Strained ring systems. XV. A regiospecific, nonconcerted semi-benzilic acid rearrangement of endo-2-methoxy-1,4-dichlorobicyclo[2.2.1]heptan-7-one
✍ Scribed by Richard N. McDonald; Cesin A. Curi
- Book ID
- 104212863
- Publisher
- Elsevier Science
- Year
- 1976
- Tongue
- French
- Weight
- 206 KB
- Volume
- 17
- Category
- Article
- ISSN
- 0040-4039
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✦ Synopsis
Of the several methods now available for preparing derivatives of bicyclo[2.2.O]hexane, the Scherer synthesis 2,3 offers certain practical advantages. The key step in the synthetic sequence is the semi-benzilic acid ring contraction of 1,4-dichlorobicyclo[2.2.l]heptan-7one (&) with powdered sodium hydroxide in THF to 4-chlorobicyclo[2.2.O]hexane-l-carboxylic acid (a). We wish to report evidence which supports the mechanistic premise that rearrangement of oxy-anion 2 X X X &,X=H 2, X=H &X=H 2. X = OCH3 4, X = OCH3 5, X = OCH3 (and 3) occurs in a two-step process via a C4 anion rather than by a direct, concerted step involving C4-C7 bond rupture with ejection of chloride ion at Cl as the Cl-C4 bond in 2 (and 2) is formed. The substrate with which we chose to examine this interesting ring contraction process was endo-2-methoxy-1,4-dichlorobicyclo[2.2.l]heptan-7-one (z).4 It would not only be reasonably representative of 2 in this reaction, but would have present in the product r2.2.01 system a versatile function at C3 for future conversions. Reaction of 2 with powdered sodium hydroxide in THF at 40" produced a mixture of acid 2 (89%) and the trisubstituted cyclohexanecarboxylic acid 2 (11%) in quantitative yield. The structure of acid 2 was arrived at from analysis of its spectral data -mass spectrum (70 eV, heated inlet): 226 m/e (Mt); pmr (CDC13 + DMSO-d6, internal TMS): r 1.50 (s, C02H, 1). 5.50 (t of d's, C4-H, l), 6.61 (6 of 8 lines, C3-H, l), 6.65 (8, OCH3. 3), and 7.2-8.3 (m, C2 5 6-H's, 6); and 13 C nmr (see Table I). Reaction of acid 2 with Pb(OAc)4 and LiCl (Kochi reAc;ion)5 gave cis-3-methoxy-1,1,4-trichlorocyclohexane (2) in 95% yield where now the full -