PPh₃-Mediated [3+2] Cycloaddition Reaction between Bis-Substituted Allenoate and N-Tosylaldimines to Construct 2-Pyrrolines

Abstract

A triphenylphosphine-promoted [3+2] cycloaddition of α,γ-bis-substituted allenoates and N-tosylaldimines followed by alkene isomerization was disclosed, affording a series of functionalized 2-pyrroline derivatives in moderate chemical yields with random diastereoselectivities.

Key words

nucleophilic catalysis - allenoate - cycloaddition - triphenylphosphine - 2-pyrroline

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• 14 See the Supporting Information for details. Other imines activated by alternative electron-withdrawing groups on the nitrogen atom failed to give satisfying results: The use of N-(tert-butoxycarbonyl)benzaldimine provided an intractable and complicated mixture, and the reaction with cyclic ketimine benzo[d]isothiazole 1,1-dioxide gave a trace amount of analogous cycloaddition product.
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• 18 General Procedure Into a solution of N-tosyl aldimine 1 (0.1 mmol) and bis-substituted allenoate 2 (0.2 mmol) in toluene (1.0 mL) was added PPh₃ (0.06 mmol) in one portion. The resulting mixture was stirred at r.t. and monitored by TLC. After the reaction was complete, the mixture was directly subjected to flash column chromatography (PE/EtOAc = 10:1 as the eluent) to furnish the corresponding product 3. Diethyl 2-(5-Phenyl-1-tosyl-4,5-dihydro-1H-pyrrol-2-yl)succinate (3a) Light yellow liquid; 78% yield; inseparable mixture of two diastereoisomers (major and minor), d.r. = 1.2:1. ¹H NMR (600 MHz, CDCl₃): δ = 7.83 (d, J = 8.2 Hz, 2 H; minor), 7.75 (d, J = 8.4 Hz, 2 H; major), 7.33 (m, 2 H; both isomers), 7.31–7.23 (m, 5 H; both isomers), 5.17 (s 1 H; both isomers), 5.24 (s, 1 H; major), 5.11 (s, 1 H; minor), 5.10–5.05 (m, 1 H; both isomers), 4.57–4.49 (m, 1 H; both isomers), 4.27–4.10 (m, 4 H; both isomers), 3.08–3.02 (m, 1 H; both isomers), 2.82–2.74 (m, 1 H; both isomers), 2.65–2.48 (m, 1 H; both isomers), 2.45 (s, 3 H; minor), 2.42 (s, 3 H; major), 2.28–2.20 (m, 1 H; both isomers), 1.28–1.21 (m, 6 H; both isomers). ¹³C NMR (150 MHz, CDCl₃): δ = 171.8, 171.3, 171.2, 171.0, 144.1, 143.9, 142.6, 142.5, 140.8, 135.5, 134.2, 129.7, 129.6, 128.7, 128.5, 127.8, 127.7, 126.0, 125.8, 114.4, 114.3, 64.7, 64.6, 61.4, 61.2, 60.9, 60.8, 42.2, 41.5, 36.7, 36.2, 35.7, 35.6, 29.7, 21.6, 21.5, 14.1, 14.0. HRMS (ESI): m/z calcd for [M]⁺(C₂₅H₂₉NO₆S): 471.1716; found: 471.1719.