Synthesis of Precursors Suitable for Pyrolytic Fragmentation to Pentatetraenone

Abstract

Reaction of 2-phenylselenopropanoyl chloride with methyl ( triphenylphosphoranylidene ) ethanoate yielded methyl 4- phenylselenopenta-2,3-dienoate (3). Distillation of the ester (3) gave dimethyl (E,E)-3′,4′-dimethylcyclobut-3′-ene-1′,2′- diylidenebisethanoate (5) and oxidation of (3) with potassium peroxymonosulfate yielded methyl 2-oxopent-3-ynoate (8). The ester (3) with cyclopentadiene at 160-180° gave a Diels-Alder adduct methyl 3- (1′-phenylselenoethylidene) bicyclo [2.2.1]hept-5-ene-2-carboxylate (4) as a mixture of four isomers. Treatment of the mixture with lithium diisopropylamide and methyl chloroformate yielded a mixture of two isomers of dimethyl 3-(1′-phenylselenoethylidene) bicyclo [2.2.1]hept-5- ene-2,2-dicarboxylate (16). Oxidation of the monoester (4) and the diester (16) with m-chloroperbenzoic acid gave the corresponding selenoxides which on thermal elimination in boiling tetrachloromethane /potassium carbonate gave endo/exo methyl 3-ethenylidenebicyclo [2.2.1]hept-5-ene-2-carboxylate(10) and dimethyl 3-ethenylidenebicyclo [2.2.1]hept-5-ene-2,2-dicarboxylate (18). Hydrolysis of these esters with lithium hydroxide gave 3-ethenylidenebicyclo [2.2.1]hept-5-ene-2-carboxylic acid (12) as a mixture of endo/exo isomers and 3-ethenylidenebicyclo[2.2.1]hept-5- ene-2,2-dicarboxylic acid (19). The diacid (19) was treated with HC=C- OEt , HC=C- OMe , Me2Si(O-C( OMe )=CMe2)2 and with (CF3CO)2O to give compounds (20)-(23) as expected pyrolytic precursors of pentatetraenone. The mixture of monoacids (12) was also treated with (CF3CO)2O to give mixed anhydride (13). The 13C-carboxyl labelled acid (12′) was similarly converted into the labelled mixed anhydride (13′) as a pyrolytic precursor of H2C=C=C=C=13C=O. The following paper reports an argon matrix infrared spectroscopic study of the pyrolysates from compounds (13), (13′), (21), (22) and (23).