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Synlett 2010(4): 575-578  
DOI: 10.1055/s-0029-1219357
LETTER
� Georg Thieme Verlag Stuttgart ˙ New York

1,5-Stereocontrol in Bismuth-Mediated Reactions between Aldehydes and Allyl Bromides: Stereoselective Synthesis of Open-Chain all-syn- and anti, anti-1,3,5-Disposed Trimethylalkanes

Norazah Basar, Sam Donnelly, Hao Liu, Eric J. Thomas*
The School of Chemistry, The University of Manchester, Manchester, M13 9PL, UK
Fax: +44(161)2754939; e-Mail: e.j.thomas@manchester.ac.uk;
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Publication History

Received 3 December 2009
Publication Date:
02 February 2010 (online)

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1,5-Stereocontrol in Bismuth-Mediated Reactions between Aldehydes and Allyl Bromides: Stereoselective Synthesis of Open-Chain all-syn- and anti, anti-1,3,5-Disposed TrimethylalkanesSynlett 2014; 25(06): 898-898
DOI: 10.1055/s-0033-1340184

Abstract

The reaction of 5-benzyloxy-2,4-dimethylpent-2-enyl bromide with the bismuth species generated by reduction of bismuth(III) iodide by zinc powder gives an intermediate which reacts with aldehydes with useful levels of stereocontrol in favour of 1,5-anti-(3E)-5-methylalk-3-enols. Manipulation of protecting groups, stereoselective hydrogenation, and tosylate displacement using a higher-order cyanomethylcuprate leads to either all-syn or anti,anti-1,3,5-disposed trimethylalkanes.

Key words

Aldehydes - organometallic reagents - allylation - hydrogenation - diastereoselectivity

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(R)-6: [α]D �5 -13.9 (c 0.0046, CHCl3).

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Zinc powder (30 mg, 0.46 mmol) was added to a solution of bismuth(III) iodide (234 mg, 0.4 mmol) in THF (1.65 mL) and the mixture stirred at r.t. for 1 h. The pent-2-enyl bromide 6 (75 mg, 0.26 mmol) and benzaldehyde (27 �L, 0.26 mmol) in THF (0.6 mL) were added and the mixture heated under reflux for 2 h. After cooling, the mixture was filtered and concentrated under reduced pressure. Column chromatography of the residue gave (1S,5R,3Z)-6-benzyl-oxy-3,5-dimethyl-1-phenylhex-3-en-1-ol (9a, 8 mg, 10%), as an oil. R f = 0.28 (light PE-Et2O, 3:1). HRMS: m/z calcd for C21H30O2N: 328.2271 [M]; found: 328.2273 [M + NH4]. IR: νmax = 3414, 1453, 1088, 1026, 752, 699 cm-�. �H NMR (300 MHz, CDCl3): δ = 0.83 (3 H, d, J = 7 Hz, 5-CH3), 1.90 (3 H, d, J = 1 Hz, 3-CH3), 2.22 (1 H, dd, J = 14, 3 Hz, 2-H), 2.78 (1 H, dd, J = 14, 10 Hz, 2-H′), 2.83 (1 H, m, 5-H), 3.18 (1 H, t, J = 9 Hz, 6-H), 3.41 (1 H, dd, J = 9, 5 Hz, 6-H′), 4.55 and 4.58 (each 1 H, d, J = 12 Hz, OHCHPh), 4.87 (1 H, dd, J = 10, 3 Hz, 1-H), 5.18 (1 H, dd, J = 10, 1 Hz, 4-H), 7.38 (10 H, m, ArH). ��C NMR (75 MHz, CDCl3): δ = 17.6, 23.8, 33.2, 43.6, 71.4, 73.3, 75.2, 125.9, 127.2, 128.0, 128.3, 128.5, 128.7, 132.6, 133.0, 138.2, 145.7. MS (CI): m/z (%) = 328 (18) [M+ + 18], 310 (9) [M+], 293 (41), 99 (100). [nl]The second fraction off the column was a mixture of the (1R,5R,3E)- and (1S,5R,3E)-6-benzyloxy-3,5-dimethyl-[nl]1-phenylhex-3-en-1-ols 7a and 8a (53 mg, 65%) as a colourless oil; 1,5-anti-7a/1,5-syn-8a = 93:7, R f = 0.25 (light PE-Et2O, 3:1). HRMS: m/z calcd for C21H30O2N: 328.2271 [M]; found: 328.2273 [M+ + NH4]. IR: νmax = 3431, 1603, 1453, 1374, 1089, 750, 649 cm-�. �H NMR (300 MHz, CDCl3): δ (major isomer) = 0.98 (3 H, d, J = 7 Hz, 5-CH3), 1.77 (3 H, d, J = 1 Hz, 3-CH3), 2.33 (1 H, dd, J = 13, 9 Hz, 2-H), 2.44 (1 H, dd, J = 13, 4 Hz, 2-H′), 2.82 (1 H, m, 5-H), 3.30 (1 H, dd, J = 9, 8 Hz, 6-H), 3.34 (1 H, dd, J = 9, 6 Hz, 6-H′), 4.55 and 4.59 (each 1 H, d, J = 12 Hz, OHCHPh), 4.77 (1 H, dd, J = 10, 4 Hz, 1-H), 5.13 (1 H, dd, J = 9, 1 Hz, 4-H), 7.39 (10 H, m, ArH). ��C NMR (75 MHz, CDCl3): δ = 16.6, 17.5, 33.5, 51.0, 71.0, 73.3, 75.4, 126.1, 127.5, 127.8, 127.9, 128.5, 128.6, 132.4, 133.1, 138.7, 144.4; δ (minor isomer) = 1.04 (3 H, d, J = 7 Hz, 5-CH3). MS (CI): m/e (%) = 328 (59) [M+ + 18], 310 (71) [M+], 293 (84), 99 (100).