Stereocontrolled Syntheses of Seven-membered Carbocycles via Tandem Allene Aziridination/[4 + 3] Reaction

Abstract

A tandem allene aziridination/[4+3]/reduction sequence converts simple homoallenic sulfamates to densely functionalized aminated cycloheptenes, where the relative stereochemistry at five contiguous asymmetric centers can be controlled through the choice of the solvent and the reductant. The products resulting from this chemistry can be readily transformed into complex molecular scaffolds that contain up to seven contiguous stereocenters.

Graphical Abstract

A tandem allene aziridination/[4 + 3]/reduction sequence converts homoallenic sulfamates to aminated cycloheptanes, where the relative stereochemistry at five contiguous chiral carbons can be controlled through the choice of solvent and reductant.

Densely functionalized seven-membered rings are valuable synthetic targets occurring in a range of bioactive natural products and their analogs (Figure 1). Several approaches to address the challenge of controlling the stereochemical outcome in the synthesis of seven-membered rings have been developed, including annulations, ring-closing metatheses and an array of cycloaddition reactions.^[1] One powerful strategy involves the [4+3] reaction of a 1,3-diene with a suitable three-carbon coupling partner; analogous to the venerable Diels-Alder reaction, potential exists to form multiple new stereocenters in a highly controlled fashion.^[2,3]

Figure 1.

Bioactive natural products with seven-membered carbocyclic cores.

Oxyallyl cations are popular coupling partners in [4+3] cyclo-additions; however, use of their nitrogen counterparts is far less common. Reactive 2-amidoallyl cations for reported intramolecular [4+3] reactions have been generated from α-chloroenamines, α-chloroimines, allenes and methylene aziridines (Scheme 1A).^[3–5] Typically, nitrogen is not retained in the product and the stereochemical outcome is dictated by the nature of the substrate. With these limitations in mind, we wanted to develop an intermolecular, stereodivergent [4+3] cycloaddition protocol with the ability to rapidly increase molecular complexity from simple allene precursors.

Scheme 1.

Stereochemical diversity via allene aziridination.

Our group has reported a suite of highly chemo-, regio- and stereoselective oxidative aminations that transform allenes into a diverse array of amine stereotriads with control over both the identity and relative stereochemistry of the three heteroatoms at C1–3.^[6] We envisaged extending this concept to controlling relative stereochemistry at each of the three original allene carbons in the synthesis of aminated cycloheptenes through a tandem aziridination/[4+3]/reduction sequence (Scheme 1B). This strategy features: 1) a one-pot allene aziridination/ring-opening of the C-N bond of 2 to yield 2-amidoallyl cation 3–4, 2) intermolecular trapping with inexpensive furan, where the reaction conditions control the relative stereochemistry at C1 and C3 in 5–6 and 3) reagent-controlled reduction of the imine to yield stereodivergent syntheses of all four stereoisomers of 7, containing functionality for further elaboration into useful building blocks.^[7]

Initial efforts to convert 8 to imines 12 and 13 through the intermediacy of 10 and 11 (Table 1) showed a 1:1 THF/furan mixture successfully transformed 9 to the endo [4+3] adduct 12 at rt (entry 2). The syn C1–C3 relationship was established by X-ray crystallography (see the SI for details). Reaction in polar aprotic solvents, such as MeCN and MeNO₂ (entries 3–4) did not occur at rt, but heating to 50 °C resulted in endo cyclization to furnish 13 with a 1,3-anti relationship. The yield of 13 was increased to 45% over two steps by heating in MeNO₂ at 65 °C, albeit in slightly lower dr.

Table 1.

Solvent screen for the formal [4 + 3] cycloaddition.

The differing stereochemical outcomes in 12 and 13 using THF or MeNO₂ were intriguing; control experiments showed neither product epimerized under the reaction conditions. Shimizu noted solvent effects on the dr of [4+3] cycloadditions of 2-oxyallyl cations; in contrast to our results, MeNO₂ favored 1,3-syn products in typical dr of 2:1, while THF:Et₂O favored the 1,3-anti products, also in low dr.^[8] We hypothesize under our reaction conditions, 12 results from rapid addition of furan to 11, while THF permits equilibration of cation 11 to 10 to relieve A^1,3 strain, resulting in the syn product 12. This could occur through rapid reversible addition of the ethereal oxygen to the allyl cation, permitting bond rotation and equilibration to 11 before addition of furan takes place.

The scope of the thermal [4+3] cycloaddition in MeNO₂ was explored first (Table 2). The intermediate imines were reduced using NaBH₃CN, resulting in attack on the imine via a pseudo-axial trajectory. Allenes 8 and 14–16, with monosubstitution at C3 (entries 1–4), delivered 13a–16a in moderate-to-good dr. The additional stereocenter α to C3 in allene 17 (entry 5) exerted an influence on the dr of 17a, resulting in only two major diastereomers in a 4.7:1 ratio.^[9,10]

Table 2.

Scope of the [4+3] reaction in MeNO₂using NaBH₃CN as reductant.

Gratifyingly, trisubstituted allene 18 (Table 2, entry 6) proved a good substrate for tandem aziridination/cycloaddition to set the all-carbon quaternary stereocenter. While NaBH₃CN gave low conversion, LiBH₄ yielded 18a in 10:1 dr. When the three-step transformation was performed in one pot, residual Rh catalyzed the reduction of the olefin with LiBH₄ to furnish 19a. Ultimately, we found olefin hydrogenation (entry 7, using Pd/C and H₂) prior to imine reduction gave reproducible high yields of 19a in excellent dr.^[10] The chemistry could even distinguish between a Me and an Et group at C3 of allene 20 to yield 20a (entry 8) or 21a (entry 9, Pd/C and H₂ to reduce the alkene) in good dr.^[10] Finally, trisubstituted allene 22, containing an additional stereocenter α to C3, gave 22a (entry 12) in dr comparable to the disubstituted allene 17 (entry 5).

Access to the 1,2-syn:2,3-anti diastereomer 13b requires the hydride to approach the 1,3-anti imine from the same face as any substituents at C3 of the allene precursor (Table 3). DIBAL-H and triisobutylaluminum gave 13b as the minor diastereomer; however, AlH₃•Me₂NEt furnished the desired stereochemical outcome. Presumably, AlH₃ binds to the oxygen of the [3.2.1] bicyclic ring to direct reduction to the hindered imine face, although coordination to the sulfamate O or to the imine is also possible. The challenge of overriding substrate control is reflected in lower dr and scope (Table 3, entries 1–3) compared to the 1,2-anti:2,3-syn diastereomers. Alane did not tolerate a Si-protected oxygen in 16 or branching α to C3 in 17 as 16b and 17b were observed only in trace amounts (entry 4 and 5). Axial reduction was noted for trisubstituted allenes (entry 6); despite the lower dr, the use of AlH₃•Me₂NEt gave more reproducible access to 18a, as compared to the use of LiBH₄ (Table 2, entry 6).

Table 3.

Scope of the [4+3] in MeNO₂ with AlH₃•Me₂NEt as the reductant.

The next goal was to improve access to 1,3-syn imines of the form 12 (Table 1). Reaction of 8 with a Rh catalyst, followed by [4+3] in THF at 50 °C (entry 2) resulted in only a 31% yield of 12. A variety of Lewis acids (see the SI for details) were tested, with 10 mol % of AgPF₆ providing optimal results. Adoption of these conditions furnished the 1,3-syn imine 30 (Table 4) in typical dr >10:1. To achieve tunable reduction of 30, we postulated that less bulky hydride sources (NaBH₃CN) should approach from the top face of 30 to favor the 1,2-anti:2,3-anti diastereomers. In contrast, bulkier reductant such as LiBHEt₃ would be expected to favor reduction from the face opposite the alkene bridge to give the all syn stereotriads. The 1,3-syn imines were much easier to reduce than the corresponding 1,3-anti imines. For example, NaBH₃CN reduced syn imines to the anti:anti products 13–16c (Table 4, entries 1, 3, 5, 7) in good-to-excellent dr. Switching the reductant to LiBHEt₃ delivered the syn:syn products 13–16d (entries 2, 4, 6, 8), also in good dr.

Table 4.

Scope of the [4 + 3] cycloaddition/reduction in THF.

In our previous work, aziridination and subsequent functionalization of enantioenriched allenes gave excellent transfer of axial-to-point chirality.^[6a,c] However, the intermediacy of an amidoallyl cation in the [4+3] precludes chirality transfer (see SI for details). Nonetheless, the ability to racemize the axial chirality of the allene during the cycloaddition process could be used to advantage to transfer chirality at the sp³ carbon of 31 to the three carbons of the initial racemic allene to yield 32 (Scheme 2) in moderate dr and excellent ee.^[10,11]

Scheme 2.

Enantioenriched cycloheptenes via [4 + 3] reactions.

The cycloheptenes from the tandem allene aziridination/formal [4+3] are flexible scaffolds for further diversification, as 12 has four primary reactive functional handles that can be manipulated (Scheme 3). For example, reduction of 12, activation of the sulfamate and nucleophilic attack with thiophenol or diethyl malonate delivers 23 and 24 in good yields.^[11,12] Ring contraction to pyrrolidine 25^[13] can be achieved or the ether bridge cleaved in an S_N2' fashion using t-BuLi as a nucleophile to give a 1:1 mixture of regioisomers 26 and 27.^[14] Dihydroxylation of the alkene yields 28 in high dr; this sequence sets seven contiguous stereocenters in high dr over four steps from a simple allene.^[10,15] Carbon nucleophiles also add to imine 12, as evidenced by a Strecker reaction to afford 29.

Scheme 3.

Synthetic utility of cycloheptene products.

a: NaBH₃CN, AcOH, MeCN; 41%, 16:1 dr from 8. b: Boc₂O, cat. DMAP, Et₃N, CH₂Cl₂, rt. c: thiophenol, K₂CO₃, CH₃CN, rt; 75% from 13. d: diethyl malonate, TBAB, Cs₂CO₃, MeCN, rt; 58% from 13. e: KOt-Bu, DMAP, CH₂CL₂, 0 °C, then Ac₂O, rt; 96%. f. Nal, DMF, 60 °C, then NaH, 40 to 60 °C ; 83%. g: t-BuLi, THF, −40 °C; 61% of a 1:1 mixture of regioisomers. h: 10 mol% OsO₄, NMO, acetone, H₂O, t-BuOH, rt; 65%, >20:1 dr. i: n-Bu₄NCN, MeCN, rt; 80% yield from 12.

In conclusion, we have described the first examples of intermolecular [4+3] reactions occurring via 2-amidoallyl cations arising from direct allene aziridination. The ability to manipulate the stereochemistry of the intermediate amidoallyl cation leads to stereodivergent syntheses of all four possible diastereomeric cycloheptenes resulting from endo cyclization. The functional group diversity of the products enables access to an array of densely functionalized synthetic building blocks in a few simple steps. While the stereoablative nature of the chemistry prevents direct transfer of axial-to-point chirality, the presence of an additional stereocenter can be employed to yield enantioenriched aminated carbocycles. Future work is focused on expanding the scope of both allenes and coupling partners, as well as applying this methodology to the syntheses of both bioactive natural products and their aminated analogs.