Versatile cyclisation reactions using selenoboranes

Abstract

Tris( phenylseleno) borane and tris(methylse1eno) borane reacted with terminal acetylenes to afford (Z)vinyl selenides. This reaction was initiated by oxygen and the intermediates were vinyl radicals. This radical reaction could be applied t o the intramolecular free radical cyclisation of enynes and some heterocycles and carbocycles were synthesised. This novel method could be also employed in the synthesis of a-kainoid derivatives.Selenyl radicals are reactive chemical species and their addition to multiple bonds is synthetically useful.' The phenylselenyl radical can be readily generated by photolysis of diphenyl diselenide or by oxidation of benzeneselenol with oxygen, and adds to olefins,2 acetylenes or a l l e n e ~. ~ Photochemical and thermal reactions'' of Se-phenyl areneselenosulfonates cause the free radical selenosulfonation to multiple bonds.On the other hand, the intramolecular addition of carbon free radicals to carbon-carbon multiple bonds constructs the fiveand six-membered rings.6 In particular, the exo-methylene substituted five-membered rings are formed by the intramolecular capture of vinyl radicals with double bonds. The vinyl radicals are generated by addition of carbon-or heteroatomcentred radicals to acetylenes7 Oshima et al. found the facile radical additions of R,GeH,* RSH or R,SnH l o to acetylenes induced by triethylborane, and developed the method to the synthesis of five-membered rings with a R,Ge-, RS-or R,Snsubstituted methylene group. However, the triethylboraneinduced hydroselenation of acetylenes did not give satisfactory results.' ' It has been reported that consecutive reactions of iodoacetylenes with bis( 1,2-dimethylpropyl)borane and with alkylselenomagnesium bromide afforded vinyl selenides. 'Recently Hevesi reported that the organoselenoboranes which are generally utilised for selenoacetalisation of aldehydes or ketones,' added to a$-unsaturated ketones. l 4 These reports prompted us to examine the reactions of organoselenoboranes with acetylenes and we revealed that organoselenoboranes caused free radical 1,2-addition to acetylenes.'5 We have been interested in the phenylselenomethylene-substituted five-membered ring compounds, since these compounds can be employed in the synthesis of biologically active compounds by transformation of the phenylselenomethylene group to other groups.I6 This paper gives full details of the addition reactions of organoselenoboranes and their application to radical cyclisation of some enynes. Results and DiscussionSelenoborane reagents 2a and 2b were treated with acetylenes in dichloromethane at room temperature to afford mono-RSesubstituted ethylenes 3 and di-RSe-substituted ethylenes 4. The addition reactions occurred regio-and stereo-selectively at the terminal acetylenic carbon to form Z-vinyl selenides in satisfactory yields. However, reactions with an acetylenic alcohol provided low yields of vinyl selenides and reactions with disubstituted acetylenes gave no addition products."The mechanism of the addition reaction was investigated by...

Gold(I)‐Catalyzed Cyclizations of 1,6‐Enynes: Alkoxycyclizations and exo/endo Skeletal Rearrangements

“…The following compounds have been described: a) Starting enynes: 1, [39] 4, [2a,b] 6, [2c] 7, [40] 8, [41] 10, [2a,b] 12, [2a,b] 16, [42] 18, [13a] 20, [19] 22, [19] 27, [52] 47, [43] 48, [44] 49, [4a] 58, [44] 67, [45] 72 a, [46] 71 b, [47] 75, [2a,b] 79, [8] and 84; [48] b) carboand heterocycles: 2 a, [2a,b] 5, [2a,b] 9, [8] 11, [2a,b] 13 a, [2a,b] 17, [2a,b] 19, [2a,b] 21, [49] 23, [50] 51, [51] 52, [10a] 53, [52] 54, [2a,b] 56, [44] 59, [29] 60, [52] 62, [52] 68, [10b] 69, [10b] 73 a, [53] 76, [54] 77, [55] 78, [10] 79, [8] 81, [53] 89, [53] 90, [53] and 91. [56] Synthesis of new enynes (E)-1,1-Bis(phenylsulfonyl)-4,8-dimethylnona-3,7-diene: Bis(phenylsulfonyl)methane (500 mg, 1.68 mmol) was added at 0 8C to a suspension of NaH (68 mg, 1.68 mmol, 60 % in mineral oil) ...…”

N‐Heterocyclic Carbene and Phosphine Ruthenium Indenylidene Precatalysts: A Comparative Study in Olefin Metathesis

“…Complexes 2 , 4 , 5 , 7 and 8 were obtained commercially. The following compounds have been previously described: 3 ,5 6 ,14 9 ,20 10 ,21 11 ,21 12 ,21 13 ,21 14 ,22 15 ,20 16 ,23 17 ,2b 19 ,24 20 ,25 21 ,26 22 ,20 23 ,27 24 ,22 25 ,22 27 ,22 28 ,20 29 ,28 32 ,20 33 ,29 36 ,30 37 ,31 39 ,28 40 ,20 41 ,20 42 ,32 44 ,33 45 ,28 46 ,34 47 ,2b 48 ,34 49 ,20 50 ,2b 51 ,2b 52 ,35 53 ,19 54 ,36 55 ,24 59 ,24 61 24…”

Cyclometalated N‐Heterocyclic Carbene‐Platinum Catalysts for the Enantioselective Cycloisomerization of Nitrogen‐Tethered 1,6‐Enynes

“…The mixture was stirred at room temperature for 1 h and hydrolyzed then with aqueous NH 4 Cl. After extraction with CH 2 Cl 2 , drying over MgSO 4 and evaporation of the solvent, the final product was purified by chromatography with a heptane/ethyl acetate gradient (from 9:1 to 1:1) to afford N-2-methylallyl-ptoluenesulfonamide; [34] yield: 1.1 g (51%). 1 Secondly, a solution of N-allyl-p-nitrobenzenesulfonA C H T U N G T R E N N U N G amide (0.50 g, 2.0 mmol), 3-phenylpropargyl bromide (0.48 g, 2.4 mmol) and potassium carbonate (0.60 g, 4.3 mmol) in MeCN (5 mL) was heated at 60 8C overnight.The reaction mixture was filtered, the solid washed with CH 2 Cl 2 and the solvents removed under vacuum.…”