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A number of olefins prepared by the phosphonate carbanion modification of the Wittig reaction were shown to have trans configurations. Vapor phase chromatography of the intact reaction mixtures showed, at most, only trace amounts of cis isomers. To determine the degree of stereospecificity of the reaction under adverse circumstances, several experiments were devised with systems which, based on the generally accepted reaction mechanism,1•2 34567should be more favorable for the preparation of cis olefins. In only one case, however, was a significant amount of a cis isomer formed [6.4% cis-l,2-{ 1,1 z-dinaphthyl)ethylene in a 1:10 cis-trans mixture], demonstrating the great stereospecific tendency of the reaction. Possible interpretations of the observed differences in stereospecificity of olefin formation from phosphonate carbanions and phosphorus ylids are discussed.

Configurationally unstable biaryl lactones of type (M)-1 ⇌ (P)-1 and ring-opened 2-acyl-2‘-hydroxy biaryl compounds of type (M)-4 ⇌ (P)-4 are versatile precursors for the atroposelective preparation of axially chiral biaryls. The activation barriers of their atropisomerization process, which constitutes a fundamental precondition for the dynamic kinetic resolution, were determined by dynamic NMR spectroscopy for rapid processes and by HPLC-monitored racemization of enantiomerically enriched material for smaller interconversion rates. For the lactones, the free activation energies ΔG ⧧ 298 increase with the steric demand of the substituent R ortho to the biaryl axis in the series H < OMe (t 1/2 ≈ ms) < Me (t 1/2 ≈ s) < Et < i-Pr (t 1/2 ≈ min) < t-Bu (t 1/2 ≈ d). The formally ring-opened 2-acyl-2‘-hydroxy biaryls, which interconvert via the lactol isomers 5 as the cyclic (and thus configurationally less stable) intermediates, have a significantly slower atropisomerization rate as a result of the high loss in activation entropy ΔS ⧧ as a consequence of the required intermediate ring closure 4 → 5.

A practicable two-step procedure for the preparation of a series of lactone-type bridged biaryls 7 as favorable substrates for subsequent atropisomer-selective ring-opening reactions is described. Due to the efficiency of the coupling step, which tolerates even a telt·butyl group next to the biaryl axis and avoids problems of regioselectivity, a variety of differentlyThe regio-and stereoselective construction of natural and unnatural hindered biaryl systems is a synthetic challenge that has successfully been approached only very recentlyP-4 1 . Exemplarily for the substitution pattern as in 1, which is found in many naturally occurring biaryls l 5 1 , we have developed a useful procedure in which the two formal partial goals of stereoselective biaryl synthesis are attained consecutively l 6-11 1 : The CC bond formation is performed by intramolecular aryl coupling of the ester-type prefixed aromatics as in 3, and the asymmetric induction at this biaryl axis is achieved by an atropdiastereo-or enantioselective ring opening of the resulting lactones 2. Hence, such bridged biaryls 2 play a crucial role in this concept: Although they already possess the biaryl axis, most of them are axially prostereogenicl6.7.12.IJJ, i.e. they do not occur as stable atropisomers (helimers) at the synthetically relevant temperatures. Apparently, the rotational barrier of 2 is drastically lower than that of the open-chain final target biaryls 1 and thus allows rapid helimerization (e.g. at room temperature).For a thorough investigation of the structures of such bridged biaryls, of their isomerization mechanism, and eso 2 00 n ~o V 3 substituted representatives is prepared. These cover a broad range of steric hindrance and thus molecular distortion. The structures are investigated mainly by NMR spectroscopy and X-ray diffraction, showing the lactones 7 to be helically distorted, depending on the size of the residues R. pecially of the unprecedented stereocontrolled ring opening process, there is a great need of appropriately substituted representatives of this stereochemically interesting class of bridged biaryls. In this paper, we describe a practicable synthetic pathway to a series of benzonaphthopyranones of the general type 7, with a broad variety of substituents of most different sizes next to the biaryl axis. Part of this work has recently been reported in preliminary form 1 9 • 10 1.The structural type 7 was chosen for different reasons: On the one hand, its substitution pattern (alkyl groups and oxygen functions in ortho-positions with respect to the axis) is similar to that of many naturally occurring biaryls. Also, the symmetrical structure of the variable phenolic component helps to avoid additional problems of regioselectivity in the intramolecular aryl coupling step. A further advantage of the lactones 7 was their supposed tendency to afford crystals suitable for X-ray structure analysis (see below). By contrast, the long-chain O-substituted derivative 7c was prepared because of its expected favorable solubility properties,...