Synlett 2005(11): 1726-1730  
DOI: 10.1055/s-2005-871555
LETTER
© Georg Thieme Verlag Stuttgart · New York

Hydride-Promoted Ring Expansion of 2-Azaspiropyrrolinium Salts: An Approach Towards the Synthesis of (-)-Nitramine

Erick Rosas Alonso, Kourosch Abbaspour Tehrani, Mark Boelens, Norbert De Kimpe*
Department of Organic Chemistry, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium
Fax: +32(9)2646243; e-Mail: norbert.dekimpe@UGent.be;
Further Information

Publication History

Received 17 March 2005
Publication Date:
28 June 2005 (online)

Abstract

An enantioselective approach toward the synthesis of the spiroalkaloid (-)-nitramine was achieved by electrophile-induced cyclization of a suitably substituted and protected chiral α-allylcyclohexanecarboxaldimine. Its hydride-promoted ring expansion after spirocyclization gave rise to the competitive formation of isomeric spiropyrrolidines and a spiropiperidine, the latter being further transformed into (-)-nitramine.

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Present address: Department of Organic Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium

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Experimental Conditions.
A solution of 470 mg (2.9 mmol) of bromine in 10 mL of CH2Cl2 was slowly added to 1.0 g (2.93 mmol) of imine 16a in 10 mL of dry CH2Cl2 at 0 °C. After 20 min of vigorous stirring at 0 °C, the solvent was evaporated in vacuo under gentle warming, resulting in 17a as a syrupy ocher mass which decomposed after standing at r.t.

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Spectral data of (6R,7S)-2-benzyl-2-azaspiro[5.5]-undecan-7-ol (19a): 1H NMR (270 MHz, CDCl3): δ = 1.04-1.80 [12 H, m, (CH2)6], 1.93-2.03 and 2.22-2.28 (1 H each, each m, NCH2CH2), 2.88 (1 H, br d, J AX = 11.5 Hz, CspiroHCHN), 3.24 (1 H, d, J AX = 11.5 Hz, CspiroHCHN), 3.40-3.49 (1 H, m, CHOH), 3.38-3.48 (1 H each, each d, J AB = 12.9 Hz, CH2C6H5), 5.32 (1 H, br s, OH), 7.17-7.39 (5 H, m, C6H5). 13C NMR (68 MHz, C6D6): δ = 21.35, 23.93, 24.60, 32.95, 37.25 and 38.40 [(CH2)4 and (CH2)2], 37.64 (Cspiro), 54.75 (NCH2CH2), 59.15 (CspiroCH2N), 63.80 (NCH2C6H5), 78.40 (CHOH), 127.56, 128.66, 129.32 and 138.40 (C6H5). MS (70 eV): m/z (%) = 259 (8) [M+], 258 (4), 231 (5), 174 (12), 168 (14), 148 (5), 146 (8), 140 (5), 134 (29), 133 (14), 132 (5), 121 (6), 120 (33), 106 (5), 92 (10), 91 (100), 67 (6), 65 (9), 55 (8), 44 (13), 43 (5), 42 (9), 41 (10). IR (NaCl): 3160-3435 (O-H) cm-1. Rf = 0.33 (CHCl3-MeOH, 95:5); [α]D 22 +15 (c 1.12, CHCl3).

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Synthesized nitramine: [α]D 23 -21.5 (c 1.51, CH2Cl2). Lit. [12a] [α]D 25 +23 (c 1.58, CH2Cl2). Enantiomeric excess values for all succeeding compounds were 91-93%.