On the Stereochemistry of the Aromatic Claisen Rearrangement. Thermal Rearrangement of erythroid and threoid ortho‐Dienones.Erythro‐ and threo‐1‐methyl‐1‐(1′‐methyl‐2′‐propynyl)‐2‐oxo‐1,2‐dihydronaph‐thalene (erythro‐ and threo‐6) as well as erythro‐ and threo‐2,6‐dimethyl‐6‐(1′‐methyl‐2′‐propynyl)‐cyclohexa‐2,4‐dien‐1‐one (erythro‐ and threo‐8) were obtained together with the corresponding aromatic ethers 5 and 7 by alkylation of 1‐methyl‐2‐naphthol and 2,6‐dimethyl‐phenol, respectively in alcoholic potassium hydroxide solution with 1‐methyl‐2‐propynyl p‐toluenesulfonate (Scheme 2). The diastereoisomeric dienones 6 and 8 were easily separated by column chromatography on silica gel and its relative configuration at C(1) or C(6) and C(1′) deduced from the chemical shifts in their 1H‐NMR.‐spectra (Table 1). Hydrogenation of 6 and 8 using Lindlar catalyst yielded the corresponding erythro‐ and threo‐configurated (1′‐methyl‐2′‐propenyl)‐dienones 10 and 13, respectively (Scheme 3) the thermal rearrangement of which were studied. The following results were obtained: threo‐10 rearranged in benzene at 85–105° preferentially via a chair‐like (C) transition state to yield 99,5% (E)‐ and 0,5% (Z)‐(2′‐butenyl) 1‐methyl‐2‐naphthyl ether ((E)‐ and (Z)‐14; ΔΔG (C/B) = −4,0 kcal/mol). On the other hand, erythro‐10 when heated at 105‐125° in benzene gave 84,7% (E)‐ and 15,3% (Z)‐14, i.e. in this case a boat‐like (B) transition state is favoured (G (C/B) = + 1,3 kcal/mol) (Scheme 5 and Table 2). The thermal rearrangement of dienones 13 led to the corresponding ethers 12 as well as p‐allyl‐phenols 11. Thus, heating of threo‐13 at 20–42° in cyclohexane resulted in the formation of 2,5% of ether 12, consisting of 98% of the (E)‐ and 2% of the (Z)‐isomer, and 97,5% of (E)‐11 which contained, at a maximum, 0,5% of the (Z)‐isomer, (Scheme 6 and Table 3). This means that both rearrangements occurred with a strong preference of the C transition state (G (C/B, phenol) = −3,3 kcal/mol). On the contrary, erythro‐13 when heated at 42–68° in cyclohexane yielded a 3:2 mixture of ether 12 and phenol 11 (Scheme 6). The ethereal part consisted of 88,0% of the (E)‐ and 12,0% of the (Z)‐isomer which again shows that the B geometry predominated in the erythro transition state leading to the ether (G (C/B)= + 1,3 kcal/mol). In the phenolic part 36–40% of the (E)‐isomer and 64–60% of (Z)‐isomer were found which means that in the para‐Claisen rearrangement of erythro‐13 the C arrangement is only slightly favoured (ΔΔG (C/B)= −0,36 kcal/mol).