Base-Dependent Regio- and Diastereoselective Alkylation of Chiral Perhydro 1,3,2-Oxazabenzophosphorinane-2-oxides Derived from (-)-8-Benzylamino Menthol

Abstract

The regioselectivity in the alkylation of chiral perhydro 1,3,2-benzoxazaphosphorinane-2-oxides is dependent on the base used for deprotonation. The ethyl oxazaphosphorinanes are ben­zylated at the nitrogen substituent after deprotonation with n-BuLi, but α to the phosphorous atom by deprotonation with LDA. On the contrary, the benzyl oxazaphosphorinane is alkylated α to the phosphorous atom after deprotonation with both, n-BuLi or LDA.

alkylation - alkyl phosphonates - asymmetric synthesis - diastereoselective reactions - 2-oxo-1,3,2-oxazaphosphorinanes

References

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Typical Experimental Procedure: To a cooled (-78 °C) solution of the corresponding 1,3,2-oxazaphosphorinane (0.9 mmol) in dry THF (20 mL) under argon atmosphere was added the base (1.8 mmol), and the mixture was stirred for 30 min. Then, benzyl bromide (1.8 mmol) was added and the stirring was continued until the reaction was finished (TLC). The reaction was quenched with H₂O (9 mL) and stirred until the mixture was reached r.t. THF was removed under reduced pressure and the aqueous mixture extracted with CH₂Cl₂ (3 × 25 mL). The combined organic layers were dried (MgSO₄), filtered and concentrated. The residue was purified by flash chromatography on silica gel and hexane-EtOAc (1:2) as eluent.

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