[60603-16-9] · C11H12OS · 2-Methoxy-3-phenylthio-1,3-butadiene · (MW 192.30)
Solubility: sol most organic solvents.
Preparative Methods: the cyclobutene precursors (e.g. 1) to the reagent and related dienes are available from cyclobutanone in three steps (eq 1).1 The ring opening of the cyclobutenes to form the title reagent (2) and related dienes may be accomplished by flash vacuum pyrolysis (eq 2), or is often carried out in situ in the presence of a dieneophile. Due to the instability of the dienes, they should be formed as needed from the precursor cyclobutenes.
Handling, Storage, and Precautions: the dienes should be both formed and used in the presence of a radical inhibitor such as BHT.
2-Alkoxy(acyloxy)-3-arylthiobuta-1,3-dienes such as (2) are highly reactive and versatile in Diels-Alder reactions.2 The Diels-Alder regiochemistry is usually controlled by the sulfur (eq 3), but the competition between dominance by the sulfur or oxygen depends on the substitution on the heteroatoms, the presence of Lewis acids, and the choice of dienophile. For example, the 2-acetoxy-3-p-anisylthiobutadiene (3) displays an increased tendency for sulfur dominance, especially in the presence of Lewis acids (eq 4). With an electron-withdrawing 2´-pyrimidyl group on the sulfur, the oxygen becomes dominant (eq 5).
When the sulfur dominates the regiochemistry, these dienes provide entries into 1,3-disubstituted cyclohexanes after hydrolysis and desulfurization, reversing the normal orientation of Diels-Alder reactions. The more readily available and stable 1-phenylthio derivatives should be considered as alternatives for this purpose (see 2-Methoxy-1-phenylthio-1,3-butadiene). The phenylthio group is also useful for directing alkylations of the ketone obtained on hydrolysis of the Diels-Alder adducts (eq 6).
Daniel A. Singleton
Texas A&M University, College Station, TX, USA