Diastereoselectivity control in photosensitized addition of methanol to (R)-(+)-limonene

Highly diastereodifferentiating bimolecular asymmetric photoreaction was achieved in the photosensitized polar addition of methanol to (R)-(+)-limonene. The diastereomeric excess (de) of the photoadduct could be controlled and fine-tuned by changing the internal/external factors such as solvent polarity, reaction temperature, and structure of the sensitizers. The de increased from 23% obtained upon xylene photosensitization in pure methanol at room temperature to >96% upon singlet sensitization with methyl benzoate at -75 degrees C in 0.5 M methanol/diethyl ether solution.