Advances in Heterocyclic Chemistry, Vol. 57 by Alan R. Katritzky

By Alan R. Katritzky

This quantity within the sequence summarizes the numerous tools of the synthesis of heterocycles from azadienes with sections protecting 1-azadienes, 2-azadienes, and 1,3-diazadienes. It covers the piperazine-2,5-diones and comparable lactim ethers - entire overview of those hugely very important intermediates for the education of a wide selection of ordinary items. The e-book explains how 1,2,4-Triazolo [1,5-a] pyrimidines are of specific value in images with different major program in prescription drugs and agrochemicals. It additionally reports tetramic acids, an importantgroup of usual items displaying signifcant organic task, in addition to a few chemistry.

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Following this strategy, the authors were able to achieve the total synthesis of piperidine alkaloid (+)-sedridine 90 in a few steps (Scheme 22); the process implies the diastereoselective intramolecular Diels-Alder reaction of the 1-azadiene derived from acrolein, lithium hexamethyldisilazane, and the homoallyl chlorocarbonate to give k& FVP 0 OAc 80 81 82 0 83 84 SCHEME 21 20 JOSE BARLUENGA AND MIGUEL TOMAS [Sec. - H2. - KOH 0 89 90 SCHEME 22 cycloadduct 89 in 30% yield, followed by reduction of the carbon-carbon double bond and cleavage of the urethane ring (91TL4371).

H2. - KOH 0 89 90 SCHEME 22 cycloadduct 89 in 30% yield, followed by reduction of the carbon-carbon double bond and cleavage of the urethane ring (91TL4371). A completely different method for the preparation of 1-acyl-I-azabutadienes, namely thermal electrocyclic ring opening of I-acyl-2-azetines, as well as for their intramolecular Diels-Alder reactions has been utilized by Jung and Choi (91JOC6729) (Scheme 23). Flash vacuum pyrolysis of 91 above 300°C at 5 mm Hg allowed isolation of azadiene derivatives 92, which in turn were cyclized to 93 on heating in refluxing benzene; the direct isolation of 93 was effected by raising the pressure at the FVP stage to 20-30 mm Hg.

When azadienes 2 (R2 = Ph; R3 = Me) and dichlorodiphenyl silane were stirred in toluene at room temperature and then heated at 60°C with dimethyl acetylenedicarboxylic acid, eight-membered heterocycles 121 were isolated in 60-71% yield. All attempts to desilylate compounds 121 preserving the ring structure failed; thus, the protodesilylation reaction induced by trifluoroacetic acid resulted in the simultaneous ring contraction to pyridines 122 in quantitative yield. The formation of 121 is now understood by nucleophilic attack of the enamine nitrogen atom-in this case attack by the Cp-enamine carbon is precluded because of methyl substitution-into the ester attached to C-3 in a rather conformationally rigid intermediate 120 [86AG(E) 181 ;92S1061.

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