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Advances in Heterocylic Chemestry. Vol 10 - Boulton A.J.

Boulton A.J. Advances in Heterocylic Chemestry. Vol 10 - Academic Press, 1969. - 347 p.
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l,5-diphenyl-2,4-pentadien-l-one (63, R = R" = Ph, R' = H) with acetophenonc and Ph3C+C104~ yields two salts: 2,6-diphenylpyrylium by dehydrogenation of the former reagent alone (cf. Section II, B, 2, e), and 2,6-diphenyl-4-styrylpyrylium by condensation of both reagents.
In the case of t-butyl-substituted x,/3-unsaturated ketones, however, no reaction with ketones occurred in the presence of triphenylmethyl fluoroborate ;379a instead, a 1,2-methyl shift in the unsaturated ketone accompanied by cyclization afforded a crystalline dihydro-furylium salt.379a,b
An interesting application with a methylene ketone was described by Boyd380 for the synthesis of pseudo azulenes from the oxalene381
378 S. A. Kodak, Belgian Patent No. 623,972 (1963); Chem,. Abstr. 63, 10102 (1965).
379 M. Simalty-Siemiatycki and R. Fugnitto, Bull. Soc. Chim. France p. 1944
379a Rundel and K. Besserer, Tetrahedron Letters p. 4333 (1968).
379b K. Dimroth and W. Maeh, Angew. Chem. 80, 490 (1968).
380 G. V. Boyd, J. Chem. Soc. p. 55 (1959).
381 W. Treibs and W. Schroth, Ann. Chem. 642, 82 (1961).
class. Substituted ideno[ 1.2-6] pyrylium salts (117, R = H or Ph) may be obtained from indan-l-ones and chalcone, or from benzylidene-indan-l-ones and acetophenone, by the action of FeCl3 +Ac20. Analogously, 3-phenylcyolopent-2-enone (118) reacts with unsaturated ketones (119, R = Ph or ?-Bu) yielding cyclopenta(6)-pyrylium chloroferrates (120), which may be reversibly deprotonated to colored anhydro bases (121).382,383
Recently, the cyclopenta[c]pyran (122) was obtained as a secondary product along with 1,2,3-tribenzoylcyclopropane from phenacyl-trimethylammonium hydroxide; a complicated sequence was proposed involving a Michael addition to 1,2-dibenzoylethylene intermediately formed.384
An indeno[l .2-&]pyrylium (123) has been reported to result from the autocondensation of chalcone (FeCl3 + Ae20).385
382 G. Y. Boyd, J. Chem. Soc. p. 1979 (1958).
383 G. V. Boyd and A. W. Ellis, J. Chem. Soc. B, 349 (1966).
384 J. Harley-Mason and C. R. Harrison, J. Chem. Soc. p. 4872 (1963).
385 G. Oosterloo, Ph.D. Thesis, University of Marburg, 1958.
R Ph
rO 0^N'Ph
h. By Dehydrogenative Condensation of Unsaturated Ketones with Aryl Acetylenes. This reaction is analogous to the previous type in Section II,C,2,g. Phenylacetylene may replace acetophenone in the condensation with chalcone, in the presence of boron trifluoride etherate.386 The yield is very low in ether (2%), but is 80% in carbon tetrachloride.
D. Three-Component Syntheses
The three possibilities of synthesizing a C5 chain, namely, C2+C1-(-C2,C2 + C2 + Ci, or Ci + C3 + Ci, lead to pyrylium salts having identical substituents in positions 2 and 4 in the second case, or 2 and 6 in the first and third cases. Despite this limitation, such syntheses are very convenient because they make the pyrylium salts easily accessible (more so than other six-membered heterocyclic aromatics) from aliphatic starting materials.
1. From C2 + Ci + 2 Units
a. From Acid Chlorides or Diazoketones and 2 Moles of Acyl-methylene-Triphenylphosphorane. Strzelecka, Simalty-Siemiatycki, and Prevost387 showed that benzoyldiazomethane (124, R' = Ph) reacts at 140 with 2 moles of benzoylmethylene-triphenylphos-phorane (125, R = Ph) giving a 6% yield of the methylenepyran 126 which may be reversibly protonated to 2,6-diphenyl-4-benzyl-pyrylium (127, R = R' = Ph).
By employing bromophenyl derivatives,88-388 the origin of the aryl residues in the reaction product was traced: the 4-aryl residue
386 W. J. T. Bos and J. F. Arens, Rec. Trav. Chim. 82, 845 (1963).
387 H. Strzelecka, M. Simalty-Siemiatycki, and C. Prevost, Gompt. Rend. 254, 696 (1962).
388 H. Strzelecka, Gompt. Rend. 255, 731 (1962).
R'COCHNa + 2RCOCH=PPh3 (124) (125)
originates from the diazoketone, and the 2- and 6-aryls from the acyl phosphine. The mechanism probably involves a Wolff rearrangement of the diazoketone yielding an aryl ketene (R'CH = C0) which then reacts with the nucleophilic acyl phosphine giving an allenie ketone. A second mole of acylmethylenephosphorane then adds in a Michael-type reaction889,390 to the activated C=C double bond affording an enolic betaine which eliminates triphenylphosphine oxide, on heating, leaving an acetylenic ketone which then cyclizes to 126.
Support for this mechanism391-393 comes from the reaction of diphenylketene with the phosphine 125 leading to 23 (Y = CPh2), from
R'CH=CO ---------------> R'CH=C=CHCOR --------------------->-
R'CH=CC -------*
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