Diels-Alder Reactions of Silyl-Substituted Masked o-Benzoquinones:
Expedient Access to Alkenylsilanes

Abstract

A one-pot synthesis of several silyl bicyclic and oxatricyclic alkenes by inter- and intramolecular Diels-Alder reactions of 4-trimethylsilyl substituted masked o-benzoquinones derived from the corresponding 2-methoxyphenols is described.

Key words

2-methoxyphenols - 4-silyl masked o-benzoquinones - Diels-Alder reaction - silyl bicyclo[2.2.2]octenes - silyl oxatricy­clo[4.3.1.0]decenes

References

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  Liao, C.-C.; Peddinti, R. K. Acc. Chem. Res., in press.

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Yen, C.-F; Peddinti, R. K.; Liao, C.-C., unpublished results.

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General procedure for intermolecular Diels-Alder reaction: To a stirred mixture of a 2-methoxyphenol (2a-c, 0.5 mmol) and dienophile (2 equiv) in MeOH (25 mL) at room temperature, was added DAIB (0.55 mmol, 1.1 equiv) at once and the reaction was continued at the same temperature. After a period of time (see Table [1] ), the reaction mixture was concentrated under reduced pressure and purified by silica gel column chromatography to get the pure adduct (4a-c to 6a-c).

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All the new compounds were satisfactorily characterized by IR, ¹H NMR (400 MHz), and ¹³C NMR (100 MHz), DEPT, and low and high resolution MS analyses.

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General procedure for intramolecular Diels-Alder reaction: To a stirred mixture of a 2-methoxyphenol (2a-c, 0.5 mmol) and an alkenol (7-10, 25 mmol, 50 equiv) at 0 °C, was added DAIB (0.55 mmol, 1.1 equiv) at once and the contents were stirred for 30 min at the same temperature [oxidations of 2-methoxyphenols in the presence of cinnamyl alcohol (9) were carried out at room temperature (entries 3, 7 and 11 in Table [2] )]. The reaction was quenched with aq NaHCO₃, extracted with ether. The organic layer was washed with brine, dried over anhyd. MgSO₄, and concentrated. The crude residue that contains mainly MOB (11a-c to 14a-c) and excess alkenol (7-10) was heated at 80 °C for 10 h. The reaction mixture was then cooled to room temperature, the excess alkenol was removed under reduced pressure and the residue was subjected to silica gel column chromatography to obtain the pure adduct (15a-c to 18a-c).

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CCDC No.: 182122.