Asymmetric Synthesis of trans-2,5-Disubstituted Pyrrolidines from Enantiopure Homoallylic Amines. Synthesis of Pyrrolidine (−)-197B

Abstract

Iodocyclization of sulfinimine-derived enantiopure homoallylic sulfonamides affords trans-2,5-disubstituted 3-iodopyrrolidines and represents valuable methodology for the asymmetric synthesis of this important heterocyclic ring system.

The trans-2,5-disubstituted pyrrolidine ring system is found in numerous natural products, medicinally relevant molecules, and in the pyrrolizidine and indolizidine alkaloids.^1,2 Enantiopure C₂-symmetrical trans-2,5-disubstituted pyrrolidines have found utility as organocatalysts,³ as chiral ligands for asymmetric catalysis,³ and as chiral auxiliaries.⁴ Although many racemic syntheses of this ring system have been reported,⁵,^1c–d,6,7,8 Furthermore, many of these methods are target specific, require difficult separations of diastereoisomers, and are multi-step. These methods also necessitate enantiomerically pure starting materials, which can be difficult to obtain. For these reasons the development of new methods for the concise asymmetric synthesis of this class of heterocycles, continues to be an important challenge.

In this context we describe an iodocyclization of sulfinimine-derived enantiopure homoallylic sulfonamides that represents a valuable new route for the asymmetric synthesis of trans-2,5-disubstituted pyrrolidines (Scheme 1). Our work was inspired by the studies of Knight et al. who explored the racemic system.⁹ In their research these workers reported that racemic E-homoallylic sulfonamides underwent the iodocyclization reaction with I₂/K₂CO₃ to give trans-2,5-disubstituted pyrrolidines in high yield and selectivity. However, the Z-homoallylic sulfonamides or multi-substituted examples resulted in poor yields and lower selectivity.

Scheme 1.

A general method for the asymmetric synthesis of trans-2,5-disubstituted pyrrolidines using the iodocyclization protocol requires easy access to structurally diverse enantiopure homoallylic sulfonamides. However, most of the methods reported for the asymmetric synthesis of homoallylic amines appear to be of limited scope and restricted to examples containing terminal olefins.¹⁰ Furthermore, many examples of substituted homoallylic amines do not provide convenient access to the requisite N-sulfonyl derivatives.¹¹

Recently we introduced a general method for the asymmetric synthesis of β-amino carbonyl compounds, including β-amino aldehydes, that involves the use of sulfinimine-derived N-sulfinyl β-amino Weinreb amides (Scheme 2).^12,13 This method involves the selective reduction of sulfinimine-derived N-sulfinyl β-amino Weinreb amides with DIBAL-H. Thus treatment of Weinreb amide (S)-(+)-1 with 5 equiv of DIBAL-H at −78 °C afforded N-sulfinyl β-amino aldehyde (S)-(+)-2 in 84%.

Scheme 2.

The aldehyde was next treated with 5.0 equiv of the Wittig reagent generated from n-butyllithium and benzyltriphenylphosphonium bromide to give the homoallylic sulfinamide (S_S,4R)-(+)-3 as an inseparable 66:34 mixture of E and Z isomers.¹⁴ The ¹H NMR chemical shifts of the two vinyl protons centered at δ 6.47 (J = 16 Hz) and δ 6.6 (J = 11 Hz) for the E and Z isomers, respectively, were used to establish the E:Z ratio. Next the mixture of homoallylic sulfinamides (+)-3 was treated with 3.0 equiv of I₂, along with 3.0 equiv of K₂CO₃ in aqueous acetonitrile (Scheme 2). However, none of the expected 3-iodopyrrolidine 5 was detected, but rather the N-sulfinyl group in 3 was oxidized to the corresponding homoallylic sulfonamide (R)-4 in 69% yield, again as an inseparable mixture of 66:34 E:Z isomers. Longer reactions times also failed to give the iodocyclization product 5. However, when 4 was re-subjected to the iodocyclization protocol (2S,3R,5R)-(−)-5 was obtained in 58% yield as the major isomer of a 95:5 mixture. The Z-homoallylic amide (S)-(+)-6 was isolated in 22% yield (Scheme 2). The structure of (−)-5 was established by conversion into the known (2R,5R)-(+)-2,5-diphenylpyrrolidine (13) as outlined below. In (+)-6 the vinyl proton centered at δ 6.6 (J = 11.7 Hz) is consistent with its structure.

Because N-sulfinyl β-amino aldehydes slowly decompose after several days at 0 °C, it was found to be more efficient to prepare the requisite N-tosyl β-amino aldehydes 9 by selective DIBAL-H reduction of the corresponding N-tosyl β-amino esters (R)-8 (Scheme 3). Oxidation of the N-sulfinyl β-amino esters 7 with 5.0 equiv of m-CPBA gave 8 in 85–99% yield. Although the N-tosyl β-amino aldehydes (R)-9 were stable to chromatography they were also observed to slowly decomposed on storage after several days. For this reason, they were quickly transformed into the corresponding homoallylic sulfonamides 4, 10, and 11 by treatment with the appropriate non-stabilized Wittig reagent at −78 °C. In all cases mixtures of inseparable E:Z isomers were obtained.

Scheme 3.

Subjecting the homoallylic sulfonamide mixtures to the iodocyclization protocol I₂/K₂CO₃/H₂O/MeCN, afforded the corresponding 3-iodo trans-2,5-disubstituted pyrrolidines 5 along with the unreacted Z-homoallylic sulfonamides 6 (Scheme 4, Table 1). Although modest yields of the 3-iodopyrrolidines 5 were obtained because mixtures of the E:Z homoallylic sulfonamide were employed, it is important to note that it was not necessary to purify the precursor sulfonamide. However, the fact that the starting homoallylic sulfonamide is not completely transformed into product could be liability in a long synthetic sequence. To address this issue we briefly explored the radical-induced photoisomerization of the Z-olefin in (R)-11 to the E-isomer.¹⁵ Photolysis of the 4:6 E:Z mixture of 11 at 300 nm in the presence of 2 equiv of phenyldisulfide for 6 h in cyclohexane improved the E:Z ratio to 8:2. Iodocyclization of (R)-11 (8:2 E:Z) afforded 5c in 71% isolated yield (Table 1, entry 4).

Scheme 4.

Table 1.

Iodocyclization of Homoallylic Sulfonamides to 3-Iodopyrrolidines 5 and Z-homoallylic Sulfonamides.

Entry	Homoallylic Sulfonamide R1,R2 (E:Z)	5 (% yield)a [dr]b	6 (% yield)a
1	4 R1 = R2 = Ph (55:45)	5a (40) [88:12]	6a (45)
2	10 R1 = Ph; R2 = Et (55:45)	5b (46) [>95:5]	6b (31)
3	11 R1 = Me; R2 = Et (4:6)	5c (39) [>95:5]	6c (49)
4	11 R1 = Me; R2 = Et (8:2)c	5c (71) [>95:5]	6c (20)

^aIsolated yield of major product.

^bDetermined by ¹H NMR.

^cPrepared using photoisomerization and PhSSPh.

The trans relationship of the 2- and 5-substituents in 5a was confirmed by treatment of (2S,3R,5R)-(−)-5a with tributyltin hydride to give (2R,5R)-2,5-diphenyl-1-tosylpyrrolidine (12) in nearly quantitative yield (Scheme 5). Reductive removal of the N-tosyl group with Na/NH₃ (liq.) afforded (2R,5R)-(+)-2,5-diphenylpyrrolidine (13) with properties identical to literature values.¹⁶ Pyrrolidine (+)-13 is a popular chiral auxiliary with C₂-symmetry, and the homoallylic sulfonamide iodocyclization protocol represents a potentially general method for the asymmetric synthesis of this valuable class of auxiliaries and ligands.^3,4,5

Scheme 5.

As shown in Table 1, E-homoallylic sulfonamides were the only active partner in the iodocyclization reaction with Z-homoallylic sulfonamides inert to the cyclization conditions. The lack of reactivity the Z-homoallylic sulfonamides 6 can be explained in terms of the chair-like transition states depicted in Figure 1. The existing A^1,3 strain in transition state TS-A for the Z-olefin compare to those in TS-B for the E-olefin makes the latter lower in energy. This results in the exclusive reaction of the E-homoallylic sulfonamide under the reaction conditions to produce the trans-2,5-pyrrolidines 5. The poorer diastereoselectivity observed in the iodocyclization of homoallylic sulfonamides having a phenyl group on the terminal carbon for R² = Ph (Table 1, entry 1) could be explained by extra stabilization of a benzylic carbocation, which could result in isomerization. In a related study, Harding et al. reported that equilibration of the products from amidomercuration of δ-alkenylcarbamates resulted in scrambling in the product.^5b

Figure 1.

The utility of our new trans-2,5-disubstituted pyrrolidinesynthesis is highlighted by a total asymmetric synthesis of (−)-pyrrolidine 197B (22), trans-2-n-butyl-5-n-pentylpyrrolidine (Scheme 6). In addition to many other alkaloids (−)-pyrrolidine 197B (22) has been detected in the poison frogs of the family Dendrobates histrionicus1a,f and in the venom of fire ants of the genus Solenopsis and Monomorium latinode.¹⁷ These alkaloids exhibit a wide range of biological activities including hemolytic and antibiotic activity,¹⁸ but because they are isolated in only minute quantities the biological properties of the majority of these alkaloids remains unknown. A single asymmetric synthesis of (−)-197B has been reported by Machinaga and Kibayashi.¹⁹ Their synthesis required in excess of 16 steps starting from C₂-symmetric diepoxides derived from D-mannitol.

Scheme 6.

Our synthesis of (−)-197B begins with the preparation of the E-homoallylic sulfonamide (R)-18 from (R)-(−)-N-(pentylidine)-p-toluenesulfinamide (14), using the standard conditions. This procedure affords (R)-18 in 50% yield for the four-step sequence (Scheme 6). The modest E:Z ratio of 18 from the Wittig reaction was improved by photoisomerization. Irradiation of compound (R)-18 at 300 nm in the presence of 2.0 equiv of PhSSPh improved the E:Z isomer ratio from 54:46 to 85:15 with an isolated yield of 84%. When (R)-18 was subjected to the iodocyclization protocol the major diastereoisomer of pyrrolidine (+)-20 was isolated in 82% yield along with 10% of the Z-olefin (R)-(+)-19. Finally, deiodination of (+)-20 with Bu₃SnH gave pyrrolidine (−)-21 in quantitative yield, which was followed removal of the N-tosyl group with Na/NH₃ (liq.) to give the target (−)-pyrrolidine 197B (22) in 62% yield (Scheme 6). While the spectral properties of (−)-22 were in agreement with literature values our specific rotation of −25.3 was considerably larger than the value of −5.8 reported by Kibayashi and co-workers.^19b This group also prepared the N-benzoyl derivative of the enantiomer of 22. Their specific rotation was +125.5 while ours was considerable higher at −231.5.^19b These discrepancies suggests that racemization may have occurred in the earlier preparations of this material. Attempts to make the Mosher amide of (−)-22 were unsuccessful.

In conclusion, we have developed a useful stereoselective synthesis of enantiopure trans-2,5-pyrrolidine derivatives from homoallylic sulfonamides using iodocyclization. The homoallylic sulfonamides were prepared from readily available sulfinimine-derived β-amino aldehydes using the Wittig reaction and were obtained as E:Z mixtures. Since only the E-homoallylic sulfonamides undergo the cyclization reaction it was not to be necessary to separate E:Z sulfonamide mixtures. The utility of this methodology was highlighted in the total asymmetric synthesis of toxic frog/fire ant venom alkaloid (−)-197 B 22.

Experimental

(S_S,3R)-(+)-N-(p-toluenesulfinyl)-3-amino N-methoxy-N-methyl-3-phenylpropionamide (1)¹², (S_S,3R)-(+)-N-(p-toluenesulfinyl)-3-amino-3-phenyl-propionaldehyde (2),¹² (S _S,R)-(+)-methyl 3-(p-toluenesulfinylamino)-3-phenylpropanoate (7a)¹² and (S_S,3S)-(+)-methyl N-(p-toluenesulfinyl)-3-aminobutanoate (7b),¹² were prepared as previously described.

(S_s,4R)-(+)-N-(p-Toluenesulfinyl)-4-amino-1,4-diphenylbut-1-en (3)

In an oven-dried, one-neck, round-bottom flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed benzyltriphenylphosphonium bromide (0.676 g, 1.56 mmol) in THF (6 mL). The solution was cooled to 0 °C and n-BuLi (0.610 mL, 1.53 mmol, 1.6 M in hexanes) was added and the solution was stirred for 1 h at rt. At this time the reaction mixture was cooled to −78 °C and a solution of (+)-2 (0.088 g, 0.305 mmol) in THF (2 mL) was added via cannula. After consumption of starting material as judged by TLC (2.5 h at 0 °C), the reaction mixture was quenched with water (1 mL) at 0 °C, warmed up to rt, and extracted with EtOAc (2 × 20 mL). The combined organic phases were washed with brine (2 × 5 mL), dried (MgSO₄), and concentrated. Chromatography (50% Et₂O/hexane) gave 0.06 g (54%) of a colorless oil as an inseparable 66:34 E:Z mixture; [α]²⁰ _D +76.4 (c 0.75, CHCl₃); ¹H NMR (CDCl₃) major isomer: δ 2.46 (s, 3H), 2.74 (m, 2H), 4.41 (d, J = 3.0 Hz, 1H), 4.73 (m, 1H), 6.06 (dt, J = 7.6, 15.5 Hz, 1H), 6.41 (d, J = 15.9 Hz, 1H), 6.47 (d, J = 15.9 Hz, 1H), 7.19–7.53 (m, 12H), 7.64 (m, 2H); minor isomer: δ 2.88 (m, 2H), 4.32 (bs, 1H), 4.68 (m, 1H), 5.61 (m, 1H), 6.57 (d, J = 11.4 Hz, 1H); ¹³C NMR (CDCl₃) major isomer: δ 21.4, 42.2, 57.7, 125.1, 125.5, 126.3, 127.5, 128.6, 128.7, 128.8, 129.59, 129.6, 134.0, 141.3, 141.46, 141.5, 142.3; minor isomer: δ 37.3, 57.9, 127.0, 127.2, 127.7, 127.9, 128.0, 128.3, 132.3, 137.0; IR (film) 3193, 3058, 3027, cm⁻¹; HRMS (EI) m/e calcd for C₂₃H₂₃NONaS (M+Na) 384.139806. Found 384.1391.

(R)-(+)-N-(1,4-Diphenylbut-3-enyl)-4-methylbenzenesulfonamide (4)

In an oven-dried one-neck round-bottom flask equipped with a magnetic stirring bar and argon inlet was placed (+)-3 (0.015 g, 0.042 mmol) and K₂CO₃ (0.02 g, 0.145 mmol) in MeCN (1 mL) and H₂O (0.01 mL), the solution was stirred for 30 min at rt, and I₂ (0.032 g, 0.145 mmol) was added. After consumption of starting material as judged by TLC analysis (1 h at rt), the mixture was quenched by addition of sat. aqueous Na₂S₂O₃ (1 mL). The resulting clear yellow solution was extracted with CH₂Cl₂ (2 × 5mL), the combined organic phases were dried (MgSO₄), and concentrated. Chromatography (10% EtOAc/hexane) gave 0.011 g (69%) of a colorless oil; [α]²⁰ _D +72.4 (c 0.55, CHCl₃); ¹H NMR (CDCl₃) major isomer δ 2.27 (s, 3H), 2.52 (dt, J = 0.8, 7.1 Hz, 2H), 4.34 (m, 1H), 4.98 (d, J = 6.4 Hz, 1H), 5.74 (dt, J = 7.4, 16.0 Hz, 1H), 6.29 (d, J = 15.6 Hz, 1H), 6.91–7.25 (m, 12H), 7.49 (d, J = 8.0 Hz, 2H); minor isomer δ 2.27 (s, 3H), 2.66 (m, 2H), 4.83 (d, J = 6.8 Hz, 1H), 5.35 (dt, J = 7.2, 12.0 Hz, 1H), 6.4 (d, J = 11.6 Hz, 1H), 7.43 (d, J = 8.4 Hz, 2H); ¹³C NMR (CDCl₃) major isomer δ 21.8, 41.6, 57.9, 124.8, 126.6, 126.9, 127.46, 127.5, 127.8, 128.8 (2C), 129.8, 134.4, 137.1, 137.8, 141.0, 143.5; minor isomer δ 30.7, 36.5, 58.3, 127.0, 127.4, 127.86, 127.9, 128.7, 129.0, 129.7, 132.7, 140.7, 143.4; IR (film) 3279, 3061, 2928, 2858, cm⁻¹. HRMS calcd for C₂₃H₂₃NO₂SNa (M+Na) 400.1347. Found 400.1352.

(2S,3R,5R)-(−)-3-Iodo-2,5-diphenyl-1-tosylpyrrolidine (5)

In an oven-dried one-neck round-bottom flask equipped with a magnetic stirring bar and argon inlet was placed (+)-4 (0.027 g, 0.072 mmol) and K₂CO₃ (0.033 g, 0.239 mmol) in MeCN (1 mL) and H₂O (0.01 mL), and the solution was stirred for 30 min at rt, and I₂ (0.061 g, 0.239 mmol) was added. After consumption of starting material as judged by TLC analysis (3 h at rt), the mixture was quenched by addition of sat. aqueous Na₂S₂O₃ (1 mL). The resulting clear solution was extracted with CH₂Cl₂ (2 × 5 mL) and the combined organic phases were dried (MgSO₄) and concentrated. Chromatography (10% EtOAc/hexane) gave 0.021 g (58%) of a light yellow solid as a mixture of inseparable diastereoisomers in a ratio of 95:5, and 0.006 g (22%) of Z homoallylic sulfonamide (+)-6; mp 164 –165 °C; [α ]²⁰ _D −7.9 (c 0.067, CHCl₃); ¹H NMR (CDCl₃) δ 2.26 (s, 3H), 2.45 (dt, J = 5.2, 15.2 Hz, 1H), 3.15 (m, 1H), 4.28 (dt, J = 8.6, 8.6 Hz, 1H), 5.19 (dd, J = 5.7, 9.6 Hz, 1H), 5.3 (d, J = 4.5 Hz, 1H), 6.85 – 7.36 (m, 14H); ¹³C NMR (CDCl₃) δ 21.5, 25.0, 45.9, 65.3, 77.2, 127.1, 127.3, 127.7, 128.1, 128.3, 128.7, 128.8, 128.9, 138.3, 139.0, 139.8, 142.4; IR (film) 3031, 2921 cm⁻¹. HRMS calcd for C₂₃H₂₂NO₂SINa (M+Na) 526.0314. Found 526.0321.

(R,Z)-(+)-1,4-Diphenyl-N-tosyulbut-3-en-1-amine (6)

[α]²⁰ _D +42.7 (c 0.33, CHCl₃); ¹H NMR (CDCl₃) δ 2.43 (s, 3H), 2.81 (m, 2H), 4.47 (q, J = 6.8 Hz, 1H), 4.77 (d, J = 6.9 Hz, 1H), 5.48 (m, 1H), 6.56 (d, J = 11.7 Hz, 1H), 7.08–7.40 (m, 12H), 7.56 (d, J = 8.1 Hz, 2H); ¹³C NMR (CDCl₃) δ 21.5, 36.2, 58.0, 126.7, 127.2, 128.5, 128.7, 129.3, 132.4, 136.8, 137.5, 138.3, 139.0, 139.8, 140.3, 142.4, 143.1; IR (film) 3276, 3026, 2923 cm⁻¹; HRMS calcd for C₂₃H₂₃NO₂SNa (M+Na) 400.1347. Found 400.1349.

Typical procedure for the preparation of N-tosyl β-amino esters. (R)-(+)-Methyl 3-(4-methylphenylsulfonamido)-3-phenylpropano-ate (8a)

In an oven-dried one-neck round-bottom flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed (+)-7a (0.157 g, 0.496 mmol) in CH₂Cl₂ (10 mL). The solution was cooled to 0 °C and m-CPBA (0.144 g, 1.984 mmol) was added and the solution was stirred for 1.5 h at 0 °C. At this time the reaction mixture was quenched by addition of sat. aqueous Na₂S₂O₃ (5 mL), diluted with CHCl₃ (10 mL), and sat. aqueous NaHCO₃ (5 mL) was added. The solution was stirred vigorously with warming to rt, and the organic phase was dried (MgSO₄) and concentrated to provide 0.140 g (85%) of a white solid, mp 100–101 °C; [α]²⁰ _D +50.6° (c 1.62, CHCl₃); ¹H NMR (CDCl₃) δ 2.43 (s, 3H), 2.87 (q, J = 15.6 Hz, 2H), 3.62 (s, 3H), 4.80 (q, J = 6.5 Hz, 1H), 5.89 (d, J = 8.1 Hz, 1H), 7.15–7.3 (m, 7H), 7.67 (d, J = 8.7 Hz, 2H); ¹³C NMR(CDCl₃) δ 21.5, 41.2, 51.9, 54.4, 126.4, 127.2, 127.8, 128.6, 129.5, 137.5, 139.4, 143.3, 171.1; IR (film) 3280, 1739 cm⁻¹; HRMS calcd for C₁₇H₁₉NO₄Na (M+Na) 356.0932. Found 356.0927.

(S)-(−)-Methyl 3-(4-methylphenylsulfonamido)butanoate (8b)

Chromatography (20% Et₂O/hexane) gave 0.247 g (88%) of a colorless oil; mp 78–79 °C; [α]²⁰ _D -22.4 (c 1.16, CHCl₃) [lit.²⁰ mp 78–79 °C, [α]²⁰ _D −27.4 (c 0.71, CHCl₃)]. Spectral properties were consistent with literature values.

Typical procedure for the preparation of N-tosyl β-amino aldehydes. (R)-(+)-4-Methyl N-(3-oxo-1-phenylpropyl)benzenesulfonamide (9a)

In an oven-dried round-bottom one-neck flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed (+)-8a (0.222 g, 0.667 mmol) in toluene (7 mL). The solution was cooled to −78 °C and added DIBAL-H (0.670 mL, 0.667 mmol, 1.0 M solution in CH₂Cl₂) was added dropwise. After consumption of starting material as judged by TLC analysis (5 h at −78 °C), the reaction mixture was quenched with by addition of sat. aqueous NaKC₄H₄O₆ · H₂O (Rochell salt) (5 mL), diluted with EtOAc (10 mL), vigorously stirred with warming to rt. The organic phase was dried (MgSO₄) and concentrated. Chromatography (50% Et₂O/hexane) gave 0.090 g (56%) of a colorless oil, and 0.095 g (43%) of starting material; [α]²⁰ _D +41.3° (c 1.37, CHCl₃); ¹H NMR (CDCl₃) δ 2.44 (s, 3H), 3.06 (m, 2H), 4.86 (q, J = 6.8 Hz, 1H), 5.52 (d, J = 7.2 Hz, 1H), 7.13 (m, 2H), 7.24 (m, 5H), 7.65 (d, J = 8.4 Hz, 2H), 9.70 (t, J = 1.2 Hz, 1H); ¹³C NMR (CDCl₃) δ 21.5, 50.1, 53.3, 126.5, 127.2, 128.0, 128.8, 129.6, 137.1, 139.3, 143.5, 199.6; IR (film) 3254, 1726 cm⁻¹. HRMS calcd for C₁₆H₁₇NO₃SNa (M+Na) 326.0827. Found 326.0820.

(S)-(−)-4-Methyl N-(4-oxobutan-2-yl)benzenesulfonamide (9b)

Chromatography (20% EtOAc/hexane) gave 0.227 g (81%) of a colorless oil and 0.047 g (15%) of starting material; [α]²⁰ _D −22.0 (c 1.98, CHCl₃); ¹H NMR (CDCl₃) δ 1.13 (d, J = 6.6 Hz, 3H), 2.46 (s, 3H), 2.65 (m, 2H), 3.81 (septet, J = 6.8 Hz, 1H), 5.51 (d, J = 7.8 Hz, 1H), 7.35 (d, J = 8.1 Hz, 2H), 7.8 (d, J = 8.1 Hz, 2H), 9.68 (s, 1H); ¹³C NMR (CDCl₃) δ 21.3, 21.6, 45.5, 50.4, 127.1, 129.9, 137.7, 143.7, 200.9; IR (film) 3275, 2978, 2739, 1715 cm⁻¹; HRMS (EI) m/e calcd for C₁₁H₁₅NO₃NaS (M+Na) 264.067035. Found 264.0668.

Typical procedure for the preparation of homoallylic sulfonamide. (R)-(+)-N-(1,4-Diphenylbut-3-enyl)-4-methyl-benzensulfonamide (4)

In an oven-dried one-neck round-bottom flask, equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed benzyltriphenylphosphonium bromide (1.49 g, 3.43 mmol) in THF (18 mL). The solution was cooled to 0 °C and n-BuLi (2.14 mL, 3.43 mmol, 1.6 M in hexanes) was added and the solution was stirred for 1 h at rt. At this time the reaction mixture was cooled to −78 °C, and a solution of (+)-9a (0.20 g, 0.686 mmol) in THF (3 mL) was added. After consumption of starting material as judged by TLC analysis (2.5 h at −78 °C), the reaction mixture was quenched with H₂O (1 mL) at 0 °C, warmed to rt, and extracted with EtOAc (3 mL). The organic phase was washed with brine, dried (MgSO₄), and concentrated. Chromatography (20% Et₂O/hexane) gave 0.15 g (61%) of a colorless oil as an inseparable 55:45 Z:E mixture of isomers; [α]²⁰ _D +23.9° (c 1.30, CHCl₃); ¹H NMR (CDCl₃) major isomer: δ 2.41 (s, 3H), 2.66 (t, J = 6.8 Hz, 2H), 4.48 (pent, J = 6.8 Hz, 1H), 5.03 (d, J = 6.8 Hz, 1H), 5.89 (dt, J = 15.6, 7.5 Hz, 1H), 6.43 (d, J = 15.9 Hz, 1H), 7.0 – 7.34 (m, 12H), 7.64 (m, 2H); minor isomer: δ 2.42 (s, 3H), 2.80 (m, 2H), 5.14 (d, J = 6.9 Hz, 1H), 5.50 (dt, J = 11.7, 7.0 Hz, 1H), 6.54 (d, J = 11.1 Hz, 1H), 7.58 (m, 2H). ¹³C NMR (CDCl₃) major isomer: δ 21.5, 41.2, 58.0, 124.5, 126.3, 126.6, 127.1, 127.2, 127.54, 128.4, 128.51, 128.52, 129.37, 134.1, 136.77, 140.6, 143.2; minor isomer: δ 36.2, 57.6, 126.5, 127.05, 127.51, 127.6, 127.7, 128.3, 128.53, 128.7, 129.43, 132.4, 136.78, 140.3, 143.1; IR (film) 3279, 3061, 3028, 2928, 2858 cm⁻¹. HRMS calcd for C₂₃H₂₃NO₂SNa (M + Na) 400.1347. Found 400.1352.

(R)-(+)-4-Methyl N-(1-phenylhex-3-enyl)benzenesulfonamide (10)

Following typical procedure, (+)-9a (0.137 g, 0.452 mmol), triphenylphosphoranepropylbromide (0.871g, 2.26 mmol), and n-BuLi (0.904 mL, 2.26 mmol) were employed. Chromatography (20% Et₂O/hexane) gave 0.111 g (75%) of a colorless oil as an inseparable 45:55 Z:E mixture of isomers; [α ]²⁰ _D +59.9 (c 2.0, CHCl₃); ¹H NMR (CDCl₃) major isomer: δ 0.97 (t, J = 5.9 Hz, 3H), 1.99 (sextet, J = 7.5 Hz, 2H), 2.43 (m, 1H), 2.44 (s, 3H), 2.53 (q, J = 7.7 Hz, 1H), 4.38 (m, 1H), 5.15 (m, 2H), 5.54 (m, 1H), 7.13 – 7.23 (m, 7H), 7.64 (d, J = 7.8 Hz, 1H); minor isomer: δ 0.93 (t, J = 5.9 Hz, 3H); ¹³C NMR (CDCl₃) major isomer: δ 15.9, 21.8, 25.9, 41.2, 57.8, 123.7, 126.95, 126.98, 127.5, 128.6, 129.7, 137.7, 140.9, 141.3, 143.4; minor isomer: δ 14.4, 21.0, 35.6, 58.1, 123.3, 127.7, 128.7, 136.2, 137.9; IR (film) 2849, 2917, 1329, 1157 cm⁻¹; HRMS (EI) m/e calcd for C₁₉H₂₂INO₂NaS (M+Na) 478.031373. Found 478.0330.

(S)-(−)-N-(hept-4-en-2-yl)-4-methylbenzenesulfonamide (11)

Following typical procedure, (−)-9c (0.274 g, 1.137 mmol), triphenylphosphoranepropylbromide (2.23 g, 5.799 mmol), and n-BuLi (2.3 mL, 5.685 mmol) were employed. Chromatography (20% Et₂O/hexane), 0.162 g (53%) of a colorless oil as an inseparable 4:6 E:Z mixture of isomers; [α ]²⁰ _D −31.4 (c 2.6, CHCl₃); ¹H NMR (CDCl₃) major isomer: δ 0.83 (t, J = 7.5 Hz, 3H), 0.98 (d, J = 6.6 Hz, 3H), 1.86 (m, 2H), 1.96 (m, 1H), 2.04 (m, 1H), 2.35 (s, 3H), 3.22 (m, 1H), 4.69 (d, J = 7.6 Hz, 1H), 5.04 (m, 1H), 5.36 (m, 1H), 7.22 (d, J = 8.2 Hz, 2H), 7.69 (m, 2H); minor isomer: δ 0.84 (t, J = 7.5 Hz, 3H), 1.0 (d, J = 6.6 Hz, 3H), 4.64 (d, J = 7.2 Hz, 1H), 5.14 (m, 1H), 5.43 (m, 1H); ¹³C NMR (CDCl₃) major isomer: δ 14.5, 21.0, 21.4, 21.8, 35.0, 50.2, 123.8, 127.5, 130.0, 135.6, 138.5, 143.5; minor isomer: δ 13.9, 21.7, 25.9, 40.5, 50.0, 124.0, 136.9; IR (film) 3279, 2967, 2933, 1329, 1184 cm⁻¹; HRMS (EI) m/e calcd for C₁₄H₂₁NO₂NaS (M+Na) 290.119071. Found 290.1195.

Typical Procedure for the Iodocyclization of Homoallylic Sulfonamide. (2S,3R,5R)-(−)-3-Iodo-2,5-diphenyl-1-(p-tolylsulfonyl)-pyrrolidine (5a)

In an oven-dried, one-neck, round-bottom flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed (+)-4 (0.053 g, 0.147 mmol) and K₂CO₃ (0.0648 g, 0.44 mmol) in CH₃CN (1.5 mL) and H₂O (0.016 mL), the solution stirred for 30 min at rt, and I₂ (0.112 g, 0.440 mmol) was added. After consumption of the starting material as judged by TLC analysis (3 h at rt), the reaction mixture was quenched with sat. aqueous Na₂S₂O₃ (1 mL), the phases were separated and the aqueous phase was extracted with CH₂Cl₂ (2 × 10 mL). The combined organic phases were dried (MgSO₄) and concentrated. Chromatography (1:9 Et₂O:hexanes) gave 0.024 g (40%) of a white solid as inseparable mixture of diastereomers in a ratio of 88:12 and 0.024 g (45%) of recovered Z homoallylic sulfonamide 6a; mp 163–164°C; [α]²⁰ _D −10.3° (c 0.76, CHCl₃); ¹H NMR (CDCl₃) major isomer: δ 2.26 (s, 3H), 2.45 (dt, J = 15.0, 5.4 Hz, 1H), 3.16 (dt, J = 14.4, 8.6 Hz, 1H), 4.28 (dt, J = 7.5, 4.7 Hz, 1H), 5.20 (dd, J = 8.9, 5.5 Hz, 1H), 5.34 (d, J = 4.2 Hz, 1H), 6.86 (d, J = 8.4 Hz, 1H), 6.96 (d, J = 8.4 Hz, 1H), 7.1–7.4 (m, 12H); Minor isomer: δ 2.57 (m, 1H), 2.74 (m, 1H), 4.12 (m, 1H), 5.21 (d, J = 9.0 Hz, 1H); ¹³C NMR (CDCl₃) δ 21.4, 24.9, 45.8, 65.2, 77.1, 126.6, 127.2, 126.9, 127.7, 128.1, 128.4, 128.6, 128.7, 138.2, 138.9, 139.7, 142.3; IR (film) 3031, 2921 cm⁻¹; HRMS calcd for C₂₃H₂₂NO₂SINa (M+Na) 526.0314. Found 526.0321.

(R,Z)-(+)-N-(1,4-Diphenylbut-3-enyl)-4-methylbenzenesulfonamide (6a)

[α]²⁰ _D +28.8° (c 1.45, CHCl₃); ¹H NMR (CDCl₃) δ 2.36 (s, 3H), 2.74 (m, 2H), 4.41 (q, J = 7.4 Hz, 1H), 5.69 (dt, J = 11.1, 7.5 Hz, 1H), 6.10 (d, J = 7.5 Hz, 1H), 6.47 (d, J = 11.4 Hz, 1H), 7.01 (d, J = 8.4 Hz, 2H), 7.05–7.35 (m, 10H), 7.62 (d, J = 8.4 Hz, 2H); ¹³C NMR (CDCl₃) δ 21.5, 36.2, 58.0, 126.7, 127.2, 128.5, 128.7, 129.3, 132.4, 136.8, 137.5, 138.3, 139.0, 139.8, 140.3, 142.4, 143.1; IR (film) 3276, 3026, 2923 cm⁻¹. HRMS calcd for C₂₃H₂₃NO₂SNa (M+Na) 400.1347. Found 400.1349.

(2S,3R,5R)-(+)-2-Ethyl-3-iodo-5-phenyl-1-tosylpyrrolidine (5b)

Chromatography (20% Et₂O/hexane) gave 0.031 g (46%) of a yellow oil and 0.015 g (31%) of recovered Z homoallylic sulfonamide 6b; [α]²⁰ _D +3.7 (c 3.1, CHCl₃); ¹H NMR (CDCl₃) δ 1.00 (t, J = 7.4 Hz, 3H), 1.67 (m, 1H), 2.12 (m, 1H), 2.24 (s, 3H), 2.37 (m, 1H), 3.18 (ddd, J = 9.7, 16.1, 7.4 Hz, 1H), 4.34 (m, 2H), 4.80 (dd, J = 9.8, 5.6 Hz, 1H), 6.87 – 7.0 (m, 5H), 7.17 (m, 4H); ¹³C NMR (CDCl₃) δ 11.0, 21.4, 21.7, 26.5, 46.4, 62.9, 76.7, 127.5, 127.8, 128.2, 129.0, 129.8, 138.5, 139.2, 142.7; IR (film) 2849, 2917, 1329, 1157 cm⁻¹; HRMS (EI) m/e calcd for C₁₉H₂₂INO₂NaS (M+Na) 478.031373. Found 478.0330.

Upon irradiation of the C-5 proton at δ 4.8, a positive NOE was observed on the C-4 proton at δ 3.18 (5.25%), confirming the cis relationship between the C-5 proton and the C-4 proton and determining the C-4 proton at δ 3.18 as H_α of two methylene C-4 protons. Upon irradiation of the C-4 proton H_α at δ 3.18, positive NOEs were observed on the C-5 proton at δ 4.8 (7.87%) and on the C-3 proton at δ 4.34 (5.55%), confirming the cis relationship between the C-5 proton and the C-3 proton. Upon irradiation of the C-2 methyl at δ 1.0, a positive NOE was observed on the C-5 proton at δ 4.80 (3.21%), confirming the trans relationship between the C-2 proton and the C-5 proton.

(R,Z)-(+)-4-methyl-N-(1-phenylhex-3-enyl)benzenesulfonamide (6b)

[α ]²⁰ _D +47.3 (c 1.5, CHCl₃); ¹H NMR (CDCl₃) δ 0.70 (t, J = 7.5 Hz, 3H), 1.74 (quint, J = 7.4 Hz, 1H), 2.20 (s, 3H), 2.32 (m, 2H), 4.14 (q, J = 7.0, 1H), 4.67 (d, J = 6.0 Hz, 2H), 4.88 (m, 1H), 5.28 (m, 1H), 6.94 (m, 7H), 7.39 (d, J = 7.8 Hz, 2H); ¹³C NMR (CDCl₃) δ 14.4, 21.0, 21.8, 35.6, 58.1, 123.2, 127.0, 127.5, 127.7, 128.7, 129.7, 136.3, 137.9, 140.9, 143.4; IR (film) 3278, 2964, 1622, 1325, 1184 cm⁻¹; HRMS (EI) m/e calcd for C₁₉H₂₃NO₂NaS (M+Na) 352.1347. Found 352.1346.

(2S,3R,5S)-(+)-2-Ethyl-3-iodo-5-methyl-1-tosylpyrrolidine (5c)

Chromatography (20% Et₂O/hexane) gave 0.026 g (39%) of a yellow oil and 0.022 g (49% recovery) of recovered Z homoallylic sulfonamide 6c; [α ]²⁰ _D +2.9 (c 1, CHCl₃); ¹H NMR (CDCl₃) δ 0.96 (t, J = 7.5 Hz, 3H), 1.42 (d, J = 6.5 Hz, 3H), 1.47 (m, 1H), 2.06 (m, 1H), 2.10 (m, 1H), 2.43 (s, 3H), 2.87 (ddd, J = 8.5, 15.0, 6.5 Hz, 1H), 4.15 (m, 1H), 4.21 (dd, J = 3.0, 10.0 Hz, 1H), 4.26 (dt, J = 6.5, 1.8 Hz, 1H), 7.30 (d, J = 8.0 Hz, 2H), 7.84 (d, J = 8.5 Hz, 2H); ¹³C NMR (CDCl₃) δ 11.1, 21.2, 21.9, 28.5, 44.1, 56.16, 56.2, 76.3, 127.9, 130.0, 140.2, 143.7; IR (film) 2971, 2932, 2876, 1457, 1338, 1157 cm⁻¹; HRMS (EI) m/e calcd for C₁₄H₂₀INO₂NaS (M+Na) 416.015723. Found 416.0148.

Upon irradiation of the C-2 methyl at δ 0.96, a positive NOE was observed on the C-3 proton at δ 4.26 (6.4%), confirming trans relationship between the C-2 proton and C-3 proton. Upon irradiation of the C-5 methyl at δ 1.42, a positive NOE was observed on the C-2 proton at δ 4.15 (2.0%), confirming trans relationship between the C-5 proton and C-2 proton.

(S,Z)-(−)-N-(Hept-4-en-2-yl)-4-methylbenzenesulfonamide (6c)

[α ]²⁰ _D −31.5 (c 1.0, CHCl₃); ¹H NMR (CDCl₃) δ 0.92 (t, J = 7.8, 3H), 1.07 (d, J = 6.5 Hz, 3H), 1.94 (dq, J = 1.5, 7.5 Hz, 2H), 2.12 (q, J = 6.5 Hz, 2H), 2.43 (s, 3H), 3.32 (quint, J = 6.8 Hz, 1H), 4.46 (d, J = 7.5 Hz, 1H), 5.13 (m, 1H), 5.46 (m, 1H), 7.30 (d, J = 8.0 Hz, 2H), 7.76 (m, 2H); ¹³C NMR (CDCl₃) δ 14.8, 21.3, 21.8, 35.2, 35.3, 50.41, 123.9, 127.8, 130.3, 136.1, 138.6, 143.9; IR (film) 3279, 2967, 2933, 1329, 1184 cm⁻¹; HRMS (EI) m/e calcd for C₁₄H₂₁NO₂NaS (M+Na) 290.119071. Found 290.1195.

Photolysis of (S)-(−)-N-(hept-4-en-2-yl)-4-methylbenzenesulfonamide and its iodocyclization

In an oven dried one-neck round bottom flask, equipped with magnetic stirring bar and rubber septum was placed PhSSPh (0.082 g, 0.375 mmol) and the mixture of 4:6 E:Z isomers of (−)-11 (0.082 g, 0.375 mmol) in cyclohexane (3 mL). The solution was irradiated by mercury lamp (300 nm, 450 W) for 6 h while stirring. The reaction was quenched with sat. aqueous NaHCO₃ (1 mL), extracted with EtOAc (3 mL), and the organic phases were dried (MgSO₄), and concentrated. Chromatography (20% Et₂O/hexane) provided 0.004 g (80%) of a colorless oil as an inseparable 8:2 E and Z mixture of 11. Following typical procedure for the iodocyclization, (−)-11 (0.04 g, 0.150 mmol, E:Z 8:2), K₂CO₃ (0.062 g, 0.45 mmol), and I₂ (0.114 g, 0.45 mmol) were employed. Chromatography (10% Et₂O/hexane) gave 0.042 g (71%) of a yellow oil and 0.008 g (20%) of recovered Z homoallylic sulfonamide 6c.

(2R,5R)-(+)-2,5-Diphenyl-1-tosylpyrrolidine (12)

In an oven dried, argon-purged 25-mL, one-neck flask attached to reflux condenser and equipped with stir bar, was placed toluene (5 mL), (−)-5a (0.047 g, 0.096 mmol), and Bu₃SnH (0.255 mL, 0.96 mmol). The reaction mixture was refluxed at 130 °C for 4 h at which time the solvent and excess Bu₃SnH were evaporated off under reduced pressure. Chromatography (3:17 Et₂O:hexanes) afforded 0.034 g (98%) of a colorless oil; [α]²⁰ _D +116.6° (c 1.10, CHCl₃); ¹H NMR (CDCl₃) δ 1.80 (m, 2H), 2.32 (s, 3H), 2.62 (m, 2H), 5.27 (d, J = 7.0 Hz, 2H), 6.93 (d, J = 8.0 Hz, 2H), 7.14 (d, J = 8 Hz, 2H), 7.16–7.27 (m, 10H); ¹³C NMR (CDCl₃) δ 34.0, 65.3, 65.5, 127.0, 127.5, 127.6, 128.9, 129.4, 139.1, 142.8, 143.5; IR (film) 3027, 2961, 2922, cm⁻¹; HRMS calcd for C₂₃H₂₃NO₂SNa (M+Na) 400.1347. Found 400.1354.

(2R,5R)-(+)-2,5-Diphenylpyrrolidine (13)

In an oven dried, argon purged 50 mL, three neck round bottom flask equipped with a magnetic stir bar and dry-ice condenser and cooled to −78 °C was collected NH₃ (~10 mL). Sodium metal (~0.9 g) was added and the solution was stirred for 15 minutes. At this time (+)-15 (0.021 g, 0.058 mmol) in THF (3 mL) was then added dropwise. The reaction was quenched after 30 min by addition of solid NH₄Cl (1.3 g), and the NH₃ was allowed to evaporated by warming to rt. Water (10 mL) was then added, and the solution extracted with Et₂O (2 X 5 mL), dried (MgSO₄), and evaporated. Chromatography (1:9 EtOAc:hexanes) afforded 0.0053 g (47%) of clear film; [α]²⁰ _D +102.1° (c 0.47, CHCl₃), [lit.^16b [α]²⁰ _D +104.5 (c 1.00, CHCl₃)]; ¹H NMR (CDCl₃) δ 1.97 (m, 2H), 2.43 (m, 2H), 2.7–3.2 (s, br, 1H), 4.59 (t, J = 7.0 Hz, 2H), 7.26 (t, J = 7.3 Hz, 2H), 7.34 (t, J = 7.3 Hz, 4H), 7.43 (d, J = 7.5 Hz, 4H); ¹³C NMR (CDCl₃) δ 35.1, 62.4, 126.3, 127.2, 128.6, 144.4; IR (film) 3400, 2921, 2851 cm⁻¹. HRMS calcd for C₁₆H₁₈N(M+H) 224.1439. Found 224.1436.

(R)-(−)-N-(Pentenylidene)-p-toluenesulfonamide (14)

In a 100-mL round-bottom, one-neck flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed valeraldehyde (0.422ml, 3.852 mmol), Ti(OEt)₄ (0.377 mL, 12.84 mmol), and (R)-(−)-p-toluenesulfonamide (0.298 g, 2.568 mmol) in CH₂Cl₂ (40 mL), and the solution was stirred for 2.5 h at 0 °C. At this time the reaction mixture was quenched with water (10 mL), filtered through Celite, rinsing with CH₂Cl₂ (20 mL), and the organic phases concentrated. Chromatography (35% Et₂O/hexane) gave 0.550 g (96%) of a colorless oil; [α]²⁰ _D −292.9 (c 0.97, CHCl₃); ¹H NMR (CDCl₃) δ 0.86 (t, J = 7.2 Hz, 3H), 1.31 (sextet, J = 7.4 Hz, 2H), 1.55 (m, 2H), 2.36 (s, 3H), 2.44 (m, 2H), 7.26 (d, J = 7.8 Hz, 2H), 7.51 (d, J = 7.8 Hz, 2H), 8.18 (t, J = 4.95 Hz, 1H); ¹³C NMR (CDCl₃) 13.1, 20.7, 21.6, 26.8, 39.0. 123.9, 129.1, 141.2, 143.7, 166.8. IR (film) 2957, 2931, 2872 cm⁻¹. HRMS calcd for C₁₂H₁₈NOS (M+H) 224.1109. Found 224.1108.

(R_S,R)-(−)-Methyl-N-(p-toluenesulfinyl)-3-aminopehtanoate (15)

In an oven-dried, 100 mL, round-bottom one-neck flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed NaHMDS (3.6 mL, 3.57 mmol, 1.0 M solution in THF) in ether (20 mL). The solution was cooled to −78 °C, and anhydrous MeCO₂Me (0.283 mL, 3.57 mmol) was added dropwise via syringe. After the solution was stirred for 1 h at this temperature, (−)-14 (0.53 g, 2.377 mmol) in Et₂O (4 mL) was added via cannula. The resulting mixture was stirred for 2 h at −78 °C, and quenched with sat. aqueous NH₄Cl (5 mL) at this temperature. The solution was warmed to rt, diluted with H₂O (10 mL), extracted with EtOAc (2 X 50 mL), and the combined organic phases were washed with brine (2 × 10 mL), dried (MgSO₄), and concentrated. Chromatography (50% Et₂O/Hexane) gave 0.648 g (92%) of a colorless oil as a single diastereomer; [α]²⁰ _D − 89 (c 1.25, CHCl₃); ¹H NMR (CDCl₃) δ 0.83 (d, J = 6.6 Hz, 3H), 1.2 – 1.7 (m, 6H), 2.32 (s, 3H), 2.51 (m, 2H), 3.57 (s, 4H), 4.59 (d, J = 9.0 Hz, 1H), 7.2 (d, J = 8.1 Hz, 2H), 7.49 (d, J = 8.1 Hz, 2H); ¹³C NMR (CDCl₃) δ 13.9, 21.3, 22.3, 28.2, 35.5, 40.4, 51.6, 52.5, 76.7, 125.5, 129.4, 141.1, 142.3, 171.9; IR (film) 3210, 2955, 2862, 1734, 1050 cm⁻¹. HRMS (EI) m/e calcd for C₁₅H₂₃NO₃NaS (M+Na) 320.129635. Found 320.1303.

(R)-(+)-Methyl 3-(4-methylphenylsulfonamido)heptanoate (16)

In an oven-dried, 100 mL one-neck round-bottom, flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed (Rs,R)-(−)-methyl-N-(p-toluenesulfinyl)-3-aminopheptanoate 15 (0.645 g, 2.172 mmol) in CH₂Cl₂ (43 mL). The solution was cooled to 0 °C m-CPBA (0.614 g, 6.515 mmol) was added. The reaction mixture was stirred for 3 h at 0 °C, quenched with sat. aqueous Na₂S₂O₃ (5 mL), diluted with CHCl₃ (20 mL), sat. aqueous. NaHCO₃ (5 mL), and stirred vigorously. The organic phases were dried (MgSO₄) and concentrated. Chromatography (50% Et₂O/hexane) gave 0.647 g (95%) of a colorless oil; [α]²⁰ _D +21.9 (c 1.05, CHCl₃); ¹H NMR (CDCl₃) δ 0.80 (d, J = 6.8 Hz, 3H), 1.19 (m, 4H), 1.46 (m, 2H), 2.37 – 2.52 (m, 5H), 3.55 (m, 1H), 3.64 (s, 3H), 5.44 (d, J = 9.0 Hz, 1H), 7.33 (d, J = 8.1 Hz, 2H), 7.80 (d, J = 7.8 Hz, 2H); ¹³C NMR (CDCl₃) δ 13.9, 21.5, 22.2, 27.8, 34.5, 38.9, 50.7, 51.7, 127.1, 129.7, 138.1, 143.4, 171.8; IR (film) 3288, 2956, 1330, 1160 cm⁻¹; HRMS (EI) m/e calcd for C₁₅H₂₃NO₄NaS (M+Na) 336.12455. Found 336.1251.

(R)-(+)-4-Methyl N-(1-oxoheptan-3-yl)benzenesulfonamide (17)

In an oven-dried, 50 mL, round-bottom one-neck flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed (R)-(+)-16 (0.645 g, 2.061 mmol) in toluene (21 mL). The solution was cooled to −78 °C then DIBAL-H (2.1 mL, 2.1 mmol, 1.0 M solution in CH₂Cl₂) was added dropwise. The reaction mixture was stirred for 3 h at −78 °C, and quenched with aq. Rochell salt (5 mL) at −78 °C. The resulting mixture was warmed to room temperature, diluted with EtOAc (10 mL) and stirred vigorously until the solution becomes two clear layers. Chromatography (35% Et₂O/hexane) provided 0.432 g (76%) of a colorless oil and 0.125 g (19%) of starting material; [α ]²⁰ _D +24.8 (c 1.0, CHCl₃); ¹H NMR (CDCl₃) δ 0.76 (d, J = 6.6 Hz, 3H), 1.14 (m, 5H), 1.42 (m, 2H), 2.44 (s, 3H), 2.62 (d, J = 5.7 Hz, 1H), 3.64 (m, 1H), 5.45 (bs, 1H), 7.33 (d, J = 8.1 Hz, 2H), 7.78 (d, J = 8.1 Hz, 2H), 9.66 (s, 1H); ¹³C NMR (CDCl₃) δ 13.8, 21.5, 22.1, 27.7 34.7, 48.8, 49.5, 127.1, 129.8, 137.8, 143.6, 201.0; IR (film) 3281, 2957, 2735, 1719, 1327, 1159 cm⁻¹; HRMS (EI) m/e calcd for C₁₄H₂₁NO₃NaS (M+Na) 306.113985. Found 306.1132.

(R)-(+)-4-Methyl N-(tridec-7-en-5-yl)benzenesulfonamide (18)

In an oven-dried, 50 mL, round-bottom one-neck flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed hexyltrophenylphosphonium bromide (0.65 g, 1.52 mmol) in THF (5 ml), and n-BuLi (0.6 mL, 1.52 mmol, 2.5 M solution in hexane) was added dropwise at 0 °C. The resulting mixture was stirred for 1 h at room temperature, cooled to −78 °C, and a solution of (R)-(+)-17 (0.086 g, 0.304 mmol) in THF (1 mL) was added. The reaction mixture stirred for 8 h at −78 °C, and quenched with H₂O (1 mL), warmed to rt and extracted with EtOAc (2 × 5 mL), dried (MgSO₄), and concentrated. Chromatography (20% Et₂O/hexane) gave 0.08 g (75%) of a colorless oil as an inseparable 54:46 E and Z mixture. In an oven-dried, one-neck, round-bottom flask equipped with a magnetic stirring bar and rubber septum was placed the E and Z mixture of (+)-18 (0.077 g, 0.219 mmol) and PhSSPh (0.096 g, 0.439 mmol) in cyclohexane (4 mL). The reaction mixture was irradiated by mercury lamp (300 nm, 450 W) for 6 h with stirring, quenched with sat. aqueous NaHCO₃ (1 mL), and extracted with EtOAc (5 mL) The organic phases were dried (MgSO₄) and concentrated. Chromatography (20% Et₂O/Hexane) provided 0.065 g (84%) of a colorless oil as an inseparable 85:15 E and Z mixture; [α]²⁰ _D +24.4 (c 1.0, CHCl₃); ¹H NMR (CDCl₃) δ 0.73 (t, J = 6.6 Hz, 3H), 0.81 (t, J = 7.0 Hz, 3H), 1.01 – 1.39 (m, 12H), 1.82 (m, 2H), 1.94 (t, J = 6.2 Hz, 2H), 2.35 (s, 3H), 3.13 (m, 1H), 4.41 (d, J = 8.0 Hz, 1H), 5.02 (m, 1H), 5.27 (m, 1H), 7.22 (d, J = 8.0 Hz, 2H), 7.68 (d, J = 8.0 Hz, 2H); ¹³C NMR (CDCl₃) δ 14.2, 14.3, 21.8, 22.7, 22.8, 27.9, 29.6, 31.7, 32.9, 34.7, 38.1, 54.0, 124.7, 127.5, 129.8, 135.5, 138.8, 143.4; IR (film) 2917, 2849 cm⁻¹; HRMS (EI) m/e calcd for C₂₀H₃₃NO₂NaS (M+Na) 374.212971. Found 374.2136.

(2R,3S,5R)-(+)-5-Butyl-3-iodo-2-pentyl-1-tosylpyrrolidine (20)

Following typical procedure for the iodocyclization, (+)-18 (0.062 g, 0.177 mmol, E:Z 85:15), K₂CO₃ (0.073 g, 0.531 mmol), and I₂ (0.135 g, 0.531 mmol) was employed. Chromatography (5% Et₂O/hexane to 20% Et₂O/hexane) gave 0.069 g (82%) of a yellow oil as a separable 84:16 mixture of diastereomers, and 0.006 g (10%) of recovered Z homoallylic sulfonamide; major diastereomer: [α]²⁰ _D +52.8 (c 3.6, CHCl₃); ¹H NMR (C₆D₆) δ 0.79 (t, J = 7.3 Hz, 3H), 0.87 (t, J = 7.0 Hz, 3H), 1.0 (m, 2H), 1.15 – 1.3 (m, 9H), 1.74 (m, 1H), 1.88 (m, 1H), 1.89 (s, 3H), 2.1 (ddd, J = 6.6, 8.6, 15.2 Hz, 1H), 2.19 (m, 2H), 3.79 (dt, J = 1.9, 6.7 Hz, 1H), 3.88 (m, 1H), 4.37 (dd, J = 1.7, 8.1, 1H), 6.83 (m, 2H), 7.98 (m, 2H); ¹³C NMR (C₆D₆) δ 14.5, 14.6, 21.5, 22.9, 23.1, 23.3, 26.6, 29.5, 32.3, 34.0, 35.3, 40.6, 60.7, 74.8, 127.9, 129.8, 141.9, 143.0; IR (film) 2956, 2927, 2859, 1339, 1156 cm⁻¹; HRMS (EI) m/e calcd for C₂₀H₃₂INO₂NaS (M+Na) 500.109623. Found 500.1099.

The C-4 proton H_β at δ 2.1 was assigned from COSY spectrum and NOE experiments. Upon irradiation of the C-4 proton H_β at δ 2.1, a positive NOE was observed on the C-3 proton at δ 3.79 (9.3%) and on the C-5 proton at δ 3.88 (11.2%), confirming the cis relationship between the C-3 proton and the C-5 proton. The trans relationship between the C-2 proton and the C-3 proton was confirmed by the trans vicinal coupling constant of J = 6.7 of the C-3 proton at δ 3.79 originating from coupling between the C-3 proton and the C-2 proton.

(R,Z)-(+)-4-Methyl-N-(tridec-7-en-5-yl)benzenesulfonamide (19)

[α ]²⁰ _D +18.8 (c 2.8, CHCl₃); ¹H NMR (CDCl₃) δ 0.85 (t, J = 6.5 Hz, 3H), 0.94 (t, J = 6.6 Hz, 3H), 1.16 – 1.53 (m, 12H), 1.84 (m, 2H), 2.08 (m, 2H), 2.48 (s, 3H), 3.29 (sext, J = 6.8 Hz, 1H), 4.61 (d, J = 8.1 Hz, 1H), 5.22 (m, 1H), 5.49 (m, 1H), 7.35 (d, J = 7.8 Hz, 2H), 7.83 (d, J = 8.1 Hz, 2H); ¹³C NMR (CDCl₃) δ 14.0, 14.1, 21.6, 22.4, 22.6, 27.4, 27.6, 29.3, 31.6, 32.6, 34.3, 53.9, 123.8, 127.2, 129.6, 133.7, 138.3, 143.2; IR (film) 2917, 2849 cm⁻¹; HRMS (EI) m/e calcd for C₂₀H₃₃NO₂NaS (M+Na) 374.212971. Found 374.2136.

Upon irradiation of one of the alkenyl protons at δ 5.49, a positive NOE was observed on the other alkenyl proton at δ 5.22 (2.9%), confirming the Z geometry of olefin.

(2R,5R)-(−)-2-Butyl-5-pentyl-1-tosylpyrrolidine (21)

In an oven-dried, 10 mL, round-bottom one-neck flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed (+)-20 (0.037 g, 0.078 mmol) in dry toluene (4 ml) and Bu₃SnH (0.194 mL, 0.73 mmol), and the reaction mixture was refluxed for 1 h at 125 °C. The resulting mixture was cooled to rt and concentrated. Chromatography (10% Et₂O/hexane) provided 0.027 g (99%) of a colorless oil; [α] ²⁰ _D −43.5 (c 2.7, CHCl₃);¹H NMR (CDCl₃) δ 0.79 (t, J = 7.2 Hz, 3H), 0.81 (t, J = 7.2 Hz, 3H), 1.0 – 1.40 (m, 12H), 1.60 (m, 2H), 1.86 (m, 4H), 2.37 (s, 3H), 3.75 (m, 2H), 7.21 (m, 2H), 7.67 (m, 2H); ¹³C NMR (CDCl₃) δ 13.9, 14.3, 16.8, 21.7, 22.9, 26.4, 26.9, 28.4, 29.0, 29.5, 32.0, 34.0, 34.2, 62.1, 127.3, 129.8, 140.6, 142.8; IR (film) 2957, 2927, 1342, 1156 cm⁻¹; HRMS (EI) m/e calcd for C₂₀H₃₃NO₂NaS (M+Na) 374.212971. Found 374.2134.

(−)-Pyrrolidine 197B (22)

In a 25-mL, two-neck, round-bottom flask equipped with a magnetic stirring bar, a rubber septum, argon inlet, and an acetone-dry ice cooled cold finger condenser, cooled to − 78°C, NH₃ (g) was passed until approximately 7 mL of liq. NH₃ (1/3 of flask volume) was collected. A solution of (−)-21 (0.023 g, 0.066 mmol) in THF (3 mL) was added and enough Na was introduced until a permanent deep navy blue color persisted. After stirring the reaction mixture for 1.5 h at −78 °C, the reaction mixture was quenched by addition of solid NH₄Cl (~1 g), and the excess NH₃ was removed by passing argon through the solution with warming to rt. At this time the residue was diluted with Et₂O (5 mL) and water (2 mL). The solution was extracted with ether (2 X 10 mL), and the combined organic phases were dried (MgSO₄). Since the product was volatile, the organic phase was concentrated carefully at 0°C. Chromatography (CHCl₃/MeOH-NH₃ 10:1) gave 0.008 g (62%) of a yellow oil; [α]²⁰ _D −25.3 (c 0.6, CHCl₃) [lit.^19b [α]²⁵_D −5.8 (c 0.61, CHCl₃)]; ¹H NMR (CDCl₃) δ 0.87 (m, 6H), 1.22 – 1.47 (m, 16H), 1.93 (m, 2H), 2.1 (bs, 1H), 3.1 (m, 2H); ¹³C NMR (CDCl₃) δ 14.4, 14.43, 23.0, 23.2, 27.4, 29.9, 32.4, 32.8 (2C), 37.1, 37.4, 58.5 (2C); IR (film) 2955, 2926, 2860 cm⁻¹. HRMS (EI) m/e calcd for C₁₃H₂₈N (M+H) 198.222175. Found 198.2218, m/e calcd for C₁₃H₂₆N (M−H) 196.206525. Found 196.2061. Spectral properties are consistent with literature values.^19b

(2R,5R)-(−)-N-Benzoyl-2-butyl-5-pentylpyrrolidinine

In an oven-dried, 5 mL round-bottom one-neck flask equipped with a magnetic stirring bar, rubber septum, and argon inlet was placed (−)-22 (0.02 g, 0.01 mmol) in CH₂Cl₂ (1 mL) and 20% aq. K₂CO₃ (1 mL, 1.45 mmol) at 0 °C. The solution was stirred for 5 min and benzoyl chloride (0.012 mL, 0.1 mmol) was added. After being stirred for 1 h at 0°C, the reaction mixture was dissolved in Et₂O (3 mL) and the organic phase was washed with brine (2 × 2 mL), dried (MgSO₄), and concentrated. Chromatography (5% EtOAc/hexane) provided 0.01 g (33%) of a colorless oil; [α]²⁰ _D −231.5 (c 0.1, CHCl₃) [lit.^19b enantiomer, [α]²⁷_D +125.5 (c 1.0, CHCl₃)]; ¹H NMR (CDCl₃) δ 0.55, 0.64 (1:1 ratio, total 6H, t, J = 7.0, 7.2 Hz, respectively, due to rotamers), 0.66 – 1.11 (m, 10H), 1.26 (m, 3H), 1.63 (m, 2H), 1.84–2.07 (m, 3), 3.89, 4.19 (1:1 ratio, total 2H, m, due to rotamers), 7.3 (m, 3H), 7.4 (m, 2H); ¹³C NMR (CDCl₃) δ 14.0, 14.1, 14.3, 21.2, 22.5, 22.9, 26.1, 26.5, 26.9, 28.6, 29.0, 32.1, 33.1, 33.4, 34.6, 34.8, 58.0, 59.7, 59.9, 105.5, 127.3, 128.5, 129.7, 140.0, 170.6. Spectral properties were consistent with literature values.^19b

Supplementary Material

1si20060126_01. Supporting Information Available.

¹H and ¹³C NMR spectra for all new compounds. This material is available free of charge via the Internet http://pubs.acs.org.