The Stereochemical Course of the Generation and Interception of aSix‐Membered Cyclic Allene: 3δ²‐1H‐Naphthalene (2,3‐Didehydro‐1,2‐dihydronaphthalene)

Abstract

Keywords: CD spectra / Cycloadditions / Cyclic allenes / Enantioselectivity / Quantum chemistry calculationsThe bromofluorocarbene adduct rac-1 of indene, possessing the fluorine atom at the endo position, is a useful substrate for the generation of the isonaphthalene 4 by the DoeringMoore-Skattebøl reaction. By resolution of rac-1, an enantiomerically pure precursor of a six-membered cyclic allene was obtained for the first time. The treatment of (+)-or (-)-1, dissolved in 2,5-dimethyl-, 2-tert-butyl-5-methyl-, or 2,5-bis-(tert-butyl)furan, with methyllithium gave rise to the [4+2] cycloadducts 6 of 4 to the furans. It was shown by means of HPLC on Chiralcel OD that the formation of 6 proceeded with about 40 % ee, and that this value was independent of the type and concentration of the furan, and the reaction temperature. The absolute configurations of the enantiomers 1, as well as those of the enantiomers 6, were determined by simulation of the CD spectra by quantum chemical methods and by the comparison of them with the experimental spectra. In the case of (+)-1, the reliability of this procedure was checked by X-ray crystal structure analysis. On the basis of

1‐Phenyl‐1,2‐cyclohexadiene: Astoundingly High Enantioselectivities on Generation in a Doering–Moore–Skattebøl Reaction and Interception by Activated Olefins

“…Therefore we propose the exclu- www.chemeurj.org sive liberation of (M)-7 from 5. As we had anticipated, [1] the stereoselectivity of the generation of a cyclic allene is impressively improved by a phenyl group at one bridgehead position of a substrate for the Doering-Moore-Skattebøl reaction.…”

“…[1] The results of carefully designed experiments led to the conclusion that (P)-and (M)-2 emerge from 1 in a ratio of about 70:30 and, without enantiomerisation, enantiospecifically add onto 3. This was unexpected, since the barrier to enantiomerisation of 2 had been estimated by quantum chemical methods to be only 11 kcal mol À1 , [2,3] which is why the reaction of 2 with 3 must be very fast.…”

“…70 %). [1] The application of this reasoning to the reaction 5!7 leads to an unambiguous conclusion, as this process requires the change of the ring sides either from the bridgehead hydrogen atom or the phenyl group. Owing to its size, the phenyl group should encounter a much larger restraint of its move by the proximal CH 2 group than the bridgehead hydrogen atom.…”

“…[1] Possibly, (M)-and (P)-2 are unable to clearly distinguish between the Re and the Si face of indene because of two hydrogen atoms at the allene termini. Clearly, the phenyl group of (M)-and (P)-7 makes the difference.…”

Halodiazoacetates as useful tools for selective halo-functionalization

“…In the reaction with indene, the cyclopropane products undergo a further, spontaneous ring expansion reaction resulting in ethyl 2-naphthoate [26,27]. This reaction goes to completion after treatment with K 2 CO 3 .…”