On the Absolute Configuration of (+)‐Indane‐1‐carboxylic Acid

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Abstract
The (R)‐configuration, attributed to (+)‐indane‐1‐carboxylic acid ((+)‐1) by Fredga [1], is unequivocally confirmed (Scheme 1). Configurational doubts, raised by an erroneous ORD. curve of (−)‐1‐methylindane ((−)‐4) published by Brewster & Buta [2], are unfounded (cf. the following paper of Brewster[3] and the corrections in (4)). This was further verified by preparing deuteriated 1‐methylindanes starting with (−) (R)‐3‐phenylbutyric acid ((−)‐(R)‐5) as well as with (+)‐(R)‐1 or (−)‐(S)‐1 (Scheme 2). The ORD. curves of the optically active 4 thus obtained were (disregarding deuteri‐um isotope effects) identical or antipodal, respectively (of. Fig. 1, 2, and 7a–e).Optically active methyl indane‐1‐carboxylates ((−)‐(R)‐14 or (+)‐(S)‐[l‐2H]‐14) show a strong solvent dependence of their ORD. and CD. spectra with a sign inver‐sion occuring in going from isooctane to methanol or benzene. The observed changes can be explained by a change in the population of conformations where the ester carbonyl group is eclipsed either with the C(1), C(2)‐ or C(1), H‐bond, with the n, π* ‐transition having a slightly different energy and the ester group an essentially enantiomeric environment with respect to its orientation relative to the benzene moiety.

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