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The intermolecular pinacol-type coupling reaction and allylation reaction of optically active imines bearing a beta-hydroxy group were performed stereoselectively with metallic samarium after treatment of the imines with trimethylaluminum.
By the promotion of samarium diiodide, thiophene-2-carboxylate reacted with 2 equiv of ketones at the C-4 and C-5 positions to give diols such as 2 and 9. Because the intermediary organosamarium species were oxophilic but not too basic, the double hydroxyalkylations with various ketone substrates, including alkyl aryl ketones, acetylthiophenes, cyclohexanone, alpha-tetralone, and alpha-phenylacetophenones, were realized without complication of side reactions. The diol products underwent an acid-catalyzed dehydration to give dienes such as 3 and 10, which were treated with DDQ to give either polysubstituted thiophenes (e.g., 4 and 11) or benzothiophenes (e.g., 5, 13, and 14) depending on the reaction conditions. Oxidative annulations of 4,5-diarylthiophenes 11 and 4,5,6,7-tetraphenylbenzothiophenes 14 were carried out by photochemical or chemical methods to give the sulfur-containing polycyclic aromatic compounds, such as phenanthro[9,10-b]thiophene-2-carboxylate, piceno[13,14-b]thiophene-2-carboxylate, and tribenzo[fg,ij,rst]pentapheno[15,16-b]thiophene-2-carboxylates. This method is applicable to the preparation of polysubstituted thiophenes, benzothiophenes, and the related compounds possessing liquid crystalline, photochromic, and other functional properties.
An easy and general sequenced elimination/reduction process by means of samarium diiodide, in the presence of D(2)O, provides an efficient method for synthesizing 2,3-dideuterioesters 2. The reaction can be also carried out in the presence of H(2)O instead of D(2)O, yielding the corresponding saturated esters 4. Other deuterated esters have been also obtained. A mechanism to explain this synthesis has been proposed.
trans-Cyclopropanamides were obtained, in high yield, from 2-chloro-3-hydroxyamides by a sequenced elimination-cyclopropanation process promoted by Samarium/diiodomethane or Samarium diiodide and Samarium/diiodomethane.
1,10-Phenanthroline reacts with aldehydes and ketones in the presence of samarium diiodide to produce 2-(1-hydroxyalkyl)-1,10-phenanthrolines. The hydroxyalkyl substituent can be functionalized in numerous ways or removed to permit further ligand variation. The carbonyl coupling reaction can also be repeated to provide 2,9-disubstituted phenanthrolines. Taken together, these operations provide ready access to a large number of phenanthroline derivatives to serve as ligand libraries for catalyst exploration.
Alkyl radicals generated by treatment of thiocarbamates of conformationally favorable 3-alkyl-3-arylpropan-1-ols with tris(trimethylsilyl)silane and AIBN efficiently undergo intramolecular ipso substitution of the methoxy group, yielding the corresponding cyclized products. In contrast, either conformationally favorable or flexible 1-arylalkan-3- or 4-ones easily cyclize into five- or six-membered condensed rings by treatment with SmI(2) via ketyl radical intermediates. The addition of HMPA as cosolvent dramatically changes the cyclization mode of the SmI(2)-induced reaction, and the para-cyclization products are exclusively formed. This "HMPA effect" can be rationalized by the strong chelating ability of HMPA with the samarium atom.
Samarium(II)-mediated spirocyclization onto an aromatic ring was achieved by the reaction of methyl 4-(4-oxoalkyl)benzoates with SmI(2) in the presence of i-PrOH and HMPA, yielding methyl 1-alkyl-1-hydroxyspiro[4.5]dec-6-ene-8-carboxylates in moderate to high yields. Utilizing this chemistry, spiro[3.5] and -[5.5] systems, and sterically congested spiro[4.5] systems, were easily synthesized. For the successful conversion, appropriate activation of the aromatic ring has proven to be extremely important: while an ester or amide functionality on the aromatic ring can promote the spirocyclization, a sulfonamide substituent causes ortho cyclization.
Highly stereoselective synthesis of aromatic alpha,beta-unsaturated amides was achieved by treatment of aromatic alpha,beta-epoxyamides with samarium diiodide. The starting compounds 1 and 3 are easily prepared by the reaction of enolates derived from alpha-chloroamides with carbonyl compounds at -78 degrees C. A mechanism to explain this transformation is proposed.
A promoted SmI2 direct reaction of dibromoacetic acid with different aldehydes, followed by an elimination reaction also promoted by samarium diiodide, affords (E)-alpha,beta-unsaturated carboxylic acids 2 with total stereoselectivity. A mechanism to explain this transformation is proposed.
An easy, direct, general, and efficient samarium diiodide-mediated preparation of 3-hydroxyacids 1 in high yield by reaction of different aldehydes or ketones with commercially available iodoacetic acid is described. The application of different esterification procedures to the crude 3-hydroxyacids so obtained afforded the corresponding 3-hydroxyesters. Also, the cyclization of crude 3-hydroxycarboxylic acids allowed the preparation of beta-lactones. A mechanism is proposed to explain the synthesis of 3-hydroxyacids 1.
A new sequential reductive coupling process promoted by samarium(II) iodide is described. Cascade epoxide ring opening and ketyl radical olefin cyclization leads to a variety of cis-1,3-cyclopentanediols and cis-1,3-cyclohexanediols in good yields and with high diastereoselectivity.
Samarium(II) iodide has been employed to promote a sequential intramolecular nucleophilic acyl substitution/intramolecular ketyl olefin coupling cyclization sequence to provide bicyclic, tricyclic, and spiro-fused oxygen heterocycles in excellent yield and with high diastereoselectivity.
Samarium(II) iodide in conjunction with a catalytic low-valent transition metal species has been employed to promote the conjugate addition reaction of primary and secondary alkyl halides onto alpha,beta-unsaturated esters and amides. The method has been determined to be quite general and hence has been extended to the cyclization reactions of alkyl halides onto alpha,beta-unsaturated lactones, lactams, and nitriles. The cyclization reactions described herein provide a very general approach to the synthesis of functionalized carbocycles from simple acyclic precursors with excellent diastereoselectivity and under very mild reaction conditions.
Samarium(II) iodide (SmI(2)) has been employed in an intramolecular sequential ketyl-olefin coupling/beta-elimination reaction. The overall process results in the net addition of an alkenyl species to a ketone carbonyl. This novel protocol for the intramolecular delivery of an alkenyl moiety avoids the basic reaction conditions typical of nucleophilic additions that are mediated by alkenylmagnesium halides and alkenyllithium reagents. A high degree of stereocontrol is imparted in the SmI(2)-mediated process as a result of the excellent facial selectivity conveyed in the initial ketyl-olefin coupling reaction. The relative asymmetric induction engendered in these addition reactions is complementary to more traditional nucleophilic addition reactions in that the alkenyl group is directed to the carbonyl center by an attached tether.
Samarium(II) iodide has been employed to promote vinylogous pinacol coupling reaction of aldehyde onto alpha,beta-unsaturated ketones. The diastereoselectivity of 6-endo products was changed by addition of a proton source and/or HMPA and by the reaction temperature. The cyclization reactions described herein provide a general approach to the syntheses of 3,3-dimethylhydrindanes with a cis-relationship between the OH at C-4 and the proton at C-3a with good diastereoselectivity and under mild reaction conditions.
A strategy for making amides and ureas using a polymer-supported hydroxylamine resin as a traceless linker is described. The cleavage of the linker by samarium(II) iodide is reported for the first time.
Samarium(III)-(R)- or (S)-propylenediaminetetraacetate complex resolved the enantiomer signals of alpha-amino acids on high-field (1)H and (13)C NMR with remarkably less line broadening than was previously reported for the Eu(III) complex of the same ligand. A widely observed regularity between the absolute configuration of enantiomers and the relative shift of their NMR signals is useful for the assignment of absolute configuration.
Methyl thiophene-2-carboxylate, methyl 3-(thien-2-yl)acrylate, and methyl 5,2'-bithiophene-2-carboxylate were utilized as the synthetic equivalents of pentanoate 5-anion, pentanoate 4,5-dianion, heptanoate 7-anion, and nonanoate-8,9-dianion. By the promotion of samarium diiodide, these thiophene-incorporating compounds reacted with aldehydes, ketones, and conjugated esters regioselectively at the thienyl rings. Long-chain esters with remote hydroxyl and carboxyl groups, including an antiarthritis agent, a shellac component, and an inhibitory agent of spore germination, were prepared after reductive desulfurization on Raney nickel.