Premium
New Heptalenes Substituted with Extended π‐Systems
Author(s) -
Houar Sarah El,
Hansen HansJürgen
Publication year - 1997
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19970800123
Subject(s) - chemistry , wittig reaction , substituent , bromide , allylic rearrangement , medicinal chemistry , isopropyl , iodide , carboxylate , stereochemistry , yield (engineering) , organic chemistry , catalysis , materials science , metallurgy
The synthesis of π‐substituted heptalenecarboxylates or ‐dicarboxylates, starting with the easily available dimethyl 9‐isopropyl‐1, 6‐dimethylheptalene‐4, 5‐dicarboxylate ( 2b ), are described. Treatment of 2b with t‐BuOK and C 2 Cl 6 at −78° leads to the chemoselective introduction of a Cl substituent in Me‐C(1) (see 5b in Scheme 1 ). Formation of the corresponding triphenylphosphonium salt 7b via the iodide 6b ( Scheme 2 ) allowed a Wittig reaction with cinnamaldehyde in the two‐phase system CH 2 Cl 2 /2N NaOH. Transformation of the 4, 5‐dicar‐boxylate of 2b into the corresponding pseudo‐ester 10b allowed the selective reduction of the carbonyl function at C(4) with DIBAH to yield the corresponding 4‐carbaldehyde 11b ( Scheme 3 ). Wittig reaction of 11b with (benzyl) triphenylphosphonium bromide led to the introduction of the 4‐phenylbuta‐1, 3‐dienyl substituent at C(4). The combination of both Wittig reactions led to the synthesis of the 1, 4‐bis(4‐phenylbuta‐1, 3‐dienyl)‐substituted heptalene‐5‐carboxylate (all‐E)‐ 17b ( Scheme 5 ). In a similar manner, by applying a Horner‐Wadsworth‐Emmons reaction, followed by the Wittig reaction, the donor‐acceptor substituted heptalene‐5‐carboxylate ( E;E )‐ 22b was synthesized ( Scheme 7). Most of these new heptalenes are in solution, at room temperature, in thermal equilibrium with their double‐bond shifted (DBS) isomers. In the case of (all‐E)‐ 17b and ( E;E )‐ 22b , irradiation of the thermal equilibrium mixture with light of λ ‐(439 ± 10) nm led to a strong preponderance ( > 90%) of the DBS isomers 17a and ( E;E )‐ 22a , respectively ( Schemes 6 and 7 ). Heating of the photo‐mixtures at 40° re‐established quickly the thermal equilibrium mixtures. Heptalenes‐carboxylates (all‐ E )‐ 17a and ( E;E )‐ 22a which represent the off‐state of a 1,4‐conjugative switch (CS) system show typical heptalene UV/VIS spectra with a bathochromically shifted heptalene band III and comparably weak heptalene bands II and I which appear only as shoulders ( Figs. 4 and 5 ). In contrast, the DBS isomers (all‐ E )‐ 17b and (all‐ E )‐ 22b , equivalent to the on‐state of a 1,4‐CS system, exhibit extremely intense heptalene bands I and, possibly, II which appear as a broad absorption band at 440 and 445 nm, respectively, thus indicating that the CSs (all‐ E )‐ 17a ⇌(all‐ E )‐ 17b and ( E;E )‐ 22a ⇌( E;E )‐ 22b are perfectly working.