nated as a "protonated cyclopropane"). By way of 1 or a properly substituted derivative, an endo-hydrogen at C-6 will migrate to the two possible configurations at C-2 with exactly or approximately equal probability, whereas in 2a, 2b, or related species it will migrate exclusively to become bound endo to C-2. We now report experiments which show that, in the 2-carboxy-3methyl-5-norbornyl cation, intramolecular transannular hydride shift is exclusively endo + endo.trans-Crotonic acid-3-d, prepared6 from acetaldehyde-1 -8 and malonic acid, reacts with cyclopentadiene to give a mixture of 3-exo-methyl-5-norbornene-2endo-carboxylic acid-3-endo-d (4b) and 3-endo-methyl-5-norbornene-2-exo-carboxylic acid-3-exo-d (Sb), which can be separated by the iodolactone method, as in the undeuterated series 4a and Sass The position of the deuterium in 4b and Sb is evident from the method of synthesis, from the collapse of the n.m.r. methyl doublets of 4a and Sa to singlets, and from the disappearance of the one-proton multiplets of the C-3 hydrogen of 4a and Sa in the spectra of 4b and Sb.Under conditions (50% by volume sulfuric acid, 21 hr.) previously reported8 to give the lactones 6a and 7a from either 4a or Sa, either deuterated acid 4b or Sb gives the unrearranged lactone 6b (that from 4b being isolated in quantity sufficient to show that deuterium is exclusively at '2-3) and the rearranged lactone 7b, with deuterium exclusively at C-2.The total deuterium content of products 6b and 7b is the same as that of the starting materials, and, accordingly, the deuterium shift, which on purely structural grounds must have occurred, is entirely intramolecular. The complete disappearance in the n.m.r. spectrum of 7b of the one-proton signal at 4.18 p.p.m. (downfield from tetramethylsilane), which is associated with the C-2 hydrogen of 7a, indicates that less than about 3 % of C-2 protio species can be present. The results are in accord with the indicated mechanism (in which the question of nonclassical or classical nature of the cations is temporarily begged).OThe intermediates or transition states for the two hydride shifts required to transform 4b to rearranged lactone 7 by way of nortricyclonium species would be represented as 10 and 11. Formation of exclusively 2-deuterio lactone 7b then becomes inexplicable; this product would arise from 11, but 10, which is formed earlier in the reaction scheme and differs from 11 only in that the positions of deuterium and the C-2 hydrogen are reversed, would have been expected to form 2-protio rearranged lactone, in conflict with experiment. l o
