The 2,6-dioxabicyclo[3.3.0]octane framework (1) is found in both naturally occurring¹ and synthetic molecules that are relevant to human health (e.g., 3; antitumor activity)² and agriculture (e.g., 4; herbicide).³ Attached-ring tetrahydrofurans (2) are displayed in a vast number of natural products, including the annonaceous acetogenins,⁴ of which asimicin (5) is a member.

Figure 1.

Examples of Biologically Active bis-Tetrahydrofurans

A variety of different approaches have been developed for the construction of these useful compounds.^4,5 Many of these strategies involve generation of the bis-tetrahydrofuran framework via sequential (or tandem) ring-closing reactions of 1,2-diols bearing pendant functional groups such as alkenes, epoxides, alcohols, or allylic acetates/halides.⁶ Although these methods effectively form the heterocyclic ring, they do not allow the simultaneous construction of a C–C bond. Thus, substituents attached to the tetrahydrofuran C2-position, such as the side chains present in 3–5, must be installed in separate steps either prior to or following ring-closure.

We felt that an alternative approach to the construction of substituted fused-ring tetrahydrofurans with the general structure 8 could be developed using sequential Pdcatalyzed carboetherification reactions^7,8 of unsaturated 1,2-diols such as 6. As shown in Scheme 1, treatment of 6 with an aryl or alkenyl halide in the presence of NaOtBu and a palladium catalyst should provide 7, which could be converted to 8 in a second catalytic transformation. This strategy could also be applied to the synthesis of attached-ring tetrahydrofurans (e.g., 9 to 10) by simply extending the tether between the alcohols and the alkenes by one methylene unit. Importantly, each carboetherification reaction would generate both a C–O bond (to form the heterocylic ring) and a C–C bond, thus providing a more concise approach to substituted bis-tetrahydrofurans compared to currently available methods.

Scheme 1.

Synthetic Strategy

In order to examine the feasibility of the strategy outlined above, we elected to examine the selective monocyclization of known diols 6⁶ⁱ and 9,⁹ which can be generated by Cu-catalyzed addition of vinylmagnesium bromide or allylmagnesium bromide to commercially available butadiene diepoxide. We also prepared mono-TBS-protected¹⁰ derivatives 11 and 12 (Figure 2), as our prior studies indicated that carboetherification reactions of mono-protected 1,2-diols are often more efficient than transformations of the corresponding unprotected diols.⁸

Figure 2.

Protection of 1,2-Diols

Preliminary attempts to effect selective monocyclization of unprotected diols 6 and 9 provided unsatisfactory results (Figure 3). Treatment of 6 with one equivalent of bromobenzene under our standard carboetherification conditions (NaOtBu, cat. Pd₂ (dba)₃/Dpe-Phos)¹¹ afforded mixtures of bis-cyclized product 13 and unreacted starting material. Treatment of 6 with four equivalents of bromobenzene led to complete consumption of starting material and the formation of 13 with >20:1 dr, albeit in only 30% yield. Efforts to achieve monocyclization of 9 did lead to the formation of desired tetrahydrofuran 14, but yields were low and isomerization of the second alkene was problematic. Use of excess aryl halide in this reaction failed to generate significant amounts of the bis-tetrahydrofuran target, and instead provided an 81% combined yield of 14 and inseparable alkene isomers.

Figure 3.

Attempted Monocyclization of 1,2-Diols

Although carboetherification reactions of 6 and 9 were generally ineffective, transformations of TBS-protected substrates 11 and 12 proceeded smoothly. As shown in Table 1, treatment of 11 with an aryl bromide in the presence of NaOtBu and a catalyst composed of Pd₂(dba)₃ and Dpe-Phos provided tetrahydrofurans 15 in good yields with excellent diastereoselectivities. Cleavage of the silyl ether protecting group was achieved under standard conditions, and carboetherification of the resulting alcohols 16 provided fused tetrahydrofurans (8) as single stereoisomers (>20:1 dr). Both cyclizations led to products that are trans−2,5-disubstituted around the tetrahydrofuran ring(s), which is consistent with our previously reported observations and stereochemical models for the conversion of γ-hydroxy alkenes to tetrahydrofurans.⁸

Table 1.

Stepwise Synthesis of Fused Tetrahydrofurans^a

entry	R1	yield 15 (%)b	yield 16 (%)b	R2	yield 8 (%)b	overall yield (%)c
1	Ph	86	92	p-F3CPh	86	68
2				p-PhC(O)Ph	94	74
3	p-Tol	91	91	3-pyridyl	91	75
4				m-MeOPh	81	67
5	p-tBuPh	71	90	p-PhPh	87	43
6				2-naphthyl	85	54
7	p-PhC(O)Ph	44	92	(E)-β-styryl	67	27
8				6-MeO-2-naphthyl	89	36

^aConditions: Steps 1 and 3: 1.0 equiv 11 or 16, 2.0 equiv ArBr, 2.0 equiv NaOtBu, 2 mol % Pd₂(dba)₃, 4 mol % Dpe-Phos, THF, 65 °C. Step 2: 1.0 equiv 15, 10 equiv TBAF, THF, rt.

^bIsolated yields (average of two or more experiments). All products were obtained with >20:1 dr.

^cYield obtained over the three step sequence from 11 to 8.

The first carboetherification reaction in this sequence (11 to 15) was sensitive to the electronic properties of the aryl bromide, and the best yields were obtained with electron-neutral substrates (entries 1–6). However, the scope of the second carboetherification reaction (16 to 8) was much broader, and a number of different aryl bromides were effectively coupled. In addition, use of β-bromostyrene in the second transformation was also successful (entry 7). Diastereoselectivities were uniformly high in all of the carboetherification reactions (>20:1 dr), and in many cases the overall yield of 8 exceeded 50% over the three-step sequence.

The synthesis of attached-ring bis-tetrahydrofurans was achieved by subjecting protected diol 12 to an analogous sequence of carboetherification (12 to 17), deprotection (17 to 18), and carboetherification (18 to 10). As observed in the transformations of 11, the scope of the second carboetherification step is considerably broader than the first (with respect to the aryl bromide component). Yields of attached-ring tetrahydrofurans were slightly lower than the corresponding fused-ring products described above. However, diastereoselectivities were excellent, and all products were obtained with >20:1 dr favoring 2,5-trans-stereochemistry around both tetrahydrofuran rings.

To further probe the synthetic utility of these transformations, we sought to determine if nonracemic starting materials could be converted to bis-tetrahydrofuran products without loss of enantiomeric purity. To this end, (–)−12 was prepared in 96% ee via asymmetric dihydroxylation of commercially available trans−1,5,9-decatriene¹² followed by mono-TBS-protection of the resulting diol. This substrate was converted to (–)−10h using a sequence of reactions identical to that shown in Table 2, entry 8, and the product was obtained with >20:1 dr and 95% ee (Figure 4).

Table 2.

Stepwise Synthesis of Attached Tetrahydrofurans^a

entry	R1	yield 17 (%)b	yield 18 (%)b	R2	yield 10 (%)b	overall yield (%)c
1	p-tBuPh	73	89	p-Tol	64	42
2				p-PhPh	63	41
3	Ph	72	91	3-pyridyl	54	35
4				2-naphthyl	61	40
5	m-MeOPh	64	91	p-F3CPh	75	44
6				p-PhC(O)Ph	72	42
7	p-MeOPh	55	96	3,5-Cl2Ph	71	37
8				6-MeO-2-naphthyl	53	28

^aConditions: Steps 1 and 3: 1.0 equiv 12 or 18, 2.0 equiv ArBr, 2.0 equiv NaOtBu, 2 mol % Pd₂(dba)₃, 4 mol % Dpe-Phos, Toluene or THF, 65 °C or 110 °C. Step 2: 1.0 equiv 17, 10 equiv TBAF, THF, rt.

^bIsolated yields (average of two or more experiments). All products were obtained with >20:1 dr.

^cYield obtained over the three step sequence from 12 to 10.

Figure 4.

Synthesis of an Enantioenriched bis-Tetrahydrofuran

The synthesis of more elaborate tetrahydrofuran products is also feasible using this method. For example, protected tetraol derivative 19 was generated from Dmannitol using standard transformations.¹³ This substrate was converted to bis-tetrahydrofuran 22 with >20:1 dr using the same reaction sequence described above (Scheme 2).

Scheme 2.

Synthesis of a Highly Substituted bis-Tetrahydrofuran

In conclusion, we have developed a concise approach to the construction of both attached-ring and fused-ring bis-tetrahydrofurans using sequential Pd-catalyzed carboetherification reactions. This strategy allows for preparation of derivatives bearing different substituents at the 2-position of each tetrahydrofuran ring, and provides access to derivatives that could not be easily generated with existing methods. Further studies on the application of these reactions to the synthesis of natural products and molecules of medicinal interest are currently underway.