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Abstract. The usefulness of molecular-mechanics calculations in the study of the conformational properties of several tetraarylporphyrins and diarylporphyrins is demonstrated. Energy-minimized conformations and rotational barriers of several substituted and unsubstituted arylporphyrins correspond very well with experimental results.

Reaction of decalones 6a-e with TosMIC gives adducts 7a-e which can be methylated concomitant with regioselective migration of the double bond to an endocyclic position. The regioselectivity of the double bond migration is determined by the presence or absence of a free hydroxyl group a t C(4). Base-catalyzed methylation of TosMIC adducts 7b-d, possessing a free hydroxyl group a t (341, and subsequent acid-catalyzed hydrolysis led preferentially to the C(6)-C(7) double bond isomers lob-d, respectively. An intramolecular deprotonation of H-6 is considered to be responsible for this regioselectivity. If the hydroxyl group a t C(4) is absent or protected as in 7a and 7e, respectively, the C(7)-C(8) double bond isomers 9a and 9c are obtained via a n intermolecular abstraction of a sterically less shielded proton a t C(8). The usefulness of this methodology is illustrated by the total synthesis of (f)-6-eudesmen-4a-ol (18) and (f)-vetiselinene (22).(34) These compounds were obtained as mixtures of diastereoisomers.

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