Facial selectivity in the Diels-Alder reaction of some inverse electron demand 1,3-cyclopentadienes and a tandem-ene approach to the synthesis of a linear triquinane

Burry, Lori Carolyn (1998) Facial selectivity in the Diels-Alder reaction of some inverse electron demand 1,3-cyclopentadienes and a tandem-ene approach to the synthesis of a linear triquinane. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Facial selectivity in the Diels-Alder reaction of 1,2,3,4,5-pentachloro1,3-cyclopentadiene and its derivatives with OCH₃, H, CH₃ and Br substituents at C-5 is detailed. These dienes which normally react by the inverse-electron-demand mode, reacted with a range of electronically different dienophiles such as N-phenylmaleimide, styrene, 4-phenyl-1,2,4-triazoline-3,5dione, and vinylene carbonate. These dienes were shown to exhibit II-facial selectivity similar to the analogous 1,3-cyclopentadienes which react by the normal-electron-demand mode. The results indicated that both the nature of the dienophile and the substituent at C-5 impart a significant influence on the reaction. -- The facial selectivity behavior was also investigated by collaborators using high level ab initio calculations. The computational work in conjunction with the experimental data described in this thesis, lead to the conclusion that the mechanism of facial selectivity can be explained on the basis of a steric interaction between the diene and the dienophile. In the transition state, the facial selectivity is a function of both size of the substituent X on the diene and the length of the bond between C-5 and the substituent X. In this way, a larger substituent with a longer C-5–X bond can provide less steric hindrance than a small substituent with a shorter C-5–X bond. This is illustrated in the case of Cl versus H, in which addition syn to chlorine was preferred with N-phenylmaleimide. in the case of OMe versus Cl, the Cieplak theory predicted addition to the face of the diene anti to the better sigma donor. It was demonstrated, however, that this is not the case. All adducts resulting from additions to 1,2,3,4,5-pentachloro-5-methoxy-1,3-cyclopentadiene (16) are anti to Cl, which is a poorer sigma donor than Me. -- Tetraene 109 could serve as the precursor for a tandem or cascade ene reaction to produce a linear polyquinane. The “metallo-ene” reaction has been utilized to form polyquinanes through an iterative process, but a cascade scheme is proposed whereby isolation of reaction intermediates would not be required. The synthetic strategy required formation of a precursor similar to tetraene 109 which would be a model to test the viability of the tandem-ene reaction. It was decided to prepare a compound having functionality like that of the triene 111. Preparation of 2,2-dimethyl-4,6-heptadienal (128) by an acid catalyzed condensation of isobutyraldehyde and 1,4-pentadien-3-ol (142) was successful. Nucleophilic attack by 3-(tert-butyidimethylsilyloxy)-1-octyne (162) onto the aldehyde (128) gave an acetylenic analogue of the required precursor (111).

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